RESUMEN
Drug addiction is a chronic disorder that is characterized by compulsive drug seeking and involves cycling between periods of compulsive drug use, abstinence, and relapse. In both human addicts and animal models of addiction, chronic food restriction has been shown to increase rates of relapse. Previously, our laboratory has demonstrated a robust increase in drug seeking following a period of withdrawal in chronically food-restricted rats compared with sated rats. To date, the neural mechanisms that mediate the effect of chronic food restriction on drug seeking have not been elucidated. However, the paraventricular nucleus of the thalamus (PVT) appears to be a promising target to investigate. The objective of the current study was to examine the role of the PVT in the augmentation of heroin seeking induced by chronic food restriction. Male Long-Evans rats were trained to self-administer heroin for 10 days. Rats were then removed from the training chambers and experienced a 14-day withdrawal period with either unrestricted (sated) or mildly restricted (FDR) access to food. On day 14, rats underwent a 1-hour heroin-seeking test under extinction conditions, during which neural activity in the PVT was either inhibited or increased using pharmacological or chemogenetic approaches. Unexpectedly, inhibition of the PVT did not alter heroin seeking in food-restricted or sated rats, while enhancing neural activity in the PVT-attenuated heroin seeking in food-restricted rats. These results indicate that PVT activity can modulate heroin seeking induced by chronic food restriction.
Asunto(s)
Conducta Animal/efectos de los fármacos , Privación de Alimentos/fisiología , Dependencia de Heroína/fisiopatología , Núcleo Hipotalámico Paraventricular/efectos de los fármacos , Núcleo Hipotalámico Paraventricular/fisiopatología , Animales , Modelos Animales de Enfermedad , Heroína/farmacología , Dependencia de Heroína/psicología , Masculino , Ratas , Ratas Long-EvansRESUMEN
The infralimbic medial prefrontal cortex (IL) is important for suppressing learned behavior after extinction, but whether this function extends to responses acquired through appetitive Pavlovian conditioning is unclear. We trained male, Long-Evans rats to associate a white-noise conditional stimulus (CS; 10 s; 14 presentations per session) with 10% liquid sucrose (0.2 mL per CS presentation), and recorded entries into the fluid port during the CS. The CS was presented without sucrose in subsequent extinction and test sessions. Increasing IL activity with pretest microinfusions of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA; 0, 0.3 nmol; 0.3 µl/side) reduced the reinstatement of CS-elicited port entries. The same result was obtained when IL neurons that expressed Channelrhodopsin-2 (ChR2) were optically stimulated during CS presentations at test (473 nm, 5 ms pulses at 20 Hz for 10.2 s, unilateral). Optical stimulation of ChR2-expressing IL neurons during CS presentations also reduced spontaneous recovery and context-induced renewal. Furthermore, optical stimulation (1) during intertrial intervals had no impact on renewal, (2) depolarized ChR2-expressing IL pyramidal neurons in vitro, and (3) preferentially increased Fos in ChR2-expressing neurons. These novel converging data highlight a critical role for the IL in suppressing the return of appetitive Pavlovian-conditioned responding following extinction.
Asunto(s)
Condicionamiento Clásico/fisiología , Extinción Psicológica/fisiología , Corteza Prefrontal/fisiología , Células Piramidales/fisiología , Animales , Masculino , Optogenética , Ratas Long-EvansRESUMEN
Local field potential (LFP) oscillations in the granule cell layer (GCL) of the cerebellar cortex have been identified previously in the awake rat and monkey during immobility. These low-frequency oscillations are thought to be generated through local circuit interactions between Golgi cells and granule cells within the GCL. Golgi cells display rhythmic firing and pacemaking properties, and also are electrically coupled through gap junctions within the GCL. Here, we tested if gap junctions in the rat cerebellar cortex contribute to the generation of LFP oscillations in the GCL. We recorded LFP oscillations under urethane anesthesia, and examined the effects of local infusion of gap junction blockers on 5-15 Hz oscillations. Local infusion of the gap junction blockers carbenoxolone and mefloquine resulted in significant decreases in the power of oscillations over a 30-min period, but the power of oscillations was unchanged in control experiments following vehicle injections. In addition, infusion of gap junction blockers had no significant effect on multi-unit activity, suggesting that the attenuation of low-frequency oscillations was likely due to reductions in electrical coupling rather than a decreased excitability within the granule cell layer. Our results indicate that electrical coupling among the Golgi cell networks in the cerebellar cortex contributes to the local circuit mechanisms that promote the occurrence of GCL LFP slow oscillations in the anesthetized rat.
Asunto(s)
Ondas Encefálicas/fisiología , Cerebelo/metabolismo , Uniones Comunicantes/metabolismo , Neuronas/metabolismo , Potenciales de Acción/efectos de los fármacos , Potenciales de Acción/fisiología , Anestésicos Intravenosos/farmacología , Animales , Ondas Encefálicas/efectos de los fármacos , Carbenoxolona/farmacología , Fármacos del Sistema Nervioso Central/farmacología , Cerebelo/citología , Cerebelo/efectos de los fármacos , Uniones Comunicantes/efectos de los fármacos , Masculino , Mefloquina/farmacología , Neuronas/citología , Neuronas/efectos de los fármacos , Ratas Sprague-Dawley , Uretano/farmacologíaRESUMEN
The superficial layers of the entorhinal cortex receive sensory and associational cortical inputs and provide the hippocampus with the majority of its cortical sensory input. The parasubiculum, which receives input from multiple hippocampal subfields, sends its single major output projection to layer II of the entorhinal cortex, suggesting that it may modulate processing of synaptic inputs to the entorhinal cortex. Indeed, stimulation of the parasubiculum can enhance entorhinal responses to synaptic input from the piriform cortex in vivo. Theta EEG activity contributes to spatial and mnemonic processes in this region, and the current study assessed how stimulation of the parasubiculum with either single pulses or short, five-pulse, theta-frequency trains may modulate synaptic responses in layer II entorhinal stellate neurons evoked by stimulation of layer I afferents in vitro. Parasubicular stimulation pulses or trains suppressed responses to layer I stimulation at intervals of 5 ms, and parasubicular stimulation trains facilitated layer I responses at a train-pulse interval of 25 ms. This suggests that firing of parasubicular neurons during theta activity may heterosynaptically enhance incoming sensory inputs to the entorhinal cortex. Bath application of the hyperpolarization-activated cation current (Ih) blocker ZD7288 enhanced the facilitation effect, suggesting that cholinergic inhibition of Ih may contribute. In addition, repetitive pairing of parasubicular trains and layer I stimulation induced a lasting depression of entorhinal responses to layer I stimulation. These findings provide evidence that theta activity in the parasubiculum may promote heterosynaptic modulation effects that may alter sensory processing in the entorhinal cortex.
Asunto(s)
Vías Aferentes/fisiología , Corteza Entorrinal/citología , Potenciales Postsinápticos Excitadores/fisiología , Hipocampo/fisiología , Sinapsis/fisiología , Análisis de Varianza , Animales , Bicuculina/análogos & derivados , Bicuculina/farmacología , Fármacos Cardiovasculares/farmacología , Estimulación Eléctrica , Antagonistas de Aminoácidos Excitadores/farmacología , Potenciales Postsinápticos Excitadores/efectos de los fármacos , Antagonistas de Receptores de GABA-A/farmacología , Masculino , Técnicas de Placa-Clamp , Ácidos Fosfínicos/farmacología , Pirimidinas/farmacología , Ratas , Ratas Long-Evans , Sinapsis/efectos de los fármacos , Factores de Tiempo , Valina/análogos & derivados , Valina/farmacologíaRESUMEN
BACKGROUND: Thalamocortical electroencephalographic rhythms in gamma (30-80 Hz) and high-gamma (80-200 Hz) ranges have been linked to arousal and conscious processes. We have recently shown that propofol causes a concentration-dependent attenuation of the power of thalamocortical rhythms in the 50 to 200 Hz range and that this effect is far more pronounced for the thalamus. To determine whether similar attenuation occurs with other anesthetics, we characterized the concentration-effect relationship of the inhaled anesthetic isoflurane on the spectral power of these rhythms. METHODS: Local field potentials were recorded from the barrel cortex and ventroposteromedial thalamic nucleus in 9 chronically instrumented rats to measure spectral power in the gamma/high-gamma range (30-200 Hz). Rats were placed in an airtight chamber and isoflurane was administered at 0.75%, 1.1%, and 1.5% concentrations. Spectral power was assessed during baseline, at the 3 isoflurane concentrations after 30 minutes for equilibration, and during recovery over 4 frequency bands (30-50, 51-75, 76-125, and 126-200 Hz). Unconsciousness was defined as sustained loss of righting reflex. Multiple linear regression was used to model the change in power (after logarithmic transformation) as a function of concentration and recording site. P values were corrected for multiple comparisons. RESULTS: Unconsciousness occurred at the 1.1% concentration in all animals. Isoflurane caused a robust (P ≤ 0.008) linear concentration-dependent attenuation of cortical and thalamic power in the 30 to 200 Hz range. The concentration-effect slope for the thalamus was steeper than for the cortex in the 51 to 75 Hz (P = 0.029) and 76 to 200 Hz (P < 0.001) ranges but not for the 30 to 50 Hz range (P = 0.320). Comparison with our previously published propofol data showed that slope for cortical power was steeper with isoflurane than with propofol for all frequency bands (P = 0.033). For thalamic power, the slope differences between isoflurane and propofol were not statistically significant (0.087 ≤ P ≤ 0.599). CONCLUSIONS: Isoflurane causes a concentration-dependent attenuation of the power of thalamocortical rhythms in the 30 to 200 Hz range, and this effect is more pronounced for the thalamus than for the cortex for frequencies >50 Hz. In comparison with propofol, isoflurane caused a greater attenuation in the cortex, but the effects on the thalamus were similar. Isoflurane and propofol cause common alterations of fast thalamocortical rhythms that may constitute an electrophysiologic signature of the anesthetized state.
Asunto(s)
Anestésicos por Inhalación/administración & dosificación , Corteza Cerebral/efectos de los fármacos , Electroencefalografía , Ritmo Gamma/efectos de los fármacos , Isoflurano/administración & dosificación , Tálamo/efectos de los fármacos , Potenciales de Acción/efectos de los fármacos , Animales , Conducta Animal/efectos de los fármacos , Corteza Cerebral/fisiología , Estado de Conciencia/efectos de los fármacos , Relación Dosis-Respuesta a Droga , Masculino , Ratas Long-Evans , Reflejo de Enderezamiento/efectos de los fármacos , Procesamiento de Señales Asistido por Computador , Tálamo/fisiología , Factores de TiempoRESUMEN
BACKGROUND: The lateral entorhinal cortex receives inputs from ventral tegmental area dopamine neurons that are activated by exposure to food-related cues, and exogenously applied dopamine is known to modulate excitatory synaptic responses within the entorhinal cortex. METHODS: The present study used in vivo synaptic field potential recording techniques to determine how exposure to cues associated with food reward modulates synaptic responses in the entorhinal cortex of the awake rat. Chronically implanted electrodes were used to monitor synaptic potentials in the entorhinal cortex evoked by stimulation of the piriform (olfactory) cortex, and to determine how synaptic responses are modulated by food-related cues. RESULTS: The amplitudes of evoked synaptic responses were reduced during exposure to cues associated with delivery of chocolate, and during delivery of chocolate for consumption at unpredictable intervals. Reductions in synaptic responses were not well predicted by changes in behavioural mobility, and were not fully blocked by systemic injection of either the D1-like receptor antagonist SCH23390, or the muscarinic receptor antagonist scopolamine. However, the reduction in synaptic responses was blocked by injection of the D2-like receptor antagonist eticlopride. CONCLUSIONS: Exposure to cues associated with palatable food results in a suppression of synaptic responses in olfactory inputs to the entorhinal cortex that is mediated in part by activation of dopamine D2 receptors.
Asunto(s)
Señales (Psicología) , Corteza Entorrinal/fisiología , Potenciales Evocados/fisiología , Receptores de Dopamina D2/fisiología , Recompensa , Animales , Benzazepinas/farmacología , Antagonistas de Dopamina/farmacología , Antagonistas de los Receptores de Dopamina D2 , Corteza Entorrinal/efectos de los fármacos , Potenciales Evocados/efectos de los fármacos , Masculino , Vías Olfatorias/efectos de los fármacos , Vías Olfatorias/fisiología , Ratas , Ratas Long-Evans , Receptores de Dopamina D1/antagonistas & inhibidores , Receptores de Dopamina D1/fisiología , Salicilamidas/farmacología , OlfatoRESUMEN
The mechanisms that regulate synapse formation and maintenance are incompletely understood. In particular, relatively few inhibitors of synapse formation have been identified. Receptor protein tyrosine phosphatase σ (RPTPσ), a transmembrane tyrosine phosphatase, is widely expressed by neurons in developing and mature mammalian brain, and functions as a receptor for chondroitin sulfate proteoglycans that inhibits axon regeneration following injury. In this study, we address RPTPσ function in the mature brain. We demonstrate increased axon collateral branching in the hippocampus of RPTPσ null mice during normal aging or following chemically induced seizure, indicating that RPTPσ maintains neural circuitry by inhibiting axonal branching. Previous studies demonstrated a role for pre-synaptic RPTPσ promoting synaptic differentiation during development; however, subcellular fractionation revealed enrichment of RPTPσ in post-synaptic densities. We report that neurons lacking RPTPσ have an increased density of pre-synaptic varicosities in vitro and increased dendritic spine density and length in vivo. RPTPσ knockouts exhibit an increased frequency of miniature excitatory post-synaptic currents, and greater paired-pulse facilitation, consistent with increased synapse density but reduced synaptic efficiency. Furthermore, RPTPσ nulls exhibit reduced long-term potentiation and enhanced novel object recognition memory. We conclude that RPTPσ limits synapse number and regulates synapse structure and function in the mature CNS.
Asunto(s)
Regulación del Desarrollo de la Expresión Génica/genética , Potenciación a Largo Plazo/genética , Neuronas/citología , Densidad Postsináptica/genética , Proteínas Tirosina Fosfatasas Clase 2 Similares a Receptores/metabolismo , Reconocimiento en Psicología/fisiología , Factores de Edad , Análisis de Varianza , Animales , Animales Recién Nacidos , Axones/efectos de los fármacos , Axones/patología , Axones/ultraestructura , Células Cultivadas , Corteza Cerebral/citología , Modelos Animales de Enfermedad , Estimulación Eléctrica , Embrión de Mamíferos , Agonistas de Aminoácidos Excitadores/toxicidad , Ácido Kaínico/toxicidad , Potenciación a Largo Plazo/efectos de los fármacos , Ratones , Ratones Endogámicos BALB C , Ratones Noqueados , Fibras Musgosas del Hipocampo/fisiología , Neuronas/efectos de los fármacos , Pruebas Neuropsicológicas , Técnicas de Placa-Clamp , Densidad Postsináptica/efectos de los fármacos , Ratas , Proteínas Tirosina Fosfatasas Clase 2 Similares a Receptores/deficiencia , Reconocimiento en Psicología/efectos de los fármacos , Tinción con Nitrato de Plata , Estado Epiléptico/inducido químicamente , Estado Epiléptico/genética , Estado Epiléptico/patologíaRESUMEN
Estrogens are thought to contribute to cognitive function in part by promoting the function of basal forebrain cholinergic neurons that project to the hippocampus and cortical regions including the entorhinal cortex. Reductions in estrogens may alter cognition by reducing the function of cholinergic inputs to both the hippocampus and entorhinal cortex. In the present study, we assessed the effects of ovariectomy on proteins associated with cholinergic synapses in the entorhinal cortex. Ovariectomy was conducted at PD63, and tissue was obtained on PD84 to 89 to quantify changes in the degradative enzyme acetylcholinesterase, the vesicular acetylcholine transporter, and muscarinic M1 receptor protein. Although the vesicular acetylcholine transporter was unaffected, ovariectomy reduced both acetylcholinesterase and M1 receptor protein, and these reductions were prevented by chronic replacement of 17ß-estradiol following ovariectomy. We also assessed the effects of ovariectomy on the cholinergic modulation of excitatory transmission, by comparing the effects of the acetylcholinesterase inhibitor eserine on evoked excitatory synaptic field potentials in brain slices obtained from intact rats, and from ovariectomized rats with or without 17ß-estradiol replacement. Eserine is known to prolong the effects of endogenously released acetylcholine, resulting in an M1-like mediated reduction of glutamate release at excitatory synapses. The reduction in excitatory synaptic potentials in layer II of the entorhinal cortex induced by 15-min application of 10 µM eserine was greatly reduced in slices from ovariectomized rats as compared to intact rats and ovariectomized rats with replacement of 17ß-estradiol. The reduced modulatory effect of eserine is consistent with the observed changes in cholinergic proteins, and suggests that reductions in 17ß-estradiol following ovariectomy lead to impaired cholinergic function within the entorhinal cortex.
Asunto(s)
Acetilcolinesterasa , Corteza Entorrinal , Animales , Colinérgicos/farmacología , Corteza Entorrinal/fisiología , Estradiol/farmacología , Estrógenos/farmacología , Potenciales Postsinápticos Excitadores , Femenino , Humanos , Ovariectomía , Fisostigmina/farmacología , Ratas , Receptor Muscarínico M1 , Transmisión Sináptica/fisiología , Proteínas de Transporte Vesicular de AcetilcolinaRESUMEN
Serotonin (5-HT) has important effects on cognitive function within the hippocampal region where it modulates membrane potential and excitatory and inhibitory synaptic transmission. Here, we investigated how 5-HT modulates excitatory synaptic strength in layers II/III of the parasubiculum in rat brain slices. Bath-application of 1 or 10⯵M 5-HT resulted in a strong, dose-dependent, and reversible reduction in the amplitude of field excitatory postsynaptic potentials (fEPSPs) recorded in the parasubiculum. The 5-HT reuptake blocker citalopram (10⯵M) also reduced fEPSP amplitudes, indicating that 5-HT released within the slice inhibits synaptic transmission. The reduction of fEPSPs induced by 5-HT was blocked by the 5-HT1A receptor blocker NAN-190 (10⯵M), but not by the 5-HT7 receptor blocker SB-269970 (10⯵M). Moreover, the 5-HT1A agonist 8-OH-DPAT induced a reduction of fEPSP amplitude similar to that induced by 5-HT. The reduction was prevented by the 5-HT1A receptor blocker NAN-190. The reduction in fEPSPs induced by either 5-HT or by 8-OH-DPAT was accompanied by an increase in paired-pulse ratio, suggesting that it is due mainly to reduced glutamate release. Our data suggest that the effects of serotonin on cognitive function may depend in part upon a 5-HT1A-mediated reduction of excitatory synaptic transmission in the parasubiculum. This may also affect synaptic processing in the entorhinal cortex, which receives the major output projection of the parasubiculum.
Asunto(s)
Citalopram/farmacología , Receptor de Serotonina 5-HT1A/metabolismo , Inhibidores Selectivos de la Recaptación de Serotonina/farmacología , Transmisión Sináptica/efectos de los fármacos , Animales , Potenciales Postsinápticos Excitadores/fisiología , Potenciales de la Membrana/efectos de los fármacos , Neuronas/efectos de los fármacos , Neuronas/metabolismo , Ratas , Receptores de Serotonina/metabolismo , Agonistas de Receptores de Serotonina/farmacología , Transmisión Sináptica/fisiologíaRESUMEN
The entorhinal cortex plays an important role in temporal lobe processes including learning and memory, object recognition, and contextual information processing. The alteration of the strength of synaptic inputs to the lateral entorhinal cortex may therefore contribute substantially to sensory and mnemonic functions. The neuromodulatory transmitter dopamine exerts powerful effects on excitatory glutamatergic synaptic transmission in the entorhinal cortex. Interestingly, inputs from midbrain dopamine neurons appear to specifically target clusters of excitatory cells located in the superficial layers of the entorhinal cortex. We have previously demonstrated that dopamine facilitates synaptic transmission through the activation of D1-like receptors. This facilitation of synaptic transmission is dependent on both activation of classical D1-like-receptors, and upon activation of dopamine receptors linked to increases in phospholipase C, inositol triphosphate (IP3), and intracellular calcium. In the present study we combined electrophysiological recordings of evoked excitatory postsynaptic currents with imaging of intracellular calcium using the fluorescent indicator fluo-4 to monitor calcium transients evoked by dopamine in electrophysiologically identified putative fan and pyramidal cells of the lateral entorhinal cortex. Bath application of dopamine (1 µM), or the phosphatidylinositol (PI)-linked D1-like-receptor agonist SKF83959 (5 µM), induced reliable and reversible increases in fluo-4 fluorescence and excitatory postsynaptic currents in fan cells, but not in pyramidal cells. In contrast, application of the classical D1-like-receptor agonist SKF38393 (10 µM) did not result in significant increases in fluorescence. Blocking release of calcium from internal stores by loading cells with the IP3 receptor blocker heparin (1 mM) or the ryanodine receptor blocker dantrolene (20 µM) abolished both the calcium transients and the facilitation of evoked synaptic currents induced by dopamine. Dopamine also induced calcium transients in fan cells when calcium was excluded from the extracellular medium, further indicating that the calcium transients are linked to release from internal stores. These results indicate that following D1-like-receptor binding, dopamine selectively induces transient elevations in intracellular calcium via activation of IP3 and ryanodine receptors, and that these elevations are linked to the facilitation of synaptic responses in putative layer II entorhinal cortex fan cells.
Asunto(s)
Calcio/metabolismo , Dopamina/metabolismo , Neuronas Dopaminérgicas/fisiología , Corteza Entorrinal/fisiología , Potenciales Postsinápticos Excitadores/fisiología , 2,3,4,5-Tetrahidro-7,8-dihidroxi-1-fenil-1H-3-benzazepina/análogos & derivados , 2,3,4,5-Tetrahidro-7,8-dihidroxi-1-fenil-1H-3-benzazepina/farmacología , Animales , Señalización del Calcio , Células Cultivadas , Agonistas de Dopamina/farmacología , Corteza Entorrinal/patología , Masculino , Ratas , Ratas Endogámicas , Receptores de Dopamina D1/metabolismoRESUMEN
The cerebellum is involved in sensorimotor, cognitive, and emotional functions through cerebello-cerebral connectivity. Cerebellar neurostimulation thus likely affects cortical circuits, as has been shown in studies using cerebellar stimulation to treat neurological disorders through modulation of frontal EEG oscillations. Here we studied the effects of different frequencies of cerebellar stimulation on oscillations and coherence in the cerebellum and prefrontal cortex in the urethane-anesthetized rat. Local field potentials were recorded in the right lateral cerebellum (Crus I/II) and bilaterally in the prefrontal cortex (frontal association area, FrA) in adult male Sprague-Dawley rats. Stimulation was delivered to the cerebellar vermis (lobule VII) using single pulses (0.2 Hz for 60 s), or repeated pulses at 1 Hz (30 s), 5 Hz (10 s), 25 Hz (2 s), and 50 Hz (1 s). Effects of stimulation were influenced by the initial state of EEG activity which varies over time during urethane-anesthesia; 1 Hz stimulation was more effective when delivered during the slow-wave state (Stage 1), while stimulation with single-pulse, 25, and 50 Hz showed stronger effects during the activated state (Stage 2). Single-pulses resulted in increases in oscillatory power in the delta and theta bands for the cerebellum, and in frequencies up to 80 Hz in cortical sites. 1 Hz stimulation induced a decrease in 0-30 Hz activity and increased activity in the 30-200 Hz range, in the right FrA. 5 Hz stimulation reduced power in high frequencies in Stage 1 and induced mixed effects during Stage 2.25 Hz stimulation increased cortical power at low frequencies during Stage 2, and increased power in higher frequency bands during Stage 1. Stimulation at 50 Hz increased delta-band power in all recording sites, with the strongest and most rapid effects in the cerebellum. 25 and 50 Hz stimulation also induced state-dependent effects on cerebello-cortical and cortico-cortical coherence at high frequencies. Cerebellar stimulation can therefore entrain field potential activity in the FrA and drive synchronization of cerebello-cortical and cortico-cortical networks in a frequency-dependent manner. These effects highlight the role of the cerebellar vermis in modulating large-scale synchronization of neural networks in non-motor frontal cortex.
RESUMEN
Dopaminergic projections to the superficial layers of the lateral entorhinal cortex can modulate the strength of olfactory inputs to the region. We have found that low concentrations of dopamine facilitate field EPSPs in the entorhinal cortex, and that higher concentrations of dopamine suppress synaptic responses. Here, we have used whole-cell current clamp recordings from layer II neurons to determine the mechanisms of the suppression. Dopamine (10 to 50 microM) hyperpolarized membrane potential and reversibly suppressed the amplitude of EPSPs evoked by layer I stimulation. Both AMPA- and NMDA-mediated components were suppressed, and paired-pulse facilitation was also enhanced indicating that the suppression is mediated largely by reduced glutamate release. Blockade of D(2)-like receptors greatly reduced the suppression of EPSPs. Dopamine also lowered input resistance, and reduced the number of action potentials evoked by depolarizing current steps. The drop in input resistance was mediated by activation of D(1)-like receptors, and was prevented by blocking K(+) channels with TEA. The dopaminergic suppression of synaptic transmission is therefore mediated by a D(2) receptor-dependent reduction in transmitter release, and a D(1) receptor-dependent increase in a K(+) conductance. This suppression of EPSPs may dampen the strength of sensory inputs during periods of elevated mesocortical dopamine activity.
Asunto(s)
Dopamina/fisiología , Corteza Entorrinal/fisiología , Transmisión Sináptica/fisiología , Animales , Dopamina/farmacología , Relación Dosis-Respuesta a Droga , Corteza Entorrinal/efectos de los fármacos , Potenciales Postsinápticos Excitadores/efectos de los fármacos , Potenciales Postsinápticos Excitadores/fisiología , Masculino , Inhibición Neural/efectos de los fármacos , Inhibición Neural/fisiología , Ratas , Ratas Long-Evans , Transmisión Sináptica/efectos de los fármacosRESUMEN
The entorhinal cortex receives a large projection from the piriform cortex, and synaptic plasticity in this pathway may affect olfactory processing. In vitro whole cell recordings have been used here to investigate postsynaptic signalling mechanisms that mediate the induction of long-term synaptic depression (LTD) in layer II entorhinal cortex cells. To induce LTD, pairs of pulses, using a 30-millisecond interval, were delivered at 1 Hz for 15 minutes. Induction of LTD was blocked by the NMDA receptor antagonist APV and by the calcium chelator BAPTA, consistent with a requirement for calcium influx via NMDA receptors. Induction of LTD was blocked when the FK506 was included in the intracellular solution to block the phosphatase calcineurin. Okadaic acid, which blocks activation of protein phosphatases 1 and 2a, also prevented LTD. Activation of protein phosphatases following calcium influx therefore contributes to induction of LTD in layer II of the entorhinal cortex.
Asunto(s)
Corteza Entorrinal/fisiología , Potenciales Postsinápticos Excitadores/fisiología , Depresión Sináptica a Largo Plazo/fisiología , Neuronas/fisiología , 2-Amino-5-fosfonovalerato/farmacología , Animales , Calcineurina/fisiología , Señalización del Calcio , Ciclosporina/farmacología , Maleato de Dizocilpina/farmacología , Corteza Entorrinal/citología , Corteza Entorrinal/efectos de los fármacos , Antagonistas de Aminoácidos Excitadores/farmacología , Potenciales Postsinápticos Excitadores/efectos de los fármacos , Depresión Sináptica a Largo Plazo/efectos de los fármacos , Masculino , Neuronas/efectos de los fármacos , Ácido Ocadaico/farmacología , Técnicas de Placa-Clamp , Proteína Fosfatasa 1/fisiología , Células Piramidales/fisiología , Ratas , Ratas Long-Evans , Receptores de N-Metil-D-Aspartato/fisiología , Tacrolimus/farmacologíaRESUMEN
Persistent firing in layer III entorhinal cortex neurons that can be evoked during muscarinic receptor activation may contribute to mechanisms of working memory. The entorhinal cortex receives strong dopaminergic inputs which may modulate working memory for motivationally significant information. We used whole cell recordings in in vitro rat brain slices to assess the effects of dopamine on persistent firing in layer III neurons initiated by depolarizing current injection. Persistent firing during pharmacological block of ionotropic excitatory and inhibitory synaptic transmission, and in the presence of the cholinergic agonist carbachol (10⯵M), was observed in 39% of layer III pyramidal cells. Addition of 1⯵M dopamine suppressed the incidence of persistent firing and similarly reduced the mean probability of induction of persistent firing at each current step, without significantly affecting the latency, duration, plateau potential, or frequency of persistent firing that was induced. These results indicate that dopamine can result in a suppression of the induction of persistent firing in layer III entorhinal neurons, while still being permissive of persistent firing once it is initiated.
Asunto(s)
Dopamina/fisiología , Corteza Entorrinal/fisiología , Neuronas/fisiología , Potenciales de Acción , Animales , Dopamina/administración & dosificación , Corteza Entorrinal/efectos de los fármacos , Masculino , Memoria a Corto Plazo/fisiología , Neuronas/efectos de los fármacos , Ratas Long-EvansRESUMEN
Synaptic plasticity in olfactory inputs to the lateral entorhinal cortex may result in lasting changes in the processing of olfactory stimuli. Changes in dopaminergic tone can have strong effects on basal evoked synaptic responses in the superficial layers of the entorhinal cortex, and the current study investigated whether dopamine may modulate the induction of long-term potentiation (LTP) and depression (LTD) in piriform cortex inputs to layer II of the lateral entorhinal cortex in awake rats. Groups of animals were pretreated with either saline or the selective dopamine reuptake inhibitor GBR12909 prior to low or high frequency stimulation to induce LTD or LTP. In saline-treated groups, synaptic responses were potentiated to 122.4+/-6.4% of baseline levels following LTP induction, and were reduced to 84.5+/-4.9% following induction of LTD. Changes in synaptic responses were maintained for up to 60min and returned to baseline levels within 24h. In contrast, induction of both LTP and LTD was blocked in rats pretreated with GBR12909. Dopaminergic suppression of synaptic plasticity in the entorhinal cortex may serve to restrain activity-dependent plasticity during reward-relevant behavioral states or during processing of novel stimuli.
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Vías Aferentes/metabolismo , Dopamina/metabolismo , Corteza Entorrinal/metabolismo , Potenciación a Largo Plazo/fisiología , Depresión Sináptica a Largo Plazo/fisiología , Vías Aferentes/efectos de los fármacos , Animales , Inhibidores de Captación de Dopamina/farmacología , Estimulación Eléctrica , Corteza Entorrinal/efectos de los fármacos , Potenciales Postsinápticos Excitadores/efectos de los fármacos , Potenciales Postsinápticos Excitadores/fisiología , Potenciación a Largo Plazo/efectos de los fármacos , Depresión Sináptica a Largo Plazo/efectos de los fármacos , Masculino , Memoria/efectos de los fármacos , Memoria/fisiología , Vías Olfatorias/efectos de los fármacos , Vías Olfatorias/fisiología , Piperazinas/farmacología , Ratas , Ratas Long-Evans , Recompensa , Transmisión Sináptica/fisiología , Área Tegmental Ventral/fisiologíaRESUMEN
The lateral entorhinal cortex receives strong inputs from midbrain dopamine neurons that can modulate its sensory and mnemonic function. We have previously demonstrated that 1 µM dopamine facilitates synaptic transmission in layer II entorhinal cortex cells via activation of D1-like receptors, increased cAMP-PKA activity, and a resulting enhancement of AMPA-receptor mediated currents. The present study assessed the contribution of phosphatidylinositol (PI)-linked D1 receptors to the dopaminergic facilitation of transmission in layer II of the rat entorhinal cortex, and the involvement of phospholipase C activity and release of calcium from internal stores. Whole-cell patch-clamp recordings of glutamate-mediated evoked excitatory postsynaptic currents were obtained from pyramidal and fan cells. Activation of D1-like receptors using SKF38393, SKF83959, or 1 µM dopamine induced a reversible facilitation of EPSCs which was abolished by loading cells with either the phospholipase C inhibitor U-73122 or the Ca2+ chelator BAPTA. Neither the L-type voltage-gated Ca2+ channel blocker nifedipine, nor the L/N-type channel blocker cilnidipine, blocked the facilitation of synaptic currents. However, the facilitation was blocked by blocking Ca2+ release from internal stores via inositol 1,4,5-trisphosphate (InsP3) receptors or ryanodine receptors. Follow-up studies demonstrated that inhibiting CaMKII activity with KN-93 failed to block the facilitation, but that application of the protein kinase C inhibitor PKC(19-36) completely blocked the dopamine-induced facilitation. Overall, in addition to our previous report indicating a role for the cAMP-PKA pathway in dopamine-induced facilitation of synaptic transmission, we demonstrate here that the dopaminergic facilitation of synaptic responses in layer II entorhinal neurons also relies on a signaling cascade dependent on PI-linked D1 receptors, PLC, release of Ca2+ from internal stores, and PKC activation which is likely dependent upon both DAG and enhanced intracellular Ca2+. These signaling pathways may collaborate to enhance sensory and mnemonic function in the entorhinal cortex during tonic release of dopamine.
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Corteza Entorrinal/metabolismo , Ácido Glutámico/metabolismo , Fosfatidilinositoles/metabolismo , Receptores Dopaminérgicos/metabolismo , Transmisión Sináptica/fisiología , 2,3,4,5-Tetrahidro-7,8-dihidroxi-1-fenil-1H-3-benzazepina/análogos & derivados , 2,3,4,5-Tetrahidro-7,8-dihidroxi-1-fenil-1H-3-benzazepina/farmacología , Animales , Proteína Quinasa Tipo 2 Dependiente de Calcio Calmodulina/metabolismo , Dopamina/farmacología , Agonistas de Dopamina/farmacología , Corteza Entorrinal/efectos de los fármacos , Potenciales Postsinápticos Excitadores/efectos de los fármacos , Potenciales Postsinápticos Excitadores/fisiología , Masculino , Neuronas/efectos de los fármacos , Neuronas/metabolismo , Ratas , Ratas Long-Evans , Transmisión Sináptica/efectos de los fármacosRESUMEN
Circadian rhythms modulate behavioral processes over a 24 h period through clock gene expression. What is largely unknown is how these molecular influences shape neural activity in different brain areas. The clock gene Per2 is rhythmically expressed in the striatum and the cerebellum and its expression is linked with daily fluctuations in extracellular dopamine levels and D2 receptor activity. Electrophysiologically, dopamine depletion enhances striatal local field potential (LFP) oscillations. We investigated if LFP oscillations and synchrony were influenced by time of day, potentially via dopamine mechanisms. To assess the presence of a diurnal effect, oscillatory power and coherence were examined in the striatum and cerebellum of rats under urethane anesthesia at four different times of day zeitgeber time (ZT1, 7, 13 and 19-indicating number of hours after lights turned on in a 12:12 h light-dark cycle). We also investigated the diurnal response to systemic raclopride, a D2 receptor antagonist. Time of day affected the proportion of LFP oscillations within the 0-3 Hz band and the 3-8 Hz band. In both the striatum and the cerebellum, slow oscillations were strongest at ZT1 and weakest at ZT13. A 3-8 Hz oscillation was present when the slow oscillation was lowest, with peak 3-8 Hz activity occurring at ZT13. Raclopride enhanced the slow oscillations, and had the greatest effect at ZT13. Within the striatum and with the cerebellum, 0-3 Hz coherence was greatest at ZT1, when the slow oscillations were strongest. Coherence was also affected the most by raclopride at ZT13. Our results suggest that neural oscillations in the cerebellum and striatum, and the synchrony between these areas, are modulated by time of day, and that these changes are influenced by dopamine manipulation. This may provide insight into how circadian gene transcription patterns influence network electrophysiology. Future experiments will address how these network alterations are linked with behavior.
RESUMEN
The parasubiculum (PaS) is a component of the hippocampal formation that sends its major output to layer II of the entorhinal cortex. The PaS receives strong cholinergic innervation from the basal forebrain that is likely to modulate neuronal excitability and contribute to theta-frequency network activity. The present study used whole cell current- and voltage-clamp recordings to determine the effects of cholinergic receptor activation on layer II PaS neurons. Bath application of carbachol (CCh; 10-50 µM) resulted in a dose-dependent depolarization of morphologically-identified layer II stellate and pyramidal cells that was not prevented by blockade of excitatory and inhibitory synaptic inputs. Bath application of the M1 receptor antagonist pirenzepine (1 µM), but not the M2-preferring antagonist methoctramine (1 µM), blocked the depolarization, suggesting that it is dependent on M1 receptors. Voltage-clamp experiments using ramped voltage commands showed that CCh resulted in the gradual development of an inward current that was partially blocked by concurrent application of the selective Kv7.2/3 channel antagonist XE-991, which inhibits the muscarine-dependent K(+) current I M. The remaining inward current also reversed near EK and was inhibited by the K(+) channel blocker Ba(2+), suggesting that M1 receptor activation attenuates both I M as well as an additional K(+) current. The additional K(+) current showed rectification at depolarized voltages, similar to K(+) conductances mediated by Kir 2.3 channels. The cholinergic depolarization of layer II PaS neurons therefore appears to occur through M1-mediated effects on I M as well as an additional K(+) conductance.
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Potenciales de la Membrana/fisiología , Neuronas/metabolismo , Células Piramidales/metabolismo , Receptor Muscarínico M1/metabolismo , Animales , Antracenos/farmacología , Carbacol/farmacología , Agonistas Colinérgicos/farmacología , Diaminas/farmacología , Canal de Potasio KCNQ2/antagonistas & inhibidores , Canal de Potasio KCNQ2/metabolismo , Canal de Potasio KCNQ3/antagonistas & inhibidores , Canal de Potasio KCNQ3/metabolismo , Masculino , Potenciales de la Membrana/efectos de los fármacos , Antagonistas Muscarínicos/farmacología , Neuronas/citología , Parasimpatolíticos/farmacología , Pirenzepina/farmacología , Células Piramidales/citología , Ratas , Ratas Long-Evans , Receptor Muscarínico M1/agonistas , Receptor Muscarínico M1/antagonistas & inhibidoresRESUMEN
The entorhinal cortex is thought to play roles in sensory and mnemonic function, and the cholinergic suppression of the strength of synaptic inputs is likely to have important impacts on these processes. Field excitatory postsynaptic potentials (fEPSPs) in the medial entorhinal cortex evoked by stimulation of the piriform cortex are suppressed during theta EEG activity in behaving animals, and cholinergic receptor activation suppresses synaptic responses both in vivo, and in layer II entorhinal neurons in vitro. Here, we have used in vitro field potential recordings to investigate the transmitter receptors that mediate the cholinergic suppression of synaptic responses in layer I inputs to layer II of the medial entorhinal cortex. Bath-application of the cholinergic agonist carbachol suppressed the amplitude of fEPSPs with an EC50 of 5.3µM, and enhanced paired-pulse ratio. The M2/M4 preferring receptor blocker methoctramine, or the M4 receptor blocker PD102807, did not prevent the cholinergic suppression. However, the M1/M4 receptor blocker pirenzepine and the M1 receptor blocker VU0255035 reduced the suppression, suggesting that the cholinergic suppression of synaptic responses in the entorhinal cortex is dependent in large part on activation of M1 receptors.