The translational inhibitor and amnestic agent emetine also suppresses ongoing hippocampal neural activity similarly to other blockers of protein synthesis.

The consolidation of memory is thought to ultimately depend on the synthesis of new proteins, since translational inhibitors such as anisomycin and cycloheximide adversely affect the permanence of long-term memory. However, when applied directly in brain, these agents also profoundly suppress neural activity to an extent that is directly correlated to the degree of protein synthesis inhibition caused. Given that neural activity itself is likely to help mediate consolidation, this finding is a serious criticism of the strict de novo protein hypothesis of memory. Here, we test the neurophysiological effects of another translational inhibitor, emetine. Unilateral intra-hippocampal infusion of emetine suppressed ongoing local field and multiunit activity at ipsilateral sites as compared to the contralateral hippocampus in a fashion that was positively correlated to the degree of protein synthesis inhibition as confirmed by autoradiography. This suppression of activity was also specific to the circumscribed brain region in which protein synthesis inhibition took place. These experiments provide further evidence that ongoing protein synthesis is necessary and fundamental for neural function and suggest that the disruption of memory observed in behavioral experiments using translational inhibitors may be due, in large part, to neural suppression.

The amnestic agent anisomycin disrupts intrinsic membrane properties of hippocampal neurons via a loss of cellular energetics.

The nearly axiomatic idea that de novo protein synthesis is necessary for long-term memory consolidation is based heavily on behavioral studies using translational inhibitors such as anisomycin. Although inhibiting protein synthesis has been shown to disrupt the expression of memory, translational inhibitors also have been found to profoundly disrupt basic neurobiological functions, including the suppression of ongoing neural activity in vivo. In the present study, using transverse hippocampal brain slices, we monitored the passive and active membrane properties of hippocampal CA1 pyramidal neurons using intracellular whole cell recordings during a brief ~30-min exposure to fast-bath-perfused anisomycin. Anisomycin suppressed protein synthesis to 46% of control levels as measured using incorporation of radiolabeled amino acids and autoradiography. During its application, anisomycin caused a significant depolarization of the membrane potential, without any changes in apparent input resistance or membrane time constant. Anisomycin-treated neurons also showed significant decreases in firing frequencies and spike amplitudes, and showed increases in spike width across spike trains, without changes in spike threshold. Because these changes indicated a loss of cellular energetics contributing to maintenance of ionic gradients across the membrane, we confirmed that anisomycin impaired mitochondrial function by reduced staining with 2,3,5-triphenyltetrazolium chloride and also impaired cytochrome c oxidase (complex IV) activity as indicated through high-resolution respirometry. These findings emphasize that anisomycin-induced alterations in neural activity and metabolism are a likely consequence of cell-wide translational inhibition. Critical reevaluation of studies using translational inhibitors to promote the protein synthesis dependent idea of long-term memory is absolutely necessary.NEW & NOTEWORTHY Memory consolidation is thought to be dependent on the synthesis of new proteins because translational inhibitors produce amnesia when administered just after learning. However, these agents also disrupt basic neurobiological functions. We show that blocking protein synthesis disrupts basic membrane properties of hippocampal neurons that correspond to induced disruptions of mitochondrial function. It is likely that translational inhibitors cause amnesia through their disruption of neural activity as a result of dysfunction of intracellular energetics.

Anxiolytic and antidepressant effects of intracerebroventricularly administered somatostatin: behavioral and neurophysiological evidence.

Somatostatin (SST) is a cyclic polypeptide that inhibits the release of a variety of regulatory hormones (e.g. growth hormone, insulin, glucagon, thyrotropin). Moreover, SST is widely distributed within the CNS, acting both as a neurotransmitter and as a neuromodulator of other neurotransmitter systems. However, despite its extensive expression in limbic areas, and its co-localization with GABA, a neurotransmitter previously implicated in emotion, the effects of SST on anxiety and depression have not been investigated. By performing intraventricular infusions in rats we demonstrate, for the first time, that SST has anxiolytic- and antidepressant-like effects in the elevated plus-maze and forced swim test, respectively. In addition, by performing local field potential recordings of hippocampal theta activity evoked by reticular stimulation in urethane-anesthetized rats we also show that SST application suppresses the frequency of theta in a similar fashion to diazepam. This neurophysiological signature, common to all classes of anxiolytic drugs (i.e. benzodiazepines, selective 5-HT reuptake inhibitors, 5-HT1A agonists) provides strong converging evidence for the anxiolytic-like characteristics of SST. Our pharmacological antagonism experiments with bicuculline further suggest that the anxiolytic effect of SST may be attributable to the interaction of SST with GABA, whereas the antidepressant-like effect of SST may be GABA-independent. In addition to contributing to the current understanding of the role of neuropeptides in mood and emotion, these findings support a clinical role for SST (or its analogues) in the treatment of anxiety and depression.

Inhibitory synaptic plasticity regulates pyramidal neuron spiking in the rodent hippocampus.

Spike-timing modifies the efficacy of both excitatory and inhibitory synapses onto CA1 pyramidal neurons in the rodent hippocampus. Repetitively spiking the presynaptic neuron before the postsynaptic neuron induces inhibitory synaptic plasticity, which results in a depolarization of the reversal potential for GABA (E(GABA)). Our goal was to determine how inhibitory synaptic plasticity regulates CA1 pyramidal neuron spiking in the rat hippocampus. We demonstrate electrophysiologically that depolarizing E(GABA) by 24.7 mV increased the spontaneous action potential firing frequency of cultured hippocampal neurons 254% from 0.12+/-0.07 Hz to 0.44+/-0.13 Hz (n=11; P<0.05). Next we used a single compartment model of a CA1 pyramidal neuron to explore in detail how inhibitory synaptic plasticity of feedforward and feedback inhibition regulates the generation of action potentials, spike latency, and the minimum excitatory conductance required to generate an action potential; plasticity was modeled as a depolarization of E(GABA), which effectively weakens inhibition. Depolarization of E(GABA) at feedforward and feedback inhibitory synapses decreased the latency to the 1st spike by 2.27 ms, which was greater that the sum of the decreases produced by depolarizing E(GABA) at feedforward (0.85 ms) or feedback inhibitory synapses (0.02 ms) alone. In response to a train of synaptic inputs, depolarizing E(GABA) decreased the inter-spike interval and increased the number of output spikes in a frequency dependent manner, improving the reliability of input-output transmission. Moreover, a depolarizing shift in E(GABA) at feedforward and feedback synapses triggered by spike trains recorded from CA1 pyramidal layer neurons during field theta from anesthetized rats, significantly increased spiking on the up- and down-strokes of the first half of the theta rhythm (P<0.05), without changing the preferred phase of firing (P=0.783). This study provides the first explanation of how depolarizing E(GABA) affects pyramidal cell output within the hippocampus.

Evidence for spatial modules mediated by temporal synchronization of carbachol-induced gamma rhythm in medial entorhinal cortex.

Fast (gamma) oscillations in the cortex underlie the rapid temporal coordination of large-scale neuronal assemblies in the processing of sensory stimuli. Cortical gamma rhythm is modulated in vivo by cholinergic innervation from the basal forebrain and can be generated in vitro after exogenous cholinergic stimulation. Using the isolated guinea pig brain, an in vitro preparation that allows for the study of an intact cerebrum, we studied the spatial features of gamma activity evoked by the cholinomimetic carbachol (CCh) in the medial entorhinal cortex (mEC). gamma activity induced by either arterial perfusion or intraparenchymal application of CCh showed a phase reversal across mEC layer II and was reduced or abolished in a spatially localized region by focal infusions of atropine, bicuculline, and CNQX. In addition, a spatially restricted zone of gamma activity could be induced by passive diffusion of CCh from a recording pipette. Finally, gamma oscillations recorded at multiple sites across the surface of the mEC using array electrodes during arterial perfusion of CCh demonstrated a decline in synchronization (coherence) as the interelectrode distance increased. This effect was independent of the signal amplitude and was specific for gamma as opposed to theta-like activity induced by CCh in the same experiments. These results suggest that CCh-induced gamma oscillations in the mEC are mediated through direct muscarinic excitation of a highly localized reciprocal inhibitory-excitatory network located in superficial layers. We propose that functional cortical modules of highly synchronous gamma oscillations may organize incoming (cortical) and outgoing (hippocampal) information in the mEC.

Electroresponsiveness of medial entorhinal cortex layer III neurons in vitro.

The entorhinal cortex funnels sensory information from the entire cortical mantle into the hippocampal formation via the perforant path. A major component of this pathway originates from the stellate cells in layer II and terminates on the dentate granule cells to activate the hippocampal trisynaptic circuit. In addition, there is also a significant, albeit less characterized, component of the perforant path that originates in entorhinal layer III pyramidal cells and terminates directly in area CA1. As a step in understanding the functional role of this monosynaptic component of the perforant path, we undertook the electrophysiological characterization of entorhinal layer III neurons in an in vitro rat brain slice preparation using intracellular recording techniques with sharp micropipettes and under current-clamp conditions. Cells were also intracellularly injected with biocytin to assess their pyramidal cell morphology. Layer III pyramidal cells did not display either the rhythmic subthreshold membrane potential oscillations nor spike-cluster discharge that characterizes the spiny stellate cells from layer II. In contrast, layer III pyramidal cells displayed a robust tendency towards spontaneous activity in the form of regular tonic discharge. Analysis of the voltage-current relations also demonstrated, in these neurons, a rather linear membrane voltage behaviour in the subthreshold range with the exception of pronounced inward rectification in the depolarizing direction. Depolarizing inward rectification was unaffected by Ca(2+)-conductance block with but was abolished by voltage-gated Na(+)-conductance block with tetrodotoxin, suggesting that a persistent Na(+)-conductance provides much of the inward current sustaining tonic discharge. In addition, in the presence of tetrodotoxin, an intermediate threshold (approximately -50 mV) Ca(2+)-dependent rebound potential was also observed which could constitute another pacemaker mechanism. A high-threshold Ca(2+)-conductance was also found to contribute to the action potential as judged by the decrease in spike duration towards the peak observed during Ca(2+)-conductance block. On the other hand, Ca(2+)-conductance block increase spike duration at the base and abolished the monophasic spike afterhyperpolarization. Analysis of the input-output relations revealed firing properties similar to those of regularly spiking neocortical cells. Current-pulse driven spike trains displayed moderate adaptation and were followed by a Ca(2+)-dependent slow afterhyperpolarization. In summary, the intrinsic electroresponsiveness of entorhinal layer III pyramidal cells suggest that these neurons may perform a rather high-fidelity transfer function of incoming neocortical sensory information directly to the CA1 hippocampal subfield. The pronounced excitability of layer III cells, due to both Na+ and Ca2+ conductances, may also be related to their tendency towards degeneration in epilepsy.

Oscillatory activity in entorhinal neurons and circuits. Mechanisms and function.

Layers II and V of the entorhinal cortex (EC) occupy a privileged anatomical position in the temporal lobe memory system that allows them to gate the main flow of information in and out of the hippocampus, respectively. In vivo studies have shown that layer II of the EC is a robust generator of theta as well as gamma activity. Theta may also be present in layer V, but the layer V network is particularly prone to genesis of short-lasting high-frequency oscillations ("ripples"). Interestingly, in vitro studies have shown that EC layers II and V, but not layer III, have the potential to act as independent pacemakers of population oscillatory activity. Moreover, it has also been shown that subgroups of principal neurons both within layers II and V, but not layer III, are endowed with autorhythmic properties. These are characterized by subthreshold oscillations where the depolarizing phase is driven by the activation of "persistent" Na+ channels. We propose that the oscillatory properties of layer II and V neurons and local circuits are responsible for setting up the proper temporal dynamics for the coordination of the multiple sensory inputs that converge onto EC and thus help to generate sensory representations and memory encoding.

Animal models of human amnesia and dementia: hippocampal and amygdala ablation compared with serotonergic and cholinergic blockade in the rat.

The behavioral effects of combined bilateral hippocampal and amygdala ablation (previously proposed as a model of human global amnesia) were compared to those seen with central blockade of the ascending cholinergic and serotonergic projections (a possible model of human global dementia) in male hooded rats. Rats were prepared with: (a) bilateral surgical lesions of the hippocampus and amygdala; (b) pharmacological blockade of central cholinergic and serotonergic function by systemic injections of scopolamine and p-chlorophenylalanine; and (c) neurotoxic lesions of the rostrally projecting serotonergic nuclei in the brainstem using intracerebral injections of 5,7-dihydroxytryptamine, later combined with scopolamine. The behavioral tests used were: an open field test, a swim-to-platform test, and a Lashley III maze. In all 3 tests, rats with either the neurotoxin lesions plus scopolamine or p-chlorophenylalanine plus scopolamine treatment showed greater impairments in comparison with controls than did the combined lesion group. These results indicate that simultaneous blockade of central serotonergic and cholinergic transmission has a greater effect on some aspects of the organization of behavior than large surgical lesions of the hippocampus and amygdala.

Effects of p-chlorophenylalanine and scopolamine on retention of habits in rats.

Rats were trained on a conventional maze test or on a swim-to-platform test. Retention of swim-to-platform performance 7 days later was severely impaired by posttraining treatment with a combination of p-chlorophenylalanine (PCPA) and scopolamine although neither drug alone had any effect. Retention of the maze habit was moderately impaired by scopolamine alone and severely impaired by a combination of scopolamine and PCPA, but was unaffected by PCPA alone. Polygraphic recordings confirmed previous reports that a combination of PCPA and scopolamine can abolish neocortical low voltage fast activity and hippocampal rhythmical slow activity. Combined blockade of central cholinergic and serotonergic neurotransmission in rats may provide a useful animal model of Alzheimer's disease.

Anxiolytic- and antidepressant-like properties of ketamine in behavioral and neurophysiological animal models.

Ketamine, a dissociative anesthetic agent, appears to have rapid antidepressant effects at sub-anesthetic doses in clinically depressed patients. Although promising, these results need to be replicated in double-blind placebo-controlled studies, a strategy thwarted by the psychoactive effects of ketamine, which are obvious to both patients and clinicians. Alternatively, demonstrations of the psychotherapeutic effects of ketamine in animal models are also complicated by ketamine's side-effects on general activity, which have not been routinely measured or taken into account in experimental studies. In this study we found that ketamine decreased "behavioral despair" in the forced swim test, a widely used rats model of antidepressant drug action. This effect was not confounded by side-effects on general activity, and was comparable to that of a standard antidepressant drug, fluoxetine. Interestingly, ketamine also produced anxiolytic-like effects in the elevated-plus-maze. Importantly, the effective dose of ketamine in the plus-maze did not affect general locomotion measures, in either the plus-maze or in the open field test. While the selective N-methyl-d-aspartic acid (NMDA) receptor antagonist MK-801 also produced antidepressant-like and anxiolytic-like effects, these were mostly confounded by changes in general activity. Finally, in a neurophysiological model of anxiolytic drug action, ketamine reduced the frequency of reticularly-activated theta oscillations in the hippocampus, similar to the proven anxiolytic drug diazepam. This particular neurophysiological signature is common to all known classes of anxiolytic drugs (i.e. benzodiazepines, 5-HT1A agonists, antidepressants) and provides strong converging evidence for the anxiolytic-like effects of ketamine. Further studies are needed to understand the underlying pharmacological mechanisms of ketamine's effects in these experiments, since it is not clear they were mimicked by the selective NMDA antagonist MK-801.

Muscarinic induction of synchronous population activity in the entorhinal cortex.

Oscillation and synchronization of neural activity is important in normal brain function but is also relevant to epileptogenesis. One of the most frequent forms of epilepsy originates in temporal lobe circuitry of which the entorhinal cortex (EC) is crucial. Because muscarinic receptor activation promotes oscillatory dynamics in EC neurons, we investigated in a brain slice preparation the effects of carbachol (CCh) on oscillatory population activity in the EC. We found that CCh produced epileptiform activity in EC, which according to field profile and current source density analysis was usually driven by layer V. In addition, localized CCh application and surgical isolation experiments demonstrated that EC layer II, but not layer III, can also independently generate synchronous population activity. Intracellular recordings from EC principal cells during epileptiform activity demonstrated large-amplitude, synaptically driven depolarizing events and bursts of action potentials synchronized to the field spikes. In layer II neurons, the depolarizing events had a multiphasic reversal potential that suggested concurrent glutamatergic and GABAergic synaptic input. Interestingly, although the epileptiform activity required activation of AMPA but not NMDA receptors, small-amplitude field spikes persisted during block of fast excitatory neurotransmission. These field spikes were correlated to large-amplitude IPSPs in layer II neurons, and both activities were abolished by GABAA-receptor antagonism. Thus, in response to muscarinic activation, pools of EC interneurons discharge synchronously by a mechanism not necessarily involving principal cell activation. Given the differential projection pattern of EC layers V and II toward the neocortex and hippocampus, respectively, their robust epileptogenic character may be of major importance in temporal lobe epilepsy.

Extrinsic modulation of theta field activity in the entorhinal cortex of the anesthetized rat.

Field recordings of the entorhinal cortex (EC) were studied and compared to those recorded concomitantly in the dentate region of the hippocampal formation (HPC) in the urethane anesthetized rat. The EC, like the HPC, showed two main variations of spontaneous field activity: a desynchronized, large amplitude irregular activity and a synchronized, rhythmic, slow frequency field activity (RSA or theta). Corroborating previous research, a phase reversal was seen across layer II of the EC and when recorded superficial to this layer, EC theta was phase-locked to that recorded from the HPC (dentate). Entorhinal cortex (and HPC) theta could be evoked by the application of moderate tail pinches (sensory stimulation), by pharmacological treatments enhancing cholinergic transmission, and by electrical stimulation of the posterior hypothalamus. Spectral analysis revealed that in all cases, theta was produced coherently across the two limbic structures. Entorhinal cortex (and HPC) production of theta could be abolished by pharmacological treatments disrupting cholinergic transmission, and by reversible procaine inactivation of the medial septal region. Therefore, it was concluded that limbic theta is modulated spontaneously, and with sensory and hypothalamic stimulation through the activity of cells in the medial septal region via muscarinic neurotransmission. It was also hypothesized that the activation of cells in the posterior hypothalamus linearly codes the frequency, and to a lesser extent the power, of EC and HPC theta. Given these findings and the coincidence and coherence of the occurrence of theta across the EC and HPC, it was postulated that it occurs via a parallel mechanism in the two areas.

Discharge patterns of hippocampal theta-related cells in the caudal diencephalon of the urethan-anesthetized rat.

1. Single-unit discharge patterns of cells in specific nuclei of the caudal diencephalon were characterized in relation to simultaneously recorded field activity from the stratum moleculare of the dentate gyrus according to the criteria that have been used previously to classify cells in the hippocampal formation (including entorhinal cortex), medial septum, and cingulate cortex. Theta (theta)-related cells were classified as 1) tonic theta-ON, if they discharged nonrhythmically and increased their discharge rates during hippocampal theta relative to large, irregular hippocampal field activity (LIA); 2) tonic theta-OFF, if they discharged nonrhythmically and decreased their discharge rates during theta relative to LIA; or 3) phasic theta-ON, if they discharged rhythmically and in phase with ongoing theta, but nonrhythmically during LIA. Cells not meeting any of the above criteria were classified as nonrelated. 2. Recordings were obtained in a total of 127 cells from the caudal diencephalon. Recordings were made in 54 cells from the posterior hypothalamic nucleus (PH), 16 from the supramammillary nucleus (SuM), 20 from the PH/SuM border, and 23 from the medial mammillary nucleus (MM). Recordings were also made from nine cells from the central medial nucleus of the thalamus (CM) and five from the dorsomedial hypothalamic nucleus (DMH). 3. Of the 54 PH cells, 43 (80%) were classified as tonic theta-ON and 11 (20%) as nonrelated. Tonic theta-ON cells in the PH discharged at significantly higher rates during theta, either occurring spontaneously (9.6 +/- 1.7 Hz, mean +/- SE) or elicited with a tail pinch (TP theta; 10.6 +/- 1.9 Hz), than during LIA (3.6 +/- 1.4 Hz). Of the nine CM cells, seven (78%) were tonic theta-ON and two (22%) were nonrelated. Tonic theta-ON cells discharged at significantly higher rates during theta (17.5 +/- 7.8 Hz) or TP theta (18.0 +/- 7.1 Hz) than during LIA (7.3 +/- 4.8 Hz). All DMH cells were nonrelated. 4. Of the 20 PH/SuM border cells, 15 (75%) were classified as tonic theta-OFF and discharged at significantly higher rates during LIA (5.3 +/- 1.5 Hz) than during theta (0.8 +/- 0.4 Hz) or TP theta (0.4 +/- 0.3 Hz). Five (25%) cells in the PH/SuM border were nonrelated. 5. All of the 16 cells (100%) recorded from the body of the SuM were phasic theta-ON. The discharge rates of these cells did not change significantly across hippocampal field states (LIA = 8.3 +/- 1.6; theta = 7.3 +/- 1.6; TP theta = 8.6 +/- 1.7 Hz).(ABSTRACT TRUNCATED AT 400 WORDS)

Classification of theta-related cells in the entorhinal cortex: cell discharges are controlled by the ascending brainstem synchronizing pathway in parallel with hippocampal theta-related cells.

Single-unit discharge patterns of entorhinal cortex (EC) cells were characterized in relation to simultaneously recorded hippocampal (HPC) field activity according to criteria used previously to classify cells in the hippocampal formation, medial septum, cingulate cortex, and caudal diencephalon. EC cells related to HPC theta field activity were classified as 1) phasic theta-on, if they discharged rhythmically, and in phase, with ongoing HPC theta, but nonrhythmically during large, irregular hippocampal field activity (LIA); 2) tonic theta-on, if they discharged nonrhythmically and increased their discharge rates during HPC theta relative to LIA; 3) phasic theta-off, if they discharged rhythmically, and in phase, with ongoing HPC theta, but increased their discharge rates during LIA; and 4) tonic theta-off, if they discharged nonrhythmically and decreased their discharge rates during HPC theta relative to LIA. Cells not meeting any of these criteria were classified as nonrelated. A total of 168 EC cells were recorded, and of these 56 (33%) were classified as theta related, with the remaining 112 (67%) classified as nonrelated. Of the 56 theta-related cells, 41 (73%) had significantly higher discharge rates during HPC theta than during LIA and were classified as theta-on cells (15 phasic theta-on cells and 26 tonic theta-on cells). Nine of the 26 tonic theta-on cells showed a phase relation of their arrhythmic discharges to simultaneously recorded HPC theta field activity. EC phasic theta-on cells did not discharge preferentially on any portion of the HPC theta field recorded from the region of the stratum moleculare of the dentate gyrus. In general, cells classified as phasic revealed a wide distribution of phase preferences. The remaining 15 (26.7%) cells were classified as theta-off cells and discharged at higher rates during HPC LIA than during HPC theta field activity (3 phasic theta-off cells and 12 tonic theta-off cells). Systemic administration of physostigmine significantly increased the discharge rate of tonic and phasic theta-on cells relative to LIA. Electrical stimulation in the posterior hypothalamic region (PH) significantly increased the discharge rate of EC theta-on cells and significantly decreased the discharge rate of EC theta-off cells relative to HPC LIA. The discharge rates of nonrelated EC cells were not influenced by electrical stimulation of the PH. Procaine microinfusion into the medical septum (MS) abolished spontaneously occurring HPC theta and theta induced with PH stimulation. In addition, 5 min after MS procaine, the ability of PH stimulation to modulate EC theta-on cell discharge was abolished. The modulation of cellular discharges produced by PH stimulation recovered by 60 min post-procaine infusion into the MS. The findings support two main conclusions: 1) theta-related cells in the EC are comprised of two main populations of cells, theta-on and theta-off, similar to other regions of limbic cortex and nuclei of the ascending brainstem synchronizing pathway; 2) the ascending brainstem synchronizing pathway exerts both similar and parallel effects on theta-related cells in entorhinal cortex and hippocampus.

In vivo intrahippocampal microinfusion of carbachol and bicuculline induces theta-like oscillations in the septally deafferented hippocampus.

In their laboratory the authors have previously demonstrated that hippocampal slices could be induced to generate trains of "theta-like" oscillations by whole-bath perfusions of carbachol. Until recently, it has not been possible to generate similar activity in the septally deafferented hippocampus of an otherwise intact brain by microinfusions of carbachol. This study presents a full report of the first demonstration of a theta-like oscillation in the in vivo, septally deafferented hippocampal formation. Rats were anesthetized with urethane and implanted with microinfusion cannulae in the region of the medial septum/vertical limb of the diagonal band of Broca (MS/vDBB) and at single or multiple sites in the stratum moleculare of the fascia dentata. The MS/vDBB was microinfused with procaine hydrochloride to produce a reversible suppression lasting for approximately 20 minutes. Intrahippocampal microinfusions of carbachol or bicuculline alone (in the postprocaine condition of the MS/vDBB) failed to produce any theta-like oscillations. The combination of carbachol and bicuculline produced trains of theta-like oscillations during suppression of the MS/vDBB very similar to those seen in the slice preparations. The oscillations were blocked by intravenous administration of atropine sulfate, and they had the same depth profile as that of theta. Theta-on cells were shown to discharge in rhythmic bursts in synchrony with the oscillations. Thus, it would appear that the essential nature of the medial septal input to the hippocampal formation, for the generation of theta field activity in the intact brain, consists of a critical balance between cholinergic and GABAergic circuitry.

Properties and role of I(h) in the pacing of subthreshold oscillations in entorhinal cortex layer II neurons.

Various subsets of brain neurons express a hyperpolarization-activated inward current (I(h)) that has been shown to be instrumental in pacing oscillatory activity at both a single-cell and a network level. A characteristic feature of the stellate cells (SCs) of entorhinal cortex (EC) layer II, those neurons giving rise to the main component of the perforant path input to the hippocampal formation, is their ability to generate persistent, Na(+)-dependent rhythmic subthreshold membrane potential oscillations, which are thought to be instrumental in implementing theta rhythmicity in the entorhinal-hippocampal network. The SCs also display a robust time-dependent inward rectification in the hyperpolarizing direction that may contribute to the generation of these oscillations. We performed whole cell recordings of SCs in in vitro slices to investigate the specific biophysical and pharmacological properties of the current underlying this inward rectification and to clarify its potential role in the genesis of the subthreshold oscillations. In voltage-clamp conditions, hyperpolarizing voltage steps evoked a slow, noninactivating inward current, which also deactivated slowly on depolarization. This current was identified as I(h) because it was resistant to extracellular Ba(2+), sensitive to Cs(+), completely and selectively abolished by ZD7288, and carried by both Na(+) and K(+) ions. I(h) in the SCs had an activation threshold and reversal potential at approximately -45 and -20 mV, respectively. Its half-activation voltage was -77 mV. Importantly, bath perfusion with ZD7288, but not Ba(2+), gradually and completely abolished the subthreshold oscillations, thus directly implicating I(h) in their generation. Using experimentally derived biophysical parameters for I(h) and the low-threshold persistent Na(+) current (I(NaP)) present in the SCs, a simplified model of these neurons was constructed and their subthreshold electroresponsiveness simulated. This indicated that the interplay between I(NaP) and I(h) can sustain persistent subthreshold oscillations in SCs. I(NaP) and I(h) operate in a "push-pull" fashion where the delay in the activation/deactivation of I(h) gives rise to the oscillatory process.