Carbachol increases locus coeruleus activation by targeting noradrenergic neurons, inhibitory interneurons and inhibitory synaptic transmission.

The locus coeruleus (LC) consists of noradrenergic (NA) neurons and plays an important role in controlling behaviours. Although much of the knowledge regarding LC functions comes from studying behavioural outcomes upon administration of muscarinic acetylcholine receptor (mAChR) agonists into the nucleus, the exact mechanisms remain unclear. Here, we report that the application of carbachol (CCh), an mAChR agonist, increased the spontaneous action potentials (sAPs) of both LC-NA neurons and local inhibitory interneurons (LC I-INs) in acute brain slices by activating M1/M3 mAChRs (m1/3 AChRs). Optogenetic activation of LC I-INs evoked inhibitory postsynaptic currents (IPSCs) in LC-NA neurons that were mediated by γ-aminobutyric acid type A (GABAA ) and glycine receptors, and CCh application decreased the IPSC amplitude through a presynaptic mechanism by activating M4 mAChRs (m4 AChRs). LC-NA neurons also exhibited spontaneous phasic-like activity (sPLA); CCh application increased the incidence of this activity. This effect of CCh application was not observed with blockade of GABAA and glycine receptors, suggesting that the sPLA enhancement occurred likely because of the decreased synaptic transmission of LC I-INs onto LC-NA neurons by the m4 AChR activation and/or increased spiking rate of LC I-INs by the m1/3 AChR activation, which could lead to fatigue of the synaptic transmission. In conclusion, we report that CCh application, while inhibiting their synaptic transmission, increases sAP rates of LC-NA neurons and LC I-INs. Collectively, these effects provide insight into the cellular mechanisms underlying the behaviour modulations following the administration of muscarinic receptor agonists into the LC reported by the previous studies.

Probing the Effect of Acidosis on Tether-Mode Mechanotransduction of Proprioceptors.

Proprioceptors are low-threshold mechanoreceptors involved in perceiving body position and strain bearing. However, the physiological response of proprioceptors to fatigue- and muscle-acidosis-related disturbances remains unknown. Here, we employed whole-cell patch-clamp recordings to probe the effect of mild acidosis on the mechanosensitivity of the proprioceptive neurons of dorsal root ganglia (DRG) in mice. We cultured neurite-bearing parvalbumin-positive (Pv+) DRG neurons on a laminin-coated elastic substrate and examined mechanically activated currents induced through substrate deformation-driven neurite stretch (SDNS). The SDNS-induced inward currents (ISDNS) were indentation depth-dependent and significantly inhibited by mild acidification (pH 7.2~6.8). The acid-inhibiting effect occurred in neurons with an ISDNS sensitive to APETx2 (an ASIC3-selective antagonist) inhibition, but not in those with an ISNDS resistant to APETx2. Detailed subgroup analyses revealed ISDNS was expressed in 59% (25/42) of Parvalbumin-positive (Pv+) DRG neurons, 90% of which were inhibited by APETx2. In contrast, an acid (pH 6.8)-induced current (IAcid) was expressed in 76% (32/42) of Pv+ DRG neurons, 59% (21/32) of which were inhibited by APETx2. Together, ASIC3-containing channels are highly heterogenous and differentially contribute to the ISNDS and IAcid among Pv+ proprioceptors. In conclusion, our findings highlight the importance of ASIC3-containing ion channels in the physiological response of proprioceptors to acidic environments.

Inhibitory interneurons regulate phasic activity of noradrenergic neurons in the mouse locus coeruleus and functional implications.

KEY POINTS: The locus coeruleus (LC) contains noradrenergic (NA) neurons that respond to novel stimuli in the environment with phasic activation to initiate an orienting response; phasic LC activation is also triggered by stimuli, representing the outcome of task-related decision processes, to facilitate ensuing behaviours and help optimize task performance. Here, we report that LC-NA neurons exhibit bursts of action potentials in vitro resembling phasic LC activation in vivo, and the activity is gated by inhibitory interneurons (I-INs) located in the peri-LC. We also observe that inhibition of peri-LC I-INs enhances prepulse inhibition and axons from cortical areas that play important roles in evaluating the cost/reward of a stimulus synapse on both peri-LC I-INs and LC-NA neurons. The results help us understand the cellular mechanisms underlying the generation and regulation of phasic LC activation with a focus on the role of peri-LC I-INs. ABSTRACT: Noradrenergic (NA) neurons in the locus coeruleus (LC) have global axonal projection to the brain. These neurons discharge action potentials phasically in response to either novel stimuli in the environment to initiate an orienting behaviour or stimuli representing the outcome of task-related decision processes to facilitate ensuing behaviours and help optimize task performance. Nevertheless, the cellular mechanisms underlying the generation and regulation of phasic LC activation remain unknown. We report here that LC-NA neurons recorded in brain slices exhibit bursts of action potentials that resembled the phasic activation-pause profile observed in animals. The activity was referred to as phasic-like activity (PLA) and was suppressed and enhanced by blocking excitatory and inhibitory synaptic transmissions, respectively. These results suggest the existence of a local circuit to drive PLA, and the activity could be regulated by the excitatory-inhibitory balance of the circuit. In support of this notion, we located a population of inhibitory interneurons (I-INs) in the medial part of the peri-LC that exerted feedforward inhibition of LC-NA neurons through GABAergic and glycinergic transmissions. Selective inhibition of peri-LC I-INs with chemogenetic methods could enhance PLA in brain slices and increase prepulse inhibition in animals. Moreover, axons from the orbitofrontal and prelimbic cortices, which play important roles in evaluating the cost/reward of a stimulus, synapse on both peri-LC I-INs and LC-NA neurons. These observations demonstrate functional roles of peri-LC I-INs in integrating inputs of the frontal cortex onto LC-NA neurons and gating the phasic LC output.

The Role of Gastrodin on Hippocampal Neurons after N-Methyl-D-Aspartate Excitotoxicity and Experimental Temporal Lobe Seizures.

Tian ma (Gastrodia elata, GE) is an ancient Chinese herbal medicine that has been suggested to be effective as an anticonvulsant and analgesic, and to have sedative effects against vertigo, general paralysis, epilepsy and tetanus. The primary active ingredient isolated from GE is termed gastrodin, which is the glucoside of 4-hydroxybenzyl alcohol (4-HBA). Gastrodin can abolish hypoxia-, glutamate- and N-methyl-D-aspartate (NMDA) receptor-induced toxicity in primary culture of rat cortical neurons, and reduces seizure severity in seizure-sensitive gerbils. We evaluated the effect of gastrodin on NMDA excitotoxicity in hippocampal slice cultures (HSCs) with propidium iodide (PI) fluorescence measurement. We also evaluated the effects of gastrodin for treating active in vivo temporal lobe seizures induced by lithium/pilocarpine. Seizure severity, time span to seizure onset, mortality rate and hippocampal histology for survivors were compared. The effect of gastrodin was evaluated for treating in vitro seizures induced by Mg²âº-free medium in hippocampal slices. Frequencies and amplitudes of epileptiform discharges were compared. The effect of gastrodin on synaptic transmission was evaluated on hippocampal CA1 Schaffer collaterals. Application of 25 µM gastrodin significantly suppressed NMDA excitotoxicity in CA3 but not in CA1 hippocampus and dentate gyrus. Intraventricular gastrodin accelerated seizure onset for 12 min after intraperitoneal pilocarpine injection (P = 0.051). Three of five rats (60%) in the gastrodin group, and three of four (75%) in the dimethyl sulfoxide (DMSO) group died within 3 days after status epilepticus (SE). Gastrodin also failed to inhibit epileptiform discharges in hippocampal slices induced by Mg²âº-free medium, believed to be NMDA receptor-mediated spontaneous activity. The frequencies of the spontaneous epileptiform discharges were similar under treatments with 25 µM gastrodin, 200 µM gastrodin and DMSO. For the evaluation of gastrodin on synaptic transmission, application of DMSO, 25 µM or 200 µM gastrodin had no significant effect on excitatory postsynaptic potential (EPSP) slopes. Gastrodin at 200 µM decreased paired-pulse facilitation (PPF) from 1.23 ± 0.04 to 1.12 ± 0.04 (P = 0.002). In conclusion, gastrodin failed to suppress in vivo and in vitro seizures in our study. Gastrodin showed no effect on hippocampal Schaffer collateral EPSP. These findings suggest that gastrodin does not interact with ionotropic glutamate receptors to inhibit NMDA receptor-facilitated seizures. However, gastrodin showed protective effects against NMDA toxicity on cultured hippocampal slices. Nevertheless, gastrodin is still a potential neuroprotective agent against NMDA excitotoxicity, with potential benefits for stroke and patients with epilepsy.

GABAB receptor-mediated tonic inhibition regulates the spontaneous firing of locus coeruleus neurons in developing rats and in citalopram-treated rats.

KEY POINTS: Noradrenaline (NA)-releasing neurons in the locus coeruleus (LC) provide NA to the forebrain and play important roles in regulating many brain functions. LC neurons are subject to tonic inhibition mediated by GABAB receptors (GABAB Rs) and that the extent of the effect varies with ambient GABA levels. GABAB R-mediated tonic inhibition can effectively tune the spontaneous firing rate (SFR) of LC neurons; it is developmentally regulated and is responsible for maintaining a constant SFR of LC neurons during development. In male, but not female rats, chronic perinatal treatment with citalopram, a selective serotonin reuptake inhibitor, results in downregulation of GABAB R-mediated tonic inhibition of LC neurons that partially accounts for increased SFR in male, but not female, rats receiving such treatment. Our results show that GABAB R-mediated tonic inhibition could be an important player in the development of normal and abnormal behaviours/brain functions associated with the LC-NA system. Noradrenaline (NA)-releasing neurons in the locus coeruleus (LC) provide NA to the forebrain. Their activity is believed to be a key factor regulating the wakefulness/arousal level of the brain. In this study, we found that the activity of NA-releasing neurons in the LC (LC neurons) was subject to γ-aminobutyric acid (GABA) tonic inhibition through GABAB receptors (GABAB Rs), but not GABAA receptors. The intensity of GABAB R tonic inhibition was found to depend on ambient GABA levels, as it was dramatically increased by blockade of GABA reuptake. It also varied with the function of GABAB Rs. The GABAB R activity on LC neurons was found to increase with postnatal age up to postnatal days 8-10, resulting in increased tonic inhibition. Interestingly, there was no significant difference in the spontaneous activity of LC neurons at different postnatal ages unless GABAB R tonic inhibition was blocked. These results show that, during postnatal development, there is a continuous increase in GABAB R tonic inhibition that maintains the activity of LC neurons at a proper level. In male, but not female, rats, chronic perinatal treatment with citalopram, a selective serotonin reuptake inhibitor, reduced GABAB R activity and tonic inhibition, which might result in the significantly higher spontaneous activity of LC neurons seen in these animals. In conclusion, our results show that GABAB R-mediated tonic inhibition has a direct impact on the spontaneous activity of LC neurons and that the extent of the effect varies with ambient GABA levels and functionality of GABAB R signalling.

Morphological and physiological evidence of a synaptic connection between the lateral parabrachial nucleus and neurons in the A7 catecholamine cell group in rats.

BACKGROUND: The descending noradrenergic (NAergic) system is one of the important endogenous analgesia systems. It has been suggested that noxious stimuli could activate descending NAergic system; nevertheless, the underlying neuronal circuit remains unclear. As NAergic neurons in the A7 catecholamine cell group (A7) are a part of the descending NAergic system and the lateral parabrachial nucleus (LPB) is an important brainstem structure that relays ascending nociceptive signal, we aimed to test whether LPB neurons have direct synaptic contact with NAergic A7 neurons. RESULTS: Stereotaxic injections of an anterograde tracer, biotinylated dextran-amine (BDA), were administered to LPB in rats. The BDA-labeled axonal terminals that have physical contacts with tyrosine hydroxylase-positive (presumed noadrenergic) neurons were identified in A7. Consistent with these morphological observations, the excitatory synaptic currents (EPSCs) were readily evoked in NAergic A7 neurons by extracellular stimulation of LPB. The EPSCs evoked by LPB stimulation were blocked by CNQX, a non-NMDA receptor blocker, and AP5, a selective NMDA receptor blocker, showing that LPB-A7 synaptic transmission is glutamatergic. Moreover, the amplitude of LPB-A7 EPSCs was significantly attenuated by DAMGO, a selective µ-opioid receptor agonist, which was associated with an increase in paired-pulse ratio. CONCLUSIONS: Taken together, the above results showed direct synaptic connections between LPB and A7 catecholamine cell group, the function of which is subject to presynaptic modulation by µ-opioid receptors.

Implication of locus coeruleus dysfunction in Prader-Willi syndrome: Insights from a mouse model.

Prader-Willi syndrome (PWS) is a multisystemic disorder. Notably, many characteristic symptoms of PWS are correlated with locus coeruleus norepinephrine system (LC-NE) dysfunction, including impairment in arousal, learning, pain modulation, and stress-induced negative affective states. Although electrophysiological experiments in necdin-deficient mice, an established PWS animal model, have revealed decreased spontaneous neuronal firing activity in the LC and impaired excitability, the behavioral phenotypes related to LC-NE dysfunction remain unexplored. In this study, heterozygous necdin-deficient mice (B6.Cg-Ndntm1ky) were bred from wild-type (WT) females to generate WT (+m/+p) and heterozygous (+m/-p) animals. Compared to WT mice, Ndn + m/-p mice demonstrated impaired visual-spatial memory in the Y-maze test, reduced social interaction, impaired sexual recognition, and shorter falling latency on the Rotarod. Using the open field test (OFT) and elevated plus maze (EPM), we observed similar locomotion activity of Ndn + m/-p and WT mice, but Ndn + m/-p mice were less anxious. After acute restraint, Ndn + m/-p mice exhibited significant impairment in stress-induced anxiety. Additionally, the plasma norepinephrine surge following exposure to acute restraint stress was also impaired. Pretreatment with atomoxetine, a norepinephrine reuptake inhibitor aimed to enhance LC function, restored Ndn + m/-p mice to exhibit a normal response to acute restraint stress. Furthermore, by employing chemogenetic approaches to facilitate LC neuronal firing, post-stress anxious responses were also partially rescued in Ndn + m/-p mice. These data strongly suggest that LC dysfunction is implicated in the pathogenesis of stress-related neuropsychiatric symptoms in PWS. Manipulation of LC activity may hold therapeutic potential for patients with PWS.

Role of extracellular signal-regulated kinase in synaptic transmission and plasticity of a nociceptive input on capsular central amygdaloid neurons in normal and acid-induced muscle pain mice.

Application of phorbol 12,13-diacetate (PDA) caused marked enhancement of synaptic transmission of nociceptive parabrachio-amygdaloid (PBA) input onto neurons of the capsular central amygdaloid (CeAC) nucleus. The potentiation of PBA-CeAC EPSCs by PDA involved a presynaptic protein kinase C (PKC)-dependent component and a postsynaptic PKC-extracellular-regulated kinase (ERK)-dependent component. NMDA glutamatergic receptor (NMDAR)-dependent long-term potentiation (LTP) of PBA-CeAC EPSCs, which was also dependent on the PKC-ERK signaling pathway, was induced by tetanus stimulation at 100 Hz. In slices from mice subjected to acid-induced muscle pain (AIMP), phosphorylated ERK levels in the CeAC increased, and PBA-CeAC synaptic transmission was postsynaptically enhanced. The enhanced PBA-CeAC synaptic transmission in AIMP mice shared common mechanisms with the postsynaptic potentiation effect of PDA and induction of NMDAR-dependent LTP by high-frequency stimulation in normal slices, both of which required ERK activation. Since the CeAC plays an important role in the emotionality of pain, enhanced synaptic function of nociceptive (PBA) inputs onto CeAC neurons might partially account for the supraspinal mechanisms underlying central sensitization.

A requirement of low-threshold calcium spike for induction of spike-timing-dependent plasticity at corticothalamic synapses on relay neurons in the ventrobasal nucleus of rat thalamus.

Relay neurons in sensory thalamus transmit somatosensory information to cerebral cortex and receive sensory and feedback corticothalamic (CT) synaptic inputs. Their duality of firing modes, in bursts and continuous, underlies state dependence of thalamic information transfer, but the impact of different firing patterns on synaptic plasticity was rarely explored. To address this issue, we made whole-cell recording from relay neurons in the ventrobasal nucleus (VBN) of rat thalamus and compared synaptic plasticity induced by pairing CT-EPSP with two different types of burst spiking: low-threshold spike (LTS)-burst spiking triggered at Vm~-70 mV, and high-frequency spiking induced at Vm~-55 mV. The latter mimics natural burst spiking of relay neurons without activation of LTS. We found that, while backpropagating APs alone were not sufficient, low-threshold calcium spike was required for the induction of spike-timing-dependent LTP at CT synapses. Our results reveal a novel role of the calcium spike plays in the induction of long-term plasticity of CT synapse. Considering the dendritic origin of LTS, this study also implies potential physiological regulations over synaptic plasticity in thalamus. We propose that this form of synaptic plasticity may be involved in the dynamic fine-tuning of thalamocortical information relay.

GABAB receptor-mediated tonic inhibition of noradrenergic A7 neurons in the rat.

Noradrenergic (NAergic) A7 neurons that project axonal terminals to the dorsal horn of the spinal cord to modulate nociceptive signaling are suggested to receive tonic inhibition from local GABAergic interneurons, which are under the regulation of descending analgesic pathways. In support of this argument, we presently report GABA(B) receptor (GABA(B)R)-mediated tonic inhibition of NAergic A7 neurons. Bath application of baclofen induced an outward current (I(Bac)) in NAergic A7 neurons that was blocked by CGP 54626, a GABA(B)R blocker. The I(Bac) was reversed at about -99 mV, displayed inward rectification, and was blocked by Ba(2+) or Tertipian-Q, showing it was mediated by G protein-activated inward-rectifying K(+) (GIRK) channels. Single-cell RT-PCR results suggested that GIRK1/3 heterotetramers might dominate functional GIRK channels in NAergic A7 neurons. Under conditions in which GABA(A) and glycine receptors were blocked, bath application of GABA inhibited the spontaneous firing of NAergic A7 neurons in a dose-dependent manner. Interestingly, CGP 54626 application not only blocked the effect of GABA but also increased the firing rate to 126.9% of the control level, showing that GABA(B)Rs were constitutively active at an ambient GABA concentration of 2.8 µM and inhibited NAergic A7 neurons. GABA(B)Rs were also found at presynaptic excitatory and inhibitory axonal terminals in the A7 area. Pharmacological activation of these GABA(B)Rs inhibited the release of neurotransmitters. No physiological role was found for GABA(B)Rs on excitatory terminals, whereas those on the inhibitory terminals were found to exert autoregulatory control of GABA release.

Bidirectional synaptic plasticity induced by conditioned stimulations with different number of pulse at hippocampal CA1 synapses: roles of N-methyl-D-aspartate and metabotropic glutamate receptors.

In the mammalian brain, the hippocampus has been established as a principle structure for learning and memory processes, which involve synaptic plasticity. Although a relationship between synaptic plasticity and stimulation frequency has been reported in numerous studies, little is known about the importance of pulse number on synaptic plasticity. Here we investigated whether the pulse number can modulate bidirectional plasticity in hippocampal CA1 areas. When a CA1 area was induced by a paired-pulse (PP) with a 10-ms interval, the strength of the synapse was altered to form a long-term depression (LTD), with a 68 ± 4% decrease in expression. The PP-induced LTD (PP-LTD) was blocked by the metabotropic glutamate receptors subtype 5 (mGluR5) antagonist MPEP, suggesting that the PP-LTD relied on the activation of GluR5. In addition, this modulation of LTD was protein kinase C (PKC)- and Group II mGluR-independent. However, when increasing the pulse number to 4 and 6, potentiated synaptic strength was observed, which was N-methyl-D-aspartate receptor (NMDAR)-dependent but mGluR5-independent. Surprisingly, when blocking mGluR, the synaptic efficacy induced by triple-pulse stimulation was altered to form a long-term potentiation (LTP) with a 142 ± 7% enhancement, and was further blocked by NMDA antagonist APV. Following treatment with APV and PKC blocker chelerythrine, the LTP expression induced by 4- and 6-pulse stimulation was switched to LTD. We suggest that CA1 synaptic plasticity is regulated by the result of competition between NMDA and mGluR5 receptors. We suggest that the pulse number can bidirectionally modulate synaptic plasticity through the activation of NMDA and mGluR5 in hippocampal CA1 areas.

Comparison of synaptic transmission and plasticity between sensory and cortical synapses on relay neurons in the ventrobasal nucleus of the rat thalamus.

Relay neurons in the ventrobasal nucleus of the thalamus transmit somatosensory information to the cerebral cortex and receive sensory and cortical (feedback) synaptic inputs via, respectively, medial lemniscal (ML) and corticothalamic (CT) fibres. Here, we report that calcium-permeable AMPA receptors are expressed at CT synapses, but not ML synapses, and that the NMDA receptor (NMDAR)-mediated/non-NMDAR-mediated synaptic current ratio is significantly larger at CT synapses than at ML synapses. Moreover, NMDAR-dependent LTP and L-type voltage-gated calcium channel-dependent LTD are readily induced at CT synapses, but not ML synapses. In particular, LTD of CT synaptic transmission is induced by spiking of postsynaptic relay neurons in continuous mode, but not burst mode, in current-clamp recordings. These results show that the strength of the cortical input to thalamic relay neurons is selectively subjected to use-dependent modification, which could be a mechanism for regulation of thalamocortical-corticothalamic interactions and the underlying sensory processing.

Neurokinin 1 receptor activates transient receptor potential-like currents in noradrenergic A7 neurons in rats.

Noradrenergic (NAergic) A7 neurons are involved in modulating nociception by releasing noradrenaline in the dorsal spinal cord. Since NAergic A7 neurons receive dense Substance P (Sub-P) releasing terminals from ventromedial medulla, here we tested the effect of Sub-P on them. Bath application of Sub-P induced an inward current (I(Sub-P)) in NAergic neurons, which was significantly blocked by Neurokinin 1 (NK1) receptor antagonist. The I(Sub-P) was reversed at approximately -20 mV, blocked by several TRP channel blockers, enhanced by OAG and negatively regulated by PKC. Immunohistochemistry staining showed that NAergic A7 neurons express high level of TRPC6 channel proteins, which is consistent with pharmacological properties of I(Sub-P) shown above, as TRPC6 channel is shown to be augmented by OAG and inhibited by PKC. In conclusion, the above results provide mechanism underlying postsynaptic action of Sub-P on NAergic A7 neurons and a role for TRPC6 channel in NAergic pain modulation.

Firing activity of locus coeruleus noradrenergic neurons decreases in necdin-deficient mice, an animal model of Prader-Willi syndrome.

BACKGROUND: Prader-Willi syndrome (PWS) is a neurodevelopmental disorder characterized by multiple respiratory, cognitive, endocrine, and behavioral symptoms, such as central apnea, intellectual disabilities, exaggerated stress responses, and temper tantrums. The locus coeruleus noradrenergic system (LC-NE) modulates a diverse range of behaviors, including arousal, learning, pain modulation, and stress-induced negative affective states, which are possibly correlated with the pathogenesis of PWS phenotypes. Therefore, we evaluated the LC-NE neuronal activity of necdin-deficient mice, an animal model of PWS. METHODS: Heterozygous necdin-deficient mice (B6.Cg-Ndntm1ky) were bred from wild-type (WT) females to generate WT (+m/+p) and heterozygotes (+m/-p) animals, which were examined of LC-NE neuronal activity, developmental reflexes, and plethysmography. RESULTS: On slice electrophysiology, LC-NE neurons of Ndntm1ky mice with necdin deficiency showed significantly decreased spontaneous activities and impaired excitability, which was mediated by enhanced A-type voltage-dependent potassium currents. Ndntm1ky mice also exhibited the neonatal phenotypes of PWS, such as hypotonia and blunt respiratory responses to hypercapnia. CONCLUSIONS: LC-NE neuronal firing activity decreased in necdin-deficient mice, suggesting that LC, the primary source of norepinephrine in the central nervous system, is possibly involved in PWS pathogenesis.

Extracellular Signal-Regulated Kinases Mediate an Autoregulation of GABAB-Receptor-Activated Whole-Cell Current in Locus Coeruleus Neurons.

The norepinephrine-releasing neurons in the locus coeruleus (LC) are well known to regulate wakefulness/arousal. They display active firing during wakefulness and a decreased discharge rate during sleep. We have previously reported that LC neurons express large numbers of GABAB receptors (GABABRs) located at peri-/extrasynaptic sites and are subject to tonic inhibition due to the continuous activation of GABABRs by ambient GABA, which is significantly higher during sleep than during wakefulness. In this study, we further showed using western blot analysis that the activation of GABABRs with baclofen could increase the level of phosphorylated extracellular signal-regulated kinase 1 (ERK1) in LC tissue. Recordings from LC neurons in brain slices showed that the inhibition of ERK1/2 with U0126 and FR180204 accelerated the decay of whole-cell membrane current induced by prolonged baclofen application. In addition, the inhibition of ERK1/2 also increased spontaneous firing and reduced tonic inhibition of LC neurons after prolonged exposure to baclofen. These results suggest a new role of GABABRs in mediating ERK1-dependent autoregulation of the stability of GABABR-activated whole-cell current, in addition to its well-known effect on gated potassium channels, to cause a tonic current in LC neurons.

GABAB receptor-mediated tonic inhibition of locus coeruleus neurons plays a role in deep anesthesia induced by isoflurane.

Noradrenergic neurons in the locus coeruleus referred to as locus coeruleus neurons, provide the major supply of norepinephrine to the forebrain and play important roles in behavior through regulation of wakefulness and arousal. In a previous study using brain slice preparations, we reported that locus coeruleus neurons are subject to tonic inhibition mediated by γ-aminobutyric acid B receptors (GABABRs) and that the extent of tonic inhibition varies with ambient GABA levels. Since ambient GABA in the locus coeruleus was reported to fluctuate during the sleep-wakefulness cycle, here we tested whether GABABR-mediated tonic inhibition of locus coeruleus neurons could be a mechanism underlying changes in brain arousal. We first demonstrated that GABABR-mediated tonic inhibition of locus coeruleus neurons also exists in vivo by showing that local infusion of CGP35348, a GABABR antagonist, into the locus coeruleus increased the firing rate of locus coeruleus neurons in anesthetized rats. We then showed that this manipulation accelerated the behavioral emergence of rats from deep anesthesia induced by isoflurane. Together, these observations show that GABABR-mediated tonic inhibition of locus coeruleus neurons occurs in vivo and support the idea that this effect may be important in regulating the functional state of the brain.

Long-term challenge of methylphenidate changes the neuronal population and membrane property of dopaminergic neuron in rats.

Attention deficit hyperactivity disorder (ADHD) has a prevalence of 7.5% in school-age children in Taiwan. A number of ADHD patients start taking medications in elementary school and continue their treatment until they are in college or adulthood. Methylphenidate is the most frequently used medication prescribed for ADHD treatment. The influence of long-term treatment of methylphenidate on neuro-development, especially dopaminergic neurons, in rats would be explored. This study investigated the impact of long-term treatment of methylphenidate on different neurons. Rats aged 1 month were divided into three groups: Normal group receiving only sucrose solution, Low-dose group receiving 2 mg/kg methylphenidate, and High-dose group receiving 10 mg/kg methylphenidate; for each group, the drug was administrated twice per day. After 7 months of the treatment period, then the alterations in number of norepinephrine, serotonergic, cholinergic and dopaminergic neurons were quantified. The number of dopaminergic neurons in the substantia nigra (SN), the serotonergic neurons in the dorsal raphe nucleus, and the cholinergic neurons in the tegmental nucleus significantly decreased as compared with Normal group, whereas the noradrenergic neurons in the locus coeruleus substantially increased. The whole-cell recording was made from dopaminergic neurons residing in the SN for examination of their firing activity. The recorded dopaminergic neurons in SN were categorized into slow and fast firing using 10 Hz as a classified index. The results displayed that the ratio of dopaminergic neurons with fast firing in the High-dose group was less as compared with those in the Normal and the Low-dose group. Furthermore, the amplitude of action potential of the dopaminergic neurons with slow firing was higher in the High-dose group than those in the Normal and Low-dose groups. The firing behavior of dopaminergic neurons and dopamine concentration in the brain is affected by the long-term challenge of methylphenidate.

Anterior nucleus of paraventricular thalamus mediates chronic mechanical hyperalgesia.

Pain-related diseases are the top leading causes of life disability. Identifying brain regions involved in persistent neuronal changes will provide new insights for developing efficient chronic pain treatment. Here, we showed that anterior nucleus of paraventricular thalamus (PVA) plays an essential role in the development of mechanical hyperalgesia in neuropathic and inflammatory pain models in mice. Increase in c-Fos, phosphorylated extracellular signal-regulated kinase, and hyperexcitability of PVA neurons were detected in hyperalgesic mice. Direct activation of PVA neurons using optogenetics and pharmacological approaches were sufficient to induce persistent mechanical hyperalgesia in naive animals. Conversely, inhibition of PVA neuronal activity using DREADDs (designer receptors exclusively activated by designer drugs) or inactivation of PVA extracellular signal-regulated kinase at the critical time window blunted mechanical hyperalgesia in chronic pain models. At the circuitry level, PVA received innervation from central nucleus of amygdala, a known pain-associated locus. As a result, activation of right central nucleus of amygdala with blue light was enough to induce persistent mechanical hyperalgesia. These findings support the idea that targeting PVA can be a potential therapeutic strategy for pain relief.

Therapeutic potential and underlying mechanism of sarcosine (N-methylglycine) in N-methyl-D-aspartate (NMDA) receptor hypofunction models of schizophrenia.

BACKGROUND: Compelling animal and clinical studies support the N-methyl-D-aspartate receptor (NMDAR) hypofunction hypothesis of schizophrenia and suggest promising pharmacological agents to ameliorate negative and cognitive symptoms of schizophrenia, including sarcosine, a glycine transporter-1 inhibitor. AIMS AND METHODS: It is imperative to evaluate the therapeutic potential of sarcosine in animal models, which provide indispensable tools for testing drug effects in detail and elucidating the underlying mechanisms. In this study, a series of seven experiments was conducted to investigate the effect of sarcosine in ameliorating behavioral deficits and the underlying mechanism in pharmacological (i.e., MK-801-induced) and genetic (i.e., serine racemase-null mutant (SR-/-) mice) NMDAR hypofunction models. RESULTS: In Experiment 1, the acute administration of 500/1000 mg/kg sarcosine (i.p.) had no adverse effects on motor function and serum biochemical responses. In Experiments 2-4, sarcosine significantly alleviated MK-801-induced (0.2 mg/kg) brain abnormalities and behavioral deficits in MK-801-induced and SR-/- mouse models. In Experiment 5, the injection of sarcosine enhanced CSF levels of glycine and serine in rat brain. In Experiments 6-7, we show for the first time that sarcosine facilitated NMDAR-mediated hippocampal field excitatory postsynaptic potentials and influenced the movement of surface NMDARs at extrasynaptic sites. CONCLUSIONS: Sarcosine effectively regulated the surface trafficking of NMDARs, NMDAR-evoked electrophysiological activity, brain glycine levels and MK-801-induced abnormalities in the brain, which contributed to the amelioration of behavioral deficits in mouse models of NMDAR hypofunction.

Xenografting of human umbilical mesenchymal stem cells from Wharton's jelly ameliorates mouse spinocerebellar ataxia type 1.

BACKGROUND: Spinocerebellar ataxia type 1 (SCA1) is an autosomal dominant neurodegenerative disorder caused by the expansion of CAG repeats in ATXN1 gene resulting in an expansion of polyglutamine repeats in the ATXN1 protein. Unfortunately, there has yet been any effective treatment so far for SCA1. This study investigated the feasibility of transplanting human umbilical mesenchymal stem cells (HUMSCs) into transgenic SCA1 mice containing an expanded uninterrupted allele with 82 repeats in the ATXN1-coding region. METHODS: 106 human umbilical mesenchymal stem cells were transplanted into the cerebella at 1 month of age. RESULTS: HUMSCs displayed significant ameliorating effects in SCA1 mice in terms of motor behaviors in balance beam test and open field test as compared with the untransplanted SCA1 mice. HUMSCs transplantation effectively reduced the cerebellar atrophy, salvaged Purkinje cell death, and alleviated molecular layer shrinkage. Electrophysiological studies showed higher amplitudes of compound motor action potentials as indicated by increasing neuronal-muscular response strength to stimuli after stem cell transplantation. At 5 months after transplantation, HUMSCs scattering in the mice cerebella remained viable and secreted cytokines without differentiating into neuronal or glia cells. CONCLUSIONS: Our findings provide hope for a new therapeutic direction for the treatment of SCA1.