Identification and characterization of glucoresponsive neurons in the enteric nervous system.

We tested the hypothesis that a subset of enteric neurons is glucoresponsive and expresses ATP-sensitive K(+) (K(ATP)) channels. The immunoreactivities of the inwardly rectifying K(+) channel 6.2 (Kir6.2) and the sulfonylurea receptor (SUR), now renamed SUR1, subunits of pancreatic beta-cell K(ATP) channels, were detected on cholinergic neurons in the guinea pig ileum, many of which were identified as sensory by their costorage of substance P and/or calbindin. Glucoresponsive neurons were distinguished in the myenteric plexus because of the hyperpolarization and decrease in membrane input resistance that were observed in response to removal of extracellular glucose. The effects of no-glucose were reversed on the reintroduction of glucose or by the K(ATP) channel inhibitor tolbutamide. No reversal of the hyperpolarization was observed when D- mannoheptulose, a hexokinase inhibitor, was present on the reintroduction of glucose. Application of the K(ATP) channel opener diazoxide or the ob gene product leptin mimicked the effect of glucose removal in a reversible manner; moreover, hyperpolarizations evoked by either agent were inhibited by tolbutamide. Glucoresponsive neurons displayed leptin receptor immunoreactivity, which was widespread in both enteric plexuses. Superfusion of diazoxide inhibited fast synaptic activity in myenteric neurons, via activation of presynaptic K(ATP) channels. Diazoxide also produced a decrease in colonic motility. These experiments demonstrate for the first time the presence of glucoresponsive neurons in the gut. We propose that the glucose-induced excitation of these neurons be mediated by inhibition of K(ATP) channels. The results support the idea that enteric K(ATP) channels play a role in glucose-evoked reflexes.     

NMDA alters rotenone toxicity in rat substantia nigra zona compacta and ventral tegmental area dopamine neurons

Previous patch-clamp studies by our laboratory showed that acute exposure to the pesticide rotenone augments inward currents evoked by N-methyl-d-aspartate (NMDA) in substantia nigra zona compacta (SNC) dopamine neurons in slices of rat brain. The present experiments were done to search for histological evidence of increased neurotoxicity produced by combined rotenone and NMDA treatments. In horizontal slices of rat midbrain, we found that a 30 min superfusion with 100 nM rotenone caused significant injury to tyrosine hydroxylase (TH)-positive proximal dendrites in dorsal and ventral regions of the SNC and ventral tegmental area (VTA). Moreover, treatment with 100 μM NMDA potentiated rotenone toxicity. In contrast, treatment with 30 μM NMDA protected against rotenone-induced injury to dendrites in the ventral SNC and ventral VTA. Interestingly, treatment with 30 μM NMDA-alone produced an apparent increase in proximal dendrite scores in ventral SNC and dorsal VTA. We conclude that NMDA has concentration-dependent actions on rotenone toxicity that differ according to regional subtype of dopamine neuron.     

Financial gain‐ and loss‐related BOLD signals in the human ventral tegmental area and substantia nigra pars compacta

Neurons in the ventral tegmental area (VTA) and substantia nigra pars compacta (SNC) play central roles in reward‐related behaviours. Nonhuman animal studies suggest that these neurons also process aversive events. However, our understanding of how the human VTA and SNC responds to such events is limited and has been hindered by the technical challenge of using functional magnetic resonance imaging (fMRI) to investigate a small structure where the signal is particularly vulnerable to physiological noise. Here we show, using methods optimized specifically for the midbrain (including high‐resolution imaging, a novel registration protocol, and physiological noise modelling), a BOLD (blood‐oxygen‐level dependent) signal to both financial gain and loss in the VTA and SNC, along with a response to nil outcomes that are better or worse than expected in the VTA. Taken together, these findings suggest that the human VTA and SNC are involved in the processing of both appetitive and aversive financial outcomes in humans.     

The acute and chronic administration of (+/-)-8-hydroxy-2-(Di-n-propylamino)tetralin significantly alters the activity of spontaneously active midbrain dopamine neurons in rats: an in vivo electrophysiological study

This study examined the effect of the acute and chronic systemic administration of (+/-)-8-Hydroxy-2-(Di-n-propylamino)Tetralin(8-OH-DPAT) on the number and firing pattern of spontaneously active dopamine (DA) neurons in the ventral tegmental area (VTA or A10) and substantia nigra pars compacta (SNC or A9) in anesthetized male rats. These parameters were measured using extracellular in vivo electrophysiology. A single s.c. injection of 0.01, 0.1, or 1 mg/kg of 8-OH-DPAT did not significantly alter the number of spontaneously active SNC DA neurons compared to vehicle-treated animals (controls). The acute administration of 0.01 or 0.1 mg/kg of 8-OH-DPAT did not significantly alter, whereas the 1 mg/kg dose significantly decreased the number of spontaneously active VTA DA neurons compared to controls. The acute administration of 8-OH-DPAT significantly increased the percentage of VTA DA neurons firing in a bursting pattern. In contrast, there was a significant decrease in the percentage of SNC DA neurons firing in a bursting pattern following the acute administration of 8-OH-DPAT. The number of spontaneously active SNC DA neurons was not significantly altered by the chronic s.c. administration of 8-OH-DPAT (0.01, 0.1, or 1 mg/kg s.c.) as compared to controls. However, the chronic s.c. administration of all doses of 8-OH-DPAT significantly decreased the number of spontaneously active VTA DA neurons compared to controls. The i.v. administration of (+)-apomorphine (50 microg/kg) did not reverse the 8-OH-DPAT-induced decrease in the number of spontaneously active VTA DA neurons, suggesting that this effect is unlikely due to depolarization blockade. The percentage of VTA DA neurons exhibiting burst firing was significantly increased by 0.01 and 0.1 mg/kg, but significantly decreased by 1 mg/kg of 8-OH-DPAT. Overall, the systemic administration of 8-OH-DPAT preferentially affects the activity of spontaneously active A10 DA neurons in rats.     

Muscarinic receptor blockade in the ventral tegmental area attenuates cocaine enhancement of laterodorsal tegmentum stimulation‐evoked accumbens dopamine efflux in the mouse

The reinforcing properties of cocaine have been related to increased extracellular concentrations of dopamine in the nucleus accumbens (NAc). M5 muscarinic acetylcholine receptors (mAChRs) on dopamine cells in the ventral tegmental area (VTA) facilitate mesoaccumbens dopamine transmission and are critically involved in mediating natural and drug reinforcement. We investigated the effects of pharmacological blockade of mAChRs in the VTA on cocaine's ability to enhance electrically evoked NAc dopamine efflux. Using fixed potential amperometry together with carbon fiber recording microelectrodes positioned in the NAc core, we quantified dopamine oxidation currents (dopamine efflux) evoked by brief stimulation (15 monophasic pulses at 50 Hz every 30 s) of the laterodorsal tegmentum (LDT) in urethane (1.5 g/kg, i.p.) anesthetized mice. Compared to predrug baseline responses, cocaine (5 or 10 mg/kg, i.p.) dose‐dependently enhanced LDT stimulation‐evoked NAc dopamine efflux, whereas the nonsubtype selective mAChR antagonist scopolamine (10 μg/0.5 μl) microinfused into the VTA diminished LDT‐evoked NAc dopamine efflux. Preinfusion of scopolamine into the VTA diminished the facilitatory actions of cocaine on LDT stimulation‐evoked NAc dopamine efflux, and when infused at the peak effect of cocaine attenuated LDT‐evoked dopamine efflux to below predrug baseline levels. These findings suggest that LDT cholinergic inputs to dopamine neurons in the VTA, via activation of mAChRs (probably of the M5 subtype), are involved in modulating the facilitatory effects of cocaine on NAc dopamine neurotransmission. They also suggest that the development of antagonists aimed at selectively disrupting M5 receptor function may be valuable in reducing abuse liability of psychostimulants. Synapse 64:216–223, 2010. © 2009 Wiley‐Liss, Inc.     

Reduced expression of Kir6.2/SUR2A subunits explains KATP deficiency in K+-depleted rats

We investigated on the mechanism responsible for the reduced ATP-sensitive K(+)(K(ATP)) channel activity recorded from skeletal muscle of K(+)-depleted rats. Patch-clamp and gene expression measurements of K(ATP) channel subunits were performed. A down-regulation of the K(ATP) channel subunits Kir6.2(-70%) and SUR2A(-46%) in skeletal muscles of K(+)-depleted rats but no changes in the expression of Kir6.1, SUR1 and SUR2B subunits were observed. A reduced K(ATP) channel currents of -69.5% in K(+)-depleted rats was observed. The Kir6.2/SUR2A-B agonist cromakalim showed similar potency in activating the K(ATP) channels of normokalaemic and K(+)-depleted rats but reduced efficacy in K(+)-depleted rats. The Kir6.2/SUR1-2B agonist diazoxide activated K(ATP) channels in normokalaemic and K(+)-depleted rats with equal potency and efficacy. The down-regulation of the Kir6.2 explains the reduced K(ATP) channel activity in K(+)-depleted rats. The lower expression of SUR2A explains the reduced efficacy of cromakalim; preserved SUR1 expression accounts for the efficacy of diazoxide. Kir6.2/SUR2A deficiency is associated with impaired muscle function in K(+)-depleted rats and in hypoPP.     

Effect of acute and chronic administration of the selective 5-HT2C receptor antagonist SB-243213 on midbrain dopamine neurons in the rat: an in vivo extracellular single cell study

In this study, we examined the effect of the acute and chronic administration of the selective 5-HT2C receptor antagonist SB-243213 (SB) on the activity of spontaneously active dopamine (DA) cells in the substantia nigra pars compacta (SNC) and ventral tegmental area (VTA) in anesthetized, albino, male Sprague-Dawley rats. This was accomplished using the technique of in vivo extracellular single cell recording. The acute i.v. administration of SB-243213 (0.025-3.2 mg/kg) did not significantly alter the basal firing rate or pattern of either spontaneously active SNC or VTA DA neurons compared to vehicle-treated controls. The acute i.p. administration of either 1 or 10 mg/kg of SB-243213 did not significantly alter the number of spontaneously active DA cells in the SNC or VTA compared to vehicle-treated controls, whereas the 3 mg/kg dose only significantly decreased the number of spontaneously active VTA DA neurons. Overall, the 1 mg/kg dose of SB-243213 did not significantly alter the firing pattern of either SNC or VTA DA neurons compared to vehicle-treated controls. In contrast, the 3 mg/kg dose significantly altered the firing pattern of SNC DA neurons, whereas the 10 mg/kg dose altered the firing pattern of DA neurons in both the SNC and VTA. The repeated i.p. administration (21 days) of 1, 3, and 10 mg/kg of SB-243213 or 20 mg/kg of clozapine produced a significant decrease in the number of spontaneously active DA cells in the VTA compared to vehicle-treated controls. The decrease in the number of spontaneously active VTA DA cells was not reversed by the i.v. administration of (+)-apomorphine (50 microg/kg). The repeated administration of either 1 or 3 mg/kg of SB-243213 had minimal effects on the firing pattern of either SNC or VTA DA neurons. In contrast, the firing pattern of VTA DA neurons was significantly altered by 10 mg/kg dose of SB-243213. Overall, our results indicate that antagonism of the 5-HT2C receptor alters the activity of midbrain DA neurons in anesthetized rats and suggest that SB-243213 has an atypical antipsychotic profile following chronic administration.     

Micturition-related electrophysiological properties in the substantia nigra pars compacta and the ventral tegmental area in cats

Parkinson's disease patients are known to have not only motor but also urinary autonomic disorders, suggesting central dopaminergic pathways being involved in the micturition function. However, there is little evidence that the substantia nigra pars compacta (SNC) and the ventral tegmental area (VTA), the major dopamine-containing nuclei in the midbrain, should participate in regulating micturition. We investigated micturition-related electrophysiological properties in the SNC and VTA. In 20 male cats under ketamine anaesthesia, in which spontaneous isovolumetric micturition reflex was generated, we performed electrical stimulation and extracellular single-unit recording in the SNC and the VTA, and correlation analysis of the neuronal firings and antidromic stimulation between the SNC/VTA and the pontine storage centre (PSC). Electrical stimulations in the SNC elicited termination of the micturition reflex, whereas those in the VTA elicited both termination and facilitation of the reflex. Forty-nine neurons in the SNC/VTA showed firing in response to the bladder storage/micturition cycles. The major neurons were tonic storage (55%) and phasic storage neurons (22%), which were found diffusely in th e SNC/VTA. The rest were tonic micturition (16%) and phasic micturition neurons (6%), which were concentrated in the caudal part (A2-4 in the Horsley-Clarke coordinates). These neuronal types were further subclassified into augmenting, constant, binary and decrementing neurons according to their temporal discharge rate change. The decrementing neurons were concentrated in the caudal part (A2-4), whereas the augmenting neurons in the rostral part (A4-6). Some of the recorded neurons had preceding firing pattern, which was more frequently found in the tonic type than in the phasic-type neurons. Twenty-four of the neuronal firings in the SNC/VTA were recorded simultaneously with those in the PSC. However, there was no apparent time-correlation between both sets of neuronal firings. In 15 of the simultaneous recording sites, electrical stimulation was applied to one site to see if antidromic response might be evoked in another site. However, there was no orthodromic or antidromic response in either SNC/VTA or PSC. In conclusion, the present study indicates that neurons in the SNC and the VTA are involved in supra-pontine control of micturition, particularly of urinary storage phase. It is also likely that the major role of the SNC is inhibition of the micturiton reflex, whereas that of the VTA is both facilitation and inhibition of the micturition reflex.     

Acute and repeated administration of the selective 5-HT(2A) receptor antagonist M100907 significantly alters the activity of midbrain dopamine neurons: an in vivo electrophysiological study

We examined the effect of the acute and repeated administration of M100907 (formerly MDL 100907), a selective 5-HT(2A) receptor antagonist, on spontaneously active dopamine (DA) neurons in the substantia nigra pars compacta (SNC) and ventral tegmental area (VTA) of rats. This was accomplished using in vivo, extracellular single unit recording. The i.v. administration of M100907 (0.01-0.64 mg/kg) did not significantly alter the basal firing rate or pattern of spontaneously active SNC and VTA DA neurons. A single injection of either 0.01 or 0.03 mg/kg i.p. of M100907 did not significantly alter the number of spontaneously active DA neurons in either the SNC or VTA areas. However, 0.1 mg/kg i.p. of M100907 significantly increased the number of spontaneously active SNC and VTA DA neurons compared to vehicle-treated animals. A single injection of all doses of M100907 significantly decreased the degree of bursting in VTA DA neurons, whereas the 0.1 mg/kg dose increased the degree of bursting in SNC DA neurons. The repeated administration (one injection per day for 21 days) of 0.03 and 0.1 mg/kg i.p. of M100907 produced a significant decrease in the number of spontaneously active SNC and VTA DA neurons compared to vehicle-treated animals. The repeated administration of M100907 did not significantly alter the firing pattern of VTA DA neurons but significantly altered the firing pattern of SNC DA neurons. The results of this study indicate that M100907 administration alters the activity of midbrain DA neurons in anesthetized rats.     

Effect of the acute and chronic administration of the selective 5-HT6 receptor antagonist SB-271046 on the activity of midbrain dopamine neurons in rats: an in vivo electrophysiological study

This study examined the effect of the acute and repeated per os (p.o.) administration of the selective 5-HT(6) receptor antagonist SB-271046, on the number, as well as the firing pattern of spontaneously active dopamine (DA) neurons in the rat substantia nigra pars compacta (SNC) and ventral tegmental area (VTA) in anesthetized male Sprague-Dawley rats. This was accomplished using the technique of extracellular in vivo electrophysiology. A single p.o. administration of either 1, 3, or 10 mg/kg of SB-271046 did not significantly alter the number of spontaneously active SNC DA neurons per stereotaxic electrode tract compared to vehicle-treated animals. The acute administration of either 1 or 3 mg/kg of SB-271046 did not significantly alter the number of spontaneously active VTA DA neurons. In contrast, a significant decrease in the number of spontaneously active VTA DA neurons was observed after a single administration of 10 mg/kg of SB-271046 compared to vehicle-treated animals. The acute p.o. administration of SB-271046 significantly altered the firing pattern parameters of all (bursting + nonbursting DA neurons) DA neurons, particularly those in the VTA, compared to vehicle-treated animals. The repeated p.o. administration (once per day for 21 days) of 1, 3, or 10 mg/kg of SB-271046 did not significantly alter the number of spontaneously active VTA DA neurons compared to vehicle-treated animals. The repeated administration of 3 or 10 mg/kg of SB-271046 significantly increased the number of spontaneously active SNC DA neurons compared to vehicle controls. Overall, the repeated administration of SB-271046 had relatively little effect on the firing pattern of midbrain DA neurons. The results obtained following the chronic administration of SB-271046 show that this compound has a profile different from that of typical or atypical antipsychotic drugs in this model. Clinical studies are required to understand what role 5-HT(6) receptor blockade might eventually play in the treatment of schizophrenia.     

Electrical stimulation of the ventral tegmental area evokes sleep‐like state transitions under urethane anaesthesia in the rat medial prefrontal cortex via dopamine D1‐like receptors

The role of dopamine in regulating sleep‐state transitions during, both natural sleep and under anaesthesia, is still unclear. Recording in vivo in the rat mPFC under urethane anaesthesia, we observed predominantly slow wave activity (SWA) of <1 Hz in the local field potential interrupted by occasional spontaneous transitions to a low‐amplitude‐fast (LAF) pattern of activity. During periods of SWA, transitions to LAF activity could be rapidly and consistently evoked by electrical stimulation of the ventral tegmental area (VTA). Spontaneous LAF activity, and that evoked by stimulation of the VTA, consisted of fast oscillations similar to those seen in the rapid eye movement (REM)‐like sleep state. Spontaneous and VTA stimulation‐evoked LAF activity occurred simultaneously along the dorsoventral extent of all mPFC subregions. Evoked LAF activity depended on VTA stimulation current and could be elicited using either regular (25–50 Hz) or burst stimulation patterns and was reproducible upon repeated stimulation. Simultaneous extracellular single‐unit recordings showed that during SWA, presumed pyramidal cells fired phasically and almost exclusively on the Up state, while during both spontaneous and VTA‐evoked LAF activity, they fired tonically. The transition to LAF activity evoked by VTA stimulation depended on dopamine D₁‐like receptor activation as it was almost completely blocked by systemic administration of the D₁‐like receptor antagonist SCH23390. Overall, our data demonstrate that activation of dopamine D₁‐like receptors in the mPFC is important for regulating sleep‐like state transitions.     

Dopamine Transporter Is a Master Regulator of Dopaminergic Neural Network Connectivity

Dopaminergic neurons of the substantia nigra pars compacta (SNC) and ventral tegmental area (VTA) exhibit spontaneous firing activity. The dopaminergic neurons in these regions have been shown to exhibit differential sensitivity to neuronal loss and psychostimulants targeting dopamine transporter. However, it remains unclear whether these regional differences scale beyond individual neuronal activity to regional neuronal networks. Here, we used live-cell calcium imaging to show that network connectivity greatly differs between SNC and VTA regions with higher incidence of hub-like neurons in the VTA. Specifically, the frequency of hub-like neurons was significantly lower in SNC than in the adjacent VTA, consistent with the interpretation of a lower network resilience to SNC neuronal loss. We tested this hypothesis, in DAT-cre/loxP-GCaMP6f mice of either sex, when activity of an individual dopaminergic neuron is suppressed, through whole-cell patch clamp electrophysiology, in either SNC or VTA networks. Neuronal loss in the SNC increased network clustering, whereas the larger number of hub-neurons in the VTA overcompensated by decreasing network clustering in the VTA. We further show that network properties are regulatable via a dopamine transporter but not a D2 receptor dependent mechanism. Our results demonstrate novel regulatory mechanisms of functional network topology in dopaminergic brain regions.SIGNIFICANCE STATEMENT In this work, we begin to untangle the differences in complex network properties between the substantia nigra pars compacta (SNC) and VTA, that may underlie differential sensitivity between regions. The methods and analysis employed provide a springboard for investigations of network topology in multiple deep brain structures and disorders.     

Cocaine disinhibits dopamine neurons in the ventral tegmental area via use‐dependent blockade of GABA neuron voltage‐sensitive sodium channels

The aim of this study was to evaluate the effects of cocaine on γ‐aminobutyric acid (GABA) and dopamine (DA) neurons in the ventral tegmental area (VTA). Utilizing single‐unit recordings in vivo, microelectrophoretic administration of DA enhanced the firing rate of VTA GABA neurons via D2/D3 DA receptor activation. Lower doses of intravenous cocaine (0.25–0.5 mg/kg), or the DA transporter (DAT) blocker methamphetamine, enhanced VTA GABA neuron firing rate via D2/D3 receptor activation. Higher doses of cocaine (1.0–2.0 mg/kg) inhibited their firing rate, which was not sensitive to the D2/D3 antagonist eticlopride. The voltage‐sensitive sodium channel (VSSC) blocker lidocaine inhibited the firing rate of VTA GABA neurons at all doses tested (0.25–2.0 mg/kg). Cocaine or lidocaine reduced VTA GABA neuron spike discharges induced by stimulation of the internal capsule (ICPSDs) at dose levels 0.25–2 mg/kg (IC₅₀ 1.2 mg/kg). There was no effect of DA or methamphetamine on ICPSDs, or of DA antagonists on cocaine inhibition of ICPSDs. In VTA GABA neurons in vitro, cocaine reduced (IC₅₀ 13 μm ) current‐evoked spikes and TTX‐sensitive sodium currents in a use‐dependent manner. In VTA DA neurons, cocaine reduced IPSCs (IC₅₀ 13 μm ), increased IPSC paired‐pulse facilitation and decreased spontaneous IPSC frequency, without affecting miniature IPSC frequency or amplitude. These findings suggest that cocaine acts on GABA neurons to reduce activity‐dependent GABA release on DA neurons in the VTA, and that cocaine’s use‐dependent blockade of VTA GABA neuron VSSCs may synergize with its DAT inhibiting properties to enhance mesolimbic DA transmission implicated in cocaine reinforcement.     

Developmental downregulation of ATP-sensitive potassium conductance in astrocytes in situ

In many neural and non-neural cells, ATP-sensitive potassium (K(ATP)) channels couple the membrane potential to energy metabolism. We investigated the activation of K(ATP) currents in astrocytes of different brain regions (hippocampus, cerebellum, dorsal vagal nucleus) by recording whole-cell currents with the patch-clamp technique in acute rat brain slices. Pharmacological tools, hypoglycemia and specific compounds in the pipette solution (cAMP, UDP), were used to modulate putative K(ATP) currents. The highest rate of K(ATP) specific currents was observed with a pipette solution containing cAMP and external stimulation with diazoxide (0.3 mM). The diazoxide-activated current had a reversal potential negative to -80 mV and was inhibited by tolbutamide (0.2 mM). We found that not all cells activated a K(ATP) current, and that the portion of cells with functional K(ATP) channel expression was developmentally downregulated. Whereas diazoxide activated K(ATP) currents in 57% of the astrocytes in rats aged 8-11 days (n = 21), the rate decreased to 38% at 12-15 days (n = 29) and to 8% at 16-19 days (n = 12). No significant difference was observed for the three brain regions. In recordings without cAMP in the internal solution, only 21% (12-15 days; n = 19) or none (16-19 days; n = 7), respectively, showed a potassium current upon diazoxide application. This metabolically regulated potassium conductance may be of importance, particularly in immature astrocytes with a complex current pattern, which have a relatively high input resistance: K(ATP) currents activated by energy depletion may hyperpolarize the cells, or stabilize a negative resting potential during depolarizing stimuli mediated, e.g., by glutamate receptors and/or uptake carriers.     

Acute and repeated administration of fluoxetine, citalopram, and paroxetine significantly alters the activity of midbrain dopamine neurons in rats: an in vivo electrophysiological study

We examined the effect of the administration of the selective serotonin reuptake inhibitors (SSRIs) fluoxetine, citalopram, and paroxetine on the activity of spontaneously active dopamine (DA) neurons in the substantia nigra pars compacta (SNC) and ventral tegmental area (VTA) in anesthetized adult male Sprague-Dawley rats. This was accomplished using the technique of in vivo extracellular recording. A single injection of 2.5 mg/kg (i.p.) of fluoxetine significantly increased the number of spontaneously active SNC and VTA DA neurons. In contrast, a single injection of either 1 mg/kg (i.p.) of paroxetine or 5 mg/kg of fluoxetine significantly increased the number of spontaneously active VTA DA neurons. The repeated administration (one injection per day for 21 days) of all of the SSRIs produced a significant increase in the number of spontaneously active VTA DA neurons. Overall, our results indicate that the systemic administration of SSRI alters the activity of midbrain DA neurons with differential effects on VTA compared with SNC DA neurons.     

Acute and chronic administration of the selective sigma1 receptor agonist SA4503 significantly alters the activity of midbrain dopamine neurons in rats: An in vivo electrophysiological study.

In this study, we examined the effect of the acute and repeated administration of the selective sigma (sigma)1 receptor agonist 1-(3, 4-dimethoxyphenethyl)-4-(3-phenylpropyl)piperazine dihydrochloride (SA4503) on the number and firing pattern of spontaneously active dopamine (DA) neurons in substantia nigra pars compacta (SNC) and ventral tegmental area (VTA) in anesthetized, male Sprague-Dawley rats. This was accomplished using the technique of in vivo extracellular single unit recording. The intravenous administration of SA4503 (0.01-1.28 mg/kg) did not significantly alter the firing rate or pattern of spontaneously active DA neurons in either the SNC or VTA. A single injection of either 0.1 or 0.3 mg/kg i.p. of SA4503 did not alter the number of spontaneously active SNC and VTA DA neurons. In contrast, a single injection of 1 mg/kg i.p. of SA4503 produced a significant decrease and increase in the number of spontaneously active SNC and VTA DA neurons, respectively. Overall, the firing pattern parameters of spontaneously active SNC DA neurons were altered more significantly than those of spontaneously active VTA DA neurons following the acute administration of SA4503. The repeated administration (one injection per day for 21 days) of 0.3 and 1 mg/kg i.p. of SA4503 produced a significant increase in the number of spontaneously active VTA DA neurons. In addition, the repeated administration of SA4503 produced a greater alteration of the firing pattern of spontaneously active VTA compared to SNC DA neurons. Our results suggest that the administration of SA4503 significantly alters the activity of spontaneously active midbrain DA neurons, particularly those in the VTA following repeated administration.     

Glucose and hippocampal neuronal excitability: role of ATP-sensitive potassium channels

Hyperglycemia-related neuronal excitability and epileptic seizures are not uncommon in clinical practice. However, their underlying mechanism remains elusive. ATP-sensitive K(+) (K(ATP)) channels are found in many excitable cells, including cardiac myocytes, pancreatic beta cells, and neurons. These channels provide a link between the electrical activity of cell membranes and cellular metabolism. We investigated the effects of higher extracellular glucose on hippocampal K(ATP) channel activities and neuronal excitability. The cell-attached patch-clamp configuration on cultured hippocampal cells and a novel multielectrode recording system on hippocampal slices were employed. In addition, a simulation modeling hippocampal CA3 pyramidal neurons (Pinsky-Rinzel model) was analyzed to investigate the role of K(ATP) channels in the firing of simulated action potentials. We found that incremental extracellular glucose could attenuate the activities of hippocampal K(ATP) channels. The effect was concentration dependent and involved mainly in open probabilities, not single-channel conductance. Additionally, higher levels of extracellular glucose could enhance neuropropagation; this could be attenuated by diazoxide, a K(ATP) channel agonist. In simulations, high levels of intracellular ATP, used to mimic increased extracellular glucose or reduced conductance of K(ATP) channels, enhanced the firing of action potentials in model neurons. The stochastic increases in intracellular ATP levels also demonstrated an irregular and clustered neuronal firing pattern. This phenomenon of K(ATP) channel attenuation could be one of the underlying mechanisms of glucose-related neuronal hyperexcitability and propagation.     

The effect of the acute and chronic administration of CP 96,345, a selective neurokinin1 receptor antagonist,on midbrain dopamine neurons in the rat: A single unit,extracellular recording study

In this study, we examined the effect of acute and chronic administration of the selective neurokinin₁ receptor antagonist CP 96,345 on the basal activity of spontaneously active dopamine (DA) neurons in the substantia nigra pars compacta (SNC) and the ventral tegmental area (VTA). This was accomplished using the technique of in vivo, extracellular single unit recording in anesthetized rats. The intravenous (i.v.) administration of CP 96,345 (0.01–1.28 mg/kg) did not significantly alter the firing rate of spontaneously active DA neurons in the SNC and VTA areas. The acute administration of 5 or 10 mg/kg, i.p., of CP 96,345 produced a significant decrease in the number of spontaneously active SNC and VTA dopamine cells compared to vehicle-treated rats. In contrast to its effect on the number of spontaneously active DA neurons, the administration of 5 mg/kg, i.p., of CP 96,345 did not significantly alter the basal firing pattern of either SNC or VTA DA neurons. The acute administration of CP 96,345 (10 mg/kg, i.p.) significantly potentiated the suppressant action of (+)-apomorphine on the basal firing rate of spontaneously active SNC and VTA DA cells. The chronic administration of CP 96,345 (5 or 10 mg/kg, i.p.) for 21 days also produced a significant decrease in the number of spontaneously active SNC and VTA DA cells compared to vehicle controls. This effect was not reversed by the systemic administration of (+)-apomorphine (50 μg/kg, i.v.), suggesting that the reduction in the number of spontaneously active DA cells produced by CP 96,345 is probably not the result of depolarization inactivation. Overall, our results indicate that the tonic activation of NK₁ receptors by substance P may be necessary to maintain the spontaneous activity of a proportion of midbrain DA neurons. © 1996 Wiley-Liss, Inc.     

Plasma membrane insertion of TRPC5 channels contributes to the cholinergic plateau potential in hippocampal CA1 pyramidal neurons

In cultured hippocampal neurons, transient receptor potential 5 (TRPC5) channels are translocated and inserted into plasma membranes of hippocampal neurons to generate nonselective cation (NSC) currents. We investigated whether TRPC5 channel translocation also contributes to the generation of NSC currents underlying the afterdepolarizations and plateau potentials (PPs) in hippocampal pyramidal cells that are induced by muscarinic receptor activation. Using a biotinylation assay to quantify the change in surface membrane proteins in acute hippocampal slices, we found that muscarinic stimulation significantly enhanced the levels of TRPC5 protein on the membrane surface but not those of TRPC1 or TRPC4 channels. We then investigated the pharmacological sensitivity of the cation current observed during muscarinic stimulation to determine if a component could be due to TRPC5 channels. The TRPC channel antagonists 2‐APB and SKF96365 strongly depressed the generation of PPs, the underlying tail currents (I_tail) and the associated dendritic Ca²⁺ influx induced by muscarinic receptor activation in pyramidal neurons. High intracellular concentrations of ATP, which specifically inhibit TRPC5 channels, depressed I_tail. In addition, pretreatment with the calmodulin (CaM) inhibitor W‐7, which depresses recombinant TRPC5 currents, inhibited both the cation current (I_tail) and the surface insertion of TRPC5 channels. Finally, the phosphatidylinositide 3‐kinase (PI₃K) inhibitor wortmannin, which blocks translocation of TRPC5 channels in cell culture, also inhibited both the I_tail and the surface insertion of TRPC5 channels. Therefore, we conclude that insertion of TRPC5 channels contributes to the generation of the prolonged afterdepolarizations following muscarinic stimulation. This altered plasma membrane expression of TRPC5 channels in pyramidal neurons may play an important role in the generation of prolonged neuronal depolarization and bursting during the epileptiform seizure discharges of epilepsy. © 2010 Wiley‐Liss, Inc.