Inhibition by talipexole, a thiazolo-azepine derivative, of dopaminergic neurons in the ventral tegmental area.

A microiontophoretic study using rats anesthetized with chloral hydrate and immobilized with gallamine triethiodide was carried out to compare the effect of talipexole (B-HT 920 CL2:2-amino-6-allyl-5,6,7,8-tetrahydro-4H-thiazolo [4,5-d]-azepine-dihydrochloride), a dopamine autoreceptor agonist, on dopaminergic neurons in the ventral tegmental area (VTA) to non-dopaminergic neurons in the VTA. VTA neurons were classified into two types according to the responses to antidromic stimulation of the nucleus accumbens (Acc): type I neurons with a long spike latency (8.69 +/- 0.24 msec) upon Acc stimulation and low spontaneous firing rate (6.80 +/- 1.34/sec), and type II neurons with a short latency (2.76 +/- 0.20 msec) and high spontaneous firing rate (26.77 +/- 7.05/sec), probably corresponding to dopaminergic and non-dopaminergic neurons, respectively. In type I neurons, microiontophoretic application of talipexole and dopamine inhibited antidromic spike generation elicited by Acc stimulation, and talipexole-induced inhibition was antagonized by domperidone (dopamine D-2 antagonist). In type II neurons, however, the antidromic spikes were not affected by either talipexole or dopamine. Furthermore, spontaneous firing was also inhibited by iontophoretically applied talipexole and dopamine in most type I neurons, but rarely affected by either drug. Inhibitory effects of talipexole were antagonized by domperidone. These results suggest that talipexole acts on dopamine D-2 receptors, thereby inhibiting the dopaminergic neurons in the VTA.     

Presynaptic inhibition of excitatory input from the substantia nigra to caudate nucleus neurons by a substituted quinolinone derivative, 7-[3-(4-(2,3-dimethylphenyl)piperazinyl)propoxy]-2(1H)-quinolinone (OPC-4392)

The effects of a newly synthesized quinolinone derivative, 7-[3-(4-(2,3-dimethylphenyl)piperazinyl) propoxy]-2(1H)-quinolinone (OPC-4392) on neuronal activities of the caudate nucleus (CN) were investigated in cats anesthetized with alpha-chloralose using a microiontophoretic method. In the CN neurons of which spikes elicited by stimulation of the pars compacta of substantia nigra (SN) were suppressed by iontophoretically applied domperidone, a dopamine D-2 receptor antagonist, application of OPC-4392 (100-200 nA) inhibited the spike generation induced by SN stimulation. Conversely, the CN neurons insensitive to domperidone were unaffected by OPC-4392. Iontophoretic application of CPC-4392 up to 200 nA did not affect glutamate-induced firing of the CN neurons, of which the firing was blocked by dopamine less than 100 nA. In addition, OPC-4392 did not inhibit firing induced by bromocriptine, a dopamine D-2 agonist; while domperidone suppressed the bromocriptine-induced firing without affecting the glutamate-induced firing. These results suggest that OPC-4392 acts on the dopaminergic nerve terminals and inhibits excitatory transmission from the SN to the CN.     

Effect of 7-(3-[4-(2,3-dimethylphenyl)piperazinyl]propoxy)- 2(1H)-quinolinone (OPC-4392), a newly synthesized agonist for presynaptic dopamine D2 receptor, on tyrosine hydroxylation in rat striatal slices

The effects of a newly synthesized compound, 7-(3-[4-(2,3-dimethylphenyl)piperazinyl]propoxy)-2(1H)-quinolinone (OPC-4392), on tyrosine hydroxylation in situ and in vitro were studied using rat striatal slices and tyrosine hydroxylase (TH) purified from bovine adrenal medulla, respectively. OPC-4392 dose-dependently inhibited L-dihydroxyphenylalanine (DOPA) formation in rat striatal slices with IC50 values of about 10(-6) M. The inhibitory effect of OPC-4392 on in situ DOPA formation was dose-dependently reversed by addition of sulpiride, a dopamine D2 receptor antagonist, whereas no change was observed by addition of nomifensine (5 X 10(-6) M), a blocker of dopamine uptake. From in vitro experiment using purified TH, OPC-4392 affected neither the enzymatic activity nor the Km value for 6-methyl-5,6,7,8-tetrahydropterin (6MPH4). These results suggest that OPC-4392 impairs in situ DOPA formation by stimulating presynaptic dopamine D2 receptor as a dopamine agonist, and not by directly inhibiting the TH activity.     

7-[3-(4-[2,3-Dimethylphenyl]piperazinyl)propoxy]-2(1H)-quinolinone (OPC-4392), a presynaptic dopamine autoreceptor agonist and postsynaptic D2 receptor antagonist

7-[3-(4-[2,3-dimethylphenyl]piperazinyl)propoxy]-2(1H)-quinolinone (OPC-4392), was synthesized in our laboratories and compared with apomorphine, 3-(3-hydroxyphenyl)-N-n-propylpiperidine (3-PPP) and dopamine antagonists in a series of tests designed to characterize dopamine receptor activation and inhibition. The assertion that OPC-4392 acts as an agonist at presynaptic dopamine autoreceptors is supported by the following behavioral and biochemical observations: OPC-4392, 3-PPP and apomorphine inhibited the reserpine-induced increase in DOPA accumulation in the forebrain of mice and in the frontal cortex, limbic forebrain and striatum of rats. In addition, the gamma-butyrolactone (GBL)-induced increase in DOPA accumulation in the mouse forebrain was also inhibited by OPC-4392, 3-PPP and apomorphine. Haloperidol antagonized the inhibitory effect of OPC-4392 in both instances. The inhibitory effect of OPC-4392 on GBL-induced DOPA accumulation lasted for at least 8 hours after oral administration to mice, while that of 3-PPP and apomorphine disappeared in 4 hours after subcutaneous injection. OPC-4392 failed to increase spontaneous motor activity in reserpinized mice, enhance spontaneous ipsilateral rotation in rats with unilateral striatal kainic acid (KA) lesions, induce contralateral rotation in rats with unilateral striatal 6-hydroxydopamine (6-OHDA) lesions and inhibit 14C-acetylcholine (Ach) release stimulated by 20 mM KCl in rat striatal slices. In addition, OPC-4392 appears to block postsynaptic D2 receptors since OPC-4392, as well as dopamine antagonists, was able to inhibit stereotyped behavior and climbing behavior induced by apomorphine in mice, displace the 3H-spiroperidol binding to rat synaptosomal membranes in vitro and reverse the inhibitory effect of apomorphine on Ach release in rat striatal slices. These results suggest that OPC-4392 acts as a dopamine agonist at presynaptic autoreceptors related to dopamine synthesis and acts as dopamine antagonist at postsynaptic D2 receptors.     

Excitation by dopamine D-2 receptor agonists, bromocriptine and LY 171555, in caudate nucleus neurons activated by nigral stimulation

Electrophysiological studies using cats anesthetized with alpha-chloralose were carried out to determine whether or not the dopamine D-2 receptor mediates the excitation of the caudate nucleus (CN) neurons activated by stimulation of the substantia nigra (SN). Microiontophoretic application of domperidone (D-2 antagonist) produced a significant inhibition of spikes elicited by SN stimulation in 20 of 27 CN neurons. When bromocriptine and LY 171555 (D-2 agonists) were iontophoretically applied to the CN neurons in which the SN-induced spikes were inhibited by domperidone, an increase in spontaneous firing rate was observed in 18 of 20 neurons and all of 10 neurons tested, respectively. However, no alterations of firing occurred with bromocriptine or LY 171555 in any 7 neurons in which the SN-induced spikes were not affected by domperidone. The increase in firing rate by the D-2 agonists was apparently antagonized during simultaneous application of domperidone and haloperidol, but not affected during application of SCH 23390 (D-1 antagonist). These results strongly suggest that the spike generation of the CN neurons upon SN stimulation is mediated by the dopamine D-2 receptor.     

Dopamine D-2 receptor-mediated excitation of caudate nucleus neurons from the substantia nigra

Microiontophoretic studies using cats anesthetized with alpha-chloralose were performed to elucidate whether the excitatory response of caudate nucleus (CN) neurons upon stimulation of the pars compacta of the substantia nigra (SN) is mediated by the dopamine D-1 or D-2 receptor. There were rare convergent inputs from the SN and motor cortex (MC) in the CN neurons. Iontophoretic application of haloperidol and domperidone (dopamine D-2 receptor antagonist) produced dose-dependent inhibition of spikes elicited by SN stimulation in 25 of 42 and 50 of 82 CN neurons, respectively, however, no alterations of spikes elicited by MC stimulation occurred in any 11 neurons tested. Iontophoretically applied SCH 23390 (D-1 antagonist) did not inhibit the SN-induced spikes in any CN neurons, of which spikes were inhibited by domperidone. These results suggest that the SN-induced spikes are mediated by dopamine, which acts on postsynaptic D-2 receptors.     

Coexistence of inhibitory dopamine D-1 and excitatory D-2 receptors on the same caudate nucleus neurons

Microiontophoretic studies using cats anesthetized with alpha-chloralose were performed to determine whether or not dopamine D-1 and D-2 receptors co-exist in the same caudate nucleus (CN) neurons that receive inputs from the substantia nigra (SN), and in which spikes elicited by SN stimulation were blocked by domperidone, a selective D-2 antagonist. Iontophoretic application of dopamine produced a dose-dependent inhibition of spontaneous firing in 2 of 4 spontaneously active CN neurons and an increase in firing in the remaining 2 neurons. However, dopamine inhibited the glutamate-induced firing in 31 of 32 CN neurons that were not spontaneously active. Similar inhibition with iontophoretically applied SKF 38393, a selective D-1 agonist, was observed in 33 of 34 spontaneously inactive neurons tested. When the effects of dopamine, SKF 38393 and bromocriptine (D-2 agonist) were examined on the same CN neurons, the inhibitory effects of both dopamine and SKF 38393 were seen in 14 of 15 neurons, and both an inhibition by SKF 38393 and an excitation by bromocriptine were observed in 15 of 17 neurons. The inhibitory effects of dopamine and SKF 38393 were antagonized by haloperidol and SCH 23390 (D-1 antagonist) without being affected by domperidone. Furthermore, the dopamine-induced inhibition was converted to an excitation during simultaneous application of SCH 23390 in 6 of 10 CN neurons, and this excitation was antagonized by domperidone. These results strongly suggest that the inhibitory D-1 and excitatory D-2 receptors co-exist on the same CN neurons receiving inputs from the SN.     

Responses of medial geniculate body neurons to auditory cortical stimulation

Extracellular and intracellular unit responses of thepars principalis of the medial geniculate body to stimulation of the first (AI), second (AII), and third (AIII) auditory cortical areas were studied in cats immobilized with D-tubocurarine. In response to auditory cortical stimulation both antidromic (45–50%) and orthodromic (50–55%) responses occurred in the geniculate neurons. The latent period of the antidromic responses was 0.3–2.5 msec and of the orthodromic 2.0–18.0 msec. Late responses had a latent period of 30–200 msec. Of all neurons responding antidromically to stimulation of AII, 63% responded antidromically to stimulation of AI also, confirming the hypothesis that many of the same neurons of the medial geniculate body have projections into both auditory areas. Orthodromic responses of geniculate neurons consisted either of 1 or 2 spikes or of volleys of 8–12 spikes with a frequency of 300–600/sec. It is suggested that the volleys of spikes were discharges of inhibitory neurons. Intracellular responses were recorded in the form of antidromic spikes, EPSPs, EPSP-spike, EPSP-spike-IPSP, EPSP-IPSP, and primary IPSP. Over 50% of primary IPSP had a latent period of 2.0–4.0 msec. It is suggested that they arose through the participation of inhibitory interneurons located in the medial geniculate body.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 8, No. 1, pp. 5–12, January–February, 1976.     

Corticofugal postsynaptic influences on red nucleus neurons

The responses of red nucleus neurons to stimulation of the sensorimotor cortex was studied on nembutal-anesthetized cats. Most of the rubrospinal neurons were identified according to their antidromic activation. Stimulation of the sensorimotor cortex was shown to evoke in the red nucleus neurons monosynaptic excitatory potentials with a latency of 1.85 msec, polysynaptic excitatory potentials (EPSP), and inhibitory postsynaptic potentials (IPSP) with a latency of 9–24 msec. The EPSP often produced spikes. The probability of generation of spreading excitation is greater with motor cortex stimulation. The monosynaptic EPSP are assumed to arise under the influence of the impulses arriving over the corticorubral neurons as a result of excitation of axodendritic synapses. The radial type of branching of red nucleus neurons facilitates the transition from electrotonically spreading local depolarization to an action potential triggered by the initial axonal segment. Polysynaptic EPSP and IPSP seem to be a result of activation of fast pyramidal neurons whose axon collaterals are connected via interneurons with the soma of the red nucleus neurons.L. A. Orbeli Institute of Physiology of the Academy of Sciences of the Armenian SSR, Erevan. Translated from Neirofiziologiya, Vol. 3, No. 1, pp. 43–51, January–February, 1971.     

Unit responses of the first and second somatosensory areas to stimulation of the ventroposterior thalamic nucleus

Single unit responses of the first (SI) and second (SII) somatosensory areas to stimulation of the ventroposterior thalamic nucleus (VP) were investigated in cats immobilized with D-tubocurarine. In response to VP stimulation 12.0% of reacting SI neurons and 9.5% of SII neurons generated an antidromic spike. In most antidromic responses of both SI and SII neurons the latent period did not exceed 1.0 msec. The minimal latent period of spike potentials during orthodromic excitation was 1.5 msec in SI and 1.7 msec in SII. Neurons with an orthodromic spike latency of not more than 3.0 msec were more numerous in SI than those with a latency of 3.1–4.5 msec. The ratio between the numbers of neurons of these two groups in SII was the opposite. In SII there were many more neurons with a latency of 5.6–8.0 msec than in SI. EPSPs appeared after a latent period of 1.1–9.0 msec in SI and of 1.4–6.6 msec in SII. The latent period of IPSPs was 1.5–6.8 msec in SI and 2.2–9.4 msec in SII. The relative importance of different pathways for excitatory and inhibitory influences of VP on SI and SII neurons is discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 8, No. 2, pp. 115–121, March–April, 1976.     

Unit responses in the auditory cortex to stimulation of geniculocortical fibers

Extracellular and intracellular single unit responses of neurons of the auditory cortex to electrical stimulation of geniculocortical fibers (GCF) were recorded in experiments on cats immobilized with tubocurarine. The latent period of responses of 15% of neurons to GCF stimulation was 0.3–1.5 msec. It is postulated that they were excited anti-dromically. The latent period of spikes generated by neurons responding to GCF stimulation orthodromically varied from 1.6 to 12 msec. In 28.6% of neurons the latent period was 1.6–2.5 msec. It is postulated that these neurons were excited monosynaptically. Intracellular recording revealed primary IPSPs in response to GCF stimulation in 63.3% of neurons, a brief EPSP followed by a prolonged IPSP in 17.7%, an EPSP-spike-IPSP complex in 12.3%, and subthreshold EPSPs in 7% of neurons. The latent period of the primary IPSPs varied from 1.8 to 11 msec, being 1.8–3.7 in 72%, 3.8–5.7 in 20.0%, and 5.8–11 msec in 8.0% of neurons. The latent period of responses beginning with an EPSP was 1–4 msec (mean 1.8 msec). Orthodromic responses arising 3–10 msec after the antidromic response, and consisting of 3–5 spikes, were recorded in some antidromically excited neurons. Hypotheses regarding the functional organization of the auditory cortex and mechanisms of inhibition in its neurons are put forward on the basis of the results obtained.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 4, No. 3, pp. 227–235, May–June, 1972.     

Connections of the second auditory cortical area with the medial geniculate body and the first auditory area

Extra — and intracellular unit responses in area AII to stimulation of geniculocortical fibers and of area AI were studied in cat immobilized with D-tubocurarine. In response to stimulation of geniculocortical fibers, antidromic mono-, di-, and polysynaptic spikes were generated by neurons in area AII. The number of antidromic responses in area AII was about half that found in area AI under the same conditions of stimulation. Most of the orthodromic responses were di- and polysynaptic. Intracellular responses also were recorded in the form of EPSPs, EPSP-IPSPs, and primary IPSPs. Stimulation of area AI evoked responses in the neurons of area AII with latent periods of 0.75–6.0, 6.1–16.0, 18.0–23.0, and 60–100 msec. Removal of the medial geniculate body led to a marked decrease in the number of responses with latent periods of 6.1–16.0 msec. Some neurons of area AII responded by spikes to stimulation of both the geniculocortical fibers and area AI. Comparison of the latent periods of responses to these two types of stimulation showed that impulses from area AI to area AII are directed both to input neurons for impulses from the medial geniculate body and to neurons at subsequent stages of the intracortical neuronal change. In response to stimulation of cortical area AI, disynaptic IPSPs appeared in many neurons of area AII. Only one IPSP with a latent period of 1.0 msec, regardable as monosynaptic, was recorded.     

"Selective" D-1 and D-2 receptor antagonists fail to differentially alter supersensitive locomotor behavior in the rat

The dopamine receptor antagonists SCH 23390 and spiperone show highly selective in vitro affinity for D-1 and D-2 dopamine receptor subtypes, respectively. We studied the effects of these selective antagonists on the supersensitive locomotor response to apomorphine in rats following 6- hydroxydopamine (6OHDA) lesions of the nucleus accumbens (N. Acc.). Both D-1 and D-2 receptor antagonists produced dose-dependent blockade of the supersensitive locomotor response at doses that did not depress baseline locomotor activity. The behavioral properties of these D-1 and D-2 receptor antagonists were further examined using a simple step-down motor task. Both antagonists produced catalepsy as evidenced by dose-dependent increases in step- down latency. These results indicate that drugs with distinct in vitro dopamine binding affinities cannot be distinguished on the basis of their ability to inhibit supersensitive locomotor activity or simple motor tasks in rats in vivo.     

Stimulation of midbrain dopaminergic structures modifies firing rates of rat lateral habenula neurons

Ventral tegmental area (VTA) and substantia nigra pars compacta (SNpc) are midbrain structures known to be involved in mediating reward in rodents. Lateral habenula (LHb) is considered as a negative reward source and it is reported that stimulation of the LHb rapidly induces inhibition of firing in midbrain dopamine neurons. Interestingly, the phasic fall in LHb neuronal activity may follow the excitation of dopamine neurons in response to reward-predicting stimuli. The VTA and SNpc give rise to dopaminergic projections that innervate the LHb, which is also known to be involved in processing painful stimuli. But it's unclear what physiological effects these inputs have on habenular function. In this study we distinguished the LHb pain-activated neurons of the Wistar rats and assessed their electrophysiological responsiveness to the stimulation of the VTA and SNpc with either single-pulse stimulation (300 μA, 0.5 Hz) or tetanic stimulation (80 μA, 25 Hz). Single-pulse stimulation that was delivered to either midbrain structure triggered transient inhibition of firing of ～90% of the LHb pain-activated neurons. However, tetanic stimulation of the VTA tended to evoke an elevation in neuronal firing rate. We conclude that LHb pain-activated neurons can receive diverse reward-related signals originating from midbrain dopaminergic structures, and thus participate in the regulation of the brain reward system via both positive and negative feedback mechanisms.     

M5 muscarinic receptors are needed for slow activation of dopamine neurons and for rewarding brain stimulation

Mesopontine cholinergic neurons (Ch5 and Ch6 cell groups) activate the cerebral cortex via thalamic projections, and activate locomotion and reward via dopamine neurons in the substantia nigra and ventral tegmental area (VTA). Nicotinic receptors in VTA activate dopamine neurons quickly, and are needed for the stimulant and rewarding effects of nicotine in rats. Muscarinic receptors in VTA activate dopamine neurons slowly, and are needed for the rewarding effects of hypothalamic stimulation, but do not increase locomotion. Antisense oligonucleotides targetting M5 mRNA, when infused into the VTA, inhibited M5 receptor binding and rewarding hypothalamic stimulation. Mutant mice with truncated M5 muscarinic receptor genes drank more water than wild-type controls. Spontaneous locomotion and locomotor responses to amphetamine and scopolamine were unchanged. Electrical stimulation near Ch6 induced dopamine release in the nucleus accumbens in two phases, an early phase (0-2 min after stimulation) dependent on nicotinic and gluatamatergic receptors in VTA, and a late phase (8-50 min after stimulation) dependent on muscarinic receptors in VTA. The late phase was lost in M5 mutant mice, while the early phase was unchanged. M5 muscarinic receptors bind slowly to muscarinic ligands, and appear to mediate slow secretions.     

Angiotensin-(1-7) increases neuronal potassium current via a nitric oxide-dependent mechanism

Actions of angiotensin-(1-7) [Ang-(1-7)], a heptapeptide of the renin-angiotensin system, in the periphery are mediated, at least in part, by activation of nitric oxide (NO) synthase (NOS) and generation NO(·). Studies of the central nervous system have shown that NO(·) acts as a sympathoinhibitory molecule and thus may play a protective role in neurocardiovascular diseases associated with sympathoexcitation, such as hypertension and heart failure. However, the contribution of NO in the intraneuronal signaling pathway of Ang-(1-7) and the subsequent modulation of neuronal activity remains unclear. Here, we tested the hypothesis that neuronal NOS (nNOS)-derived NO(·) mediates changes in neuronal activity following Ang-(1-7) stimulation. For these studies, we used differentiated catecholaminergic (CATH.a) neurons, which we show express the Ang-(1-7) receptor (Mas R) and nNOS. Stimulation of CATH.a neurons with Ang-(1-7) (100 nM) increased intracellular NO levels, as measured by 4-amino-5-methylamino-2',7'-difluorofluorescein diacetate (DAF-FM) fluorescence and confocal microscopy. This response was significantly attenuated in neurons pretreated with the Mas R antagonist (A-779), a nonspecific NOS inhibitor (nitro-L-arginine methyl ester), or an nNOS inhibitor (S-methyl-L-thiocitrulline, SMTC), but not by endothelial NOS (eNOS) or inhibitory NOS (iNOS) inhibition {L-N-5-(1-iminoethyl)ornithine (L-NIO) and 1400W, respectively}. To examine the effect of Ang-(1-7)-NO(·) signaling on neuronal activity, we recorded voltage-gated outward K(+) current (I(Kv)) in CATH.a neurons using the whole cell configuration of the patch-clamp technique. Ang-(1-7) significantly increased I(Kv), and this response was inhibited by A-779 or S-methyl-L-thiocitrulline, but not L-NIO or 1400W. These findings indicate that Ang-(1-7) is capable of increasing nNOS-derived NO(·) levels, which in turn, activates hyperpolarizing I(Kv) in catecholaminergic neurons.     

Unit responses of the reticular and ventral anterior thalamic nuclei of cats to orbitofrontal cortical stimulation

Responses of 98 neurons of the reticular (R) and 72 neurons of the ventral anterior (VA) thalamic nuclei to stimulation of various zones of the orbitofrontal cortex were investigated in acute experiments on cats immobilized with D-tubocurarine. Not all zones of this cortex were found to be connected equally closely with R and VA. Most of the R (82.7%) and VA (66.7%) neurons responded to stimulation of the proreal gyrus, and fewest (37.3 and 48.9%, respectively) to stimulation of the posterior orbital gyrus. Among the responding neurons, 85.2–86.3% of R cells and 78.2–81.2% of VA cells were excited by cortical stimulation and the rest were inhibited. Excitation was expressed as the appearance of a single spike or of discharges of varied duration in response to each stimulus. The latent period of the spike responses varied from 0.5 to 55.0 msec and the minimal latent period of the discharges was 0.8 msec and its maximal value over 500 msec. The spike frequency in the discharge was 120–250/sec. Unit responses consisting of spikes with a latent period of under 1.3 msec and, it is assumed, some of the responses with a latent period of under 4.0 msec were antidromic. The axons of some R and VA neurons were shown to form branches terminating in different zones of the orbitofrontal cortex.     

Characteristics of the functional properties of the spinal cord motoneurons of old rats]

The mean membrane potential (MP) of old rats did not differ significantly from that in young mature rats ((58.4 +/- +/-1,4 mV and 56.6 +/- 1.26 mV, respectively). At the same time the frequency of detection of motor neurons with the MP OF 70 mV and more fell by 18.6%, and with the MP of 50-59 mV -increased by 14.2% in the old, in comparison with the young animals. The direct excitability threshold in old rats decreased (3.0 +/- 3-10(-9) in young mature and 2.0 +/- 0.2-10(-9) a in old rats; P less than 0.02). The number of discharges per 50 msec of the neuron poliarization reached 4-5, constituting 80-100 pulse/min. When determined by the first two intervals the action potential frequency reached 125 pulse/sec, and in the young mature rats--over 300 pulse/sec. The duration of antidromic spikes was increased (1.02 +/- 0.09 msec in young mature animals and 1.65 +/- 0.14 msec in the old animals; P less than 0.001). The antidromic spikes of the neurons in old mature rats, as a rule, had no delayed depolarization.     

Cortical Regulation of Subcortical Dopamine Release: Mediation via the Ventral Tegmental Area

Abstract: In vivo microdialysis was used to determine the extent to which ionotropic glutamate receptors in the ventral tegmental area (VTA) regulate dopamine release in the nucleus accumbens. Coapplication of 2-amino-5-phosphonopentanoic acid (AP5; 200 µ M ) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 50 µ M ) to the VTA via reverse dialysis decreased extracellular concentrations of dopamine in the nucleus accumbens by ∼30%. In accordance with previous results, electrical stimulation of the prefrontal cortex increased dopamine release by 60%. Application of AP5 and CNQX to the VTA during cortical stimulation blocked the effect of stimulation on dopamine release. These results indicate that ionotropic glutamate receptors in the VTA are critically involved in basal and evoked dopamine release in the nucleus accumbens and suggest that a glutamatergic projection from the prefrontal cortex regulates the activity of dopaminergic neurons in the VTA.     

5-HT(2C) receptors localize to dopamine and GABA neurons in the rat mesoaccumbens pathway

The serotonin 5-HT(2C) receptor (5-HT(2C)R) is localized to the limbic-corticostriatal circuit, which plays an integral role in mediating attention, motivation, cognition, and reward processes. The 5-HT(2C)R is linked to modulation of mesoaccumbens dopamine neurotransmission via an activation of γ-aminobutyric acid (GABA) neurons in the ventral tegmental area (VTA). However, we recently demonstrated the expression of the 5-HT(2C)R within dopamine VTA neurons suggesting the possibility of a direct influence of the 5-HT(2C)R upon mesoaccumbens dopamine output. Here, we employed double-label fluorescence immunochemistry with the synthetic enzymes for dopamine (tyrosine hydroxylase; TH) and GABA (glutamic acid decarboxylase isoform 67; GAD-67) and retrograde tract tracing with FluoroGold (FG) to uncover whether dopamine and GABA VTA neurons that possess 5-HT(2C)R innervate the nucleus accumbens (NAc). The highest numbers of FG-labeled cells were detected in the middle versus rostral and caudal levels of the VTA, and included a subset of TH- and GAD-67 immunoreactive cells, of which >50% also contained 5-HT(2C)R immunoreactivity. Thus, we demonstrate for the first time that the 5-HT(2C)R colocalizes in DA and GABA VTA neurons which project to the NAc, describe in detail the distribution of NAc-projecting GABA VTA neurons, and identify the colocalization of TH and GAD-67 in the same NAc-projecting VTA neurons. These data suggest that the 5-HT(2C)R may exert direct influence upon both dopamine and GABA VTA output to the NAc. Further, the indication that a proportion of NAc-projecting VTA neurons synthesize and potentially release both dopamine and GABA adds intriguing complexity to the framework of the VTA and its postulated neuroanatomical roles.