Pharmacology of sensory stimulation-evoked increases in frontal cortical acetylcholine release.

Recent research has demonstrated that a variety of sensory stimuli can increase acetylcholine release in the frontal cortex of rats. The aim of the present experiments was to investigate the pharmacological regulation of sensory stimulation-induced increases in the activity of basal forebrain cholinergic neurons. To this end, the effects of agonists and antagonists at a variety of neurotransmitter receptors on basal and tactile stimulation-evoked increases in frontal cortical acetylcholine release were studied using in vivo brain microdialysis. Tactile stimulation, produced by gently stroking the rat's neck with a nylon brush for 20 min, significantly increased frontal cortical acetylcholine release by more than 100% above baseline. The noradrenergic alpha2 agonist clonidine (0.1 or 0.2 mg/kg) and alpha1 antagonist prazosin (1 mg/kg) failed to affect basal cortical acetylcholine release; however, both compounds significantly reduced the increases evoked by sensory stimulation. In contrast, the alpha2 antagonist yohimbine (3 mg/kg) increased basal cortical acetylcholine release, thereby preventing meaningful investigation of its effects on tactile stimulation-evoked increases. The benzodiazepine agonist diazepam (5 mg/kg) reduced, and the GABA(A) receptor antagonist picrotoxin (2 mg/kg) increased basal cortical acetylcholine release; in addition, diazepam attenuated the increases in cortical acetylcholine release evoked by tactile stimulation. While dopaminergic D1 (SCH 23390, 0.15 mg/kg) and D2 (raclopride, 1 mg/kg) receptor antagonists did not by themselves significantly influence the increases evoked by tactile stimulation, their co-administration produced a significant reduction. The opioid receptor antagonist naltrexone (1.5 mg/kg) failed to affect either basal or tactile stimulation-evoked increases in acetylcholine overflow. Finally, the non-competitive N-methyl-D-aspartate receptor antagonist, dizocilpine maleate (MK-801; 0.025 and 0.05 mg/kg) increased basal cortical acetylcholine release. These results confirm that cortically projecting cholinergic neurons are activated by sensory stimuli, and indicate that the increases in cortical acetylcholine release produced by tactile stimulation are inhibited by stimulation of alpha2 or blockade of alpha1 noradrenergic receptors, and by enhanced GABAergic transmission. In addition, simultaneous blockade of dopamine D1 and D2 receptors appears necessary to achieve a significant reduction of sensory stimulation-evoked acetylcholine release in the frontal cortex. The results are consistent with the hypothesis that cortical acetylcholine release is a component of the neurochemistry of arousal and/or attention and indicate that this is modulated by GABAergic, noradrenergic and dopaminergic systems. In contrast, endogenous opioid actions do not appear to be involved.

Phenotypic characterization of neuroleptic-sensitive neurons in the forebrain: contrasting targets of haloperidol and clozapine.

The prototypical neuroleptic haloperidol and the atypical antipsychotic clozapine induce distinctly different patterns of c-fos expression in the forebrain. While haloperidol appears to increase c-fos expression via its D2 dopamine receptor antagonist properties, the receptor mechanisms by which clozapine produces its unique pattern of c-fos expression are not known. The present experiments sought to address this question by determining the phenotypes of neurons in which clozapine increases Fos-like immunoreactivity (FLI). Fos immunostaining combined with in situ hybridization histochemistry using a cDNA oligonucleotide probe for D3 receptor mRNA indicated that the great majority (95%) of clozapine-induced FLI neurons in the major island of Calleja (ICjM) express D3 receptors. Similarly, in the nucleus accumbens (NAc) and lateral septal nucleus (LSN), the majority of clozapine-induced FLI neurons express D3 receptor mRNA (NAc 69%; LS 73%). In marked contrast, haloperidol-induced FLI neurons failed to express D3 receptors in any brain region. Studies with oligonucleotide probes for enkephalin (ENK) and dynorphin (DYN) indicated that clozapine increases c-fos expression in both ENK and DYN containing neurons in the NAc (ENK 40%, DYN 53%) and LSN (ENK 32%, DYN 59%). Haloperidol also increases c-fos expression in ENK and DYN containing neurons, albeit in a different pattern (striatum: ENK 93%, DYN 20%; nucleus accumbens: ENK 46%, DYN 36%; lateral septum: ENK 29%, DYN 18%). The present results demonstrate that haloperidol and clozapine target different populations of neurons even in regions such as the NAc and LSN, where they both increase c-fos expression. In addition, the fact that the majority of clozapine-sensitive neurons in NAc, LSN, and ICjM express D3 receptors suggests that activity at these receptors may contribute to the unique clinical profile of this antipsychotic agent. These data indicate that D3 receptors may represent novel targets in the pharmacotherapy of schizophrenia.

Opioid receptor modulation of feeding-evoked dopamine release in the rat nucleus accumbens.

Feeding is associated with increases in the activity of the mesolimbic dopamine (DA) system which originates in the ventral tegmental area (VTA) and projects heavily to the nucleus accumbens. The present study used in vivo brain microdialysis to assess the contribution of opioid receptors in feeding-evoked DA release in the nucleus accumbens. Feeding in 18 h food-deprived rats increased DA release by about 50% above baseline. Systemic injection of the opioid receptor antagonist naltrexone (1 mg/kg, s.c.) blocked the effect of feeding on DA release and reduced the amount of food consumed. Unilateral application of naltrexone (100 microM) in the VTA via a microdialysis probe failed to affect the DA response to feeding, the amount of food consumed, or the latency to eat. In contrast, intra-VTA naltrexone significantly reduced the effect of systemic heroin (0.5 mg/kg, s.c.) on accumbal DA release. These results indicate that: (1) opioid receptor activation is a component of the neural substrates of deprivation-induced feeding: (2) opioid receptors in the VTA do not contribute significantly to feeding-associated increases in DA release in the nucleus accumbens; and (3) heroin-induced increases in accumbal DA release are mediated, at least in part, by opioid receptors in the VTA.

Dopaminergic regulation of striatal acetylcholine release: the critical role of acetylcholinesterase inhibition.

This study examined the effects of different levels of acetylcholinesterase (AChE) inhibition on dopaminergic regulation of striatal acetylcholine (ACh) release as estimated by in vivo brain microdialysis. Systemic administration of d-amphetamine (2 or 10 mg/kg) increased the striatal output of ACh when the AChE inhibitor neostigmine (0.1 microM) was present in the perfusion fluid. In contrast, when the same experiments were conducted at 0.01 microM neostigmine, d-amphetamine failed to affect (2 mg/kg) or significantly decreased (10 mg/kg) striatal ACh output. The inhibitory action of the D2 receptor agonist quinpirole (0.2 mg/kg) was significantly greater at 0.01 microM than at 0.1 microM neostigmine. Similarly, there was a nonsignificant trend for the D2 antagonist raclopride (1 mg/kg) to stimulate ACh release to a greater extent at the low neostigmine concentration. In contrast, the stimulant effects of systemic administration of the D1 agonist A-77636 (1.46 mg/kg) on striatal ACh release were the same at the two neostigmine concentrations. These results demonstrate that the concentration of an AChE inhibitor in the perfusion solution can quantitatively and even qualitatively influence the manner in which dopaminergic agents regulate ACh overflow in the striatum. On comparing the present results with earlier reports concerning the effects of d-amphetamine on tissue concentrations of ACh, it is tentatively concluded that a low neostigmine concentration is the more physiologically relevant condition. Under such conditions, at moderate doses d-amphetamine does not appear to alter striatal ACh release, with this likely being due to the opposing actions of D1 and D2 receptors. Nevertheless, until the endogenous interstitial concentrations of striatal ACh can be measured by other methods, the physiological relevance of ACh microdialysis studies in the striatum will remain uncertain.

Acute and prolonged effects of clocinnamox and methoclocinnamox on nucleus accumbens dopamine overflow.

The mu opioid antagonist clocinnamox (CCAM) insurmountably inhibits opioid self-administration. In contrast, CCAM's prodrug, methoclocinnamox (MCCAM), acts as a weak partial agonist in this paradigm when given acutely and inhibits opioid self-administration for up to 5 days. In vivo microdialysis was employed to determine if these effects are paralleled in basal and opioid-stimulated dopamine (DA) overflow in the rat nucleus accumbens (NAC). When given acutely, CCAM (10 mg/kg s.c.) was essentially without effect. CCAM also markedly attenuated the overflow of DA induced by heroin (0.5 mg/kg s.c.; 200% of DA baseline) 24 h later. In contrast, MCCAM (10 mg/kg s.c.) acutely increased NAC DA overflow to 200-245% baseline within 30 min. NAC DA remained at this elevated level for the whole 3-h period of the experiment. Even after 24 h, NAC DA overflow of MCCAM-pretreated animals remained elevated at 165% of VEH-treated animals. Administration of heroin did not result in any further elevation of NAC DA release under these conditions. Thus, the suggested therapeutic profile of MCCAM, i.e., an acute partial agonistic reinforcing effect followed by antagonism of the reinforcing effects of subsequently abused opioids, was confirmed in NAC DA overflow, a neurochemical correlate of the reinforcing effects of drugs of abuse. The most parsimonious explanation for MCCAM's effect on NAC DA overflow is that it acted as an essentially irreversible partial agonist.

Nicotine and heroin augment cocaine-induced dopamine overflow in nucleus accumbens.

The current public debate on nicotine concentrates on the abuse potential of nicotine per se. However, little is known about the interaction of nicotine with other drugs of well-established abuse liability such as cocaine. Indeed, cigarette smoking increases the intake of cocaine and other drugs of abuse. In order to test if these epidemiological data are reflected in a neurochemical correlate of the reinforcing effects of drugs of abuse, i.e., dopamine overflow in the nucleus accumbens, in vivo brain microdialysis was used to examine the effects of nicotine and cocaine either alone or in combination in freely moving rats. Furthermore, the effects of the nicotine + cocaine combination were compared to another drug combination of high abuse potential, i.e., heroin + cocaine ('speedball'). Both nicotine + cocaine as well as heroin + cocaine stimulated nucleus accumbens dopamine overflow in an additive manner. Repeated intermittent administration of nicotine did not significantly alter the effects of a subsequent challenge with the nicotine + cocaine combination. These data suggest that the clinical-epidemiological findings on either drug combination are reflected in a stimulatory interaction on nucleus accumbens dopamine overflow that is additive. No significant tolerance seems to develop to this effect of nicotine. These neurochemical findings support behavioral data suggesting that the reinforcing effects of cocaine and heroin are additive and predict that nicotine will enhance the reinforcing effects of cocaine.

The putative atypical antipsychotic drug amperozide preferentially increases c-fos expression in rat medial prefrontal cortex and lateral septum.

The effects of acute, systemic administration of the putative atypical antipsychotic drug amperozide on c-fos expression in the rat forebrain were studied by means of Fos immunohistochemistry. Amperozide significantly increased the number of Fos-immunoreactive nuclei in the medial prefrontal cortex and the lateral septum but not in the nucleus accumbens (shell or core), the striatum, or the amygdala. With the exception of the nucleus accumbens-shell, where amperozide failed to produce statistically significant increases, the regional distribution of Fos immunoreactivity following amperozide was similar to that induced by atypical, but not by typical, antipsychotic drugs. In addition, after amperozide the number of Fos-positive nuclei was higher in the nucleus accumbens than in the dorsolateral striatum, a characteristic that is common to all known atypical antipsychotic agents.

Nonstriatal dopamine D1 receptors regulate striatal acetylcholine release in vivo.

The role of dopamine (DA) D1 receptors in the regulation of acetylcholine (ACh) release in the striatum was studied using in vivo microdialysis in freely moving rats. Systemic administration of the full D1 DA receptor agonist A-77636 (4 micromol/kg) increased striatal ACh release by 53% above the base line and decreased DA release by 33%. Local application of A-77636 (10 and 100 microM) by reverse dialysis was without effect on either striatal ACh or DA release. Systemic administration of the D1 DA receptor antagonist SCH 23390 (0.74 micromol/kg) or SCH 39166 (1.42 micromol/kg) blocked the stimulation of striatal ACh release produced by systemic A-77636 (4 micromol/kg). Local perfusion of either SCH 23390 or SCH 39166 did not decrease basal ACh release. Furthermore, when applied locally via the dialysis probe, SCH 23390 (12 microM) or SCH 39166 (50 microM) failed to attenuate the stimulation of striatal ACh release produced by systemic A-77636. Similarly, d-amphetamine (5.42 micromol/kg)-induced increases in striatal ACh release were not modified by simultaneous local perfusion with SCH 39166 (50 microM). These findings are consistent with the hypothesis that D1 receptor activation stimulates ACh release in the striatum. However, because local application of D1 receptor agonists and antagonists fail to influence ACh release, the relevant D1 receptors are not located in the striatum. The use of unphysiological dialysis conditions (high concentrations of acetylcholinesterase (AChE) inhibitors, high Ca++ concentrations and an absence of Mg++ in the perfusion fluid) may account for some earlier suggestions that local D1 receptors regulate ACh release in the striatum.

Activation of the mesocortical dopamine system by feeding: lack of a selective response to stress.

There is wide agreement that catecholamine systems in the prefrontal cortex are activated by stressful stimuli. To date, however, the extent to which other stimuli can increase the activity of these systems has received little attention. In the present study, the effects of tail pinch stress and feeding on dopamine and noradrenaline release in the prefrontal cortex of rats were examined using in vivo brain microdialysis. Both stimuli increased dopamine release, with peak effects reaching 212% above baseline for tail pinch and 165% above baseline for feeding. The effects of the two stimuli on peak dopamine release were not significantly different. Both stimuli also significantly increased noradrenaline release, with peak effects reaching 128% above baseline for tail pinch and 98% above baseline for feeding. The effects of the two stimuli on peak noradrenaline release were not significantly different. These results indicate that activation of catecholaminergic afferents to the prefrontal cortex is not specific to stress, but also occurs in response to non-stressors with positive motivational valence.

Amphetamine sensitization enhances regional c-fos expression produced by conditioned fear.

Chronically administered amphetamine can result in a paranoid psychosis that can be re-induced in former amphetamine abusers by psychological stressors. In an attempt to investigate the neurobiological correlates of this phenomenon, the present study examined the effects of prior D-amphetamine sensitization on regional c-fos expression induced by a psychological stressor. Rats received intermittent footshock in a distinctive environment for 30 min/day for three days. Three days after the last fear conditioning session, the animals received injections of saline or D-amphetamine (4 mg/kg, i.p.) once every second day for 16 days (eight injections in total). After a 14-day drug abstinent period, the animals were placed in the fear conditioning apparatus but without footshock. The amphetamine sensitization procedure significantly enhanced the effects of conditioned fear on c-fos expression in several brain regions. These included the cingulate cortex area 3, agranular insular cortex (layers 2 and 3), claustrum, piriform cortex, the shell region of the nucleus accumbens, medial striatum, ventral lateral septum, and CA3 and polymorphic layer of the hippocampal formation. These results indicate that D-amphetamine sensitization can have long-lasting effects on the neural circuitries activated by conditioned stressors.

Feeding-evoked dopamine release in the nucleus, accumbens: regulation by glutamatergic mechanisms.

The extent to which glutamate receptors in the nucleus accumbens and ventral tegmental area regulate feeding-evoked increases in dopamine release in the nucleus accumbens was determined using in vivo brain microdialysis in the rat. In some animals a second dialysis probe was implanted in the ventral tegmental area ipsilateral to the nucleus accumbens probe. The feeding protocol involved access to standard rat chow after 18 h of food deprivation. Under these conditions rats began eating approximately 30 s after the introduction of food and consumed 7-8 g, resulting in a 50% increase in dopamine release. Application of the glutamate receptor antagonist kynurenate (1 mM) in the nucleus accumbens potentiated the feeding-evoked increase in dopamine release by 80%. Application of the metabotropic glutamate receptor agonist trans-1S,3R-1-amino-1,3-cyclopentanedicarboxylic acid (100 microM) in the nucleus accumbens blocked the feeding-evoked increase in dopamine release. Application of a combination of the ionotropic glutamate receptor antagonists 2-amino-5-phosphopentanoic acid (200 microM) and 6-cyano-7-nitroquinoxaline-2,3-dione (50 microM) through the dialysis probe in the ventral tegmental area reduced basal dopamine output in the nucleus accumbens by 20% and markedly attenuated (by 70%) the effect of feeding on dopamine release. None of the treatments affected the latency to eat or the volume of food consumed. These results indicate that glutamatergic afferents to the ventral tegmental area mediate feeding-induced increases in dopamine release in the nucleus accumbens. In contrast, at physiological concentrations, glutamate in the nucleus accumbens appears to decrease dopamine release via actions on ionotropic and metabotropic receptors.

Effects of chronic haloperidol on stress- and stimulation-induced increases in dopamine release: tests of the depolarization block hypothesis.

There is considerable neurophysiological evidence that chronically administered neuroleptics can, under certain circumstances, decrease the activity of mesencephalic dopaminergic neurons. This finding, referred to as depolarization inactivation or depolarization block, has led to the hypothesis that the delayed therapeutic effects of neuroleptic drugs are due to a graduate silencing of mesolimbic dopaminergic neurons. One prediction of depolarization inactivation is that dopamine neurons in this state should be resistant to activation by excitatory stimuli. As a test of this prediction, rats that had been treated chronically with either saline or haloperidol (0.5 mg/kg x 21 days) were exposed to either acute mild stress or electrical stimulation of the prelimbic region of the prefrontal cortex while extracellular levels of dopamine in the nucleus accumbens were monitored by in vivo microdialysis. A 10-minute exposure to acute stress via tail pinch increased dopamine release by 20% and 18% in the saline and haloperidol groups, respectively. Similarly, 20 minutes of cortical stimulation increased dopamine release by 51% and 56% in rats treated chronically with saline or haloperidol, respectively. These results indicate that contrary to a prediction of the depolarization block hypothesis, mesolimbic dopaminergic neurons can be activated in neuroleptic-treated animals.

Glutamate receptor agonists decrease extracellular dopamine in the rat nucleus accumbens in vivo.

Intracerebral microdialysis was used to investigate the effects of local application of L-glutamate, N-methyl-D-aspartate, and the glutamate uptake inhibitor 1-trans-pyrrolidine-2,4-dicarboxylic acid (PDC) in the nucleus accumbens (NAc) on extracellular dopamine (DA) concentrations. The effects of locally applied PDC on extracellular glutamate concentrations were also examined. Glutamate produced a concentration-dependent decrease in extracellular DA that could be blocked by concurrent, local application of the broad spectrum ionotropic glutamate receptor antagonist kynurenic acid (KYN:1 mM). N-Methyl-D-aspartate had a concentration-dependent effect on DA release, with a low concentration (0.1 mM) producing a decrease and a higher concentration (1.0 mM) resulting in an increase. Both effects were blocked by KYN. PDC (1 mM) increased extracellular glutamate concentrations to 102% above baseline. The same concentration of PDC decreased extracellular DA concentrations, and coapplication of KYN attenuated this effect. These results indicate that glutamate receptor agonists can have both facilitatory and inhibitory effects on extracellular DA concentrations. However, the effects of PDC indicate that inhibition of DA release is the more physiologically relevant effect. Furthermore, the results of these and other experiments suggest that glutamate's inhibitory effects on DA release in the NAc are not due to direct actions of this excitatory amino acid on DA terminals. A multisynaptic model that accounts for glutamate's actions on DA release is proposed.

Conditioned and unconditioned stimuli increase frontal cortical and hippocampal acetylcholine release: effects of novelty, habituation, and fear.

Recent evidence showing that basal forebrain cholinergic neurons with projections to the frontal cortex and hippocampus are activated by behaviorally salient stimuli suggests that these neurons are involved in arousal and/or attentional processes. We sought in the present experiments to test this hypothesis by examining whether unconditioned stimuli (a tone and flashing light) that normally increase cortical nad hippocampal acetylcholine (ACh) release would fail to do so after habituation (i.e., repeated presentation with no programmed consequences). In addition, the extent to which presentation of these stimuli would continue to increase ACh release when they had previously been paired with an aversive stimulus was investigated. Three experimental groups were used: habituation, novel stimuli, and conditioned fear. Subjects in each of these groups were placed in a training apparatus for twelve 200 min sessions. While the habituation group received extensive exposure to the tone and light during the training sessions, subjects in the novel stimuli group were placed in the apparatus but were never exposed to the tone or light during these sessions. The conditioned fear group was treated identically to the habituation group, with the addition that the tone and light were paired with footshock. On completion of these training schedules, all animals were implanted with microdialysis probes in the frontal cortex and hippocampus. Two days later, they were placed in the apparatus and the tone and light were presented to all subjects during microdialysis. In the novel stimuli group, the tone and light (unconditioned stimuli) produced significant increases in frontal cortical and hippocampal ACh release. Similarly, in the conditioned fear group, presentation of the tone and light (conditioned stimuli) also significantly increased ACh release in frontal cortex and hippocampus. In contrast, in the habituation group the tone and light failed to significantly enhance ACh release in either structure. During the test session, the tone and light elicited a variety of arousal- and fear-related behaviors in the novel stimuli and conditioned fear groups. In contrast, subjects in the habituation group generally failed to respond to these stimuli. These data indicate that cortically and hippocampally projecting basal forebrain cholinergic neurons are activated by conditioned and unconditioned stimuli that produce arousal in rats (novelty or conditioned fear). In contrast, presentation of these stimuli to habituated animals fails to enhance ACh release. These findings are consistent with a growing body of information indicating that ACh release in the cortex and hippocampus is reliably activated by behaviorally relevant stimuli. They also provide strong support for the hypothesis that cholinergic neurons in the basal forebrain are involved in arousal and/or attentional processes.

Chronic lithium attenuates dopamine D1-receptor mediated increases in acetylcholine release in rat frontal cortex.

The effects of chronic lithium treatment on methylphenidate-, D1 dopamine receptor agonist (A-77636)-, and tactile stimulation-induced increases in frontal cortical acetylcholine release were studied in the rat using in vivo brain microdialysis. Cortical acetylcholine release in control rats was maximally stimulated by methylphenidate (1.25 and 2.5 mg/kg) to 173% and 212% above baseline, respectively. The effect of methylphenidate (2.5 mg/kg) was blocked by pretreatment with the dopamine D1 receptor antagonist SCH 23390 (0.3 mg/kg). Chronic treatment with lithium chloride (3-4 weeks) produced plasma lithium concentrations of 0.45 +/- 0.02 meq/l. Chronic lithium significantly reduced increases in cortical acetylcholine release produced by methylphenidate. Stimulation of dopamine D1 receptors with the full D1 receptor agonist A-77636 (0.73 mg/kg) increased cortical acetylcholine release. Chronic lithium significantly reduced this effect of A-77636. In contrast, lithium failed to influence the increases of cortical acetylcholine release produced by tactile stimulation. These results suggest that while lithium does not influence normal, arousal-related increase in cortical acetylcholine release, this ion selectively attenuates dopamine mediated increases and/or abnormally large increases, which in the present circumstances were pharmacologically induced. The relevance of these findings to the antimanic actions of lithium is discussed.

Effects of olanzapine on regional C-Fos expression in rat forebrain.

Compared to typical antipsychotic drugs, clozapine produces a unique pattern of Fos-like immunoreactive neurons in the rat forebrain. It has been proposed, therefore, that this approach may be useful in identifying other agents with clozapine's therapeutic profile. In the present study, we examined the ability of olanzapine to increase the number of Fos-like immunoreactive neurons in the striatum, nucleus accumbens, lateral septal nucleus, and prefrontal cortex. Olanzapine (5, 10 mg/kg) produced dose-dependent increases in the number of Fos-positive neurons in the nucleus accumbens and lateral septal nucleus, important components of the limbic system that may mediate some of the therapeutic actions of neuroleptics. Olanzapine also produced dose-dependent increases in the number of Fos-positive neurons in the dorsolateral striatum, an effect that correlates with the ability of neuroleptics to produce extrapyramidal side-effects. The effects of olanzapine on regional c-fos expression are not therefore identical to clozapine, which is without effect in the dorsolateral striatum. However, olanzapine-induced increases in the dorsolateral striatum were considerably smaller than those generated in the nucleus accumbens suggesting that at low, potentially therapeutic doses olanzapine may not generate significant extrapyramidal side effects. Olanzapine also increased the number of Fos-positive neurons in medical prefrontal cortex, an action unique to clozapine and a few other atypical antipsychotics. These findings are consistent with the hypothesis that olanzapine is an atypical antipsychotic in the sense that it does not produce significant extrapyramidal side-effects at low therapeutic doses. However, extrapyramidal side-effects at higher doses can be predicted by these results. Finally, olanzapine's actions in the medial prefrontal cortex may be predictive of a clozapine-like profile with respect to actions on negative symptoms in schizophrenia. Additional clinical experience with olanzapine and other new antipsychotics is required to test the validity of these hypotheses.

Serotonergic regulation of acetylcholine release in rat frontal cortex.

The extent to which serotonin regulates the activity of cortically projecting cholinergic neurons was studied using in vivo microdialysis to monitor interstitial concentrations of acetylcholine in the frontal cortex of freely moving rats. Systemic administration of the serotonin release-inducing agent fenfluramine (3 or 10 mg/kg, i.p.) increased acetylcholine release by 110-130%. The fenfluramine-induced increase in acetylcholine release was significantly attenuated by pretreatment with the selective serotonin uptake inhibitor fluoxetine (10 mg/kg, i.p.). Pretreatment with the selective dopamine D1 receptor antagonist SCH-23390 (0.3 mg/kg, s.c.) failed to prevent the fenfluramine-induced increase in acetylcholine release. In contrast, the serotonin 5-HT2A receptor antagonist ketanserin (5 mg/kg, i.p.) blocked fenfluramine-induced increases in acetylcholine release. In contrast to previous studies that have concluded that serotonin has inhibitory actions on cortical acetylcholine release, the present results indicate that fenfluramine increases cortical acetylcholine release in vivo by its ability to enhance serotonin transmission and that serotonin produces these effects at least in part via actions at serotonin 5-HT2A receptors.

Cortical regulation of subcortical dopamine release: mediation via the ventral tegmental area.

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 microM) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 50 microM) to the VTA via reverse dialysis decreased extracellular concentrations of dopamine in the nucleus accumbens by approximately 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.