Lumateperone-mediated effects on prefrontal glutamatergic receptor-mediated neurotransmission: A dopamine D1 receptor dependent mechanism.

Lumateperone is a novel drug approved for the treatment of schizophrenia in adults and depressive episodes associated with bipolar depression in adults, as monotherapy and as adjunctive therapy with lithium or valproate treatment in the United States. Lumateperone simultaneously modulates key neurotransmitters, such as serotonin, dopamine, and glutamate, implicated in serious mental illness. In patients with schizophrenia, lumateperone was shown to improve positive symptoms along with negative and depressive symptoms, while also enhancing prosocial behavior. Moreover, in patients with bipolar I or II disorder, lumateperone improved depressive symptoms as well. To further understand the mechanisms related to lumateperone's clinical response, the aim of this study was to investigate the effect of lumateperone on dopaminergic- and glutamatergic signaling in the rat medial prefrontal cortex (mPFC). We used the conditioned avoidance response (CAR) test to determine the antipsychotic-like effect of lumateperone, electrophysiology in vitro to study lumateperone's effects on NMDA- and AMPA-induced currents in the mPFC, and the neurochemical techniques microdialysis and amperometry to measure dopamine- and glutamate release in the rat mPFC. Our results demonstrate that lumateperone; i) significantly suppressed CAR in rats, indicating an antipsychotic-like effect, ii) facilitated NMDA and AMPA receptor-mediated currents in the mPFC, in a dopamine D1-dependent manner, and iii) significantly increased dopamine and glutamate release in the rat mPFC. To the extent that these findings can be translated to humans, the ability of lumateperone to activate these pathways may contribute to its demonstrated effectiveness in safely improving symptoms related to neuropsychiatric disorder including mood alterations.

EPA guidance on tobacco dependence and strategies for smoking cessation in people with mental illness.

Tobacco dependence is the most common substance use disorder in adults with mental illness. The prevalence rates for tobacco dependence are two to four times higher in these patients than in the general population. Smoking has a strong, negative influence on the life expectancy and quality of life of mental health patients, and remains the leading preventable cause of death in this group. Despite these statistics, in some countries smokers with mental illness are disadvantaged in receiving intervention and support for their tobacco dependence, which is often overlooked or even tolerated. This statement from the European Psychiatric Association (EPA) systematically reviews the current evidence on tobacco dependence and withdrawal in patients with mental illness and their treatment. It provides seven recommendations for the core components of diagnostics and treatment in this patient group. These recommendations concern: (1) the recording process, (2) the timing of the intervention, (3) counselling specificities, (4) proposed treatments, (5) frequency of contact after stopping, (6) follow-up visits and (7) relapse prevention. They aim to help clinicians improve the care, health and well-being of patients suffering from mental illness.

Nitric oxide is involved in nicotine-induced burst firing of rat ventral tegmental area dopamine neurons.

In the present study, using single cell recordings in vivo and intracellular recordings in vitro from midbrain slices, the role of N-methyl-d-aspartate (NMDA) receptor signaling on firing activity in ventral tegmental area dopamine neurons elicited by nicotine was investigated in the rat. In accordance with previous studies, systemic nicotine (0.5 mg/kg s.c.) increased both firing rate and burst firing of dopamine neurons in vivo, and bath-applied nicotine (10 microM) increased firing rate in vitro. The competitive NMDA receptor antagonist CGP39551 (2.5 mg/kg i.p.) inhibited nicotine's effects on burst firing and also attenuated the nicotine-induced increase in firing rate. Moreover, although the nitric oxide (NO)-synthase inhibitor N-nitro-l-arginine-methyl-ester (l-NAME; 5.0 mg/kg i.p.) had no effect on cell firing by itself, it prevented the response to nicotine in vivo. In contrast, l-NAME (100 microM) did not influence nicotine's effect on dopamine cell firing in vitro, suggesting that the effect of l-NAME seen in vivo is dependent on presynaptic afferent input. The present study confirms previous results suggesting that the effect of systemically administered nicotine is in part presynaptic and mediated via NMDA receptors. The data also indicate that NO plays an important role in the previously demonstrated, indirect, glutamate-mediated excitation of these neurons by nicotine. By inference, our results provide additional support for the involvement of NO in nicotine dependence.

Selective noradrenaline reuptake inhibition enhances serotonergic neuronal activity and transmitter release in the rat forebrain.

Present pharmacotherapy of major depression is, in principle, based on enhancement of central monoaminergic neurotransmission. Clinical studies utilizing depletion experiments indicate that antidepressants which primarily enhance serotonergic or noradrenergic central activity, i.e. serotonin or nor-adrenaline reuptake inhibitors, largely work by two separate neuronal pathways. However, experimental studies have shown that noradrenaline may regulate serotonergic neurotransmission both at the serotonin cell body and nerve-terminal level. We therefore investigated the effects of the selective NRI reboxetine on serotonergic neuronal activity and extracellular levels of transmitter in the nerve-terminal area. In vivo electrophysiological experiments showed that low doses of reboxetine significantly enhance the firing rate of serotonergic neurons in the dorsal raphe nucleus of anaesthetized rats. Also, in the medial prefrontal cortex reboxetine (3 mg/kg s.c.) enhanced, whereas citalopram (3 mg/kg s.c.) reduced, extracellular concentrations of serotonin measured by means of microdialysis in awake rats, using a low dose of citalopram (0,5 micro M) in the perfusion solution. Local administration of reboxetine only induced an increase in cortical serotonin levels at very high concentrations (1000 micro M). Hence, NRIs may cause a secondary enhancement of central serotonergic activity by a mechanism separate from 5-HT reuptake inhibition; an effect that may contribute to their clinical antidepressant efficacy.

Biochemical effects in brain of low doses of haloperidol are qualitatively similar to those of high doses.

Typical and atypical antipsychotic drugs (APD) show differential effects in brain on both dopamine output and activation of Fos-like immunoreactivity (FLI) in dopamine nerve terminal regions. Typical APD increase dopamine output preferentially in the core and atypical APD increase dopamine output preferentially in the shell of the nucleus accumbens (NAC). Whereas both typical and atypical APD increase FLI in NAC, typicals cause FLI activation in the striatum and atypicals cause FLI activation in the prefrontal cortex. Clinically, low doses of haloperidol cause less side-effects than higher doses, and low-dose haloperidol has been suggested as a cost-effective alternative to atypical APD. Here, in vivo voltammetry in anaesthetised, pargyline-pretreated rats was used to measure dopamine output in the two subdivisions of the NAC and, in addition, immunohistochemistry was used to assess FLI activation in dopamine target areas, following acute haloperidol administration. Haloperidol, 0.001 and 0.01 mg/kg i.v., caused a significantly higher dopamine output in the core than in the shell of the NAC. Moreover, haloperidol 0.05 and 0.5, but not 0.005, mg/kg s.c. increased FLI in the NAC and the striatum, but not in the medial prefrontal cortex. Thus, even extremely low doses of haloperidol generate, in principle, the same biochemical effects in brain as higher doses, and these effects remain different from those of atypical APD. These biological data indicate that clinical differences between haloperidol and atypicals are qualitative rather than dose-dependent. Consequently, our results do not support the use of low-dose haloperidol as replacement for atypical APD in the treatment of schizophrenia.

Active immunization against nicotine suppresses nicotine-induced dopamine release in the rat nucleus accumbens shell.

BACKGROUND: Tobacco smoking is the largest preventable cause of morbidity and premature mortality in the world. Although its medical consequences are well documented, 20-50% of the population even in developed countries remain tobacco smokers. The drugs presently used in smoking cessation have limited efficiency and, therefore, there is a need for alternative and improved treatments. One novel approach in this regard may be provided by immunization against nicotine. OBJECTIVE: The present study in male Wistar rats investigated if active immunization with a novel nicotine immunogen, IP18-KLH, may generate nicotine-selective antibodies and, furthermore, whether this treatment might prevent nicotine from exerting its stimulating effect on the mesolimbic, dopaminergic reward system in the brain. METHODS: Enzyme-linked immunosorbent assay (ELISA) was used to determine the titer of nicotine antibodies in plasma after immunization with IP18-KLH in Freund's adjuvant. Competitive ELISA was used to assess the selectivity of the antibodies. Finally, we used in vivo voltammetry to investigate whether active immunization with IP18-KLH could prevent nicotine-induced dopamine release in the shell of nucleus accumbens (NAC(shell)). RESULTS: The present study shows that active immunization with IP18-KLH generates antibodies that are highly selective for nicotine. Furthermore, immunization with IP18-KLH prevented the nicotine-induced increase in dopamine release in the NAC(shell), a biochemical correlate to the rewarding properties of nicotine. CONCLUSIONS: Active immunization with IP18-KLH prevents a central effect of nicotine that is considered critical for the induction of nicotine dependence. Consequently, active immunization may provide long-term protection against initiation of tobacco dependence, an effect that may prove particularly advantageous in relapse prevention.

Active immunization against nicotine prevents reinstatement of nicotine-seeking behavior in rats.

BACKGROUND: The presently available pharmaceutical aids in smoking cessation possess a rather limited effectiveness. Therefore, we have synthesized a series of immunoconjugates that stimulate the induction of antibodies which may bind nicotine in the blood, thereby preventing it from passing the blood-brain barrier. Thus, the reinforcing action of nicotine in the brain, which is the driving force in tobacco smoking, should be abolished. OBJECTIVE: The present study was undertaken to test this notion in a long-term relapse model in rats, measuring the reinstatement of nicotine-seeking behavior, following active immunization with IP18-KLH, one of our immunoconjugates. METHODS: Male Wistar rats were immunized with a nicotine-KLH conjugate (nicotine immunogen) and Freund's adjuvant after having been trained to meet the criteria of stable nicotine self-administration on a fixed ratio (FR3) schedule. The rats were subsequently extinguished from nicotine self-administration behavior and finally, as extinction was completed, they were exposed to small, priming doses of nicotine, which previously have been shown to reinstate the nicotine-seeking behavior. The antibody titers were measured by ELISA. RESULTS: It was found that rats with high titers (>1:10,000) of antibodies against nicotine, in contrast to those with low/no nicotine selective antibodies, do not reinstate nicotine self-administration behavior when they are exposed to nicotine. CONCLUSIONS: Our findings indicate that active immunization against nicotine may effectively abolish the reinforcing action of nicotine in brain, an effect which is critical for relapse in nicotine dependence. These data suggest the potential utility of active immunization in smoking cessation programs.

Reboxetine modulates the firing pattern of dopamine cells in the ventral tegmental area and selectively increases dopamine availability in the prefrontal cortex.

Central dopaminergic neurons have been suggested to be involved in the pathophysiology of several psychiatric disorders, including depression, and appear to be modulated by noradrenergic activity both at the nerve terminal level and at the somatodendritic level. In recent years reboxetine, a selective noradrenaline reuptake inhibitor that differs from tricyclic antidepressants by its low affinity for muscarinic, cholinergic and alpha(1)-adrenergic receptors, has been introduced clinically. In the present study the effect of reboxetine on the function of the mesolimbocortical dopamine system was investigated by means of single cell recording and microdialysis in rats following administration of reboxetine in doses that appear to yield clinically relevant plasma concentrations. Reboxetine (0.625--20 mg/kg intravenously) induced an increase in burst firing, but not in average firing frequency of dopamine (DA) cells in the ventral tegmental area (VTA). Moreover, reboxetine (0.15--13.5 mg/kg intraperitoneally) caused a significantly enhanced DA output in the medial prefrontal cortex, whereas no effect was observed in the nucleus accumbens. Local administration of reboxetine (333 microM, 60 min), by means of reversed microdialysis into these brain regions, caused a significant increase in DA output in both brain regions. However, local administration of reboxetine into the VTA (333 microM, 60 min) did not affect DA availability in these terminal areas. Our results imply that clinical treatment with reboxetine may result in facilitation of both prefrontal DA output and the excitability of VTA DA neurons, effects that may contribute to its antidepressant action, especially on drive and motivation.

Effects of competitive and non-competitive NMDA receptor antagonists on dopamine output in the shell and core subdivisions of the nucleus accumbens.

The effects of acute intravenous administration of the non-competitive NMDA receptor antagonists, phencyclidine (PCP), dizocilpine (MK-801; (+)-5-methyl-10,11-dihydroxy-5H-dibenzo(a,b)cyclohepten-5,10-imine), and the competitive NMDA receptor antagonist CGP 39551 (DL-(E)-2-amino-4-methyl-5-phosphono-3-pentanoic acid) on extracellular dopamine concentrations were analyzed in the shell and core subdivisions of the nucleus accumbens (NAC), associated with limbic and motor functions, respectively. Extracellular dopamine concentrations were assessed utilizing differential normal pulse voltammetry in chloral hydrate anesthetized, pargyline pretreated rats. Intravenous administration of PCP (0.5 mg/kg) or MK-801 (0.1 mg/kg) both significantly elevated extracellular dopamine levels in the NAC shell but not in the core. However, administration of relatively low doses of the competitive NMDA receptor antagonist CGP 39551 (2.5 mg/kg) failed to affect dopamine output in either region. However, when a higher dose (10 mg/kg) was administered a significant elevation in dopamine output was obtained in the shell compared to the core. Our data demonstrate that non-competitive NMDA receptor antagonists evoke an accumbal dopamine output that is selective to limbic cortical related NAC regions. This profile is shared also by competitive NMDA receptor antagonists when given in high, but not low doses. Our results are compatible with the reported elicitation of PCP-like behavioral effects by competitive NMDA receptor antagonists when administered in relatively high doses. Moreover, these findings suggest that differences in the regional accumbal dopamine output between competitive and non-competitive NMDA receptor antagonists may be essentially attributable to the relative degree of NMDA receptor antagonism achieved by the drugs. This experimental model may afford a biochemical means to assess the psychotomimetic liability of NMDA receptor antagonists, a side effect that may reduce their usefulness as neuroprotective agents.

Receptor-mediated regulation of serotonin output in the rat dorsal raphe nucleus: effects of risperidone.

OBJECTIVES: The present study was undertaken to characterize the regulation of serotonin (5-HT) efflux and neuronal activity in the dorsal raphe nucleus (DRN) as well as to examine the potential ability of the antipsychotic drug risperidone to interfere with these mechanisms. METHODS AND RESULTS: By using microdialysis in freely moving rats, it was found that administration of the alpha2 adrenoceptor antagonist idazoxan (0.25 mg/kg, SC), the 5-HT1B/D receptor antagonist GR 127,935 (1.0 mg/kg, SC) and risperidone (0.6 or 2.0 mg/kg, SC) increased 5-HT output in the DRN. Local DRN perfusion with GR 127,935 or risperidone via reversed dialysis (100 or 10-100 microM, respectively) enhanced 5-HT efflux in this area, whereas idazoxan (10-100 microM) failed to affect this parameter. Both systemic administration and reversed DRN dialysis of the D2/3 and 5-HT2A receptor antagonists raclopride (2.0 mg/kg, SC or 10-100 microM) and MDL 100,907 (1.0 mg/kg, SC or 10-100 microM), respectively, were without effect. Intraraphe dialysis of the 5-HT1B/D receptor agonist CP 135,807 (0.2 microM) decreased the efflux of 5-HT in the DRN, an effect which was antagonized by co-administration of either GR 127,935 or risperidone (10 and 3.3 microM, respectively). By using single-cell recording, it was found that administration of GR 127,935 (50-400 microg/kg, IV) decreased, whereas CP 135,807 (2.5-20 microg/kg, IV) increased firing of 5-HT cells in the DRN. CONCLUSIONS: Our findings suggest a regulatory role of local 5-HT1B/D receptors on 5-HT efflux as well as cell firing in the DRN and indicate that risperidone may interfere with the regulation of 5-HT availability in this area primarily via blockade of 5-HT1D receptors.

Catalepsy induced by the 5-HT(1A) receptor antagonist WAY 100635 in rats pretreated with the selective serotonin reuptake inhibitor citalopram.

Neither a high dose of the selective serotonin reuptake inhibitor citalopram (100 micromol kg(-1) s.c.), nor the 5-HT(1A) receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohexane carboxamide 3HCl (WAY 100635) (0.1--0.4 micromol kg(-1) s.c.) produced any evidence of catalepsy in adult male rats. When combined with citalopram, however, WAY 100635 produced a dose-dependent, and statistically significant, catalepsy in the inclined grid test.

Enhancement of antipsychotic-like effects by combined treatment with the alpha1-adrenoceptor antagonist prazosin and the dopamine D2 receptor antagonist raclopride in rats.

Blockade of central alpha1-adrenoceptors has been implicated as a possible factor contributing to the atypical antipsychotic profile of clozapine. Thus, in the present study we examined the effects of concomitant alpha1-adrenoceptor and dopamine D2 receptor blockade on conditioned avoidance response performance, as an index of antipsychotic-like activity, and on the induction of catalepsy, as a test for extrapyramidal side effect liability, in rats. It was found that pretreatment with the alpha1-adrenoceptor antagonist prazosin (0.2mg kg(-1) s.c.) caused an enhancement of a suppression of conditioned avoidance response in the presence of the dopamine D2 receptor antagonist raclopride (0.05-0.20 mg kg(-1) s.c.). The effect was most prominent at a subthreshold dose of raclopride (0.05 mg kg(-1)). At these doses, prazosin or raclopride by themselves, or in combination, did not produce catalepsy. In addition, pretreatment with prazosin (0.2mgkg(-1) s.c.) did not alter the catalepsy produced by a higher dose of raclopride (1.0 mg kg(-1) s.c.). It is suggested that, in the presence of low dopamine D2 receptor occupancy, additional alpha1-adrenoceptor blockade might improve antipsychotic efficacy, and thereby improve the therapeutic window with regard to parkinsonism.

Effects of acute and chronic electroconvulsive stimuli on cAMP and cGMP efflux in the rat striatum and hippocampus.

The effects of acute and chronic electroconvulsive stimuli (ECS) on extracellular concentrations of the cyclic nucleotides, cAMP and cGMP, from the striatum and hippocampus of awake rats were studied with in vivo microdialysis in conjunction with radioimmunoassay. Acute ECS, but not acute sham-ECS, significantly increased cAMP and cGMP efflux from the striatum by about 75 and 50%, respectively. Chronic ECS did not influence significantly basal efflux of cAMP or cGMP from the striatum or the hippocampus in comparison to control animals receiving chronically sham-ECS. Administration of a challenge ECS in animals treated chronically with sham-ECS resulted in an increase in cAMP and cGMP concentrations in the striatum by 20%, but it failed to affect significantly efflux of these nucleotides in animals treated chronically with ECS. Similarly, in the hippocampus, administration of a challenge ECS in animals treated chronically with sham-ECS resulted in an increase in cAMP and cGMP concentrations by about 40 and 65%, respectively, whereas it failed to affect significantly efflux of these nucleotides in animals treated chronically with ECS. Thus, acutely administered ECS increases cAMP and cGMP efflux in the striatum and hippocampus of rats, an effect that is greatly diminished in animals chronically receiving ECS. These findings suggest changes in the cAMP and cGMP signal transduction mechanisms in response to acute and chronic ECS that may be related to the therapeutic effects of this antidepressant and antipsychotic treatment.

Atypical antipsychotic-like effect of AMPA receptor antagonists in the rat.

Systemic administration of two chemically different AMPA receptor antagonists, GYKI52466, 20mg/kg, and LY326325, 18mg/kg, given subcutaneously, caused a selective suppression of conditioned avoidance response in the rat with preservation of escape behavior. The number of intertrial crosses was not affected and no catalepsy was observed. These experimental results indicate, in principle, an antipsychotic effect of AMPA receptor antagonists with a low liability for extrapyramidal side effects and, consequently, a pharmacological profile consonant with atypical antipsychotic drugs.

Intraaccumbal mecamylamine infusion does not affect dopamine output in the nucleus accumbens of chronically nicotine-treated rats.

Recently we have shown that the nicotinic receptor (nAChR) antagonist mecamylamine both when administered systemically and locally into the ventral tegmental area (VTA) to chronically nicotine-treated rats reduces dopamine (DA) output in the nucleus accumbens (NAC) and elicits behavioral withdrawal signs. However, the putative contributory role of nAChRs in the NAC in mediating these effects of systemic mecamylamine has not been clarified. Therefore, we here investigated the effect on extracellular levels of DA in the NAC of local intraaccumbal administration of mecamylamine to chronically nicotine-treated rats and its putative behavioral correlates. In these experiments local application of mecamylamine into the NAC, in a concentration that increased NAC DA levels in control rats, did not affect DA output or behavior in the nicotine-treated animals. These results provide further support for the contention that nAChRs in the VTA, but not in the NAC, are of major importance for the mesolimbic DA reduction and associated behavioral signs in nicotine withdrawal.

Putative role of presynaptic alpha7* nicotinic receptors in nicotine stimulated increases of extracellular levels of glutamate and aspartate in the ventral tegmental area.

We have previously provided evidence that the stimulatory action of systemic nicotine on dopamine release in the rat nucleus accumbens is initiated in the ventral tegmental area (VTA), and that it appears to be mediated partly through an indirect, presynaptic mechanism. Thus, it was found that blockade of N-methyl-D-aspartate (NMDA) receptors in the VTA attenuates the enhancing effect of nicotine on extracellular levels of dopamine in the nucleus accumbens. Moreover, the nicotine-induced dopamine output in the nucleus accumbens was found to be blocked by pretreatment with methyllycaconitine (MLA) in the VTA, indicating a role for alpha7* nicotinic acetylcholine receptors (nAChRs) in this mechanism. Thus, nicotine may exert its effects in the VTA through stimulation of alpha7* nAChRs localized on excitatory amino acid (EAA)ergic afferents. To test this hypothesis, we here measured extracellular concentrations of glutamate and aspartate in the VTA in response to systemic nicotine, with or without concurrent infusion of MLA in the VTA, using microdialysis in anaesthetized rats. Since the medial prefrontal cortex is an important source of EAA input to the VTA, we also assessed the density of alpha-bungarotoxin binding sites in the VTA in rats lesioned bilaterally in the prefrontal cortex with ibotenic acid and in sham-lesioned rats by means of quantitative autoradiography. Nicotine (0.5 mg/kg, s.c.) significantly increased extracellular levels of both aspartate and glutamate in the VTA. MLA (0.3 mM) infused locally in the VTA prevented the nicotine-induced increase in glutamate and aspartate levels. Ibotenic acid lesions of the prefrontal cortex decreased the density of alpha-bungarotoxin binding sites in the VTA by about 30%. These data indicate that nicotine increases the extracellular levels of excitatory amino acids in the VTA through stimulation of nAChRs in the VTA and that part of the alpha7* nAChR population in the VTA is localized on neurons originating in the prefrontal cortex.

Effects of antipsychotic drugs on cholecystokinin and preprotachykinin (substance P) mRNA expression in the rat hippocampal formation.

To assess the involvement of substance P (SP) and cholecystokinin (CCK) in the effects of antipsychotic drugs, preprotachykinin-A (PPT-A) and CCK mRNA expression was studied in the hippocampal formation using in situ hybridisation following 21 daily i.p. injections with the typical antipsychotic drug haloperidol (1 mg/kg) and the atypical drug clozapine (15 mg/kg). PPT-A mRNA levels were increased in the hippocampal CA3 subregion and in the entorhinal cortex after haloperidol, whereas a decrease was observed in the CA1 after clozapine. CCK mRNA levels increased in the CA1, the entorhinal cortex and in hilus, following both haloperidol and clozapine. It is suggested that earlier findings of increased SP levels in the hippocampal formation of schizophrenics may be a consequence of haloperidol treatment and that reduced hippocampal CCK and CCK mRNA levels found earlier in schizophrenics do not result from antipsychotic drug treatment. These results are consonant to the hypothesis that increased cortical CCK transmission may be beneficial in the treatment of psychosis.

Nicotine-induced excitation of midbrain dopamine neurons in vitro involves ionotropic glutamate receptor activation.

Presynaptic nicotinic acetylcholine receptors (nAChR) on glutamatergic as well as GABAergic synaptic terminals are considered to play a major role in mediating nicotinic effects on neurons in many parts of the brain. However, to what extent the excitatory effect of nicotine on the dopamine (DA) neurons in the ventral tegmental area (VTA) is mediated via their glutamatergic input remains unclear. The excitatory effect of nicotine on these cells was therefore studied by means of intracellular recordings from a midbrain slice preparation in the presence of antagonists to NMDA and non-NMDA receptors and compared to the effect of nicotine alone. Our results show that the excitatory effect of nicotine is markedly reduced both in the presence of 2-amino-5-phosphonopentanoic acid (AP5) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), i.e., from 115 +/- 14.3% to 63.4 +/- 11.0% and 63.2 +/- 13.6%, respectively. The coapplication of both antagonists did not have an additional effect in reducing the nicotine-induced increase in firing frequency. These findings clearly indicate that ionotropic glutamate receptor activation partly, but not entirely, mediates the excitatory effect of nicotine on DA neurons in VTA. In addition, we have pharmacologically characterized the nicotinic effect by the use of different nAChR antagonists, i.e., dihydro-beta-erythroidine (DHBE), mecamylamine, and methyllycaconitine (MLA). DHBE and mecamylamine but not MLA completely blocked the effect of nicotine, indicating that nAChRs other than alpha(7)-subtype are involved in the nicotine-induced excitation of the dopamine neurons in the brain slice preparation.

Role of alpha7 nicotinic receptors in nicotine dependence and implications for psychiatric illness.

It has previously been shown that the reinforcing and dependence-producing properties of nicotine depend to a great extent on activation of nicotinic receptors within the ventral tegmental area (VTA), i.e. the site of origin of the mesolimbocortical dopaminergic projection. Based on the data reviewed in the present study, it is suggested that nicotine by stimulating presynaptic alpha7 nicotinic receptors within the VTA, that are probably localized on glutamatergic afferents from the medial prefrontal cortex, produces sequentially an increase in glutamate concentrations, stimulation of NMDA receptors found on dopamine (DA)-containing neurons in the VTA, enhanced firing activity of VTA-DA neurons, augmented DA release in the nerve terminal regions, and enhanced c-fos expression in the dopaminergic projection areas through activation of D1-DA receptors. In addition, it appears that alpha7 nicotinic receptors within the VTA are directly involved in nicotine-related reward and withdrawal responses. These data may be instrumental in understanding how nicotine interacts with the mesolimbocortical dopaminergic system, which is perhaps the most important component of the neural mechanisms underlying nicotine dependence. These results may also contribute to unraveling the cellular basis of nicotine's association with neuropsychiatric disorders, thereby offering the prospect of new therapeutic advances for their treatment.

Partial 5-HT1A receptor agonist properties of (-)pindolol in combination with citalopram on serotonergic dorsal raphe cell firing in vivo.

RATIONALE: Pindolol has been reported to shorten the onset of antidepressant action of selective serotonin (5-hydroxytryptamine; 5-HT) reuptake inhibitors (SSRIs) as well as to increase their efficacy. It has been postulated that pindolol enhances the antidepressant effect of SSRIs by blocking somatodendritic 5-HT1A autoreceptors, thus antagonizing the SSRI-induced feedback inhibition of midbrain 5-HT cell firing. A recent study, however, found that pindolol suppresses the firing rate of central 5-HT cells, suggesting that the compound possesses agonistic activity at 5-HT1A autoreceptors. OBJECTIVES: The purpose of the present study was to investigate if acute administration of (-)pindolol antagonizes the decrease in firing rate of dorsal raphe 5-HT cells produced by acute treatment with the SSRI citalopram. METHODS: Extracellular recordings of single 5-HT neurons in the dorsal raphe nucleus in anaesthetized rats. RESULTS: Administration of 0.25 or 3.0 mg/kg (IV) (-)pindolol alone decreased the firing rate of a majority of the 5-HT cells studied, an effect that was reversed by the 5-HT1A receptor antagonist WAY 100635 (0.1 mg/kg, IV). Neither 0.25 nor 3.0 mg/kg (-)pindolol reversed the citalopram (0.1-0.2 mg/kg. IV)-induced suppression of 5-HT cell activity, but produced a further decrease in firing rate. In contrast, WAY100635 (0.1 mg/kg) completely reversed this effect of citalopram. Yet, pretreatment with 3.0, but not 0.25 mg/kg (-)pindolol significantly attenuated the acute inhibitory effect of citalopram on serotonergic cell firing. CONCLUSIONS: The present findings support the previous notion that (-)pindolol possesses prominent agonistic activity at somatodendritic 5-HT1A autoreceptors, but also indicate that it may possess a weak antagonistic action at these receptors.