Chronic nicotine enhances basal and nicotine-induced Fos immunoreactivity preferentially in the medial prefrontal cortex of the rat

In the present study, expression of the immediate early gene protein products Fos and Jun-B within the dorsolateral striatum, the core and shell of the nucleus accumbens (NAC), the medial prefrontal cortex (mPFC), and the ventrolateral orbital cortex was examined. Rats were injected s.c. with either saline or nicotine (0.5 mg/kg) once daily for 12 days. On day 13, animals received a challenge injection of either saline or nicotine (0.5 or 1.0 mg/kg, s.c.) and 2 h later their brains were examined for Fos-like (FLI) and Jun-B-like (JLI) immunoreactivity. Chronic nicotine significantly increased basal expression of FLI selectively in the mPFC. Nicotine challenge significantly increased FLI in the mPFC of saline-treated animals and even further increased FLI in the mPFC of nicotine-treated animals. In the shell of the NAC, nicotine challenge also increased FLI in nicotine-treated animals, whereas it increased JLI only in saline-treated animals. After chronic nicotine treatment, injection of D1 receptor antagonist SCH 23390 (0.1 mg/kg, i.p.) 10 min before a nicotine challenge (0.5 mg/kg, s.c.), significantly attenuated the nicotine-induced FLI in the mPFC and the shell of the NAC. These results suggest that the regionally selective effect of nicotine challenge on FLI is due to enhanced dopaminergic transmission, mediated via stimulation of D1 receptors.

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.

Risperidone dose-dependently increases extracellular concentrations of serotonin in the rat frontal cortex: role of alpha 2-adrenoceptor antagonism

We have previously shown that risperidone, an antipsychotic drug with high affinity for 5-hydroxytryptamine (5-HT)2A and dopamine (DA)2 receptors, as well as for alpha 2- and alpha 2-adrenoceptors, enhances 5-HT metabolism selectively in the rat frontal cortex (FC). To further study the influence of risperidone on central 5-HT systems, we compared its effects on dialysate 5-HT in the FC, as assessed by microdialysis, with those obtained with other antipsychotic drugs, i.e., clozapine, haloperidol, and amperozide, as well as with the selective alpha 2- or 5-HT2A receptor antagonists idazoxan or MDL 100,907, respectively. The underlying mechanism for risperidone's effect on 5-HT output in the FC was also investigated using single-cell recording in the dorsal raphe nucleus (DRN). Administration of risperidone (0.2, 0.6, and 2.0 mg/kg, SC) dose-dependently increased 5-HT levels in the FC. This stimulatory action was mimicked by amperozide (10 mg/kg, SC) and, to some extent, by idazoxan (0.25 mg/kg, SC). In contrast, clozapine (10 mg/kg, SC), haloperidol (2.0 mg/kg, SC), and MDL 100,907 (1.0 mg/kg, SC) exerted only minor effects on 5-HT output in brain. Local administration of risperidone or idazoxan (1.0-1000 mumol/L) in the FC dose-dependently increased dialysate levels of 5-HT in this region. On the other hand, risperidone 25-800 micrograms/kg, IV) dose-dependently decreased the firing rate of 5-HT cells in the DRN, an effect that was largely antagonized by pretreatment with the selective 5-HT1A receptor antagonist WAY 100,635 (5.0 micrograms/kg, IV). These results indicate that the risperidone-increased 5-HT output in the FC may be related to its alpha 2-adrenoceptor antagonistic action, a property shared with both amperozide and idazoxan, and that this action probably is executed at the nerve terminal level. The inhibition of 5-HT cell firing by risperidone is probably secondary to increased 5-HT availability, e.g., in the DRN, since it could be antagonized by a 5-HT1A receptor antagonist. The enhanced 5-HT output in the FC by risperidone may be of particular relevance for the treatment of schizophrenia when associated with depression and in schizoaffective disorder.

Effect of chronic antipsychotic drug treatment on preprosomatostatin and preprotachykinin A mRNA levels in the medial prefrontal cortex, the nucleus accumbens and the caudate putamen of the rat

In situ hybridization histochemistry was used to study the expression of preprosomatostatin (PPSOM) and preprotachykinin A (PPT-A) mRNA in the medial prefrontal cortex (mPFC), the nucleus accumbens (NAC) and the caudate putamen (CP) of the rat after chronic (21 days) treatment with the classical antipsychotic drug haloperidol (1 mg/kg i.p.), the atypical antipsychotic drugs clozapine (15 mg/kg i.p.) and amperozide (5 mg/kg i.p.), and the selective dopamine (DA)-D2/D3 receptor antagonist raclopride (2 mg/kg i.p.). Whereas amperozide markedly elevated the numerical density of PPSOM mRNA expressing neurons in the mPFC (52%), the other drugs did not significantly affect PPSOM mRNA levels in any of the brain regions studied. Amperozide also altered PPT-A mRNA expression in the mPFC, i.e. a decrease (22%) was found. Of the other drugs tested only haloperidol significantly decreased PPT-A mRNA levels in the NAC shell (14%), in the dorso-lateral CP (19%) and in the medial CP (15%). In view of the differences between amperozide and the other drugs studied, as regards both pre-clinical and clinical characteristics, we suggest that the specific effects of amperozide on PPSOM and PPT-A mRNA in the mPFC may be related to its 5-HT releasing action in the frontal cortex, an effect possibly caused by its alpha2-adrenoceptor blocking activity. This effect, in turn, may be related to an antidepressant-like action that this compound exhibits in animal studies. The decrease in PPT-A mRNA levels seen after the haloperidol treatment is probably due to its potent DA-D2 receptor antagonism and may be related to side-effects, rather than therapeutic effects of this drug.

Ventral pallidum self-stimulation induces stimulus dependent increase in c-fos expression in reward-related brain regions

Neuronal expression of Fos, the protein product of the immediate early gene c-fos has been used as a high resolution metabolic marker for mapping polysynaptic pathways in the brain. We used Fos immunohistochemistry to reveal neuronal activation following self-stimulation of the ventral pallidum. Four groups of rats were allowed to self-stimulate for 30 min with 0.4 s trains of cathodal rectangular pulses of constant intensity (0.4 mA) and duration (0.1 ms). Each group was assigned a different pulse frequency, (3, 17, 24 and 50 pulses/stimulation train). Which was preselected from within each animal's rate-frequency function. The subjects that were assigned three pulses failed to self-stimulate and were considered as controls. The subjects that were assigned 17 pulses self-stimulated at half-maximal rate, whereas those that were assigned 24 and 50 pulses self-stimulated at maximal rates. The animals were sacrificed 90 min after the self-stimulation session and their brains were processed for Fos-like immunoreactivity. Fos-like immunoreactivity was found to increase as a function of pulse frequency in several brain regions known to be involved in drug and/or brain stimulation reward (medial prefrontal cortex, lateral septum, nucleus accumbens; lateral hypothalamus and ventral tegmental area), whereas it was not affected in structures devoid of such involvement (substantia nigra reticulata and dorsolateral striatum). The level of Fos expression induced by trains of 50 pulses was considerably higher than that produced by 24 pulses although both frequencies supported the same (maximal) self-stimulation rate. This finding indicates that Fos expression correlated with reward magnitude (known to increase between these frequencies), not with bar-pressing rate, thus suggesting the presence of a reward-specific effect. The finding of a frequency-dependent Fos expression in a behavioural paradigm can be considered analogous to a pharmacological dose-response curve and, as such, our results may open new avenues for the use of Fos immunohistochemistry in quantitative neurobehavioural studies.

Stimulation of beta-adrenoceptors with isoprenaline inhibits small intestinal activity fronts and induces a postprandial-like motility pattern in humans

AIMS

To investigate the effects of beta-adrenoceptor stimulation, using the agonist isoprenaline and the antagonist propranolol, on migrating motor complexes in the upper intestine of 16 healthy human volunteers.

METHODS

Fasting motility was monitored using a tube with water perfused side holes connected to a pneumohydraulic system. Continuous eight hour recordings were obtained from each volunteer after a 12 hour fasting period. In all experiments, saline was given as control for the first four hour period and beta-adrenergic agents for the next four hours. In separate control studies, saline was given for the whole eight hour period.

RESULTS

Isoprenaline (2.5 micrograms/kg/min) reduced the number of activity fronts (phase III) of migrating motor complexes from 3 (2-4) in controls to 1 (0-2) during isoprenaline infusion (p < 0.01). Also, phase II-like activity replaced the regular motility pattern (p < 0.01). By contrast, propranolol (25 micrograms/kg/min) did not induce any significant changes in phase III compared with controls. Saline alone had no effect on motor activity.

CONCLUSIONS

Isoprenaline inhibited activity fronts in the human proximal small intestine and induced a postprandial-like motility pattern, whereas propranolol did not affect motor patterns. Stimulation of beta-adrenoceptors is of importance in the control of motor activity of the human small intestine, especially under stressful conditions with high adrenergic activity.

Behavioral manifestations of the nicotine abstinence syndrome in the rat: peripheral versus central mechanisms

The nicotine abstinence syndrome was studied in the rat utilizing a modified rating scale of the opiate abstinence syndrome. Rats were infused with 10.27 mg/kg per day nicotine hydrogen tartrate for 7 days via subcutaneous minipumps. The behavior of each animal was observed before, during and after termination of the nicotine infusion. The abstinence signs in the withdrawal sessions included gasps, genital licks, ptosis, shakes, teeth chatter, yawns and changes in locomotor activity. Abstinence was induced through surgical removal of the pump or through administration of a nicotinic receptor antagonist, acting either centrally and peripherally (mecamylamine 1 mg/kg s.c.) or peripherally only (chlorisondamine 1 mg/kg s.c.). Statistical evaluation revealed a significant increase in overall abstinence signs both at 16 (P < 0.05) and 40 h (P < 0.01) after termination of the nicotine infusion, as compared to the number of signs in the nicotine treated animals' baseline sessions and to the number of signs in control animals (P < 0.05). There was also a significant reduction in locomotor activity during both withdrawal sessions. Animals injected with mecamylamine or chlorisondamine displayed a larger increase in the abstinence score (P < 0.001) than the spontaneously abstinent animals. Acute administration of different doses of nicotine or of the peripherally acting nicotinic receptor agonist tetramethylammonium (0.8 mg/kg s.c.) reversed the behavioral nicotine abstinence syndrome. Our results show that a nicotine abstinence syndrome can be elicited in rats on a chronic nicotine regimen either by acute withdrawal of nicotine or by the administration of nicotinic receptor antagonists and that peripheral nicotinic receptors may contribute significantly to the overall withdrawal reaction.

Differential effects of acute and chronic nicotine on dopamine output in the core and shell of the rat nucleus accumbens

Like several drugs of abuse, nicotine increase dopamine (DA) release in the nucleus accumbens (NAC). In the present study, the effects of acute and chronic nicotine on DA output in two subdivisions of the NAC, the core and the shell, which are largely associated with motor control and limbic functions, respectively, were examined by means of in vivo differential normal pulse voltammetry in anesthetized, pargyline-treated rats. In the first experiment, acute administration of nicotine (25, 50 and 100 micrograms/kg, cumulative doses; i.v.) was found to increase DA levels in the NACshell to 163% of baseline, whereas DA output in the NACcore was not significantly affected. In the second experiment, animals were pretreated with twelve daily injections of saline or nicotine (0.5 mg/kg, i.p.); about 24 hours after the last injection, the animals were challenged with nicotine (50 micrograms/kg and 100 micrograms/kg, cumulative doses; i.v.). Under these conditions, nicotine increased DA output in the NACshell in saline-pretreated animals to 248% and in nicotine-pretreated rats to 180%. Also, nicotine increased DA output in the NACcore in saline-pretreated animals to 185%, whereas no significant effect was observed in nicotine-pretreated rats. The results of the present experiments indicate (i) that acutely administered nicotine or nicotine challenge in chronically pretreated animals with either saline or nicotine consistently increases DA release to a greater extent in the NACshell than in the NACcore, and (ii) that chronic nicotine pretreatment reduces the stimulatory-action of nicotine on DA output in either the shell or the core subdivision of the NAC.

The psychotomimetic drugs D-amphetamine and phencyclidine release calcitonin gene-related peptide in the limbic forebrain of the rat

Calcitonin gene-related peptide (CGRP) is the major product of the calcitonin gene in brain and exerts a number of actions in the central nervous system (CNS). In particular the finding that CGRP affects dopamine (DA) release and metabolism has raised the possibility that it may play a role in several neuropsychiatric disorders. Consequently, we have here studied the effects of two psychotomimetic drugs, namely, d-amphetamine (AMPH) and phencyclidine (PCP), on CGRP concentrations in brain microdialysates from freely moving rats. The animals were stereotaxically implanted with vertical concentric probes in the medial prefrontal cortex (mPFC), the ventral striatum (vSTR), or the hippocampus; and the experiments were performed 48 hr after surgery. The dialysis probes were perfused with a modified Ringer's solution at the rate of 5 microliters/min. AMPH 1.5 mg/kg, PCP 2.5 mg/kg, or NaCl 0.9% were injected s.c.; and the perfusates were collected at 60 min intervals before and after the injections and used for CGRP-like immunoreactivity (-LI) determination by radioimmunoassay (RIA). In separate experiment, KCl (100 mM), veratridine (50 microM), or tetrodotoxin (2 microM), were added to the perfusate and infused in the vSTR. Baseline levels of CGRP-LI were detected in dialysates from all three regions. Both AMPH and PCP caused a significant and sustained increase (maximum about 300%) in CGRP-LI concentrations, in particular from the mPFC and vSTR, while saline had no effect. KCl and veratridine also increased CGRP-LI in dialysates during the first posttreatment period, while tetrodotoxin induced a significant but delayed decrease in CGRP-LI levels. Finally, cervical dislocation also elevated CGRP-LI in dialysates from the mPFC and the vSTR. Our findings demonstrate that 1) CGRP-LI can be measured in vivo in microdialysates from mPFC, vSTR, and hippocampus; 2) the release in vSTR is action potential-dependent; and 3) systemic administration of AMPH or PCP results in a long-lasting release of CGRP-LI in the mPFC and vSTR, thus demonstrating a novel action of these drugs in the brain. Since other studies have shown that major antipsychotic drugs appear to reduce CGRP release in brain, our study provides, in principle, support for a role of CGRP in psychotic disorders.

Nicotine injections into the ventral tegmental area increase locomotion and Fos-like immunoreactivity in the nucleus accumbens of the rat

Systemic administration of nicotine has been shown to increase locomotor activity in rats, an effect which is enhanced by chronic pretreatment with the drug. Furthermore, administration of nicotine either systemically, or locally within the ventral tegmental area (VTA), increases extracellular levels of dopamine (DA) in the nucleus accumbens (NAc). In the present study, we examined the effect of local, bilateral injections into the VTA of nicotine (0.02, 0.2, 2.0 and 8.0 micrograms/0.5 microliter/side) on locomotor activity of rats in an open field. Nicotine (8.0 micrograms/side) significantly increased forward locomotion within 20 min after injection, whereas rearing was not affected. The stimulatory effect of locally applied nicotine was completely blocked by pretreatment with mecamylamine (1.0 mg/kg, s.c.). Repeated intra-tegmental injections of a subthreshold dose of nicotine (2.0 micrograms/side every 2 days), gradually increased locomotion, compared to the effect of acute intra-tegmental administration or control injections of saline, after the fifth and sixth injection. The effects of intra-tegmental injections of nicotine were further investigated on cells in several target areas for the VTA-DA neurons through determination of c-fos expression by means of Fos immunohistochemistry. Intra-tegmental injections of nicotine (8.0 micrograms/side) increased Fos-like immunoreactivity in the NAc, but did not affect the number of Fos-positive nuclei in the medial prefrontal cortex or in the dorsolateral striatum. The increase in accumbal Fos-like immunoreactivity was attenuated by pretreatment with mecamylamine (1.0 mg/kg, s.c.). Our data demonstrate that locomotor activating effects similar to those evoked by systemically administered nicotine, including behavioral sensitization, can be produced by intra-tegmental nicotine administration. Moreover, such local VTA administration of the drug was found to significantly affect neurons within DA target areas. Our findings support the notion that the effects of systemically administered nicotine in mesolimbic target areas are largely dependent on stimulation of nicotinic receptors in the VTA.

Haemodynamic changes in the small intestine correlate to migrating motor complex in humans

OBJECTIVE

To evaluate local intestinal blood flow and its relationship to fasting gut motility in humans.

DESIGN AND METHODS

Regional laser Doppler flowmetry (LDF) was carried out for 8 h in 14 tasted volunteers. LDF measured as perfusion units (PU) was performed using two single-fibre microprobes attached to a small intestinal manometry tube, which monitored migrating motor complexes (MMCs). Luminal pressures of small intestine were registered in analogue and digital recordings.

RESULTS

During phase 1 of MMC, PU values of 65 (33-95) and 37 (20-100) in proximal and distal duodenum were measured. During phase 2, PU values increased by 17% and 38%, respectively (each P < 0.001). At phase 3 of MMC, corresponding PU values increased by 363% and 443% (each P < 0.001) and remained at high levels until termination of phase 3. During phase 3 there was aboral propagation of LDF activity. Computerized recordings allowed detailed analysis of relationships between LDF pattern and luminal pressures. At pressures below 48 (42-54) mmHg, individual LDF cycles and contraction cycles were phase displaced at 180 degrees, indicating low perfusion during contractions. When pressures exceeded 48 (42-54) mmHg, a decrease in PU was registered and the cycling pattern of LDF was abolished. Then, as luminal pressure decreased below 33 (25-41) mmHg, PU increased by 246%.

CONCLUSION

Our findings show a relationship between fasting motility and blood flow in the human gut, in which the blood flow exhibited a pattern similar to phase 1, 2 and 3 of the MMC.

Prazosin inhibits MK-801-induced hyperlocomotion and dopamine release in the nucleus accumbens

This study examined the putative inhibitory effect of the alpha 1-adrenoceptor antagonist prazosin (1-(4-amino-6,7-dimethoxy-2-quinazolinyl)-4-(2-furanylcarbonyl)pip erazine) on changes evoked by the psychotomimetic, non-competitive NMDA receptor antagonist, MK-801((+)-5-methyl-10,11-dihydroxy-5H-dibenzo-(a,d)cyclohepten-5, 10-imine), in locomotor activity and extracellular concentrations of dopamine and its metabolites, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), and the serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA) in the nucleus accumbens as assessed by microdialysis in freely moving rats. MK-801 (0.1 and 0.3 mg/kg, s.c.) induced a significant, dose-dependent increase in horizontal locomotor activity but did not affect rearing. Prazosin administration alone (1 mg/kg, s.c.) only slightly reduced horizontal activity during an initial 10 min measurement period, although it consistently reduced rearing. However, pretreatment with prazosin effectively suppressed the locomotor stimulation caused by either dose of MK-801 throughout the whole observation period, i.e. 40 min. Both doses of MK-801 significantly increased extracellular levels of dopamine in the nucleus accumbens up to approximately 90%. In addition, MK-801 dose dependently increased dopamine metabolite concentrations in the nucleus accumbens, but 5-HIAA was significantly increased only by the high dose of MK-801. When given alone, prazosin did not affect either dopamine, DOPAC, HVA or 5-HIAA levels. However, prazosin pretreatment effectively blocked MK-801-evoked increases in dialysate dopamine concentrations. Consequently, the potent and selective alpha 1-adrenoceptor antagonist prazosin was found to specifically suppress MK-801-evoked, but not basal dopamine release in the nucleus accumbens, while effectively blocking MK-801-evoked locomotor stimulation with only negligible effects on basal locomotor activity. Thus, alpha 1-adrenoceptor antagonism may act by reducing the sensitivity of the mesolimbic dopamine system to pharmacological or environmental challenge. Since most antipsychotic drugs exhibit both dopamine D2 receptor and alpha 1-adrenoceptor antagonistic properties, they may alleviate psychosis not only through blockade of postsynaptic dopamine receptors, but also presynaptically on the mesolimbic dopamine system, through their alpha 1-adrenoceptor antagonistic action. This latter action may contribute to reduce evoked dopamine hyperactivity, e.g. in response to stress.

Beta-adrenoceptors regulate myoelectric activity in the small intestine of rats: stimulation by beta 2 and inhibition by beta 3 subtypes

Using beta-adrenergic agonists and antagonists this study investigated the importance of three different adrenoceptor subtypes for the regulation of migrating myoelectric complexes (MMCs) of the upper small intestine in conscious, naive rats. After a control period of 60 min with four activity fronts, agonists were given as an intravenous infusion for another 60 min. The non-selective beta-adrenoceptor agonist isoprenaline (1 microgram kg-1 min-1) inhibited MMCs and induced irregular spiking during the infusion period. This effect was blocked by intravenous administration of a bolus dose of either the non-selective beta-adrenoceptor antagonist propranolol (1 mg kg-1), or the beta 2-antagonist ICI 118 551 (1 mg kg-1), both given prior to isoprenaline. However, acebutolol (1 mg kg-1), a selective beta 1-antagonist, failed to antagonize the effect of isoprenaline. Furthermore, prenalterol, a selective beta 1-agonist (12.5-800.0 micrograms kg-1 min-1), had no effect on the MMC pattern, whereas the beta 2-selective agonist ritodrine (25-100 micrograms kg-1 min-1) induced a myoelectric pattern similar to one induced by isoprenaline. The partial beta 3-adrenoceptor agonist D7114 (50-100 micrograms kg-1 min-1), disrupted the MMCs and induced quiescence. Neither of the antagonists, i.e. propranolol (1 mg kg-1), acebutolol (1 mg kg-1) nor ICI 118 551 (1 mg kg-1), given alone induced changes in the MMC pattern. In conclusion, beta 2-adrenoceptors in particular but also beta 3-adrenoceptors seem to be of importance in the regulation of small intestinal motility by disrupting the regular MMC pattern in rats.

The 5-HT1A receptor antagonist (S)-UH-301 augments the increase in extracellular concentrations of 5-HT in the frontal cortex produced by both acute and chronic treatment with citalopram

In a recent study, utilizing single cell recording techniques, we have shown that administration of 5-HT1A receptor antagonists, e.g. (S)-UH-301, to rats concomitantly treated, acute or chronically, with the selective serotonin reuptake inhibitor (SSRI) citalopram significantly increases the activity of 5-hydroxytryptamine (5-HT) containing neurons in the dorsal raphe nucleus (DRN). Here we report correlative experiments using microdialysis in freely moving animals to measure extracellular levels of 5-HT and its metabolite 5-hydroxyindole acetic acid (5-HIAA) in the frontal cortex, a major projection area for DRN-5-HT neurons. Acute administration of (S)-UH-301 (2.5 mg/kg s.c.) or citalopram (2.0 mg/kg s.c.) increased 5-HT concentrations with a maximum of about 70% and 185%, respectively, above baseline. However, when (S)-UH-301 was administered 30 min before citalopram the maximal increase in 5-HT levels was approximately 400%. In rats chronically treated with citalopram (20 mg/kg/day i.p. for 14 days) basal 5-HT concentrations in the frontal cortex were significantly increased and 5-HIAA concentrations were decreased when measured 10-12 h, but not 18-20 h, after the last injection of citalopram, as compared to basal 5-HT and 5-HIAA concentrations in chronic saline-treated rats. When (S)-UH-301 (2.5 mg/kg s.c.) was administered 12 h, but not 20 h, after the last dose of citalopram it produced a significantly larger increase in extracellular concentrations of 5-HT than in control rats. However, in rats pretreated with a single, very high dose of citalopram, 20 mg/kg i.p., administration of (S)-UH-301 at 12 h after citalopram did not increase 5-HT levels. The augmentation by (S)-UH-301 of the increase in brain 5-HT output produced by acute administration of citalopram is probably due to antagonism of the citalopram induced feedback inhibition of 5-HT cells in the DRN, as previously suggested. However, the capacity of (S)-UH-301 to further increase the already elevated extracellular concentrations of 5-HT in brain in animals maintained on a chronic citalopram regimen, in which significant tolerance to the initial feedback inhibition of DRN-5-HT cells and developed, represents a novel finding. Generally, the reduced feedback inhibition of 5-HT neurons obtained with chronic citalopram treatment, and the associated elevation of brain 5-HT concentrations, may be related to functional desensitization of somatodendritic 5-HT1A autoreceptors in the DRN. This phenomenon may also largely explain the larger increase in 5-HT output produced by (S)-UH-301 in chronic citalopram treated animals as compared to its effect in control animals. Yet, a contributory factor may be a slight, remaining feedback inhibition of the 5-HT cells caused by residual citalopram at 12, but not 20 h after its last administration. Previous clinical studies suggest that addition of a 5-HT1A receptor antagonist to an SSRI in the treatment of depression may accelerate the onset of clinical effects. Moreover, in therapy-resistant cases maintained on SSRI treatment, addition of a 5-HT1A receptor antagonist may improve clinical efficacy. Since the therapeutic effect of SSRIs in depression has been found to be critically linked to the availability of 5-HT in brain, our experiments results support, in principle, both of the above clinically based notions.

The 5-HT1A receptor antagonist (S)-UH-301 decreases dopamine release in the rat nucleus accumbens and striatum

In this study we employed in vivo microdialysis to examine the effects of the selective 5-HT1A receptor antagonist (S)-5-fluoro-8-hydroxy-2-(dipropylamino)tetralin [(S)-UH-301] on extracellular concentrations of dopamine (DA) and its metabolites dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the nucleus accumbens (NAC) and dorsal striatum of awake freely moving rats. Systemic administration of (S)-UH-301 (1.25, 2.5, 5.0 mg/kg s.c.) dose-dependently decreased extracellular concentrations of DA, DOPAC and HVA in the NAC. (S)-UH-301 (2.5 mg/kg s.c.) also decreased DA, but not DOPAC and HVA, concentrations in the striatum. Infusion of low concentrations (1, 10 microM) of (S)-UH-301 into either the NAC or the striatum did not affect DA levels, while only the highest concentration (1,000 microM) significantly decreased DA levels in both areas. Similarly, infusion of the selective 5-HT1A receptor agonist (R)-8-hydroxy-2-(di-n-propylamino)tetralin [(R)-8-OH-DPAT] only in high concentrations (100, 1,000 microM) decreased DA levels in both regions. These data suggest that (S)-UH-301 decreases DA release both in the NAC and the striatum probably indirectly via its purported DA-D2/D3 receptor agonistic properties. However, the observed inhibitory effect of (S)-UH-301 on DA release in the studied brain regions may also be explained, at least partly, by a serotonergic influence on the DA systems, acting at 5-HT1A receptor sites located elsewhere in the brain.

Burst stimulation of the medial forebrain bundle selectively increase Fos-like immunoreactivity in the limbic forebrain of the rat

The present study was designed to evaluate the postsynaptic functional consequences of different presynaptic activity patterns in midbrain dopamine systems using electrical stimulation of the rat medial forebrain bundle and subsequent determination of c-fos expression, used as a marker for neuronal activation, in dopamine target areas, by means of Fos immunohistochemistry. Nerve terminal dopamine release evoked by electrical stimulation of the medial forebrain bundle was monitored in the same animals using in vivo voltammetry. A 5 Hz stimulation consisting of 60 trains of five pulses and lasting 1 min was applied to the medial forebrain bundle. This stimulation was repeated 15 times every 3 min. Its pattern was defined by the interpulse interval which was either 70 ms or 200 ms for burst or regularly spaced stimulation, respectively. Our results show that burst stimulation of the medial forebrain bundle, which increase release of dopamine in target areas, increases the basal Fos-like immunoreactivity in the stimulated hemisphere, while regular stimulation does not affect expression of this protein. Moreover, the increase in Fos-like immunoreactivity induced by burst stimulation is restricted to limbic related structures, i.e. nucleus accumbens shell and intermediate aspect of the lateral septum, and the major island of Calleja, but is not observed in motor related structures (nucleus accumbens core and striatum). Pretreatment with the D1 dopamine receptor antagonist, SCH23390 (0.1 mg/kg, i.p.), blocked the increase in Fos-like immunoreactivity induced by burst stimulation of the medial forebrain bundle, suggesting a role for these receptors in the observed effects. Pretreatment with the 5-hydroxytryptamine2A/2C receptor antagonist ritanserin (0.4 mg/kg, i.p.) did not affect the increase in Fos-like immunoreactivity induced by burst stimulation in the nucleus accumbens shell or in the lateral septum, although it blocked the stimulated enhancement of Fos-like immunoreactivity in the major island of Calleja. The present data indicate that, rather than the absolute mean discharge rate of midbrain dopamine neurons, the temporal organization of the action potentials they generate conveys information to their target areas.

Condition-independent sensitization of locomotor stimulation and mesocortical dopamine release following chronic nicotine treatment in the rat

Chronic nicotine (NIC) pretreatment has been shown to enhance NIC-induced locomotor stimulation, an effect that seems critically dependent on activation of brain dopamine (DA) systems. In the present study the effects of chronic, intermittent NIC treatment were examined in the rat to establish whether such behavioral sensitization is associated with specific, regional changes in brain dopaminergic activity. Male rats received daily injections in their home cage with either saline (SAL) or NIC (0.5 mg/kg, s.c.) for 12 days. Twenty-four hours later, the locomotor activity of the animals subjected to NIC challenge as well as the functional responsiveness of the mesolimbocortical dopaminergic system were assessed. To this end, microdialysis experiments were performed in awake animals, measuring extracellular concentrations of DA and its metabolites in the prefrontal cortex (PFC) and the nucleus accumbens (NAC). Extracellular single cell recordings from DA neurons in the ventral tegmental area (VTA) were also performed in anesthetized animals. NIC (0.5 mg/kg, s.c.) increased all measured parameters of locomotor activity, with the exception of rearing, in SAL-pretreated animals; these effects were substantially enhanced after pretreatment with NIC. Nicotine (0.5 mg/kg, s.c.) increased DA release in both the PFC and the NAC in SAL-treated animals. Nicotine pretreatment significantly enhanced this effect in the PFC, whereas it did not affect the response in the NAC. Low doses of intravenously administered NIC dose-dependently increased burst activity, starting at 12 micrograms/kg in the SAL pretreated animals and at 6 micrograms/kg in the NIC-pretreated animals, and also dose-dependently increased firing rate in SAL as well as NIC-pretreated animals, although starting at a higher dose level, i.e., 25 micrograms/kg. These results demonstrate that behavioral sensitization after chronic NIC treatment is accompanied by an enhanced dopamine release specifically within the PFC. This phenomenon may be highly significant for the dependence-producing effects of NIC, particularly in association with major psychiatric disorder, such as schizophrenia.

Differential actions of typical and atypical antipsychotic drugs on dopamine release in the core and shell of the nucleus accumbens

The effects of acute administration of typical and atypical antipsychotic drugs on extracellular dopamine (DA) concentrations in brain were examined in two subdivisions of the nucleus accumbens (NAC), the core and the shell, which are largely associated with motor control and limbic functions, respectively, by using in vivo differential normal pulse voltammetry in anesthetized, pargyline pretreated rats. The following drugs were studied: haloperidol (0.1 and 1.0 mg/kg), clozapine (1.0 and 5.0 mg/kg), amperozide (1.0 and 2.0 mg/kg), risperidone (0.1 and 1.0 mg/kg), the selective 5-HT2A/5-HT2C receptor antagonist ritanserin (1 mg/kg) and the selective DA-D2/D3 receptor antagonist raclopride (10 and 320 micrograms/kg). Drugs with predominantly high 5-HT2 receptor antagonistic action, such as amperozide and ritanserin, as well as low doses of either risperidone or clozapine increased DA concentrations to a greater extent in the shell than in the core subdivision of the NAC. In contrast, drugs with a more potent D2 receptor antagonistic action, such as haloperidol and raclopride, as well as high doses of either risperidone or clozapine, elicited a larger DA increase in the core than in the shell. Consequently, atypical antipsychotics characterized by potent 5-HT2 receptor antagonism can be differentiated from typical antipsychotic drugs on the basis of their preferential effect on DA transmission in the shell region of the NAC.

Risperidone: regional effects in vivo on release and metabolism of dopamine and serotonin in the rat brain

The antipsychotic drug risperidone shows high affinity for both central serotonin (5-HT)2A and dopamine (DA)-D2 receptors in vivo. By employing microdialysis in freely moving rats, the effects of acute risperidone administration on regional brain DA and 5-HT release and metabolism were compared with the corresponding effects of the atypical antipsychotic drug clozapine as well as amperozide, the selective DA-D2 receptor antagonist raclopride and the selective 5-HT2A/5-HT2C receptor antagonist ritanserin. Risperidone (0.2 or 2.0 mg/kg, SC) was found to increase DA release and metabolism to about the same extent in three major projection areas of the mesotelencephalic dopaminergic system, i.e. the nucleus accumbens (NAC), the medial prefrontal cortex (MPC) and the lateral striatum (STR). In contrast, clozapine and amperozide (both 10.0 mg/kg, SC), as well as raclopride (2.0 mg/kg, SC), were all found differentially to affect DA release and metabolism in the three projections areas. Specifically, clozapine and amperozide enhanced DA release in the MPC to a greater extent than in the NAC or the STR, whereas raclopride instead preferentially increased DA release in the NAC and the STR but not in the MPC. Ritanserin (3.0 mg/kg, SC) did not exert any major effects on DA metabolism in the three areas studied. In contrast to the regionally rather homogenous activation of brain DA systems caused by risperidone, the drug was found to enhance brain 5-HT metabolism preferentially in the MPC, as indicated by the elevated extracellular concentration of 5-hydroxyindoleacetic acid (5-HIAA) in this region. A similar elevation of the 5-HIAA level in the MPC was observed after amperozide and, to some extent, after clozapine and ritanserin administration. The risperidone-induced (2.0 mg/kg, SC) elevation of 5-HIAA concentrations in the frontal cortex was found to be paralleled by an increased 5-HT release in this brain area. Consequently, our findings demonstrate a pharmacological profile of risperidone, as reflected in brain DA metabolism, in between that of clozapine and the DA-D2 antagonists. The preferential activation of 5-HT release and metabolism in frontal cortical areas might be of particular relevance for the ameliorating effect of risperidone on negative symptoms in schizophrenia, especially when associated with depression.

Effects of D-amphetamine and phencyclidine on behavior and extracellular concentrations of neurotensin and dopamine in the ventral striatum and the medial prefrontal cortex of the rat

The effects of systemically administered phencyclidine (PCP; 2.5 mg/kg, s.c.) and D-amphetamine (1.5 mg/kg, s.c.) on the extracellular concentrations of neurotensin-like immunoreactivity (NT-LI) and dopamine (DA) in the ventral striatum (vSTR) and the medial prefrontal cortex (mPFC) were studied in freely moving rats using microdialysis. In separate animals, the effects of PCP and D-amphetamine on open field activity were also analyzed. PCP, but not D-amphetamine, caused a significant increase (156% over baseline) of NT-LI levels in the vSTR which was relatively short lasting, i.e., of less than 2 h duration. In contrast, both drugs significantly increased NT-LI concentrations in the mPFC by almost 100% during the same period. PCP and D-amphetamine also significantly increased extracellular levels of DA in the vSTR by 83 and 364%, respectively. However, the peak effect of PCP on DA appeared later than that of D-amphetamine, i.e., at 150 and 60 min, respectively, after drug administration. Also in the mPFC, both PCP and D-amphetamine significantly increased DA concentrations by 98 and 284%, respectively. Generally, effects on DA levels of both PCP and D-amphetamine were, in contrast to their effects on NT-LI levels, clearly more long-lasting, i.e., of 3-4 h duration. Behaviorally, D-amphetamine produced a more pronounced, general activation than PCP, with a faster onset of activation, i.e. within 30 vs 90 min after administration. However, both drugs produced long-lasting effects on the spatial organization of behavioral activity, which lasted for 3-4 h. In conclusion, the more pronounced behavioral stimulation by D-amphetamine (1.5 mg/kg, s.c.) vs PCP (2.5 mg/kg, s.c.) in the rat may largely be explained by its more potent DA-releasing effect in the brain. Initial behavioral suppression by PCP, e.g., of rearing, as well as its rather poor locomotor stimulant action in general, might relate to release of NT in the vSTR. The long-lasting, behavioral disorganization by both PCP and D-amphetamine may, however, be related to increased release of DA rather than NT in the mesolimbocortical areas.

Ritanserin potentiates the stimulatory effects of raclopride on neuronal activity and dopamine release selectivity in the mesolimbic dopaminergic system

The atypical profile of clozapine and some other new antipsychotic drugs has been attributed to a relatively selective effect on the mesolimbic dopaminergic system, as well as to their potent serotonin 5-HT2 receptor antagonism and high ratio of 5-HT2 to dopamine D2 receptor affinities. It is unclear, however, how concurrent 5-HT2 and D2 receptor antagonism specifically affects the mesoaccumbens and the mesocortical dopaminergic systems. The present study examined the effect of pretreatment with the 5-HT2 receptor antagonist, ritanserin, on changes in midbrain dopamine neuronal activity as well as in forebrain, extracellular concentrations of dopamine, induced by relatively low doses of the D2 receptor antagonist raclopride, utilizing in vivo extracellular single cell recording techniques and voltammetry in anesthetized rats, as well as microdialysis in freely moving rats. Raclopride alone (10-2560 microgram/kg, i.v.) induced a dose-dependent increase in three parameters of neuronal activity, i.e. burst firing, firing rate and variation coefficient, of midbrain DA neurons. This effect of raclopride was more pronounced in cells of the ventral tegmental area than in cells of the substantia nigra-zona compacta. Ritanserin alone (1.0 mg/kg, i.v.) also increased all three parameters of neuronal activity in dopamine cells of the ventral tegmental area, but only firing rate in the cells of the substantia nigra. Ritanserin pretreatment (30 min) significantly enhanced the stimulatory effects of low doses of raclopride (10-20 micrograms/kg, s.c.) increased extracellular concentrations of dopamine in the medial prefrontal cortex and the dorsolateral striatum by 75 and 110%, respectively, as measured by microdialysis. Ritanserin alone (1.5 mg/kg, s.c.) did not significantly affect cortical and striatal extracellular dopamine concentrations; however, pretreatment (40 min) with ritanserin elevated the raclopride-induced increase of dopamine concentrations in the medial prefrontal cortex of about 250%, but failed to affect the action of raclopride on striatal dopamine levels. Raclopride alone (10 and 320 micrograms/kg, i.v.) dose-dependently increased extracellular concentrations of dopamine in the nucleus accumbens and the dorsolateral striatum to about 500%, as determined by voltammetry. Ritanserin alone (1.0 mg/kg, i.v.) did not significantly affect the voltammetric dopamine signal in the nucleus accumbens or the dorsolateral striatum; however, ritanserin pretreatment (30 min) enhanced the raclopride-induced increase in accumbal but not striatal dopamine concentrations to about 1600%. The stimulatory effect of the combined ritanserin plus raclopride treatment on neuronal activity and DA release was more pronounced in the mesolimbic than the nigrostriatal dopaminergic system. The present data indicate that concurrent 5-HT2 and D2 receptor antagonism selectively affects the activity of the mesolimbic dopaminergic system. These findings provide an experimental basis for the notion that combined 5-HT2 and D2 receptor antagonism may underlie the limbic mode of action of at least some atypical antipsychotic drugs and consequently contribute to their unique therapeutic effects.

5-HT1A receptor antagonists increase the activity of serotonergic cells in the dorsal raphe nucleus in rats treated acutely or chronically with citalopram

In this study we have examined the acute effects of systemic administration of the selective serotonin reuptake inhibitor (SSRI), citalopram, in combination with either of the two selective 5-HT1A receptor antagonists, [(S)-5-fluoro-8-hydroxy-2-(dipropylamino)-tetralin [(S)-UH-301] or (+)-N-tertbutyl 3-(4-(2-methoxyphenyl)piperazin-1-yl)-2-phenyl-propionamide dihydrochloride [(+)-WAY100135], on the activity of single 5-HT neurons in the dorsal raphe nucleus (DRN) of anesthetized rats using extracellular recording techniques. Acute administration of citalopram (0.3 mg/kg i.v.) significantly decreased the firing rate of DRN-5-HT cells most likely as a result of indirect stimulation of inhibitory somatodendritic 5-HT1A autoreceptors located on 5-HT cells in the DRN. This effect of citalopram was completely reversed by (S)-UH-301 (0.5 mg/kg i.v.) and partly by (+)-WAY100135 (0.5 mg/kg i.v.). Furthermore, the inhibitory effect of citalopram on the activity of 5-HT neurons was significantly attenuated by pretreatment with (S)-UH-301 (0.25 mg/kg i.v.) or (+)-WAY100135 (0.25 mg/kg i.v.). We have also studied the effects of (S)-UH-301 (0.03-0.50 mg/kg i.v.) on the firing rate of single DRN-5-HT cells in rats chronically treated with citalopram (20 mg/kg/day i.p. x 14 days). Administration of (S)-UH-301 significantly and dose-dependently increased the activity of 5-HT cells in citalopram-treated rats, but did not affect these neurons in saline-treated (1 ml/kg/day i.p. x 14 days), control rats. Our results thus suggest that 5-HT1A receptor antagonists can augment both the acute and chronic effects of citalopram on central serotonergic neurotransmission.(ABSTRACT TRUNCATED AT 250 WORDS)

Serotonin in rat oral tissues: role of 5-HT1 receptors in sympathetic vascular control

In this study we examined whether the indoleamine, serotonin (5-hydroxytryptamine, 5-HT), is contained in the rat incisor pulp and gingiva as well as its possible role in regulation of blood flow in these tissues. Tissue biochemical analysis, by means of high performance liquid chromatography coupled to electrochemical detection, revealed the presence of 5-HT and its metabolite, 5-hydroxyindoleacetic acid (5-HIAA), as well as the catecholamine, dopamine, in both pulp and gingiva. Unilateral surgical sympathectomy or resection of the inferior alveolar nerve failed to affect 5-HT levels in either tissue while dopamine contents in the pulp and gingiva were diminished following sympathectomy. Electrical stimulation of the sympathetic trunk induced a frequency-dependent vasoconstriction in the pulp and gingiva as measured by laser Doppler flowmetry. This vasoconstriction was unaffected by infusion of 5-HT2 or 5-HT3 receptor antagonists or dopamine receptor antagonists but it was significantly reduced in both tissues after alpha 1-adrenoceptor blockade. During this blockade the remaining vasoconstriction induced by high frequency stimulation (16 Hz) was reduced in gingiva by the 5-HT1 receptor blocker, methiothepin. The results indicate an involvement of 5-HT1 receptors and alpha 1-adrenoceptors in the sympathetic vascular control in the gingiva.

Nicotine dependence, midbrain dopamine systems and psychiatric disorders

Compelling evidence exists that tobacco-smoking represents a form of drug addiction to nicotine. Like several drugs of abuse, nicotine activates the mesolimbic dopamine system and this effect appears to be of critical importance for the reinforcing properties of the drug. Specifically, nicotine has been shown to increase burst activity in dopamine neurones of the ventral tegmental area, i.e. a mode of firing pattern in these cells which is physiologically associated with basic motivational processes underlying learning and cognitive behaviour. The stimulatory action of nicotine on mesolimbic dopamine neurones is exerted both at the somatodendritic and at the terminal levels. Yet, the release of dopamine in the nucleus accumbens induced by systemically administered nicotine is abolished by the nicotine receptor antagonist, mecamylamine when administered locally in the ventral tegmental area, but not in the nucleus accumbens. Whereas continuous infusion of nicotine into the ventral tegmental area produces a long-lasting increase in accumbal dopamine release, analogously to the effect of systemically administered nicotine, continuous infusion of nicotine into the nucleus accumbens produces a very short-lasting dopamine release. Thus, nicotinic receptors in the ventral tegmental area appear to be more significant than those located in the nucleus accumbens for mediating the stimulatory effect of nicotine on dopamine release in the nucleus accumbens. The effect of nicotine on midbrain dopamine systems may help to explain the extremely high prevalence of tobacco-smoking in schizophrenics, who frequently display so-called hypofrontality, i.e. a reduced functional activity in the prefrontal cortex which provides a direct input to the ventral tegmental area dopamine cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Mode of action of atypical neuroleptics in relation to the phencyclidine model of schizophrenia: role of 5-HT2 receptor and alpha 1-adrenoceptor antagonism [corrected]

In experiments in rats, by the use of single-cell recordings from midbrain dopamine (DA) neurons of the ventral tegmental area (VTA), the systemic administration of the schizophrenomimetic N-methyl-D-aspartate receptor antagonists phencyclidine (PCP) or dizocilpine (MK-801) caused an increased firing rate but reduced the variability of firing in VTA DA neurons. Burst firing was increased in cells predominantly located in the paranigral nucleus, a subdivision of the VTA largely projecting to the nucleus accumbens and other limbic regions, but reduced in DA cells predominantly located in the parabrachial pigmented nucleus, another subdivision of the VTA that projects largely to the prefrontal cortex (PFC). Thus, a severely impaired signal-to-noise ratio within the PFC DA projection was obtained, concomitant with an overactive mesolimbic DA system. The administration of high doses of ritanserin or atypical neuroleptics with prominent serotonin (5-hydroxytrypyamine) 5-HT2 receptor antagonist action, such as clozapine or amperozide, produced preferential activation of the PFC DA projection. In contrast, the selective D2 receptor antagonist raclopride caused a greater activation of the subcortical than cortical DA projections, as assessed by microdialysis experiments in vivo from our laboratory. Adding ritanserin treatment to raclopride markedly enhanced the raclopride-induced increase in DA levels in the medial PFC, an effect probably mediated by augmentation of the raclopride-induced increase in the burst firing of meso-cortical DA neurons, but failed to affect the action of raclopride on striatal DA levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Infusion of nicotine in the ventral tegmental area or the nucleus accumbens of the rat differentially affects accumbal dopamine release

The present study examined the effects of acute, continuous infusion of nicotine in either the ventral tegmental area or the nucleus accumbens on extracellular concentrations of dopamine in the nucleus accumbens by applying in vivo microdialysis in freely moving rats. Nicotine (1000 microM) infusion for 80 min. in the ventral tegmental area produced a long-lasting increase in accumbal dopamine, whereas similar nicotine infusion in the nucleus accumbens increased dopamine levels only within the first 20 min. of administration. This effect was blocked by systemic pretreatment with the nicotinic receptor antagonist mecamylamine (1 mg/kg, subcutaneously). In contrast to the effects of nicotine, N-methyl-D-aspartate infusion in the ventral tegmental area as well as in the nucleus accumbens produced a long-lasting increase in accumbal dopamine levels. The more procounced effect of infusion of nicotine in the ventral tegmental area as compared to the nucleus accumbens on accumbal dopamine release may be due to a lower rate or even lack of tolerance to nicotine's stimulatory action in the ventral tegmental area. These results support the notion that nicotinic receptors in the ventral tegmental area may be of greater importance than those located in the nucleus accumbens for mediating some of the stimulatory effects of nicotine on the reward-related mesoaccumbens dopamine system.

The 5-HT1A receptor antagonist (S)-UH-301 blocks the qR)-8-OH-DPAT-induced inhibition of serotonergic dorsal raphe cell firing in the rat

(S)-UH-301 [(S)-5-fluoro-8-hydroxy-2-(dipropylamino)-tetralin, 0.5-4.0 mg/kg i.v.] did not significantly alter the firing rate of 5-hydroxytryptamine (5-HT) containing neurons in the dorsal raphe nucleus (DRN) as a group, although some individual cells were activated whereas others were depressed. However, (S)-UH-301 (2.0 mg/kg i.v.) consistently reversed the inhibition of DRN-5-HT cells produced by the selective 5-HT1A receptor agonist (R)-8-OH-DPAT (0.5 microgram/kg i.v.) and the dose-response curve for this effect of (R)-8-OH-DPAT was markedly shifted to the right by pretreatment with (S)-UH-301 (1.0 mg/kg i.v.). These results support the notion that (S)-UH-301 acts as an antagonist at central 5-HT1A receptors.

Systemic administration of amperozide, a new atypical antipsychotic drug, preferentially increases dopamine release in the rat medial prefrontal cortex

The putative atypical antipsychotic drug amperozide (APZ) shows high affinity for serotonin 5-HT2 receptors but only low affinity for dopamine (DA) D2 receptors. By employing microdialysis, we examined the effects of APZ on extracellular concentrations of DA in the nucleus accumbens (NAC), the dorsolateral striatum (STR) and the medial prefrontal cortex (MPC) of awake rats. A 5.0 mg/kg (SC) dose of APZ failed to affect DA concentrations in the NAC, while it increased DA outflow in the STR (by 46%) and the MPC (by 207%). A higher dose of APZ (10 mg/kg, SC) enhanced dialysate DA from the NAC and the STR by 30%, and from the MPC by 326%. Similarly, clozapine (2.5 and 10 mg/kg, SC) produced a greater release of DA in the MPC (+ 127 and + 279%) than in the NAC (+ 52 and + 98%). The selective 5-HT2 receptor antagonist ritanserin (1.5 and 3.0 mg/kg, SC) also produced a slightly higher increase of DA output in the MPC (+ 25 and + 47%) compared with the NAC (+ 19 and + 21%). In contrast, the selective D2 receptor antagonist raclopride (0.5 and 2.0 mg/kg, SC) increased DA release in the NAC (+ 65 and + 119%) to a greater extent than in the MPC (+ 45 and + 67%). These data suggest that the 5-HT2 receptor antagonistic properties of APZ and clozapine may contribute to their preferential effects on DA transmission in the MPC. Infusion of low doses (1, 10 microM, 40 min) of APZ through the probe in the DA terminal areas did not affect significantly DA outflow, while infusion of high doses (100, 1000 microM, 40 min) resulted in a more pronounced elevation of DA levels in the NAC (up to 961%) and the STR (up to 950%) than in the MPC (up to 316%). These findings indicate that the selective action of systemically administered APZ on DA in the MPC is most likely mediated at a level other than the terminal region. Taken together, the present results provide support for the notion that 5-HT2 receptor antagonism may be of considerable significance for the action of atypical antipsychotic drugs on mesolimbocortical dopaminergic neurotransmission.

Prazosin modulates the changes in firing pattern and transmitter release induced by raclopride in the mesolimbic, but not in the nigrostriatal dopaminergic system

Most antipsychotic drugs are, in addition to being dopamine (DA) D2 receptor antagonists, also relatively potent alpha 1 adrenoceptor antagonists. Here, we have studied the effects of the selective DA D2 receptor antagonist raclopride, alone and in combination with the selective alpha 1 adrenoceptor antagonist, prazosin, on midbrain DA neurons utilizing extracellular single cell recording techniques. As a reference compound, haloperidol (0.05-1.6 mg/kg, i.v.), a potent antagonist at both DA D2 receptors and alpha 1 adrenoceptors, was included in the electrophysiological part of the study. In addition, in vivo voltammetry was used to measure extracellular DA concentrations in the nucleus accumbens (NAC) and the dorsolateral striatum (STR) in anesthetized, pargyline pretreated rats treated with the above drugs. Raclopride (10-5120 micrograms/kg, i.v.) induced a dose dependent increase in firing rate of DA neurons in the ventral tegmental area (VTA), that was significant already at 10 micrograms/kg, and in the substantia nigra-zone compacta (SN-ZC), that reached significance at 2560 micrograms/kg. Burst firing of DA neurons was also increased in the VTA at 40 micrograms/kg, as well as in the SN-ZC at 640 micrograms/kg. A low dose of raclopride (80 micrograms/kg, cumulated dose) induced a significant increase in extracellular DA concentrations in NAC to 490% and in STR to 220%. A high dose of raclopride (2560 micrograms/kg, cumulated dose) induced a 930% increase in extracellular DA concentrations in NAC, but only a 280% increase in STR. These data demonstrate that raclopride exerts a relatively selective action on mesolimbic DA neurons. Prazosin (0.3 mg/kg, i.v.) decreased burst firing of VTA, but not SN-ZC DA neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Systemic nicotine-induced dopamine release in the rat nucleus accumbens is regulated by nicotinic receptors in the ventral tegmental area

Stimulation of the mesolimbic dopamine (DA) system is considered of major importance for the rewarding and dependence producing properties of nicotine (NIC). To identify the site of this stimulatory action, simultaneous microdialysis was performed in the ventral tegmental area (VTA) and the ipsilateral nucleus accumbens (NAC) of awake rats. Extracellular concentrations of DA and its metabolites were measured in the NAC. NIC (0.5 mg/kg, s.c.) increased DA and its metabolites by approximately 50%. Concomitant infusion of the nicotinic receptor antagonist mecamylamine (MEC, 100 microM) through the VTA probe, starting 40 min before NIC injection, antagonized the NIC induced increases of DA and its metabolites. In contrast, similar MEC pretreatment (40 or 140 min) in the NAC did not affect DA or metabolite responses to systemic NIC. Infusion of NIC (1,000 microM) in the NAC or the VTA increased DA release by 49% and 48%, respectively, whereas only the VTA infusion increased metabolite concentrations by approximately 25%. MEC infusion (1-1,000 microM) in the VTA did not affect DA or its metabolites, whereas the 1,000 microM concentration infused in the NAC increased DA by 77%. These results suggest that nicotinic receptors in the somatodendritic region may be of greater importance than those located in the terminal area for the stimulatory action of systemic NIC on the mesolimbic DA system. Furthermore, our findings support the notion that the mesolimbic dopaminergic system is phasically rather than tonically regulated by nicotinic receptor activation within the VTA.

Prefrontal cortex regulates burst firing and transmitter release in rat mesolimbic dopamine neurons studied in vivo

The influence of the medial prefrontal cortex (PFC) on mesolimbic dopamine activity was studied with electrophysiological techniques and in vivo voltammetry in the chloral hydrate-anesthetized male rat. Glutamate injected into the PFC selectively increased burst firing of single dopamine cells in the ventral tegmental area and enhanced the release of dopamine from nerve terminals in the nucleus accumbens. PFC injection of the local anesthetic lidocaine produced the opposite effects on burst firing and terminal release. This selective modulation of the dynamic activity of mesolimbic dopamine neurons by the prefrontal cortex might be important in motivation, learning and schizophrenia.

The 5-HT1A receptor selective ligands, (R)-8-OH-DPAT and (S)-UH-301, differentially affect the activity of midbrain dopamine neurons

The effects of the selective 5-HT1A receptor agonist (R)-8-hydroxy-2(di-n-propylamino)tetralin [(R)-8-OH-DPAT] and the novel 5-HT1A antagonist (S)-5-fluoro-8-hydroxy-2-(dipropylamino)-tetralin [(S)-UH-301] were studied with regard to the firing pattern of single mesencephalic dopamine (DA) neurons with extracellular recording techniques in chloral hydrate anesthetized male rats. Neuronal activity was studied with respect to firing rate, burst firing and regularity of firing. In the ventral tegmental area (VTA) low doses of (R)-8-OH-DPAT (2-32 micrograms/kg i.v.) caused an increase in all three parameters. The effect on firing rate of DA neurons was more pronounced in the parabrachial pigmentosus nucleus than in the paranigral nucleus, the two major subdivisions of VTA. In the substantia nigra zona compacta (SN-ZC), (R)-8-OH-DPAT (2-256 micrograms/kg i.v.) had no effect on firing rate and regularity of firing and only slightly increased burst firing. High doses of (R)-8-OH-DPAT (512-1024 micrograms/kg i.v.) decreased the activity of DA cells in both areas, an effect that was prevented by pretreatment with the selective DA D2 receptor antagonist raclopride. (S)-UH-301 (100-800 micrograms/kg i.v.) decreased both firing rate and burst firing without affecting regularity of DA neurons in the VTA. In the SN-ZC, (S)-UH-301 decreased the firing rate but failed to affect burst firing and regularity of firing. These effects of (S)-UH-301 were blocked by raclopride pretreatment. Local application by pneumatic ejection of 8-OH-DPAT excited the DA cells in both the VTA and the SN-ZC, whereas (S)-UH-301 inhibited these cells when given locally. These results show that 5-HT1A receptor related compounds differentially affect the electrophysiological activity of central DA neurons. The DA receptor agonistic properties of these compound appear to contribute to the inhibitory effects of high doses of (R)-8-OH-DPAT and (S)-UH-301 on DA neuronal activity. Given the potential use of 5-HT1A receptor selective compounds in the treatment of anxiety and depression their effects on central DA systems involved in mood regulation and reward related processes are of considerable importance.

Prazosin modulates the firing pattern of dopamine neurons in rat ventral tegmental area

Previous studies have indicated a noradrenergic modulation of midbrain dopamine cell activity. The effects of systemic administration of the alpha 1-adrenoceptor antagonist prazosin and the alpha 2-antagonist idazoxan on midbrain dopamine cell firing were now studied with extracellular recording from single dopamine neurons in the ventral tegmental area of chloral hydrate-anaesthetized male rats. Prazosin (0.15-0.6 mg/kg i.v.) dose dependently decreased burst firing and regularized the firing pattern of dopamine neurons, while the firing rate was unaffected. The prazosin-induced effects were abolished by pretreatment with reserpine. Idazoxan (0.5-2.0 mg/kg i.v.) increased firing rate and burst firing and made the firing pattern less regular, probably by increasing adrenergic transmission via blockade of presynaptic alpha 2-adrenoceptors. The effects of idazoxan were blocked by prazosin. The present results indicate that noradrenergic neurons modulate the dopamine cell firing pattern via excitatory postsynaptic alpha 1-adrenoceptors. This mechanism might be involved in the pathogenesis and pharmacological treatment of schizophrenia.

Effects of dizocilpine (MK-801) on rat midbrain dopamine cell activity: differential actions on firing pattern related to anatomical localization

The effects of the non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist dizocilpine ((+)-MK-801) on the firing pattern of midbrain dopamine neurons were studied with single cell recording techniques in male albino rats anaesthetized with chloral hydrate. The extracellularly recorded electrical activity of single, identified dopamine neurons was studied with respect to firing rate, burst firing and regularity of firing. MK-801 (0.01-1.0 mg/kg IV) induced different effects in different subgroups of midbrain dopamine neurons. In the substantia nigra, firing rate was increased while the pattern was regularized and burst firing slightly increased. In the ventral tegmental area, firing rate and regularity of firing was also increased while effects on burst firing were bidirectional. Histological inspections revealed that neurons which responded with an increase in burst firing were mainly located in the nucleus paranigralis subdivision of the ventral tegmental area, while cells responding with a decrease were predominantly found in the nucleus parabrachialis pigmentosus subdivision. The effects of MK-801 were similar to previously described effects of phencyclidine, another non-competitive NMDA antagonist. The present effects of MK-801 might shed some light on the mechanisms involved in psychotic symptoms induced by phencyclidine and other non-competitive NMDA antagonists.

Tonic activation of NMDA receptors causes spontaneous burst discharge of rat midbrain dopamine neurons in vivo

Midbrain dopamine neurons in vivo discharge in a single-spike firing pattern or in a burst-firing pattern. Such activity in vivo strikingly contrasts with the pacemaker activity of the same dopamine neurons recorded in vitro. We have recently shown that burst activity in vivo of midbrain dopamine neurons is due to the local activation of excitatory amino acid receptors, as microapplication of the broad-spectrum antagonist of excitatory amino acids, kynurenic acid, strongly regularized the spontaneous firing pattern of these dopamine neurons. In the present study, we investigated which subtypes of excitatory amino acid receptors are involved in the burst-firing of midbrain dopamine neurons in chloral hydrate-anaesthetized rats, iontophoretic or pressure microejections of 6-cyano, 7-nitroquinoxaline-2,3-dione (CNQX), a non-N-methyl-D-aspartate (NMDA) receptor antagonist, did not alter the spontaneous burst firing of dopamine neurons (n = 36). In contrast, similar ejections of (+-)2-amino,5-phosphonopentanoic acid (AP-5), a specific antagonist at NMDA receptors, markedly regularized the firing pattern by reducing the occurrence of bursts (n = 52). In addition, iontophoretic ejections of NMDA, but not kainate or quisqualate, elicited a discharge of these dopamine neurons in bursts (n = 20, 12 and 14, respectively). These data suggest that burst-firing of midbrain dopamine neurons in vivo results from the tonic activation of NMDA receptors by endogenous excitatory amino acids. In view of the critical dependency of catecholamine release on the discharge pattern of source neurons, excitatory amino acid inputs to midbrain dopamine neurons may constitute a major physiological substrate in the control of the dopamine level in target areas.

Noradrenergic modulation of midbrain dopamine cell firing elicited by stimulation of the locus coeruleus in the rat

Electrical stimulation techniques were employed in the chloral hydrate anaesthetized male rat to evaluate if the pontine noradrenergic nucleus locus coeruleus can influence the activity of midbrain dopamine neurons in the ventral tegmental area and zona compacta, substantia nigra. Single-pulse locus coeruleus stimulation evoked an excitation, followed by an inhibition, of the electrical activity of single midbrain dopamine neurons. Neither of these responses were observed in animals pretreated with reserpine, implicating noradrenaline as a mediator. The alpha 1-adrenoceptor antagonist prazosin decreased the excitation, while other adrenoceptor antagonists were without general effect. Burst-type stimulation produced only a more long-lasting inhibition. The influence from the locus coeruleus on midbrain dopamine neurons could be important in behavioural situations involving novelty and reward, and might also be of importance for the actions of psychotropic drugs.

Dihydropyridine calcium channel antagonists disrupt migrating myoelectric complexes and counteract intestinal disorders associated with morphine withdrawal diarrhea

The effects of two dihydropyridine (DHP) calcium channel antagonists, nifedipine and nimodipine, on migrating myoelectric complexes (MMCs) of the small intestine were studied in naive and morphine-dependent rats. In addition, the effects of two other calcium channel antagonists, verapamil and diltiazem, on the MMCs were investigated. Nifedipine (1.0-4.0 mg kg-1 intravenously) or nimodipine (1.0-4.0 mg kg-1 intravenously) had an inhibitory effect on the spontaneously occurring MMCs, whereas verapamil (2.5-5.0 mg kg-1 intravenously) or diltiazem (2.5-5.0 mg kg-1 intravenously) had no effect. Bay K 8644 (0.25 mg kg-1 intravenously), a DHP calcium channel agonist, instantly reversed the inhibition induced by nifedipine or nimodipine. When given alone, Bay K 8644 induced irregular spiking activity. In morphine-dependent rats with regular MMCs naloxone (1.0 mg kg-1 intravenously) induced intense spiking activity and profuse diarrhea. Nifedipine (2.0 and 4.0 mg kg-1 intravenously) and nimodipine (2.0 and 4.0 mg kg-1 intravenously) given before naloxone prevented the intense, abstinence-evoked spiking and associated diarrhea. In healthy volunteers nimodipine at an infusion rate of 2 mg h-1 for 4 h did not inhibit the fasting motility pattern. Our findings indicate that DHP-binding sites are involved in the regulation of MMC in the rat and that drugs acting as antagonists at these sites can be used to suppress morphine withdrawal diarrhea and, tentatively, other functional disorders of the intestine.

In vivo release of somatostatin from rat hippocampus and striatum

Rats were implanted with microdialysis probes in hippocampi and striata, and somatostatin-like immunoreactivity (SS-LI) was measured in outflows obtained from awake, freely moving animals 48 and 72 h post implantation. SS-LI was measurable in all dialysates under basal conditions; concentrations were stable and within a narrow range, about 3-6 fmol/ml. Cysteamine (300 mg/kg, s.c.) markedly reduced basal SS-LI concentrations in outflows from hippocampus (P < 0.00001). KCl (100 mM, 10 min) or veratridine (50 microM, 10 min) infusion elevated hippocampal SS-LI output by 55 and 106%, respectively (P's < 0.05). EGTA (10 mM) or tetrodotoxin (2 microM) infusion inhibited the SS-LI release elicited by KCl and veratridine, respectively, without affecting the basal SS-LI outflow. Thus, our results demonstrate that SS-LI is released from rat hippocampus and striatum in vivo, and provide evidence that the peptide may be released in hippocampus by both action potential dependent and independent processes.

Potency mismatch for behavioral and biochemical effects by dopamine receptor antagonists: implications for the mechanism of action of clozapine

Clozapine (3.8-60.0 mumol kg-1) did not produce any alterations in DOPA accumulation (following inhibition of cerebral aromatic L-amino acid decarboxylase) in the prefrontal cortex or in three regions of the neostriatum, i.e. the ventral, the dorso-lateral and the posterior regions, in the rat. In contrast, clozapine produced a reduction in the 5-HTP accumulation in all these brain areas, except for the prefrontal cortex. Raclopride (0.08-20.0 mumol kg-1) produced a marked increase in DOPA accumulation in all four brain regions and an increase in 5-HTP accumulation in the dorso-lateral neostriatum (2.5-20.0 mumol kg-1), but not in the other forebrain regions. Treatment with SCH-23390 (0.4-1.6 mumol kg-1) resulted in increased DOPA accumulation in the ventral and posterior parts of the neostriatum. No other changes in the DOPA or 5-HTP accumulation were seen with SCH-23390. Considering the doses of these three compounds needed for suppression of conditioned avoidance behavior and for the induction of cataleptic rigidity, it is concluded that raclopride produces an increased DA synthesis at much lower doses than those needed for behavioral effects. In contrast, the behavioral effects of SCH-23390 or clozapine precedes effects on brain DA synthesis on the dose-effect curve. In fact, the only biochemical effect of clozapine, which was observed in low, yet behaviorally active doses, was a decrease in forebrain 5-HTP accumulation. In conclusion, the present results demonstrate a mismatch, in different directions for raclopride and SCH-23390, as regards to the doses needed to produce effects on brain dopamine synthesis and on behavior.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of amperozide on psychostimulant-induced hyperlocomotion and dopamine release in the nucleus accumbens

N-Ethyl-4-[4',4'-bis(p-fluorophenyl)-butyl]-1-piperazine carboxamide [amperozide (APZ)] is a novel atypical neuroleptic that appears to selectively act on the limbic system. The present study investigated behavioral and biochemical effects of APZ on either d-amphetamine (AMPH)- or cocaine (COC)-treated rats. Behavior was assessed by locomotor activity measurements. Compared to saline controls, APZ (5 and 10 mg/kg, SC) decreased spontaneous locomotion. AMPH (1.0 mg/kg, SC)- or COC (10 mg/kg, IP)-induced hyperlocomotion was markedly reduced by APZ administered 20 min earlier. Biochemical data were obtained by in vivo microdialysis in freely moving animals. APZ dose dependently increased interstitial concentrations of dopamine (DA, +25%) and its metabolite, homovanillic acid (HVA, +20%), in the nucleus accumbens (NAC). While either AMPH or COC alone increased DA levels (450 and 270%, respectively), pretreatment with APZ had no effect on these increases. In contrast, APZ pretreatment dose dependently attenuated the reduction of DA metabolites induced by both AMPH and COC. Thus, APZ blocked hyperlocomotion induced by psychostimulants without producing correlative changes in DA concentrations in the NAC.

Nicotinic and muscarinic components of rat brain dopamine synthesis stimulation induced by physostigmine

The role of a putative cholinergic control of ascending midbrain dopamine neurons was studied with biochemical methods in the unanaesthetized male albino rat. Post-mortem catechols were measured with high performance liquid chromatography with electrochemical detection. The acetylcholinesterase inhibitor physostigmine (0.5 mg/kg s.c.) enhanced L-dihydroxyphenylalanine (DOPA) and 3,4-dihydroxyphenylacetic acid (DOPAC) levels in both the corpus striatum and limbic areas (nucleus accumbens) after inhibition of aromatic amino acid decarboxylase with NSD-1015, indicating an enhanced synthesis of dopamine in these brain regions. The effect of physostigmine was blocked both in the corpus striatum and in limbic areas by the centrally penetrating muscarinic antagonist scopolamine (1.0 mg/kg s.c.). In contrast, the nicotinic antagonist mecamylamine (1.0 mg/kg s.c.) significantly reduced the stimulatory effect of physostigmine in limbic areas, but not in the corpus striatum. The present results suggest that ascending dopamine neurons are influenced by cholinergic synaptic transmission being mediated mainly by muscarinic receptors as regards the nigrostriatal system, and by both nicotinic and muscarinic receptors as regards the mesolimbic system. The nicotinic influence appears to primarily control phasic activity of the dopamine neurons.

Substance P injection into the dorsal raphe increases blood pressure and serotonin release in hippocampus of conscious rats

Microinjections of substance P (SP, 100 pmol) into the dorsal raphe nucleus (DRN) in conscious rats increased blood pressure and heart rate for 30-40 min. Concomitantly, the extracellular levels of 5-hydroxytryptamine (5-HT) in the ventral hippocampus, monitored by microdialysis, increased by 30% for 20 min compared with the vehicle control. Pretreatment with the 5-HT2 receptor antagonist, ritanserin (1 mg/kg i.v.), prevented the pressor response to SP but not the increase in heart rate. Pretreatment with the partial 5-HT1A receptor agonist, 8-methoxy-2-(N-2-chloroethyl-N-n-propyl)amino tetralin (8-MeO-CLEPAT, 10 micrograms/kg i.v.) prevented the increase in both blood pressure and heart rate. It is suggested that microinjections of SP into the DRN increase blood pressure through activation of serotonergic DRN neurons and that the postsynaptic receptor responsible for the pressor response is of the 5-HT2 type.

The novel 5-HT1A receptor antagonist (S)-UH-301 prevents (R)-8-OH-DPAT-induced decrease in interstitial concentrations of serotonin in the rat hippocampus

Previous studies have demonstrated that the novel 8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)tetralin) analogue (S)-5-fluoro-8-hydroxy-2-(dipropylamino)tetralin ((S)-UH-301) is able to antagonize several behavioural and biochemical effects of the 5-HT1A receptor agonist 8-OH-DPAT in the rat. In the present study in vivo microdialysis was used to evaluate the effects of (S)-UH-301 on interstitial concentrations of 5-hydroxytryptamine (5-HT), its metabolite 5-hydroxyindoloacetic acid (5-HIAA), and the catecholamine metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the dorsal hippocampus of freely moving rats. Furthermore, the effects of (S)-UH-301 on (R)-8-OH-DPAT-induced changes in dialysate hippocampal concentrations of 5-HT and metabolites were examined. Neither 5-HT nor metabolites were significantly influenced by (S)-UH-301 (1.25, 2.5, 5.0 mg/kg s.c.). In contrast, (R)-8-OH-DPAT (100 micrograms/kg s.c.) decreased interstitial concentrations of 5-HT (to 45% of baseline) and 5-HIAA (to 75%), and increased concentrations of DOPAC (to 165%) and HVA (to 155%). Pretreatment with (S)-UH-301 (2.5 mg/kg s.c.) 20 min before (R)-8-OH-DPAT (100 micrograms/kg s.c.) abolished the 5-HT and metabolite response to (R)-8-OH-DPAT. These data indicate that (S)-UH-301 is able to antagonize (R)-8-OH-DPAT-induced biochemical effects in vivo without producing any effects when given alone. Thus, the present study contributes to the characterization of (S)-UH-301 as a 5-HT1A receptor antagonist with low intrinsic activity.

Decreased sensory responsiveness of noradrenergic neurons in the rat locus coeruleus following phencyclidine or dizocilpine (MK-801): role of NMDA antagonism

The effects of the schizophrenomimetic compound phencyclidine (PCP) on baseline activity and sensory-evoked responses of noradrenergic locus coeruleus neurons were studied with extracellular single-cell recording techniques in the chloral hydrate-anaesthetized male albino rat. PCP dose-dependently decreased firing rate, induced a more regular firing pattern of the neurons, and decreased neuronal responses to a peripheral sensory stimulus (electrical stimulation of the hindpaw). These effects of PCP were significantly decreased by pretreatment with reserpine or yohimbine, indicating that the effects of PCP were largely indirect and mediated through noradrenaline, i.e. by inhibition of its re-uptake, resulting in stimulation of alpha 2 autoreceptors. The effects of PCP were, however, mimicked by dizocilpine (MK-801), a selective non-competitive antagonist at excitatory amino acid receptors of the N-methyl-D-aspartate (NMDA) sub-type, suggesting a role also for NMDA receptors in the suppression of sensory responsiveness of locus coeruleus neurons by PCP. In view of the purported physiological role of the locus coeruleus, this effect of PCP may well contribute to the psychotomimetic properties of the drug.

Amperozide and clozapine but not haloperidol or raclopride increase the secretion of oxytocin in rats

The aim of the present study was to investigate whether amperozide, an antipsychotic drug which possesses anti-aggressive and anxiolytic-like properties, stimulates the secretion of oxytocin and if so, by which receptor mechanism. For this purpose, female or male Sprague Dawley rats were given amperozide (0.5, 2.5 and 5.0 mg/kg IP), ritanserin (5.0 mg/kg), raclopride (2.0 mg/kg) and prazosin (1.0 mg/kg) and were subsequently decapitated for collection of blood (30 and 120 min) after injection. Oxytocin levels were measured with radioimmunoassay. Amperozide 2.5 and 5 mg/kg increased plasma levels of oxytocin significantly (P < 0.05 and < 0.001). The effect appeared maximal about 30 min after injection of the drug and oxytocin levels were almost back to basal within 120 min. Similar effects were obtained in female and male rats as well as in animals that were freely fed or food deprived for 24 h. CSF levels of oxytocin were also increased. Ritanserin, a 5-HT2-receptor antagonist but not the D2 receptor antagonist raclopride or the alpha 1-adrenoceptor antagonist prazosin stimulated oxytocin release. In addition, clozapine, a neuroleptic with potent HT2-antagonistic properties, was a potent releaser of oxytocin, whereas haloperidol was without effect. A possible role for oxytocin in the behavioural effects of amperozide and clozapine remains to be explored.

MK-801 prevents the enhanced behavioural response to apomorphine elicited by repeated electroconvulsive treatment in mice

Repeated administration of electroconvulsive stimuli (ECS) to mice once daily for a period of 7 days results in an enhanced locomotor response induced by apomorphine (1.0 mg/kg, IP). Pretreatment (30 min) with the N-methyl-D-aspartate (NMDA) receptor antagonist, MK-801 (0.01-1.0 mg/kg IP), suppressed ECS-induced seizure activity in a dose-dependent manner. MK-801 (0.01 and 0.033 mg/kg, IP) given 30 min before each ECS dose-dependently decreased apomorphine-mediated responses. Administration of MK-801 (0.033 mg/kg IP) 30 min after each convulsion had the same effect. These results indicate that MK-801 can abolish the ECS-induced enhancement of dopamine-mediated behaviour possibly by interfering with postictal processes. Thus, NMDA receptors seem to be involved in the behavioural changes and presumably also in the neural adaptations produced by repeated ECS.

Chronic continuous nicotine treatment causes decreased burst firing of nigral dopamine neurons in rats partially hemitransected at the meso-diencephalic junction

Chronic continuous administration of nicotine (0.125 mg/kg/h, 14 days) to male Sprague-Dawley rats with a partial hemitransection at the meso-diencephalic junction caused a significant reduction in burst firing of remaining dopamine (DA) neurons in the zona compacta, substantia nigra, whereas neither the firing rate nor the number of spontaneously active DA cells per track were altered in comparison with saline-treated, hemitransected controls. The reduced functional activity of the remaining DA cells subjected to nicotine treatment provides a physiological correlate to the previously observed, reduced DA utilization in these neurons. It may also help to explain the increased nigral DA cell survival found after chronic nicotine treatment in similar lesion experiments.