A persistent deficit of serotonin neurons in the offspring of ethanol-fed dams: protective effects of maternal ipsapirone treatment

An earlier study from this laboratory found a significant reduction in the density of serotonin (5-HT) neurons in the dorsal and median raphe and in the B9 complex of postnatal day 5 (PN5) offspring of female rats that consumed ethanol on a chronic basis prior to parturition. In addition, we demonstrated that maternal treatment with the 5-HT(1A) agonist ipsapirone (3 mg/kg) prevented the ethanol-associated reduction in 5-HT neurons. The present investigation examined whether there was a persistent deficit of 5-HT-immunopositive neurons in the dorsal and median raphe of the offspring of ethanol-fed dams. We also evaluated whether a lower ipsapirone dose (1 mg/kg) was protective to developing 5-HT neurons in the offspring of ethanol-fed dams. The offspring of ethanol-fed dams exhibited an apparent lasting reduction in the density of 5-HT neurons in the dorsal and median raphe. The density of 5-HT neurons in control offspring was comparable at PN5 and PN19, but at both ages the offspring of ethanol-fed dams had a significant deficit of 5-HT neurons in the dorsal and median raphe. The lack of recovery in the density of 5-HT-immunopositive neurons in the offspring of ethanol-fed dams between PN5 and PN19 suggests and that the reduction was long lasting. The protective effects of ipsapirone appeared to be dose dependent. The density of 5-HT neurons in the dorsal and median raphe of PN5 (prior study) and PN19 offspring of ethanol-fed dams that were treated with 3 mg/kg of ipsapirone between gestational day 13 (G13) and G20 was comparable to that of control offspring. However, the effects of maternal treatment of ethanol-fed dams with the 1 mg/kg dose were variable, and some abnormalities were detected in the offspring of ipsapirone-treated control dams.     

Modafinil enhances the increase of extracellular serotonin levels induced by the antidepressant drugs fluoxetine and imipramine: a dual probe microdialysis study in awake rat

In view of a postulated role of the vigilance-promoting drug modafinil in depression, the interaction of modafinil and two classical antidepressant drugs, fluoxetine and imipramine, were studied in 5-HT levels in the dorsal raphe-cortical system using dual-probe microdialysis. Fluoxetine (1-10 mg/kg) dose-dependently increased dorsal raphe-cortical 5-HT levels. Modafinil at a very low dose (3 mg/kg), by itself ineffective, enhanced the fluoxetine (5 mg/kg)-induced increases of 5-HT levels in both brain areas. A synergistic interaction was observed in the prefrontal cortex with fluoxetine (1 mg/kg) in terms of 5-HT release, but not in the dorsal raphe. Imipramine (1.3 mg/kg) increased 5-HT levels in the dorsal raphe, but not in the prefrontal cortex, while the higher doses (10.9-21.8 mg/kg) caused substantial increases in both brain areas. Modafinil (3 mg/kg), injected before imipramine (1.3 mg/kg), which by itself was ineffective on cortical 5-HT levels, increased cortical 5-HT levels. On other hand, modafinil failed to affect the high-dose imipramine (10.9 mg/kg)-induced increase of 5-HT levels in the prefrontal cortex and the imipramine (1.3; 10.9 mg/kg)-induced increase of 5-HT levels in the dorsal raphe nucleus. These results demonstrate that modafinil in low doses enhances the acute effects of fluoxetine and imipramine on 5-HT levels in the dorsal raphe nucleus (fluoxetine only) and especially in the prefrontal cortex of the awake rat. These findings suggest a therapeutic potential of low doses of modafinil in the treatment of depression when combined with low doses of classical antidepressants, especially by increasing 5-HT transmission in cortical regions.     

Differential responsiveness of the rat dorsal and median raphe 5-HT systems to 5-HT1 receptor agonists and p-chloroamphetamine

The dorsal and median raphe 5-HT neurons give rise to projections that differ in axon morphology and in vulnerability to certain amphetamine derivatives. The present study was undertaken to determine if these two 5-HT systems possess different functional properties. To this end, we studied the effects of selective 5-HT1A or 5-HT1A/5-HT1B receptor agonists and of p-chloroamphetamine on extracellular levels of indoleamines, as measured by differential pulse voltammetry with extracellular levels of indoleamines, as measured by differential pulse voltammetry with electrochemically pretreated carbon fiber electrodes, in cell body and nerve terminal regions of these subsets of 5-HT neurons in the rat brain. The selective 5-HT1A agonist 8-OH-DPAT produced a gradual decrease in the height of the 300 mV oxidation peak in the dorsal raphe and in the frontal cortex, reaching a maximum of 60% 3 h after the i.v. injection of 30 micrograms/kg. However, the same dose of 8-OH-DPAT was ineffective in the median raphe and in the dentate gyrus that receives its 5-HT innervation exclusively from the median raphe. A higher dose of 8-OH-DPAT (150 micrograms/kg, i.v.) produced a 60% decrease in the height of the 300 mV oxidation peak in the median raphe, whereas only a 20% decrease was obtained in the dentate gyrus. In contrast, the non-selective 5-HT1 agonist RU 24,969 (10 mg/kg, i.p.) caused a 70% reduction of the 300 mV peak height in both the dorsal and median raphe and a 50% decrease in both the frontal cortex and the dentate gyrus. Moreover, although a high dose of 8-OH-DPAT (150 micrograms/kg, i.v.) given alone reduced by 20% the amplitude of the oxidative peak in the dentate gyrus, subsequent administration of RU 24,969 (10 mg/kg, i.p.) caused a further 30% diminution of the oxidative peak height. The greater responsiveness of dorsal as compared to median raphe 5-HT systems to 5-HT1A receptor agonists was confirmed in two further series of experiments. First, the microiontophoretic application of 8-OH-DPAT directly onto 5-HT neurons was three times more potent in suppressing the firing rate of dorsal raphe 5-HT neurons than that of their median raphe congeners. Second, 8-OH-DPAT and buspirone were ten and four times, respectively, more potent in decreasing 5-HT synthesis in the frontal cortex than in the hippocampus.(ABSTRACT TRUNCATED AT 400 WORDS)     

Effects of short-term serotonin depletion on the efficacy of serotonin neurotransmission: electrophysiological studies in the rat central nervous system

The effects of short-term serotonin (5-HT) depletion by p-chlorophenylalanine (PCPA) on the firing activity of dorsal raphe nucleus 5-HT neurons, on the responsiveness of dorsal hippocampus pyramidal neurons to microiontophoretically applied 5-HT and on the efficacy of the electrical stimulation of the ascending 5-HT pathway in suppressing the firing activity of CA3 dorsal hippocampus pyramidal neurons were assessed in chloral hydrate-anesthetized rats. PCPA (250 mg/kg/day i.p. for 2 days) reduced the 5-HT content of the dorsal hippocampus by 90%. However, the number of spontaneously active 5-HT neurons per microelectrode trajectory through the dorsal raphe or their average rate of firing was unaltered. The effect of afferent 5-HT pathway stimulation was reduced in only 40% of treated rats, whereas the sensitivity of CA3 pyramidal neurons to microiontophoretic 5-HT was not modified. The function of the terminal 5-HT autoreceptor was assessed using methiothepin, an autoreceptor antagonist. Methiothepin (1 mg/kg, i.v.) significantly enhanced the efficacy of the stimulation in PCPA-treated rats, although the degree of enhancement was much less than in controls. A greater reduction of the effectiveness of the stimulation was obtained by increasing the dose of PCPA (350 mg/kg/day i.p. for 2 days). This regimen reduced the 5-HT content of the dorsal hippocampus by 95%. In these rats, the sensitivity of the terminal 5-HT autoreceptor was assessed by increasing the frequency of the stimulation from 1 to 5 Hz. This procedure reduced to a similar extent the efficacy of the stimulation in treated and control rats, suggesting that the reduced effectiveness of methiothepin in enhancing 5-HT synaptic transmission in PCPA-treated rats is due to a lower degree of activation of the terminal 5-HT autoreceptor. The present results showing that the 350 mg/kg/day regimen of PCPA, but not the 250 mg/kg/day regimen, reduced the efficacy of the stimulation of the ascending 5-HT pathway suggest that a greater than 90% depletion is required to affect 5-HT neurotransmission significantly. The reduced level of activation of terminal 5-HT autoreceptors in rats treated with the lower dose of PCPA may facilitate the release of the remaining 5-HT per stimulation-triggered action potential, ensuring a virtually unaltered synaptic efficacy.     

Neurochemical and electrophysiological evidence that 5-HT4 receptors exert a state-dependent facilitatory control in vivo on nigrostriatal, but not mesoaccumbal, dopaminergic function

In this study we investigated, using in vivo microdialysis and single unit recordings, the role of serotonin4 (5-HT4) receptors in the control of nigrostriatal and mesoaccumbal dopaminergic (DA) pathway activity. In freely moving rats, the 5-HT4 antagonist GR 125487 (1 mg/kg, i.p.), without effect on its own, significantly reduced the enhancement of striatal DA outflow induced by 0.01 (-35%) and 0.1 (-66%), but not 1 mg/kg, s.c. haloperidol (HAL). Intrastriatal infusion of GR 125487 (1 microM) had no influence on basal DA outflow, but attenuated (-49%) the effect of 0.01 mg/kg HAL. Systemic administration of GR 125487 modified neither basal nor 0.01 mg/kg HAL-stimulated accumbal DA outflow. In halothane-anaesthetized rats, 1 or 10 mg/kg GR 125487, without effect by itself, failed to modify the changes in accumbal and striatal DA outflow elicited by electrical stimulation (300 microA, 1 ms, 20 Hz, 15 min) of the dorsal raphe nucleus. Finally, GR 125487 (444 microg/kg, i.v.), whilst not affecting basal firing of DA neurons within either the substantia nigra or the ventral tegmental area, reduced HAL-stimulated (1--300 microg/kg, i.v.) impulse flow of nigrostriatal DA neurons only. These results indicate that 5-HT4 receptors exert a facilitatory control on both striatal DA release and nigral DA neuron impulse flow only when nigrostriatal DA transmission is under activated conditions. Furthermore, they indicate that the striatum constitutes a major site for the expression of the control exerted by 5-HT4 receptors on DA release. In contrast, 5-HT4 receptors have no influence on mesoaccumbal DA activity in either basal or activated conditions.     

Modulation of the firing activity of rat serotonin and noradrenaline neurons by (+/-)pindolol.

BACKGROUND: (+/-)Pindolol is a beta-adrenergic/5-HT1A receptor antagonist used in combination with certain antidepressant drugs to accelerate the onset of the antidepressive response. METHODS: The aim of the present study was to assess, using an in vivo electrophysiologic paradigm, the effect of (+/-)pindolol on the spontaneous firing activity of rat dorsal raphe serotonin (5-HT) and locus coeruleus noradrenaline (NA) neurons. RESULTS: (+/-)Pindolol did not modify the firing activity of dorsal raphe 5-HT neurons at low doses (10 and 200 micrograms/kg, i.v.), but it prevented the suppressant effect of the 5-HT autoreceptor agonist lysergic acid diethylamide (LSD, 10 micrograms/kg, i.v.) but not that of the 5-HT1A receptor 8-hydroxy-N,N-dipropyl-aminotetralin (8-OHDPAT, 5 micrograms/kg, i.v.). At a higher dose (500 micrograms/kg, i.v.), (+/-)pindolol decreased 5-HT neuronal firing and this effect was reversed by the selective 5-HT1A receptor antagonist WAY 100635 (100 micrograms/kg, i.v.), suggesting that it could act as a partial 5-HT1A autoreceptor agonist. In the locus coeruleus, the high dose of (+/-)pindolol decreased the firing activity of NA neurons and this effect was reversed by the 5-HT2A receptor antagonist MDL 100907 (200 micrograms/kg, i.v.). Finally, both a lesion of NA neurons and the administration of MDL 100907 prevented the suppressant effect of (+/-)pindolol on the firing of 5-HT neurons. CONCLUSIONS: It is suggested that, at low doses, (+/-)pindolol acts as a somatodendritic 5-HT1A autoreceptor antagonist whereas at a higher dose, it decreases the tonic excitatory input from NA neurons to 5-HT neurons.     

Activation of postsynaptic 5-HT(1A) receptors by fluoxetine despite the loss of firing-dependent serotonergic input: electrophysiological and neurochemical studies

Systemic doses of fluoxetine slow dorsal raphe cell firing by blocking the reuptake carrier located in the cell body region with the surplus 5-HT thus generated activating inhibitory autoreceptors. The concurrent actions of fluoxetine on postsynaptic receptors in raphe projection areas has not been as thoroughly investigated, although it is presumed that a reduction in cell firing should curtail these targeted effects. The goal of the present studies was to assess the degree of postsynaptic receptor activation obtained with fluoxetine and relate it to cell body autoreceptor activation and the level of extracellular 5-HT obtained at the nerve endings. Changes in firing rates of CA3 hippocampal neurons following systemic administration of fluoxetine were used as a marker of SSRI-dependent changes in postsynaptic 5-HT receptor activation; monitoring of unit activity of neurons in the dorsal raphe nucleus served to gauge the degree of serotonergic input in a parallel series of animals. Estimates of the corresponding changes in terminal 5-HT release in the CA3 region were analyzed by microdialysis. The results indicate that fluoxetine inhibits hippocampal cell firing in a dose-dependent manner (ED(50) = 4.4 mg/kg i.v.) and one sensitive to pretreatment with the 5-HT(1A) antagonist WAY-100,635. Within the same dose range, increases in hippocampal extracellular 5-HT approaching 300% above basal levels were achieved. Both the changes in hippocampal neuronal activity and extracellular 5-HT are evident at doses of fluoxetine in excess of that required to inhibit dorsal raphe cell firing (ED(50) = 1.1 mg/kg i.v.). Taken together, these data suggest that increases in extracellular levels of 5-HT on the order of that observed are sufficient to alter postsynaptic excitability and that this accumulation of synaptic 5-HT and the subsequent activation of postsynaptic 5-HT(1A) receptors are achievable despite loss of firing-dependent 5-HT release.     

Pindolol excites dopaminergic and adrenergic neurons, and inhibits serotonergic neurons, by activation of 5-HT1A receptors

Pindolol accelerates the clinical actions of selective serotonin reuptake inhibitors (SSRIs) in man, and modulates extracellular levels of monoamines in corticolimbic structures in rats. Herein, we examined its influence upon electrical activity of serotonergic, dopaminergic and adrenergic perikarya in the dorsal raphe nucleus (DRN), ventral tegmental area (VTA) and locus coeruleus (LC) of anaesthetized rats. In analogy to the serotonin1A (5-HT1A) agonist, 8-OH-DPAT (-100%), pindolol dose-dependently (0.063- 1.0 mg/kg) decreased (-70%) the firing rate of serotonergic neurons. The inhibitory action of pindolol was abolished by the selective 5-HT1A antagonist, WAY-100,635 (0.031 mg/kg). In contrast, 8-OH-DPAT (+26%) and pindolol (0.25-4.0 mg/kg, +60%) dose-dependently increased the firing rate of dopaminergic cells. Of 57 neurons recorded (pindolol, 2.0 mg/kg), 36 (63%) were excited, 11 (19%) were unaffected and 10 (18%) were inhibited. This variable influence could be attributed to regularly firing neurons in the parabrachial subdivision, inasmuch as all neurons in the paranigral subnucleus were excited. The facilitation of firing by pindolol was accompanied by an increase in burst firing throughout the VTA. Both the increases in burst firing and in firing rate were reversed by WAY-100,635 (0.031 mg/kg). Finally, the electrical activity of adrenergic neurons was dose-dependently enhanced by 8-OH-DPAT and pindolol (+99% and +83%, respectively). WAY-100,635 reversed this excitation and, itself, inhibited the activity of adrenergic neurons. In conclusion, via engagement of 5-HT1A receptors, pindolol inhibits serotonergic, and activates dopaminergic and adrenergic, neurons in anaesthetized rats. Such actions may contribute to its influence upon mood, both alone and in association with antidepressant agents.     

Opposite effects of low versus high dose haloperidol treatments on spontaneous and apomorphine induced motor behavior: evidence that at a very low dose haloperidol acts as an indirect dopamine agonist

Anti-psychotic drugs are antagonists at the dopamine D2 receptors and repeated administration can lead to the development of dopamine receptor supersensitivity. In two experiments, separate groups of rats were administered 10 daily low or high doses of the typical anti-psychotic drug haloperidol (0.03 or 1.0 mg/kg). The high dose decreased locomotion whereas, the low dose increased locomotion. After 5 days of withdrawal, all groups received 2.0 mg/kg apomorphine on 5 successive days. The apomorphine treatments given to the vehicle group generated a progressive locomotion sensitization effect and this effect was potentiated by pre-exposure to 0.03 mg/kg haloperidol. Initially, the prior high dose of haloperidol exaggerated the apomorphine locomotor stimulant effect but with repeated apomorphine treatments desensitization developed. Following a 5-day withdrawal period an apomorphine challenge test was conducted and apomorphine sensitization was absent in the haloperidol high dose pre-exposure group but potentiated in the low dose pre-exposure group. In the second replication experiment a conditioning test instead of a sensitization challenge test was conducted 5 days after completion of the 5-day apomorphine treatment protocol. The repeated apomorphine treatments induced conditioned hyper- locomotion and this conditioned effect was prevented by the prior high dose haloperidol pre-exposure but enhanced by the prior low dose haloperidol pre-exposure. Two new key findings are (a) that a low dose haloperidol regimen can function as a dopamine agonist and these effects persist after withdrawal and (b) that repeated apomorphine treatments can desensitize D2 receptors previously sensitized by a high dose haloperidol treatment regimen.     

Differential enhancement of dialysate serotonin levels in distinct brain regions of the awake rat by modafinil: possible relevance for wakefulness and depression

The present in vivo microdialysis study evaluates the possible existence of a differential regulation of serotonergic transmission by the antinarcoleptic drug modafinil [(diphenyl-methyl)-sulfinyl-2-acetamide; Modiodal] among various brain regions of the awake rat. The results show that, in the cerebral cortex, the central amygdala, and the dorsal raphe nucleus, modafinil in the dose range of 10-100 mg/kg i.p. dose-dependently increases dialysate serotonin (5-HT) levels. In other brain areas, such as the medial preoptic area and the posterior hypothalamus, the modafinil-induced increase in dialysate 5-HT levels is observed only at tenfold higher doses (100 mg/kg), 10-30 mg/kg being ineffective. Together these data suggest that, in the frontal cortex, the amygdala, and the dorsal raphe, modafinil is more potent in enhancing extracellular 5-HT levels and presumably 5-HT transmission than in the medial preoptic area and the posterior hypothalamus. In view of the role of ascending 5-HT pathways in arousal and depression, it seems likely that the antinarcoleptic drug modafinil may also have an antidepressant potential in addition to its wakefulness-promoting action, both actions involving enhancement of 5-HT neurotransmission.     

Assessment of the serotonin reuptake blocking property of YM992: electrophysiological studies in the rat hippocampus and dorsal raphe.

YM992 is a selective serotonin (5-HT) reuptake inhibitor and a 5-HT(2A) antagonist with potential antidepressant activity. As expected from a 5-HT reuptake inhibitor, which induces an accumulation of 5-HT in the dorsal raphe, YM992 inhibited the firing activity of these 5-HT neurons (ED50: 2.0+/-0.2 mg/kg, i.v.). This effect was reversed by the 5-HT(1A) antagonist WAY 100635. YM992 also dose-dependently prolonged the time for CA3 neurons to recover 50% of their firing rate following microiontophoretic applications of 5-HT, a reliable index of the function of the 5-HT reuptake carrier. In a second series of experiments, the adaptative properties of 5-HT neurons were examined during sustained administration of YM992 (20 mg/kg/day, s.c., delivered by osmotic minipumps) after 2 days of treatment. YM992 decreased by more than 60% the firing activity of the 5-HT neurons. There was a partial recovery of firing after 7 days and a complete one after 14 days of treatment in the presence of the minipump still delivering the drug. In a third series of experiments, the sensitivity of pre- and postsynaptic 5-HT(1A) receptors in the dorsal raphe and the dorsal hippocampus were assessed. The results showed that YM992 attenuated the inhibitory effect of intravenous administration of LSD and the 5-HT(1A) agonist 8-OH-DPAT on the firing activity of 5-HT neurons. As did the selective 5-HT reuptake inhibitor fluvoxamine, YM992 markedly increased the effectiveness of the electrical stimulation of ascending 5-HT fibres on firing activity of the postsynaptic hippocampus pyramidal neurons. This enhancement of 5-HT neurotransmission by YM992 was attributable to a desensitization of the terminal 5-HT(1B) autoreceptors since the postsynaptic 5-HT(1A) receptors in the hippocampus remained normosensitive.     

Remoxipride, a new selective D2 antagonist, and haloperidol in cebus monkeys

1. Nine Cebus monkeys, 6 with mild spontaneous oral dyskinesia (tongue protrusions), were tested with two dopamine D2 antagonists, remoxipride (a new substituted benzamide) and haloperidol, and with two dopamine agonists, methylphenidate and apomorphine. 2. Remoxipride 4 and 8 mg/kg and haloperidol 0.01 and 0.02 mg/kg given alone induced identical dystonic-dyskinetic syndromes. 3. Methylphenidate 0.5 mg/kg caused increased arousal, but reduced oral dyskinesia, while apomorphine 0.25 mg/kg slightly increased arousal and induced/aggravated oral dyskinesia. 4. Remoxipride 2 and 4 mg/kg and haloperidol 0.005 and 0.01 mg/kg equally antagonized the methylphenidate- and apomorphine-induced arousal, but not oral dyskinesia. 5. Marked sedation was seen when apomorphine was given together with either D2 receptor antagonists. 6. It is concluded that remoxipride and haloperidol have a similar qualitative effect in motor behavior in Cebus monkeys, but the quantitative difference between the dystonia-inducing dose levels of the two drugs compared with the antipsychotic dose levels (estimated from clinical studies) suggests that remoxipride may cause relatively few extrapyramidal side-effects in human.     

Hypothalamic alpha-melanocyte-stimulating hormone (alpha-MSH) is not under dopaminergic control

A possible dopaminergic regulation of hypothalamic proopiomelanocortin (POMC)-containing neurons has been investigated in rats by means of in vivo and in vitro approaches. Acute or 3-weeks chronic in vivo treatments with the dopaminergic agonists apomorphine (1 mg/kg: s.c.) and 2-Br-alpha-ergocriptine (2.5 mg/kg; s.c.) or the dopaminergic antagonist haloperidol (0.15-3 mg/kg; i.p.) had no significant effect on the concentration of alpha-melanocyte-stimulating hormone (alpha-MSH) in two hypothalamic regions: arcuate nucleus (AN) and dorsolateral area (DLH). In the same way, chronic administration of the dopaminergic agonists or antagonist did not induce any change in hypothalamic contents of beta-endorphin, another peptide derived from POMC. Reverse-phase high-performance liquid chromatographic analysis revealed that acetic acid extracts of AN and DLH both contained two major forms of alpha-MSH-like peptides: deacetylated alpha-MSH and authentic alpha-MSH. The ratio between these two forms was not altered after acute haloperidol treatment (3 mg/kg, i.p.). The possible effect of dopamine on the release of hypothalamic alpha-MSH was studied in vitro using perifused rat hypothalamic slices. Infusion of dopamine (10(-7)-10(-5)M) or its antagonist haloperidol (10(-5)M) had no effect on spontaneous alpha-MSH release from hypothalamic tissue. In addition, none of these drugs had any effect on potassium (50 mM)-induced alpha-MSH release. It is concluded that dopaminergic neurons are not involved in the regulation of synthesis, post-translational processing (acetylation) or release of hypothalamic alpha-MSH.     

R-citalopram prevents the neuronal adaptive changes induced by escitalopram

This study examined the long-term effects of the antidepressant escitalopram on rat serotonin (5-HT) neuronal activity and hippocampal neuroplasticity. In the dorsal raphe nucleus, a 2-week treatment with escitalopram (10 mg/kg/day, subcutaneous) did not modify the firing activity of 5-HT neurons, whereas a cotreatment with R-citalopram (20 mg/kg/day, subcutaneous) decreased it. In the dentate gyrus of dorsal hippocampus, escitalopram increased significantly (57%) the number of de novo cells and this was prevented by a cotreatment with R-citalopram. The present results support the role of the allosteric modulation of the 5-HT transporter in the regulation of the recovery of 5-HT neuronal activity and long-lasting hippocampal cellular plasticity induced by escitalopram, two adaptive changes presumably associated with the antidepressant response.     

Immunocytochemical survey of haloperidol-induced immunoreactive changes of [Met]enkephalin-Arg6-Gly7-Leu8 in the rat forebrain.

It has already been demonstrated that chronic treatment with the dopamine receptor blocker, haloperidol, results in an increase of proenkephalin-A-derived peptides in the caudate-putamen (CP). To examine this phenomenon at the cellular level, we used immunocytochemistry to investigate the effects of haloperidol on [Met]enkephalin-Arg6-Gly7-Leu8 (MEAGL) immunoreactivity in the rat forebrain. After daily haloperidol (5 mg/kg, IP, for 6 days) or haloperidol decanoate (70 mg/kg, IM, given once or twice) treatment, immunoreactive neurons appeared diffusely in the whole CP and in the core part of the nucleus accumbens (Acb) and less frequently in the outer shell part of the Acb and the cell-dense layer of the tuberculum olfactorium (TuO). Increase of MEAGL-immunoreactive fibers in the CP, Acb, and TuO was also detected after these treatments, a particularly prominent increase being found in the striopallidal terminals in the globus pallidus and ventral pallidum. Haloperidol or haloperidol decanoate had no effect on MEAGL immunoreactivity in the cerebral cortex, amygdala, or hypothalamus. Reserpine treatment (5 mg/kg, IP, for 6 days) caused similar effects on the dorsal and ventral striopallidal system, and the direct injection of 6-hydroxydopamine (10 micrograms/5 microliters) into the CP led to the appearance of MEAGL-immunoreactive neurons in accordance with the depleted dopaminergic terminal area. These findings suggest that haloperidol influences enkephalinergic neurons region specifically and that in the dorsal and ventral striopallidal enkephalinergic system haloperidol increases MEAGL immunoreactivity in cell bodies, fibers, and terminals by blocking intrastriatal dopaminergic neurotransmission.     

The effect of lysergic and diethylamide (LSD) and 2-bromolysergic acid diethylamide (BOL) on the striatal DOPA accumulation: influence of central 5-hydroxytryptaminergic pathways.

Selective chronic lesions of the dorsal raphe nucleus or combined lesions of the dorsal and median raphe nuclei did not significantly change the in vivo tyrosine hydroxylation in the striatum as measured by the DOPA accumulation after decarboxylase inhibition. Neither did acute combined lesions of the raphe nuclei, nor did electrical stimulation of the dorsal raphe nucleus have any significant effect. p-Chloroamphetamine (PCA, 20 mg/kg, i.p.) and p-chlorophenylalanine (PCPA, 400 mg/kg, i.p.), known inhibitors of the 5-hydroxytryptamine (5-HT) synthesis, significantly decreased the DOPA accumulation. The increase in DOPA accumulation observed after LSD (0.5 mg/kg, i.p.) or BOL (0.5 mg/kg, i.p.) was seemingly unaffected by pretreatment with PCA or PCPA and also after lesion of the dorsal raphe nucleus. The results suggest that the effect of LSD or BOL on the DOPA accumulation in the striatum is not mediated via a 5-hydroxytryptaminergic control mechanism originating in the dorsal raphe nucleus. A control mediated via the median raphe nucleus cannot be excluded, since LSD did not increase the DOPA accumulation after combined chronic raphe lesions. Such a control would also be in agreement with our previous results suggesting that hte DOPA generation after LSD is controlled by 5-HT receptors.     

Clozapine and haloperidol differentially alter the constitutive activity of central serotonin2C receptors in vivo.

BACKGROUND: Central serotonin2C (5-HT2C) receptors are known to play a role in the mechanism of action of the antipsychotic drugs (APDs) clozapine and haloperidol. However, evidence for the involvement of the constitutive activity of 5-HT2C receptors in the dopamine (DA)ergic effects of APDs is lacking in vivo. METHODS: Using in vivo microdialysis in halothane-anesthetized rats, we assessed the ability of selective 5-HT2C compounds to modulate the release of DA induced by haloperidol and clozapine in the nucleus accumbens and striatum. RESULTS: Both APDs induced a dose-dependent increase in accumbal and striatal DA extracellular levels. The effect of .01 mg/kg haloperidol was potentiated by the 5-HT2C inverse agonist SB 206553 (5 mg/kg) but unaltered by the 5-HT2C antagonists SB 243213 and SB 242084 (1 mg/kg). Conversely, the effect of 1 mg/kg clozapine, a dose able to reverse the decrease in DA outflow induced by the 5-HT2C agonist Ro 60-0175 (3 mg/kg), was unaffected by SB 206553 but blocked by SB 243213 (1 mg/kg) and SB 242084 (.3 and 1 mg/kg). CONCLUSIONS: These results show that clozapine and haloperidol differentially alter the constitutive activity of 5-HT2C receptors and suggest that clozapine behaves as a 5-HT2C inverse agonist in vivo.     

Agonist-antagonist interaction on dopamine receptors in brain,as reflected in the rates of tyrosine and tryptophan hydroxylation

Summary The effect of haloperidol and apomorphine, and both drugs in combination, on the first steps in the synthesis of catecholamines and 5-hydroxytryptamine (5-HT) has been studied in three rat brain regions. The rate of formation of dopa and 5-hydroxytryptophan (5-HTP) was studied by measuring the accumulation of these amino acids during 30 min after administration of the inhibitor of the aromaticL-amino acid decarboxylase, NSD 1015 (3-hydroxybenzylhydrazine HCl).Haloperidol caused an increase in dopa and no change in 5-HTP formation. The threshold dose was severalfold higher in the noradrenalinepredominated hemisphere portion than in the dopamine-rich striatal and limbic regions, suggesting a higher affinity of haloperidol for dopamine than for noradrenaline receptors.Apomorphine caused a decrease in dopa formation in all three brain regions studied, although the effect was much more pronounced in the regions dominated by dopamine. The threshold dose was about 30g/kg,i.e. an order of magnitude lower than the threshold dose for apparent postsynaptic dopaminergic: receptor activation. This discrepancy is suggested to be due to preferential activation of inhibitory dopaminergic autoreceptors by low apomorphine doses. This phenomenon may also contribute to explain the complex dose-response curves of apomorphine.Low doses of apomorphine caused a decrease and high doses an increase in 5-HTP formation. These effects, like those on noradrenaline synthesis, are suggested to be secondary to activation of dopaminergic pre-and postsynaptic receptors.The interaction between apomorphine and haloperidol with respect to dopa formation appears to be largely explicable on the assumption of a competition between an agonist and an antagonist for dopaminergic receptors. However, very large doses of apomorphine cause a haloperidolresistant inhibition of tyrosine, and probably also tryptophan, hydroxylation, which may be due to a direct inhibition of the aromatic amino acid hydroxylase involved.     

Regulators of G-protein signaling 4: modulation of 5-HT1A-mediated neurotransmitter release in vivo

Regulators of G-protein signaling (RGS) play a key role in the signal transduction of G-protein-coupled receptors (GPCRs). Specifically, RGS proteins function as GTPase accelerating proteins (GAPs) to dampen or "negatively regulate" GPCR-mediated signaling. Our group recently showed that RGS4 effectively GAPs Galpha(i)-mediated signaling in CHO cells expressing the serotonin-1A (5-HT(1A)) receptor. However, whether a similar relationship exists in vivo has yet to be identified. In present studies, a replication-deficient herpes simplex virus (HSV) was used to elevate RGS4 mRNA in the rat dorsal raphe nuclei (DRN) while extracellular levels of 5-HT in the striatum were monitored by in vivo microdialysis. Initial experiments conducted with noninfected rats showed that acute administration of 8-OH-DPAT (0.01-0.3 mg/kg, subcutaneous [s.c.]) dose dependently decreased striatal levels of 5-HT, an effect postulated to result from activation of somatodendritic 5-HT(1A) autoreceptors in the DRN. In control rats receiving a single intra-DRN infusion of HSV-LacZ, 8-OH-DPAT (0.03 mg/kg, s.c.) decreased 5-HT levels to an extent similar to that observed in noninfected animals. Conversely, rats infected with HSV-RGS4 in the DRN showed a blunted neurochemical response to 8-OH-DPAT (0.03 mg/kg, s.c.); however, increasing the dose to 0.3 mg/kg reversed this effect. Together, these findings represent the first in vivo evidence demonstrating that RGS4 functions to GAP Galpha(i)-coupled receptors and suggest that drug discovery efforts targeting RGS proteins may represent a novel mechanism to manipulate 5-HT(1A)-mediated neurotransmitter release.     

Mirtazapine enhances frontocortical dopaminergic and corticolimbic adrenergic, but not serotonergic, transmission by blockade of alpha2-adrenergic and serotonin2C receptors: a comparison with citalopram

Mirtazapine displayed marked affinity for cloned, human alpha2A-adrenergic (AR) receptors at which it blocked noradrenaline (NA)-induced stimulation of guanosine-5'-O-(3-[35S]thio)-triphosphate ([35S]-GTPgammaS) binding. Similarly, mirtazapine showed high affinity for cloned, human serotonin (5-HT)2C receptors at which it abolished 5-HT-induced phosphoinositide generation. Alpha2-AR antagonist properties were revealed in vivo by blockade of UK-14,304-induced antinociception, while antagonist actions at 5-HT2C receptors were demonstrated by blockade of Ro 60 0175-induced penile erections and discriminative stimulus properties. Mirtazapine showed negligible affinity for 5-HT reuptake sites, in contrast to the selective 5-HT reuptake inhibitor, citalopram. In freely moving rats, in the dorsal hippocampus, frontal cortex (FCX), nucleus accumbens and striatum, citalopram increased dialysate levels of 5-HT, but not dopamine (DA) and NA. On the contrary, mirtazapine markedly elevated dialysate levels of NA and, in FCX, DA, whereas 5-HT was not affected. Citalopram inhibited the firing rate of serotonergic neurons in dorsal raphe nucleus, but not of dopaminergic neurons in the ventral tegmental area, nor adrenergic neurons in the locus coeruleus. Mirtazapine, in contrast, enhanced the firing rate of dopaminergic and adrenergic, but not serotonergic, neurons. Following 2 weeks administration, the facilitatory influence of mirtazapine upon dialysate levels of DA and NA versus 5-HT in FCX was maintained, and the influence of citalopram upon FCX levels of 5-HT versus DA and NA was also unchanged. Moreover, citalopram still inhibited, and mirtazapine still failed to influence, dorsal raphe serotonergic neurons. In conclusion, in contrast to citalopram, mirtazapine reinforces frontocortical dopaminergic and corticolimbic adrenergic, but not serotonergic, transmission. These actions reflect antagonist properties at alpha2A-AR and 5-HT2C receptors.