Denervation supersensitivity to serotonin in rat forebrain: single cell studies

Mapping the physiological and molecular markers of stress and SSRI antidepressant treatment in S100a10 corticostriatal neurons

In mood disorders, psychomotor and sensory abnormalities are prevalent, disabling, and intertwined with emotional and cognitive symptoms. Corticostriatal neurons in motor and somatosensory cortex are implicated in these symptoms, yet mechanisms of their vulnerability are unknown. Here, we demonstrate that S100a10 corticostriatal neurons exhibit distinct serotonin responses and have increased excitability, compared with S100a10-negative neurons. We reveal that prolonged social isolation disrupts the specific serotonin response which gets restored by chronic antidepressant treatment. We identify cell-type-specific transcriptional signatures in S100a10 neurons that contribute to serotonin responses and strongly associate with psychomotor and somatosensory function. Our studies provide a strong framework to understand the pathogenesis and create new avenues for the treatment of mood disorders.

Failure to mount adaptive responses to stress results in dysregulation and cell death in the midbrain raphe

Stress is a common trigger in affective disorder onset, yet the mechanism and predisposing factors of vulnerability remain unknown. Effective disease prevention requires a critical balance of responses within the serotonergic raphe nucleus, including a coordination of corticotropin-releasing factor (CRF) actions at both of its receptors, CRF receptor-1 and CRF receptor-2. Mice deficient in CRF receptor-2 (R2KO) were used as a model of maladaptive stress responsivity to examine the physiological and molecular markers of stress dysregulation within the raphe in the absence of this receptor. After chronic stress, R2KO mice failed to display the robust stress-mediated adaptations characteristic of control mice, including elevations in tryptophan hydroxylase-2 and CRF receptor-1 expression and concordant increases in behavioral arousal. As a further indication of failed homeostatic mechanisms, R2KO mice displayed indices of cell death in the raphe after stress exposure, with elevations in proapoptotic factors but a failure to mount adaptive increases in antiapoptotic factors found in control mice. In vitro electrophysiological characterization of the specific influence of CRF on the raphe revealed both basal differences and a failure to respond to CRF administration in R2KO mice. These results support a requirement for homeostatic maintenance in response to stress in the raphe, where dysregulation may be a critical predictor of affective disorder onset.

Sodium- and magnesium-valproate in vivo modulate glutamatergic and GABAergic synapses in the medial prefrontal cortex

RATIONALE

Valproic acid (VPA) is a psychoactive drug currently used for the treatment of epilepsy. Recently it has been introduced in psychiatry for the treatment of bipolar disorders, aggression, impulsivity, and resistant schizophrenia, although the mechanism by which VPA acts on these psychiatric diseases remains still unknown.

OBJECTIVES

The aim of this study was to analyze the distinct effects of sodium-(Na-) and magnesium-valproate (Mg-VPA) in pyramidal neurons of the medial prefrontal cortex (mPFC) and their interactions with gamma-aminobutyric acid (GABA) and excitatory amino acid responses.

MATERIALS AND METHODS

In vivo electrophysiology and microiontophoresis techniques were used to attend these goals.

RESULTS

Both VPA salts decreased spontaneous neuronal firing activity in greater than 60% of recorded pyramidal neurons as well as potentiated GABA inhibitions. When injected at equal concentrations and currents, Mg-VPA blocked the excitatory responses induced by N-methyl-D-aspartate (NMDA) more frequently than Na-VPA. Both VPA salts equally blocked the excitatory responses of quisqualate and kainate.

CONCLUSIONS

These data suggest that VPA salts significantly modulate the activity of excitatory amino acid at mPFC pyramidal neurons and this mechanism should explain the therapeutic effects of valproate in psychiatric diseases involving NMDA, AMPA, and kainate receptors at the mPFC level.

Effects of sustained administration of the serotonin and norepinephrine reuptake inhibitor venlafaxine: I. in vivo electrophysiological studies in the rat

The effect of a 21-day treatment with the dual 5-HT and NE reuptake blocker venlafaxine (delivered s.c. by osmotic minipumps) was assessed on the time required for a 50% recovery (RT(50)) of the firing activity of dorsal hippocampus CA(3) pyramidal neurons from the suppression induced by microiontophoretic applications of 5-HT and NE. The RT(50) values for 5-HT were increased by both 10 and 40 mg/kg/day regimens of venlafaxine, whereas those for NE were increased only by the 40 mg/kg/day regimen, indicative of a greater potency of venlafaxine in blocking 5-HT reuptake. The sensitivity of the postsynaptic 5-HT(1A) and alpha(2)-adrenergic receptors was altered by neither regimen of venlafaxine. Using a paradigm by which the 5-HT(1A) antagonist WAY 100635 can induce a disinhibition of firing activity of CA(3) pyramidal neurons, it was demonstrated that the high, but not the low, dose of venlafaxine led to an enhanced tonic activation of postsynaptic 5-HT(1A) receptors in the dorsal hippocampus. The duration of the suppressant effect of the firing activity of CA(3) hippocampus pyramidal neurons produced by the electrical stimulation of the ascending 5-HT pathway was significantly reduced when the frequency of the stimulation was enhanced from 1 Hz to 5 Hz in control rats and in rats treated with 10 mg/kg/day, but not with 40 mg/kg/day of venlafaxine. Hence, venlafaxine induced a desensitization of the terminal 5-HT(1B) autoreceptor only at the high dose. A 2-day treatment with 10 mg/kg/day of venlafaxine induced a suppression of the firing activity of 5-HT neurons of the dorsal raphe. The firing activity of these neurons was back to control level in rats that had been treated for 21 days with the same dose of venlafaxine. The suppressant effect of the i.v. administration of the 5-HT autoreceptor agonist LSD on the firing activity of dorsal raphe 5-HT neurons was reduced in rats that had been treated for 21 days with 10 mg/kg/day of venlafaxine. A 2-day treatment with 40 mg/kg/day of venlafaxine, unlike the 10 mg/kg/day regimen, induced a marked suppression of the firing activity of locus coeruleus NE neurons. However, in contrast to 5-HT neurons, NE neurons did not recover their firing activity after a 21-day treatment. Taken together, the results from this study indicate that the low dose of venlafaxine blocked selectively the reuptake of 5-HT, whereas the high dose blocked the reuptake of both 5-HT and NE. Moreover, an enhancement of serotonergic neurotransmission by venlafaxine was only achieved under conditions whereby the desensitization of the terminal 5-HT(1B) autoreceptor is appended to that of the somatodendritic 5-HT(1A) receptor.

Clozapine blocks dopamine, 5-HT2 and 5-HT3 responses in the medial prefrontal cortex: an in vivo microiontophoretic study

Clozapine is an atypical antipsychotic drug active on both positive and negative symptoms of schizophrenia which has a unique serotonergic and dopaminergic profile. Given the putative role of the medial prefrontal cortex (mPFC) in negative symptoms of schizophrenia, the aim of this study was to assess the effects of clozapine on the dopamine- and serotonin-responsive neurons in that particular brain structure. D1 and D2 agonists (SKF 38393 and quinpirole) as well as 5-HT2 and 5-HT3 agonists (1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane, DOI, and phenylbiguanide) were applied by microiontophoresis alone and concurrently with clozapine while recording extracellularly mPFC neurons. Dopamine ejections inhibited firing activity while D1 and D2 agonists were ineffective. Clozapine did not change basal firing by itself, but was able to suppress the inhibition produced by dopamine and by the 5-HT2/5-HT3 receptor agonists. It is concluded that clozapine at the mPFC level exerts a complex modulatory activity on dopamine receptors, that is directly at the dopaminergic receptors and through 5-HT receptors on the same neurons.

Effects of 5,7-dihydroxytryptamine depletion of tissue serotonin levels on extracellular serotonin in the striatum assessed with in vivo microdialysis: relationship to behavior

Effects of i.c.v. administration of 5,7-dihydroxytryptamine (5,7-DHT) on biochemistry and behavior were studied in awake Sprague-Dawley rats. It was found that 5,7-DHT depletion of striatal tissue levels of serotonin (5-HT) does not diminish extracellular levels until substantial depletions occur. This finding is similar to those observed after 6-hydroxydopamine lesions of the brain dopamine systems. Although varying amounts of 5,7-DHT produced serotonin depletions in striatal tissue, decreases in extracellular levels were only observed at tissue depletions greater than 60% compared to saline-injected control subjects. Thus, the effects of serotonin lesions which produce only moderate depletions may not be the result of decreased extracellular serotonin, but instead may be the result of compensatory changes in remaining neurons which maintain normal extracellular serotonin concentrations. Different degrees of striatal serotonin depletion were associated with opposite behavioral effects. Moderate levels of serotonin depletion (50-75%) produced evidence of increased anxiety, while these effects were no longer seen in rats with more severe 5-HT depletions (>75%).

Venlafaxine: discrepancy between in vivo 5-HT and NE reuptake blockade and affinity for reuptake sites

Using an in vivo electrophysiological paradigm, venlafaxine and paroxetine displayed similar potency for suppressing the firing activity of dorsal raphe 5-HT neurons (ED50: 233 and 211 microg/kg i.v., respectively), while venlafaxine was three times less potent than desipramine (ED50: 727 and 241 microg/kg i.v., respectively) to suppress the firing activity of locus coeruleus NE neurons. The selective 5-HT1A receptor antagonist WAY 100635 (100 microg/kg, i.v.) reversed the suppressant effect of venlafaxine and paroxetine on the firing activity of 5-HT neurons and the alpha2-adrenoceptor antagonist piperoxane (1 mg/kg, i.v.) reversed those of venlafaxine and desipramine on the firing activity of NE neurons. The ED50 of venlafaxine on the firing activity of 5-HT neurons was not altered (ED50: 264 microg/kg) in noradrenergic-lesioned rats, while the suppressant effect of venlafaxine on the firing activity of NE neurons was greater in serotonergic-lesioned rats (ED50: 285 microg/kg). Taken together, these results suggest that, in vivo, venlafaxine blocks both reuptake processes, its potency to block the 5-HT reuptake process being greater than that for NE. Since the affinities of venlafaxine for the 5-HT and NE reuptake carriers are not in keeping with its potencies for suppressing the firing activity of 5-HT and NE neurons, the suppressant effect of venlafaxine on the firing activity of 5-HT and NE neurons observed in vivo may not be mediated solely by its action on the [3H]cyanoimipramine and [3H]nisoxetine binding sites. In an attempt to unravel the mechanism responsible for this peculiarity, in vitro superfusion experiments were carried out in rat brain slices to assess a putative monoamine releasing property for venlafaxine. (+/-)Fenfluramine and tyramine substantially increased the spontaneous outflow of [3H]5-HT and [3H]NE, respectively, while venlafaxine was devoid of such releasing properties.

Blockade of 5-hydroxytryptamine and noradrenaline uptake by venlafaxine: a comparative study with paroxetine and desipramine

1. Venlafaxine is an antidepressant agent which blocks in vitro the reuptake of both 5-HT and NA. The present in vivo electrophysiological studies were undertaken, in the rat, to compare the effects of venlafaxine on 5-HT and NA reuptake to those of the selective 5-HT reuptake inhibitor paroxetine and the selective NA reuptake inhibitor desipramine. 2. Administered acutely, venlafaxine dose-dependently prolonged the time required for a 50% recovery (RT50) of the firing activity of dorsal hippocampus CA3 pyramidal neurons from the suppression induced by microiontophoretic applications of 5-HT and NA. Venlafaxine and paroxetine increased with a similar potency the RT50 values for 5-HT, while desipramine was more potent than venlafaxine at increasing the RT50 values for NA. Moreover, venlafaxine demonstrated a greater potency at increasing the RT50 values for 5-HT compared to that of NA. 3. A two-day treatment with venlafaxine (delivered s.c. by osmotic minipumps) increased the RT50 values for both 5-HT and NA applications. The RT50 values for 5-HT were significantly increased at a dose of 10 mg kg(-1) day(-1), whereas those for NA were increased at a dose of 20 mg kg(-1) day(-1), consistent with the data obtained following the acute administration of venlafaxine. 4. Taken together, these results indicate that, in vivo, venlafaxine blocks both reuptake processes, with a potency greater for the 5-HT than for the NA reuptake process. This dual action, combined with the differential potency of venlafaxine, might constitute the biological substratum responsible for its apparent unique clinical efficacy in major depression.

The neurobiology of tryptophan depletion in depression: effects of intravenous tryptophan infusion

BACKGROUND

Previous work has suggested that acute depletion of the serotonin (5-HT) precursor tryptophan (TRP) causes transient compensatory changes in the 5-HT system that might be exploited for their antidepressant effects. In this study, neuroendocrine and mood responses to intravenous (i.v.) infusion of TRP were examined in order to evaluate central 5-HT function in depressed patients undergoing acute TRP depletion.

METHODS

Thirty-eight drug-free patients with DSM-III-R major depression participated. Each patient underwent two randomized, double-blind TRP depletion tests, one sham and one active. At the estimated time of maximum TRP depletion, each patient received an i.v. infusion of TRP 100 mg/kg. Blood was obtained for serum cortisol, prolactin, and growth hormone. Mood was assessed using standardized rating scales.

RESULTS

The cortisol response to i.v. TRP was significantly greater during TRP depletion than during sham depletion. Depressive symptoms showed a tendency to decrease after i.v. TRP following active, but not sham, TRP depletion.

CONCLUSIONS

These findings are consistent with the present hypothesis and previous evidence that acute TRP depletion in drug-free depressed patients induces compensatory upregulation of postsynaptic 5-HT receptors. These changes are insufficient to serve as a means of effecting clinical improvement, but suggest that the antidepressant properties of rapid, marked manipulations of 5-HT function warrant further study.

Repeated exposure to methylenedioxymethamphetamine (MDMA) alters nucleus accumbens neuronal responses to dopamine and serotonin

The purpose of this experiment was to investigate the effects of repeated exposure to methylenedioxymethamphetamine (MDMA) on responses of neurons in the nucleus accumbens of anesthetized rats to microiontophoretically-applied dopamine and serotonin. In tests conducted 1-4 days or 9-15 days following the last injection of MDMA (20 mg/kg, s.c., twice daily for 4 days), the inhibitory effects of both dopamine and serotonin on glutamate-evoked firing of nucleus accumbens cells were significantly attenuated compared to effects in control rats that were pretreated with saline injections. The inhibitory effect of the D1 receptor agonist SKF38393 was also significantly attenuated in the MDMA-pretreated rats. In contrast, the amount of inhibition of glutamate-evoked firing produced by application of GABA did not significantly differ between the MDMA-pretreated and the saline-pretreated rats. The neurotoxicity of the MDMA treatment regimen was confirmed by demonstrating that 3H-paroxetine binding was significantly decreased in the medial prefrontal cortex and the nucleus accumbens of the MDMA-pretreated rats. The mechanisms that produce the attenuated inhibitory responses to dopamine and serotonin following repeated injections of MDMA are not known. However, the results of these experiments indicate that repeated MDMA administration induces long-lasting changes in dopaminergic as well as serotonergic neurotransmission in the nucleus accumbens.

5-HT3-like receptors in the rat medial prefrontal cortex: further pharmacological characterization

The aim of the study was to further characterize the pharmacological properties of 5-hydroxytryptamine (5-HT)3-like receptors in the rat medial prefrontal cortex (mPFC) using combinations of biochemical and electrophysiological approaches. Phenylbiguanide (PBG) and three chlorinated derivatives, ortho-chloro-PBG (oCPBG), meta-chloro-PBG (mCPBG) and para-chloro-PBG (pCPBG), dose-dependently stimulated phosphoionositide (PI) turnover in fronto-cingulate cortical slices. All three chloro-isomers of PBG were equipotent in stimulating PI turnover. SR 57227A ((4-amino)-(6-chloro-2-pyridyl) L-piperidine hydrochloride, a novel compound with high affinity and selectivity for peripheral and central 5-HT3 receptors) dose-dependently stimulated PI turnover in fronto-cingulate cortical slices. The rank order of potency of all the 5-HT3 receptor agonists tested in the PI assay as compared to 5-HT was: 5-HT > 2-Me-5-HT > SR57227A > PBG = mCPBG = oCPBG = mCPBG. 5-HT and 5-HT receptor agonists depressed the firing rate of both spontaneously active and glutamate-activated quiescent mPFC cells in a current (dose)-dependent fashion. The rank order of effectiveness of these compounds was: 5-HT > SR57227A = 2-Me-5-HT = mCPBG = oCPBG = pCPBG = PBG. Unlike its action on the 5-HT3 receptors in the periphery or cultured cell lines, D-tubocurarine chloride appears to be non-specific in blocking the depressant action of 2-Me-5-HT, gamma-aminobutyric acid and dopamine. Our results combined support the view that the pharmacological properties of 5-HT3-like receptors in the mPFC are not identical to those located in peripheral tissues and in cultured cell lines.

Inhibition of catecholamine synthesis with alpha-methyl-p-tyrosine apparently increases brain serotoninergic activity in the rat: no influence of previous chronic immobilization stress

The functional relationship between brain catecholamines and serotoninergic function was studied in stress-naive and chronically immobilized rats after blockade of catecholamine synthesis with alpha-methyl-p-tyrosine (alpha MpT). The levels of noradrenaline (NA), serotonin, and 5-hydroxyindole acetic acid (5-HIAA) in pons plus medulla, brainstem, hypothalamus, hippocampus, and frontal cortex, and those of 3-methoxy, 4-hydroxyphenile-tileneglicol sulphate (MHPG-SO4) in the hypothalamus were measured by HPLC. Chronic immobilization (IMO) resulted in higher NA levels in pons plus medulla and hypothalamus, the latter area (the only one in which the NA metabolite was determined) also showing slightly elevated MHPG-SO4 levels as compared to stress-naive rats. Chronic IMO did not alter either serotonin or 5-HIAA levels, but acute stress consistently increased 5-HIAA levels in all areas, independently of previous chronic stress. Administration of alpha-MpT drastically reduced NA and increased 5-HIAA levels in all brain regions excepting the frontal cortex. The effect of the drug on serotoninergic function was not altered by previous chronic exposure to IMO. These data suggest that the noradrenergic system appears to exert a tonic inhibitory effect on serotoninergic activity in the brain, with the intensity of the effect depending on the brain area studied. In addition, chronic stress does not appear to alter the functional relationship between noradrenergic and serotoninergic activities, although interactions might exist in more restricted brain areas; this deserves further study.

Electrophysiological evidence for a functional interaction between 5-HT1A and 5-HT2A receptors in the rat medial prefrontal cortex: an iontophoretic study

In this study, we examined the interaction of 5-HT1A and 5-HT2A receptors in the rat medial prefrontal cortex (mPFc) using the techniques of extracellular single unit recording and microiontophoresis. The iontophoresis of the selective 5-HT1A receptor agonist (+-)-8-hydroxy-2-(di-n-propylamino) tetralin (8-OHDPAT) produced a current-dependent suppression (2.5-20 nA) of the basal firing rate of spontaneously active mPFc cells. The iontophoretic (5-10 nA) and systemic administration (0.1-0.5 mg/kg, i.v.) of the 5-HT2A/5-HT2C receptor antagonist ritanserin and the selective 5-HT2A receptor antagonist MDL 28727 significantly potentiated and prolonged 8-OHDPAT's suppressant action. In addition, the systemic administration of another selective 5-HT2A antagonist MDL 100907, but not its less active enantiomer MDL 100009, also potentiated and prolonged 8-OHDPAT's action. The potentiating effect of the 5-HT2A receptor antagonists on the action of 8-OHDPAT is specific in that neither the iontophoresis of ritanserin nor MDL 28727 altered the suppressant action produced by the iontophoresis of the 5-HT3 receptor agonist 2-methylserotonin onto mPFc cells. Moreover, the suppressant action of 8-OHDPAT was not altered by the systemic administration of the selective 5-HT3 receptor antagonist granisetron (0.1-0.5 mg/kg, i.v.). On the other hand, the iontophoresis of a low current (0.5 nA) of the 5-HT2A,2C receptor agonist (+-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) potentiated the excitation induced by the iontophoresis of l-glutamate on quiescent mPFc cells.(ABSTRACT TRUNCATED AT 250 WORDS)

The induction of serotonin3-like receptor supersensitivity and dopamine receptor subsensitivity in the rat medial prefrontal cortex after the intraventricular administration of the neurotoxin 5,7-dihydroxytryptamine: a microiontophoretic study

This study examines the effect of intraventricular administration of the neurotoxin 5,7-dihydroxytryptamine on serotonin1A, serotonin2 and serotonin3 receptors in the rat medial prefrontal cortex using in vivo extracellular single cell recording and iontophoresis. Iontophoresis of the serotonin1A, serotonin1C,2 and serotonin3 receptor agonists (+-)-8-hydroxy-(di-n-propyl)aminotetralin, (+-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane and 2-methylserotonin, respectively, produces a current-dependent (5-80 nA) suppression of the basal firing rate of medial prefrontal cortical cells in sham- and 5,7-dihydroxytryptamine-lesioned rats. The suppression produced by 2-methylserotonin and serotonin was significantly greater in 5,7-dihydroxytryptamine-lesioned rats than in control rats. No significant difference in the spontaneous activity of medial prefrontal cortex cells was observed between experimental and control rats after iontophoresis of (+-)-8-hydroxy-(di-n-propyl)aminotetralin or (+-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane. There was no significant difference between the groups regarding the response of medial prefrontal cortex cells to the iontophoresis of GABA, whereas the response of medial prefrontal cortex cells to the iontophoresis of dopamine was significantly attenuated in animals pretreated with 5,7-dihydroxytryptamine compared to controls. Our results indicate that 5,7-dihydroxytryptamine-induced denervation selectively enhances the sensitivity of serotonin3-like receptors in the medial prefrontal cortex, which could, at least partially, account for the serotonin denervation supersensitivity. Moreover, the finding that the response of medial prefrontal cortical cells to the iontophoresis of dopamine is attenuated in 5,7-dihydroxytryptamine pretreated rats is consistent with the view that the inhibitory action of dopamine in the medial prefrontal cortex is dependent upon serotonin tone.

Serotonin3 receptor agonists attenuate glutamate-induced firing in rat hippocampal CA1 pyramidal cells

The techniques of extracellular single cell recording and microiontophoresis were used to study the effect of 5-HT3 receptor agonists on glutamate-activated firing of CA1 hippocampal pyramidal cells. Iontophoretic application of 5-HT3 receptor agonists 2-methyl-5-HT and SR 57227A produced a current (dose)-dependent suppression of the firing of CA1 pyramidal cells; SR 57227A was more effective than 2-methyl-5-HT. The suppressant action of 2-methyl-5-HT and SR 57227A had a slow onset and showed little or no desensitization. This effect was markedly attenuated or completely blocked by the 5-HT3 receptor antagonist BRL 46470A but not by the nonspecific 5-HT1 and 5-HT2 receptor antagonist metergoline or by the 5-HT1A antagonist WAY 100478. Intravenous administration of SR 57227A was effective in reducing the firing rate of CA1 pyramidal cells and this effect was prevented by BRL 46470A administered either i.v. or iontophoretically. Iontophoresis of 2-methyl-5-HT also diminished CA1 postsynaptic field potentials evoked by electrical stimulation of the Schaffer collaterals. Again, BRL 46470A but not metergoline prevented the suppressant action of 2-methyl-5-HT. Taken together, our results indicate that activation of 5-HT3-like receptors in the hippocampal CA1 region effectively reduces the efficacy of glutamatergic neurotransmission.

The effects of BTS 54,505, a metabolite of sibutramine, on monoamine and excitatory amino acid-evoked responses in the rat dorsolateral geniculate nucleus in vivo

1. The effects of BTS 54,505, the primary amine metabolite of the non-tricylic putative antidepressant sibutramine, on the responses evoked by visual stimulation and ionophoretic application of noradrenaline (NA), 5-hydroxytryptamine (5-HT) and excitatory amino acids (EAAs) in the rat dorsolateral geniculate nucleus (dLGN) have been investigated. 2. Ionophoretic application of 5-HT to dLGN neurones attenuated visually-evoked (n = 46), NMDA-evoked (n = 21) and AMPA-evoked responses (n = 21), while ionophoretic application of NA potentiated visually-evoked activity in these cells (n = 27). 3. Simultaneous application of BTS 54,505 with 5-HT (over 120 s) resulted in a prolongation of the recovery time (i.e. the period required by a neurone to recover by 50%, RT50) from the 5-HT-mediated suppression of discharge activity (approximately 275% increase in RT50). BTS 54,505 also prolonged the recovery time from a NA-mediated potentiation of firing (approximately 450% increase in RT50). These effects on recovery time are attributed to the inhibition of uptake of both 5-HT and NA by BTS 54,505. The amplitude of the response to 5-HT or NA was unaffected by co-ejection of BTS 54,505. 4. Ionophoretic application of N-methyl-D-aspartate (NMDA) produced a current-dependent increase in neuronal firing, as did application of the non-NMDA receptor agonist alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA). A simultaneous 120 s application of BTS 54,505 inhibited the NMDA response in all cells studied (mean ED50 = 16 +/- 5 nA) but had no effect on AMPA-evoked activity in the majority of the same cells (n = 15/21).5. Short 10 s applications of BTS 54 505, at ejecting currents (>30 nA) that attenuated NMDA-evoked activity in all cells studied, had no effect on either response amplitude or recovery time from ionophoretic application of 5-HT, suggesting that inhibition of NMDA-evoked activity by BTS 54 505 was not mediated by 5-HT uptake blockade.6. These results suggest that BTS 54 505 inhibits NMDA-evoked activity, and the observation that this effect is unlikely to be due to raised levels of endogenous 5-HT following monoamine uptake blockade indicate that BTS 54 505 may interact directly with the NMDA receptor ionophore complex.

Changes of D1 and D2 receptors in adult rat neostriatum after neonatal dopamine denervation: quantitative data from ligand binding, in situ hybridization and iontophoresis

The specific binding of [3H]SCH23390 to D1 and of [3H]raclopride to D2 dopamine receptors was measured by autoradiography in the rostral and caudal halves of neostriatum and in the substantia nigra of adult rats subjected to near total destruction of nigrostriatal dopamine neurons by intraventricular 6-hydroxydopamine soon after birth. Three months after this lesion, [3H]SCH23390 binding (D1 receptors) was slightly but significantly decreased in the rostral neostriatum (22%), but unchanged in its caudal half and in the substantia nigra. In contrast, [3H]raclopride binding (D2 receptors) was considerably increased throughout the neostriatum (10-40%), while markedly decreased in the substantia nigra (80%). In the rostral neostriatum, there were no parallel changes in D2 receptor messenger RNA levels, as measured by in situ hybridization on adjacent sections. Caudally, however, slight but significant increases in D2 messenger RNA could be observed (10-20%). As assessed by quantitative iontophoresis, there was a marked enhancement (63%) of the inhibitory responsiveness of spontaneously firing units in the rostral neostriatum to dopamine and the D1 agonist, SKF38393, in neonatally lesioned compared to control rats. On the other hand, responsiveness to PPHT, a potent D2 agonist, appeared to be unchanged. Such opposite changes in the number of D1 and D2 binding sites, dissociated from the expression of D2 receptor messenger RNA and from the sensitivity to dopamine and D1 and D2 agonists, suggested independent adaptations of these various parameters following the neonatal dopamine denervation of neostriatum. They also provided further evidence for mechanisms other than the dopamine innervation in the control of the expression of neostriatal D2 receptor messenger RNA during ontogenesis, and emphasized that the effects of dopamine and its D1 and D2 agonists in neostriatum do not depend strictly on the number of D1 and D2 primary ligand recognition sites.

Effects of serotonin agonists and melatonin on photic responses of hamster intergeniculate leaflet neurons

Retinal input to the suprachiasmatic nuclei (SCN) and the intergeniculate leaflet (IGL) is involved in photic entrainment of mammalian circadian rhythms. The activating effects of light on firing rates of IGL cells may be regulated by serotonin (5-HT), since the IGL receives a dense serotonergic input from the midbrain raphe. We investigated the effects of 5-HT agonists and melatonin (a derivative of 5-HT) on single-unit discharges of light-sensitive cells in the hamster IGL area, using a microiontophoretic technique. 5-HT and a 5-HT1A-selective agonist, 8-OH-DPAT, potently suppressed both spontaneous and light-induced activity of IGL cells in a dose-related manner. This suppression was unchanged or potentiated by concurrently applied Mg2+, suggesting a direct action. Furthermore, the suppressive effects of both agonists were antagonized by a nonselective 5-HT antagonist, metergoline, and a 5-HT1A-directed antagonist, pindobind-5-HT1A. However, other putative 5-HT1A antagonists were weak (propranolol) or ineffective (pindolol and spiperone) in blocking the effects of 8-OH-DPAT. Neither of two 5-HT2 antagonists tested was able to block the effects of 5-HT. Melatonin generally mimicked the effects of 5-HT agonists on IGL cells, but these effects were not attenuated by 5-HT antagonists. The results indicate that both 5-HT and melatonin exert inhibitory effects on spontaneous activity and photic responses of cells in the hamster IGL, and that these effects are mediated via a 5-HT1A-like receptor and a melatonin receptor, respectively.

Effect of 5,7-dihydroxytryptamine lesion on mianserin-induced conditioned place aversion and on 5-hydroxytryptamine1C receptors in the rat brain

The lesion of serotonergic neurons (by an intraventricular injection of 5,7-dihydroxytryptamine) potentiated the conditioned place aversion induced by the 5-hydroxytryptamine1C/5-hydroxytryptamine2 antagonist mianserin in rats. This effect was selective for mianserin as the same lesion suppressed the conditioned place aversion induced by the benzodiazepine inverse agonist FG-7142. Previous results had shown the involvement of the 5-hydroxytryptamine1C receptors in the conditioned place aversion induced by mianserin [Rocha et al. (1993) Behav. Pharmac. 4, 101-106]. It was thus of interest to investigate the effect of the lesion on these receptor binding sites. Autoradiographic binding studies showed that the lesion significantly increased the concentration of the 5-hydroxytryptamine1C binding sites in various brain regions, including the amygdala, the hippocampus and the nucleus accumbens. Contrastingly, in these same brain regions, in situ hybridization histochemistry did not reveal an alteration of the level of messenger RNA coding for these receptors. On the one hand, correlating potentiation of the aversive effects of mianserin and increase of 5-hydroxytryptamine1C binding sites in the limbic system represent an interesting step in the comprehension of the molecular and motivational effects of serotonergic drugs. On the other hand, showing a dissociation between the expression of 5-hydroxytryptamine1C receptors and their corresponding messenger RNA, suggest that post-transcriptional mechanisms are involved in the regulation of these receptors.

The role of 5-hydroxytryptamine (5-HT) receptor subtypes and plasticity in the 5-HT systems in the regulation of nociceptive sensitivity

This review shows that the role of 5-hydroxytryptamine (5-HT) in the regulation of nociception depends on the 5-HT receptor subtypes involved and on long-term functional changes in the 5-HT receptors. Stimulation of the 5-HT1 receptors, as well as of the 5-HT2 and 5-HT3 receptors, may reduce nociceptive sensitivity. In addition, activation of 5-HT2 and 5-HT3 receptors may also enhance nociceptive sensitivity. Up- or down-regulation of the 5-HT receptors may result in long-lasting changes, plasticity, in the 5-HT systems. Lesioning of 5-HT neurons induces denervation supersensitivity to 5-HT, and prolonged stimulation of 5-HT receptors may produce subsensitivity to 5-HT. In the spinal cord denervation supersensitivity to 5-HT may depend on reduced release of substance P (SP). An increase in the release of SP, on the other hand, may reduce the effects of 5-HT receptor activation. Long-term treatment with antidepressants which are used in clinical pain therapy appears to up-regulate the 5-HT1 receptors and to down-regulate the 5-HT2 receptors.

Physiopharmacological interactions between stress hormones and central serotonergic systems

The present review tries to delineate some mechanisms through which the sympathetic nervous system (SNS) and the hypothalamo-pituitary-adrenal (HPA) interact with central serotonergic systems. The recent progress in 5-hydroxytryptamine (5-HT) receptor pharmacology has helped to define the means by which central serotonergic activity may alter the respective activities of the SNS (sympathetic nerves and adrenomedulla) and of the HPA axis. These pharmacological findings have also helped to characterize the differential effects of central 5-HT upon different branches of the SNS and the numerous sites at which 5-HT exerts stimulatory influences upon the HPA axis. Although relevant to stress-related neuroendocrinology, the extent to which these interactions are involved in the antidepressant/anxiolytic properties of some serotonergic agents still remains to be clarified. Beside these findings, there is also abundant evidence for a tight control of central serotonergic systems by stress hormones. Activation of the SNS increases, by numerous means, central availability of tryptophan, whereas glucocorticoids exert differential actions upon the intra- and the extraneuronal regulation of 5-HT function. Actually, a significant number of these mechanisms is involved in the maintenance of homeostasis during stressful events, thereby conferring to these mechanisms a key role in adaptation processes.

Up-regulation of 5-hydroxytryptamine2 and neurokinin-1 receptors associated with serotonin/substance P hyperinnervation in the rat inferior olive

The fate of serotonin and substance P receptors following serotonin/substance P hyperinnervation of CNS tissue was investigated in the inferior olivary complex of adult rats subjected to earlier intraventricular administration of 5,6-dihydroxytryptamine. [3H]8-hydroxy-2-(Dl-n-propylamino)tetralin, [3H]5-hydroxytryptamine, [3H]ketanserin and [125I]Bolton-Hunter-substance P were respectively used to label 5-hydroxytryptamine1A, 5-hydroxytryptamine1B, 5-hydroxytryptamine2 and neurokinin-1 receptor sites for quantitative ligand binding autoradiography. Only 5-hydroxytryptamine2 and neurokinin-1 sites were detected in the normal or serotonin/substance P-hyperinnervated inferior olivary complex. In the normal inferior olivary complex, the density of [3H]ketanserin binding (5-hydroxytryptamine2 receptors) was relatively low, being the highest in pars a of the caudal medial accessory olive and the principal olive; moderate in pars c of the caudal medial accessory olive; truly low in the medial and the lateral dorsal accessory olive, nucleus b and pars b of the caudal medial accessory olive; and negligible in the middle medial accessory olive, rostral medial accessory olive and the smaller subnuclei. [125I]Bolton-Hunter-substance P binding (neurokinin-1 receptors) appeared denser, being highest in nucleus beta and the middle medial dorsal accessory olive; moderate in the three portions of the caudal medial accessory olive, the lateral dorsal accessory olive and the dorsal cap of Kooy; low in the rostral medial accessory olive, the ventrolateral outgrowth and the dorsomedial cell column; and very low or null in the principal olive and the medial dorsal accessory olive. After serotonin/substance P hyperinnervation, there were striking increases in the apparent density of both populations of receptor. [3H]Ketanserin binding was now stronger in the most olivary subnuclei, including some in which it had not been found in the normal, such as the middle and the rostral medial accessory olive. [125I]Bolton-Hunter-substance P binding showed even greater elevations in a few subnuclei, such as the principal olive and the dorsomedial cell column; it was now detectable in the medial dorsal accessory olive, unchanged in the dorsal cap of Kooy and the ventrolateral outgrowth, and slightly decreased in the lateral dorsal accessory olive. The normal and altered distributions of both ligands did not match the respective patterns of serotonin and substance P innervation and hyperinnervation previously demonstrated with immunocytochemistry. In some sectors of the inferior olivary complex where both transmitters are presumably co-localized, there was no overlap in the distribution of the respective binding sites either in the normal or in the hyperinnervated state.(ABSTRACT TRUNCATED AT 400 WORDS)

A review and reevaluation of the role of serotonin in the modulation of lordosis behavior in the female rat

The role of serotonin (5-HT) in the modulation of sexual receptivity (lordosis) in the female rat is reviewed and reevaluated. The effects on lordosis of drug treatments that decrease or increase the activity and availability of central 5-HT are first discussed, and this is followed by an evaluation of the effects of drugs that act directly at 5-HT receptors. In order to shed light on the physiological significance of effects of serotonergic drugs on lordosis, there is also a review of what is known of changes in levels of serotonergic activity and densities of 5-HT receptors in the female rat brain that take place through the estrous cycle and in response to administration of behaviorally effective doses of gonadal steroids. Serotonin has generally been thought to have a tonic, inhibitory effect on lordosis. However, it is concluded that 5-HT can either inhibit or facilitate lordosis depending on which subtypes of central 5-HT receptors become activated. Because of a lack of consistent or compelling evidence of effects of ovarian hormones on serotonergic activity or 5-HT receptors in critical areas of the brain, it is stated that there is at present no basis to conclude that the effects of pharmacological manipulations of serotonergic activity on lordosis reflect an important, physiological role of 5-HT in the modulation of lordosis behavior in the female rat.

Chronic cocaine enhances serotonin autoregulation and serotonin uptake binding

Repeated cocaine intoxication can result in the development of behavioral sensitization in animals and psychosis in humans, phenomena that have been associated with alterations in dopamine (DA) function. Using electrophysiologic and autoradiographic techniques, modifications of central serotonin (5-hydroxytryptamine; 5-HT) systems were investigated in rats treated with a regimen of cocaine administration that produced behavioral sensitization. The inhibitory response of single 5-HT neurons in the dorsal raphe (DR) to (-)-cocaine, the 5-HT uptake inhibitor fluoxetine or the 5-HT1A agonist 8-hydroxy-2-[di-N-propylamino]tetralin (8-OHDPAT) was significantly enhanced in cocaine-treated rats. Furthermore, several brain areas that contain either cell bodies (DR) or terminals for 5-HT (medial and sulcal prefrontal cortex, frontal cortex) showed cocaine-induced elevations in [3H]imipramine-labeled 5-HT uptake sites, while [3H]-8-OHDPAT-labeled 5-HT1A receptors were decreased only in the central medial amygdala. These results suggest that modifications of autoregulatory mechanisms secondary to alterations of 5-HT uptake processes may contribute to the development of cocaine sensitization.

Action of serotonin in the medial prefrontal cortex: mediation by serotonin3-like receptors

In the present study, we investigated the effects of various serotonin (5-HT) antagonists on 5-HT's action on medial prefrontal cortical cells (mPFc) using the techniques of single cell recording and microiontophoresis. The microiontophoretic application of 5-HT (10-80 nA) produced a current-dependent suppression of mPFc cell firing and this effect was blocked by the selective 5-HT3 receptor antagonists (+/-)-zacopride, ICS 205930 and granisetron at currents of 5-20 nA. Furthermore, the intravenous (i.v.) administration of (+/-)-zacopride (5-50 micrograms/kg) markedly attenuates the suppressive action of 5-HT on mPFc cell firing. In contrast, the microiontophoresis of 5-HT1 and 5-HT2 receptor antagonists such as (+/-)-pindolol, spiperone, metergoline, and ritanserin (10-20 nA) failed to block 5-HT's effect. In fact, in some cells, spiperone and ritanserin potentiated 5-HT's action and prolonged neuronal recovery. In addition, the intravenous administration of either ritanserin (5-2,000 micrograms/kg) or metergoline (4-2,400 micrograms/kg) failed to alter 5-HT's action. The electrical stimulation of the caudal linear raphe nucleus (CLi) suppressed the spontaneous activity of 83% of the mPFc cells tested by 45 +/- 2%. This suppression was significantly attenuated by the iontophoresis of granisetron (2.5-5 nA) but not by the 5-HT2 and 5-HT1C receptor antagonist ritanserin or the relatively selective 5-HT2 receptor antagonist (+)-MDL 11,939 (10-40 nA). However, the i.v. administration of ritanserin (0.5-1.5 mg/kg) or S-zacopride (0.1 mg/kg) significantly blocked the suppression of mPFc cell firing produced by CLi stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of the effects of various typical and atypical antipsychotic drugs on the suppressant action of 2-methylserotonin on medial prefrontal cortical cells in the rat

In this study, we report the effects of various typical and atypical antipsychotic drugs (APDs) on the suppressant action of microiontophoretically applied 2-methylserotonin (2-Me-5HT, a 5-HT3 agonist) on medial prefrontal cortical (mPFc) cells. The microiontophoresis of 2-Me-5HT (10-80 nA) produced a current-dependent suppression of mPFc cells' firing, and this effect was blocked by various 5-HT3 antagonists. The microiontophoresis of the atypical APDs clozapine and a structurally related compound, RMI 81,582, mimicked the action of the 5-HT3 antagonists. In addition, the intravenous administration of clozapine and RMI 81,582 antagonized the suppressant action produced by the iontophoretic application of 2-Me-5HT on mPFc cells. However, the suppressant action of 2-Me-5HT was not blocked by the typical APDs haloperidol and chlorpromazine. The putative atypical APDs risperidone, setoperone, CL 77328, SCH 23390, CGS 10746B, 1-sulpiride, and thioridazine were ineffective in antagonizing 2-Me-5HT's action. Overall, our results suggest that the majority of putative atypical APDs do not interact with 5-HT3 binding sites in the brain. Whether the interaction of clozapine and RMI 81,582 with 5-HT3 sites is correlated with their therapeutic efficacy or lower potential to induce neurological side effects remains to be determined.

5-HT3-like receptors in the rat medial prefrontal cortex: an electrophysiological study

In this study, we have identified and characterized 5-HT3-like receptors in the rat medial prefrontal cortex (mPFc), an area with a moderate density of 5-HT3 binding sites, using the techniques of single unit recording and microiontophoresis. The microiontophoresis of the 5-HT3 receptor agonist 2-methylserotonin (2-Me-5HT), similar to the action of 5-HT, produced a current-dependent (10-80 nA) suppression of the firing rate of both spontaneously active and glutamate (GLU)-activated (quiescent) mPFc cells. Phenylbiguanide (PBG), another 5-HT3 receptor agonist, suppressed the firing rate of mPFc cells but was less effective compared to 2-Me-5HT. The continuous iontophoresis (10-20 min) of 1 M magnesium chloride markedly attenuated the suppressant effect produced by electrical stimulation of the ascending 5-HT pathway, but did not alter 2-Me-5HT's action, suggesting that the action of 2-Me-5HT is a direct one. The suppressant action of 2-Me-5HT on mPFc cells was blocked by a number of structurally diverse and selective 5-HT3 antagonists, with a rank order of effectiveness as follows: ICS 205930 = (+/-)-zacopride greater than granisetron = ondansetron = LY 278584 greater than MDL 72222. Furthermore, the intravenous administration of (+/-)-zacopride antagonized the action of 2-Me-5HT and PBG on mPFc cells. In contrast to the effects of the 5-HT3 receptors antagonists, other receptor antagonists such as metergoline (5-HT1A,1B,1C.2), (+/-)-pindolol (5-HT1A,1B, beta), SCH 23390 (5-HT1C.2, D1), l-sulpiride (D2) or SR 95103 (GABAA) failed to block 2-Me-5HT's action. These results combined suggest that 2-Me-5HT's suppressive action on mPFc cells is mediated directly by 5-HT3-like receptors.

The action of (+/-)-MDMA on medial prefrontal cortical neurons is mediated through the serotonergic system

The mechanism of action of systemically administered (+/-)-MDMA (3,4-methylenedioxymethamphetamine) on spontaneously active neurons in the medial prefrontal cortex (mPFc) of chloral hydrate anesthetized rats was examined using standard single unit extracellular recording techniques. Intravenously administered MDMA dose-dependently decreased the firing rates of the majority of mPFc neurons in control rats. In contrast, in rats that were pretreated with p-chlorophenylalanine (PCPA), which depletes the brain serotonin (5-hydroxytryptamine, 5-HT) content by inhibiting tryptophan hydroxylase, the rate-limiting enzyme in the synthesis of 5-HT, MDMA was largely ineffective in inhibiting the firing of mPFc cells. In PCPA-treated animals, the administration of 5-hydroxytryptophan (5-HTP), which presumably restored the brain 5-HT content, but not L-DOPA, reinstated MDMA's inhibitory action in PCPA-treated rats. In rats that were pretreated with alpha-methyl-p-tyrosine (AMPT), which depletes the brain dopamine (DA) content by inhibiting tyrosine hydroxylase, the rate-limiting enzyme in the synthesis of DA, MDMA inhibited the firing of all of the mPFc cells. MDMA's effect on mPFc neurons was reversed by 5-HT receptor antagonists such as granisetron and metergoline. These results strongly suggest that MDMA exerts its action on mPFc cells indirectly by releasing endogenous 5-HT.

Depletion of brain serotonin differently affects behaviors induced by 5HT1A, 5HT1C, and 5HT2 receptor activation in rats

5-hydroxytryptamine (5HT)-depleted rats were subjected to behavioral experiments in which the response to activation of 5HT1A, 5HT1c, and 5HT2 receptor subtypes was measured. Depletion of 5HT was produced by unilateral intracerebroventricular injection of 5,7-dihydroxytryptamine (100 micrograms/rat) or by systemic injection of p-chlorophenylalanine (150 mg/kg injected intraperitoneally 72, 48, and 24 h before the test). The dose-response curve of the 5HT1A-mediated, 8-hydroxy-2-(di-n-propylamino)tetralin (0.022-0.46 mg/kg)-induced lower lip retraction was not changed after depletion, nor was the dose-response curve of the 5HT2 receptor-mediated (+-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (0.046-1.0 mg/kg)-induced head shake response. The dose-response curve for penile erections, a 5HT1c receptor-mediated response after mCPP (0.1-1.0 mg/kg), a direct 5HT1c agonist, is shifted to the left after 5HT depletion, whereas the response to indirect activation of the 5HT1c receptor with the 5HT reuptake inhibitors citalopram (2.2-4.6 mg/kg) and paroxetine (0.22-2.2 mg/kg) was inhibited after 5HT depletion. These results suggest that 5HT1c receptors are more subject to denervation supersensitivity than 5HT1A and 5HT2 receptors. This lesion model may be useful to discriminate behaviorally between direct and indirect activation of the 5HT1c receptor.

Tryptophan hydroxylase inhibition increases preprotachykinin mRNA in developing and adult medullary raphe nuclei

In order to study the regulation of co-localized monoamine and peptide neurotransmitters in the medullary raphe nuclei (MRN), we determined whether inhibition of serotonin (5-HT) synthesis affected levels of preprotachykinin (PPT; the prohormone precursor of substance P) mRNA in the MRN. Adult rats received p-chlorophenylalanine (pCPA), an irreversible inhibitor of tryptophan hydroxylase (TPH), via Alzet minipumps. TPH activity was inhibited by 70-80% for 3 weeks following pump implantation. During this period Northern mRNA analysis revealed that PPT mRNA levels in the MRN were increased 1.5-2-fold. The pCPA-induced increase was specific for PPT mRNA since no change was detected in mRNA coding for neuron-specific enolase (NSE; a constitutive neuronal protein) or 28 S ribosomal RNA. To determine whether fetal inhibition of 5-HT synthesis affected development of PPT mRNA in the MRN, pregnant rats were administered pCPA via Alzet minipump implanted on embryonic day 8. In pCPA-treated litters TPH activity was decreased by 60-70% from E16 to postnatal day 3 (P3), returning to control levels by P8. Northern mRNA analysis revealed that PPT mRNA levels increased 2.4-fold of control levels at P1. Infusion of pCPA for one week resulted in an earlier increase in PPT mRNA levels, suggesting that birth was not required to elicit the surge in PPT message. These results support the hypothesis that alterations in 5-HT metabolism have regulatory consequences for co-localized substance P formation in the MRN.

Behavioral reactivity following 5-MeODMT administration in 5,7-DHT-pretreated killer rats

Grouped, nonkiller and killer animals were centrally injected either with vehicle or with 5,7-dihydroxytryptamine. After a period of 7-10 days, forepaw treading and hindlimb abduction were induced by 5-MeODMT administration in all sham and lesioned rats. As expected, behavioral supersensitivity was observed in grouped and nonkiller lesioned rats. A reduced increase in 5-MeODMT-induced behaviors was obtained in killer lesioned animals. 5-HT uptake studies showed a comparable reduction of 5-HT uptake within all the lesioned rats. This evidence suggests an altered capacity to promote adaptive changes on 5-HT sites in killer rats following the destruction of central serotonergic fibers.

Effects of antipsychotic drugs on 5-HT2 receptors in the medial prefrontal cortex: microiontophoretic studies

Microiontophoretic application (10-80 nA) of the relatively selective serotonin 2 (5-HT2) agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane [+/-)-DOI), suppressed medial prefrontal cortical (mPFc) cell's firing rate in a dose-dependent manner. This effect was antagonized by the antipsychotic drugs (APDs) clozapine (CLOZ) and spiperone (SPIP), with CLOZ displaying a greater potency in blocking DOI's action in the mPFc. In contrast, the typical APDs haloperidol (HAL) and L-sulpiride (SUL) failed to block DOI's effect. These results suggest that certain APDs interact with 5-HT2 receptors. Although 5-HT2 antagonism alone may not distinguish typical versus atypical APDs, it is possible that their 5-HT2/dopamine 2 (D2) binding ratio is important.

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.

Characterization of 5-hydroxytryptamine3 receptors in the medial prefrontal cortex: a microiontophoretic study

The microiontophoretic application of the selective 5-hydroxytryptamine3 (5-HT3) agonist 2-methylserotonin suppresses medial prefrontal cortex cell firing. This effect is blocked by the 5-HT3 antagonists BRL 43694 and ICS205930, but not by metergoline or (+/-)-pindolol. Continuous microiontophoretic administration of magnesium chloride or the gamma-aminobutyric acidA antagonist SR 95103 did not alter 2-methylserotonin's suppressant action, suggesting that this effect is direct. Our results suggest that 5-HT3 receptors have a functional role in the medial prefrontal cortex.

Effects of (+/-)-DOI on medial prefrontal cortical cells: a microiontophoretic study

The relatively selective 5-HT2 receptor agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane [+/-)-DOI) ejected by microiontophoresis at low currents potentiates glutamate (GLU)-induced excitation and at higher currents (greater than 20 nA) invariably inhibits both spontaneous and GLU-induced activity of cells in the medial prefrontal cortex (mPFc). The inhibitory action of DOI is blocked by the 5-HT2 antagonists, ritanserin, metergoline and spiperone, but not other receptor antagonists. Microiontophoresis of 1 M Mg2+ for 5-20 min did not alter DOI's inhibitory effect suggesting that DOI's action is a direct one. These results show that DOI predominantly inhibits mPFc neuronal activity and this inhibitory action is mediated by 5-HT2 receptors.

Hippocampal neuronal responsiveness to different neurotransmitters in the adult rat after neonatal interference with noradrenaline transmission

In order to test the functional significance of noradrenergic neurotransmission for brain maturation, rat pups were injected daily with clonidine from 8-21 days of postnatal life. In adulthood, single cell responses to microiontophoretically applied neurotransmitters were studied in different layers of the hippocampus. No significant differences were found in neuronal responsiveness to glutamate, GABA, or acetylcholine in the hippocampus of clonidine-treated rats as compared with age-matched controls. However, there was a significantly stronger depression of glutamate-evoked activity of CA1 pyramidal neurons by noradrenaline in the clonidine-treated rats.

Electrophysiological and biochemical characterization of the development of alpha 1-adrenergic and 5-HT1 receptors associated with dorsal raphe neurons

Electrophysiological and quantitative autoradiographic receptor binding approaches were used to characterize the postnatal development of alpha 1-adrenergic and serotonergic receptors in the midbrain dorsal raphe nucleus (DRN) in rats. Dose-response data were obtained from extracellular records of the firing of DRN neurons recorded in midbrain slices perfused with artificial cerebral spinal fluid containing varying concentrations of the drugs investigated. These data indicated that DRN neurons, at all postnatal ages, are equally sensitive to the excitatory effects of phenylephrine (an alpha 1-norepinephrine agonist) and the inhibitory effects of serotonin (5-HT). In contrast, LSD (a 5-HT agonist) was considerably more potent in neonates as compared to adults. With one exception, the autoradiographic binding data was found to be in agreement with the electrophysiological findings. There were no observed changes in the number of alpha 1-adrenergic ([3H]prazosin) binding sites at different ages suggesting that cells in this region exhibit functionally mature alpha 1-adrenergic responses at birth. Similarly, the number of [3H]LSD binding sites was found to increase in older animals consistent with the increased potency of LSD in older animals. Paradoxically, the number of [3H]5-HT sites was observed to increase with age, a finding inconsistent with the functional data obtained using 5-HT in the slice. Hypotheses for the observed discrepancy are examined.

Increased behavioural response to intrathecal serotonin after lesion of serotonergic pathways with 5,7-dihydroxytryptamine seems not to be due to depletion of serotonin

The behavioural response to intrathecal i.th. serotonin (5-HT) was examined in mice pretreated with the neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) or the 5-HT synthesis inhibitor p-chlorophenylalanine (PCPA) which produce similar extensive depletion of central 5-HT levels. Intrathecal 5-HT (0.4 micrograms) elicited a behavioural response consisting of reciprocal hindlimb scratching and biting or licking of the hindquarters indicative of nociceptive stimulation. The response to i.th. 5-HT was markedly increased 5 days after intracerebroventricular (i.c.v.) injection of 5,7-DHT (80 micrograms base per mouse). On the other hand, 24 h after the last pretreatment injection of PCPA (400 mg kg-1 for 6 consecutive days), the response to i.th. 5-HT was unaltered. These results indicate that i.c.v. 5,7-DHT produces supersensitivity to 5-HT. Since PCPA failed to alter the effect of 5-HT, the supersensitivity seems not to be due to depletion of 5-HT levels after the lesion.

Validation of the I.T50 method for assessing neuronal responsiveness to microiontophoretic applications: a single-cell recording study

The I.T50 method consists of determining the charge required to obtain a 50% depression of firing activity of neurons recorded extracellularly with microiontophoretic applications of inhibitory agents. This method has been used successfully to detect modification of neuronal responsiveness, but the limits of its validity had never been determined. In the present study, it was found that the use of microiontophoretic currents greater than 3 nA yielded consistent I.T50 values when serotonin (5-HT) was applied to rat hippocampal pyramidal neurons. The departure from linearity of I.T50 values measured from applications carried out with a very low current (0.5 nA) of 5-HT is probably due to the relatively important contribution of the leak when a minimal ejecting current is used. The responsiveness to 5-HT was not altered by the activation of the recorded neuron produced by acetylcholine.

5-HT depletion with 5,7-DHT, PCA and PCPA in mice: differential effects on the sensitivity to 5-MeODMT, 8-OH-DPAT and 5-HTP as measured by two nociceptive tests

Depletion of 5-hydroxytryptamine (5-HT) in mice was produced by intracerebroventricular injection of 5,7-dihydroxytryptamine (5,7-DHT, 80 micrograms) or by systemic injections of p-chloroamphetamine (PCA, 3 X 40 or 4 X 40 mg/kg), p-chlorophenylalanine (PCPA, 5 X 400 or 14 X 400 mg/kg) or combined PCA (3 X 40 mg/kg) + PCPA (11 X 400 mg/kg). Neither of the pretreatments altered nociception in the increasing temperature hot-plate test, whereas hyperalgesia was demonstrated in 5,7-DHT lesioned animals in the tail-flick test. 5,7-DHT-pretreatment enhanced the antinociceptive effect of the 5-HT agonists 5-methoxy-N,N-dimethyltryptamine (5-MeODMT), 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and 5-hydroxytryptophan (5-HTP). This effect was observed after 2, 5 and 8 days in the tail-flick test and after 5 and 8 days in the hot-plate test. However, pretreatment with PCPA or PCA failed to alter the antinociception elicited by the 5-HT agonists, although a tendency towards enhancement of antinociception was found after combined treatment with PCA and PCPA. It is suggested that the injection of 5,7-DHT induces denervation supersensitivity of post-synaptic 5-HT receptors. The lack of such supersensitivity after PCPA-pretreatment which induces similar 5-HT depletion to 5,7-DHT, may suggest that other factors than the absence of 5-HT may contribute to the development of denervation supersensitivity. Alternatively, the three 5-HT depleting agents may produce a qualitatively different reduction of 5-HT.

Responsiveness of suprachiasmatic and ventral lateral geniculate neurons to serotonin and imipramine: a microiontophoretic study in normal and imipramine-treated rats

The suprachiasmatic nuclei (SCN) are a major pacemaker of circadian rhythms in mammals. The SCN receive a direct retinal projection and a second optic input via the ventral lateral geniculate nucleus (vLGN). Both visual pathways mediate the entrainment of circadian rhythms, whereas both the SCN and the vLGN receive serotonergic afferents from the raphe nuclei. We investigated the effects of microiontophoretically applied serotonin (5HT) on SCN and vLGN cells in normal rats and rats chronically treated with the 5HT reuptake blocker imipramine (IMI). In the SCN of both groups over 40% of all recorded cells (N = 80) responded to 5HT with a dose-dependent suppression of their spontaneous or glutamate-evoked discharge, while twenty percent were tonically light-responsive. Except for one cell with an inconsistent 5HT response, none of the visual SCN neurons were 5HT-sensitive. In the vLGN of normal and IMI-treated rats about 60% of the cells recorded (N = 42) were inhibited by 5HT. In IMI-treated rats a few cases of excitation by 5HT were encountered in the vLGN. Visual as well as non-visual vLGN cells were responsive to 5HT. Microiontophoretic application of IMI resulted in suppression of electrical activity in both brain regions and enhanced the response induced by 5HT. Chronic IMI-treatment produced a significant increase in the sensitivity of cells in the SCN and vLGN to iontophoresed 5HT, without affecting the relative magnitude of the inhibition. The recovery from 5HT-induced inhibition was slow in these animals. Interestingly, the spontaneous discharge rate of both 5HT-sensitive and 5HT-insensitive SCN and vLGN cells was significantly lower in the imipramine-treated group.(ABSTRACT TRUNCATED AT 250 WORDS)

Hypothermia induced by the putative 5-HT1A agonists LY165163 and 8-OH-DPAT is not prevented by 5-HT depletion

The putative 5-HT1A agonist 1-[2-(4-aminophenyl)ethyl]-4-(3-trifluoromethylphenyl)piperazine (LY165163, PAPP) (1, 2, 4, 10 mg/kg s.c.) caused a significant and dose-dependent hypothermia in rats, 30 and 60 min after injection. The decreases of temperature were less marked than that caused by 8-OH-DPAT 1 mg/kg s.c.). Depletion of brain serotonin (5-HT) by 91% following pretreatment with p-chlorophenylalanine (pCPA) (150 mg/kg i.p. on three successive days) significantly enhanced the hypothermic effects of both 8-OH-DPAT (0.25 mg/kg s.c.) and LY165163 (4 mg/kg s.c.). LY165163-induced hypothermia was also somewhat enhanced following depletion of hypothalamic 5-HT by 76% after infusion of 5,7-dihydroxytryptamine (5,7-DHT) (150 micrograms) into the third ventricle. Results indicate that the hypothermia induced by the putative 5-HT1A agonists LY165163 and 8-OH-DPAT in the rat is not dependent on presynaptic 5-HT stores and is therefore probably mediated by postsynaptic 5-HT receptors.

Differential effects of acute clozapine and haloperidol on the activity of ventral tegmental (A10) and nigrostriatal (A9) dopamine neurons

Extracellular single-unit recording techniques were used to evaluate the effects of acute intravenous (i.v.) clozapine (CLOZ) and haloperidol (HAL) on the activity of dopamine (DA) neurons of the ventral tegmental area (VTA or A10) and the substantia nigra pars compacta (SNC or A9). CLOZ increased the firing rate of A10 but not A9 cells, and drove 9/23 (39%) of A10 cells into an apparent depolarization blockade. HAL, on the other hand, produced a rate elevation and, at higher doses, depolarization inactivation in both subpopulations of DA neurons. Cell firing was restored in inactivated cells with i.v. apomorphine (APO) or iontophoretic GABA. CLOZ always fully reversed APO-induced suppression of A10 DA activity, but in many cases only partially reversed suppression of A9 DA neurons. Scopolamine did not mimic the effects of CLOZ on A10 neurons, and it also failed to block the activating effect of HAL on A9 units, indicating that the selective action of CLOZ cannot be interpreted simply by its anticholinergic properties. After hemi-transections of the diencephalon, which severed the medial forebrain bundle and other feedback pathways to the DA somata, CLOZ was still ineffective in altering A9 DA activity. This suggests that the lack of effect on CLOZ on A9 cells is not due to the inhibitory influence of forebrain feedback pathways. This hemi-transection also left intact the activation of A10 neurons produced by HAL and CLOZ, but it did prevent the excitatory action of HAL on most A9 neurons sampled. This indicates that forebrain feedback pathways are less critical in mediating the action of APDs on A10 DA neurons. Finally, iontophoretic application of CLOZ and HAL into the vicinity of DA cell bodies blocked the rate-reducing effects of locally applied DA, but not those of GABA. This suggests that both APDs block somatodendritic DA autoreceptors. However, HAL was considerably more potent than CLOZ in producing this blockade. It is suggested that the different pharmacological and clinical properties of HAL and CLOZ may be partially explained by a differential mode of action on the A10 and A9 subpopulations of DA cells. The data also provide pharmacological evidence that these 2 groups of DA cells are regulated by different mechanisms.

Enhancement of clonidine-induced analgesia by lesions induced with spinal and intracerebroventricular administration of 5,7-dihydroxytryptamine

The role of serotonin (5-HT) in analgesia induced by clonidine was examined by determining the effect of intraspinal (i.s.) and intracerebroventricular (i.c.v.) injections of 5,7-dihydroxytryptamine (5,7-DHT) on analgesia produced by clonidine in the tail-flick and hot plate tests. Depletion of amines was verified by high performance liquid chromatography analysis. Intraspinal injections of 5,7-DHT potentiated the action of clonidine in both tests for analgesia and caused depletion of 5-HT in the spinal cord. Intracerebroventricularly injected 5,7-DHT also increased the action of clonidine and depleted 5-HT in brain as well as in the spinal cord. In the groups given intracerebroventricular injections, there appeared to be a biphasic increase in the action of the clonidine. Significant hyperalgesia from pretreatment with neurotoxin was observed only on a limited number of occasions. The present results indicate that 5-HT mechanisms in the CNS are important mediators of the analgesic action of clonidine. Interactions between clonidine and 5-HT systems at both spinal and supraspinal sites are considered.

Diurnal rhythms in the responsiveness of hippocampal pyramidal neurons to serotonin, norepinephrine, gamma-aminobutyric acid and acetylcholine

Radioligand binding studies have revealed the existence of endogenous circadian rhythms in the number of several receptors in the rat brain. The present microiontophoretic study was undertaken to assess diurnal rhythms in the responsiveness of rat hippocampal pyramidal neurons to serotonin (5-HT), norepinephrine (NE), gamma-aminobutyric acid (GABA), and acetylcholine (ACh). Between December and April, there was a significant diurnal variation in the responsiveness of hippocampal pyramidal neurons to 5-HT and ACh. Between May and August, the responsiveness to NE and ACh showed a diurnal variation. There was no diurnal variation in the responsiveness to GABA in either period of the year. Short-term exposure to constant light or darkness produced a phase-shift of the serotoninergic and cholinergic rhythms, suggesting their endogenous nature and their synchronization to clock-time by the light-dark cycle. The diurnal rhythms in responsiveness to 5-HT and NE underwent phase-shifts from the December-April to the May-August period in rats entrained to 12:12 light-dark cycle, suggesting the existence of seasonal modulation of these rhythms. These circadian rhythms in the postsynaptic responsiveness of hippocampal pyramidal cells and their seasonal fluctuation may be related to the diurnal variation of mood seen in major depression as well as to the seasonal incidence of this illness.

Adaptive responsiveness of some central receptors to the denervation of heterologous afferent fibers: functional significance and possible behavioral consequences

As shown in the rat brain, the development of the classical "denervation supersensitivity" of receptors does not always occur despite the complete destruction of the corresponding afferent fibers. This is due to the concomitant destruction of heterologous fibers which directly or indirectly contribute to the regulation of the denervated receptors. In this review, on the basis of data obtained in rats with lesions of ascending DA neurons, three examples of hetero-regulation of receptors will be provided. In the prefrontal cortex, the concomitant destruction of afferent noradrenergic fibers (NA) prevents the development of the denervation supersensitivity of DA receptors of the D1 type. In the nucleus accumbens, the development of the denervation supersensitivity of D1 receptors seems to be dependent on the state of activity of the cortico-nucleus accumbens glutamatergic projection. Finally, in cortico-limbic structures, the prolonged interruption of DA transmission leads to an increased density of 125I-neurotensin binding sites. These hetero-regulations reflect the existence of possible functional relationships between the two types of neuronal populations under investigation. As an example, it will be shown that some dysfunctions seen following the degeneration of the mesocortico-prefrontal and mesolimbic DA neurons disappear following the combined destruction of ascending NA neurons.