Claude de Montigny

Effects of a selective 5-HT reuptake blocker, citalopram, on the sensitivity of 5-HT autoreceptors: Electrophysiological studies in the rat brain

SummaryCitalopram (CIT), is a selective serotonin (5-HT) reuptake blocker and a clinically effective antidepressant. The present electrophysiological studies were undertaken to investigate in vivo the acute and long-term effects of CIT administration on 5-HT neurotransmission. In a first series of experiments, a single dose of CIT (0.05–0.5 mg/kg) was administered intravenously to naive rats while recording the activity of a 5-HT-containing neuron in the nucleus raphe dorsalis. A dose-response relationship of the inhibitory effect of CIT on the firing activity of 5-HT neurons was obtained with an ED50 of 0.23±0.03 mg/kg. In a second series of experiments, rats were treated with CIT (20 mg/kg/day, i.p.) for 2, 7 and 14 days. In rats treated for 2 days, there was a marked reduction in the firing activity of 5-HT neurons in the nucleus raphe dorsalis; there was a partial recovery after 7 days and a complete recovery after 14 days of treatment. The response of 5-HT neurons to intravenously administered LSD was decreased in rats treated for 14 days with CIT, indicating a desensitization of the somatodendritic 5-HT autoreceptor. In a third series of experiments, carried out in rats treated with CIT (20 mg/kg/day, i.p.) for 14 days, the suppression of firing activity of CA3 hippocampal pyramidal neurons produced by microiontophoretically-applied 5-HT and by the electrical activation of the ascending 5-HT pathway was measured. Long-term treatment with CIT did not modify the responsiveness of these neurons to microiontophoretically-applied 5-HT; however, the effect of the electrical activation of the ascending 5-HT pathway on these same neurons was enhanced. To determine if 5-HT reuptake blockade could be responsible for this enhancement, CIT (1 mg/kg) was injected intravenously in naive rats while stimulating the ascending 5-HT pathway; it failed to modify the effectiveness of the stimulation. To assess the involvement of the 5-HT terminal autoreceptor, methiothepin, a 5-HT autoreceptor antagonist, was injected intravenously (1 mg/kg) in naive rats and in rats treated for 14 days with CIT while stimulating the ascending 5-HT pathway. Methiothepin enhanced the effect of the stimulation in naive rats but failed to do so in the CIT-treated rats. It is concluded that long-term CIT treatment enhances 5-HT neurotransmission by desensitizing both the somatodendritic and terminal 5-HT autoreceptors.

Short-term lithium administration enhances serotonergic neurotransmission: electrophysiological evidence in the rat CNS

Rats received lithium-containing chow for 48 h. Brain and plasma lithium concentrations ranged from 0.4 to 1.0 mEq/l. A first series of experiments served to assess the responsiveness of CA3 hippocampal pyramidal neurons to microiontophoretically applied serotonin (5-HT) and norepinephrine (NE). The response of the same neurons to electrical stimulation of the ventro-medial 5-HT pathway was measured after lithium treatment. The responsiveness to 5-HT and NE was not modified whereas the effect of activation of the ascending 5-HT pathway was increased two-fold by the lithium treatment. These neurons were activated to their physiological firing rate by means of small ejection currents of acetylcholine. A pretreatment with the 5-HT neurotoxin 5,7-dihydroxytryptamine abolished the response to the electrical stimulation in lithium-treated rats. In a second series of experiments, unitary recordings of 5-HT neurons were obtained from the dorsal raphe nucleus. The lithium treatment modified neither the number of spontaneously active 5-HT neurons nor their mean firing rate. These results provide direct electrophysiological evidence for the enhancement of 5-HT neurotransmission by short-term lithium treatment through its presynaptic action on 5-HT terminals.

Effects of the two antidepressant drugs mianserin and indalpine on the serotonergic system: single-cell studies in the rat

Several antidepressant treatments enhance serotonergic neurotransmission. The present electrophysiological studies were undertaken to assess the effect of mianserin and indalpine, two antidepressant drugs with different pharmacological profiles, on serotonergic neurotransmission. In a first series of experiments, the responsiveness of hippocampal pyramidal neurons to microiontophoretic applications of serotonin (5-HT), norepinephrine (NE) and γ-aminobutyric acid (GABA) was assessed following mianserin, imipramine (5 mg/kg/day IP) or saline administration for 14 days. At 48 h after the last dose of mianserin, responsiveness to 5-HT was increased whereas that to NE and GABA was not modified. The degree of sensitization to 5-HT was the same as that produced by imipramine. Acute IV administration of mianserin (up to 10 mg/kg) did not decrease the firing rate of dorsal raphe 5-HT neurons. In a second series of experiments, long-term administration of indalpine (5 mg/kg/day IP for 14 days) did not modify the responsiveness of hippocampal pyramidal neurons to microiontophoretically applied 5-HT, NE and GABA whereas imipramine treatment (5 mg/kg/day IP) increased selectively their sensitivity to 5-HT when compared to indalpine-treated rats. In keeping with its potent reuptake-blocking property, acute IV indalpine produced a marked decrease in the firing rate of dorsal raphe 5-HT neurons (ED50 0.33 mg/kg). The firing rate of dorsal raphe 5-HT neurons was assessed following 2-, 7- and 14-day treatments with indalpine (5 mg/day IP). After 2 days, the firing rate of 5-HT neurons was greatly reduced, after 7 days it had recovered partially and after 14 days it had returned to normal. At this point, the responsiveness of 5-HT neurons to IV LSD, an agonist of the 5-HT autoreceptor, and to microiontophoretically-applied 5-HT was decreased twofold, indicating desensitization of the autoreceptor. In conclusion, it is proposed that long-term treatment with mianserin, as with tricyclic antidepressant drugs and electroconvulsive shocks, increases 5-HT neurotransmission via sensitization of postsynaptic neurons to 5-HT whereas long-term treatment with indalpine, as with zimelidine, results in the same final effect via its presynaptic effect on 5-HT neurons presumably by blocking 5-HT reuptake. These data further support the notion that enhancing 5-HT neurotransmission might have an antidepressant effect.

Effects of PK 8165, a partial benzodiazepine receptor agonist, on cholecystokinin-induced activation of hippocampal pyramidal neurons: A microiontophoretic study in the rat

Benzodiazepines (BZD) have been reported to suppress cholecystokinin-8S (CCK-8S)-induced activation. PK 8165, a ligand of BZD receptors, is an anxiolytic devoid of sedative and anticonvulsant effects. PK 8165, applied microiontophoretically or administered i.v. at low doses, suppressed CCK-8S-induced activation of hippocampal pyramidal neurons, whereas, at high doses it antagonized the effect of microiontophoretic applications of flurazepam. These results indicate that PK 8165 acts as a mixed agonist-antagonist at BZD receptors and suggest that the suppression of CCK-8S-induced activation by BZD might be related to their anxiolytic property rather than to their sedative or anticonvulsant activity.

Sensitization of rat forebrain neurons to serotonin by adinazolam, an antidepressant triazolobenzodiazepine.

Adinazolam , a triazolobenzodiazepine , has been reported to be an effective antidepressant treatment in major depression. In the present study, a 14-day (but not a five-day) treatment with adinazolam enhanced the responsiveness of rat hippocampal pyramidal neurons to microiontophoretically applied 5-HT but not to NE. A 14-day treatment with diazepam failed to induce sensitization to either 5-HT or NE. Acute intravenous administration of adinazolam did not modify dorsal raphe 5-HT neuron firing rate. It is proposed that the antidepressant activity of adinazolam might be mediated by a heightened 5-HT neurotransmission.

Effect of repeated amiflamine administration on serotonergic and noradrenergic neurotransmission: electrophysiological studies in the rat CNS

SummaryAmiflamine is a selective and reversible inhibitor of monoamine oxidase (MAO) type A which exerts a preferential effect on serotonin (5-HT) catabolism. The present studies were undertaken to compare the effects of repeated administration of amiflamine (2 mg/kg, twice daily) on several aspects of the functioning of the 5-HT and norepinephrine (NE) systems in the rat CNS. The activity of MAO-A and B was assessed in forebrain slices and the whole brain contents of the neurotransmitters and their metabolites were determined by HPLC after 2-, 7- and 21-day treatments. MAO-A was inhibited by about 50% 2 h after the last dose and its activity was back to normal in rats sacrificed 12 h after the last dose. The activity of MAO-B was unaffected two or 12 h after the last dose. Whole brain concentration of 5-HT was increased to a greater degree than that of NE following repeated administration of amiflamine. These increases in 5-HT and NE were accompanied by decreased levels of their respective metabolites 5-hydroxy-indoleacetic acid and 3-methoxy-4-hydroxyphenylethyleneglycol. The firing activity of dorsal raphe 5-HT neurons, but not that of NE neurons, was markedl decreased 2–6 h after the last dose of a 2-day treatment. However, 2–6 h after a 21-day treatment, the firing activity of 5-HT neurons was back to normal, whereas that of NE neurons was decreased by 30%. The recovery of firing activity of 5-HT neurons following long-term amiflamine is attributable to a desensitization of the somatic 5-HT autoreceptor as indicated by the decreased responsiveness of 5-HT neurons to intravenous LSD. In contrast, the sensitivity of the somatic NE autoreceptor was not modified, as verified by the unchanged responsiveness of NE neurons to clonidine, an α2 agonist. The duration of the suppression of firing activity of hippocampus pyramidal neurons produced by the electrical activation of the ascending 5-HT pathway was prolonged in amiflamine-treated rats as compared to controls. This increased synaptic efficacy was attributable to a presynaptic modification since the responsiveness of the same neurons to microiontophoretically-applied 5-HT was not changed. The efficacy of the stimulation of the dorsal NE bundle, as well as that of the microiontophoretic applied of NE, were unchanged in the same rats. Since the efficacy of the stimulation of the 5-HT pathway is increased at the time 5-HT neurons have regained their normal firing rate, it is concluded that 5-HT neurotransmission is enhanced following the long-term treatment with amiflamine.

Diurnal rhythms in the responsiveness of hippocampal pyramidal neurons to serotonin, norepinephrine, γ-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.

A Comparative Double-blind Controlled Study of Trimipramine and Amitriptyline in Major Depression: Lack of Correlation with 5-hydroxytryptamine Reuptake Blockade

Thirty-four hospitalized patients with major depression were enrolled in a 3-week double-blind parallel comparative study of trimipramine and amitriptyline. Following a 1-week washout period, patients randomly received one of the two drugs up to 100 mg twice daily on a fixed increment dosage schedule. Both treatments produced a rapid significant clinical improvement that occurred in a predominantly linear fashion. The pattern of improvement was very similar with both drugs. There was no significant correlation between plasma levels of trimipramine and desmethyl-trimipramine and clinical improvement. A negative correlation between amitriptyline plasma levels and clinical improvement was found, whereas a positive correlation occurred with the nortripty-line levels. Amitriptyline, and to a lesser extent trimipramine, prolonged intracardiac conduction. In the amitriptyline group only, this effect was accompanied by significant increases of heart rate and blood pressure. Platelet serotonin content was decreased by 57% by the amitriptyline treatment but remained unchanged in the trimipramine group. This finding constitutes the first clinical evidence that trimipramine does not exert its anti-depressant effect through 5-hydroxytryptamine reuptake blockade. It is proposed that neuronal sensitization to 5-hydroxytryptamine might mediate the therapeutic effect of tricyclic antidepres-sant drugs.

Antagonism of Cholecystokinin-induced Activation by Benzodiazepine Receptor Agonists

Cholecystokinin (CCK) has been found in the central nervous system, with a particularly high density in the cerebral cortex, the amygdala, and the hippocampus of all the mammalian species studied so far;’-5 and, specific CCK binding sites have been demonstrated in the rat, guinea pig, and human Of the 39-, 33-, 13-, 8-, and 4-amino acid-residue molecular forms of CCK, the sulfated octapeptide (CCK-8(s)) seems to be the most abundant.238-’0 Microiontophoretic application of this peptide exerts a consistent excitatory effect on deep cortical and hippocampal pyramidal neurons.’ ‘-I6 Specific binding sites for benzodiazepines have been known to exist for several These binding sites meet the generally accepted criteria for “receptors.” Several molecules have been proposed as possible endogenous ligands of these receptors; however, to our knowledge, none has yet fulfilled all criteria to be considered as such. Recent progress in the field has made it possible to distinguish between different types of BZD receptors, the most important distinction being between the “neuronal” and “peripheral” types.I9 There is now ample evidence that the former type mediates the anxiolytic effect of BZD’S.”.’~ In the last few years, several non-BZD molecules have been found to bind to BZD receptors. Interestingly, these molecules have proven to either mimic or block the behavioral effect of BZD’s (such as in the anticonflict model), indicating that, despite their different structure, their binding to BZD receptors enables them to assume the properties of agonistic or antagonistic BZD’s. However, some of these new non-BZD molecules appear to have mixed agonist-antagonist (or partial agonist) properties. This is exemplified by the PK 8 165 compound, a quinoleic acid derivative, which exerts, a t low doses, an anticonflict activity2’.22 but blocks, a t high doses, the effect of d ia~epam.’~ Furthermore, in several biochemical paradigms, PK 8 165 also behaves like a mixed agoni~t-antagonist.~~~’~ Interestingly, recent clinical studies have demonstrated the clinical anxiolytic effect of PK 8165 in generalized anxiety (Le Fur, personal communication).

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.