Tricyclic antidepressants: long-term treatment increases responsivity of rat forebrain neurons to serotonin

Prevention of the serotonin syndrome in rats by repeated administration of monoamine oxidase inhibitors but not tricyclic antidepressants

The serotonin syndrome, a behavioral response produced by the activation of serotonin receptors, and 3H-serotonin binding were examined after repeated treatment of rats with different types of antidepressant drugs. The serotonin syndrome was produced by the direct-acting serotonin receptor agonists 5-methoxy-N,N-dimethyltryptamine (5-MeDMT) or d-lysergic acid diethylamide (LSD). Repeated, but not acute treatment of rats with monoamine oxidase inhibitors (nialamide, pargyline, and phenelzine) prevented the serotonin syndrome in response to either 5-MeDMT or LSD and also reduced 3H-serotonin binding in the brain stem and spinal cord. Pretreatment of rats with p-chlorophenylalanine blocked the ability of nialamide treatment to inhibit the serotonin syndrome caused by 5-MeDMT. By contrast, neither the serotonin syndrome or 3H-serotonin binding was affected significantly by the repeated administration of tricyclic antidepressants (amitriptyline, desmethylimipramine, and chlorimipramine) or iprindole. Repeated monoamine oxidase inhibitor treatments may prevent the serotonin syndrome by causing a reduction of 3H-serotonin receptor binding sites in the brain stem and/or spinal cord.

Quantitation of iprindole in plasma by GLC

A specific, sensitive, rapid, and reproducible method for iprindole in human plasma was developed using gas chromatography with trimipramine as internal standard. The sensitivity of the method is of 5 ng ml-1 and a linearity was obtained for concentrations ranging from 12.5 ng ml-1 to 100 ng ml-1 with a regression coefficient of 0.9872. The plasma levels of iprindole were determined in five healthy volunteers following the administration of a single oral dose of 60 mg and in four patients admitted for endogenous depression after a 3-week administration of 90 mg d-1. Following a single oral dose to healthy volunteers, the maximum concentrations occurred between 2 and 4 h after administration of the drug and the mean half-life value was 52.5 h. In patients the steady-state concentrations of iprindole ranged between 18 and 77 ng ml-1.

Brain monoamine oxidase activity after in vivo-chronic iprindole treatment

The effects of chronic administration of three different doses of iprindole on the monoamine oxidase activity and neurotransmitter content were studied in the cerebral cortex, the mesencephalon and the cerebellum of mouse brain. The treatment inhibited mitochondrial monoamine oxidase activity of all three brain regions studied, although a dose-response inhibitory effect was found only in the mesencephalon. Brain regional serotonin contents were markedly increased after iprindole treatment. The 5-hydroxyindoleacetic acid contents showed a significant decrease only in the cerebral cortex. Neither dopamine nor norepinephrine brain regional contents were significantly altered. On the basis of these results it is suggested that a substrate-specific inhibition of monoamine oxidase is involved in the mechanism of action of this tricyclic antidepressant.

Enhancement of responsiveness of the central serotonergic system and serotonin-2 receptor density in rat frontal cortex by electroconvulsive treatment

The effect of a series of 10 electroconvulsive shocks (ECS) on the hyperthermic response of heat-adapted rats to a serotonergic stimulant, 3-chlorophenylpiperazine (CPP), and on radioligand binding to serotonin-1 and serotonin-2 receptors in frontal cortical membranes was investigated. Chronic ECS augmented the hyperthermic response of rats to CPP, did not affect [3H]serotonin binding to serotonin-1 receptors, and increased [3H]spiroperidol binding to serotonin-2 receptors. The treatment increased the density of serotonin-2 receptors by about 40%. It is suggested that the increased responsiveness of the central serotonergic system may be an important factor in the antidepressant action of chronic ECS.

Thyrotropin releasing hormone potentiates the effects of imipramine on brain serotonergic system

Daily administration of thyrotropin-releasing hormone (TRH) (20 mg/kg) for 10 days in 2 equally divided doses increased 5-hydroxytryptamine and/or 5-hydroxyindoleacetic acid levels in certain brain areas. Chronic imipramine treatment (10 mg/kg) for 10 days significantly decreased the levels of brain 5-hydroxytryptamine and 5-hydroxyindoleacetic acid. When TRH was administered concurrently with imipramine (10 mg/kg), this neuropeptide significantly potentiated the effects of imipramine on lowering of endogenous levels of 5-hydroxytryptamine (striatum, hypothalamus) and 5-hydroxyindoleacetic acid (striatum, midbrain, pons-medulla). Our data provide a possible neurochemical basis for the reported potentiation of tricyclic antidepressant action by TRH.

Chemistry, pharmacology, pharmacokinetics, adverse effects, and efficacy of the antidepressant maprotiline hydrochloride

Maprotiline, a tetracyclic antidepressant with sedative properties, exhibits strong inhibitory effects on norepinephrine uptake across nerve cell membranes but interferes relatively little with serotoninergic mechanisms. The biological half-life of unchanged maprotiline in blood averages 43 hours. Though several studies suggest a more rapid onset of antidepressant effects with maprotiline than with amitriptyline or imipramine, this issue remains unresolved. The adverse effect profile of maprotiline is similar to that of the tricyclic antidepressants, except that rashes are about twice as frequent with maprotiline as with amitriptyline or imipramine. The most frequent adverse reactions are anticholinergic effects and sedation. Data suggest less frequent and severe anticholinergic side effects with maprotiline than with amitriptyline. Maprotiline may be less likely to induce orthostatic hypotension and tachycardia than standard tricyclic antidepressants, but clinically important differences in cardiovascular effects remain to be conclusively demonstrated. Many patients benefit from the convenience of once daily dosing. Maprotiline is comparable in antidepressant efficacy to the tricyclic antidepressants.

The sensitivity of cortical neurons to serotonin: effect of chronic treatment with antidepressants, serotonin-uptake inhibitors and monoamine-oxidase-blocking drugs

The sensitivity of rostral and cingulate cortical neurons to microiontophoretically administered serotonin (5-HT) was compared in groups of rats treated either acutely or chronically for different periods with various drugs. The drugs used were: desipramine (10 mg/kg), clomipramine (10 mg/kg), CGP 6085 (10 mg/kg), clorgyline (0.3 mg/kg), and deprenyl (1 mg/kg). Serotonin and, in some instances, gamma-aminobutyric acid (GABA) were applied microiontophoretically over periods of 60 sec with various ejection currents to spontaneously active neurons in the rostral and cingulate cortex. Of all the compounds tested, only clorgyline produced a marked desensitization to 5-HT in both cortical areas. After prolonged treatment with all the other drugs, no change in the sensitivity to serotonin was observed. The desensitization to 5-HT induced by clorgyline developed after 4 to 10 days of treatment. The responsiveness of these cells to GABA was unchanged after chronic exposure to clorgyline. The present results are consistent with those biochemical studies showing that chronic treatment with 5-HT-uptake-blocking compounds has no effect on 5-HT-binding characteristics, as well as with the observation that prolonged treatment with the monoamine-oxidase A-type blocker clorgyline reduces the number of 5-HT-binding sites.

Relevance of tryptophan and tyrosine availability in endogenous and 'non-endogenous' depressives treated with imipramine or clomipramine

This study was performed on 65 depressed in-patients who were included in previously reported trials of imipramine and clomipramine. Before and during treatment, blood samples were collected for estimation of the availability of tryptophan and tyrosine by measurement of their plasma ratios to competing amino acids, and for determination of plasma steady-state concentrations of imipramine, clomipramine and their demethylated metabolites. The patients were classified as endogenous or 'non-endogenous' depressives by means of diagnostic rating scales, and therapeutic efficacy was evaluated by means of the Hamilton rating scales. Neither imipramine nor clomipramine increased the availability of tryptophan or tyrosine. Three biochemical regions were defined: a low region including mostly patients with subnormal availability of both tryptophan and tyrosine, a medium region, and a high region including mostly patients with supernormal precursor availabilities. Endogenous depressives showed about the same biochemical distribution as controls whereas there tended to be a proportionately higher number of 'non-endogenous' depressives within the low region. Patients in the low region, irrespective of diagnostic classification, improved faster and more on imipramine than patients in the medium and high regions with comparable plasma drug levels. Patients on clomipramine tended to show a relationship between precursor availabilities and clinical response but no definite conclusion could be drawn from these data. The results suggest that determination of the pre-treatment tryptophan and tyrosine availability may be superior to diagnostic classification in predicting response to imipramine. The possible mode of action of tricyclic antidepressants is briefly discussed.

alpha 1-Adrenoceptor-mediated responses in the lateral geniculate nucleus are enhanced by chronic antidepressant treatment

Responses of single dorsal lateral geniculate neurons to iontophoretic noradrenaline, serotonin, and carbachol were studied following acute or chronic administration of tricyclic antidepressants. Long-term (15-20 day) treatment of rats with a variety of clinically effective tricyclics, including the atypical iprindole, significantly enhanced alpha 1-adrenoceptor mediated activations induced by noradrenaline. This change appears to require chronic treatment, since acute and short term (6-8 day) tricyclic regimens fail to consistently affect noradrenaline's action. Long term antidepressant treatment was also effective in enhancing geniculate neuron sensitivity to serotonin, in accord with previous studies, but failed to modify responses to the cholinergic agonist carbachol. It is suggested that the modulation of noradrenaline and serotonin receptor activity may represent a slowly developing action of tricyclic antidepressants which correlates with their delayed clinical effects.

REM sleep deprivation changes behavioral response to catecholaminergic and serotonergic receptor activation in rats

The effects of REM sleep deprivation (REMD) on apomorphine-induced aggressiveness and quipazine-induced head twitches in rats were determined. Forty-eight hr of REMD increased apomorphine-induced aggressiveness, and reduced (immediately after completing of REMD) or increased (96 hr after completing of REMD) quipazine-induced head twitches. Results are discussed in terms of similarity to pharmacological effects of other antidepressive treatments.

Central pre- and postsynaptic monoamine receptors in antidepressant therapy

Activation of postsynaptic noradrenergic alpha 1-receptors may be involved in the mediation of psychomotor activating effects of tricyclic antidepressant (TCA) drugs. On the other hand, the pronounced sedative properties of some TCA drugs seem to be correlated with their alpha 1-receptor blocking capacity. The presynaptic alpha 2-receptors probably mediate the feed back inhibition of central NE neurons seen after administration of TCA drugs, particularly the secondary amines. Yet other antidepressants such as mianserin are potent antagonists at central alpha 2-receptors, a phenomenon which can even cause activation of brain NE neurons and form a basis for their therapeutic action. beta-Receptor activation in the brain is also suggested to participate in the therapeutic effect of several drugs, e.g. mianserin and the putative antidepressant agent salbutamol, a beta 2- receptor agonist. A reduced central beta-receptor activation may, accordingly, contribute to depressive symptoms associated with treatment with beta-adrenoceptor blocking drugs, both by their action per se as well as by secondary effects on the monoamine systems, which we recently have demonstrated. Facilitation of brain 5-HT neurotransmission seems to be achieved with several TCA drugs not only via inhibition of reuptake but also through sensitization of postsynaptic 5-HT receptors, developing during repeated treatment. In contrast the "presynaptic" 5-HT receptors do not show increased sensitivity during chronic TCA drug treatment, thus allowing for an enhanced synaptic effect of 5-HT induced by TCA drugs.

The pharmacology of zimelidine: a 5-HT selective reuptake inhibitor

Zimelidine (ZIM) and its main active metabolite norzimelidine (NZIM) have been shown to preferentially inhibit 5-hydroxytryptamine (5-HT) neuronal uptake both in vitro and in vivo while having much less effect on noradrenaline (NA) uptake. ZIM in vivo blocked the 5-HT uptake mechanism in the cerebral cortex, hippocampus, striatum, hypothalamus and spinal cord, thus indicating effects on both the ascending and descending 5-HT pathways. ZIM is devoid of a 5-HT releasing action, MAO-inhibitory properties and effects on dopamine (DA) uptake. ZIM failed to reduce NA turnover even in high doses, but markedly reduced 5-HT turnover in very low doses in the rat. ZIM also enhanced 5-HT mediated behaviours in mice in doses related to the inhibition of 5-HT uptake. In contrast to amitriptyline (AMI) and mianserin (MIAN), ZIM only in extremely high doses displayed a 5-HT receptor blocking action in vitro and failed to block 5-HT mediated behaviour. ZIM was practically devoid of action on histamine H1 and H2 receptors, and had also a neglible action on noradrenergic alpha 1- and alpha 2-receptors, and on beta-receptors. Unlike the tricyclic antidepressants (TAD's) ZIM had a negligible action on muscarinic receptors and failed to affect cholinergic induced activity. Long-term treatment with ZIM did not result in any attenuation of the 5-HT uptake blocking potency or the reduction of 5-HT turnover. This long-term treatment slightly reduced the number of beta-receptors in the brain. However, repeated ZIM-treatment induced a new 5-HT receptor binding site characterized by a low affinity and with a high number of binding sites and decreased the number of high affinity 5-HT receptor binding sites. Unlike the TAD's zimelidine failed to block the action of reserpine. Metabolic and behavioural interactions studies in mice showed that ZIM was devoid of any significant interactions with ethanol, barbiturates and benzodiazepines. It is concluded that ZIM markedly differs from both the TAD's and new antidepressants such as mianserin and nomifensine. ZIM seems preferentially to effect the presynaptic 5-HT reuptake mechanism while having a negligible action on noradrenergic, 5-HT, acetylcholine and histamine receptors in the brain.

Differential effect of chronic desipramine and amitriptyline treatment on rat brain adrenergic and serotonergic receptors

Serotonergic and adrenergic receptors were examine in rat brains by direct binding assays after chronic treatment with tricyclic antidepressants. Chronic amitriptyline (AMT) treatment (10 mg/kg/day i.p. for 21 days) decreased specific 3H-spiperone binding in the cortex and not in the caudate nuclei. Specific 3H-dihydroalprenolol (3H-DHA) binding was reduced in cerebellar but not in cerebral cortex. Chronic desipramine (DMI) treatment given in the same dose schedule, on the other hand, decreased specific 3H-DHA binding in both cerebellar and cerebral cortex. Specific 3H-spiperone binding in the cerebral cortex was also reduced but to a lesser extent than that in the AMT treated group. Scatchard analysis showed that reductions in 3H-spiperone or 3H-DHA binding in all cases were due to decreases in number of binding sites (Bmax) and not to changes in dissociation constants (KD). No change was observed in 3H-serotonin (3H-5HT), 3H-clonidine, or 3H-WB-4101 binding. The results show that there is no single common change in brain adrenergic and serotonergic receptors after chronic AMT and DMI treatment.

Effect of antidepressants, lithium and electroconvulsive treatment on rat serum prolactin levels

The ability of antidepressant drugs, electroconvulsive treatment (ECT), or lithium chloride (LiCl), to modify prolactin secretion in the rat was studied. Chlorimipramine, citalopram, fluoxetine, imipramine and zimelidine potentiated the low dose 5-hydroxytryptophan (5-HTP)-induced increase in prolactin secretion, suggesting inhibition of serotonin (5-HT) uptake by these drugs. Amitriptyline, doxepin, iprindole, mianserin and trazadone inhibited the prolactin stimulating effects of high doses of 5-HTP and quipazine, suggesting that these drugs have 5-HT receptor blocking properties. Tandamine inhibited only 5-HTP-induced increase in prolactin secretion. Chronic administration of imipramine, potentiated the effect of low dose 5-HTP significantly more than an acute dose. Amitriptyline, produced similar inhibition of the 5-HTP-induced increase in prolactin secretion after both acute and chronic administration. The ability of bupropion and mazindol to inhibit alpha-methylparatyrosine-induced prolactin secretion, and of nomifensine to inhibit reserpine-induced prolactin secretion, is consistent with other evidence that these agents are indirect dopamine (DA) agonists. Desipramine, acutely, had no effect on any of the above paradigms but after chronic administration, potentiated the effect of low dose 5-HTP on prolactin secretion. Nortriptyline had no effect on prolactin secretion after acute or chronic treatment. ECT for 10 days did not affect the ability of a 5-HT agonist or d-amphetamine to modify prolactin secretion. However, chronic, but not acute, treatment with LiCl markedly enhanced the prolactin response to 5-HT agonists and reserpine while shifting the dose response curve for d-amphetamine and apomorphine to the right. These results are discussed in light of current theories of the role of 5-HT and DA in depression.

Subchronic treatment with the tricyclic antidepressant DMI increases isolation-induced fighting in rats

Male rats treated with desmethylimipramine (DMI) (20 mg/kg for 7 days) were more likely than controls to attack an intruder rat placed in their home cage; they were also more likely to submit when attacked by the intruder. These behavioural changes were not seen at lower doses of DMI. Similar results were obtained in experiments in which is drugged animal and a control were placed together in a "neutral" cage; in this paradigm it was also found that lower doses of DMI were effective, provided that either the period of drug treatment was increased, or a delay of 3-4 days after withdrawal of DMI preceded behavioural testing. A dose dependent resistance to handling developed during drug treatment; drugged animals also showed weight loss and decreased open-field activity. In previous studies, acute treatment with tricyclic antidepressants has not been found to increase fighting; the present results underline the importance of chronic drug studies.

Some current problems related to the mode of action of antidepressant drugs

An important relationship between antidepressant drugs and the monoamine transmitters of the brain is generally recognized. Major groups of established antidepressants are inhibitors of monoamine inactivation (by reuptake or monoamine oxidase) or monoamine precursors. However, the tricyclic antidepressants have many actions in addition to the blockade of monoamine reuptake, e.g. blockade of muscarinic, alpha-adrenergic and histaminic receptors and general membrane effects which may lead to cardiotoxicity. The discovery that certain antihistaminic agents devoid of condensed ring systems are fairly potent inhibitors of monoamine reuptake led to the synthesis of zimelidine, a potent and selective 5-HT reuptake inhibitor with little affinity for receptors and almost devoid of cardiotoxicity. Careful pharmacological studies and clinical trials with such selective monoaminergic agents are of considerable value for testing the possible role of monoaminergic mechanisms in various aspects of human and animal behaviour and for central nervous disorders, including depression.

Effects of antidepressant drugs on different receptors in the brain

Radioligand receptor binding techniques were used to characterize the effects of different structural types of antidepressant drugs on neurotransmitter receptors. The tricyclic antidepressants more or less potently inhibited the binding to rat brain preparations of several different radiolabelled ligands [3H]WB4101, [3H]QNB, [3H]-d-LSD, [3H]mepyramine). The potency of the nontricyclic antidepressants varied greatly. Mianserin, potently displaced [3H]mepyramine, [3H]d-LSD and [3H]WB4101 while it was very weak on [3H]QNB-binding. Nomifensine and the specific 5-HT uptake inhibitors zimelidine and alaproclate had very low affinity for these receptors. All the antidepressants tested were practically devoid of activity on [3H]DHA binding, [3H]spiroperidol binding, [3H]flunitrazepam binding, [3H]muscimol binding and [3H]naloxone binding. The implications of these findings for biogenic amine theories of affective disorders are discussed.

Lithium induces rapid relief of depression in tricyclic antidepressant drug non-responders

Eight patients suffering from a major unipolar depression and having field to respond to treatment for three weeks or more with tricyclic antidepressants were given lithium. All eight patients experienced a remarkable relief of their depression within 48 hours. This rapid antidepressant effect of lithium in "treatment-resistant' patients might be due to the enhancement of the efficacy of the central serotoninergic system, unveiling the tricyclic antidepressant-induced sensitization of the serotoninergic postsynaptic receptors.

A pilot study of the mechanism of action of desipramine

To test the hypothesis the desipramine alters alpha adrenoceptor function in depressed patients, the effects of clonidine upon growth hormone sedation and blood pressure have been measured in depressed patients before and after treatment with desipramine. After three weeks of treatment the hypotensive and sedative effects of clonidine were inhibited in all patients even though plasma desipramine concentrations at this time varied from 42 to 560 micrograms/l. Growth hormone responses to clonidine were enhanced in five of the six patients but this effect was not statistically significant. These findings are consistent with the hypothesis that in these patients desipramine altered alpha adrenoceptor function: other explanation are discussed.

Differential effects of clomipramine and clorgyline on the sensitivity of cortical neurons to serotonin: effect of chronic treatment

The sensitivity of rostral and cingulate cortical neurons to microiontophoretically administered serotonin (5-HT) was compared in groups of rats treated either acutely or chronically with clomipramine (10 mg/kg i.p.) or with clorgyline (0.3 mg/kg i.p.). The sensitivity of cortical neurons to 5-HT was significantly reduced in both cortical areas of animals treated chronically with clorgyline. By contrast, no change of sensitivity was observed in the clomipramine-treated animals. These findings are consistent with biochemical studies showing a decrease in 5-HT receptors following the chronic treatment with clorgyline and an absence of effect on 5-HT receptors following the chronic treatment with clomipramine.

Monoaminergic denervation of the rat hippocampus: microiontophoretic studies on pre- and postsynaptic supersensitivity to norepinephrine and serotonin

The responsiveness of hippocampal CA3 pyramidal neurons to microiontophoretic applications of serotonin (5-HT), norepinephrine (NE), gamma-aminobutyric acid (GABA) and isoproterenol (ISO) was assessed in rats following 5,7-dihydroxy-tryptamine (5,7-DHT) and 6-hydroxydopamine (6-OHDA) pretreatments and bilateral locus coeruleus lesions. The intraventricular administration of 200 micrograms (free base) of 5,7-DHT and of 6-OHDA produced 89% and 93% decreases of 5-HT and NE respectively. None of these pretreatments modified the inital responsiveness to, or recovery from iontophoretic application of 5-HT. In 6-OHDA pretreated and locus-lesioned rats, the initial effectiveness of NE was not altered but its effect was markedly prolonged. However, there was no such prolongation of the effect of ISO which is not a substrate for the high affinity NE reuptake. The effect of GABA was not affected by these pretreatments. Acute pharmacological blockade of the NE reuptake with desipramine (5 mg/kg, i.p.) similarly induced a prolongation of the effect of iontophoretically applied NE, while fluoxetine (10 mg/kg, i.p.) a 5-HT reuptake blocker, failed to alter the recovery of pyramidal cells from iontophoretic application of 5-HT. It is concluded that 5-HT denervation induces neither pre- nor postsynaptic types of supersensitivity in hippocampal pyramidal cells, contrasting with the previously shown supersensitivity of ventral lateral geniculate and amygdaloid neurons following 5-HT denervation. NE denervation fails to induce a postsynaptic type of supersensitivity but leads to a marked prolongation of the response to NE indicative of a presynaptic mechanism. These results underscore the necessity for regional studies of neurotransmitters and drug action.

Effect of chronic tricylic antidepressant treatment on the serotoninergic autoreceptor: a microiontophoretic study in the rat

Chronic treatment with tricyclic antidepressant (TCA) drugs has been shown to enhance the responsiveness of rat forebrain neurons to serotonin (5-HT). In the present study, imipramine (5 mg and 10 mg/kg), iprindole (2.5 mg/kg), desipramine and femoxetine (5 mg/kg) were administered daily for 14 days. The response of dorsal raphe neurons to intravenous injection of LSD (4 microgram/kg) and to microiontophoretic applications of 5-HT and LSD was assessed 24 h after the last dose. The responsiveness to intravenous LSD and the effectiveness of microiontophoretic applications of 5-HT and LSD were not altered by TCA drug pretreatments. Furthermore, the treatments did not change the mean firing rate of these 5-HT neurons. Those results suggest that chronic treatment with TCA drugs does not alter the sensitivity of the 5-HT autoreceptor. Thus, the effect of the previously reported increase of postsynaptic neuron responsiveness to 5-HT would not be dampened by a decreased activity of the presynaptic neurons.

Long-term antidepressant treatment decreases spiroperidol-labeled serotonin receptor binding

Antidepressants compete at several neurotransmitter receptor binding site, but drug affinities do not correlate with clinical efficacy. Long-term, but not short-term, antidepressant treatment decreases the numbers of both serotonin and beta-adrenergic receptors. The decrease in the number of receptor sites is most marked for [3H]spiroperidol-labeled serotonin receptors and is characteristic for antidepressants of several classes.

Repeated tricyclics induce a progressive dopamine autoreceptor subsensitivity independent of daily drug treatment

Dopamine (DA) has largely been ignored in considering possible mechanisms underlying the therapeutic effects of tricyclic anti-depressants (TCA). Most previous work done has focused on the ability of some TCAs to block the in vitro re-uptake of DA--an effect which unfortunately requires very high doses. Recently, however, Serra et al. proposed that TCAs may exert their therapeutic effects by inducing a subsensitivity of presynaptic receptors located on the dendrites and soma of DA neurones (DA autoreceptors). This hypothesis was directly tested by examining the influence of TCAs on the demonstrated ability of the DA agonist, apomorphine, to depress selectively the spontaneous activity of single DA cells. We now report that repeated administration of both typical and atypical TCAs induces a progressive subsensitivity of DA autoreceptors, and that this gradual augmentation of DA autoreceptor subsensitivity depends on the passage of time rather than daily TCA administration. The latter finding suggests that daily drug administration may not be therapeutically necessary.

Imipramine normalizes naturally-occurring and drug-induced differences in the exploratory activity of rats

1 Exploratory activity of female hooded rats was measured in a Y maze on two occasions, 1 week apart. Locomotion (maze arm entries), rearing, and head-dipping into pots were scored for 5 min at each trial. 2 In control rats, differences between individuals in the amount of locomotion and rearing were consistent, as shown by significant test-retest correlations (r = +0.55, and +0.83 respectively). There was no correlation between head-dipping scores obtained in the two tests. 3 Imipramine (Imip) pretreatment before the second trial (10 mg/kg i.p on the 3 preceding days, and 2.5 mg/kg 1 h before) abolished these correlations. The scatter of the scores about the mean was also reduced by Imip, but there was no significant change in mean scores. Thus Imip appeared to have a 'normalizing' effect on locomotion and rears: after pretreatment, scores tended to be more uniform, and no longer reflected naturally-occurring individual differences. 4 Imip abolished the changes in exploratory activity produced by drugs which alter brain 5-hydroxytryptamine metabolism: p-chlorophenylalanine (100 mg/kg 24 h before testing) increased and DL-5-hydroxytryptophan (12.5 mg/kg 1 h before testing) decreased the fall in activity over the trial in saline-treated rats but not that in Imip-treated rats. In this case, Imip also produced an overall reduction in activity scores. 5 The normalizing effects of Imip on rat behaviour may be analogous to its therapeutic effects in human depressive disorders.

Symptom patterns in unipolar and bipolar depression correlating with monoamine metabolites in the cerebrospinal fluid: I. General patterns

The symptom scores on the Schedule for Affective Disorders and Schizophrenia (SADS) of 21 unipolar and 12 bipolar depressive patients diagnosed with Research Diagnostic Criteria (RDC) were correlated with the monoamine metabolites homovanillic acid (HVA), 3-methoxy-4-hydroxyphenylglycol (MHPG), and 5-hydroxyindoleacetic acid (5HIAA) in the cerebrospinal fluid (CSF). For the unipolar group, multiple regression analyses revealed strong multiple correlations (from r = 0.92 to 0.97) to the effect that high- and low-HVA, high- and low-MHPG, and high- and low-5HIAA syndromes, respectively, could be isolated. The bipolars were too few for the same analyses to work well, but there is evidence for the high- and low-monoamine syndromes to be characterized by differential symptomatology in bipolar and unipolar patients. Through the comparison of monoamine metabolite values predicted from a total of 18 SADS symptom items with the true CSF values, a computer program was able to classify 20 of the 21 unipolar and all the 12 bipolars correctly. The results are consistent with a hypothesis of the pathoplastic role of individually set (genetically determined?) brain monoamine homeostases in shaping the profile of an affective episode.

Neurochemical modulation of sensory-motor reactivity: acoustic and tactile startle reflexes

The present review argues that the startle reflex is particularly well suited as a model system to analyze how drugs alter stimulus reactivity and reflex excitability. It then reviews all the literature to date on how drugs or lesions that are thought to alter neurochemical transmitter systems affect acoustic and/or tactile startle. Hypotheses are presented to account for how serotonin, dopamine, norepinephrine, acetylcholine, and opiates modulate startle. Effects on startle plasticity such as habituation, sensitization, and potentiation resulting from prior associative learning are also included.