Chronic antidepressant therapy and associated changes in central monoaminergic receptor functioning

Brain beta-adrenoceptor binding sites in antidepressant-free depressed suicide victims

beta-Adrenoceptor binding sites were quantitated by saturation binding of [3H]CGP 12177 in 9 brain regions from 21 suicide victims, with a firm retrospective diagnosis of depression, who had not recently received antidepressant drugs, and 20 age- and sex-matched controls. In depressed suicides the number of total beta-adrenoceptors was significantly lower in temporal cortex (Brodmann area 38, by 19%) and beta 1-adrenoceptors (Brodmann area 21/22, by 17%) compared to controls. Suicides who died by violent means had significantly lower numbers of total beta- and beta 1-adrenoceptors in frontal cortex than matched controls (by 23 and 25%, respectively) and than non-violent suicides (by 20 and 22%, respectively) and lower numbers of beta 1-adrenoceptors in temporal cortex (Brodmann area 21/22) than matched controls (by 16%). Depressed suicides who died by non-violent means had lower numbers of total beta-adrenoceptors in occipital cortex than matched controls (by 24%) and than violent suicides (by 18%), and lower numbers of total beta- and beta 1-adrenoceptors in temporal cortex (Brodmann area 38) than matched controls (by 27 and 24%, respectively). Depression in suicide victims is associated with deficits in beta-adrenoceptor binding sites, largely restricted to cortical areas.

Central dopaminergic transmission is selectively increased in the limbic system of rats chronically exposed to antidepressants

Repeated electroconvulsive shock (ECS) exposure produced a decrease of [3H]SCH 23390 binding sites and a reduced response of adenylate cyclase activity to dopamine D-1 receptor stimulation in the rat limbic area analogous to that previously observed in rats chronically treated with imipramine. These effects were completely prevented by the repeated administration of a small dose of alpha-methyl-p-tyrosine (alpha-MPT), associated with the tricyclic compound. Increased dopaminergic transmission seems to be involved in the mechanism of antidepressant action. Rats chronically treated with imipramine showed a decrease of dihydroxyphenylacetic acid (DOPAC) concentration restricted to the limbic area. Finally, both imipramine and desipramine blocked the uptake of [3H]dopamine in the limbic system with a 100-fold greater potency than that observed in the basal ganglia.

Effect of clenbuterol on sexual behavior in male rats

The intraperitoneal injection of clenbuterol, a beta-2 adrenoceptor agonist which readily penetrates into the brain, modified the copulatory pattern of male rats. In sexually vigorous rats, the acute administration of clenbuterol at the dose of 0.1 or 1 mg/kg reduced the number of mounts and/or intromissions and increased the postejaculation interval, while the repeated administration (7 days) of a dose of 0.1 mg/kg/day had no effect. In sexually sluggish rats, both the acute (0.01 or 0.1 mg/kg) and repeated (0.1 mg/kg/day x 7 days) administration of clenbuterol increased the percent of animals achieving ejaculation and succeeding in initiating a new copulatory series within the respective cutoff times, and reduced mount, intromission and ejaculation latencies, as well as the postejaculation interval. These data show that clenbuterol negatively affects the copulatory behavior of sexually vigorous male rats, but improves that of sexually sluggish ones, and provide further evidence that central beta-receptor activation produces behavioral effects.

Depression and antidepressant therapy: receptor dynamics

1. The classical norepinephrine (NE) and serotonin (5-HT) theories of depression have been abandoned in light of recent chronic antidepressant drug studies. 2. The new NE and 5-HT theories of depression focus on the dynamics of receptor subtypes in depression and chronic antidepressant treatments. 3. Recent studies in molecular genetics suggest a reclassification of monoamine receptors based on receptor structural homologies in DNA and amino acid sequences rather than receptor affinity for ligands. 4. Electrophysiologic studies in rats suggest that 5-HT1 receptor function is facilitated by chronic antidepressant treatment. 5. Preclinical studies employing a range of 5-HT1 mediated behavioral models also suggest that chronic antidepressant treatment facilitates transmission at central 5-HT1 receptors. 6. Patient studies, employing a 5-HT1 mediated neuroendocrine model, suggest that depression is associated with decreased transmission at CNS 5-HT1 receptors; and that chronic antidepressant treatment facilitates 5-HT1 receptor responsiveness in depressed patients. 7. New 5-HT1 selective agonists have been developed and found to be clinically effective antidepressants. 8. The above clinical and preclinical data suggest that some forms of depression are related to a decreased responsiveness of 5-HT1 receptors which is reversed by chronic antidepressant treatment. 9. Beta adrenergic and NE-stimulated cyclic AMP studies suggest that chronic antidepressant treatment decreases the responsiveness of central beta-adrenergic receptors, particularly beta-1 receptors. 10. A novel approach to antidepressant drug development focuses on identifying centrally active beta-1 agonists, which like clinically proven antidepressants, decrease beta-1 receptor responsiveness with chronic treatment. 11. 5-HT2 receptor binding studies and initial studies of 5-HT2 receptor coupled PI turnover suggest that chronic antidepressant treatment decreases 5-HT2 receptor number and function. 12. The development of new atypical antidepressants with 5-HT2 receptor related mechanisms of action suggest that 5-HT2 receptors may be associated with certain types of depression and their clinical treatment.

Effect of prolonged 5-hydroxytryptamine uptake inhibition by paroxetine on cortical beta 1 and beta 2-adrenoceptors in rat brain

The effects of prolonged (21 day) oral administration of the antidepressants paroxetine (0.9 to 8.9 mg/kg/day) and amitriptyline (2.7 to 27 mg/kg/day), on rat brain cortical beta 1- and beta 2-adrenoceptor numbers and affinities were investigated using [3H]-CGP 12177. Although amitriptyline, 27 mg/kg, caused a significant (p less than 0.05) 20% reduction in the number of beta 1-adrenoceptors, paroxetine, at doses up to 8.9 mg/kg p.o., did not influence binding of [3H]-CGP 12177 to cortical beta 1- or beta 2-adrenoceptors. This study with paroxetine provides further evidence that the down-regulation of central beta 1-adrenoceptors in rat brain after repeated administration is not a property of all antidepressant drugs.

Brain 5-HT1 binding sites in depressed suicides

5-HT1 and 5-HT1A binding sites were measured in brain tissue obtained at postmortem from 19 suicides, with definite evidence of depression, and 19 sex and age-matched controls. Thirteen of the depressed suicides had not been prescribed psychoactive drugs recently (drug-free suicides); six had been receiving antidepressant drugs, alone or in combination with other drugs (antidepressant-treated suicides). No significant differences were found in the number or affinity of 5-HT1 and 5-HT1A binding sites in frontal or temporal cortex between drug-free suicides and controls. The number of 5-HT1 sites was significantly lower (by 20%), affinity unaltered, in hippocampus and the affinity significantly lower (by 33%), number unaltered, in amygdala of drug-free suicides than controls. The number of 5-HT1 binding sites tended to be higher and the affinity lower in the antidepressant-treated compared to drug-free suicides, and significantly so in hippocampus. The present results, together with our previous studies, provide no evidence of altered cortical 5-HT markers in depressed suicides, but further emphasise abnormalities in the hippocampus.

[3H]yohimbine binding to platelet alpha 2-adrenoceptors in depression

[3H]Yohimbine binding to platelet alpha 2-adrenoceptors was studied in depressed patients and healthy volunteers. Where possible platelet binding measurement was repeated in depressed patients following treatment. Bmax of [3H]yohimbine binding did not differ significantly between depressed patients and control subjects and did not change with treatment in depressed patients. KD was significantly lower in female depressed patients, particularly in those who were post-menopausal. Multivariate analysis showed significant effects on KD of depression, season of testing and assay protein concentration.

Disruption of FR-40 by 5-HT agonists. II. Effects of chronic phenelzine or isocarboxazid

Effects of chronic treatment with the monoamine oxidase inhibitors phenelzine and isocarboxazid on disruption of FR-40 operant responses by 5-HT agonists have been studied. Three groups of rats that were trained in the FR-40 operant schedule showed marked disruption by 0.1 mg/kg IP lysergic acid diethylamide (LSD), 2 mg/kg IP quipazine (Q), 0.05 mg/kg SC 8-hydroxy-2-(di-n-propylamino)tetralin (8-OHDPAT), and 1 mg/kg SC (m-trifluoromethyl-phenyl)piperazine (TFMPP), administered twice weekly in random order. Subsequently, one group received daily IP injection of phenelzine (5 and 10 mg/kg), the second group received 5 mg/kg IP of isocarboxazid, and the third group received vehicle (0.5% methyl cellulose) for 24 days (Period 1 and Period 2). For these periods and 12 days after discontinuing the MAOI treatments (Washout Period), test doses of 5-HT agonists were evaluated for their effects to decrease reinforcements (R) and increase pauses (P). No change in sensitivity to the LSD, Q and TFMPP effects on FR-40 behavior was observed in the vehicle-treated group. However, an attenuated effect of 8-OHDPAT was found in this group. In phenelzine- and isocarboxazid-treated rats the disruption of FR-40 responses by LSD and 8-OHDPAT were significantly reduced during Period 1, Period 2 and Washout Period. A significantly less effect on disruption in FR-40 responses by quipazine and TFMPP during Period 2 and the Washout Period was also seen. Since MAO inhibitors appear to down-regulate both 5-HT1 and 5-HT2 binding sites in brain, the attenuated effects of the 5-HT agonists were anticipated.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of brain serotonin in the electroconvulsive shock-induced changes in behavioural effects of intra-hippocampally injected clonidine

The influence of central serotonin depletion upon behavioural effects of intra-hippocampally injected clonidine in the electroconvulsive shock-treated rats (ECS), was studied. Repeated ECS significantly attenuated the depressive influence of clonidine upon the locomotor activity of the rats in the open field test. Chemical lesions to the median raphe nucleus (MR) did not significantly affect ECS-induced changes in clonidine activity in this test. In the forced swimming the MR lesions revealed the stimulatory potency of clonidine microinjections upon rat active behaviour. In animals pretreated with repeated ECS, clonidine also significantly potentiated swimming activity, but no evident synergism of ECS and MR lesion could be observed. Taking into account these and other data it is concluded that central serotonin depletion might differentially affect the adaptive processes occurring in the alpha 2-adrenoceptors in the course of treatment with tricyclic antidepressants and ECS, but it does not seem to be a strong phenomenon. Moreover, it is suggested that clonidine effects in the open field and forced swimming tests may be mediated by different neuronal substrates within the rat hippocampus.

The effects of acute and chronic desipramine on the thermogenic and hypoactivity responses to alpha 2-agonists in reserpinized and normal mice

1. The effects of acute and chronic (14 day) administration of the noradrenaline uptake inhibitor, desipramine (DMI), on the thermogenic responses to clonidine in reserpine-treated mice, and on the hypothermic and hypoactivity responses to the alpha 2-agonist, UK-14,304, in untreated mice were examined. 2. Taking the capacity of DMI to delay the onset of reserpine-induced hypothermia as an indicator of noradrenaline (NA) uptake inhibition, the lowest dose of DMI to inhibit uptake significantly for 12 h in the mouse was shown to be between 10 and 20 mg kg-1 orally. 3. Chronic (every 12 h for 14 days), but not acute treatment with DMI (15 mg kg-1, orally), attenuated the thermogenic responses to low doses (0.02-0.225 mg kg-1, i.p.) of clonidine (injected 20 h after the last dose of DMI) in reserpinized mice. 4. Acute DMI administration slightly attenuated the hypothermia and hypoactivity induced by UK-14,304 (0.25-1.0 mg kg-1, i.p.) when injected 2h, but not when injected 18-21h before the agonist. In contrast, 18-21h after withdrawal from chronic DMI both of these responses to UK-14, 304 were markedly attenuated. 5. As the thermogenic response to clonidine in reserpinized mice appears to involve central post-synaptic alpha 2-adrenoceptors, these results suggest that prolonged inhibition of NA uptake decreases the sensitivity of postsynaptic alpha 2-adrenoceptors. The results of the studies using UK-14,304 indicate that central alpha 2-adrenoceptors involved in mediating other behavioural and pharmacological responses to alpha 2-agonists are also down-regulated by chronic inhibition of NA uptake.

Long-term imipramine treatment potentiates m-chlorophenylpiperazine-induced changes in prolactin but not corticosterone or growth hormone levels in rats

Intravenous administration of m-chlorophenylpiperazine (m-CPP, a selective 5-HT agonist) to rats produced increases in plasma prolactin and corticosterone and a decrease in plasma growth hormone concentrations. Long-term but not short-term imipramine treatment potentiated m-CPP's effect on plasma prolactin, but not its effects on corticosterone or growth hormone. Short-term or long-term imipramine treatment did not produce significant changes in baseline levels of prolactin, corticosterone or growth hormone. These findings are compatible with development of functional supersensitivity of 5-HT receptors mediating prolactin release. Lack of potentiation of m-CPP's effects on corticosterone and growth hormone following long-term imipramine treatment suggests either differential regulation of these hormones by serotonergic and possibly other mechanisms, or different 5-HT receptor subtypes mediating the release of these hormones. Alternatively, adaptive changes in other aminergic neurotransmitter mechanisms such as the noradrenergic system may account for the differential effect of long-term imipramine treatment on m-CPP-induced neuroendocrine changes.

Noradrenergic and opioidergic influences on the antinociceptive effect of clomipramine in the formalin test in rats

Although tricyclic antidepressants are especially useful in the treatment of chronic pain conditions, most of the work about its mechanism of action has been made on acute pain tests. The present study was aimed at studying the role played by noradrenergic and opioidergic influences on the antinociceptive activity of subchronically administered clomipramine in the formalin test (a tonic pain model) in rats. Clomipramine produced antinociception after 7 days, administration (2.5 mg/kg/day), an effect equivalent to that obtained by acute morphine (5 mg/kg). The antinociceptive effect of clomipramine was inhibited by the following: nonspecific blocking of alpha 1- and alpha 2-adrenoceptors by phentolamine, specific blocking of alpha 1-adrenoceptors by prazosin; stimulation of alpha 2 receptors by clonidine; and blocking of the opioid receptors by naloxone. Blocking the alpha 2-receptors with yohimbine did not antagonize the effect of clomipramine. These results suggest that clomipramine produces antinociception in this test, partly via the participation of the endogenous opioid system and partly by further activating or potentiating previously activated noradrenergic pathways which are involved in the control of pain information.

Species dependence of adaptations at the pre- and postsynaptic serotonergic receptors following long-term antidepressant drug treatment

Rats were treated daily for 3 weeks with the antidepressant amitriptyline, and adaptations following this treatment at the level of the postsynaptic 5-HT2 receptor were studied, as well as the presynaptic serotonin re-uptake and the 5-HT autoreceptor functioning. Rabbits were treated chronically with one of the antidepressants amitriptyline, imipramine, chlorimipramine and mianserin, and the occurrence of the different pre- and postsynaptic adaptations were compared to what was observed in rat brain. Postsynaptic 5-HT2 receptors were down-regulated following a long-term antidepressant drug treatment in rat prefrontal cortex, but were unchanged in rabbit brain. Two markers for presynaptic 5-HT uptake were used to evaluate differences between control and treated animals: in rat brain a decreased number of [3H]imipramine binding sites was observed, however, without any change in the kinetics of the [3H]5-HT accumulation. In rabbit brain, both [3H]imipramine binding and [3H]5-HT accumulation remained unchanged. The function of the presynaptic serotonergic autoreceptor was affected, although differentially, in both rat and rabbit brain, following the long-term antidepressant drug treatment. In rat brain, these autoreceptors were down-regulated, whereas in rabbit brain, the results indicated that the autoreceptors were only no longer activated by endogenously released serotonin. The authors hypothesize that the different presynaptic adaptations at the level of the 5-HT autoreceptor are responsible for the absence or presence of a postsynaptic 5-HT2 receptor down-regulation in rat and rabbit brain following a long-term antidepressant treatment.

Role of serotonergic input in the down-regulation of beta-adrenoceptors following long-term clorgyline treatment

Administration of the selective monoamine oxidase (MAO) type A-inhibiting antidepressant clorgyline (1 mg/kg per day) to rats for 21 days caused a significant decrease in cortical [3H]dihydroalprenolol binding. Selective lesioning of central serotonergic axons by 5,7-dihydroxytryptamine (5,7-DHT; confirmed by the presence of the serotonin syndrome in response to a 40 mg/kg dose of 5-hydroxytryptophan (5-HTP) or inhibition of 5-HT synthesis by parachlorophenylalanine (PCPA) caused significant 5-HT and 5-HIAA depletions in the cortex without much effect on NE and DA concentrations, but did not have any significant effect on beta-adrenoceptor density, and furthermore failed to attenuate clorgyline-induced decreases in beta-adrenoceptor density. Clorgyline treatment partially antagonized 5-HT depletion by the 5,7-DHT lesion or PCPA treatment. These findings suggest that due to their ability to raise 5-HT concentrations, MAO-inhibiting antidepressants may be a better alternative than the tricyclics in treating depressed patients with reduced 5-HT if down-regulation of beta-adrenoceptors is critical for antidepressant efficacy.

Chronic imipramine diminishes the nuclear size of neurons in the locus coeruleus and cingular cortex but not in the hippocampus of the rat brain

The effect of an antidepressant drug--imipramine--on the nuclear volume of the rat brain neurons was studied. Imipramine was administered per os, 10 mg/kg acutely or chronically (twice a day, for 14 days). A reduction in the nuclear volume was observed after chronic treatment in neurons of the locus coeruleus and cingular cortex, but not in the hippocampus. The diminution in the nuclear volume of the affected cells suggests a decrease in their activity.

Effects of trazodone treatment on serotonergic function in depressed patients

Changes in serotonergic (5HT) neurotransmission may mediate the therapeutic actions of some antidepressant drugs. In the present study, the 5HT precursor L-tryptophan (L-TRP) was administered intravenously to nine depressed patients before and during treatment with the triazolopyridine antidepressant trazodone (TRZ). Neuroendocrine, subjective mood, and cardiovascular responses to L-TRP were assessed. Unlike tricyclic antidepressants and monoamine oxidase inhibitors, TRZ did not enhance the prolactin response to L-TRP and had little effect on other measures. Since other studies indicate that the TRP-induced increase of prolactin in humans may reflect 5HT function, the present study suggests that TRZ treatment does not enhance net 5HT function in depressed patients.

Effects of prenalterol on beta adrenergic responsiveness and receptors in the cerebral cortex of the rat

The activity of the beta-adrenoceptor agonist, prenalterol, at beta adrenoceptors in the cerebral cortex of the rat and the effect of chronic intraperitoneal infusion of prenalterol on the biochemical responsiveness and density of cerebral cortical beta adrenoceptors was studied. Whereas isoproterenol caused a four-fold rise in the content of cyclic AMP in slices of cerebral cortex, prenalterol did not produce a significant increase in cyclic AMP. However, prenalterol inhibited the isoproterenol-stimulated increase in cyclic AMP in cortical slices in a concentration-dependent manner. Using an in vivo binding technique, prenalterol (3.8 mg/kg/hr) infused intraperitoneally through osmotic minipumps, penetrated the brain and significantly inhibited the binding of the beta adrenoceptor antagonist, [125I]iodopindolol (125I-IPIN), to cortical beta adrenoceptors. Infusion of prenalterol (3.8 mg/kg/hr) for 7 days resulted in a small (20%), but significant, reduction in the ability of isoproterenol to stimulate maximally the accumulation of cyclic AMP in slices of cerebral cortex. No alteration in the Bmax or KD of the binding of [125I]iodopindolol was observed in homogenates of cortex obtained from prenalterol-treated rats. Furthermore, no change was observed in the binding of the hydrophilic ligand [3H]CGP-12177 in homogenates of cortex. In this study, then, prenalterol exhibited properties in vitro of a beta adrenoceptor antagonist, but did cause modest desensitization of beta adrenoceptor responsiveness when administered continuously in vivo.

Evaluation of the role of pre- and postsynaptic serotonergic receptors in electroconvulsive shock therapy

Despite the well-established clinical efficacy of electroconvulsive therapy (ECT) in the treatment of affective disorders, there is no generally accepted theory regarding its mode of action. Pre- and postsynaptic serotonin (5-hydroxytryptamine, 5-HT) receptors were studied following treatment of rabbits with electroconvulsive therapy (8 shocks). No change was observed in the binding at the postsynaptic 5-HT2 receptor in frontoparietal cortex, as indicated by an unchanged affinity and by the total number of receptors. In control rabbits, exogenously added serotonin and metitepin modulated the electrically evoked 3H-serotonin release, probably by acting on a presynaptic serotonin autoreceptor. Following electroconvulsive therapy, exogenous serotonin still inhibited the 3H-serotonin release, whereas metitepin had lost its enhancing effect. This observation provides evidence for a decreased amount of endogenous neurotransmitter present in the synaptic cleft at stimulation. The results indicate that postsynaptic receptor changes following antidepressant treatments are related to the presynaptic autoreceptor changes.

Effect of long-term rolipram administration on the sensitivity of alpha 2-adrenoceptors in rat brain

The effects of single and chronic doses of rolipram on the sensitivity of alpha 2-adrenoceptors have been compared with the phosphodiesterase inhibitors, isobutylmethylxanthine (IBMX) and ICI 63,197, and the antidepressant, desipramine. While pretreatment with a single dose of rolipram, ICI 63,197 or IBMX administered either 1 or 24 h prior to clonidine (0.1 mg/kg) enhanced clonidine-induced hypothermia and hypoactivity, chronic dosing (twice daily for 14 days) with desipramine (10 mg/kg) or rolipram (5 mg/kg) antagonized these behavioural effects. In contrast, chronic dosing with IBMX or ICI 63,197 failed to antagonize clonidine-induced hypothermia and hypoactivity. In binding studies neither ICI 63,197, IBMX, rolipram nor desipramine induced changes in the binding of 3H-labelled clonidine to rat cerebral cortical membranes following chronic administration. The failure of ICI 63,107 and IBMX to antagonize clonidine-induced hypothermia and hypoactivity suggests that the antidepressant effect of rolipram is independent of its phosphodiesterase inhibitor property.

Brain 5-HT2 receptor binding sites in depressed suicide victims

5-HT2 receptor binding sites were measured (by saturation binding of [3H]ketanserin) in brain tissue obtained at postmortem from 19 suicide victims with definite evidence of depression and 19 sex and age-matched control subjects. Five of the suicide victims were receiving antidepressant drugs prior to death; 13 suicide victims had not been prescribed antidepressant or other psychoactive drugs recently and none were found in their blood at postmortem. The number of serotonin-2 (5-HT2) binding sites in frontal, temporal and occipital cortex and amygdala did not differ significantly between the depressed suicide victims and controls, either in the total suicide group or in the antidepressant drug-free suicides. The number of 5-HT2 binding sites in the hippocampus did not differ from controls in the total suicide group but was significantly lower (by 23%) in the antidepressant-free suicide group. The affinity of [3H]ketanserin binding did not differ from controls in the antidepressant-free suicides but was lower (increased Kd) in those subjects receiving antidepressant drugs. No correlation was found between the time of death and storage of tissue or the duration of tissue storage prior to assay and the number or affinity of 5-HT2 binding sites. A significant negative correlation was found between age of the subject and the number of 5-HT2 binding sites in the frontal and occipital cortex. The present study of suicide victims with definite evidence of depression do not confirm previous studies of increased numbers of 5-HT2 binding sites in suicide victims and suggest that these previous findings may be related to factors other than depression.

Effects of chronic oral administration of the antidepressants, desmethylimipramine and zimelidine on rat cortical GABAB binding sites: a comparison with 5-HT2 binding site changes

1. The effects of chronic oral administration of desmethylimipramine (DMI) or zimelidine (1.25 and 5 mg kg-1 twice daily for 21 days) were studied on rat whole cortical gamma-aminobutyric acidB (GABAB) binding sites. No changes in receptor affinity or number were found with either drug. 2. A subsequent study of GABAB binding sites using higher doses of these drugs (5 and 10 mg kg-1) and rat frontal cortex was also without effect, when investigated 24 h after termination of drug administration or 72 h after DMI administration (5 mg kg-1). 3. The number of frontal cortical 5-hydroxytryptamine2 (5-HT2) binding sites was significantly and dose-dependently decreased after both drugs, whereas the number of hippocampal 5-HT2 binding sites was not significantly altered after either drug. 4. As the number of frontal cortical GABAB binding sites was unaltered whereas the number of 5-HT2 binding sites was significantly decreased under identical study conditions, it may be concluded that the effects of antidepressant administration upon GABAB binding sites is a less consistent observation than their effects on 5-HT2 binding sites.

Cocaine and central monoaminergic neurotransmission: a review of electrophysiological studies and comparison to amphetamine and antidepressants

Psychomotor stimulants (e.g. cocaine and amphetamine) and many antidepressants are believed to elicit their psychotropic actions by interacting primarily with central monoaminergic neurons. The acute central neuronal effects of amphetamine and antidepressants have been extensively investigated in rats utilizing extracellular single unit electrophysiological and microiontophoretic techniques in vivo. In recent years the chronic effects of these compounds on the above neuronal systems have also been reported. Such investigations have proliferated because of the realization that the mechanisms underlying the psychotomimetic effects (e.g. amphetamine and cocaine) and mood elevation (i.e. antidepressants) observed with the administration of these drugs are more accurately reflected in chronic studies. For many years it has been assumed that cocaine and amphetamine produce very similar if not identical psychotropic effects through their actions on central monoaminergic neurotransmission. In terms of effects on single monoaminergic neurons, this assumption had gone by untested until two years ago, when the first report of the electrophysiological effects of cocaine on central monoaminergic (locus ceruleus) neurons appeared in the literature (61). This review discusses recent electrophysiological studies with cocaine at the level of single identified monoaminergic neurons and compares such data with that previously reported for amphetamine and antidepressants. In addition to identifying some of the similarities and differences between these compounds, this review also highlights some of the gaps in our knowledge regarding the effects of these drugs on central monoaminergic neurotransmission.

Delayed behavioral response to antidepressant drugs following selective damage to the hippocampal noradrenergic innervation in rats

The present study was undertaken to investigate in rats the role of the noradrenergic innervation of the hippocampus in the reversal by antidepressant drugs of escape failures caused by previous exposure to inescapable shocks (learned helplessness design). Rats were either sham-operated or given a bilateral infusion of 6-hydroxydopamine (6-OHDA, 8.1 micrograms of free base in 0.8 microliter saline containing 0.02% ascorbic acid) into the dorsal hippocampus. Three weeks later, sham and lesioned rats were exposed (experimental rats) or not exposed (control rats) to 60 randomized, inescapable footshocks (0.8 mA, 15 s duration) and, 48 h later, experimental and control rats were subjected to daily consecutive shuttle-box sessions (30 trials/day; intertrial interval = 30 s). Separate groups of experimental animals were given twice daily injection (i.p.) of clomipramine (total daily dose: 16 or 32 mg/kg), desipramine (24 mg/kg), imipramine (32 mg/kg), nialamide (16 mg/kg) or saline. After behavioral testing, animals were sacrificed and [3H]noradrenaline uptake was assayed in synaptosomal preparations from the hippocampus, the cerebral cortex and the septum, [3H]serotonin uptake being assessed in hippocampal synaptosomes. We found that hippocampal 6-OHDA infusion resulted in a mean 70% reduction in local [3H]noradrenaline uptake and a marked delay in the response to antidepressants (reversal of escape failures over consecutive shuttle-box sessions). The 6-OHDA infusion was found not to alter hippocampal [3H]serotonin uptake but to decrease (30%) [3H]noradrenaline uptake in the cerebral cortex.(ABSTRACT TRUNCATED AT 250 WORDS)

The modulatory effect of neurotensin on [3H]dopamine release from rat nucleus accumbens slices is enhanced after chronic desipramine treatment

In the presence of the reuptake blocker, nomifensine (1 microM), neurotensin caused a dose-dependent increase in the 25 mM K+-evoked release of [3H]dopamine ([3H]-DA) from rat nucleus accumbens slices. Chronic desipramine (DMI)-treatment (20 mg DMI/kg/day, orally for 28 days) caused a significant reduction in the K+-stimulated release of [3H]DA from rat nucleus accumbens slices. This could possibly have resulted from increased DA autoreceptor-mediated inhibition of DA release. Superfusion of the DMI-treated rat tissue with buffer-containing neurotensin (100 nM) resulted in an increase in the K+-evoked release of [3H]DA to a level which was higher than that observed with tissue obtained from untreated rats. A similar enhanced effect of neurotensin on DA release was observed when the DA agonist, apomorphine (APO), and ascorbate were included in the superfusion buffer. These findings are consistent with DMI treatment giving rise to impaired DA release possibly due to DA autoreceptor-mediated inhibition of DA release and cancellation of this effect by neurotensin.

Are increases in GABAB receptors consistent findings following chronic antidepressant administration?

Desmethylimipramine and zimelidine (10 mg/kg) were administered to rats once daily for 21 days. Chronic desmethylimipramine significantly reduced the number of 5HT2 binding sites in the frontal cortex but not hippocampus while chronic zimelidine did not affect 5HT2 binding in either brain region. The number and affinity of GABAB binding sites in the frontal cortex were unaltered by either drug.

Long-term imipramine treatment enhances locomotor and food intake suppressant effects of m-chlorophenylpiperazine in rats

1 Administration of the 5-HT1B receptor agonist m-chlorophenylpiperazine (m-CPP) to rats produces dose-dependent decreases in locomotor activity and food intake. 2 The locomotor suppressant effect of m-CPP was inhibited by the 5-hydroxytryptaminergic antagonist, metergoline, but not by phentolamine, propranolol, clonidine, or haloperidol. 3 The locomotor suppressant effects of m-CPP were enhanced following long-term (but not short-term) treatment with imipramine, possibly reflecting the postulated development of a functional supersensitivity of 5-HT1B receptors mediating locomotion during longer-term antidepressant drug treatment. 4 The food intake suppressant effects of m-CPP were enhanced following both short (3-5 days) and longer-term (21 days) treatment with imipramine. Rapidly developing 5-hydroxytryptamine uptake inhibition may be responsible for this change, or it may represent an earlier adaptive change in the 5-HT1B receptors mediating food intake compared to more complexly modulated motor responses.

Chronic treatment with antidepressants decreases the number of [3H]SCH 23390 binding sites in the rat striatum and limbic system

Binding of [3H]SCH 23390 to dopamine D-1 receptors and of [3H]spiperone to D-2 sites was measured in identical membrane preparations of the striatum and limbic system of rats treated chronically (twice daily, for two weeks) with antidepressants. Chronic administration of imipramine, amitriptyline, mianserin, citalopram, bupropion, iprindole and electroconvulsive shocks, but not benztropine or cyproheptadine (non-antidepressants) decreased the number of [3H]SCH 23390 binding sites, while no change in the parameters of [3H]spiperone binding was observed. The serotonin2 receptor antagonist ketanserin when added to the incubation medium had no effect on [3H]SCH 23390 binding to D-1 sites. The results suggest that D-1 receptor subsensitivity is a component of the therapeutic effect of antidepressants.

Clinical and treatment effects on 3H-clonidine and 3H-imipramine binding in elderly depressed patients

3H-clonidine and 3H-imipramine binding were measured in depressed patients, 55 years and older. There was no significant difference in either 3H-clonidine or 3H-imipramine binding between depressed patients and age- and sex-matched controls. There was no significant correlation between 3H-clonidine or 3H-imipramine binding and severity of depression before treatment. There was a significant negative correlation between the KD of 3H-imipramine binding sites and Hamilton score over seven weeks of antidepressant treatment. There was no significant difference between receptor data of responders and nonresponders to antidepressant treatment.

The timing, specificity and clinical prediction of tricyclic drug effects in depression

This research was aimed at studying the rate of action of tricyclic drugs in depressive disorders, specifying the behavioural effects associated with recovery, and predicting clinical response. The research design involved comparison of a recovered group with a group treated for the equivalent four weeks, who showed minimal to no response. The findings indicated significant differences in baseline characteristics between responders and non-responders. Further, the drugs were found to act early in the responders, within the first week of treatment. Specific changes at one week which distinguished responder and non-responder groups occurred in the disturbed affects, and in cognitive functioning. Improvements also occurred in somatic symptoms, but these latter changes were general and not associated with later recovery. At 2 1/2 weeks, all facets of the depressed condition showed positive change in the responders. Implications of the results for assessing rate of tricyclic drug actions, their effects on the interaction of affect and neurochemistry, and the practical application of the results for the clinical situation, are discussed.

Chronic treatment with antidepressant drugs and ECT differentially modifies the hypothermic action of clonidine and guanfacine

The hypothermia inducing action of clonidine and guanfacine was abolished by yohimbine and idazoxan pretreatment which suggests an alpha 2-adrenoceptor involvement in this effect. The effects of acute and chronic treatment with the antidepressant drugs desipramine (DMI), amitriptyline (AMI), maprotiline (MAP), mianserin (MIAN), iprindol (IPR), alaproclate (ALA) and electroconvulsive treatment (ECT) on the hypothermic action of the alpha 2-adrenoceptor agonists clonidine and guanfacine were studied. Acute administration of MIAN potentiated clonidine induced hypothermia whereas acute MIAN, IPR and ALA potentiated guanfacine induced hypothermia. Repetitive DMI, AMI and MAP treatment attenuated clonidine-induced hypothermia whereas guanfacine-induced hypothermia was potentiated by chronic treatment with DMI, AMI, MAP and MIAN, ECT applied without anaesthesia attenuated both clonidine and guanfacine hypothermia, however, under ethyl ether anaesthesia ECT was effective only towards guanfacine hypothermia. This discrepancy is discussed in terms of the relative selectivity of the agonists used, the reliability of agonist studies for indexing receptor function, and possible pharmacokinetic interaction.

Beta-adrenoceptors on lymphocytes of patients with major depressive disorder

3H-Dihydroalprenolol binding to lymphocyte membranes of patients with primary, unipolar major depressive disorder was compared to that of a normal, healthy control population. No significant difference could be demonstrated between the Kd values of the two different groups, but the Bmax values of the depressed patients were significantly lower than those of the controls. Positive correlations were observed between the lymphocyte beta-adrenoceptor Bmax values of the patients and their Beck self-evaluation and Hamilton depression ratings. We propose that decreased lymphocyte beta-adrenoceptor Bmax values may be used as a biological marker for major depressive disorder.

Antidepressant-like action of 5-HT1A agonists and conventional antidepressants in an animal model of depression

Previous results have suggested that behavioural adaptation to restraint might be promoted by post-restraint stimulation of 5-HT1A receptors. Therefore, rats were restrained for 2 h and injected with vehicle or 60-1,000 micrograms/kg of the 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) s.c. Vehicle-treated restrained rats showed reduced locomotor activity and increased defaecation in an open field test the day after the end of restraint. A single injection of 250 or 1,000 micrograms/kg 8-OH-DPAT attenuated these effects. The above locomotor deficits were also attenuated by chronic pretreatment with the antidepressants desipramine and sertraline but not by a single treatment with desipramine or the benzodiazepine anxiolytic drugs chlordiazepoxide and diazepam; none of these treatments unambiguously reversed stress-induced increases in defaecation. Evidence suggests that the above action of 8-OH-DPAT is mediated by 5-HT1A receptors since it was antagonised by the 5-HT1A antagonist spiperone but not by the 5-HT2 antagonist ketanserin and was not mimicked by the 5-HT1B agonist RU 24969. However, the 5-HT1A agonists buspirone and TVXQ 7821 (ipsapirone) and the non-specific 5-HT agonist quipazine all possess similar properties to 8-OH-DPAT in this test. The results suggest that 5-HT1A agonists may have rapid antidepressant properties.

Acute effects of nomifensine on in vivo uptake and metabolism of dopamine, noradrenaline and serotonin in the rat brain

Nomifensine, in contrast to all other antidepressants, inhibits the neuronal uptake of dopamine. The effect of this drug (10 mg/kg intraperitoneally) on the metabolism of dopamine, serotonin and noradrenaline was studied in the rat brain. After 1.5 hours, nomifensine increased the concentrations of homovanillic acid (HVA) in corpus striatum and total (free and conjugated) 3-methoxy, 4-hydroxyphenylglycol (MOPEG) in mesencephalon, but had no effect on the 5-hydroxyindoleacetic acid (5-HIAA) in pons/medulla oblongata and mesencephalon. The effect was identical with that of desipramine (25 mg/kg) on MOPEG and of imipramine (25 mg/kg) on 5-HIAA. Two methods ordinarily used for estimating turnover rates of monoamines were compared: accumulation of acid metabolites after probenecid (measuring efflux of metabolites from the brain) and accumulation of monoamine precursors after decarboxylase inhibition (measuring amine synthesis). The efflux was reduced for 5-HIAA and MOPEG but increased for HVA after nomifensine. Imipramine had the same effect on 5-HIAA and desipramine on MOPEG. Desipramine decreased the efflux of HVA from corpus striatum. In contrast, nomifensine did not change the synthesis of noradrenaline and serotonin significantly. Imipramine reduced the synthesis of serotonin in pons/medulla oblongata. In corpus striatum nomifensine, unlike imipramine, increased the concentration of 5-HIAA and synthesis of serotonin in spite of a decrease in efflux, probably because of a secondary effect from the dopaminergic action. The conclusion was made that there were more than one compartment of monoamine metabolites. The antidepressants could to some extent lead to a shift in the metabolism to sites more distant to the transport mechanism.

Different pharmacological interaction of clonidine and guanabenz with antidepressive drugs

A study was made of the effect of several antidepressive drugs with different mechanisms of action on cardiovascular response induced by clonidine (CLO) or guanabenz (GUA) in anaesthetized normotensive rats. The acute pretreatment (1 mg.kg-1) with desipramine (DMI), maprotiline (MAP) and mianserin (MIA) reduced the hypotensive and bradycardiac responses induced by CLO (10 micrograms.kg-1), while only MIA prevented these central actions when elicited by GUA (30 micrograms.kg-1). Only chronic treatment with DMI reduced the hypotensive effect of CLO, but none of long-term antidepressant administrations decreased the fall in blood pressure induced by GUA. The long-term administration of DMI, MAP and MIA abolished the bradycardia from both agonists, CLO and GUA. The different pharmacological interaction of CLO and GUA with (acute or chronic) antidepressive treatments could be explained by a structural difference between the two agonists, and also by differences in localization and/or subtype of the alpha-adrenoceptors involved in their central cardiovascular effects.

Marked enhancement by clorgyline of nocturnal and daytime melatonin release in rhesus monkeys

The type A monoamine oxidase (MAO)-inhibiting antidepressant clorgyline (1 mg/kg/24 days) administered to rhesus monkeys increased night-time cerebrospinal fluid (CSF) melatonin concentrations 3-fold and day-time maltonin values 5-fold. Other circadian parameters of melatonin release, including the peak time and duration of nocturnal melatonin elevation measured during continuous CSF collection periods of 90 min duration over 24-h cycles, were unaffected by clorgyline. While pinealocytes are thought to contain only MAO-B, treatment with the selective MAO-B inhibitor deprenyl (2 mg/kg/24 days) did not alter day or night-time melatonin concentrations. These results are consistent with MAO-A and non-selective MAO inhibitors acting via blockade of degradation of the preferential substrates of MAO-A, serotonin and/or norepinephrine, in adrenergic neurons entering the pineal gland. Further study is needed to evaluate the relative contributions of an increased availability of the melatonin precursor, serotonin, or a sustained net increase in alpha 1-or beta adrenoceptor-mediated input on pinealocytes to these marked changes in melatonin production.

The effect of 5,7-DHT-induced lesions of the median raphe nucleus and chronic desipramine administration upon motor behaviour of rats given intrahippocampal clonidine injection

The role of serotonergic (5-HT) innervation of rat hippocampus in clonidine- induced behavioural effects was studied in naive and desipramine-pretreated animals. 5,7- DHT-lesions of the median raphe nucleus (MR) produced about 50 per cent decrease of serotonin and 5-hydroxyindoloacetic acid (5-HIAA) content in the rat cortex and hippocampus. Lesions of the MR also accelerated the attenuating effect of desipramine upon locomotor inhibition produced by intrahippocampal clonidine injections. In the forced swim test (FST) the MR lesion revealed the stimulatory potency of clonidine upon rat active behaviour, which was statistically significant. However, no further potentiation by MR lesions of DMI effects upon behavioural action of clonidine could be observed in this test. It is concluded that hippocampal serotonergic innervation plays a minor role in the antidepressant-induced changes in behavioural effects of clonidine, which probably occur via modulation of local alpha-adrenoceptors. It is also conceivable that the stimulatory effect of intrahippocampal clonidine injections in the forced swim test in MR-lesioned rats, may be due to the increase in the alpha-(1)-adrenoceptor function in this brain area.

Reserpine augmentation of desipramine in refractory depression: clinical and neurobiological effects

Early studies showed dramatic improvement in some depressed patients when a brief course of parenteral reserpine was added to ineffective tricyclic antidepressant (TCA) treatment. We treated eight patients with DSM-III melancholic major depression with desipramine (DMI) greater than or equal to 2.5 mg/kg/day (plasma levels greater than 125 ng/ml) for at least 4 weeks. All patients failed to respond and received reserpine 5 mg IM b.i.d. over 2 days, in seven cases as a placebo-controlled, double-blind trial. One patient had dramatic resolution of depressive and psychotic symptoms within 48 h, but relapsed within 2 weeks; two other patients had transient hypomanic symptoms. Depression ratings did not significantly change for the sample as a whole, but plasma and cerebrospinal fluid (CSF) levels of 3-methoxy-4-hydroxyphenylethyleneglycol (MHPG) decreased and CSF levels of homovanillic acid (HVA) and 5-hydroxy-indoleacetic acid (5-HIAA) increased. Despite robust effects on central monoamine metabolism, reserpine augmentation appears insufficiently effective for routine use in managing refractory depression.

Female rats are more vulnerable than males in an animal model of depression: the possible role of serotonin

A single 2-h restraint stress reduces locomotion and increases defaecation of male rats placed in an open field 24 h later. After daily 2-h restraints for 5 days these effects were no longer observed. This adaptation was associated with enhanced sensitivity to the serotonin agonist 5-methoxy-N,N-dimethyltryptamine. Female rats were less affected by a single restraint but failed to adapt to the repeated stress procedure and did not exhibit enhanced sensitivity to 5-methoxy-N,N-dimethyltryptamine. Furthermore, females but not males killed 24 h after the final restraint period had decreased brain regional 5-hydroxyindoleacetic acid concentrations particularly in the frontal cortex. No sex differences in hypothalamic and striatal dopamine metabolism were observed. The above differences between male and female rats were unaffected by adult gonadectomy. Similar differences could be involved in the higher incidence of depressive illness in women.

Chronic treatment with antidepressant drugs and the analgesia induced by 5-methoxy-N,N-dimethyltryptamine: attenuation by desipramine

The effect of chronic and acute oral or intraperitoneal treatment with the antidepressant drugs, desipramine, amitriptyline, alaproclate and iprindole, upon pain thresholds in the tail flick, hot plate and shock titration tests of nociception in saline- and 5-MeODMT-treated rats was studied. Chronic desipramine treatment increased the pre-test tail flick latencies. In the saline-treated rats, chronic oral desipramine treatment increased tail flick latencies, whereas chronic oral amitriptyline treatment decreased tail flick latencies. In 5-MeODMT-treated rats, chronic oral desipramine treatment attenuated the effects of 5-MeODMT (1 mg/kg) in all three tests of nociception, whereas chronic amitriptyline caused a potentiation in the tail flick and hot plate tests. Chronic oral iprindole treatment attenuated 5-MeODMT-induced analgesia in the hot plate test. Chronic intraperitoneal desipramine treatment attenuated 5-MeODMT analgesia in the tail flick and shock titration tests. In a different chronic treatment experiment, oral desipramine treatment attenuated 5-MeODMT analgesia in the tail flick test and zimeldine did for both the tail flick and hot plate tests, whereas mianserin potentiated 5-MeODMT-induced analgesia in both the tail flick and hot plate tests. In the saline-treated rats, acute treatment with all four drugs, desipramine, amitriptyline, iprindole and alaproclate, elevated the shock thresholds, whereas in 5-MeODMT-treated rats, desipramine and amitriptyline elevated shock thresholds. Two main conclusions can be drawn: chronic desipramine caused a quite consistent attenuation of 5-MeODMT-induced analgesia and the effects of acute treatment differed strongly from that of the chronic treatment. The effects of chronic administration with these antidepressants were compared with other findings using different measures of behavioural and receptor function.

Characterization of serotonin receptors and lack of effect of antidepressant therapy on monoamine functions in various regions of the rabbit brain

The effects of single and long-term administration of the antidepressants imipramine, desimipramine, amitriptyline, zimelidine and maprotiline were studied in the rabbit brain. Special attention was given to the brain serotonin (5-HT) receptors. Our results show that in different areas of the rabbit brain, the binding sites for 5-HT display pharmacological characteristics very similar to those of the 5-HT1 and 5-HT2 receptors described for the rat brain. No significant correlation could be shown between the distribution of either of the receptors and the distribution of serotonergic nerve terminals (as measured by the 5-HT content and the [3H]5-HT accumulation). Addition of antidepressants to rabbit brain slices, in vitro, caused an inhibition of the [3H]5-HT accumulation. The compounds only weakly inhibited the binding of [3H]5-HT and [3H]ketanserin as compared to the inhibition caused by serotonergic agonists and antagonists. The [3H]5-HT accumulation in brain slices was markedly reduced 2 h after a single i.p. injection of imipramine. After a two-week administration of the antidepressants, the specific binding of neither [3H]5-HT nor [3H]ketanserin was significantly altered. The simultaneous determination of monoamine metabolites and of dopamine-beta-hydroxylase in the cerebrospinal fluid of these treated rabbits did not reveal any significant difference from the control animals.

The lesion of serotonergic neurons does not prevent antidepressant-induced reversal of escape failures produced by inescapable shocks in rats

The present study was aimed at testing the hypothesis (based mainly on biochemical evidence) of the implication of brain serotonergic neurons in the induction of learned helplessness (escape deficit) and its reversal by antidepressants in rats. After desipramine (25 mg/kg IP)-pretreatment rats were either sham-operated or infused with 5,7-dihydroxytryptamine (5,7-DHT, 3 micrograms of free base in 0.4 microliter saline containing 0.02% ascorbic acid) into the midbrain raphe area. Three weeks later, experimental animals were exposed to 60 randomized inescapable shocks (0.8 mA; 15 sec duration), control rats being not shocked, and 48 hr later, they were subjected to daily shuttle-box sessions (30 trials/day; ITI = 30 sec) on 3 consecutive days in order to assess escape deficits. After inescapable shock pretreatment separate groups of rats were given twice daily injections of clomipramine (total daily dose: 32 mg/kg), desipramine (24 mg/kg), imipramine (32 mg/kg), nialamide (32 mg/kg) or saline. After behavioral testing, animals were sacrificed, and tryptophan hydroxylase activity was assayed in the cerebral cortex, the hippocampus and the striatum. We found that damage to serotonergic neurons associated with a 70% loss of tryptophan hydroxylase activity altered neither escape deficits produced by prior exposure to inescapable shock, nor the ability of either antidepressant studied to reverse escape failures in the shuttle-box paradigm. These findings cast some doubts on the hypothesized crucial role of serotonergic neurons in helpless behavior and its reversal by antidepressants.

Long-term amitriptyline treatment alters the affinity state of alpha 2 adrenoceptors in rabbit hindbrain

The effects of 7 and 21 days amitriptyline treatment on brain stem alpha 2 adrenoceptor number and affinity state and on the in vivo depressor response to intracisternal clonidine were examined in male New Zealand white rabbits.

The effects of tricyclic and 'atypical' antidepressants on spontaneous locomotor activity in rodents

With the exception of amineptin, buproprion and nomifensine all tricyclic and 'atypical' antidepressants have been reported to reduce spontaneous motor activity in rodents, after both acute and chronic administration. However, with the diversity of chemical actions of these drugs it is unlikely that a single neurochemical mechanism is underlying this one behavioral effect. These widespread sedative effects have implications for interpreting behavioral changes in other test situations, since sedation generally occurs at doses that fall within the dose-range effective in other tests. We also review the effects on spontaneous motor activity of withdrawal from chronic antidepressant treatment.

Delayed up-regulation of alpha-adrenoceptor populations in particular regions of the rat brain after stimulation of the nucleus locus coeruleus

We previously showed that electrical stimulation of the nucleus locus coeruleus resulted 4 weeks later in greatly improved performance in the acquisition and extinction of a food-reinforced operant task. To ascertain whether adrenergic receptors of particular brain regions were involved in this long term behavioral modification, we studied the characteristics of alpha 1, and alpha 2 and beta-binding sites after stimulation of the locus coeruleus. In the first experiment these characteristics were studied, 4 weeks after treatment, in cortex, hippocampus, hypothalamus and the brainstem. Neither the number, nor the affinity of beta-receptors ([125I]iodocyanopindolol binding sites) was modified in any brain region. A significant increase in the number of alpha 1-receptors ([3H]prazosin binding sites) was observed in the cortex (62%). The number of alpha 2-receptors, ([3H]yohimbine binding sites), was significantly increased in cortex (99%), hippocampus (33%) and hypothalamus (113%). No significant alteration of the alpha 1, alpha 2 and beta-adrenoceptors was observed in the brainstem. To investigate the time course of these adrenoceptor changes, the characteristics of alpha 1, alpha 2 and beta-adrenoceptors were studied 2 weeks after stimulation using the same ligands and in the same brain regions. The only significant modifications observed were an increase of the alpha 2-adrenoceptors in the cortex (19.4%) and in the hypothalamus (54%). Furthermore, in both experiments, the increase in the number of alpha 1 and alpha 2-receptors was associated with a significant decrease in affinity. These results are discussed in relation to our previous behavioral and pharmacological findings.

Antidepressants and protein kinases: desipramine treatment affects pineal gland cAMP-dependent protein kinase activity

To further describe the molecular mechanisms involved in reductions in noradrenergic responsiveness induced by antidepressants, the effects of antidepressant treatment on the rat pineal gland cAMP-dependent protein kinase system were examined. The concentration of cyclic AMP-dependent protein kinase activity was reduced 24 h after acute treatment with desipramine, as well as in animals treated repeatedly with desipramine. Assays performed in the presence of cAMP protein kinase inhibitor showed no significant effects of either acute or repeated desipramine treatment on the concentration of cAMP-independent protein kinase activity. Neither acute nor repeated treatment with other antidepressants (zimelidine, iprindole or fluoxetine) significantly altered the concentration of cAMP-dependent or cAMP-independent protein kinase activity. Using activity ratios to judge the extent of activation of cAMP-dependent protein kinase in vivo, it was found that isoproterenol-induced increases in cAMP protein kinase activity were similar in control and acutely-treated animals, but were reduced with repeated desipramine treatment. The extent of protein kinase activation was also elevated by both acute and repeated treatment in the absence of isoproterenol. In further studies, desipramine (10 microM) did not directly affect activation of the kinase by cAMP or maximum kinase catalytic activity. These results show that the concentration and extent of activation of cAMP protein kinase is altered following desipramine treatment in the rat pineal gland and that modulation of cAMP protein kinase may be a locus of regulation for desipramine-mediated reduction in noradrenergic responsiveness.

Human plasma melatonin is elevated during treatment with the monoamine oxidase inhibitors clorgyline and tranylcypromine but not deprenyl

Melatonin was measured in plasma collected between 8:00 and 8:30 a.m. from 27 depressed patients studied before and after 21- to 24-day treatment with three monoamine oxidase (MAO) inhibitors. Baseline plasma melatonin concentrations determined by radioimmunoassay were 4.0 +/- SD 4.7 pg/ml. Tranylcypromine, a nonselective MAO inhibitor given in doses of 20-40 mg/day for 3 weeks, significantly elevated plasma melatonin to 10.6 +/- SD 2.0 pg/ml. Clorgyline, given in doses of 15-30 mg/day for 3 weeks, produced a significant, approximately three-fold increase in plasma melatonin (13.6 +/- SD 13.5 pg/ml). This clorgyline dose was selective for MAO type A inhibition, as MAO-B activity measured in platelets from the same blood samples was unaffected by clorgyline. In contrast, the selective MAO-B inhibitor deprenyl (10-30 mg/day for 3 weeks) led to a 96 +/- 4% inhibition of platelet MAO-B activity but no significant change in plasma melatonin (5.1 +/- SD 4.2 pg/ml). As both serotonin and norepinephrine are preferentially metabolized by MAO-A rather than MAO-B, an increased availability of serotonin (the precursor of melatonin) or enhanced noradrenergic function might mediate the melatonin changes observed to follow MAO-A but not MAO-B inhibition.