Postsynaptic serotonergic action of antidepressive drugs

Causes of changes in brain noradrenaline systems and later effects on responses to social stressors in rhesus monkeys: the cascade hypothesis

Disruption of social attachments in social primates produces a protest-despair response. In rhesus monkeys, the response is probably adaptive in the feral environment, although the despair stage resembles human depression in many respects. The severity of the response varies between individuals and is affected by deprivation of certain classes of social stimuli during development. Social deprivation is associated with differences in the concentrations of noradrenaline (NA) in cerebrospinal fluid and in responses to agents that affect catecholamine systems. Thus, early rearing conditions and pre-existing genetic or perinatal differences between monkeys can have long-term effects on the response to social separation, and NA system release and/or receptor mechanisms are involved. NA systems appear to mediate adaptation to the environment from the level of perception to reorganization of neural tissue. Adaptation to the social environment may involve a cascade of changes that begins with behavioural coping attempts and terminates in structural reorganization of regions of the cerebral cortex. Processes at each level occur within environmentally appropriate but neurobiologically constrained time-frames. The cerebral NA system may be an adaptive mechanism that can fail or be damaged. Behavioural changes caused by such damage or failure would be manifested by inappropriate responses to environmental contingencies and inability to change behaviour to adapt to the prevailing environment. These features of NA system disorder could be common to depression and several other forms of human psychopathology.

Response suppression in rats after bilateral microinjection of 5-hydroxytryptophan in lateral hypothalamus

Studies using the 5-hydroxytryptophan (5-HTP) animal model of depression have led to the development of the hypersensitive postsynaptic serotonin receptor theory of depression. To demonstrate more clearly that the 5-HTP-induced suppression is a centrally mediated phenomenon, rats were implanted with bilateral cannulae in the lateral hypothalamus and received microinjections of D,L-5-HTP (100-500 ng) 15 min after the start of a VI operant session (milk reinforcement). Significant decreases in responding were observed that were comparable to those obtained after a systemic injection of 50 mg/kg D,L-5-HTP. Rats receiving a microinjection of 5-HTP in the posterior hypothalamus did not exhibit a behavioral effect. Rats working on shock-avoidance schedules did not demonstrate response suppression following microinjection of 5-HTP into the lateral hypothalamus, which is the same result as that following systemic 5-HTP administration. These data support the important role previously assigned to central 5-HT mechanisms in the 5-HTP animal model of depression.

Blockade of a 5-hydroxytryptophan-induced animal model of depression with a potent and selective 5-HT2 receptor antagonist (LY53857)

To test the hypothesis that a new potent and selective 5-HT2 receptor antagonist would be an excellent blocker of D,L-5-hydroxytryptophan (5-HTP)-induced response suppression in an animal model of depression, we administered LY53857 60 min prior to 5-HTP injections into rats working on an operant schedule for milk reinforcement. As predicted, LY53857 pretreatment significantly blocked 5-HTP depression (90%) in doses as low as 0.1 mg/kg ip. When the dose of LY58357 was further reduced to 0.025 mg/kg, blockade of 5-HTP-induced depression was still greater than 30%. In doses as high as 5.0 mg/kg, LY53857 alone had no effect on the baseline performance of rats working a VI 1 schedule. Pretreatment with desipramine (2.5 mg/kg), an antidepressant characterized as having major noradrenergic effects, did not significantly block the 5-HTP-induced depression. These data suggest that the 5-HTP-induced depression is mediated by serotonergic mechanisms involving 5-HT2 receptors, as LY53857 is a selective antagonist of these receptors. These data also support the suggestion, based on other published data from this laboratory, that some antidepressants are antagonizing 5-HT2 receptors in our animal model of depression and may also act in a similar manner in depressed patients. Thus, this new drug could be of interest as a possible antidepressant agent of the general type that was proposed earlier by Aprison and Hingtgen (1981).

Postsynaptic serotonergic blockade following chronic antidepressive treatment with trazodone in an animal model of depression

Acute pretreatment with clinically equivalent doses of antidepressive drugs has been observed to block D,L-5-hydroxytryptophan (5-HTP) induced behavioral depression in rats working on a food-reinforced operant schedule. Data from studies designed to distinguish presynaptic from postsynaptic events, indicated that the antidepressants were acting in part as blockers of postsynaptic serotonergic receptors. Using the same 5-HTP model of depression, we studied both the chronic and acute effects of a recently introduced antidepressant, triazolopyridine compound. Rats working for milk reinforcement and exhibiting behavioral depression following administration of 50 mg/kg 5-HTP were pretreated (one hr before 5-HTP) with 1, 2, or 4 mg/kg trazodone with resulting blockade of 5-HTP induced depression of 35, 62 and 70% respectively. Chronic administration of trazodone (2 mg/kg trazodone/day) also resulted in a significant blockade of the 5-HTP effect (75%). Neither 2 mg/kg or 4 mg/kg trazodone was found to potentiate the shorter period of depression following 25 mg/kg 5-HTP. Chronic treatment with the antidepressant drugs, amitriptyline or mianserin also blocked 5-HTP depression. Thus, as in our earlier studies, these data suggest an important postsynaptic mechanism associated with chronic administration of trazodone, amitriptyline and mianserin which could be implicated in the therapeutic effectiveness of these drugs. The potency of trazodone in relation to other antidepressant drugs in our behavioral model of depression paralleled their potency in displacing radioligand binding to 5-HT receptors, and gives additional support for the new hypersensitive postsynaptic serotonin receptor theory of depression.