Neurotransmitters and depression: too much, too little, too unstable - or not unstable enough?

The paradox of tianeptine

The classical biochemical hypothesis of depression posits a functional deficit in central neurotransmitter systems, particularly serotonin (5-HT) and/or noradrenaline. The major support for this theory was that antidepressants increase the amount of neurotransmitters in the synaptic cleft, by inhibiting reuptake mechanisms (tricyclics) or inhibiting enzymatic catabolism (MAOIs). The major role suggested for 5-HT in this theory led to the development of a large number of compounds which selectively inhibit 5-HT reuptake, such as fluvoxamine, fluoxetine, citalopram, sertraline, paroxetine, etc. Numerous clinical studies have demonstrated the antidepressant activity of such types of agents, supporting 5-HT deficit as the main origin of depression. Tianeptine is active in classical animal models of antidepressants. Its antidepressant efficacy has been established in controlled trials involving a large number of patients. Several biochemical studies however demonstrated that tianeptine induces in acute as well as in chronic conditions, a presynaptic increase of 5-HT reuptake, both in animal and human platelets and animal CNS. Therefore, as a 5-HT reuptake enhancer, tianeptine exhibits a mechanism of action totally opposite to 5-HT reuptake blockers such as fluoxetine but, paradoxically, both mechanisms of action are associated with a therapeutic activity in depressive disorders. Several hypotheses to explain these paradoxical findings and different methodologies to test them clinically are proposed.

Depression: The predisposing influence of stress

Aversive experiences have been thought to provoke or exacerbate clinical depression. The present review provides a brief survey of the stress-depression literature and suggests that the effects of stressful experiences on affective state may be related to depletion of several neurotransmitters, including norepinephrine, dopamine, and serotonin. A major element in determining the neurochemical changes is the organism's ability to cope with the aversive stimuli through behavioral means. Aversive experiences give rise to behavioral attempts to cope with the stressor, coupled with increased utilization and synthesis of brain amines to contend with environmental demands. When behavioral coping is possible, neurochemical systems are not overly taxed, and behavioral pathology will not ensue. However, when there can be no behavioral control over the stressful stimuli, or when the aversive experience is perceived as uncontrollable, increased emphasis is placed on coping through endogenous neurochemical mechanisms. Amine utilization increases appreciably and may exceed synthesis, resulting in a net reduction of amine stores, which in turn promotes or exacerbates affective disorder. The processes governing the depletions may be subject to sensitization or conditioning, such that exposure to traumatic experiences may have long-term repercussions when the organism subsequently encounters related stressful stimuli. With continued uncontrollable stimulation, adaptation occurs in the form of increased activity of synthetic enzymes, and levels of amines approach basal values. It is suggested that either the initial amine depletion provoked by aversive experiences or a dysfunction of the adaptive processes, resulting in persistent amine depletion, contributes to behavioral depression. Aside from the contribution of behavioral coping, several organismic, experiential, and environmental variables will influence the effects of aversive experiences on neurochemical activity, and may thus influence vulnerability to depression.

Central and peripheral contributions to the enhancement of amphetamine anorexia by desmethylimipramine (DMI)

Intrahypothalamic administration of amphetamine to rats increased food intake, but pre-treatment with the alpha-receptor antagonist phentolamine unmasked an anorexic effect commensurate with that seen after peripheral amphetamine administration. Pretreatment with systemic DMI increased anorexia after peripheral or central amphetamine administration, but the enhancement of centrally-induced anorexia was small. It is concluded that enhancement of the anorexic effect of peripherally administered amphetamine by DMI is primarily a peripheral phenomenon, with interactions within the central nervous system making a relatively minor contribution.

Changes in amphetamine-induced anorexia and stereotypy during chronic treatment with antidepressant drugs

Amphetamine-induced anorexia and stereotyped behaviour were studied in rats, following pretreatment with the antidepressants DMI, inprindole and mianserin. A complex drug-dependent and dose-dependent pattern of results was obtained. Acute pretreatment with DMI and iprindole enhanced amphetamine anorexia and stereotypy; at high doses only, the enhancement of anorexia disappeared during chronic treatment. Mianserin had no effects acutely, but chronic treatment with high doses attenuated anorexia and enhanced stereotypy. High doses of all three drugs attenuated anorexia and enhanced stereotypy during withdrawal. The most parsimonious account of these results is that the acute affects of DMI and iprindole are artefactual, and that chronic administration of all three antidepressants increased dopaminergic function and decreased beta-adrenergic function.

The neurochemistry and pharmacology of extinction behavior

The role of various neurotransmitter systems in the brain in extinction behavior is examined. An attempt is made to suggest psychological mechanisms (such as attention, secondary reinforcement or internal inhibition) by which the neurotransmitter systems or drugs act to produce the observed alteration in extinction behavior. The putative neurotransmitters acetylcholine, noradrenaline, dopamine, serotonin, endorphins and the peptides are reviewed, as are pharmacological agents such as the benzodiazepines, the barbiturates, the psychodelics, the neuroleptics, the psychomotor stimulants and cannabinoids. Other treatments and factors are considered such as peripheral hormones and the adrenal-pituitary axis. It is suggested that the noradrenergic system may be involved in the expression of extinction behavior by a role in selective attention, the dopamine system via an involvement with secondary reinforcement, the cholinergic system by a mechanism of response inhibition and the barbiturates and benzodiazepines by a block of nonreward.

Behavioural evidence that chronic treatment with the antidepressant desipramine causes reduced functioning of brain noradrenaline systems

A behavioral system sensitive to the net functional activity of the locus coeruleus noradrenergic system, with characteristics of a beta-adrenoceptor mediated response, has been developed based on the duration of thiopentone anaesthesia in the rat. The effects of acute and chronic treatment with the tricyclic antidepressant desipramine (DMI) were determined. Acute DMI from 5 to 25 mg/kg increased thiopentone sleeping-time in a dose-dependent fashion. This was due to an action on noradrenergic systems, since it was mimicked by treatment with the selective neurotoxin 6-hydroxydopamine, which itself increased thiopentone sleeping-time and prevented any additional effect of DMI. Chronic treatment with DMI had no effect on thiopentone sleeping-time when carried out for 2 or 5 days but markedly prolonged it when carried out for 10 or 20 days, thus paralleling the time course of clinical action of the drug.

Evidence suggesting that DMI-induced resistance to extinction is not mediated by the dorsal noradrenergic bundle

Rats were trained to press a lever for food rewards, then given a 5 week break, followed by a single extinction session. Animals which received 14 daily desmethylimipramine (DMI) injections, ending 4 days before the extinction session, showed resistance to extinction; no effect was seen in animals which received DMI during acquisition. The opposite pattern of results would be predicted if the effect were mediated by changes in the efficacy of the dorsal noradrenergic bundle.

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.