Clinical and biochemical effects of catecholamine depletion on antidepressant-induced remission of depression

Recent advances in antidepressant drug therapy

Many new antidepressants are available for use. The acute effects of administration are mainly manifested as side-effects, whereas the chronic effects take 2-3 weeks to set in and are responsible for therapeutic action. Such changes may include formation and release of neurotransmitters, changes in pre-synaptic uptake, and changes in receptor density and sensitivity. The noradrenergic and serotonergic systems are not separate, but are entwined with each other. The newer SSRI citalopram, and drugs from the SNRI, NaSSA and NAN classes, have been shown in controlled trials to be equivalent to, or more efficacious than, TCAs and/or SSRIs. They have an improved side-effect profile, especially when compared to TCAs, and hence are better tolerated.

Possible serotonergic mechanisms underlying the antidepressant and anti-obsessive-compulsive disorder responses

Considerable evidence is now available to support the pivotal role of the serotonin (5-HT) system is exerting the antidepressant response in humans. Different type of antidepressant treatments enhance 5-HT neurotransmission via different pre- or postsynaptic mechanisms. The time course for the occurrence of these adaptive changes in the brain of laboratory animals is consistent with the delayed onset of the antidepressant response in humans. The drugs effective in obsessive-compulsive disorder (OCD) also enhance 5-HT neurotransmission in brain regions involved in mediating OCD symptoms but with a more prolonged delay, consistently with the larger time necessary to obtain therapeutic effect in OCD than in depression. The elucidation of these mechanisms of action lead to the development of new pharmacologic strategies to potentiate the therapeutic effect of the drugs currently available and the identification of novel targets to accelerate and further improve treatment response in depression and OCD.

What determines primary breast cancer patients' hope to recover

The main purpose was to offer evidence for the hypothesis that the stronger an acute real life stressor, namely, hearing from the physician that one has breast cancer and that one has to undergo mastectomy, the greater the induced noradrenaline (NA) depletion in the central nervous system (CNS) and the more the patient loses hope to recover. The data were derived from answers to interviews, questionnaires, and analyses of blood samples obtained from the patients on the day of admission to the hospital for a biopsy and 24 hours after the surgeon communicated the results of the biopsy to the patients. Analysis showed that a decline in 3-Methoxy 4-Hydroxy Methoxy 4-Hydroxy Phenylethylene Glycol (MHPG) concentration in blood plasma samples after being informed of the diagnosis is associated with less hope of recovery. MHPG is the main metabolite of CNS noradrenaline.

The neurobiology of tryptophan depletion in depression: effects of intravenous tryptophan infusion

BACKGROUND

Previous work has suggested that acute depletion of the serotonin (5-HT) precursor tryptophan (TRP) causes transient compensatory changes in the 5-HT system that might be exploited for their antidepressant effects. In this study, neuroendocrine and mood responses to intravenous (i.v.) infusion of TRP were examined in order to evaluate central 5-HT function in depressed patients undergoing acute TRP depletion.

METHODS

Thirty-eight drug-free patients with DSM-III-R major depression participated. Each patient underwent two randomized, double-blind TRP depletion tests, one sham and one active. At the estimated time of maximum TRP depletion, each patient received an i.v. infusion of TRP 100 mg/kg. Blood was obtained for serum cortisol, prolactin, and growth hormone. Mood was assessed using standardized rating scales.

RESULTS

The cortisol response to i.v. TRP was significantly greater during TRP depletion than during sham depletion. Depressive symptoms showed a tendency to decrease after i.v. TRP following active, but not sham, TRP depletion.

CONCLUSIONS

These findings are consistent with the present hypothesis and previous evidence that acute TRP depletion in drug-free depressed patients induces compensatory upregulation of postsynaptic 5-HT receptors. These changes are insufficient to serve as a means of effecting clinical improvement, but suggest that the antidepressant properties of rapid, marked manipulations of 5-HT function warrant further study.

Rapid depletion of plasma tryptophan: a review of studies and experimental methodology

Evidence that the neurotransmitter serotonin (5-hydroxytryptamine, 5-HT) plays a role in the pathophysiology of mood disorders has been accumulating over the past three decades. Recent studies on this neurotransmitter have extended across the spectrum of psychiatric disorder, suggesting a role for 5-HT in psychosis, aggression, eating disorders and addiction. However, much of the evidence has come from post-mortem examination of the brain or measures of peripheral rather than central 5-HT function. The technique of tryptophan depletion allows investigation of brain 5-HT function in living subjects by examining the behavioural responses to this pharmacological challenge. This review considers the current status of tryptophan depletion as an experimental technique and discusses the implications of findings both in affective disorders and in a range of other psychiatric syndromes. MEDLINE and PSYCHLIT searches were completed for the years 1966 to November 1996 using the key words 'serotonin', '5-hydroxytryptamine', 'tryptophan' and 'depletion'. In addition relevant journals were hand-searched for the period from 1980 to December 1996. Forty-four double-blind studies in humans and three clinical case reports were identified; these cover a range of psychiatric disorders including mood disorders and psychoses, anxiety and eating disorders and specific behaviours such as appetite, aggression and craving. The studies reviewed utilized a variety of differing methodologies reducing the extent to which results can be generalized. A series of studies in depressed patients (before and after treatment with antidepressants) and their first-degree relatives have shown the importance of an intact 5-HT system in the action of antidepressants and offer new insights into the biology of affective disorder. The mood change induced by tryptophan depletion may predict those patients likely to respond to 5-HT-specific drugs. Rapid tryptophan depletion has also been reported to exacerbate both panic and aggression in vulnerable individuals. Effects in other disorders are conflicting and further research is needed to clarify these findings.

Effects of fluoxetine administration on mood response to tryptophan depletion in healthy subjects

Short-term reduction in plasma tryptophan (tryptophan depletion) produces a relapse of depressive symptoms in 60% of previously depressed patients recently recovered with serotonin reuptake inhibitor treatment. Tryptophan depletion does not consistently increase depressive symptoms in unmedicated depressed patients or in depressed patients whose symptoms are remitted with a norepinephrine reuptake inhibitor. These data suggest that serotonin reuptake inhibitor treatment itself may confer vulnerability to the development of depressive symptoms during tryptophan depletion. In order to further investigate this possibility, six healthy individuals underwent double-blind placebo-controlled tryptophan depletion before and following six weeks of treatment with fluoxetine 20 mg/day. No increased vulnerability to the mood-lowering effects of tryptophan depletion occurred as a result of fluoxetine treatment. Additionally, fluoxetine treatment itself was not associated with changes in mood or quality of life in these healthy volunteers. These data indicate that serotonin reuptake inhibitor treatment alone does not produce the depressive effects of tryptophan depletion that are observed in serotonin reuptake inhibitor-treated depressed and obsessive compulsive disorder patients.

Is there a role for a pure noradrenergic drug in the treatment of depression?

Depression is thought to result from a dysfunction in the noradrenergic or serotonergic systems. The noradrenergic system appears to be associated with increased drive, whereas the serotonergic system relates more to changes in mood and it is possible that the different symptoms of depression may benefit from drugs acting mainly on one or other of the neurotransmitter systems. A series of studies has shown that interruption of serotonin synthesis compromises the efficacy of serotonin but not noradrenaline reuptake inhibitors, and interruption of noradrenaline synthesis compromises the efficacy of noradrenaline but not serotonin reuptake inhibitors (SSRIs). This suggests that the two classes of drugs owe their activity to functional changes in different neurotransmitter systems. Reboxetine represents a new class of drugs-the selective noradrenaline reuptake inhibitors (NARIs). It acts specifically at noradrenergic sites unlike the non-selective tricyclic antidepressants (TCAs). NARIs have a role in the treatment of depression, either alone or as adjunctive therapy.

Pharmacology of antidepressants

This review covers mechanisms of action, efficacy, side effects, and toxicity of various classes of antidepressants: tricyclic antidepressants, monoamine oxidase inhibitors, second-generation antidepressants including the selective inhibitors of serotonin reuptake, and novel drugs such as mirtazapine, nefazodone, and venlafaxine.

Lack of behavioral effects of monoamine depletion in healthy subjects

This study was designed to determine the behavioral effects of a reduction in catecholamine and indoleamine function in healthy subjects. Eight healthy subjects received the tyrosine hydroxylase inhibitor, alpha-methyl-para-tyrosine (AMPT) in combination with a full-strength tryptophan-depleting amino acid drink during one 4-day test session, and AMPT and tryptophan-supplemented amino acid drink (n = 2), or a 25% strength tryptophan-depleting amino acid drink (n = 6) during a second 4-day test session. The combined administration of AMPT and the tryptophan-free amino acid drink did not produce statistically significant or even clinically noticeable changes in mood among the healthy subjects. The implications of these observations for the monoamine hypotheses of depression are discussed.