Lithium and 5-HT1A receptor sensitivity: a neuroendocrine study in healthy volunteers

Drugs and HPA axis

This paper outlines the interferences of the most widely used drugs with hypothalamo-pituitary-adrenal function and the related laboratory parameters, with the purpose of providing practical help to clinicians during testing for hypo- or hypercortisolemic states.

Effects of acute tryptophan depletion on mood and suicidal ideation in bipolar patients symptomatically stable on lithium

BACKGROUND

Previous studies suggest that brain serotonin neurotransmission may mediate the actions of lithium carbonate. Acute tryptophan depletion reduces brain serotonin and allows the study of this neurotransmitter in patient groups.

AIMS

To examine the effects of acute tryptophan depletion on mood and suicidal ideation in bipolar patients who were symptomatically stable on lithium.

METHOD

Nineteen subjects satisfying DSM-IV criteria for bipolar I disorder participated in a within-subject, double-blind, placebo-controlled random-order crossover study. Symptoms were evaluated following acute tryptophan depletion, which was induced by a 100 g amino acid drink following an overnight fast.

RESULTS

Plasma tryptophan fell significantly after the depleting drink, but not after the control drink (P < 0.05, paired t-test, mean reduction 83%). No significant changes in mood or suicidality scores were recorded after acute tryptophan depletion.

CONCLUSIONS

Acute tryptophan depletion does not reverse lithium's effects on mood and suicidality in bipolar disorder.

Serotonin in mania and in the mechanism of action of mood stabilizers: a review of clinical studies

OBJECTIVES

Serotonin (5-hydroxytryptamine, 5-HT) was implicated in the pathophysiology of manic-depressive illness as early as 1958. Although extensive evidence has accumulated since then to support 5-HT's role in depression, relatively fewer studies examined its role in mania. The purpose of this paper was to review and summarize the current state of knowledge on the role of 5-HT in mania and its treatment.

METHODS

We systemically reviewed clinical studies of 1) 5-HT function in mania and 2) 5-HT in the mechanism of action of mood stabilizers, including lithium and anticonvulsants.

RESULTS

Review showed that cerebrospinal fluid, postmortem, platelet, neuroendocrine challenge, and tryptophan depletion studies provided some evidence to support the hypothesis that a 5-HT deficit is involved in mania and that enhancement of 5-HT neurotransmission exerts a mood-stabilizing effect.

CONCLUSIONS

There is some evidence from clinical studies for the contribution of 5-HT in mania and in the mechanism of action of mood stabilizers. However, it is very likely that other neurotransmitters also play important roles. Future directions for research include 1) in vivo study of 5-HT receptor subtypes using positron emission tomography, 2) investigation of the interaction between 5-HT and other neurotransmitter systems, and 3) determination of the relationships between diagnostic subtypes of mania and 5-HT function and other neurotransmitter systems.

Effects of lamotrigine on the 5-HT1A receptor function in healthy human males

BACKGROUND

Lamotrigine, a new anticonvulsant, has recently been reported to be effective in treating patients with bipolar mania, depression, and schizoaffective disorder, suggesting that it is perhaps a mood stabilizer with antimanic and antidepressant properties. However, the mechanism of action underlying its efficacy in mood disorders is still unknown.

METHODS

To explore the role of 5-HT1A receptors in the mechanism of action of lamotrigine, we measured the body temperature and plasma cortisol responses to a challenge with a selective 5-HT1A receptor agonist ipsapirone in ten healthy human males. Each subject received 0.3 mg/kg of ipsapirone hydrochloride tablets at time "0". Body temperature readings and blood samples for cortisol levels were obtained at 0, 30, 60, 90, 120, 150, 180 min. The ipsapirone challenge tests were repeated after 1 week treatment with lamotrigine (100 mg/day).

RESULTS

Treatment with lamotrigine for 1 week did not significantly alter the hypothermic or cortisol responses to ipsapirone.

LIMITATIONS

The limitations of this study included small sample size, low treatment dose, short treatment interval, and lack of placebo control.

CONCLUSIONS

Our findings might suggest that 5-HT1A receptor function is not involved in the mechanism of action of lamotrigine in humans. Further placebo-controlled studies with a higher lamotrigine dose and a longer treatment interval in a larger number of subjects are needed to verify this.

Clomipramine challenge responses covary with Tridimensional Personality Questionnaire scores in healthy subjects

Cloninger's Unified Biosocial Theory of Personality postulates a relationship between the relative functional activity of central serotonergic, dopaminergic, and noradrenergic neurotransmitter systems, and the strength of three elemental dimensions of personality. These dimensions are Harm Avoidance, Novelty Seeking, and Reward Dependence, respectively. Accordingly, we predicted that neuroendocrine responses to serotonergic challenge would correlate with Harm Avoidance scores, but not with Novelty Seeking or Reward Dependence scores. We examined the relationship between the prolactin and cortisol responses to a 12.5-mg intravenous clomipramine challenge and these personality dimensions as measured by Cloninger's Tridimensional Personality Questionnaire in 32 healthy subjects. The cortisol response correlated only with Harm Avoidance scores, as predicted; however, prolactin response did not correlate with Harm Avoidance scores. Instead, it demonstrated an inverse relationship with Novelty Seeking scores. There was a positive relationship of baseline prolactin with Harm Avoidance in a post hoc analysis. Cortisol response to serotonergic challenge may be a better indicator for responsivity of serotonergic systems as they relate to the personality dimension of Harm Avoidance than is prolactin. Prolactin responses may be overly affected by dopaminergic influences; however, baseline prolactin may still be a valid indicator of serotonergic tone.

5-HT brain function in affective disorder: d,l-fenfluramine-induced hormone release and clinical outcome in long-term lithium/carbamazepine prophylaxis

Prolactin (PRL) and Cortisol (CORT) responses to d,l-fenfluramine (FEN) challenge (60 mg) were examined in patients with affective disorders on two occasions under euthymic conditions: drug-free before admission to prophylactic treatment and after about 9 months of medication with lithium or carbamazepine. Response to treatment was assessed by a complex algorithm using continuous ratings in outpatient clinic over a period of 2 years. In general, treatment resulted in a delayed and diminished CORT release (n.s.); subgroup analysis revealed an attenuated CORT response (P < 0.05) in responders, whereas nonresponders showed no change in CORT secretion pattern except an enhanced CORT baseline value (P < 0.05). Cross-sectional comparison of responders with nonresponders under medication yielded a trend for greater CORT stimulation in nonresponders. This result was not affected by FEN/NorFEN or lithium/carbamazepine serum levels, baseline CORT values, age, sex, diagnostic distribution, number of appointments to the outpatient clinic or duration of medication at the time of FEN test session. Before onset of prophylactic medication responders and nonresponders could not be discriminated significantly regarding stimulated hormone release, probably due to the small sample size (n = 17). CORT response to FEN was increased in drugfree unipolar patients compared to bipolar (P < 0.05) and to schizoaffective patients (P < 0.1). In accordance with its well-documented presynaptic 5-HT-agonistic action lithium medication resulted in a significantly greater increase in CORT release than carbamazepine (P < 0.05). Evaluation of PRL stimulation showed patterns of secretion quite similar to those of CORT, without reaching statistical significance in most cases. Perhaps due to methodological differences in assessing treatment response, these data do not confirm former results, which supposed an enhanced 5-HT net activity in long-term prophylactic lithium treatment. Because of high interindividual variances of hormone parameters, the FEN-test procedure is not a useful tool for the prediction of therapeutical outcome in terms of clinical routine use. Relations of stimulated hormone response as a marker of central serotoninergic activity and clinical outcome are discussed.

Effect of the placebo control condition in neuroendocrine challenge studies

Neuroendocrine challenge studies are frequently used to make inferences about the pathophysiology of psychiatric illness or about the effects of chronic psychotropic drug treatment on brain monoamine function. The outcome of these exploratory neuroendocrine tests, however, may be influenced by the effects of confounding variables, such as specific stress effects and variations in basal hormone levels. The inclusion of a placebo control condition minimizes the variability due to these variables. A number of recent neuroendocrine studies have omitted the placebo control condition. Based on experimental data, it can be shown that failure to include a placebo control condition may result in both Type I and Type II errors of inference. These results suggest a placebo control condition should be included in exploratory neuroendocrine challenge studies.

Actions of lithium on the cyclic AMP signalling system in various regions of the brain--possible relations to its psychotropic actions. A study on the adenylate cyclase in rat cerebral cortex, corpus striatum and hippocampus

It has been estimated that in most industrialized countries 1 person out of every 1000 in the population is undergoing lithium treatment to stabilize their episodic mood disturbances due to manic-depressive illness. Lithium may stabilize mood swings by altering the action of certain neurotransmitters at the synaptic level in the brain. Recent research suggests that lithium alters neurotransmission by affecting neurotransmitter-coupled second messenger systems. A major second messenger system is the adenylate cyclase, which generates intracellular cAMP from ATP. The adenylate cyclases (type I-IV) are regulated by stimulatory and inhibitory receptors, which either stimulate or inhibit the adenylate cyclase activity. The stimulatory and inhibitory neurotransmitter-receptor signals are transferred to the catalytic unit of the adenylate cyclase by Gs and Gi, respectively. The activated receptor induces GTP stimulation of the heterotrimeric G protein, leading to a dissociation of the protein into the active alpha*GTP and the beta gamma complex. The former stimulates the catalytic unit of adenylate cyclase. The stimulation is terminated by a GTPase located on the alpha subunit that converts GTP to inactive GDP. At present, G proteins are known to play a central role in coupling receptors to effector proteins. In addition to extracellular regulation due to neurotransmitters, some adenylate cyclases (type I, III) are regulated by CaM as a consequence of enhanced intracellular concentrations of free Ca2+. The Ca(2+)-dependent stimulation of adenylate cyclase by CaM is assumed to occur by a direct effect on the catalytic unit. The catalytic units sensitive to Ca(2+)-CaM are also subjected to regulation by stimulatory and inhibitory neurotransmitter stimuli. Magnesium is essential for adenylate cyclase activity, since MgATP2- is the enzyme substrate. Furthermore, one Mg2+ site located on the G protein regulates both the receptor agonist affinity and the dissociation of the G protein during the activation cycle. A second Mg2+ site on the catalytic unit is responsible for Mg2+ regulation of the catalytic activity. The present work aimed at investigating the mechanisms by which lithium in vitro and after chronic treatment (ex vivo) affects adenylate cyclase activities in various regions of the rat brain. Lithium in vitro and ex vivo inhibited the selective stimulation of adenylate cyclase by Ca(2+)-CaM in the cerebral cortex. Furthermore, lithium in vitro interacted directly with the catalytic unit of adenylate cyclase.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of lithium on the prolactin response to D-fenfluramine in healthy subjects

We assessed the effect of two regimens of lithium treatment (3 days and 2 weeks) on brain serotonin (5-HT) function in healthy subjects by measuring the prolactin (PRL) responses to the 5-HT releasing agent, d-fenfluramine (30 mg) before and after lithium administration. Neither regime of lithium treatment altered d-fenfluramine-induced PRL release. These results are in contrast to the facilitation produced by lithium of the PRL responses to the 5-HT precursor L-tryptophan. The data suggest that the ability of lithium to increase brain 5-HT neurotransmission in humans may depend on interactions with particular 5-HT receptor subtypes or specific mechanisms of 5-HT release.

Neuroendocrine probes of serotonergic function: a critical review

Neuroendocrine probes of serotonergic function are based on the premise that serotonin (5-HT) exhibits an excitory influence on the release of certain anterior pituitary hormones and that the extent of release of these hormones following a challenge with a 5-HT agonist would provide an index of central 5-HT activity. This paper critically reviews studies published to date on healthy volunteers to assess the validity of tests of serotonergic function. Using standardized criteria, it is concluded that although extensive data is available on 5-HT-neuroendocrine probes, there is no test that is currently available that would fulfil all the proposed requirements for a test to be acceptable, although ipsapirone, m-chlorophenylpiperazine (m-CPP), and possibly fenfluramine challenge tests come very close. The rapid development and availability of more specific and stereo-selective 5-HT agonists/antagonists seem to indicate that it will not be too long before a valid neuroendocrine test for the assessment of serotonergic function emerges.

Selective 5-HT1A and 5-HT2 receptor-mediated adrenocorticotropin release in the rat: effect of repeated antidepressant treatments

The 5-HT receptor agonists, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) produced dose-dependent increases in plasma adrenocorticotropin (ACTH) in the male rat by activation of 5-HT1A and 5-HT2 receptors respectively. The ACTH response to DOI was enhanced by repeated administration of electroconvulsive shock (five over 10 days) but abolished by the tricyclic antidepressant, amitriptyline (20 mg/kg for 14 days). In contrast 21 days lithium treatment failed to alter DOI-induced ACTH release. Neither repeated electroconvulsive shock, nor amitriptyline, nor lithium altered the ACTH response to 8-OH-DPAT. These data are consistent with results from ligand binding and behavioural studies which suggest that the sensitivity of brain 5-HT2 receptors is increased by repeated electroconvulsive shock but attenuated by tricyclic antidepressant treatment. In contrast, our data suggest that the antidepressant treatments studied do not alter the sensitivity of the 5-HT1A receptors involved in ACTH release.

Endocrine and receptor pharmacology of serotonergic anxiolytics, antipsychotics and antidepressants

Several classes of drugs that modify serotonin (5-HT) neurotransmission are either currently used, or are being evaluated for their potential use in the treatment of anxiety, schizophrenia, and depression. 5-HT1A agonists are considered potential anxiolytics, while some atypical antipsychotics are potent 5-HT2 antagonists (and also have modest dopamine D2 affinity). Furthermore, there is a diverse group of serotonergic drugs that may be effective antidepressants. Secretion of ACTH, corticosterone/cortisol, prolactin, renin, oxytocin and vasopressin are stimulated by activation of different 5-HT receptor subtypes, while other neurotransmitter receptors also influence the secretion of these hormones. We compared the receptor binding profiles of 5-HT anxiolytics, antipsychotics and antidepressants with their endocrine effects. These comparisons could aid in understanding both the therapeutic and side effects of these drugs.