Circadian studies of 5HT2 receptors: effects of clorgyline administration

5-HT2A and 5-HT2C receptors and their atypical regulation properties

The 5-HT(2A) and 5-HT(2C) receptors belong to the G-protein-coupled receptor (GPCR) superfamily. GPCRs transduce extracellular signals to the interior of cells through their interaction with G-proteins. The 5-HT(2A) and 5-HT(2C) receptors mediate effects of a large variety of compounds affecting depression, schizophrenia, anxiety, hallucinations, dysthymia, sleep patterns, feeding behaviour and neuro-endocrine functions. Binding of such compounds to either 5-HT(2) receptor subtype induces processes that regulate receptor sensitivity. In contrast to most other receptors, chronic blockade of 5-HT(2A) and 5-HT(2C) receptors leads not to an up- but to a (paradoxical) down-regulation. This review deals with published data involving such non-classical regulation of 5-HT(2A) and 5-HT(2C) receptors obtained from in vivo and in vitro studies. The underlying regulatory processes of the agonist-induced regulation of 5-HT(2A) and 5-HT(2C) receptors, commonly thought to be desensitisation and resensitisation, are discussed. The atypical down-regulation of both 5-HT(2) receptor subtypes by antidepressants, antipsychotics and 5-HT(2) antagonists is reviewed. The possible mechanisms of this paradoxical down-regulation are discussed, and a new hypothesis on possible heterologous regulation of 5-HT(2A) receptors is proposed.

Chronopharmacology of psychotropic drugs: circadian rhythms in drug effects and its implications to rhythms in the brain

The effects of many kinds of psychotropic drugs have been shown in animal studies to follow a circadian rhythm. Trials for the clinical application of this circadian rhythm have already been undertaken. Although the mechanisms underlying this phenomenon are still unclear, chronological changes in the levels of drugs in the blood and brain suggest that it is primarily due to rhythms in the brain's susceptibility to drugs. Rhythms are present in the level of intracerebral neurotransmitters, receptors and second messengers. Each of these rhythms may cause other rhythms within each system of neurotransmitters, which in turn induces a rhythm in the susceptibility to drugs.

Functional activity of 5-HT2 receptors in the modulation of the sleep/wakefulness states

In the light of recent pharmacological investigations using agonists and antagonists that have potent actions on 5-hydroxytryptamine-2 (5-HT2) receptors, the possible functional role of 5-HT2 receptors in the modulation of the sleep/wakefulness states was examined. Data obtained from animals and from clinical studies suggest that serotonin may exert an inhibitory control on deep slow-wave sleep (SWS) through 5-HT2 receptors. In further investigations, the existence of a diurnal variation in the functional activity of 5-HT2 receptors, that depends on the day/night cycle and/or the melatonin rhythm, was revealed. Questions remain with regard to the physiological significance, of the 5-HT2 receptor-mediated deep SWS regulation, the anatomical site(s) of 5-HT2 receptors involved in this regulation and the mechanism underlying diurnal fluctuations in the functional activity of 5-HT2 receptors.

Circadian and seasonal rhythms of 5-HT receptor subtypes, membrane anisotropy and 5-HT release in hippocampus and cortex of the rat

Specific serotonin binding (5-HT1, 5-HT1A, and 5-HT2 subtypes) and membrane anisotropy were measured at 2 h intervals over a 24 h period in the hippocampus and cortex of Wistar WU rats, housed under a 12 h light-dark cycle, with lights on at 07.00. All experiments were performed both in March and December. In the hippocampus significant circadian rhythms could be ascertained for 5-HT1 binding sites in March and December while for 5-HT1A (subtype of 5-HT1) binding sites the circadian rhythm was only significant in March. The membrane anisotropy also showed significant variations only in March. Circadian rhythms were also found in the cortex for 5-HT1 (December) and 5-HT2 (March and December) binding sites as well as for the membrane anisotropy (December). A correlation was found between membrane anisotropy and 5-HT1 and 5-HT2 binding sites in hippocampus and cortex, respectively. A circadian rhythmicity was also observed for serotonin release as measured by in vivo voltammetry in both brain areas. The results obtained on the diurnal variations of serotonin receptor subtypes and serotonin release and the probable inverse relationship of these two parameters may be relevant in understanding the coupling of pre- and postsynaptic activity.

5HT2 receptor changes in rat cortex and platelets following chronic ritanserin and clorgyline administration

Chronic administration of clorgyline or ritanserin to adult rats for 28 days followed by a 3 day drug-free period results in a significant decrease in 5HT2 receptor number (Bmax) in rat frontal cortex from 315.23 +/- 10.72 fmol/mg protein to 249.63 +/- 13.99 fmol/mg protein and 222.55 +/- 17.17 fmol/mg protein, respectively. On rat blood platelets, ritanserin significantly increases recept number from 26.18 +/- 3.83 fmol/mg protein to 50.94 +/- 7.96 fmol bound/mg protein, whereas clorgyline has no significant effect (21.32 +/- 4.78 fmol/mg protein). Following both drug regimens, the affinity (Kd) of the respective ligands for the receptor is not significantly different from controls: the mean Kd value of the three groups for [3H]ketanserin is 1.57 +/- 0.05 nM in cortex and 0.83 +/- 0.25 nM for [125I]iodolysergic acid diethylamide (LSD) on platelets. Clorgyline increases serotonin (5HT) and noradrenaline (NA) levels in cerebellum, and decreases 5-hydroxyindole acetic acid (5HIAA) and homovanillic acid (HVA): ritanserin does not change the levels of the amines or their metabolites. The data shows that platelet and brain changes are not comparable after ritanserin administration. The receptor binding data demonstrates that curve fitting to two data points provides information which is comparable to and as statistically robust as that obtained from eight point saturation curves. Thus, if pilot studies show that the data follows a rectangular hyperbola, two point assays (optimal at 0.1 Kd and 3 Kd) can be used to obtain estimates of Bmax and Kd.

An examination of experimental design in relation to receptor binding assays

1. Real and computer-simulated data were used to examine the efficiency of designs for receptor binding assays. 2. Initially, several different concentrations of [3H]-ketanserin were used in receptor binding studies, using membrane preparations from rat cerebral cortex to establish the form of the binding curves and to investigate the relationship between the variance of the binding measurements and their means. 3. The data showed that the specific binding could be modelled by a simple rectangular hyperbola, and were consistent with the assumption that the binding measurements have a constant coefficient of variation (0.1). 4. Computer-simulated data, with a coefficient of variation of 0.1, were then used to look at the precision of estimates of KD and Bmax obtained through the use of assay designs based on replicate incubations at two radioligand concentrations, or through the use of a geometric sequence of 5-7 radioligand concentrations. In each case, the influence of varying amounts (3.8-50%) of non-specific binding at KD on the precision of these estimates was monitored. 5. The results (a) illustrate the problems which arise in the analysis of receptor binding data when there are relatively high amounts of non-specific binding in combination with a constant coefficient of variation, (b) calculate the errors involved and (c) quantitate the relative merits of 2, 5, 6 and 7 point saturation curves in the estimation of Bmax and KD.