Alessandra C. Rovescalli

Effect of Different Photoperiod Exposure on [3H]Imipramine Binding and Serotonin Uptake in the Rat Brain

Abstract: Seasonal rhythmicity in the occurrence of acute depressive episodes and the therapeutic efficacy of light exposure suggest the possible involvement of the pineal gland or other biological oscillators in the pathophysiology of depressive illness. We have performed studies to clarify whether different light/dark (LD) cycle schedules may induce changes in the biochemical targets of antidepressants in the rat CNS. In particular, we have investigated the effect of short‐ (LD 8: 16) or long‐day (LD 14:10) photoperiods on different biochemical parameters of serotonergic neurons. A significant increase in the density of [3H]imipramine ([3H]IMI) binding and in the Vmax of 5‐[3H]hydroxytryptamine (5‐[3H]HT) uptake was found in the hypothalamus of LD 8:16‐ with respect to LD 14:10‐exposed rats, whereas no difference was found in the kinetic properties of postsynaptic 5‐HT receptors and in 5‐HT metabolism in the hypothalami and cerebral cortices of rats exposed to the two different photoperiods. A seasonal rhythm of [3H]IMI binding sites and 5‐HT uptake seems to exist only in certain brain areas, such as the hypothalamus, because no differences were found in the cerebral cortex of LD 14:10‐ and LD 8:16‐accustomed rats. [3H]IMI binding and 5‐HT uptake were significantly increased in the hypothalamus of rats accustomed to a light/dark‐inverted cycle (DL 10:14) and killed 6 h after the stopping of lighting in comparison to rats exposed to normal LD 14:10 cycles and killed 6 h after the beginning of lighting. Therefore, a circadian modification of the serotonergic presynaptic sites seems to be present and related to light/dark exposure. Because the existence of endogenous compounds able to modulate [3H]IMI binding and 5‐HT uptake, other than 5‐HT, has been postulated in the mammalian brain, the involvement of these substances in the periodic changes observed could be suggested.

Heterologous sensitization of adenylate cyclase activity by serotonin in the rat cerebral cortex

In vitro exposure of rat cerebrocortical slices to microM concentrations of serotonin (5HT) results in an increased response of adenylate cyclase to isoproterenol (ISO). No change in the affinity of the beta-adrenoceptor toward the agonist was found after 5HT exposure when measuring ISO displacement of [3H]CGP 12177 binding. A similar increase of adenylate cyclase response was also found when using VIP as a stimulatory agent. The dose-response curve of adenylate cyclase to the GTP analogue, GppNHp, was modified by 5HT, which promotes a significantly higher maximal response without altering the potency of GppNHp. Forskolin-stimulated adenylate cyclase activity was not affected by 5HT. Serotonergic 5HT2 receptors are involved in the sensitization of adenylate cyclase to GppNHp, since the selective 5HT2 antagonist ketanserin inhibits the effect of 5HT, whereas the 5HT2 agonist DOI mimics 5HT. The involvement of 5HT2 receptor-coupled activation of protein kinase C is also demonstrated: direct protein kinase C activators such as phorbol esters and s,n-dioctanoylglycerol behave in the same manner as 5HT, while the protein kinase C inhibitor CGP 41251 prevents 5HT from increasing adenylate cyclase responsiveness to GppNHp. Moreover, in vitro exposure of cortical slices to 5HT results in reduced inhibition of adenylate cyclase by somatostatin. Since no change was observed at the receptor level and in the direct stimulation of the catalytic subunit of the enzyme, we propose that 5HT might accomplish the sensitization of adenylate cyclase through protein kinase C by inactivating the inhibitory coupling protein Gi and facilitating the interaction of the exogenous GppNHp with the stimulatory coupling protein Gs.

Absence of [3H]SCH 23390 specific binding sites in anterior pituitary: Dissociation from effects on prolactin secretion

We extended a previous study that had shown the selective D1 receptor antagonist SCH 23390, at relatively high doses, to stimulate prolactin (PRL) secretion in the rat and weakly inhibit [3H]spiperone binding to striatum and anterior pituitary (AP) membranes. No specific [3H]SCH 23390 binding sites, up to the micromolar range, were detected in rat AP while specific, saturable [3H]SCH 23390 binding sites (low nanomolar range) were observed in the striatum. In vivo SCH 23390 (1 mg/kg s.c.) induced higher plasma PRL levels, not reversible by the D1 agonist SKF 38393. Similarly the postsynaptic serotonin (5-HT) antagonists metergoline and cyproheptadine did not influence the SCH 23390 effect on PRL. SCH 23390 was also unable to antagonize the decrease of PRL secretion induced by the selective D2 agonist LY 171555. However this latter compound prevented SCH 23390 as well as sulpiride from increasing the PRL concentrations above the control values. These data rule out the possibility that D1 or 5-HT receptors mediate the stimulation of PRL release by SCH 23390. This effect is more likely to be due to a weak indirect interaction with AP-D2 receptors, as indicated by the non-competitive inhibition of [3H]spiperone binding to AP exerted by SCH 23390. Alternatively, non-specific mechanisms triggered by the multiple behavioral changes elicited by such high doses of SCH 23390 may be involved.

Effects of repeated tiapride administration on anterior pituitary dopamine receptors and prolactin release in the rat

The substituted benzamides tiapride and sulpiride, and the classic neuroleptic haloperidol, were studied in the rat to assess their interaction with the anterior pituitary (AP) dopamine (DA) receptors both in vitro ([3H]spiperone binding) and in vivo prolactin-PRL-release). Tiapride weakly inhibited [3H]spiperone binding in both pituitary and striatal membranes with affinity 5-7 times lower than sulpiride and 400-300 times lower than haloperidol. All three drugs were more potent in displacing [3H]spiperone from striatum than from AP. In vivo, tiapride produced weak and transient stimulation of PRL release reaching a full effect at 2 mg/kg i.p. Similar doses of sulpiride produced longer-lasting effects. Haloperidol was more potent than both benzamides. In prolonged treatments (15 or 60 days), tiapride, given twice daily at 0.5 mg/kg i.p., did not modify [3H]spiperone binding in either AP or striatum, nor did it induce significant changes of basal PRL levels. The challenge with a low threshold dose of TIA (0.2 mg/kg ip) produced similar increases of PRL release in the group either treated with TIA or saline. The data indicate that the benzamides examined have low potency for interaction with DA receptors in pituitary and striatum. In particular, tiapride displayed weaker affinity for AP-DA receptors than the other drugs and induced only slight stimulation of PRL levels. Results from repeated tiapride administration indicate that the drug, at a clinically relevant dose, is unable to modify either kinetic characteristics of DA receptors in the pituitary or plasma PRL levels.

Endocrine effects of 5-methoxytryptoline, 5-hydroxytryptoline and tryptoline, putative modulators of rat serotonergic system

The effects of tryptolines (THβCs), putative endogenous compounds acting on the serotonergic function, have been studied on endocrine parameters in rats. In particular, graded doses of 5-methoxytryptoline (5-MeOT), 5-hydroxytryptoline (5-OHT) and tryptoline (Tp) were ip or iv administered to the animals and the circulating titers of prolactin (PRL), growth hormone (GH) and luteinizing hormone (LH) were assayed either at 20 min or at various times after the injection of the compounds. The data herein reported show that THβCs exert endocrine effects, at least in a pharmacological condition. However, the three compounds unequally affect anterior pituitary function. In fact, while 5-MeOT, 5-OHT and Tp all enhance plasma PRL concentrations in a quick, short lasting and dose-related manner, 5-MeOT and Tp induce also decreases of serum LH levels in ovariectomized rats, and 5-MeOT only is able to diminish plasma GH titers. These findings underline an endocrine effect for THβCs in pharmacological conditions and may suggest a functional role for these compounds in the control of anterior pituitary function.

Monophenolic 2-amino-1-indanone and 2-amino-1-tetralone derivatives: synthesis and assessment of dopaminergic activity

Abstract Monophenolic 2-amino-1-indanones and 2-amino-1-tetralones having various substituents on the nitrogen atom were prepared and their activities in vitro on D1 and D2 dopaminergic receptors were assayed. All compounds were inactive in binding assays and this inactivity is discussed.

Chronobiological Studies of Serotonin Uptake Complex: Relevance to Antidepressant Action

The kinetic properties of |3H|-imipramine binding to the hypothalamic membranes of rats have been evaluated at different time intervals throughout the 24 hour-daily cycle, either in control rats or in rats repeatedly treated with desmethylimipramine (DMI) and accustomed to constant darkness or to a 14:10 light-dark photoperiod. The density of |3H|-imipramine binding in the hypothalamus of constant darkness exposed rats fluctuates with peaks in the early morning and late afternoon. Strong variations in correspondence with the turning of light were observed both in LD 14:10 exposed rats and in constant darkness exposed rats subjected to single pulses of light. The comparison between the circadian patterns of |3H|-imipramine binding in saline and DMI-treated rats indicates that drug exposure modifies the daily pattern differently depending on the light-dark schedule. Moreover, the response to a single pulse of light is altered in animals chronically treated with DMI.

Light-Dark-Related Changes in the Serotonin Uptake Molecular Complex in Rat Brain: Involvement in Antidepressant Action

Clinical studies have clearly indicated that acute episodes and relapses of major depressive illness are characterized by a rhythmic cycle during the year, their incidence being highest during the spring and autumn.1 In addition, circadian modifications in the mood of depressed patients and in the efficacy of antidepressive therapies have been observed2–4 together with disruption or desynchronization of hormonal circadian patterns.5 Based on the above observations, it has been postulated that in those depressed patients susceptible to seasonal bouts of the illness, a circadian and/or seasonal desynchronization occurs and may be corrected by antidepressant drugs1,6 or light therapy.4