Long-term lithium treatment in rats attenuates m-chlorophenylpiperazine-induced decreases in food intake but not locomotor activity

The effects of antipsychotics with 5-HT(2C) receptor affinity in behavioral assays selective for 5-HT(2C) receptor antagonist properties of compounds

Many antipsychotics have marked antagonist effects at 5-hydroxytryptamine (5-HT(2C)) receptors in vitro, which, however, have been difficult to show in behavioral assays. Here, we used two assays - hypolocomotion and hypophagia induced by the 5-HT(2C) receptor agonist 1-(3-chlorophenyl)piperazine (mCPP) - to try to characterize the 5-HT(2C) receptor antagonist properties of antipsychotics in vivo. Clozapine, olanzapine, pipamperone, and trans-5-chloro-2-methyl-2,3,3a,12b-tetrahydro-1H-dibenz-[2,3:6, 7]oxepino[4,5-C] pyrrolidino maleate (ORG 5222), modestly, but significantly, attenuated mCPP (10 mg/kg)-induced hypolocomotion. In contrast, risperidone and loxapine were inactive. The putative antipsychotic ORG 5222 significantly attenuated mCPP (5 mg/kg)-induced hypophagia, whereas the other antipsychotics were inactive. Selective antagonists at dopamine D(2)-like receptors, alpha(1)-adrenoceptors, alpha(2)-adrenoceptors, or muscarinic receptors were not able to antagonize the effects of mCPP in either assay. The results suggest that mCPP-induced hypolocomotion can be used to characterize the 5-HT(2C) receptor antagonist properties of antipsychotics, whereas mCPP-induced hypophagia appeared to be sensitive only to compounds highly selective for 5-HT(2C) receptors. Together, these assays may help to characterize functional, in vivo, 5-HT(2C) receptor antagonist properties of antipsychotics.

Effects of the serotonin agonists 8-OH-DPAT, buspirone, and DOI on water maze performance

We have previously reported that the serotonin 5-HT1A agonist 8-OH-DPAT and the 5-HT2c agonist TFMPP impair performance on a water maze. In the present report we extended those studies by examining a second 5-HT1A agonist, buspirone, to see whether its effects paralleled those of 8-OH-DPAT, and by testing the effects of the 5-HT2 agonist DOI. Unlike the open pool Morris water maze, the maze used in these experiments has alleys and doorways. The maze can be easily reconfigured to present rats with both previously learned or new maze challenges. Performance is assessed by time to reach the maze exit platform and the number of wrong doorways entered (errors). At doses that did not affect performance in a previously learned maze, the 5-HT1A agonists 8-OH-DPAT (0.1 mg/kg) and buspirone (1 mg/kg) slowed acquisition of a new maze configuration as measured by both swim time to the exit platform and errors committed. A higher dose of buspirone (10 mg/kg) completely blocked acquisition of a novel maze. In contrast. DOI slowed performance as assessed by swim time on both a well-learned maze as well as acquisition of a new maze, but did not affect error rate on either task, suggesting that this 5-HT2 agonist impaired performance by depressing motor activity. These experiments demonstrate that serotonin agonists, especially the 5-HT1A subtype, can impair learning.

Long-term antidepressant treatment restores clonidine's effect on growth hormone secretion in a genetic animal model of depression

We have recently demonstrated that various doses of clonidine failed to increase growth hormone (GH) in Fawn-hooded (FH) rats (a rat strain suggested to represent a genetic model of depression). In the present study, we investigated whether long-term antidepressant treatment might normalize clonidine's effect on GH secretion in male FH rats. Long-term (16 days) treatment with the tricyclic antidepressant, imipramine (5 mg/kg/day), the 5-HT uptake inhibiting antidepressant, fluoxetine (2.5 mg/kg/day), and the noradrenergic uptake inhibiting antidepressant, desipramine (5 mg/kg/day), accentuated clonidine's effect on GH levels. On the other hand, long-term treatment with the monoamine oxidase type-A inhibiting antidepressant, clorgyline (1 mg/kg/day) and the alpha 2-noradrenergic antagonists, yohimbine and 1-phenylpiperazine (1 mg/kg/day, each) did not modify clonidine's effect. These findings suggest enhancement of 5-HT2c receptor-mediated function following long-term treatment with uptake inhibiting antidepressants in a genetic animal model of depression.

Effects of the serotonin receptor agonists 8-OH-DPAT and TFMPP on learning as assessed using a novel water maze

We evaluated the effects of two drugs active at serotonin receptors, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT, a 5-HT1A agonist) and N-3-trifluoromethylphenyl)piperazine hydrochloride (TFMPP, a 5-HT2C agonist) on learning using a novel water maze previously characterized in our laboratory. The water maze utilized is a traditional type of maze with alleyways and doors through which the rats learn to swim to reach a platform, unlike the open pool Morris water maze task. Performance is assessed by swim time required to reach the platform and errors committed. Following initial training on maze configuration A, rats were assigned to saline, TFMPP and 8-OH-DPAT treatment groups and tested for performance once per dose, 30 min after administration of drug (0.25, 0.5, and 1.0 mg/kg IP). Swim times were significantly increased as compared to saline for all doses for both drugs. The error rate was increased for 8-OH-DPAT at all doses, while TFMPP had no effect on error rate at any dose. Next, rats were challenged to learn new mazes following daily administration of 0.25 or 0.5 mg/kg of each drug 30 min prior to each daily swim trial. Rats given 0.25 mg/kg of 8-OH-DPAT learned new maze C more slowly than saline-treated rats, while TFMPP had no effect at this dose. At the higher dose of 0.5 mg/kg, tested on new maze B, TFMPP administration significantly increased swim times but not errors, while this dose of 8-OH-DPAT markedly increased both swim time and errors. Finally, rats from all groups were tested on maze E after drug administration was discontinued, and there were no performance differences among groups. These data suggest that serotonin1A receptors may inhibit learning.

Neuroendocrine responses to serotonergic agonists as indices of the functional status of central serotonin neurotransmission in humans: a preliminary comparative analysis of neuroendocrine endpoints versus other endpoint measures

The status of central serotonergic neurotransmission and of specific serotonin (5-HT) receptor subtype sensitivity has been inferred from neuroendocrine and other endpoint responses to serotonergic agents given to humans. The question of whether changes in neuroendocrine responsivity to the 5-HT2C partial agonist, meta-chlorophenylpiperazine (m-CPP), are accompanied by similar changes in other endpoints (temperature, behavior) is addressed in this brief review of published studies. These studies were selected based on the following criteria: (1) neuroendocrine (cortisol, prolactin increases) and at least one other endpoint (behavior and/or temperature increases) were measured in the same populations, and (2) statistically significant changes were observed after m-CPP in the healthy volunteer control or pre-long-term-treatment subjects. Parenthetically, in the 13 of 14 studies that reported both prolactin and cortisol responses, the results were congruent for the two neuroendocrine measures in 12 of the 13 (92%). However, neuroendocrine versus behavioral results were in agreement in fewer (7 of the 13) studies (54%). Neuroendocrine vs. temperature results were non-concordant in all 4 of the studies which included temperature measurements. These generally disparate findings suggest that these different endpoints may reflect brain serotonin neuroanatomic and receptor subsystem complexity and/or m-CPP's complex pharmacological properties. Thus, these neuroendocrine response measures cannot at this time be considered a general index of the other response measures, nor necessarily an index of the functional status of central serotonergic neurotransmission until this is established by more direct experimental investigations.

Long-term antidepressant treatments alter 5-HT2A and 5-HT2C receptor-mediated hyperthermia in Fawn-Hooded rats

We have recently demonstrated that hyperthermia induced by 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) and m-chlorophenylpiperazine (m-CPP) are separately mediated by selective stimulation of 5-HT2A and 5-HT2C receptors, respectively in Wistar rats. Furthermore, hyperthermia induced by either DOI or m-CPP was found to be significantly less in Fawn-Hooded rats (a rat strain suggested to represent a genetic model of depression) relative to Wistar rats. In the present study, we studied the effects of long-term antidepressant treatments on DOI (2.5 mg/kg)-induced and m-CPP (2.5 mg/kg)-induced hyperthermia in male Fawn-Hooded rats. Long-term (21 days) treatment with the tricyclic antidepressants, imipramine or clomipramine (each 5 mg/kg/day), attenuated DOI-induced hyperthermia, while m-CPP-induced hyperthermia was accentuated. On the other hand, long-term (21 days) treatment with the monoamine oxidase type-A inhibiting antidepressant, clorgyline (1 mg/kg/day), did not modify m-CPP-induced hyperthermia, but significantly attenuated DOI-induced hyperthermia. These findings demonstrate that long-term antidepressant treatments alter 5-HT2A and 5-HT2C receptor-mediated hyperthermia in a genetic animal model of depression.

Enhanced anorexic responses to m-chlorophenylpiperazine during lithium administration to fawn-hooded rats

In the present study, we investigated whether functional adaptational changes in the serotonergic neurotransmitter mechanisms regulating food intake following long-term lithium treatment in Fawn-Hooded rats (a rat strain suggested to represent a genetic model of depression) were different or similar to those previously observed in Wistar rats. Long-term (21-25 days) lithium treatment accentuated m-chlorophenylpiperazine (m-CPP, a 5-HT agonist) induced decreases in food intake. There was no significant difference in either brain m-CPP concentrations or hypothalamic norepinephrine, dopamine and 5-hydroxyindoleacetic acid concentrations between control and long-term lithium-treated rats following m-CPP. However, hypothalamic serotonin concentrations were significantly higher in long-term lithium-treated compared to saline-treated animals. This finding contrasts with our previous report demonstrating attenuation of m-CPP-induced anorexia in Wistar rats following similar long-term lithium treatment, and therefore suggests a differential adaptation in the serotonergic neurotransmitter mechanisms regulating food intake in a genetic animal model of depression.

Lithium treatment restores clonidine's effect in an animal model of depression

IP administration of various doses of clonidine produces significant increases in growth hormone levels in the Wistar rats but not in the Fawn-Hooded (FH) rats, a rat strain suggested to be a genetic model of depression. However, short-term lithium treatment restores clonidine's effect on growth hormone levels in the Fawn-Hooded rats. Potentiation of clonidine's effect on growth hormone levels following short-term lithium treatment appears most likely due to increased serotonergic function as a consequence of enhanced 5-HT concentrations at postsynaptic 5-HT1C receptor sites. Thus, the reversal of a deficit state in Fawn-Hooded rats by lithium treatment supports earlier studies suggesting this rat strain to represent a genetic model of depression.

Chronic treatment with meta-chlorophenylpiperazine (m-CPP) alters behavioral and cerebral metabolic responses to the serotonin agonists m-CPP and quipazine but not 8-hydroxy-2(di-N-propylamino)tetralin

The effects of the serotonin (5-HT) agonists meta-chlorophenylpiperazine (m-CPP), quipazine and 8-hydroxy-2(di-n-propylamino)tetralin (DPAT) on behavior and on regional cerebral metabolic rates for glucose (rCMRglc) were measured in control rats or in rats pretreated for 2 weeks with continuous infusion of saline or m-CPP (2.5 mg/kg/day, subcutaneously). rCMRglc was measured in 71 brain regions, using the quantitative autoradiographic [14C]2-deoxy-D-glucose technique, at 15 min after acute administration of m-CPP 2.5 mg/kg, 60 min after quipazine 20 mg/kg, or 10 min after DPAT 1 mg/kg. Behavioral effects were assessed for m-CPP with an activity monitor, for quipazine by counting head shakes and for DPAT by scoring the serotonin syndrome. Chronic m-CPP pretreatment produced tolerance to hypolocomotion induced by acute m-CPP and to head shakes caused by acute quipazine, but did not alter the serotonin syndrome produced by DPAT. m-CPP 2.5 mg/kg IP produced widespread rCMRglc reductions in control rats but failed to modify rCMRglc in any region after chronic m-CPP pretreatment. Quipazine increased rCMRglc in 4 regions in control rats, but reduced rCMRglc in 14 brain areas of chronically m-CPP-pretreated animals. DPAT altered rCMRglc to the same degree in control (25 regions affected) and in chronically m-CPP-pretreated rats (28 regions affected). Reduced behavioral and metabolic effects of acute m-CPP in chronically m-CPP-pretreated rats were not due to pharmacokinetic alterations. These results demonstrate that chronic administration of m-CPP produces behavioral and metabolic tolerance to acute administration of m-CPP, but not of DPAT.(ABSTRACT TRUNCATED AT 250 WORDS)

m-Chlorophenylpiperazine as a probe of serotonin function

m-Chlorophenylpiperazine (mCPP) is the most extensively used probe of serotonin function in psychiatry. This article reviews its in vitro and in vivo properties in animals, normal human subjects, and psychiatric patients. mCPP is a safe, reliable, direct 5-hydroxytryptamine (5HT) agonist, which may be used to evaluate 5HT receptor sensitivity. It causes a consistent, dose-dependent elevation of ACTH, cortisol, and prolactin levels in both animals and humans, as well as increased body temperature in man. It also causes a variety of behavioral effects, depending on the population studied. These effects are probably 5HT receptor-related, although specific 5HT receptor subtype mechanisms have not yet been established. mCPP may be considered an important addition to armamentarium of 5HT receptor probes, which is especially useful until more selective 5HT receptor agonists have been tested.

Differential effects of long-term lithium and carbamazepine administration on Gs alpha and Gi alpha protein in rat brain

The G protein subunits, Gs alpha, Gi alpha, and Go alpha, have been quantitated in various rat brain regions using enzyme-linked immunosorbent assay (ELISA) techniques. Chronic (3-week) treatment with carbamazepine decreased Gs alpha in several brain regions reaching significance in the neostriatum, while chronic lithium treatment had no unequivocal effect. Lithium significantly increased Gi alpha in the hypothalamus and hippocampus, whereas carbamazepine decreased Gi alpha in the frontal cortex. Both treatments had no consistent effects on Go alpha. We conclude that long-term treatment with lithium and carbamazepine exerts differential effects on G protein alpha subunits, and that this modification of signal transduction represents a potential mechanism of antibipolar drug-induced neural plasticity.

Potencies of antagonists indicate that 5-HT1C receptors mediate 1-3(chlorophenyl)piperazine-induced hypophagia

1. 1-3(Chlorophenyl)piperazine (mCPP) (5 mg kg-1, i.p.) inhibited 2 h food intake in rats previously deprived of food for one day. Ten 5-hydroxytryptamine (5-HT) antagonists given s.c. opposed this hypophagic response. Calculated ID50 values correlated significantly with reported affinities (r = 0.81, n = 10, P less than 0.01) for 5-HT1C but not for 5-HT2, 5-HT1A, 5-HT1B or 5-HT1D receptors. 2. ID50 values of the ten antagonists against 5-hydroxytryptophan (5-HTP) + carbidopa-induced head shakes (a 5-HT2-mediated response) correlated significantly (r = 0.81, n = 10, P less than 0.01) with their affinities for 5-HT2 but not for 5-HT1A, 5-HT1B, 5-HT1C or 5-HT1D receptors. 3. ID50 values for inhibition of hypophagia and head shakes did not correlate significantly with each other. 4. Ratios of ID50 values against hypophagia and 5-HT2-mediated head shakes gave indices of relative in vivo potencies independent of differences in drug metabolism and disposition. These ratios correlated highly significantly (r = 0.91, n = 10, P less than 0.001) with the ratios of the affinities of the drugs for 5-HT1C (but not for 5-HT1A, 5-HT1B or 5-HT1B or 5-HT1D receptors) and with their affinities for 5-HT2 receptors. These results strongly support the hypothesis that mediation of mCPP-induced hypophagia is by stimulation of 5-HT1C receptors and the mediation of 5-HTP-induced head twitches by 5-HT2 receptors.

Differential effect of lithium treatment on fenfluramine-induced decreases in food intake and locomotor activity in rats

Administration of fenfluramine to rats decreased 1-h food intake and locomotor activity. Short-term (2-6 days) but not long-term (21-25 days) lithium treatment potentiated fenfluramine-induced suppression of food intake. However, neither short-term nor long-term lithium treatment had any significant effect on fenfluramine-induced suppression of locomotor activity. These findings demonstrate a differential effect of lithium treatment on fenfluramine-induced suppression of food intake and locomotor activity. Furthermore, these findings are consistent with results from several other animal and clinical studies demonstrating a differential effect of lithium treatment on two different serotonin-mediated functions.

Lithium-selective alteration of the function of brain versus cardiac Gs protein

Lithium was recently demonstrated to inhibit the coupling of both muscarinic cholinergic receptors and beta-adrenergic receptors to pertussis toxin-sensitive and cholera toxin-sensitive G proteins, respectively, thus suggesting alteration of the function of G protein by lithium, as the single site for both the antimanic and antidepressant effects of this drug. One of the most puzzling aspects of the ability of lithium to ameliorate the manic-depressive condition, is its relatively selective action upon the central nervous system (CNS). In the present study, it was shown that lithium selectively attenuated the function of Gs proteins in the CNS, assessed through isoproterenol-induced increases in the binding of guanosine triphosphate (GTP) to these proteins. Therapeutic concentrations of lithium (1-1.5 mM in vitro) inhibited the function of Gs protein in the cerebral cortex of the rat, while 4- to 6-fold larger concentrations of lithium were required to alter the function of Gs protein equivalently in the cardiac ventricles of the rat. Chronic administration of lithium via rat chow, containing lithium carbonate, to rats totally abolished the effect of isoproterenol on the binding of GTP in the CNS but did not affect the function of peripheral cardiac Gs protein. The lithium-selective action on the function of Gs protein in the CNS may stem from the heterogeneity of the alpha s subunit proteins: in the heart, the major species is a 45 kDa molecule, while in the brain, a 52 kDa molecular weight species predominates. The heterogeneity in alpha s subunits may thus be the biochemical basis for the selective action of lithium on the CNS and for the scarcity of peripheral side effects.