Differential effects of long-term antidepressant treatments on 8-OHDPAT-induced increases in plasma prolactin and corticosterone in rats

Interactions between serotoninergic and aminoacidergic pathways in the control of PRL secretion in prepubertal male rats

Prolactin secretion is controlled by the hypothalamus through different neurotransmitters which interact with multiple receptor subtypes. The discovery of different families of receptors for serotonin (5-HT1-5-HT7) and excitatory aminoacids (NMDA, KA, AMPA and metabotropic receptors) ilustrates the complexity of this regulation. Moreover, in the rat the role of different neurotransmitters changes during pubertal development. Present experiments were carried out to analyse the interactions between AMPA and serotoninergic receptors in the control of prolactin secretion in prepubertal male rats. For this purpose, 16 and 23-day old male rats were treated with 5-hydroxytryptophan (5-HTP, precursor of serotonin synthesis) plus fluoxetine (blocker of serotonin reuptake), 8-OH-DPAT (agonist of 5-HT1A receptors), DOI and alpha-Me-5-HT (agonists of 5-HT2 receptors), 1-phenylbiguanide (agonist of 5-HT3 receptors) alone or in combination with AMPA (agonist of AMPA receptors). The results obtained indicate that: (a) activation of 5-HT1A receptors stimulated PRL secretion on day 16 and inhibited it on day 23; activation of 5-HT2 receptors stimulated PRL secretion on days 16 and 23, whereas activation of 5-HT3 receptors inhibited PRL release only on day 23; (b) activation of AMPA receptors inhibited PRL secretion on day 23, but not on day 16 and (c) a cross-talk is apparent between 5-HT2 and AMPA receptors in the regulation of PRL secretion, the stimulatory effect of DOI being blocked by AMPA.

Prenatal stress prevents the desensitization of the corticosterone response to TFMPP by desmethylimipramine, but not by phenelzine, in adult male offspring

Gravid female rats were subjected to one hour of restraint stress twice daily or left undisturbed from days 14-21 of gestation. Adult 105-day old male non-stressed (NS) and stressed (S) offspring were treated once daily with saline, desipramine (DMI) (10 mg/kg, sc) or phenelzine (5.0 mg/kg, sc) for 14 days. Twenty-four hours after the last injection, animals were challenged with saline or 1-(m-trifluoromethylphenyl)piperazine (TFMPP) (5.0 mg/kg, sc), a serotonin1B/2C (5-HT1B/2C) agonist, and plasma prolactin and corticosterone concentrations were measured one hour later. As compared to acute saline administration, TFMPP significantly increased prolactin and corticosterone concentrations in all groups. In NS offspring, both DMI and phenelzine treatment augmented the prolactin response, but blunted the corticosterone response, to TFMPP. In S offspring, the prolactin response to TFMPP also was augmented by phenelzine or DMI treatment, whereas the corticosterone response to TFMPP was blunted during phenelzine treatment. However, DMI treatment was not able to desensitize the corticosterone response to TFMPP in the S rats. The results indicate the adaptive capacity of 5-HT systems to DMI administration was compromised in adult animals exposed to stress in utero.

Effects of 8-OH-DPAT on corticosterone after acute and chronic administration of antidepressants

1. Serotonin has a facilitary role in the role of corticosterone secretion. 8-Hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), a selective 5-HT1A agonist, dose dependently (0.25- 1.0 mg/kg i.p.) increased rat plasma corticosterone concentration. 2. 3 days parachlorophenylalanine (PCPA) (150 mg/kg) administration did not effect the 8-OH-DPAT-induced corticosterone secretion. 3. Corticosterone responses to 8-OH-DPAT (0.5 mg/kg) were significantly attenuated by pretreatment with propranolol (5 mg/kg). Ketanserin (2 mg/kg), haloperidol (0.2 mg/kg), prazosin (0.1 mg/kg), and ICS-205930 (30 mu/kg) failed to antagonize the corticosterone response to 8-OH-DPAT. 4. 8-OH-DPAT-induced corticosterone were investigated in male rats after treatment with mianserin (2, 10 mg/kg), imipramine (5 mg/kg), desipramine (5 mg/kg), doxepine (5 mg/kg) for 1 day or 3 weeks. Chronic mianserin (10 mg/kg) and doxepine (5 mg/kg) did significantly increase 8-OH-DPAT-induced corticosterone response. Acute antidepressant, chronic imipramine, desipramine and mianserin (2 mg/kg) treatment did not change it. 5. These findings demonstrate that chronic treatment of some antidepressants potentiates 8-OH-DPAT-induced increase in plasma corticosterone, by actions at 5-HT-1A receptors located postsynaptically on 5-HT neurones.

Increased mRNA for corticotrophin releasing hormone in the amygdala of fawn-hooded rats: a potential animal model of anxiety

Compared to the outbred Wistar rat strain, the Fawn-hooded rat strain has several characteristics which suggest that the Fawn-hooded strain is hyperaroused. Fawn-hooded rats exhibit more freezing behavior in response to stress, have an increased preference for alcohol, develop adult onset hypertension, and have elevated urinary catecholamine levels. We used quantitative in situ hybridization to investigate central components of the corticotropin releasing hormone (CRH), arginine vasopressin (AVP) and noradrenergic stress response and arousal systems in these rats. We also measured basal corticosterone levels and adrenal weights to assess tonic hypothalamic-pituitary-adrenal axis activity. Compared to Wistar rats, Fawn-hooded rats had significantly increased CRH mRNA in the central nucleus of the amygdala and reduced CRH mRNA in the paraventricular nucleus of the hypothalamus. Fawn-hooded rats also bad reduced AVP mRNA expression in the parvocellular cells of the hypothalamic paraventricular nucleus. There were no differences between strains in glucocorticoid receptor mRNA in the hippocampus or the paraventricular nucleus or in mineralocorticoid receptor mRNA in the hippocampus. There was also no difference between strains in tyrosine hydroxylase mRNA in the locus ceruleus. Finally, adrenal weights were significantly reduced in the Fawn-hooded rats while basal corticosterone levels were similar in the two strains, which suggests central hypoactivity of the hypothalamic-pituitary-adrenal axis in Fawn-hooded rats compared to Wistar rats. Increased CRH mRNA in the central nucleus of the amygdala and reduced tonic hypothalamic-pituitary-adrenal axis activity may play a role in the unique behavioral and physiological characteristics of Fawn-hooded rats.

Gonadotropin and prolactin secretion in prepubertal female rats treated with 8-hydroxy-2-(di-n-propylamino) tetralin

The effect of serotoninergic activation on gonadotropin and prolactin release were analysed in 16-day-old intact female rats. In the first experiment, females were decapitated 30 min after i.p. administration of 100 mg/kg of 5-hydroxytryptophan (5-HTP) or vehicle; in the second experiment the rats were decapitated 15 and 30 min after i.p. injection of vehicle or some doses (0.1, 1 and 10 mg/kg) of 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), a selective agonist of the serotonin (5-HT)1A receptors. We found that: 1) serum follicle-stimulating hormone (FSH), luteinizing-hormone (LH) and prolactin concentrations increased after 5-HTP administration; 2) serum LH and prolactin concentrations and pituitary prolactin content increased after administration of 8-OH-DPAT. Our results indicate that in prepubertal rats, activation of serotoninergic system stimulated gonadotropin and prolactin release, and that 5-HT1A receptors are involved in this effect. In addition, the simultaneous increase in serum and pituitary prolactin content suggests that 8-OH-DPAT enhances prolactin synthesis.

Effect of lofepramine on 5-HT function and sleep

We studied the effect of the tricyclic antidepressant lofepramine (140-210 mg daily for 16 days) on 5-hydroxytryptamine 1A (5-HT1A) receptor sensitivity in healthy volunteers, using a buspirone neuroendocrine challenge paradigm (30 mg orally). We also studied the effect of lofepramine on platelet 5-HT content and sleep architecture. Lofepramine treatment did not alter the hypothermic, endocrine or amnesic effects of buspirone but significantly lowered platelet 5-HT content and decreased rapid eye movement sleep. Our findings suggest that at clinically used doses, lofepramine inhibits the uptake of 5-HT and produces changes in sleep architecture characteristic of tricyclic antidepressants. However, lofepramine does not appear to alter the sensitivity of 5-HT1A receptors.

Attenuation of hypercortisolemia in fawn-hooded rats by antidepressant drugs

Long-term (21 days) treatment with imipramine, clomipramine (tricyclic antidepressants) and clorgyline (monoamine-oxidase type A inhibiting antidepressant) produced significant decreases in plasma corticosterone levels in fawn-hooded (FH) rats. In contrast, plasma adrenocorticotropic hormone (ACTH) levels were not altered by chronic imipramine or clorgyline treatment but were significantly higher in chronic clomipramine-treated FH rats. These findings demonstrate a differential effect of chronic antidepressant treatment on plasma ACTH and corticosterone concentrations in FH rats and, furthermore, support the results of earlier studies suggesting that the FH rat strain may represent a genetic model of depression.

Physiopharmacological interactions between stress hormones and central serotonergic systems

The present review tries to delineate some mechanisms through which the sympathetic nervous system (SNS) and the hypothalamo-pituitary-adrenal (HPA) interact with central serotonergic systems. The recent progress in 5-hydroxytryptamine (5-HT) receptor pharmacology has helped to define the means by which central serotonergic activity may alter the respective activities of the SNS (sympathetic nerves and adrenomedulla) and of the HPA axis. These pharmacological findings have also helped to characterize the differential effects of central 5-HT upon different branches of the SNS and the numerous sites at which 5-HT exerts stimulatory influences upon the HPA axis. Although relevant to stress-related neuroendocrinology, the extent to which these interactions are involved in the antidepressant/anxiolytic properties of some serotonergic agents still remains to be clarified. Beside these findings, there is also abundant evidence for a tight control of central serotonergic systems by stress hormones. Activation of the SNS increases, by numerous means, central availability of tryptophan, whereas glucocorticoids exert differential actions upon the intra- and the extraneuronal regulation of 5-HT function. Actually, a significant number of these mechanisms is involved in the maintenance of homeostasis during stressful events, thereby conferring to these mechanisms a key role in adaptation processes.

Regional brain expression of serotonin transporter mRNA and its regulation by reuptake inhibiting antidepressants

Regional expression and antidepressant drug-induced regulation of mRNA encoding the serotonin (5-HT) transporter were studied in rat brain. While 5-HT transporter mRNA is abundantly expressed in the midbrain raphe complex, lower concentrations were also found in frontal cortex, hippocampus, and neostriatum using a combination of reverse transcriptase-polymerase chain reaction (RT-PCR), Southern hybridization, and sequence analysis. Long-term administration of antidepressants which inhibit 5-HT reuptake, but not monoamine oxidase inhibitors or 5-HT receptor agonists, decrease 5-HT transporter mRNA steady-state concentrations. Based on these observations, we conclude that (1) mRNA coding for the 5-HT transporter is present in several brain areas associated with ascending HT pathways, and (2) chronic treatment with reuptake inhibiting antidepressants may be associated with regulation of the 5-HT transporter at the level of gene expression which may contribute to the neuroadaptive mechanisms that likely underlie their therapeutic efficacy.

Effects of repeated mild stress and two antidepressant treatments on the behavioral response to 5HT1C receptor activation in rats

This study investigated the possible involvement of 5HT1C receptors in the development of depressive states and in the mode of action of antidepressants. The effects of repeated unpredictable mild stress (a regimen known to induce an anhedonic state in the rat) and of chronic administration of either of two recognized antidepressant treatments (sleep deprivation or inhibition of monoamine oxidase type A) in rats were studied on a 5HT1C receptor initiated response, i.e. mCPP-induced penile erection. A 3-week period of repeated, but unpredictable exposure to mild stressors induced a shift to the left of the dose-response curve for mCPP-induced penile erection. In contrast, 72-h REM sleep deprivation resulted in a shift to the right of the mCPP dose-response curve and 10-day administration of the monoamine oxidase type A inhibitor moclobemide (20 mg/kg IP bid) also resulted in a decreased number of mCPP-induced penile erections. These findings support the hypothesis that neuronal activities initiated via 5HT1C receptor stimulation may play a role in the pathophysiology and treatment of depression.

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.

A review and reevaluation of the role of serotonin in the modulation of lordosis behavior in the female rat

The role of serotonin (5-HT) in the modulation of sexual receptivity (lordosis) in the female rat is reviewed and reevaluated. The effects on lordosis of drug treatments that decrease or increase the activity and availability of central 5-HT are first discussed, and this is followed by an evaluation of the effects of drugs that act directly at 5-HT receptors. In order to shed light on the physiological significance of effects of serotonergic drugs on lordosis, there is also a review of what is known of changes in levels of serotonergic activity and densities of 5-HT receptors in the female rat brain that take place through the estrous cycle and in response to administration of behaviorally effective doses of gonadal steroids. Serotonin has generally been thought to have a tonic, inhibitory effect on lordosis. However, it is concluded that 5-HT can either inhibit or facilitate lordosis depending on which subtypes of central 5-HT receptors become activated. Because of a lack of consistent or compelling evidence of effects of ovarian hormones on serotonergic activity or 5-HT receptors in critical areas of the brain, it is stated that there is at present no basis to conclude that the effects of pharmacological manipulations of serotonergic activity on lordosis reflect an important, physiological role of 5-HT in the modulation of lordosis behavior in the female rat.

Sex difference for tolerance of 5-HT1A receptor-mediated temperature and corticosterone responses in mice

Repeated treatment with 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) resulted in significant attenuation of 8-OH-DPAT-induced hypothermia and adrenocorticol effect in mice of both sexes, while it did not affect the 8-OH-DPAT-induced decrease in 5-hydroxyindoleacetic acid in the hypothalamus in either sex. The attenuated responses developed more rapidly in female than in male mice, indicating sex differences in the adaptive regulation of the 5-HT1A receptor-mediated responses.

Neuroendocrine response to clonidine and 8-OH-DPAT in rats following chronic administration of desipramine or sertraline

1. Rats were administered either desipramine (DMI) or sertraline daily at doses 7.5 mg kg-1 or 10 mg kg-1, i.p., respectively and the effects on the functional state of hypothalamic neuroendocrine control mechanisms assessed by measurements of plasma hormones following acute drug challenge. The effects of treatment on gross behaviour and brain adrenoceptor density were also determined. 2. Both DMI and sertraline caused significant reduction in activity measured as ambulation and rearing at 14 days of treatment. 3. All animals were chronically cannulated after 14 days of treatment and tested for neuroendocrine response to acute i.v. clonidine (50 micrograms kg-1) or 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT, 250 micrograms kg-1) after 21 or more days of treatment. 4. Rats treated with DMI but not sertraline showed a virtually complete suppression of the growth hormone (GH) secretion elicited by clonidine in controls, while the secretion of corticosterone was augmented. 5. Treatment with DMI but not sertraline led to a significantly greater 8-OH-DPAT-induced secretion of prolactin than in the control rats, while the plasma concentrations of corticosterone following 8-OH-DPAT were not influenced by either DMI or sertraline treatment. 6. The density (but not the affinity) of cerebral cortical binding of [3H]-dihydroalprenolol was significantly reduced by DMI treatment. 7. These results show that DMI treatment blunted the sensitivity of post-synaptic alpha 2-adrenoceptors, accompanied by complex interactions manifested as increased responsiveness of alpha 1-adrenoceptors and 5-HT1A receptors. Sertraline had no significant neurendocrine effects at a dose which significantly reduced gross activity.

Desipramine, administered chronically, influences 5-hydroxytryptamine1A-receptors, as measured by behavioral tests and receptor binding in rats

The 5-hydroxytryptamine1A (5-HT1A) receptor subtype seems to be of importance in the pathogenesis of depression and in the mode of action of antidepressants. In this study, behavioural experiments were performed in rats after oral administration of desipramine for 18-20 days, followed by an acute injection of the selective 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), either systemically or intrathecally. Chronic administration of desipramine prolonged the behavioural 5-HT syndrome in the animals injected systemically with 8-OH-DPAT. Treatment with desipramine was also found to potentiate and prolong the antinociceptive effect of an acute injection, systemically or intrathecally, of 8-OH-DPAT in the increasing temperature hot plate test. After systemic administration of 8-OH-DPAT, the colonic temperature was lowered similarly in the desipramine-treated group and in controls, whereas an intrathecal injection of 8-OH-DPAT resulted in a fall in the colonic temperature in the desipramine-treated group only. In vitro receptor binding studies, using [3H]8-OH-DPAT as the ligand, showed a statistically significant reduction of Kd and Bmax in the frontal cortex and of Kd in the spinal cord, after treatment with desipramine. No changes of Kd and Bmax were found in the hippocampus after this treatment. Thus, desipramine, administered chronically, resulted in a functional up-regulation of the 5-HT1A-receptors, both spinally and supraspinally, whereas in the in vitro receptor binding, a slight down-regulation or no change was found. It seems therefore that the results of in vitro receptor binding studies do not necessarily reflect the functional state of the neuronal system.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of G proteins by chronic antidepressant drug treatment in rat brain: tricyclics but not clorgyline increase Go alpha subunits

The effect of long-term (3-week) administration of various antidepressant drugs on the steady-state concentrations of G protein alpha subunits, Gs alpha, Gi alpha, and Go alpha, has been investigated in rat brain using an enzyme-linked immunosorbent assay. Tricyclic antidepressants and clorgyline decreased Gs alpha and, to a lesser extent, Gi alpha in several brain regions, while Go alpha was increased by tricyclics but not clorgyline. We conclude that long-term treatment with antidepressant drugs exerts differential effects on G protein alpha subunits, and that antidepressant efficacy may potentially be based on functional modifications of signal transduction.

Repeated treatment with the 5-HT1A receptor agonist, ipsapirone, does not affect 8-OH-DPAT- and stress-induced increases in plasma adrenaline levels in the rat

The aim of this study was to investigate whether 5-HT1A receptor-mediated adrenal catecholamine release undergoes rapid desensitisation. Thus, we measured plasma adrenaline either following the acute administration of the 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT, 0.3 mg/kg i.v.), or during immobilisation stress, in rats pretreated repeatedly with saline or with the 5-HT1A receptor agonist, ipsapirone (10 mg/kg i.p., t.i.d. for 7 days). Plasma corticosterone and glucose were measured concomitantly. Neither body weight nor basal plasma adrenaline and corticosterone levels were significantly affected by ipsapirone treatment. Conversely, the latter diminished basal plasma glucose levels. While the 8-OH-DPAT-induced elevations in plasma adrenaline remained unaffected by ipsapirone, the 8-OH-DPAT-induced elevations in plasma corticosterone and glucose tended to be diminished by ipsapirone. Ipsapirone treatment modified the kinetics, but not the amount of adrenaline released by stress. On the other hand, stress-induced activation of the corticotropic axis was amplified by ipsapirone. Lastly, ipsapirone treatment again tended to diminish the hyperglycemic response to stress. These results indicate that (i) 5-HT1A receptor-mediated activation of adrenaline release is not desensitised by short-term ipsapirone treatment, (ii) the anxiolytic/antidepressant effect of ipsapirone may not be explainable in terms of tolerance to some neuroendorinological consequences of stress.

Effects of repeated administration of mifepristone and 8-OH-DPAT on expression of preproneuropeptide Y mRNA in the arcuate nucleus of obese Zucker rats

Neuropeptide Y (NPY) is an important hypothalamic regulator of feeding behavior. In this study we have investigated the regulation of the expression of preproNPY mRNA in male obese and lean Zucker rats by in situ hybridization. These animals represent a model of genetic obesity with hyperphagia, hyperinsulinemia and altered endocrine functions. Obese Zucker rats, treated for 12 days with 0.9% saline, had about 210% higher level of basal preproNPY mRNA expression in the arcuate nucleus when compared to their lean littermate controls. Repeated administrations of 8-hydroxy-dipropylaminotetralin (8-OH-DPAT), a serotonergic 5-HT1A agonist, or mifepristone, a glucocorticoid receptor antagonist, did not modify the basal expression of preproNPY mRNA in the Zucker phenotypes. The 8-OH-DPAT treatment significantly reduced hyperinsulinemia in obese Zucker rats without changing plasma glucose levels. The mifepristone treatment significantly increased plasma corticosterone levels in lean animals, but not in obese animals. The present study demonstrates enhanced expression of preproNPY mRNA in the arcuate nucleus in obese Zucker rats suggesting an involvement of NPY in the pathophysiology of the hyperphagic syndrome and genetically determined obesity in Zucker rats. Neither the antagonism of glucocorticoid receptors by mifepristone, nor repeated treatment with 8-OH-DPAT resulting in reduced insulin levels in obese Zucker rats, modified the basal expression of preproNPY mRNA in the arcuate nucleus.

Long-term fluoxetine treatment decreases 5-HT1A receptor responsivity in obsessive-compulsive disorder

Fluoxetine (FLX) is a selective serotonin (5-HT) reuptake inhibitor with therapeutic benefit in patients with obsessive-compulsive disorder (OCD). To evaluate the effect of chronic FLX treatment on 5-HT1A receptor responsivity, hypothermic, neuroendocrine, and behavioral responses to the selective 5-HT1A receptor ligand ipsapirone (IPS) were examined in patients with primary OCD. A single dose of 0.3 mg/kg of IPS or placebo were given under double-blind, random-assignment conditions to ten patients before and during FLX treatment. The ability of IPS to induce hypothermia and ACTH/cortisol release was significantly attenuated during chronic FLX as compared to the pretreatment IPS challenge. The behavioral effects of IPS, though minimal, were less pronounced during FLX treatment. While FLX was effective in reducing the severity of OC symptoms, no significant correlation between attenuation of 5-HT1A receptor-mediated functional measures and FLX-induced improvement in OC symptoms was detected. These findings are consistent with the development of adaptive hyporesponsivity of the 5-HT1A receptor-effector system complex possibly involving subsensitivity of the 5-HT1A receptor itself and/or decreased functional activity of the postreceptor signal transduction. Modulation of 5-HT1A receptor-effector system function may be critical to the antidepressant/anti-OC efficacy of 5-HT reuptake inhibitors.

5-HT1A receptor responsivity in anxiety disorders and depression

1. Azapirones, selective partial agonists at the 5-HT1A receptor subtype, induce hypothermia and corticotropin (ACTH)/cortisol release as specific functional correlates of central 5-HT1A receptor activation. 2. Compared to controls, hypothermic and ACTH/cortisol responses to the azapirone ipsapirone are attenuated in patients with unipolar depression and panic disorder but not in patients with obsessive-compulsive disorder. The impaired thermic and neuroendocrine responses are associated with increased basal cortisol secretion in depressed patients but not in patients with panic disorder. 3. Chronic treatment with the selective 5-HT reuptake inhibitor fluoxetine decreases 5-HT1A receptor-mediated responses in patients with obsessive-compulsive disorder, while long-term treatment with the tricyclic antidepressant amitriptyline further decreases hypothermia following ipsapirone but has no effect on ACTH/cortisol release. 4. Alteration of the 5-HT1A receptor and/or its signal transduction pathways may play a role in the pathophysiology and treatment of anxiety disorders and depression.

Corticosterone and prolactin response to TFMPP in rats during repeated antidepressant administration

The corticosterone and prolactin response to acute administration of the 5-HT agonist 1-(m-trifluoromethylphenyl) piperazine (TFMPP) (10 mg kg-1) was assessed in rats treated for 10 days with either saline, amitriptyline (20 mg kg-1 day-1) or nialamide (40 mg kg-1 day-1). For all groups, TFMPP significantly increased both serum corticosterone and prolactin concentrations compared with control animals challenged with saline. However, the corticosterone response to TFMPP was attenuated significantly by nialamide pretreatment, while the prolactin response to TFMPP was enhanced significantly by amitriptyline pretreatment. These results support previous reports that antidepressants differentially affect 5-HT-ergic systems involved in the regulation of corticosterone and prolactin secretion.

Effects of 5-HT1A receptor agonists on hypothalamo-pituitary-adrenal axis activity and corticotropin-releasing factor containing neurons in the rat brain

Corticotropin-releasing factor (CRF) is the major physiological regulator of the hypothalamo-pituitary-adrenal axis. There is evidence that CRF release from the hypothalamus is under stimulatory serotonergic control. The specific 5-HT receptor subtypes that mediate this effect is unclear. Administration of the 5-HT1A agonists, 8-OH-DPAT (1 mg/kg) and ipsapirone (4 mg/kg), to rats resulted in activation of the HPA axis as evidenced by increased plasma ACTH and corticosterone concentrations in acutely treated rats and increased plasma corticosterone concentrations in both acutely and chronically treated rats. However, chronic administration of these compounds failed to alter CRF concentrations in the medium eminence or CRF receptor number of affinity in the anterior pituitary. Chronic administration of both compounds resulted in increased CRF concentrations in the piriform cortex and hippocampus, whereas 8-OH-DPAT alone increased CRF concentrations in the amygdala and entorhinal cortex. These results suggest that both hypothalamic and extrahypothalamic CRF neurons are influenced by activation of 5-HT1A receptors.

Effects of systemic 8-OH-DPAT on the feeding induced by hypothalamic NE infusion

Past research suggests that activating brain serotonin (5-hydroxytryptamine or 5-HT) systems can inhibit feeding induced by activating brain norepinephrine (NE) systems. To explore this interaction more fully, we tested the capacity of the endogenous 5-HT release inhibitor, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), to enhance feeding stimulated by infusing NE into the medial hypothalamus. All experiments were conducted using ad lib-fed adult male rats with indwelling cannulae aimed at the paraventricular nucleus (PVN). In the first study, proven PVN-NE responders were tested for 40-min food intake after receiving 20 nanomoles (nmol) 1-NE or saline in the PVN following subcutaneous (SC) pretreatment with 250 micrograms/kg 8-OH-DPAT or saline. Both drugs produced equivalent, reliable increments in feeding compared to PVN-saline. However, no additivity or synergy was seen when they were combined. Short-term water intake was unaffected by these treatments as was subsequent food or water intake over the next 22 hr. In a second study, additional proven PVN-NE responders were tested under two comparable conditions when 1) the 8-OH-DPAT dose was left at 250 micrograms/kg but the NE dose was lowered to 10 nmol, and 2) the 8-OH-DPAT dose was lowered to 120 micrograms/kg and the NE dose was increased to 40 nmol. In the first case, no reliable feeding was seen in response to either agent alone or combined. In the second case, NE alone enhanced feeding but 8-OH-DPAT did not. The combination of both produced the same enhanced feeding as seen with NE alone.(ABSTRACT TRUNCATED AT 250 WORDS)

Repeated treatment with antidepressant drugs and/or electroconvulsive shock (ECS) does not affect the quinpirole-induced elevation of the serum corticosterone concentration in rats

The selective dopamine D2 receptor agonist quinpirole (0.03-1 mg/kg intra peritoneally) increased dose-dependently the serum corticosterone level in rats. The effect of the maximum dose of quinpirole (0.3 mg/kg intraperitoneally) was antagonized in a dose- dependent manner by the selective D2 receptor antagonist sulpiride (3-30 mg/kg intra peritoneally), but not by the selective D(1) receptor antagonist SCH-23390 (3 mg/kg intra peritoneally). Imipramine and amitriptyline (10 mg/kg per os), administered acutely or repeatedly (twice daily for 14 days), did not affect the corticosterone response to quinpirole (0.1 mg/kg intraperitoneally). The response was modified neither in animals treated repeatedly with electroconvulsive shock (ECS) (seven shocks every 2 days), nor after their combined repeated treatment with imipramine and ECS. The above results indicate that repeated treatment with antidepressant drugs and/or ECS does not affect the sensitivity of dopamine D2 receptors involved in the corticosterone response to quinpirole.