Serotonin, stress and corticoids

Central 5-HT receptor hypersensitivity in migraine without aura

Serotonin has long been implicated as a key neurotransmitter in migraine. There is a dearth of research specifically examining 5-HT1A receptor sensitivity in migraine despite the importance of this receptor in regulating central serotonergic tone. In this study we examined the hypothesis that migraine without aura is associated with hypersensitivity of central 5-HT1A receptors, using a 5-HT1A neuroendocrine challenge drug and comparing serum prolactin responses between a test group with migraine and a matched group of healthy controls. Twelve female subjects fulfilling International Headache Society (IHS) criteria for migraine without aura were evaluated. Following an overnight fast, subjects presented for testing at 9am. An intravenous canula was inserted and serum prolactin was assessed at baseline and every 30 min for 3 h following a single dose of 30 mg oral buspirone, a 5-HT1A-receptor agonist. Subjects were assessed during the first 5 days of the menstrual cycle. No subjects were taking psychotropic medication or migraine prophylactic treatment. Patients with current or previous psychiatric disorder, daily headache or analgesic overuse were excluded. 16 healthy female volunteers matched for age and menstrual status were also evaluated and served as controls. There was no difference in baseline prolactin between groups. There was a significant rise in prolactin following buspirone in both groups. Subjects with migraine had a significantly increased prolactin response to buspirone (delta max) compared to controls (P < 0.001). This study supports the hypothesis that migraine without aura is associated with a relative hypersensitivity of central 5-HT1A receptors. This is of relevance given the role of the 5-HT1A receptor in controlling raphe 5-HT tone and in the possible association between migraine and anxiety and depression.

Rat strain differences in peripheral and central serotonin transporter protein expression and function

Female Fischer 344 (F344) rats have been shown to display increased serotonin transporter (5-HTT) gene expression in the dorsal raphe, compared to female Lewis (LEW) rats. Herein, we explored, by means of synaptosomal preparations and in vivo microdialysis, whether central, but also peripheral, 5-HTT protein expression/function differ between strains. Midbrain and hippocampal [3H]paroxetine binding at the 5-HTT and hippocampal [3H]serotonin (5-HT) reuptake were increased in male and female F344 rats, compared to their LEW counterparts, these strain differences being observed both in rats of commercial origin and in homebred rats. Moreover, in homebred rats, it was found that these strain differences extended to blood platelet 5-HTT protein expression and function. Saturation studies of midbrain and hippocampal [3H]paroxetine binding at the 5-HTT, and hippocampal and blood platelet [3H]5-HT reuptake, also revealed significant strain differences in Bmax and Vmax values. Although F344 and LEW rats differ in the activity of the hypothalamo-pituitary-adrenal (HPA) axis, manipulations of that axis revealed that the strain differences in hippocampal [3H]paroxetine binding at 5-HTTs and [3H]5-HT reuptake were not accounted for by corticosteroids. Hippocampal extracellular 5-HT levels were reduced in F344 rats, compared to LEW rats, with the relative, but not the absolute, increase in extracellular 5-HT elicited by the local administration of citalopram being larger in F344 rats. Because the aforementioned strain differences did not lie in the coding sequences of the 5-HTT gene, our results open the promising hypothesis that F344 and LEW strains model functional polymorphisms in the promoter region of the human 5-HTT gene.

Fox odour affects corticosterone release but not hippocampal serotonin reuptake and open field behaviour in rats

Group-housed Sprague-Dawley (SD) rats exposed for 1 h to 2,5-dihydro-2,4,5-trimethylthiazoline (TMT, a component of fox feces) did not display changes in hippocampal serotonin (5-HT) metabolism and [3H]5-HT reuptake, compared to water or butyric acid. Such an observation extended to isolated SD and Fischer 344 rats. When group-housed SD rats were tested 1 week after a 1-h exposure to TMT, hippocampal 5-HT metabolism, [3H]5-HT reuptake, and [3H]paroxetine binding at the 5-HT transporter remained unchanged. This study questions TMT as a specific predatory stimulus as both butyric acid and TMT increased plasma corticosterone levels whilst leaving intact open field behaviour (at least in group-housed SD rats).

Forced swim stress activates rat hippocampal serotonergic neurotransmission involving a corticotropin-releasing hormone receptor-dependent mechanism

Serotonin is important for adequate coping with stress. Aberrant serotonin function is implicated in the aetiology of major depression and anxiety disorders. Dysregulation of the hypothalamic-pituitary-adrenocortical axis, involving elevated corticotropin-releasing hormone (CRH) activity, also plays a role in these stress-related illnesses. Here we studied the effects of stress on hippocampal serotonin and the role of the CRH system using in vivo microdialysis. First, rats were subjected to a forced swim stress, resulting in a dramatic increase in hippocampal serotonin (1500% of baseline), which was associated with the occurrence of diving behaviour. The diving-associated increase in serotonin depended on activation of CRH receptors, as it was antagonized by intracerebroventricular pretreatment with D-Phe-CRH12-41. Secondly, the effects of intracerebroventricular administration of CRH and urocortin (0.03-1.0 microg) were studied. Both CRH and urocortin caused a dose-dependent rise in hippocampal serotonin (maximally 350% of baseline) and 5-hydroxyindoleacetic acid levels, suggesting the involvement of CRH receptor type 1. Because the effects of urocortin were prolonged, CRH receptor type 2 could play a role in a later phase of the neurotransmitter response. Experiments using adrenalectomized rats showed that CRH-induced serotonin changes were adrenally independent. These data suggest that the raphe-hippocampal serotonin system is able to mount, CRH receptor-dependent, responses to specific stressful situations that surpass the usually observed maximal increases of about 300% of baseline during stress and enhanced vigilance.

Effects of neonatal handling on basal and stress-induced monoamine levels in the male and female rat brain

Neonatal handling has pervasive effects on the rat brain leading to increased ability to cope with and adapt to stressful stimuli. We determined the effects of neonatal handling on the dopaminergic and serotonergic system, in the male and female rat brain, under basal conditions before and after puberty and after short- and long-term forced swimming stress. Exposure of animals to neonatal handling resulted in sex-dependent changes in the concentration and turnover of monoamines in the different brain areas. In the prepubertal brain, the effect of neonatal handling was manifested as an increase in dopamine turnover in the females, particularly in the hypothalamus, an increase in serotonin levels and a decrease in its turnover in all three brain regions examined of both males and females. Certain of the handling-induced effects observed in the prepubertal brain were reversed in the postpubertal animals. Thus, in the postpubertal brain, the handling-induced changes in serotonin levels and its turnover observed in both sexes before puberty were abolished. On the other hand, the handling-induced increase in hypothalamic dopamine turnover was maintained. After exposure to short-term stress, the effect of handling was manifested on one hand as decreased striatal dopamine levels in the females, and decreased dopamine turnover in the hypothalamus of both males and females, and on the other, as increased serotonin levels in the hypothalamus. After exposure to long-term stress, handled females had decreased dopamine turnover in the hypothalamus and the striatum, but there was no effect of handling on the serotonergic system. Our results provide some neurobiological evidence supporting the determinant role of the mother-infant relationship in the development of psychopathology. Neonatal handling, which modifies normal mother-pup interactions, results in alterations in brain dopaminergic and serotonergic systems, both of which are involved in the etiopathogenesis of major psychoses. Exposure to either short- or long-term stress in adult life results in sex-dependent changes in brain monoamines, which are affected by handling thus making coping more efficient and rendering the stressful stimulus less noxious.

Functional subsets of serotonergic neurones: implications for control of the hypothalamic-pituitary-adrenal axis

Serotonergic systems play an important role in the regulation of behavioural, autonomic and endocrine responses to stressful stimuli. This includes modulation of both the hypothalamic-pituitary-adrenal (HPA) axis and the hypothalamic-spinal-adrenal (HSA) axis, which converge at the level of the adrenal cortex to regulate glucocorticoid secretion. Paradoxically, serotonin can either facilitate or inhibit HPA axis activity and stress-related physiological or behavioural responses. A detailed analysis of the brainstem raphé complex and its ascending projections reveals that facilitatory and inhibitory effects of serotonergic systems on glucocorticoid secretion may be due to influences of topographically organized and functionally diverse serotonergic systems. (i) A serotonergic system arising from the middle and caudal dorsal raphé nucleus and projecting to a distributed central autonomic control system and a lateral 'emotional motor system'. Evidence suggests that serotonin can sensitize this subcortical circuit associated with autonomic arousal, anxiety and conditioned fear. (ii) A serotonergic system arising from the median raphé nucleus and projecting extensively and selectively to a ventral subiculum projection system. Evidence suggests that serotonin facilitates this limbic circuit associated with inhibition of ultradian, circadian and stress-induced activity of both the HPA axis and the HSA axis. These new perspectives, based on functional anatomical considerations, provide a hypothetical framework for investigating the role of serotonergic systems in the modulation of ultradian, circadian and stress-induced neuroendocrine function.

Enhancement of stress-induced pituitary hormone release and cardiovascular activation by antidepressant treatment in healthy men

A randomized, double-blind, placebo-controlled trial was performed to verify the suggestion that (i) in accordance with the results of animal studies, treatment with antidepressants inhibits hormone release in response to stressful stimulation in humans and (ii) drugs with opposing effects on brain serotonine (citalopram and tianeptine) exert similar modulatory effects on neuroendocrine activation during stress. Healthy male volunteers were treated with citalopram (20 mg), tianeptine (37.5 mg) or placebo for 7 days. As a stress stimulus, insulin-induced hypoglycaemia was used. Measurement of hormone concentrations revealed an enhanced release of adrenocorticotropic hormone and growth hormone in response to stress of hypoglycaemia in subjects treated with both antidepressants used. A similar augmentation was observed in systolic blood pressure. Stress-induced prolactin release was potenciated by citalopram only. Plasma renin activity, epinephrine, norepinephrine and cortisol levels failed to be modified by antidepressants. The present study demonstrates that (i) repeated antidepressant treatment in healthy men does not inhibit, but enhances, neuroendocrine activation during stress and (ii) such effects were observed after treatment with antidepressants having opposing actions on brain serotonin, indicating involvement of nonserotoninergic mechanisms.

Corticosterone modulation of somatodendritic 5-HT1A receptor function in mice

Corticosteroid modulation of serotonergic function may play a central role in mood disorders. 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) produces a hypothermia in mice that serves as an in-vivo model of somatodendritic 5-HT1A autoreceptor function. Daily injections (s.c.) of 50 mg/kg of corticosterone (CORT) for 3 days attenuates 8-OH-DPAT hypothermia tested 24 h later. This study sought to further clarify the nature of the CORT-mediated attenuation of somatodendritic 5-HT1A receptor function. Mice underwent various CORT manipulations prior to an 8-OH-DPAT challenge. Neither 14-day bilateral adrenalectomy (ADX), nor CORT 50 mg/kg/day, administered continuously by osmotic minipump over 72 h had any effect on the 8-OH-DPAT hypothermic response. In contrast, daily injections of CORT over three consecutive days caused a significant attenuation in 8-OH-DPAT hypothermia when tested 24 h later. However, administration of an additional dose of CORT 2 h prior to the 8-OH-DPAT challenge occluded this CORT-mediated attenuation in a dose-dependent fashion. The findings demonstrate that CORT modulates somatodendritic 5-HT1A receptor function in a complex manner. Attenuation is seen only after intermittent administration of CORT. In addition, the degree of attenuation depends on CORT concentrations at the time of testing. These findings may have implications regarding mechanisms of adaptation to stress.

Effects of cortisol on aggression and locomotor activity in rainbow trout

Noninvasive administration of cortisol through the diet resulted in relatively rapid (<1.5 h) and highly reproducible increases in plasma cortisol in rainbow trout, comparable to changes seen in fish subjected to substantial stress. Juvenile rainbow trout were reared in isolation for 1 week, before their daily food ration was replaced by a meal of cortisol-treated food corresponding to 6 mg cortisol kg(-1). All fish were observed for 30 min, beginning at 1 or 48 h following the introduction of cortisol-treated food. Additional cortisol (75% of the original dose on Day 2, and 50% on Day 3) was administered to the long-term cortisol-treated group. The resulting blood plasma concentrations of cortisol were similar in short- and long-term treated fish, and corresponded to those previously seen in stressed rainbow trout. Controls were fed similar food without cortisol. Half of the fish from each treatment group (controls and short- and long-term cortisol) were subjected to an intruder test (a smaller conspecific introduced into the aquarium), while half of the fish were observed in isolation. In fish challenged by a conspecific intruder, short-term cortisol treatment stimulated locomotor activity, while long-term treatment inhibited locomotion. Aggressive behavior was also inhibited by long-term cortisol treatment, but not by short-term exposure to cortisol. Cortisol treatment had no effect on locomotor activity in undisturbed fish, indicating that the behavioral effects of cortisol were mediated through interaction with other signal systems activated during the simulated territorial intrusion test. This study demonstrates for the first time that cortisol has time- and context-dependent effects on behavior in teleost fish.

Alteration in 5-hydroxytryptamine agonist-induced behaviour following a corticosterone implant in adult rats

Hypercortisolism and altered serotonergic function may account for the pathological symptoms seen in depression. This study examines the impact of 4 days continuous corticosterone treatment on 5-HT agonist-induced behaviour to delineate changes in 5-HT receptor function in the adult rat. The flat body posture, reciprocal forepaw treading, elevated corticosterone, hyperglycaemia, hypothermia and reduced hippocampal 5-HT induced by the 5-HT(1A) agonist 8-OHDPAT (0.3 mg/kg ip) were all significantly attenuated by the corticosterone implant. The elevation in plasma corticosterone and back muscle contractions evoked by the 5-HT(2A) agonist DOI (1 mg/kg ip) were attenuated, whilst wet-dog shakes were enhanced by corticosterone treatment. 5-HT(2B) agonist-induced behaviour and the hypolocomotion and hypophagia induced by the 5-HT(2C) agonist m-CPP (2.5 mg/kg ip) were unaltered but the mCPP-induced elevation in corticosterone was abolished by corticosterone treatment. Hypothalamic 5-HT receptors mediating corticosterone- and 5-HT(1A) receptors, whether on serotonergic nerve terminals or postsynaptic neurones, were downregulated by corticosterone treatment. In contrast, 5-HT(2A) receptors may be up- or downregulated dependent on whether they are on supraspinal or spinal neurones, respectively. A comparison of the brain region-dependent alteration in serotonergic function produced by hypercorticosterone in the rat with that seen in depression is discussed.

Repeated experience of social defeats increases serotonin transporter and monoamine oxidase A mRNA levels in raphe nuclei of male mice

Serotonin transporter (SERT) and monoamine oxidase A (MAOA) mRNA levels in the raphe nuclei area of the midbrain were measured by the multiplex reverse transcription-polymerase chain reaction method in male mice with repeated experience of social victories (winners) and defeats (losers) in ten daily agonistic confrontations. Experiments revealed enhanced SERT and MAOA mRNA levels in the losers compared with the winners and controls. It has been supposed that SERT and MAOA genes are involved in enhancement of serotonin inactivation in response to the increase of serotonergic activity shown earlier in the losers. A positive correlation between MAOA and SERT mRNA levels in the raphe nuclei area of the midbrain was shown.

Sex differences in the effects of neonatal handling on the animal's response to stress and the vulnerability for depressive behaviour

Neonatal handling is known to affect the programming of the hypothalamic-pituitary-adrenal axis and, as a result, the ability of the organism to respond to stress. We determined the effect of neonatal handling on the animal's response in three animal models of depression, as well as to either (a) acute or (b) chronic forced swimming stress. Neonatal handling resulted in a significant increase in the immobility time in the Porsolt forced swimming test in both sexes, and in the 8-hydroxy-2-(di-n-propylamino) tetralin-induced hypothermia in the males. On the other hand, handling had sex-dependent effects when animals were exposed to a chronic stressor. After exposure to chronic restraint stress, statistically more handled than non-handled females failed to adapt, while no such difference was found in the males. In the chronic forced swimming stress, handled males had shorter immobility times, and higher plasma corticosterone levels, while the opposite held true in the females. Furthermore, neonatal handling significantly decreased basal plasma corticosterone levels in both pre- and post-pubertal animals. Thus, the early experience of handling provides males with a greater capacity to actively face chronic stressors, while in the females it increases their susceptibility to express 'depressive' behaviour since they are unable to cope and adopt a 'passive, despaired' behaviour.

Differences in behaviour between rainbow trout selected for high- and low-stress responsiveness

Two F1 lines of rainbow trout Oncorhynchus mykiss, divergent for plasma cortisol responsiveness, were generated by individual selection for post-stress cortisol values within the F0 generation. Adult females of the F1 generation were transferred to rearing in social isolation in observation tanks. After 6 days, locomotor activity in high-responding (HR) and low-responding (LR) individuals was quantified as time spent moving during a 20 min observation period. Behavioural observations were repeated the next day with a smaller conspecific intruder present in each observation tank. Differential hypothalamus–pituitary–interrenal axis activity in the two lines was subsequently confirmed by a standardised confinement stress test, which resulted in significantly higher plasma cortisol concentrations in HR than LR fish. HR fish displayed higher levels of locomotor activity than LR fish in the presence of an intruder, but not when in isolation. Aggressive behaviour towards the intruder was not seen, suggesting either a state-dependent lack of territorial aggression, or chronic stress in the experimental fish. A significantly higher incidence of feed intake was seen in LR trout when held in observation tanks (40 % versus 0 % of the fish took food when in isolation), suggesting that these fish acclimated more successfully to the experimental conditions than HR fish did. These results suggest that selection for stress responsiveness in salmonid fish leads to behavioural alterations, which are of potential importance to the performance of these fish in aquaculture rearing operations.

Long-term behavioral and neurochemical effects of chronic stress exposure in rats

Rats exposed to acute unavoidable stress develop a deficit in escaping avoidable aversive stimuli that lasts as long as unavoidable stress exposure is repeated. A 3-week exposure to unavoidable stress also reduces dopamine (DA) output in the nucleus accumbens shell (NAcS). This study showed that a 7-day exposure to unavoidable stress induced in rats an escape deficit and a decrease in extraneuronal DA basal concentration in the NAcS. Moreover, animals had reduced DA and serotonin (5-HT) accumulation after cocaine administration in the medial pre-frontal cortex (mPFC) and NAcS, compared with control animals. After a 3-week exposure to unavoidable stress, escape deficit and reduced DA output in the NAcS were still significant at day 14 after the last stress administration. In the mPFC we observed: (i) a short-term reduction in DA basal levels that was back to control values at day 14; (ii) a decrease in DA accumulation at day 3 followed by a significant increase beyond control values at day 14; (iii) a significant reduction in 5-HT extraneuronal basal levels at day 3, but not at day 14. Finally, a significant decrease in 5-HT accumulation following cocaine administration was present in the NAcS and mPFC at day 3, but not at day 14. In conclusion, a long-term stress exposure induced long-lasting behavioral sequelae associated with reproducible neurochemical modifications.

The molecular neurobiology of stress--evidence from genetic and epigenetic models

Knowledge of the genetic and molecular events underlying the neuroendocrine and behavioural sequelae of the response to stress has advanced rapidly over recent years. The response of an individual to a stressful experience is a polygenic trait, but also involves non-genetic sources of variance. Using a combination of top-down (quantitative trait locus [QTL] and microarray analysis) and bottom-up (gene targeting, transgenesis, antisense technology and random mutagenesis) strategies, we are beginning to dissect the molecular players in the mediation of the stress response. Given the wealth of the data obtained from mouse mutants, this review will primarily focus on the contributions made by transgenesis and knockout studies, but the relative contribution of QTL studies and microarray studies will also be briefly addressed. From these studies it is evident that several neuroendocrine and behavioural alterations induced by stress can be modelled in mouse mutants with alterations in hypothalamic-pituitary-adrenal axis activity or other, extrahypothalamic, neurotransmitter systems known to be involved in the stress response. The relative contribution of these models to understanding the stress response and their limitations will be discussed.

Maternal deprivation increases 5-HT(1A) receptor expression in the CA1 and CA3 areas of senescent Brown Norway rats

Maternally-deprived male Brown Norway rats were classified as non-impaired or impaired according to their performance in the water maze when 3 and 30-32 months old. Age and spatial learning ability did not affect the pattern and density of hippocampal 5-HT(1A)-receptor mRNA in mother-reared control rats. However, senescent maternally-deprived rats with impaired spatial learning ability showed increased expression of 5-HT(1A)-receptor mRNA in the hippocampal CA1 (14%) and CA3 (13%) areas but not in the dentate gyrus.

Beta-amyloid(1-42)-induced cholinergic lesions in rat nucleus basalis bidirectionally modulate serotonergic innervation of the basal forebrain and cerebral cortex

Ample experimental evidence suggests that beta-amyloid (A beta), when injected into the rat magnocellular nucleus basalis (MBN), impels excitotoxic injury of cholinergic projection neurons. Whereas learning and memory dysfunction is a hallmark of A beta-induced cholinergic deficits, anxiety, or hypoactivity under novel conditions cannot be attributed to the loss of cholinergic MBN neurons. As mood-related behavioral parameters are primarily influenced by the central serotonergic system, in the present study we investigated whether A beta(1-42) toxicity in the rat MBN leads to an altered serotonergic innervation pattern in the rat basal forebrain and cerebral cortex 7 days postsurgery. A beta infusion into the MBN elicited significant anxiety in the elevated plus maze. A beta toxicity on cholinergic MBN neurons, expressed as the loss of acetylcholinesterase-positive cortical projections, was accompanied by sprouting of serotonergic projection fibers in the MBN. In contrast, the loss of serotonin-positive fiber projections, decreased concentrations of both serotonin and 5-hydroxyindoleacetic acid, and decline of cortical 5-HT(1A) receptor binding sites indicated reduced serotonergic activity in the somatosensory cortex. In conclusion, the A beta-induced primary cholinergic deficit in the MBN and subsequent cortical cholinergic denervation bidirectionally modulate serotonergic parameters in the rat basal forebrain and cerebral cortex. We assume that enhanced serotonin immunoreactivity in the damaged MBN indicates intrinsic processes facilitating neuronal recovery and cellular repair mechanisms, while diminished cortical serotonergic activity correlates with the loss of the subcortical cholinergic input, thereby maintaining the balance of neurotransmitter concentrations in the cerebral cortex.

Stress-induced attenuation of the hypercapnic ventilatory response in awake rats

To test the hypothesis that stress alters the performance of the respiratory control system, we compared the acute (20 min) responses to moderate hypoxia and hypercapnia of rats previously subjected to immobilization stress (90 min/day) with responses of control animals. Ventilatory measurements were performed on awake rats using whole body plethysmography. Under baseline conditions, there were no differences in minute ventilation between stressed and unstressed groups. Rats previously exposed to immobilization stress had a 45% lower ventilatory response to hypercapnia (inspiratory CO(2) fraction = 0.05) than controls. In contrast, stress exposure had no statistically significant effect on the ventilatory response to hypoxia (inspiratory O(2) fraction = 0.12). Stress-induced attenuation of the hypercapnic response was associated with reduced tidal volume and inspiratory flow increases; the frequency and timing components of the response were not different between groups. We conclude that previous exposure to a stressful condition that does not constitute a direct challenge to respiratory homeostasis can elicit persistent (> or =24 h) functional plasticity in the ventilatory control system.

Stress and inflammatory disease: widening roles for serotonin and substance P

Serotonin has been implicated in mediating the hypothalamo-pituitary-adrenal (HPA) axis response to stress and is an important therapeutic target for a number of psychiatric disorders including depression. The neurokinin substance P has been shown to inhibit stress-induced HPA axis activity and we have demonstrated that endogenous substance P is able to reduce the duration of the HPA axis response to stress suggesting an important role in the termination of the stress response. This may be important in controlling the transition from acute to chronic stress and substance P has recently attracted attention as a potential antidepressant.In addition to these central effects, serotonin and substance P are considered to be pro-inflammatory agents. Despite being implicated in mediating inflammation there have been few studies investigating the effects of manipulations of serotonergic or substance P systems on chronic inflammatory disease. Treatment of rats with adjuvant-induced arthritis(AA), a model of chronic inflammatory stress, with a substance P antagonist specific for the NK1 receptor subtype resulted in a reduction in hind paw inflammation suggesting substance P may influence inflammation. We have noted that depletion of whole body serotonin and selective central depletion of serotonin results in a decrease in the severity of inflammation in rats with adjuvant arthritis. Furthermore, treatment with a selective serotonin reuptake inhibitor results in an earlier onset and increased severity of inflammation in adjuvant arthritis, confirming a pro-inflammatory role for serotonin. Serotonin is also present in the immune tissues and concentrations in the spleen fall following the development of inflammation in adjuvant arthritis. Concentrations of serotonin are significantly higher in normal female spleen than in males, and this may underlie the greater predisposition of females to certain autoimmune diseases.There is increasing evidence of a role for transmitters such as serotonin and substance P,both centrally and peripherally, in mediating a wide variety of inflammatory and psychiatric disorders. A better understanding of the mechanisms of action of these transmitters and the development of suitable drugs targeting specific receptor subtypes has great potential to impact on clinical practice in the near future. The purpose of this review is to consider the separate roles of serotonin and substance P in relation to HPA axis stress responses, in the context of a model of chronic inflammatory disease, highlighting novel directions of current research for each of these transmitters.

Functional effects of corticosterone on 5-HT(1A) and 5-HT(1B) receptor activity in rat brain: in vivo microdialysis studies

Glucocorticoid hormones are known to be elevated in depression, and to interact with serotonin 5-HT(1A) receptors at both the presynaptic and postsynaptic levels. Since one of the presumed mechanisms of action of antidepressant drugs is induction of changes in sensitivity of 5-HT(1A) and also 5-HT(1B) receptors, the effects of repeated administration of corticosterone (50 mg/kg s.c. b.i.d. for 10 days) on activities of these receptors were determined using in vivo microdialysis in freely moving rats. Presynaptic 5-HT(1A) receptor activity, as measured by the effect of a challenge dose (0.2 mg/kg s.c.) of the 5-HT(1A) agonist 8-hydroxy-2 (di-n-propylamino) tetralin (8-OH-DPAT) to reduce 5-HT levels in the hypothalamus, was not affected by corticosterone administration. Presynaptic 5-HT(1B) receptor activity, as measured by the effect of the 5-HT(1B) receptor antagonist (N-[4-methoxy-3-(4-methyl-1-piperizinyl)phenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiazole-3-yl)[1,1'-biphenyl]-carboxamide (GR 127935) (5 mg/kg s.c.) to increase 5-HT levels, was increased in hypothalamus but not hippocampus of corticosterone-treated rats. Postsynaptic 5-HT(1A) receptor activity, as measured by the effect of 8-OH-DPAT to increase cyclic AMP levels in the hippocampus, was not affected by corticosterone administration. The decrease in presynaptic 5-HT(1B) receptor activity after chronic administration of antidepressant drugs complements the increases in 5-HT(1B) receptor number observed in animal models of depression.