Psychological stress increases serotonin release in the rat amygdala and prefrontal cortex assessed by in vivo microdialysis

c-fos expression, behavioural, endocrine and autonomic responses to acute social stress in male rats after chronic restraint: modulation by serotonin

The effects in male rats of serotonin depletion (using the neurotoxin 5,7-dihydroxytryptamine) on the cross-sensitization of an acute social stress (defeat by a larger resident male) by previous repeated restraint stress (10 days, 60 min per day) was studied. Previous restraint increased freezing responses during social defeat in sham-operated rats, but this was not observed in those with depleted serotonin (83% or more in different regions of the brain). In contrast, neither heart rate (tachycardia) nor core temperature responses (hyperthermia) were accentuated in previously restrained rats (i.e. neither showed heterotypical sensitization), and neither adapted to repeated restraint (there is a hypothermic core temperature response during restraint). Corticosterone levels, which did adapt, nevertheless did not show accentuated responses to social defeat in previously restrained rats, though samples could only be taken 60 min after defeat. c-fos expression in the central nucleus of the amygdala 60 min after social defeat was increased by previous restraint. No other areas examined in the hypothalamus (e.g., paraventricular nucleus) or brainstem (e.g., solitary nucleus) showed differences related to previous restraint. Serotonin depletion reduced the expression of c-fos in the frontal cortex, lateral preoptic area, medial amygdala, central gray, medial and dorsal raphe, and locus coeruleus after social stress, but this was not altered by previous restraint. These results show that serotonin depletion has selective effects on the cross-sensitization of responses in previously stressed rats to a heterotypical stressor.

5-HT-moduline controls serotonergic activity: implication in neuroimmune reciprocal regulation mechanisms

The serotonergic neurotransmission is known as a neuromodulatory system exerting its activity in the central nervous system (CNS) as well as at the periphery. The anatomical and morphological organization of the system based on a marked centralization of the cellular bodies and the large, almost ubiquitary, presence of axonal projections of the neurons is in good agreement with this modulatory role. Furthermore, a very high number of varicosities located along the axonal branches are capable of releasing serotonin (5-HT). The amine stimulates a number of different specific receptor types which allows 5-HT to exert different activities on its various cellular targets. Among these receptors, the 5-HT1B subtypes play a particular role as they are autoreceptors located on 5-HT neurons terminals and heteroreceptors located on non-serotonergic terminals where they control the release of the neurotransmitter. 5-HT-moduline, an endogenous tetrapeptide, regulates the efficacy of these 5-HT1B receptors, hence, is able to control the serotonergic activity in a synchronous manner for the various varicosities from a single neuron and thus may favour the differential effect of that neuron on distinct cerebral functions. Accordingly, the peptide allows the 'fine tuning' of the cerebral activity by the serotonergic system to elaborate the response given by the brain to a particular stimulus, that is, stress situations. At the periphery, the serotonergic system also appears to possess a regulatory activity via 5-HT1B receptors. In particular, the receptors located on immunocompetent cells control their activity and are themselves regulated by 5-HT-moduline likely originating from adrenal medulla and released after acute stress. The serotonergic system appears to play a major role in the reciprocal signalling existing between the neuronal and the immune system. The participation of 5-HT-moduline is likely in physiological functions as well as in pathological disorders affecting central and peripheral activities.

The effect of lumbar puncture stress on dopamine and serotonin metabolites in human cerebrospinal fluid

In order to examine concentrations of cerebrospinal fluid (CSF) neurochemicals, the technique of lumbar puncture is typically used. However, the effect of the intrinsic stress of undergoing a lumbar puncture on CSF monoamine concentrations in humans has not yet been established. We used lumbar puncture followed 3 h later by continuous CSF sampling to examine the effect of lumbar puncture on levels of the dopamine and serotonin metabolites homovanillic acid (HVA) and 5-hydroxyindoleacetic acid (5-HIAA), respectively. Additionally, we examined the effect of lumbar puncture on the CSF HVA to 5-HIAA ratio. Immediately post lumbar puncture, CSF concentrations of HVA and 5-HIAA were, respectively, only 51 and 54% of the mean levels detected hours later. However, the HVA to 5-HIAA ratio remained stable during lumbar puncture. While HVA and 5-HIAA levels in CSF obtained via lumbar puncture reflect highly variable responses to the stress of the procedure, the ratio of these metabolites is unaffected.

Dopaminergic input is required for increases in serotonin output produced by behavioral activation: an in vivo microdialysis study in rat forebrain

Previous research has demonstrated that pharmacological stimulation of postsynaptic dopamine D2 receptors produces increases in serotonin output. The present study explored whether this relationship also holds under physiological conditions. Accordingly, we examined the effects of D2 receptor blockade or unilateral dopamine depletion on behaviorally induced increases in extracellular serotonin levels in the corpus striatum and prefrontal cortex of freely moving rats using in vivo microdialysis. Extracellular levels of dopamine and serotonin, as well as behavioral activity, were increased by both mild tail pinch and the light-dark transition. Tail pinch-induced increases in serotonin levels (39+/-3% and 53+/-5% in the corpus striatum and prefrontal cortex, respectively), but not the accompanying behavioral changes, were blocked by local application of the D2 receptor antagonist raclopride (10 microM). D2 receptor blockade also disrupted the positive relationship between striatal serotonin levels and behavioral activity of animals across the light-dark transition (r=0.93 without raclopride, r=0.24 in presence of raclopride). Unilateral 6-hydroxydopamine lesion of the nigrostriatal dopaminergic system also abolished increases in striatal serotonin output induced by both tail pinch and light-dark transition. A negative correlation was observed between the degree of striatal dopamine depletion and tail pinch-induced increases in serotonin efflux (r= - 0.88). Thus, both a local blockade of postsynaptic D2 receptors and striatal dopamine depletion prevented increases in serotonin output that normally accompany behavioral activation. These data indicate that the increases in the forebrain serotonin output produced by two distinct physiological/environmental manipulations appear to be largely dependent upon intact local dopaminergic neurotransmission.

Reboxetine attenuates forced swim test-induced behavioural and neurochemical alterations in the rat

The forced swim test is a behavioural paradigm that is predicative of antidepressant activity in rodents. Until recently, research has focused on the ability of antidepressant drugs to decrease immobility in the forced swim test paradigm, but the neurochemical sequelae induced by swim stress, or the neurochemical basis of antidepressant-induced behavioural changes have received little attention. In this regard, we have recently demonstrated that forced swim test exposure increases serotonergic activity in the amygdala, frontal cortex and hippocampus and dopamine turnover in the striatum. In addition, forced swim test-exposure activates the hypothalamic pituitary adrenal axis. The purpose of the present study was to examine the effect of treatment with the selective noradrenaline reuptake inhibitor reboxetine (3, 10 and 30 mg/kg; i.p.) on immobility and defaecation scores in the forced swim test, and on forced swim test-induced neurochemical and hypothalamic pituitary adrenal axis changes in the rat. Reboxetine treatment (10 and 30 mg/kg) significantly decreased immobility and defaecation in the forced swim test in dose dependent manner. Furthermore, reboxetine produced a dose dependent attenuation of forced swim test-induced increases in serotonin turnover in the amygdala and frontal cortex and dopamine turnover in the striatum. Reboxetine (30 mg/kg) produced a modest, but non-significant, attenuation of forced swim test-induced increases in serum corticosterone concentrations. These data demonstrate that, in addition to the behavioural activity of reboxetine in the rat forced swim test paradigm, a dose-dependent attenuation of swim stress-induced increases in serotonergic and dopaminergic activity occurred in a region specific manner. These are the first data to demonstrate that treatment with the selective noradrenaline reuptake inhibitor, reboxetine can impact on the activity of other neurotransmitter systems in response to stress. Moreover, these data further demonstrate that this paradigm is a valuable tool in studying the effect of antidepressants, on both behaviour and swim stress-related alterations in central neurotransmitter function and hypothalamic pituitary adrenal axis activity in the rat.

Presynaptic serotonergic inhibition of GABAergic synaptic transmission in mechanically dissociated rat basolateral amygdala neurons

1. The basolateral amygdala (ABL) nuclei contribute to the process of anxiety. GABAergic transmission is critical in these nuclei and serotonergic inputs from dorsal raphe nuclei also significantly regulate GABA release. In mechanically dissociated rat ABL neurons, spontaneous miniature inhibitory postsynaptic currents (mIPSCs) arising from attached GABAergic presynaptic nerve terminals were recorded with the nystatin-perforated patch method and pharmacological isolation. 2. 5-HT reversibly reduced the GABAergic mIPSC frequency without affecting the mean amplitude. The serotonergic effect was mimicked by the 5-HT1A specific agonist 8-OH DPAT (8-hydroxy-2-(di-n-propylamino)tetralin) and blocked by the 5-HT1A antagonist spiperone. 3. The GTP-binding protein inhibitor N-ethylmaleimide removed the serotonergic inhibition of mIPSC frequency. In either K+-free or Ca2+-free external solution, 5-HT could inhibit mIPSC frequency. 4. High K+ stimulation increased mIPSC frequency and 8-OH DPAT inhibited this increase even in the presence of Cd2+. 5. Forskolin, an activator of adenylyl cyclase (AC), significantly increased synaptic GABA release frequency. Pretreatment with forskolin prevented the serotonergic inhibition of mIPSC frequency in both the standard and high K+ external solution. 6. Ruthenium Red (RR), an agent facilitating the secretory process in a Ca2+-independent manner, increased synaptic GABA release. 5-HT also suppressed RR-facilitated mIPSC frequency. 7. We conclude that 5-HT inhibits GABAergic mIPSCs by inactivating the AC-cAMP signal transduction pathway via a G-protein-coupled 5-HT1A receptor and this intracellular pathway directly acts on the GABA-releasing process independent of K+ and Ca2+ channels in the presynaptic nerve terminals.

Serotonergic modulation of neurotransmission in the rat basolateral amygdala

Whole cell patch-clamp recordings were obtained from projection neurons and interneurons of the rat basolateral amygdala (BLA) to understand local network interactions in morphologically identified neurons and their modulation by serotonin. Projection neurons and interneurons were characterized morphologically and electrophysiologically according to their intrinsic membrane properties and synaptic characteristics. Synaptic activity in projection neurons was dominated by spontaneous inhibitory postsynaptic currents (IPSCs) that were multiphasic, reached 181 +/- 38 pA in amplitude, lasted 296 +/- 27 mS, and were blocked by the GABAA receptor antagonist, bicuculline methiodide (30 microM). In interneurons, spontaneous synaptic activity was characterized by a burst-firing discharge patterns (200 +/- 40 Hz) that correlated with the occurrence of 6-cyano-7-nitroquinoxaline-2,3-dione-sensitive, high-amplitude (260 +/- 42 pA), long-duration (139 +/- 19 mS) inward excitatory postsynaptic currents (EPSCs). The interevent interval of 831 +/- 344 mS for compound inhibitory postsynaptic potentials (IPSPs), and 916 +/- 270 mS for EPSC bursts, suggested that spontaneous IPSP/Cs in projection neurons are driven by burst of action potentials in interneurons. Hence, BLA interneurons may regulate the excitability of projection neurons and thus determine the degree of synchrony within ensembles of BLA neurons. In interneurons 5-hydroxytryptamine oxalate (5-HT) evoked a direct, dose-dependent, membrane depolarization mediated by a 45 +/- 6.9 pA inward current, which had a reversal potential of -90 mV. The effect of 5-HT was mimicked by the 5-HT2 receptor agonist, alpha-methyl-5-hydroxytryptamine (alpha-methyl-5-HT), but not by the 5-HT1A receptor agonist, (+/-) 8-hydroxydipropylaminotetralin hydrobromide (8-OH-DPAT), or the 5-HT1B agonist, CGS 12066A. In projection neurons, 5-HT evoked an indirect membrane hyperpolarization ( approximately 2 mV) that was associated with a 75 +/- 42 pA outward current and had a reversal potential of -70 mV. The response was independent of 5-HT concentration, blocked by TTX, mimicked by alpha-methyl-5-HT but not by 8-OH-DPAT. In interneurons, 5-HT reduced the amplitude of the evoked EPSC and in the presence of TTX (0.6 microM) reduced the frequency of miniature EPSCs but not their quantal content. In projection neurons, 5-HT also caused a dose-dependent reduction in the amplitude of stimulus evoked EPSCs and IPSCs. These results suggest that acute serotonin release would directly activate GABAergic interneurons of the BLA, via an activation of 5-HT2 receptors, and increase the frequency of inhibitory synaptic events in projection neurons. Chronic serotonin release, or high levels of serotonin, would reduce the excitatory drive onto interneurons and may act as a feedback mechanism to prevent excess inhibition within the nucleus.

Tryptophan hydroxylase mRNA levels are elevated by repeated immobilization stress in rat raphe nuclei but not in pineal gland

Repeated stress triggers a wide range of adaptive changes in the central nervous system including the elevation of serotonin (5-HT) metabolism and an increased susceptibility to affective disorders. To begin to examine whether these changes are mediated by alterations in gene expression for tryptophan hydroxylase (TPH), the rate-limiting enzyme in 5-HT biosynthesis, we quantitated its mRNA levels by competitive reverse transcription-polymerase chain reaction (RT-PCR). Repeated immobilization stress (2 h, 7 days) elicited a six- or ten-fold rise in TPH mRNA in median raphe nucleus (MRN) and dorsal raphe nucleus (DRN), respectively, without significantly altering TPH mRNA levels in the pineal gland. In contrast, there was little change in mRNA levels for GTP cyclohydrolase I (GTPCH), the rate limiting enzyme in synthesis of the tetrahydrobiopterin (BH4), the obligate cofactor for TPH. This is the first study to reveal stress-elicited activation of TPH gene expression.

Neurochemical characteristics and behavioral responses to psychological stress in ovariectomized rats

The present study was designed to clarify the time-dependent changes in brain monoamine turnover in the frontal cortex, hypothalamus, hippocampus, septum and amygdala after ovariectomy, and the difference in behavioral responses to psychological stress between sham-operated and ovariectomized (OVX) rats. At 2 and 4 weeks after ovariectomy, the turnover rates of dopamine and norepinephrine in all of the brain regions examined did not differ significantly between the sham-operated and OVX rats. However, 5-hydroxytryptamine (5-HT) turnover in all of the brain regions at 2 weeks after OVX was significantly lower than that in sham-operated rats. This difference was greater in the hypothalamus than in other brain regions. At 4 weeks after ovariectomy, 5-HT turnover in all of the brain regions examined was not significantly different between sham-operated and OVX rats. At 2 and 4 weeks after ovariectomy, exploratory behaviour (e.g., locomotor activity, head- dipping, crossing and rearing behaviours) in a non-stressed ovariectomy group did not differ from that in a non-stressed sham-operation group. Locomotor activity and the number of head-dips and crossings significantly (P<0.05) increased after repeated exposure to psychological stress for 5 days in sham-operated rats, but not in those at 2 weeks after OVX. At 4 weeks after ovariectomy, locomotor activity and the number of crossings and rearings in sham-operated and OVX rats were not significantly different in the psychological stress and non-stress groups. However, the number of head-dips significantly (P<0.05) increased with psychological stress in the sham-operated rats, but not in OVX rats. These results suggest that female gonadal hormones may play an important role in the regulation of brain 5-HTergic systems. These interactions between gonadal hormones and 5-HT metabolism may be related to 5-HT-related neuropsychiatric disorders.

Differential effect of immobilization stress on in vivo synthesis rate of monoamines in medial prefrontal cortex and nucleus accumbens of conscious rats

We have used microdialysis to measure the in vivo hydroxylation level of tyrosine and tryptophan in the medial prefrontal cortex and nucleus accumbens of conscious rats that were subjected to immobilization. The brain was perfused with an inhibitor of aromatic L-amino acid decarboxylase, 3-hydroxybenzylhydrazine, and the amount of 3,4-dihydroxyphenylalanine (DOPA) and 5-hydroxytryptophan (5-HTP) accumulating in the dialysate was measured as an index of the in vivo hydroxylation rate of tyrosine and tryptophan. One hour of immobilization caused a significant increase in extracellular DOPAin the medial prefrontal cortex but not nucleus accumbens. The same manipulation produced a significant and more prolonged elevation in extracellular 5-HTP in the nucleus accumbens as well as medial prefrontal cortex. The observed profile of stress-induced 5-HTP response was comparable in two brain regions. The results suggest that in vivo catecholamine synthesis is heterogenous, whereas in vivo serotonin synthesis is homogenous, with respect to responsiveness to stress in the medial prefrontal cortex and nucleus accumbens.

Acute stress induces a differential increase of 5-HT-moduline (LSAL) tissue content in various rat brain areas

5-HT-moduline is an endogenous cerebral tetrapeptide (LSAL) which specifically interacts as an allosteric modulator with 5-HT1B receptors controlling serotonergic activity [O. Massot, J.C. Rousselle, M.P. Fillion, B. Grimaldi, I. Cloez-Tayarani, A. Fugelli, N. Prudhomme, L. Seguin, B. Rousseau, M. Plantefol, R. Hen, G. Fillion, 5-Hydroxytryptamine-moduline, a new endogenous cerebral peptide, controls the serotonergic activity via its specific interaction with 5-hydroxytryptamine1B/1D receptors, Mol. Pharmacol. 50 (1996) 752-762; J.C. Rousselle, O. Massot, M. Delepierre, E. Zifa, G. Fillion, Isolation and characterization of an endogenous peptide from rat brain interacting specifically with the serotonergic1B receptor subtypes, J. Biol. Chem. 271 (1996) 726-735; J.C. Rousselle, M. Plantefol, M.P. Fillion, O. Massot, P.J. Pauwels, G. Fillion, Specific interaction of 5-HT-moduline with human 5-HT1b as well as 5-HT1d receptors expressed in transfected cultured cells, Naunyn-Schmiedeberg's Arch. Pharmacol. 358 (1998) 279-286]. Cerebral tissue contents of 5-HT-moduline were determined in various rat brain areas after an acute restraint stress, and after repetition of this stress, to examine whether or not mechanisms involving this peptide could be affected by stress situations. The measurement of the peptide was carried out using specific polyclonal antibodies [B. Grimaldi, M.P. Fillion, A. Bonnin, J.C. Rousselle, O. Massot, G. Fillion, Immunocytochemical localization of neurons expressing 5-HT-moduline in the mouse brain, Neuropharmacology 36 (1997) 1079-1087] in a dot-ELISA (enzyme-linked-immunosorbent assay) assay in cortex, hippocampus, hypothalamus, substantia nigra, striatum and in adrenal glands. Tissue contents of 5-HT-moduline progressively and transiently increased in most studied brain regions and reached a maximal value 20 min after the beginning of the restraint stress. The increase in 5-HT-moduline tissue contents represented 323% of the value observed in unstressed control animals in the cortex, 207% in the hippocampus, 149% in the hypothalamus and 156% in the substantia nigra. Thereafter, the peptide content of the latter tissues diminished during the last 20 min of restraint and returned to control values within 1 h after the end of the stress period. The striatum did not show any significant variation of 5-HT-moduline content during restraint stress. In adrenal glands, the 5-HT-moduline content rapidly decreased (60% of controls) after the beginning of the restraint stress, the effect of this stress being progressively less pronounced, still representing 80% of controls after 40 min. Repetition of the restraint stress daily for 3 weeks totally abolished the effect of the stress on variations of 5-HT-moduline tissue content in all the studied brain regions. These results show that an acute restraint stress induces a rapid and significant increase in the amount of 5-HT-moduline contained in various brain areas. This phenomenon is likely to be related to the stress-induced 5-HT1B receptor desensitization which was previously demonstrated.

Low platelet-poor plasma concentrations of serotonin in patients with combat-related posttraumatic stress disorder

BACKGROUND

Combat-related posttraumatic stress disorder (CR-PTSD) is associated with a dysregulation of various neurotransmitter systems.

METHODS

We assessed levels of platelet-poor plasma (PPP) norepinephrine (NE), and serotonin (5-HT), and 24-hour urinary excretion of NE, dopamine (DA), and homovanillic acid (HVA) in 17 male outpatients with untreated chronic CR-PTSD (age, 33.1 +/- 7.4 years) and 10 normal control subjects (age, 35.8 +/- 2.7 years).

RESULTS

Compared with the control subjects, the PTSD patients showed significantly lower PPP 5-HT levels, elevated PPP NE levels, and significantly higher mean 24-hour urinary excretion of all three catecholamines (NE, DA, and HVA). The 24-hour urinary HVA values of the CR-PTSD patients correlated significantly and positively with the total Impact of Event Scale scores and the avoidance symptoms cluster scores, and the PPP 5-HT levels correlated negatively with the Hamilton Anxiety Rating Scale scores. The PPP NE/5-HT ratio was significantly higher in the study group than in the control subjects.

CONCLUSIONS

We believe this combined enhanced noradrenergic activity and diminished 5-HT activity may be relevant to the neurobiology of CR-PTSD.

Dopaminergic innervation of the amygdala is highly responsive to stress

The amygdala has been implicated in the neuronal sequelae of stress, although little is known about the neurochemical mechanisms underlying amygdala transmission. In vivo microdialysis was employed to measure extracellular levels of dopamine in the basolateral nucleus of the amygdala in awake rats. Once it was established that impulse-dependent release of dopamine could be measured reliably in the amygdala, the effect of stress, induced by mild handling, on amygdala dopamine release was compared with that in three other dopamine-innervated regions, the medial prefrontal cortex, nucleus accumbens, and caudate nucleus. The magnitude of increase in dopamine in response to the handling stimulus was significantly greater in the amygdala than in the nucleus accumbens and prefrontal cortex. This increase was maximal during the application of stress and diminished after the cessation of stress. In contrast, the increases in extracellular dopamine levels in other regions, in particular the nucleus accumbens, were prolonged, reaching maximal values after the cessation of stress. These results suggest that dopaminergic innervation of the amygdala may be more responsive to stress than that of other dopamine-innervated regions of the limbic system, including the prefrontal cortex, and implicate amygdalar dopamine in normal and pathophysiological processes subserving an organism's response to stress.

Opioid-dependent effects of inescapable shock on escape behavior and conditioned fear responding are mediated by the dorsal raphe nucleus

Manipulations of the dorsal raphe nucleus (DRN) modulate the behavioral effects of exposure to inescapable shock (IS). Opiate agonists and antagonists also influence the impact of IS, but the role of the DRN in mediating these effects is unknown. The opiate antagonist naltrexone micro-injected into the region of the DRN immediately prior to IS prevented both the escape deficit and the enhancement of fear conditioning that occur 24 h later. Intra-DRN naltrexone administered at the time of later behavioral testing reduced, but did not eliminate, these effects of prior IS. Conversely, the opiate agonist morphine, in combination with a subthreshold number of 20 IS trials, induced an escape deficit and enhanced conditioned fear 24 h later. Microinjections of naltrexone into the dorsolateral periaqueductal gray area did not alter the effects of IS and electrolytic lesions of the DRN prevented the effect of the morphine-20 IS trial combination. The role of opioids in mediating the behavioral effects of IS is discussed.

Serotonin modulation of sensory inputs to the lateral amygdala: dependency on corticosterone

The lateral nucleus of the amygdala (LA) receives excitatory (glutamatergic) inputs from thalamic and cortical sensory processing areas and is believed to be involved in evaluation of the affective significance of sensory events. We examined whether serotonin (5-HT) affects excitatory transmission in auditory afferents to the LA and, if so, whether this modulation of sensory transmission is regulated by the stress hormone corticosterone (CORT). Neuronal activity in the LA was elicited via iontophoretic ejection of L-glutamate or synaptically via electrical stimulation of auditory afferent pathways. In the intact rat, iontophoretically applied 5-HT inhibited both synaptically and glutamate-evoked action potentials in most neurons examined. However, after adrenalectomy (ADX), which eliminates endogenous CORT, 5-HT no longer inhibited evoked activity in the LA. High-CORT doses given to ADX animals reinstated the inhibition of excitatory transmission of 5-HT, whereas low-CORT doses had little effect. Immunocytochemical labeling of the glucocorticoid receptor in the intact rat demonstrated nuclear staining throughout several amygdala regions, including the LA. However, after ADX, no nuclear labeling was visible. With a high replacement dose of CORT (5 or 10 mg) after ADX, dense nuclear staining returned, but with a low replacement dose (1 mg/kg), there was only light nuclear staining. Thus, the ability of 5-HT to modulate glutamatergic activity in auditory pathways to the amygdala is dependent on the presence of CORT and possibly glucocorticoid activation. Via this mechanism, 5-HT modulates the processing of sensory information within the LA and thus may regulate amygdala-related functions.

Escapable and inescapable stress differentially alter extracellular levels of 5-HT in the basolateral amygdala of the rat

The effects of escapable and yoked inescapable electric tailshocks on extracellular levels of serotonin (5-HT) in the basolateral amygdala were measured by in vivo microdialysis. Inescapable, but not escapable, shock increased extracellular 5-HT in the amygdala relative to restrained controls. Basal levels of 5-HT were elevated 24 h after inescapable shock, and previously inescapably shocked subjects exhibited an exaggerated 5-HT response to two brief footshocks. Levels of extracellular 5-HIAA did not follow any particular pattern and were not correlated with the changes in 5-HT.

Escapable and inescapable stress differentially and selectively alter extracellular levels of 5-HT in the ventral hippocampus and dorsal periaqueductal gray of the rat

The effects of escapable and yoked inescapable electric tailshocks on extracellular levels of serotonin (5-HT) in the ventral hippocampus and dorsal periaqueductal gray (dPAG) were measured by in vivo microdialysis. Inescapable, but not escapable shock increased extracellular 5-HT in the ventral hippocampus relative to restrained controls. Basal levels of 5-HT were elevated 24 h after inescapable shock, and previously inescapably shocked subjects exhibited an exaggerated 5-HT response to 2 brief footshocks. In contrast, escapable, but not inescapable shock, increased extracellular 5-HT in the dPAG, increased basal 5-HT in the dPAG 24 h later, and led to an enhanced 5-HT response to subsequent brief footshock.

Conditioned release of 5-hydroxytryptamine in vivo in the nucleus accumbens following isolation-rearing in the rat

This study examined the effect of isolation-rearing in the Lister hooded rat on extracellular 5-hydroxytryptamine in the medial nucleus accumbens following footshock and in relation to a conditioned emotional response. Inescapable mild footshock was associated with an immediate and prolonged increase in extracellular 5-hydroxytryptamine in the medial nucleus accumbens of isolation-reared rats. In group-reared rats (footshock-treated) and control groups (no footshock) there was no significant change in extracellular 5-hydroxytryptamine levels. When exposed to the contextual stimulus 140 min later (testing box without shock) there was an immediate and long-lasting increase in extracellular 5-hydroxytryptamine in the nucleus accumbens of the isolation-reared rats, however, the contextual stimulus did not significantly affect extracellular 5-hydroxytryptamine in the medial nucleus accumbens of group-reared rats. The results show that exposure to footshock and conditioning to context are not normally associated with a change in extracellular 5-hydroxytryptamine in the medial nucleus accumbens, however, in rats exposed to social isolation from weaning, both stimuli increase extracellular 5-hydroxytryptamine. The isolation-induced increase in presynaptic serotonergic function in the medial nucleus accumbens contrasts with previous reports of reduced 5-hydroxytryptamine release in the hippocampus and therefore suggests that isolation-rearing differentially affects the function of serotonergic neurons in the brain. The changes in 5-hydroxytryptamine function in the medial nucleus accumbens may represent physiological adaptations to stress or may occur secondary to changes in the function of another neurotransmitter, possibly dopamine.

Exposure to inescapable but not escapable shock increases extracellular levels of 5-HT in the dorsal raphe nucleus of the rat

The effects of escapable and yoked inescapable electric tailshocks on extracellular levels of serotonin (5-HT) in the dorsal raphe nucleus were measured by in vivo microdialysis. In comparison to either control rats or to their own preshock baseline, rats exposed to inescapable shock showed an increase in extracellular 5-HT within 25 min of shock initiation, and 5-HT levels continued to rise during the remainder of the shock session. Rats that were exposed to comparable shock treatment, but that were given the opportunity to escape, did not show an increase in 5-HT. Rats that were restrained but not shocked also did not show an increase in 5-HT. These results add further support to suggestions that serotonergic changes occur in the dorsal raphe nucleus during inescapable shock and that such changes may contribute to the behavioral effects of inescapable shock.

Kainate microinjection into the dorsal raphe nucleus induces 5-HT release in the amygdala and periaqueductal gray

Earlier results obtained in one of our laboratories showed that microinjection into the dorsal raphe nucleus (DRN) of the excitatory amino acid kainic acid, the benzodiazepine (BZD) inverse agonist FG 7142, and the 5-HT1A receptor agonist 8-OHDPAT changed the behavior of rats in the elevated T-maze, an animal model of anxiety. The present study investigates biochemical correlates of these results in awake rats by measuring 5-HT release with in vivo microdialysis in two brain structures innervated by the DRN-the amygdala (Am) and the dorsal periaqueductal gray matter (DPAG)-that have been implicated in anxiety. Microinjection of kainic acid (60 pmol) into the DRN significantly increased 5-HT release in both the Am and the DPAG. In the DPAG, the increase was 14-fold higher with respect to the baseline and occurred only at the first sample, which was collected 30 min after the injection. In the Am, the increase was less pronounced (nearly fourfold) but persistent, lasting until the fourth sample, which was collected 120 min from the injection. FG 7142 (40 pmol) and 8-OH-DPAT (8 nmol) were ineffective. Because only intra-DRN kainate both increased inhibitory avoidance and decreased one-way escape in the elevated T-maze, the present behavioral results support the suggestion that 5-HT facilitates conditioned fear in the Am and inhibits unconditioned fear in the DPAG.

The effects of different stressors on extracellular 5-hydroxytryptamine and 5-hydroxyindoleacetic acid

The effects of application of five different stressors on extracellular 5-hydroxytryptamine and 5-hydroxyindoleacetic acid in the striatum and hippocampus were compared using in vivo microdialysis. Forced swimming for 30 min elevated extracellular 5-hydroxytryptamine to 90% above basal levels and reduced 5-hydroxyindoleacetic acid to 45% of basal levels in the striatum during the swim session. In contrast, hippocampal 5-hydroxytryptamine was not altered significantly by forced swimming but 5-hydroxyindoleacetic acid levels were reduced to 60% of basal levels. Tail pinch for 5 min elevated 5-hydroxytryptamine to 55% above basal levels in striatum and to 35% above basal levels in hippocampus. Tail pinch had no effect on 5-hydroxyindoleacetic acid in either brain region. In contrast to forced swimming and the tail pinch, the other three stressors, immobilization stress for 100 min, exposure to a cold environment (4 degrees C) for 2 h, and forced motor activity on a rotarod for 30 min, failed to alter extracellular 5-hydroxytryptamine in either the striatum or the hippocampus. All five stressors increased plasma corticosterone levels: tail pinch, 246%; cold stress, 432%; immobilization, 870%; forced motor activity, 1030%; and forced swimming, 1530%. These results suggest that individual stressors produce different effects on extracellular 5-hydroxytryptamine in different brain regions. In addition, there does not appear to be a relationship between the effects of stressors on the 5-hydroxytryptamine system and the magnitude of their ability to activate the hypothalamic-pituitary-adrenal axis.

Comparative study in the rat of the actions of different types of stress on the release of 5-HT in raphe nuclei and forebrain areas

The effects of several stress procedures on the release of 5-HT in the dorsal and median raphe nuclei (DRN and MRN, respectively) and in forebrain structures of the rat brain innervated by both nuclei have been studied using intracerebral microdialysis. Handling for 30 sec, a saline injection and forced swimming for 5 min elevated significantly the 5-HT output in the MRN. The 5-HT output in the DRN was also enhanced by a saline injection. With regard to the forebrain structure examined, handling and forced swimming increased dialysate 5-HT in the amygdala. The injection of saline induced a slight, but significant, elevation of 5-HT in the medial prefrontal cortex. In contrast, the outflow of 5-HT was significantly reduced in the ventral hippocampus and medial prefrontal cortex following forced swimming and this effect persisted well beyond the cessation of the swim session. These results indicate that the efflux of 5-HT in the MRN appears to respond to different forms of stress, whereas that in the DRN only increases after the injection of saline. The release of 5-HT in the forebrain structures is also dependent on the type of stress procedure and the region studied.

A critical review of 5-HT brain microdialysis and behavior

Serotonin (5-HT) has been implicated in many central nervous system-mediated functions including sleep, arousal, feeding, motor activity and the stress response. In order to help establish the precise role of 5-HT in physiology and behavior, in vivo microdialysis studies have sought to identify the conditions under which the release of 5-HT is altered. Extracellular 5-HT levels have been monitored in more than fifteen regions of the brain during a variety of spontaneous behaviors, and in response to several physiological, environmental, and behavioral manipulations. The vast majority of these studies found increases (30-100%) in 5-HT release in almost all brain regions studied. Since electrophysiological studies have shown that behavioral arousal is the primary determinant of brain serotonergic neuronal activity, we suggest that the increase in 5-HT release seen during a wide variety of experimental conditions is largely due to one factor, namely an increase in behavioral arousal/motor activity associated with the manipulation.

Androgen inhibits neurotransmitter turnover in the medial prefrontal cortex of the rat following exposure to a novel environment

Previous studies have demonstrated that gonadal steroid hormones affect the neuroendocrine response to a novel environment and other stressors. Introduction to a novel environment also increases neurotransmitter turnover in the medial prefrontal cortex (MPFC). In this study, we examined the possibility that gonadal steroid hormones could similarly modulate the neurotransmitter response to a novel environment in the MPFC of the male rat. Male Fischer 344 rats at 3 months of age were gonadectomized (GDX'd) and implanted with Silastic capsules containing dihydrotestosterone propionate (DHTP, a non-aromatizable form of androgen), 17 beta-estradiol (E), or placebo. Control animals were left intact. Each of these groups was further divided into a group introduced to a novel environment or a home cage control group. Animals exposed to a novel environment were killed after spending 20 min in a novel open field, whereas control animals were killed immediately upon removal from their home cage. Using high performance liquid chromatography, the MPFC was assayed for tissue levels of dopamine (DA) and its metabolites, 3,4-dihydroxyphenylalanine (DOPAC) and homovanillic acid (HVA); norepinephrine (NE) and its metabolite 3-methoxy-4-hydroxyphenylglycol (MHPG); or serotonin (5-HT) and its metabolite 5-hydroxyindole acetic acid (5-HIAA). The introduction to a novel environment caused significant increases in turnover of all three neurochemicals examined as estimated by metabolite/precursor ratios. These increases were characterized by increases in DOPAC, HVA, MHPG and 5-HIAA coupled with decreases in DA, NE and 5-HT. There was no effect of GDX on neurotransmitter turnover, however, treatment of GDX animals with DHTP prevented the open field induced increase in DOPAC/DA, MHPG/NE, and 5-HIAA/5-HT ratio. Treatment of GDX animals with estrogen had the opposite effect of DHTP, DOPAC/DA and MHPG/NE ratios increased to a greater level following the introduction to a novel environment than in GDX or intact animals. Examination of behavior in the open field showed significant decreases in activity in the DHTP-treated group but not in any other behavioral parameter (rears, nose pokes). Since the non-aromatizable androgen, DHTP, is presumably acting via androgen receptors, and E is presumably acting via estrogen receptors, these data suggest that, in the MPFC of male rats, androgen and estrogen receptors act in an opposing fashion to modify neurotransmitter turnover. This suggests that local changes in the relative levels of androgen and estrogen can have profound effects on the neurobiological response of the medial prefrontal cortex to stimuli.

Release of serotonin in the rat locus coeruleus: effects of cardiovascular, stressful and noxious stimuli

To investigate the function of serotonergic neurons within the locus coeruleus, this brain nucleus of conscious, freely moving rats was superfused with artificial cerebrospinal fluid through a push-pull cannula and the extracellular concentration of serotonin was determined in the superfusate. Serotonin release was increased by depolarization with veratridine (5 microM) or 80 mM K+, while superfusion with tetrodotoxin (1 microM) or systemic administration of 8-hydroxy-2-(di-n-propylamino)tetralin substantially diminished the release rate of serotonin in the locus coeruleus. The pressor response to intravenous infusion of noradrenaline (4 micrograms/kg/min) was associated with a pronounced increase in the release rate of serotonin. Superfusion of the locus coeruleus with tetrodotoxin (1 microM) abolished the increase in serotonin release evoked by the pressor response. A fall of blood pressure produced by intravenous administration of nitroprusside (150 micrograms/kg/min) or chlorisondamine (3 mg/kg) diminished the release rate of serotonin. Immobilization, noise (95 dB) or tail pinch increased the release of serotonin in the locus coeruleus and slightly elevated blood pressure. Chlorisondamine abolished the rise in blood pressure elicited by tail pinch without influencing the increased serotonin release. Tail pinch-induced serotonin release was abolished by superfusion with tetrodotoxin. The findings demonstrate that neuronal serotonin release in the locus coeruleus responds to cardiovascular and sensory stimuli, suggesting a function of serotonergic neurons in central blood pressure regulation, as well as in the modulation of locus coeruleus activity by stress and noxious stimuli.

A microdialysis examination of serotonin release in the rat forebrain induced by behavioral/environmental manipulations

Serotonin (5-HT) has been implicated in a variety of behavioral and physiological processes mediated by the central nervous system. However, the exact nature of 5-HT release under naturalistic or physiologic conditions remains unclear. The present study investigated this issue by employing in vivo microdialysis to examine 5-HT release in the hippocampus, corpus striatum, amygdala, and prefrontal cortex of the rat during manipulations that induced varying behavioral and physiological responses. In each rat, two sites were examined simultaneously during two of the following manipulations: tail pinch, tail pinch with food present, feeding, cat exposure, floating, and swimming. Results indicate that 5-HT levels increased 20-65% above baseline levels in response to each manipulation in all forebrain areas examined. The only significant difference found across manipulations was that tail pinch with food present produced a greater increase in 5-HT than cat exposure, which may be attributable to differences in the degree of general activation induced by these manipulations rather than to specific physiological or behavioral aspects. Furthermore, there was a strong positive correlation between 5-HT release and time spent in alert waking. Finally, there was an overall significantly smaller increase in 5-HT release in the corpus striatum compared to the other three sites. These data suggest that the release of 5-HT in the forebrain is closely related to behavioral state, not to specific behaviors, and that the magnitude of 5-HT release may reflect the degree of activation. In addition, this study suggests there can be some degree of differential pattern of release of 5-HT in the forebrain.

Evaluation of perospirone (SM-9018), a novel serotonin-2 and dopamine-2 receptor antagonist, and other antipsychotics in the conditioned fear stress-induced freezing behavior model in rats

We studied the effects of perospirone (SM-9018), a novel serotonin-2 (5-HT2) and dopamine-2 (D2) receptor antagonist (SDA), on conditioned fear stress (CFS)-induced freezing behavior in rats and compared its actions with those of other antipsychotics. Exposure of rats to the environment previously paired with foot shock induced marked freezing behavior, which was reduced by the anxiolytic diazepam (0.1-3 mg/kg, p.o.) or antidepressants, desipramine and imipramine (10-100 mg/kg, p.o.). Perospirone at 0.3-3 mg/kg, p.o. significantly attenuated the CFS-induced freezing behavior in a dose-dependent manner, while the effect was reduced at the higher dose of 6 mg/kg. Other SDA-type antipsychotics, clozapine (1-30 mg/kg, p.o.) and risperidone (0.03-1 mg/kg, p.o.), and selective 5-HT2 antagonists, ritanserin (0.1-1 mg/kg, p.o.) and ketanserin (0.3-1 mg/kg, p.o.), all reduced the freezing behavior with U-shaped dose-response curves. However, neither conventional antipsychotic, haloperidol (0.1-3 mg/kg, p.o.), chlorpromazine (3-100 mg/kg, p.o.), thioridazine (3-100 mg/kg, p.o.), mosapramine (3-100 mg/kg, p.o.) nor tiapride (30-1000 mg/kg, p.o.) reduced the CFS-induced freezing behavior. In addition, subacute treatment of rats with perospirone (1-10 mg/kg/day) or imipramine (30 mg/kg/day) for 2 weeks prevented the induction of the freezing behavior by CFS. These findings suggest that SDA-type antipsychotics including perospirone are effective for the treatment of mood disturbances such as anxiety and depressive mood associated with schizophrenia and have a broader efficacy profile as compared with the conventional antipsychotics.

Conflict situation increases serotonin release in rat dorsal hippocampus: in vivo study with microdialysis and Vogel test

The release of serotonin (5-hydroxytryptamine, 5-HT) in the dorsal hippocampus was measured using an in vivo microdialysis method in rats subjected to the Vogel type conflict test. The conflict situation significantly increased 5-HT release in the dorsal hippocampus. Midazolam (0.75 and 1.5 mg/kg i.p.) suppressed the dosage-dependently this increased 5-HT release, an inhibition closely associated with the attenuation of conflict behavior. These findings suggest that the activation of serotonergic neuronal activity in the dorsal hippocampus is linked to mediation of anxiety-related behavior.

5-HT1A and benzodiazepine receptors in the basolateral amygdala modulate anxiety in the social interaction test, but not in the elevated plus-maze

In order to investigate the role of the 5-HT1A receptors of the amygdala in modulating anxiety, rats were implanted with bilateral cannulae aimed at the basolateral nucleus of the amygdala complex and infused with either artificial cerebrospinal fluid (aCSF) or the selective 5-HT1A receptor agonist 8-OH-DPAT (50-200 ng) and tested in two animal models of anxiety. In the elevated plus-maze test, no significant effects were detected in this dose range. In contrast, 8-OH-DPAT caused an overall reduction in levels of social investigation, thus indicating anxiogenic actions in the social interaction test. At 50 ng, 8-OH-DPAT had a selective action on anxiety, while at 200 ng there was a concomitant reduction in locomotor activity and, in some animals, signs of the 5-HT1A syndrome. Evidence that the anxiogenic effect of 8-OH-DPAT (50 ng) was due to activation of 5-HT1A receptors came from the finding that (-)-tertatolol, a 5-HT1A receptor antagonist, reversed this effect at a dose (1.5 micrograms) which was silent when given alone. The benzodiazepine receptor agonist, midazolam (1 and 2 micrograms) was bilaterally administered into the basolateral nucleus of the amygdala and evoked clear-cut anxiolytic effects in the social interaction test. These data indicate that the agonist activation of post-synaptic 5-HT1A receptors in the basolateral nucleus of the amygdala may produce anxiogenic effects, while agonist activation of BDZ receptors in the same areas evokes anxiolytic effects. Our results from the social interaction test are similar to those previously reported from tests of anxiety using punished paradigms, but contrast with those found in the elevated plus-maze. Thus, it is concluded that either the two tests have different sensitivities to midazolam and 8-OH-DPAT or more intriguingly, the tests are evoking fundamentally different states of anxiety, with that evoked by the plus-maze being mediated via brain areas or receptors different from those studied here.

The effects of two chronic intermittent stressors on brain monoamines

The effects of chronic exposure (27 days) to two different stressors on brain monoaminergic activity was studied in adult male rats. The stressors used were restraint in tubes (RES) and immobilization in wooden boards (IMO). Both chronically stressed and stress naive (control) rats were subjected to 0, 15, and 60 min of the same stressor to which they were chronically exposed. Previous chronic exposure to either RES or IMO significantly reduced ACTH response to the same stressor. Monoaminergic response to these stressors was studied by measuring the levels of noradrenaline (NA), serotonin (5-HT) and their metabolites: 3-methoxy,4-hydroxyphenyletileneglycol sulfate (MHPG-SO4) and 5-hydroxyindoleacetic acid (5-HIAA), respectively. The regions studied were: pons plus medulla, midbrain, hypothalamus, hippocampus, and frontal cortex. Previous chronic exposure to the stressors induced only few changes in the resting levels of the monoamines and their metabolites. In addition, monoaminergic response to the same stressor to which they were chronically exposed was always similar in control and chronically stressed rats. These data indicate that brain NA and 5-HT metabolism is less sensitive than ACTH to the process of habituation to a repeated stressor, at least in the gross areas of the brain analyzed in the present study.

Neurochemical and behavioural interactions between ibogaine and nicotine in the rat

1. In vivo brain microdialysis has been employed to investigate the effects of ibogaine on nicotine-induced changes in dopamine overflow in the nucleus accumbens (NAc) of freely moving rats. The effects of the compound on locomotor responses to nicotine and behaviour in the elevated plus-maze were also examined. 2. No changes were observed in the dopamine overflow or the locomotor activity of the animals following the administration of ibogaine (40 mg kg-1, i.p.). However, ibogaine, administered 22 h earlier, significantly (P < 0.01) attenuated the increase in dopamine overflow but not the hyperlocomotion, evoked by nicotine. 3. In the elevated plus-maze test, significant reductions in the open:total runway entries in both saline-treated controls (P < 0.05) and nicotine-treated (P < 0.01) rats were obtained when the animals were tested 22 h after pretreatment with ibogaine (40 mg kg-1, i.p.). The total activity was significantly (P < 0.01) greater in the nicotine-treated rats but this response was not affected by ibogaine pretreatment. 4. Administration of ibogaine was associated with reductions in the tissue levels of 5-hydroxyindoleacetic acid (5-HIAA) in the NAc (P < 0.01) and striatum (P < 0.05) and an increase in the level of this metabolite in the medial prefrontal cortex (mPFC) (P < 0.01) while the levels of dopamine and 5-hydroxytryptamine (5-HT) in the mPFC were reduced (P < 0.05). The DOPAC/dopamine (P < 0.05) and 5-HIAA/5-HT (P < 0.01) ratios were significantly increased in the mPFC for at least 7 days after a single treatment with ibogaine. 5. Ibogaine attenuates the nicotine-induced increases in dopamine overflow in the NAc and may, therefore, inhibit the rewarding effects of this drug. However, the long lasting anxiogenesis induced by ibogaine warrant further investigation before its use could be recommended for smokers.

Effects of stress on the functional properties of pre- and postsynaptic 5-HT1B receptors in the rat brain

Numerous studies have clearly shown that the turnover and release of serotonin (5-hydroxytryptamine, 5-HT) are increased under acute stressful conditions. Inasmuch as this latter process is under the control of a feedback mechanism involving the stimulation of presynaptic 5-HT1B autoreceptors, we have investigated the possible effects of acute restraint (40 min) on the functional properties of 5-HT1B receptors. The efficacy of the selective 5-HT1B receptor agonist 3-[1,2,5,6-tetrahydropyrid-4-yl]pyrrolo-[3,2-b]pyrid-5-one (CP-93,129) in inhibiting in vitro the K+-evoked release of [3H]5-HT, was significantly reduced in stressed rats as compared to naive animals. Similarly, the responsiveness of 5-HT1B receptors inhibiting the release of [3H]acetylcholine (presynaptic 5-HT1B heteroreceptors), was reduced by restraint. These effects were observed in the hippocampus, but using the inhibitory effect of CP-93,129 on forskolin-stimulated adenylyl cyclase activity as an index of 5-HT1B receptor function, it could be shown that the 5-HT1B receptors located in the substantia nigra are also desensitized by stress. The number as well as the apparent affinity constant of 5-HT1B binding sites labelled by [125I]iodocyanopindolol, as measured by quantitative autoradiography and membrane binding, were similar in naive and restraint-stressed rats suggesting that the stress-induced desensitization of 5-HT1B receptors is not due to a reduced number of 5-HT1B binding sites. As stress is thought to be a causal factor for the etiology of anxiety and depression, these results support the potential involvement of 5-HT1B receptor dysfunction in the development of these neurological disorders.

Long-term effects of chronic social stress on serotonergic indices in the prefrontal cortex of adult male cynomolgus macaques

We examined the effects of chronic social stress and social rank on monamine concentrations in the prefrontal cortex (PFC) in adult male cynomolgus macaques (Macaca fascicularis). Seventy-five animals were housed in five-member social groups for 28 months and were exposed to three experimental conditions. A 'no-stress' condition was comprised of animals housed in groups of stable membership throughout the study. Animals assigned to a 'past-stress' condition had their group memberships reorganized at monthly intervals during the first (but not last) 14 months of the study, and a third 'recent-stress' condition consisted of social groups reorganized only during the last 14 months. At necropsy, the brains were collected and frozen at -70 degrees C until analyzed. Prefrontal orbital cortex was assayed for monoamines (serotonin (5-HT), dopamine (DA), norepinephrine (NE)), metabolites (5-hydroxyindoleacetic acid (5-HIAA), homovanillic acid (HVA), 3-methoxy-4-hydroxyphenethyleneglycol (MHPG)), and tryptophan using high-performance liquid chromatography with electrochemical detection. Animals in the past-stress condition had significantly lower PFC 5-HIAA concentrations compared to those in the no-stress condition (P < 0.05). PFC 5-HT concentrations of animals in the past-stress condition were significantly lower than those in the no-stress and recent-stress conditions (P < 0.01). The concentrations of DA, HVA, NE and MHPG were not altered. These data suggest that exposure to chronic social stress is associated with long-term selective reductions in serotonergic activity in the PFC. This effect may underlie the association in human beings between reduced serotonergic function and conditions such as pathological grief and posttraumatic stress disorder.

Serotonin and stress-induced increases in renin secretion are not blocked by sympathectomy/adrenal medullectomy but are blocked by beta antagonists

The present study examined the role of the sympathetic nervous system as a mediator of the message from the CNS to the kidneys to stimulate the secretion of renin. Two procedures that increase the secretion of renin were tested: administration of the serotonin releaser fenfluramine, which increases renin release without altering blood pressure [53], and subjecting the rats to the 'psychological' stressor of conditioned emotional response (CER) stress. Pretreatment of rats with either the beta antagonist sotalol or the beta 1-selective antagonist atenolol completely prevented the increase in plasma renin activity and concentration caused by fenfluramine (5 mg/kg i.p.) injection. However, chemical sympathectomy with 6-hydroxydopamine (6-OHDA) combined with surgical adrenal medullectomy did not prevent the increase in plasma renin activity and concentration following fenfluramine injection. Since beta-antagonists have been previously shown to prevent the renin response to CER stress, we also tested whether the sympathetic nervous system mediates the renin response to CER stress. Chemical sympathectomy combined with adrenal medullectomy did not prevent the effect of CER stress on renin release. The completeness of the sympathectomy/adrenal medullectomy was verified biochemically by measuring plasma epinephrine and both plasma and renal norepinephrine concentrations. Plasma epinephrine and renal norepinephrine levels were reduced to below 1% of control while plasma norepinephrine was reduced to below 8% of control values. In conclusion, our data support previous reports suggesting that activation of CNS pathways increases the secretion of renin. However, the message from the brain to release renin from the kidneys does not exclusively involve either the sympathetic innervation of the kidneys or adrenal epinephrine. Although beta 1 receptors are involved in mediating this phenomenon, their location or mechanism remains unknown and will be discussed.

Brain microdialysis and its application for the study of animal behaviour

Microdialysis is a sampling method that is used to determine the extracellular concentration of neurotransmitters in the brain. The method can be applied to conscious and unrestrained animals and is very suitable for the study of the chemistry of endogenous behaviour. This article reviews the contribution that microdialysis made to our understanding of the chemistry of behaviour. Methodological and practical considerations such as the implantation time and the use of guide cannulas are reviewed. The question whether neurotransmitters and related metabolites in dialysates reflect true synaptic release is critically discussed. There is much evidence that dopamine, noradrenaline, acetylcholine and serotonin in dialysates are related to neurotransmission, but there is serious doubt whether this is the case with amino acid transmitters such as GABA, glutamate and aspartate. Until now far over 100 papers appeared that used microdialysis in behavioural studies. Behavioural activation, the sleep-awake cycle and diurnal rhythms were subject of several of these studies. Various workers have described neurochemical changes in the brain that are related to feeding. Other studies were concerned with sexual behaviour and the sexual cycle in females. Parturition, maternal behaviour and offspring recognition have been studied in a series of microdialysis studies carried out in sheep. An overview is given of the microdialysis studies that were carried out to understand the biochemistry of stress. In this respect dopamine and noradrenaline have received much attention. A great number of microdialysis studies dealt with the role of dopamine in self-stimulation, reward and aversive emotions. It is concluded that microdialysis is at presently the most versatile and practical method to study the chemistry of behaviour and it is to be expected that it will soon be a routine methodology in behavioural research. Finally, perspectives and possible future developments of the methods are discussed.

Previous stress increases in vivo biogenic amine response to swim stress

In vivo microdialysis was used to determine biogenic amines in medial prefrontal cortex of rats exposed to eight minutes of swim stress on two consecutive days. On the first day of stress, norepinephrine (NE) efflux increased by 183% over baseline after stress, while dopamine (DA) and serotonin (5-HT) remained stable throughout. On the second day of stress, a robust increase was observed in all 3 neurotransmitters measured, with (NE), (DA), and (5-HT) increasing by 310%, 441% and 496% respectively, and remaining elevated for an hour or more after stress. This suggests that the first exposure to swim stress, while not causing dramatic changes in biogenic amine release, may sensitize biogenic amines in medial prefrontal cortex to subsequent swim stress. Our results also serve as preliminary data concerning the neurochemical changes which might underlie the forced swimming model of "behavioral despair".