5-HT and mechanisms of defence

Further evidence that the mouse defense test battery is useful for screening anxiolytic and panicolytic drugs: effects of acute and chronic treatment with alprazolam

The Mouse Defense Test Battery (MDTB) has been designed to investigate defensive responses of Swiss-Webster mice confronted with a natural predator, a rat. These behaviors include flight, avoidance, defensive threat/attack responses, and risk assessment activities. Previous studies with the MDTB have suggested that this model may have some utility for the investigation of panicogenic and antipanic compounds. In the present study the MDTB was used to investigate the effects of acute (0.05-1 mg/kg, i.p., 30 min) or chronic (0.5-2 mg/kg, one daily i.p. injection during 10 days) treatment with the benzodiazepine receptor (BZPR) full agonist and panicolytic agent alprazolam. At non motor-impairing doses (0.05-0.5 mg/kg), acute alprazolam failed to alter the avoidance distance between the subject and the predator, the number of avoidances when the rat is approaching, predator assessment activities, defensive threat/attack responses when contact is forced between the subject and the predator or contextual escape attempts after the predator was removed. This was in contrast to chronic treatment which decreased both avoidance variables at 0.5 and 1 mg/kg, defensive threat/attack responses at all doses, and predator assessment responses at 0.5 mg/kg. In addition, the latter treatment reduced post-predator potentiation of escape attempts at 2 mg/kg. These results (1) confirm previous findings with the BZPR full agonist chlordiazepoxide, indicating that these compounds generally attenuate antipredator defensive responses in Swiss-Webster mice; (2) support recent data indicating that panic-altering drugs modulate flight/escape reactions, and suggest that the primary mechanism of action of drugs with efficacy against panic disorder may involve neural systems controlling flight; (3) confirm that the MDTB may be useful for the investigation of panicolytic as well as anxiolytic agents.

Differential effects of forebrain 5-hydroxytryptamine depletions on Pavlovian aversive conditioning to discrete and contextual stimuli in the rat

The experiments examined the effects of depleting forebrain 5-hydroxytryptamine (5HT) on Pavlovian aversive conditioning to discrete and contextual stimuli. Rats were lesioned with intracerebroventricular injections of the neurotoxin 5,7-dihydroxytryptamine and then conditioned in a distinctive environment (termed the context) to a 30 s auditory stimulus. In 50% of animals the interval between the offset of the discrete auditory stimulus and the reinforcer, a mild foot-shock (0.5 mA, 0.5 s), was 5 s (the short-trace group) and in the other 50%, 30 s (the long-trace group). Theory predicts that animals in the short-trace condition will learn more about the discrete stimulus as a predictor of shock and become strongly conditioned, while those in the long-trace condition learn relatively more about the context. The extent of conditioning to the discrete and contextual stimuli was assessed separately, in extinction, using lick-suppression and place-preference measures respectively. Under these conditions sham subjects exhibited the expected dissociation with respect to trace interval. However, lesioned animals exhibited a specific impairment in contextual conditioning. The results are discussed in terms of the behavioural, neurochemical and neuroanatomical specificity of 5HT function in aversive conditioning and the implications for general theories of the role of 5HT in aversive processes.

Role of 5-HT receptor subtypes in the modulation of dorsal periaqueductal gray generated aversion

To explore the role of 5-HT receptor subtypes in controlling aversion, we measured the effect of 5-HT1A and 5-HT2A/2C receptor agonists microinjected into the dorsal periaqueductal gray (DPAG) of rats on aversive behavior induced by electrical stimulation of the same brain area. The 5-HT1A agonists 8-OH-DPAT (4-16 nmol) and BAY-R-1531 (4-16 nmol) raised the threshold of aversive electrical stimulation in a dose-dependent way. Similarly, microinjection of the 5-HT2A/2C agonist DOI (4-16 nmol) increased the aversive mCPP (16 and 32 nmol) was ineffective. Previous intra-DPAG administration of the 5-HT1A receptor blocker NAN-190 (40 nmol) antagonized the antiaversive effect of 8-OH-DPAT (8 nmol), whereas pretreatment with the 5-HT2A receptor blocker spiperone (10 nmol) antagonized the effect of DOI (16 nmol). Spiperone also counteracted the effect of 8-OH-DPAT and NAN-190 counteracted the effect of DOI. These results indicate that activation of 5-HT1A and 5-HT2A receptors inhibits aversion in the DPAG and that both receptors have to be functional for the expression of each one's activation to occur.

Dorsal periaqueductal gray-induced aversion as a simulation of panic anxiety: elements of face and predictive validity

Neurosurgical stimulation of the dorsal periaqueductal gray (dPAG) matter in man induces acute signs of autonomic arousal and feelings of subjective anxiety; those signs have phenomenological similarity with the symptom profile characterizing a panic attack. Animals undergoing dPAG stimulation show comparable physical signs of autonomic activation and sudden fear-suggestive behavioral reactions that can be shaped into operant self-interruption behavior. Drugs known to acutely reduce (alprazolam, clonazepam) or precipitate (yohimbine, caffeine) panic attacks in patients were found to acutely and dose-dependently reduce or enhance, respectively, aversion induced by dPAG stimulation in rats. When considered as an animal model of panic anxiety, the dPAG model simultaneously meets criteria of face validity (symptomatic homology) and predictive validity (pharmacological homology under short-term treatment); aspects of its construct validity (theoretical rationale supporting the model) are discussed. It is suggested that dPAG stimulation-induced aversion may represent a model of some aspects of panic disorders.

Differential modulation of antipredator defensive behavior in Swiss-Webster mice following acute or chronic administration of imipramine and fluoxetine

The Mouse Defense Test Battery (MDTB) has been designed to assess defensive reactions in Swiss-Webster mice to situations associated with a natural predator, the rat. Primary measures taken before, during and after predator confrontation comprise escape attempts, predator assessment, defensive attack and flight. Previous reports from this laboratory have shown that the panic-promoting drug yohimbine potentiated flight behavior, while long-term treatment with the panicolytic agent alprazolam reduced this response. In order to evaluate further the possibility that the MDTB may represent an effective animal model of panic attacks, the present study investigated the behavioral effect of imipramine and fluoxetine, two serotonin reuptake inhibitors (SRIs) known to alleviate panic symptoms when given on a repeated basis. Both drugs were administered acutely and chronically (one daily IP injection for 21 days) at 5, 10 and 15 mg/kg. Our results showed that a single dose of imipramine or fluoxetine strongly potentiated flight reactions in response to an approaching predator and increased defensive attack toward the rat. This was in contrast to chronic treatment with each drug which dramatically decreased flight responses and defensive attack behaviors. In addition, long-term administration with both SRIs produced a reliable attenuation of predator assessment activities. Taken together, these findings suggest an acute anxiogenic-like effect of imipramine and fluoxetine followed by a fear/anxiety reducing effect after repeated administrations. These results support clinical observations revealing an acute anxiogenic effect of SRIs followed by an anxiolytic and/or panicolytic effect after chronic use, and support previous results suggesting that the MDTB may be useful for the investigation of panic-modulating agents.

5-HT1A agonists modulate mouse antipredator defensive behavior differently from the 5-HT2A antagonist pirenperone

The mouse defense test battery (MDTB) has been designed to investigate defensive reactions in Swiss-Webster mice to situations associated with a natural predator, the rat, such as flight, avoidance, defensive threat, defensive attack, and risk assessment activities. The present study evaluated the ability of 8-OH-DPAT (0.05-10 mg/kg, SC, 5) and gepirone (2.5-10 mg/kg, IP, 30), a full- and a partial agonist at 5-HT1A sites, as well as pirenperone (0.25-1 mg/kg, IP, 30), a preferential 5-HT2A receptor antagonist, to exert an anxiolytic-like action in the MDTB. The most consistent effect of both 5-HT1A receptor agonists across tests was a marked reduction in predator assessment activity and defensive attack behavior. In contrast, neither of the two ligands was able to reduce flight responses to the approaching predator, and both failed to reduce in a specific manner contextual defense behaviors after the predator was removed. The 5-HT2A receptor antagonist pirenperone did not provide significant indication of an anxiolytic effect on predator assessment activity and postpredator potentiation of contextual defense responses, and had negligible influence on antipredator defensive behavior. The most interesting exception to this profile was a dose-related reduction in flight-related measures. In view of previous results indicating that the panic-promoting drug yohimbine increases flight/escape reactions and that the panicolytic compound alprazolam reduces these responses, we tentatively suggest that the preferential 5-HT2A receptor antagonist pirenperone may have some efficacy in improving panic attacks. In addition, the lack of effect of the 5-HT1A receptor agonists on these flight responses is consistent with clinical findings indicating that these agents are of limited use in the treatment of panic disorder. These findings suggest that the MDTB provides behavioural measures capable of differentiating between various classes of antianxiety drugs.

Computer analysis and quantification of periaqueductal grey-induced defence behaviour

The pharmacological study of midbrain-evoked aversive behaviours is commonly measured in terms of electrical escape thresholds, although studies which examine the actual expression of the behaviour are being introduced. A computer-driven automated tracking system has been developed to record and analyse the very rapid locomotor activity produced by the defence response. Stimulation of the midbrain periaqueductal grey (PAG) matter in rats with the excitatory amino acid D,L-homocysteic acid (DLH) produced very rapid and short-lasting escape behaviour which was tracked by computer. The system, VideoTrack, determines the speed and distance travelled by the animal and stores and statistically analyses the data. VideoTrack was able to detect a pharmacological modifications of the response; anti-aversive effects produced by intra-PAG pretreatment with the 5-HT1A agonist 8-hydroxy 2-(di-n-propylamino)tetralin (8-OHDPAT) and its reversal by peripheral pretreatment with the 5-HT1A antagonist N-[2-[-(2-methoxyphenyl)-1-piperazinyl]ethyl-N- (pyridinyl)cyclohexanecarboxanide (WAY 100635).

5-Hydroxytryptamine pathways in anxiety and its treatment

The effects of manipulating 5-hydroxytryptamine (5HT) neuronal function in humans and in animals are reviewed. 5HT pathways do not have a unitary function in modulating anxiety. It is proposed that, rather than acting as input or output channels for brain aversive systems, these pathways provide information concerning waking/motor status, which is crucial to the organisation of appropriate responses to threat. Each terminal region can make use of this information in different ways. Globally, the influence of 5HT neurones on higher centres appears predominantly to facilitate information processing relevant to threat, while their major influence on brainstem centres may be a restraining one.

Effects of combined or separate 5,7-dihydroxytryptamine lesions of the dorsal and median raphe nuclei on responding maintained by a DRL 20s schedule of food reinforcement

Previous studies have shown that long-term 5-hydroxytryptamine (5-HT) depletion induced by combined dorsal and median raphe injections of 5,7-dihydroxytryptamine (5,7-DHT) leads to impairments in the acquisition and performance of behaviour maintained under a differential-reinforcement-of-low-rate (DRL) schedule of reinforcement. The present studies examined the relative importance of dorsal versus median raphe 5-HT projections in mediating these effects by investigating the impact of separate versus combined infusions of 5,7-DHT into these sites on DRL responding. Adult male rats received injections of 3 micrograms 5,7-DHT into both dorsal raphe and median raphe, dorsal raphe only, or median raphe only. Sham-operated controls received vehicle injections. Animals were then trained to respond on a DRL 20s schedule for 36 days. Combined dorsal raphe and median raphe 5,7-DHT infusions depleted striatal and hippocampal 5-HT by > 90%, increased responding, decreased the number of reinforcers earned, lowered the mean inter-response time (IRT), and shifted the frequency distribution of IRTs to the left. Median raphe 5,7-DHT infusions induced similar behavioural effects and reduced hippocampal 5-HT by 64%. Dorsal raphe 5,7-DHT lesions, that depleted striatal 5-HT by 56%, and hippocampal 5-HT by 30%, had no effect on behaviour. In a final experiment it was shown that providing an external cue light to signal food availability, and that removed the need to rely on internal timing processes, resulted in rapid acquisition of responding, and prevented the behavioural deficits in 5,7-DHT-lesioned rats. Following removal of this cue, responding deteriorated, and the 5,7-DHT treated animals performed worse than controls. These results suggest that destruction of 5-HT neurons arising from the median raphe is sufficient to disrupt behaviour maintained by a DRL 20s schedule, and that the behavioural changes observed may involve a deficit in timing.

Dieting decreases plasma tryptophan and increases the prolactin response to d-fenfluramine in women but not men

We studied the effect of 3 weeks of moderate calorie restriction on 5-HT-mediated prolactin (PRL) release in healthy volunteers using the 5-HT-releasing agent d-fenfluramine. In women, dieting significantly lowered plasma total and free tryptophan (TRP) and increased the PRL response to d-fenfluramine. None of these measures were altered in men who dieted. These findings add to the data indicating that dieting alters brain 5-HT function in women, perhaps as a consequence of reducing the availability of plasma TRP.

A comparison of augmenting central serotonin and noradrenaline function in healthy subjects: implications for studies on the neurochemistry of anxiety

Biochemical and psychophysiological effects of augmenting serotonergic and noradrenergic function were compared in 12 normal volunteers. Fluvoxamine (100 mg), a serotonin (5-HT) re-uptake inhibitor, maprotiline (75 mg), a noradrenaline re-uptake inhibitor, and placebo were given for 7 days each to each subject. Subjects were tested pre-drug on days 1, 4 and 8 of each treatment. Catecholamines in 24 h urine, 'platelet-rich' plasma 5-HT and hormones, EEG, auditory evoked response, skin conductance, and bodily and psychological responses were monitored. Augmentation of central 5-HT by fluvoxamine was demonstrated by the decrease of 5-HT plasma levels. Fluvoxamine also reduced urinary dopamine, indicating a decrease in dopamine metabolism in response to augmentation of 5-HT function. Decrease in pulse rate, loss of appetite and a mild arousing effect were also detected with administration of fluvoxamine. Thus, further investigations on the neuro chemistry of anxiety disorders should include the study of mechanisms of interaction of neurotransmitters.

Toward a unified theory of serotonin dysregulation in eating and related disorders

Patients with eating disorders (EDs) exhibit several clinical features and biologic findings indicative of serotonin (5-hydroxytryptamine, 5-HT) dysregulation. These include feeding disturbances, depression and suicide, impulsivity and violence, anxiety and harm avoidance, obsessive-compulsive features, seasonal variation of symptoms, as well as disturbances in neuroendocrine and vascular tissues, as well as other neurochemical systems linked to 5-HT, such as temperature. This review attempts to integrate available results from controlled studies in humans, with particular focus on cerebrospinal fluid (CSF), platelet and plasma studies, as well as pharmacologic challenge strategies using a variety of serotonergic agents. Taken together, these findings support the concept of altered post-synaptic, hypothalamic 5-HT receptor sensitivity in bulimia nervosa (BN), regardless of the presence of anorexia nervosa (AN) or major depression (MD), although these conditions may be associated with other disturbances in 5-HT function, perhaps pre-synaptic ones. The observation that different response measures of 5-HT function in the same subjects may be simultaneously increased, decreased and no different in patients compared to controls is consistent with a 5-HT dysregulation hypothesis. It may be that a variety of psychobiological stressors, such as dieting, binge-eating, purging, drug abuse, photoperiodic changes, as well as psychosocial-interpersonal stressors, perturb and interact with an already vulnerable 5-HT system.

The influence of 5-hydroxytryptamine re-uptake blockade on CCK receptor antagonist effects in the rat elevated zero-maze

In this study, the elevated zero-maze model of anxiety was used to investigate CCK receptor antagonist effects on the behaviour of male Lister-hooded rats and to demonstrate, by administering antagonists in the presence or absence of selective 5-hydroxytryptamine (5-HT) re-uptake inhibitors, the involvement of 5-HT in the mediation of these effects. Devazepide, a selective CCKA receptor antagonist, L-365,260 (3R(+)-N-2,3-dihydro-1-methyl-2-oxo-5-phenyl-1H-1,4-benzodiazepin- 3-yl-N1- (3-methyl-phenyl)urea) or CI-988 (4-([2-[[3-(1H-indol-3-yl)-2-methyl-1- oxo-2-[[(tricyclo[3.3.1.1.(3.7)]-dec-2-yloxy)-carbonyl]-amin o]- propyl]-amino]-1-phenylethyl]-amino)-4-oxo-[R-(R*,R*)]-butanoate- N-methyl-D-glucamine), both selective CCKB receptor antagonists, were administered 30 min prior to testing. Behavioural analysis during testing included measures of risk-assessment behaviours (e.g. stretched-attend posture) in addition to time spent on the open quadrants. Devazepide induced significant anxiolytic effects, whereas CI-988 produced inconsistent results and L-365,260 was ineffective. When administered simultaneously with the 5-HT re-uptake inhibitors zimelidine or Wy 27587 (N-[[[1-[(6- fluoro-2-naphthalenyl)methyl]-4-piperidinyl]amino] carbonyl]-3-pyridine carboxamide methyl sulphonate salt), the significant anxiolytic effect induced by devazepide was dose-dependently and significantly attenuated. Zimelidine and Wy27587 had little effect alone on zero-maze behaviour at the lower of two doses given. These data show that the elevated zero-maze, in conjunction with the analysis of 'risk-assessment' behaviours, is an anxiety model which is sensitive to the anxiolytic effects of CCK receptor antagonism.(ABSTRACT TRUNCATED AT 250 WORDS)

Facilitated acquisition of a temporal discrimination following destruction of the ascending 5-hydroxytryptaminergic pathways

This experiment examined the effect of destroying the 5-hydroxytryptaminergic (5HTergic) pathways on the acquisition and performance of discrimination between two brief time intervals. Rats that had received injections of 5,7-dihydroxytryptamine into the dorsal and median raphe nuclei, and sham-lesioned control rats were trained in a series of discrete trials to press lever A following a 200-ms presentation of a light stimulus and lever B following an 800-ms presentation of the same stimulus. Both groups gradually acquired accurate performance, attaining 80%-85% accuracy by the end of 40 sessions. The lesioned group learnt the task significantly faster than the control group. When stable performance had been attained, "probe" trials were introduced in which the light was presented for intermediate durations. Both groups showed sigmoid functions relating percent choice of lever B to log stimulus duration. The bisection point (duration corresponding to 50% choice of lever B) did not differ significantly between the two groups; however, the Weber fraction was significantly smaller in the lesioned group than in the control group. The levels of 5HT and 5-hydroxy-indole-acetic acid were markedly reduced in the brains of the lesioned rats, but the levels of noradrenaline and dopamine were not altered. The results indicate that destruction of the 5HTergic pathways facilitates acquisition of a temporal discrimination. The lack of an effect of the lesion on the bisection point contrasts with our previous finding using longer stimulus durations; it is suggested that different behavioural processes may underlie millisecond-range and second-range temporal discrimination, and that these may be differently affected by 5HT depletion.

Effects of two stressors on behaviour in the elevated X-maze: preliminary investigation of their interaction with 8-OH-DPAT

Effects of water deprivation and restraint were compared in the rat elevated X-maze. Water deprivation for 12-48 h increased corticosterone and had a duration-dependent "anxiolytic" effect in the elevated X-maze, increasing the ratio of open/total arm entries (OTR) and the proportion of time spent on the open arms (% time) without affecting total entries. Brain 5HIAA/5HT was increased only after 24 or 48 h deprivation. Restraint for 15 min also increased plasma corticosterone and brain 5HIAA/5HT but had no effect on behaviour in the elevated X-maze when rats were tested immediately afterwards. However, 1 h restraint was "anxiogenic" in the elevated X-maze immediately after release, reducing OTR and % time, but with a less consistent reduction in total entries; reductions in OTR and % time were still present 24 h later. The 5HT1A agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) (0.1-0.2 mg/kg), administered 10 min before testing in the elevated X-maze, had "anxiogenic" actions in non-stressed rats. The effect of 0.1 mg/kg 8-OH-DPAT was not significantly altered by 24 or 48 h water deprivation but was abolished by restraint for 1 h immediately beforehand, despite the "anxiogenic" effect of restraint alone. Similar mutual antagonism of 8-OH-DPAT and restraint occurred when the dose of 8-OH-DPAT was increased to 0.2 mg/kg. Twenty-four hours after restraint, restrained rats which had received 8-OH-DPAT (0.1-0.2 mg/kg) still did not show any significant "anxiogenic effect" compared with non-restrained vehicle treated controls. Restraint-induced deficits in elevated X-maze exploration may prove a useful model with which to study the pharmacology of depression-related anxiety. However, the effects of the stressors examined, and their interaction with 8-OH-DPAT in the elevated X-maze, appear to depend on the nature of the stressor.

Effects of hydrocortisone on brain 5-HT function and sleep

The effects of hydrocortisone administration (20 mg, orally, twice daily) on the sensitivity of brain 5-HT1A receptors in healthy volunteers were studied using a buspirone challenge paradigm. The effects of hydrocortisone administration on sleep architecture were also studied. Hydrocortisone treatment significantly attenuated the hypothermic and cortisol responses to buspirone; however, the prolactin and growth hormone responses were unchanged. Hydrocortisone also decreased the amount of rapid eye movement sleep (REM). The ability of hydrocortisone to attenuate 5-HT1A receptor mediated hypothermia and decrease REM sleep is shared by certain antidepressant treatments and may be related to the effects of corticosteroids on mood.

Adaptive changes in 5-HT1A receptor-mediated hippocampal inhibition in the alert rat produced by repeated 8-OH-DPAT treatment

1. The effect of acute and repeated treatment with 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), a 5-HT1A receptor ligand, on excitatory amino acid-mediated synaptic transmission was examined in the stratum radiatum CA1 region of the dorsal hippocampus of alert, gently restrained, rats. 2. Acute administration of 8-OH-DPAT transiently reduced the amplitude of the field excitatory postsynaptic potential (e.p.s.p.) in a dose-dependent (25-75 micrograms kg-1, i.p.) manner. This effect was blocked by the postsynaptic 5-HT1A receptor antagonist, MDL 73005EF (2 and 4 mg kg-1, i.p.). 3. 8-OH-DPAT (25 micrograms kg-1, i.p.) administered daily for 7 days produced a gradual reduction in the 24 h pre-injection baseline field e.p.s.p. amplitude. The reduction reached its lowest level after 7-8 days and was transiently reversed by acute injection of MDL 73005EF (2 mg kg-1, i.p.) on day 8. The field e.p.s.p. baseline amplitude recovered fully 5-8 days after cessation of drug treatment. 4. 8-OH-DPAT (25 micrograms kg-1, i.p.) administered daily for 7 days produced a marked reduction in acute response to 8-OH-DPAT (25 and 50 micrograms kg-1, i.p.) which did not recover until between day 36 and day 80 of the study. 5. It was concluded that repeated treatment with 8-OH-DPAT produced adaptive changes which resulted in a reduction in the dynamic range of 5-HT1A receptor-mediated transmission in the hippocampus.

Effects of BOC-CCK-4 and L 365.260 on cortical 5-HT release in guinea-pigs on exposure to the elevated plus maze

The elevated plus maze is a well-established model of anxiety, with previous results showing that guinea-pigs handled daily from birth exhibit behaviour in this test similar to rats. In the present microdialysis study exposure of the guinea-pig to the elevated plus maze increased extracellular 5-HT in the lateral prefrontal cortex. The CCK-B receptor agonist BOC-CCK-4 (10 micrograms/kg) produced 'anxious' behaviour and potentiated the rise in 5-HT observed on exposure to the X-maze. The basal release of cortical extracellular 5-HT was not affected by BOC-CCK-4. Pretreatment with the selective CCK-B antagonist L 365.260 (100 micrograms/kg) antagonized both the 'anxious' behaviour and the neurochemical changes induced by BOC-CCK-4 while L 365.260 alone produced 'anxiolytic' behaviour, decreased basal extracellular 5-HT and prevented the increase in extracellular 5-HT seen when the guinea-pigs were exposed to the X-maze. Our results show that CCK-B receptor stimulation and blockade induce changes in central extracellular 5-HT levels associated with 'anxious' and 'anxiolytic' behaviour, respectively.

Evidence that mCPP-induced anxiety in the plus-maze is mediated by postsynaptic 5-HT2C receptors but not by sympathomimetic effects

1-(3-Chlorophenyl)piperazine (mCPP) (0.125-1.0 mg/kg i.p.), previously shown to inhibit social interaction, dose-dependently reduced exploration of the open arms of an elevated plus-maze. These findings suggest anxiogenic properties. The effect of mCPP was more potently inhibited by 1-(1-naphthyl)piperazine than by ketanserin, indicative of its mediation via activation of 5-HT2C rather than 5-HT2A receptors. The 5-HT1B receptor agonist CGS 12066B did not antagonise the anxiety-like response to mCPP, and further reduced exploration at the highest dose tested (10 mg/kg i.p.). Depletion of serotonin (5-HT) by p-chlorophenylalanine (PCPA, 150 mg/kg/day x 3) did not prevent the response, although PCPA itself increased open arm exploration. The 5-HT1A/B and beta-adrenoceptor antagonist 1-propanolol (5 mg/kg i.p.) and the peripheral beta 1-receptor antagonist atenolol (20 mg/kg i.p.) showed no significant activity on the plus-maze either alone or against the anxiogenic effect of mCPP. These results indicate that mCPP induces anxiety in the rat in the elevated plus-maze primarily by stimulation of postsynaptic 5-HT2C receptors, and suggest that sympathomimetic effects of mCPP are not involved.

Effects of imipramine on serotonergic and beta-adrenergic receptor binding in a realistic animal model of depression

Chronic exposure to mild unpredictable stress (CMS) has previously been found to cause an antidepressant-reversible decrease in the consumption of palatable sweet solutions. In the present study, in addition to confirming these behavioural observations, the binding properties of cortical beta-adrenergic and 5HT2 receptors, and hippocampal 5HT1A receptors were studied (using the ligands [3H]-dihydroalprenolol, [3H]-ketanserin and [3H]-8-OH-DPAT, respectively), following 7 weeks of CMS and 4 weeks of imipramine treatment (10 mg/kg per day). CMS increased Bmax for all three receptor systems. Imipramine decreased Bmax, reversing the effect of CMS, for beta-adrenergic and 5HT2 receptor binding, but increased Bmax for 5HT1A receptor binding. KDs were unaffected by either treatment. The beta-receptor and 5HT2 receptor binding data are consistent with accounts of antidepressant action derived from studies in normal animals, but the 5HT1A receptor binding data are more difficult to reconcile. In no case was there a good correlation between receptor binding and behavioural data.

Thresholds of electrically induced defence reaction of the rat: short- and long-term adaptation mechanisms

The thresholds of electrically induced defence reaction of the rat were studied through the logistic fitting of the response output. When stepwise increasing stimuli were applied at the dorsal midbrain, hierarchically organized mean thresholds, spaced 10 microA apart, were observed for immobility, running and jumping defensive behaviours. The parallel threshold functions of these responses, ranked in the above order, denote that they have distinct output probabilities when induced with sequential stepwise increasing stimuli. In contrast, when single daily stimuli were given in a random order, virtually superimposed threshold functions were obtained for these defensive behaviours. In this case, since the same output probabilities would be expected for immobility, running and jumping behaviours, the defence system seems to operate in a state of maximum entropy. The above data suggest that the dorsal midbrain, including the deep collicular layers and the periaqueductal gray, may encode hierarchical or non-hierarchical defensive patterns which, respectively, mimic either the attentive behaviour of the prey watching the approaching predator or its chaotic behaviour when cornered by a sudden attack. On the other hand, whereas quite stable thresholds were observed for the somatic defensive responses when 5 stimulation sessions were repeated over 15 days, the defecation and micturition output underwent a marked and progressive lessening. Since these autonomic responses have long been considered as reliable indexes of fear, their attenuation throughout the repeated sessions could express the rat adaptation to fear by the recurrence of the aversive experience. Taken together, these data suggest that while short-term neuronal adaptation could be responsible for the hierarchical threshold structure of the short interval stepwise stimulation, long-term neuronal adaptation could underlie the selective decrease of defecation and micturition responses over repeated sessions of intracranial stimulation.

Hippocampal 5-HT receptors and consolidation of stressful memories

It has been suggested that postsynaptic 5-HT1A receptors in the hippocampus, innervated by 5-HT neurons localized in the median raphe nucleus, mediate adaptive or coping responses to aversive events and that dysfunction of this system is related to symptoms of depression. To test this hypothesis we investigated the expression of c-fos mRNA in animals submitted to immobilization stress. The results showed that c-fos mRNA expression is significantly increased in the dentate gyrus and CA1-CA3 regions of the hippocampus after 30 min of forced restraint, suggesting that this structure is activated during stress. To investigate the role of 5-HT neurotransmission in the hippocampus on adaptation to aversive events we immobilized rats for 2 h and tested them 24 h later in an elevated plus-maze. Our results showed that the previous restraint period decreases exploration of open arms in the maze. This effect was reversed by bilateral microinjection of zimelidine (20 and 100 nmol), a 5-HT re-uptake blocker, or 8-OH-DPAT (3 nmol), a 5-HT1A agonist, into the dorsal hippocampus immediately after restraint. These results are compatible with the idea that postsynaptic 5-HT1A receptors located in the hippocampus participate in the development of tolerance to aversive events.

Role of the amygdala and periaqueductal gray in anxiety and panic

The amygdala (AM) and the periaqueductal gray (PAG) represent the rostral and the caudal pole, respectively, of a longitudinally organized neural system, that is responsible for the integration of behavioral and physiological manifestations of defensive reactions against innate and learned threats. Microinjection of benzodiazepine (BZD) anxiolytics, GABAA receptor agonists or 5-HT receptor antagonists into the AM has anxiolytic effects in conflict tests and other models of conditioned fear, while similar administration of 5-HT or of a 5-HT1A receptor agonist has anxiogenic effects. On the other hand, in the test of electrical stimulation of the PAG, microinjection of 5-HT, 5-HT mimetics, or of drugs that enhance the action of endogenous 5-HT into the same brain area has an antiaversive effect, like BZD and GABAA agonists. Furthermore, microinjection of midazolam, of the NMDA receptor antagonist AP-7, or of the 5-HT1A/1B receptor blocker propranolol increased the exploration of the open arms of the elevated plus-maze, having therefore an anxiolytic effect. These results point to an inhibitory role of the GABA-BZD system in both the AM and the PAG. In contrast, 5-HT seemingly enhances conditioned fear in the AM, while inhibiting unconditioned fear in the PAG. Thus, 5-HT2/1C antagonists reportedly release punished behavior when injected into the AM, whereas they antagonized the antiaversive effect of 5-HT, zimelidine and 5-HT1A/1B receptor blockers in the PAG. Since reported clinical studies revealed that one of such compounds, ritanserin, relieves generalized anxiety but tends to aggravate panic disorder, a relationship may be established between the AM and anxiety and the PAG and panic.

Multiple serotonin mechanisms in animal models of anxiety: environmental, emotional and cognitive factors

Responses to serotonergic drugs in animal models of 'anxiety' are reviewed with emphasis on the elevated X-maze. Evidence for the 'classic' hypothesis, that decreasing serotonergic function is anxiolytic and increasing it anxiogenic, is most consistent in models of behavioural inhibition where the stimulus inhibits an approach response (conflict models). However, paradoxical drug effects are also frequent, especially where the aversive stimulus evokes an active response. Both types of drug effect are equally frequent in the elevated X-maze. 'Anxiety' models may detect multiple sites and mechanisms of action of the same drug; this may indicate multiple anxiety-related neurological mechanisms in the brain. However, not all drug effects in 'anxiety' models are necessarily related to anxiety itself. It is possible that cognitive factors may affect stimulus evaluation, and response inhibition by an aversive stimulus may be a special case of a wider role for serotonin in behavioural control. Clinical implications of these observations are considered.

Role of resocialization and of 5-HT1A receptor activation on the anxiogenic effects induced by isolation in the elevated plus-maze test

Rats were isolated for periods varying from 1 h to 2 weeks and the exploratory activity of these animals on the elevated plus-maze was studied. Rats isolated from periods of 2 h on displayed a significant reduction in the number of entries and time spent in the open arms of the plus-maze compared to socially housed controls. This effect was not correlated with the decrease in the total number of entries also produced by isolation. Acute treatment with midazolam or resocialization for a 24-h period clearly reversed these responses produced by prior 2-h isolation in the elevated plus-maze. It is suggested that exposure to a 2-h isolation period could be a useful nonpharmacological means of generating anxiety in laboratory rodents. Chronic treatment, but not acute treatment, with gepirone, a 5-HT1A agonist, inhibited the anxiogenic effects caused by a 2-week period of isolation. The reduction in aversiveness promoted by resocialization may be due to a recovery in the 5-HT activity depressed by isolation in a much faster way than observed with chronic gepirone administration.

Viruses and behavioural changes: a review of clinical and experimental findings

This review focuses on behavioural neurovirology. Profound changes in behaviour are observed following infection of the central nervous system by some viruses. Irritability, insomnia, hyperactivity and learning disability are some of the behavioural disturbances that have been described in both humans and animals with central nervous system infection. The reticular core neurons which innervate the entire brain play an important role in regulating behaviour. Some of these neurons--locus coeruleus, raphe and diagonal bands--send projections to the olfactory bulbs and can be targets for exogenous agents attacking the olfactory epithelium. In infant rats, vesicular stomatitis virus is transported along the olfactory pathway by retrograde transport and reaches the reticular core neurons causing destruction of raphe, diagonal bands and, to a lesser extent, the locus coeruleus. As the neurons degenerate, the viral antigens disappear and the animals sustain severe deficits in neurotransmitter levels and behaviour. Such a "hit and run" effect of the virus suggests the possibility that a similar mechanism may be operating in some human disorders. Apart from their intrinsic interest as possible aetiological factors, viruses may provide valuable tools in experimental work seeking to correlate behaviour, morphology and neurotransmitter function.

An assessment of the elevated X-maze for studying anxiety and anxiety-modulating drugs

The elevated X-maze has strong claims to validity as an animal model of anxiety, both in theoretical basis and drug responses. The model is sensitive to actual and putative anxiolytics, but because insufficient time has elapsed since its discovery, no agent first predicted to be anxiolytic in the elevated X-maze has been brought into general use yet. It has an advantage in detecting anxiolytic and anxiogenic agents under the same operating conditions. The design and execution of experiments with the model is discussed and it is shown that baseline arm preference and the size or direction of drug effects differ in the procedural factors affecting them. Because it presents features of both passive and active avoidance and approach/avoidance conflict, it may prove able to detect drug effects in different forms of human anxiety and aid in understanding their neurobiology.

A comparison of the effects of dorsal or median raphe injections of 8-OH-DPAT in three operant tasks measuring response inhibition

Experiments were conducted to examine the effects of reducing the activity of 5-hydroxytryptamine (5-HT) containing neurons originating in the dorsal raphe (DR) or median raphe (MR) on behavioural inhibition resulting from non-reward. Groups of rats equipped with guide cannulae aimed at either the DR or MR were trained to bar press for food reward in three different operant paradigms. The 5-HT agonist 8-OH-DPAT was then infused into either the DR or MR to suppress the activity of 5-HT neurons and the effects on response inhibition resulting from the omission of reward were measured. At doses of 0.2 and 1 microgram 8-OH-DPAT injected into the MR increased responding during extinction of a continuously reinforced response. Similar injections into the DR failed to alter responding. In an omission training paradigm, requiring animals to withhold responding for a period of at least 20 s to receive free reward, rats treated with 1 microgram 8-OH-DPAT in the MR showed significantly higher levels of responding. Normal inhibition was observed following DR injections of 8-OH-DPAT. In a third paradigm both DR and MR injections of 8-OH-DPAT impaired accuracy of responding in a signalled go/nogo successive discrimination. In the case of DR treatment this resulted from a decrease in responding during reward periods signalled by S+. However, 5 micrograms 8-OH-DPAT in the MR significantly increased responding during periods of non-reward, signalled by S-. The pattern of results across these paradigms shows that reducing 5-HT activity at the level of the MR results in a failure to demonstrate normal behavioural inhibition induced by non-reward. These results support the notion that MR 5-HT neurons may be involved in controlling behavioural inhibition by detecting signals of non-reward, which then act to suppress ongoing behaviour. Serotonergic neurons arising from the DR do not appear to be involved in mediating behavioural inhibition resulting from omission of reward.

Yohimbine potentiates active defensive responses to threatening stimuli in Swiss-Webster mice

Yohimbine HCl, an antagonist at alpha 2-noradrenaline receptors with putative panicogenic effects in human subjects, was administered to Swiss-Webster mice at doses of 0.5, 1.0, and 2.0 mg/kg. Animals were then tested in two defense test batteries. Yohimbine produced increases in flight from an approaching/contacting human and potentiated animals' reactions to dorsal contact. During a 5-min exposure to a cat (separated from the mouse by a wire-mesh screen) and the 15-min period thereafter, yohimbine produced a dose-dependent pattern of changes in defensive behaviors that included increases in locomotion, transits from one segment of the test chamber to another, fore- and hindpaw wall climbing, screen climbing and hanging, and roof pushing. Crouching (relative immobility while in a hunched-back posture) was notably decreased at all doses. During the postcat period, two different response patterns, "high-escape" and "low-escape," characterized in part by high and low frequencies of wall climbing, were observed in cat-exposed groups. In yohimbine-injected mice, the low-escape behavior pattern also included a tendency to avoid the segment of the test chamber closest to the cat compartment. Both patterns differed from the crouching and immobility generally exhibited by vehicle-injected, cat-exposed controls. It was suggested that yohimbine effected these behavioral changes by either potentiating neural mechanisms mediating flight or inhibiting mechanisms mediating freezing. This model may have some utility for the investigation of panicogenic and antipanic compounds and may contribute insights into the etiology of panic disorder.

Effect of diazepam on cortical 5-HT release and behaviour in the guinea-pig on exposure to the elevated plus maze

Previous studies have used the elevated plus maze to test for "anxiolytic" drugs in rats. The present study demonstrates that guinea-pigs handled daily from birth exhibit similar behaviour to rats on the plus maze. Pretreatment with diazepam (1.0 mg/kg) significantly increased the time the animals spent in the open arms and amount of entries into the open arms. Using intra-cortical microdialysis on exposure of the guinea-pig to the elevated plus maze resulted in increased extracellular 5-HT in the frontal cortex. Diazepam reduced, but not significantly, the increase in extracellular 5-HT and produced an "anxiolytic" profile of behaviour. Pretreatment with the benzodiazepine antagonist flumazenil (10.0 mg/kg) fully antagonised the behavioural effects of diazepam. Flumazenil also reduced the effect of diazepam on the increase in extracellular 5-HT observed on exposure of the guinea-pig to the elevated plus maze. Flumazenil alone decreased basal extracellular cortical 5-HT but had no effect on behaviour in the elevated plus maze. The results show that an increase in extracellular 5-HT occurs in the guinea-pig exposed to aversive conditions. While it remains to be determined whether the "anxiolytic" effects of diazepam in the guinea-pig are causally associated with decreased extracellular 5-HT, it is of interest that the selective benzodiazepine antagonist also prevented the increase in basal extracellular 5-HT produced by the exposure to the elevated plus maze but had no effect on behaviour. Results indicate that there is no simple relationship between inhibition of 5-HT release and the "anxiolytic" action of benzodiazepines.

Clomipramine, a better reference drug for panic/agoraphobia. I. Effectiveness comparison with imipramine

An 8-week, double-blind, flexible-dose trial comparing low doses of clomipramine (mean=50 mg) with moderate doses of imipramine (mean=113.8 mg and propanteline (active placebo), was carried out in 60 out-patients with panic disorder with or without agoraphobia. Efficacy evaluation included global, anxiety and depression rating scales, and the determination of rates of relapse over up to 10 weeks of single-blind placebo follow-up. Both tricyclics were significantly more effective than propanteline, but clomipramine tended to act faster and more consistently than imipramine on most measures. Given the degree of blindness achieved and the significantly lower doses of clomipramine, this seems a better reference drug than imipramine for clinical trials in panic/agoraphobia.

Clomipramine and initial worsening in panic disorder: beyond the 'jitteriness syndrome'

The time course and the phenomenology of the initial response to clomipramine (10-20 mg/day) was investigated in 70 patients with panic disorder with or without agoraphobia. Nineteen (27.1%) patients reported worsening of their clinical state which, on average, began 28 h after the first dose and lasted for 5 days. Increase in the frequency and severity of panic attacks was the most frequent finding (14 patients), followed by psycho- stimulant (jitteriness; n=9), depressive (n=8) and tonic anxiety symptoms (n=7). This pattern of initial worsening is different from the 'jitteriness syndrome' described for other antidepressants. Its implications for the understanding of the pathophysiology of panic disorder are discussed.

5HT drugs in animal models of anxiety

It has been widely accepted that 5HT neurones promote anxiety, in humans as well as in animal models. This could be termed the "classic" hypothesis and it has led to a determined search for drugs which reduce 5HT function, especially agents which have selective actions at 5HT receptor subtypes. However, these novel agents tend to have weak and/or variable effects in animal models and more detailed examination of their actions suggests that not all findings are accounted for by the classic hypothesis. There appear to be circumstances in which increased 5HT activity can reduce anxious behaviour. There is increasing evidence for multiple anxiety mechanisms, which may be able to explain differential patterns of drug effects within and between models. Animal models of anxiety may also detect non-anxiety factors: effects on cognition or on impulsivity could be reflected in some models. This could be important in the light of recent evidence that 5HT-selective reuptake inhibitors are effective in impulsivity disorders. The classic hypothesis of 5HT function in anxiety may be only one part of an increasingly complex story. Unravelling the rest of this story is likely to lead to new insights in our understanding of anxiety and related disorders.

5-HT 1A receptors and changes in extracellular 5-HT in the guinea-pig prefrontal cortex: involvement in aversive behaviour

The study demonstrates that guinea-pigs handled daily from birth exhibit similar behaviour to rats on the elevated plus maze. Simultaneously performed intra-cortical microdialysis showed that placing guinea-pigs on the elevated plus maze results in an increase in extracellular 5-HT in the frontal cortex. 8-OH-DPAT (0.3 mg/kg s.c.) reduced significantly the increase in cortical extracellular 5-HT and resulted in an 'anxiolytic' profile of behaviour. Pre-treatment with the 5-HT(1) antagonist methiothepin (10.0 mg/kg) antagonized the behavioural effects and prevented the increase in cortical extracellular 5-HT normally produced by exposure to the elevated plus maze. Methiothepin alone had no effect on behaviour in the elevated plus maze. 5-CT (0.1 mg/kg i.p.) abolished the rise in extracellular 5-HT, but did not produce an 'anxiolytic' profile of behaviour on the elevated plus maze. The results show that an increase in cortical extracellular 5-HT occurs in the saline- treated guinea-pig exposed to aversive conditions. It remains to be determined whether the 'anxiolytic' effect of 8-OH-DPAT in the guinea-pig is causally associated with decreased basal extracellular 5-HT. The results however indicate that there is no simple relationship between inhibition of 5-HT release and behaviour indicative of 'anxiolytic' drug actions.

Effects of beta-adrenoceptor ligands in the elevated X-maze 'anxiety' model and antagonism of the 'anxiogenic' response to 8-OH-DPAT

Effects of β-adrenoceptor agonists and antagonists have been examined on open/total arm entry ratio (OTR) over a wide range of doses in the rat elevated X-maze 'anxiety' model. For clenbuterol, terbutaline and adrenaline, OTR was reduced only at doses at or above those reducing total entries; dobutamine was inactive. Neither the β(1)-adrenoceptor antagonist metoprolol nor the β( 2)-adrenoceptor antagonist ICI 118,551 affected OTR or total entries. The peripherally acting β(1)-antagonist practolol was also inactive. The elevated X-maze therefore does not appear to detect β-adrenoceptor-mediated changes in 'anxiety'. Among the β-adrenoceptor antagonists which also bind to 5-HT(1) receptors, sotalol and timolol were inactive but restricted doses of alprenolol (0.1 mg/kg) and pindolol (0.1-0.25 mg/kg) caused an anxiolytic-like increase in OTR while propranolol (5-10 mg/kg) and pindolol (1.0 mg/kg) reduced OTR without affecting total entries. These effects may be attributable to the activity of these agents at 5-HT( 1) receptors. Since metoprolol (3.0 mg/kg) and ICI 118,551 (1.0 mg/kg) did not alter the fall in OTR caused by the selective 5-HT(1A) agonist 8-OH-DPAT, the antagonism of this fall by alprenolol (0.5 mg/kg), pindolol (0.5 mg/kg), propranolol (3.0 mg/kg) and timolol (3.0 mg/kg) is most likely to represent a 5-HT(1) receptor antagonist effect of these agents.

Effect of lesions of the ascending 5-hydroxytryptaminergic pathways on choice between delayed reinforcers

The possible involvement of the ascending 5-hydroxytryptaminergic (5HTergic) pathways in determining the effectiveness of delayed positive reinforcers was investigated using Mazur's (1984) adjusting-delay paradigm. Fourteen rats received injections of 5,7-dihydroxytryptamine into the dorsal and median raphe nuclei; 12 rats received sham lesions. The rats made repeated choices in a two-lever operant conditioning chamber between a smaller reinforcer delivered after a 2-s delay and a larger reinforcer delivered after a variable delay, the length of which was determined by the subject's previous choices. When the two reinforcers consisted of one and two food pellets, the "indifference point" (the delay to the larger reinforcer that rendered the two reinforcers equally effective) was shorter in the lesioned group than in the control group. Increasing the sizes of the reinforcers to three and six pellets reduced the indifference point in both groups and abolished the between-group difference. The levels of 5HT and 5-hydroxyindoleacetic acid (5HIAA) in the parietal cortex, hippocampus, amygdala, nucleus accumbens and hypothalamus were greatly reduced in the lesioned group, but the levels of noradrenaline and dopamine were not significantly affected. The results are consistent with the suggestion that the 5HTergic pathways play a role in maintaining the effectiveness of delayed reinforcers.

Actions and some interactions of 5-HT1A ligands in the elevated X-maze and effects of dorsal raphe lesions

Effects of 5-HT1A agonists and partial agonists on open/total arm entry ratio (OTR) have been examined in the elevated X-maze anxiety model. 8-OH-DPAT (0.05-0.2 mg/kg), RU 24969 (0.5-2.0 mg/kg) and BAY R 1521 (0.1-1.2 mg/kg) produced dose-dependent reductions in OTR, signifying anxiogenic effects. Buspirone reduced OTR only at doses (0.25-5.0 mg/kg) decreasing total entries; gepirone (0.1-5.0 mg/kg) was inactive. Ipsapirone (0.25-5.0 mg/kg) increased OTR and at 1.0 mg/kg antagonised the anxiogenic action of 8-OH-D-PAT, RU 24969 and BAY R 1531. Gepirone (2.5 mg/kg) failed to antagonise 8-OH-DPAT, but the dose was limited by its effect on total entries. The anxiogenic effect of a low dose of 8-OH-DPAT was also prevented by p-chlorophenylalanine (p-CPA) pretreatment and reversed to anxiolytic by 5,7-dihydroxytryptamine lesions of dorsal raphe, which spared median raphe. These lesions also abolished the anxiolytic effect of ipsapirone without affecting the anxiogenic response to yohimbine. This study provides preliminary evidence that 8-OH-DPAT may be capable of acting as an agonist and ipsapirone as an antagonist at a presynaptic site related to dorsal raphe which is separate from the site of action of yohimbine. 5-HT1A agonists and partial agonists may have multiple sites and/or mechanisms of action in the elevated X-maze.