5-HT and mechanisms of defence

Central serotonin depletion impairs both the acquisition and performance of a symmetrically reinforced go/no-go conditional visual discrimination

The involvement of central serotonin systems in behavioural disinhibition in the rat was assessed using a symmetrically reinforced go/no-go conditional visual discrimination task. Selective central 5-HT depletion (generally averaging more than 90% in neocortex, hippocampus and striatum) was induced by intracerebroventricular administration of the neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) following pretreatment with both a noradrenergic and dopaminergic re-uptake inhibitor. The lesioned animals failed to acquire the conditional visual discrimination. This deficit was due to an inability to withhold responding and thus correctly complete the no-go trials. The lesioned animals responded faster both correctly, during go trials, and incorrectly during no-go trials. Impulsive early responding during the initial 1.2 s of the stimulus presentation was also increased by 5-HT depletion. Subjects that were lesioned after stable performance of the task had been acquired showed a similar, but smaller effect. These animals displayed more accurate performance of the go trials, but poorer performance of the no-go trials. Once again, go trial response latencies were faster and early responses during the no-go trials were increased by the lesion. The results suggest that previous accounts of impulsive responding induced by 5-HT depletion fail to recognise the pervasive nature of this effect, which affects multiple behavioural indices of response disinhibition and can impede the acquisition and performance of discrimination tasks depending on their precise response requirements.

Forebrain serotonin depletion facilitates the acquisition and performance of a conditional visual discrimination task in rats

Three experiments examined the effects of depleting forebrain 5-hydroxytryptamine (5-HT) on the acquisition and performance of an operant conditional discrimination in the visual modality. In the first experiment, rats with 5-HT lesions induced by infusing the neurotoxin 5,7-dihydroxytryptamine intracerebroventricularly acquired the conditional visual discrimination more rapidly than the sham-operated controls. Following acquisition, a series of manipulations of the task parameters tested the effects of the lesion on cognitive, sensory and motivational aspects of performance. In experiment two, the performance of rats that had acquired the task to asymptote before receiving lesions was assessed. The performance of this second group of serotonin-lesioned rats was similar to that of the pre-acquisition lesioned group following all but one manipulation of the task parameters. When the rate of stimulus presentations was increased, rats with forebrain 5-HT depletions were protected from the disruptive effects on performance seen in the sham-operated controls. This latter finding was also observed in a third experiment, in which the infusion of the 5-HT1A receptor partial agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OHDPAT), directly into the dorsal raphe nucleus improved the performance of unlesioned rats following an increase in the rate of stimulus presentations. The results are discussed in terms of the behavioural, neurochemical and neuroanatomical specificity of serotonin function in appetitive learning and the implications for general theories of the function of serotoninergic processes in cognition.

Evaluation of the elevated T-maze as an animal model of anxiety in the mouse

The elevated T-maze has been developed as an animal model of anxiety to generate both conditioned and unconditioned fears in the same rat. This study explores a version of the elevated T-maze fit for mice. Inhibitory (passive) avoidance- conditioned fear-is measured by recording the latency to leave the enclosed arm during three consecutive trials. One-way escape- unconditioned fear-is measured by recording the time to withdraw from open arms. The results showed that mice do not appear to acquire inhibitory avoidance in the standard T-maze, since their latencies to leave enclosed arm did not increase along trials. Nevertheless, the open arms seemed to be aversive for mice, because the latency to leave the enclosed arm after the first trial was lower in a T-maze with the three enclosed arms than in the standard elevated T-maze. In agreement, the exposure of mice to an elevated T-maze without shield, that reduces the perception of openness, increased significantly the latencies to leave the enclosed arm. However, the absence of the shield also increased the time taken to leave the open arms when compared to that recorded in standard T-maze. Systematic observation of behavioral items in the enclosed arm has shown that risk assessment behavior decreases along trials while freezing increases. In the open arms, freezing did not appear to influence the high latency to leave this compartment, since mice spend only about 8% of their time exhibiting this behavior. These results suggest that mice acquire inhibitory avoidance of the open arms by decreasing and increasing time in risk assessment and freezing, respectively, along three consecutive trials. However, one-way escape could not be characterized. Therefore, there are important differences between mice (present results) and rats (previously reported results) in the performance of behavioral tasks in the elevated T-maze.

Escape-related behaviours in an unstable elevated and exposed environment. I. A new behavioural model of extreme anxiety

The aim of this study was to validate a new behavioural test for extreme anxiety, designed to elicit unconditioned escape from animals on an unstable, elevated and exposed plus maze. The animals were also run on a standard elevated plus maze, so that well-validated anxiety measures could be compared with the animals' propensity to escape. The first experiment compared the behavioural repertoire of three rat strains; more escape behaviours were elicited from the Sprague Dawleys (SDs) and dark agouti (DAs) than from the hooded listers (HLs) on the unstable maze. Similarly, the SDs and DAs exhibited higher anxiety profiles than the HLs on the standard plus maze. There was a clear correlation, therefore, between the propensity to escape and well-validated anxiety measures. A further group of rats [HLs] was tested on both the unstable and standard plus mazes (Experiment 2) and principal components analyses carried out on all behavioural measures. The extracted factors identified separate and independent factors relating to escape-linked behaviours on the unstable plus maze and anxiety on the standard plus maze. The escape-linked behaviours exhibited on the unstable plus maze appear to reflect a different type of anxiety from that exhibited on the standard plus maze. The bias towards escape behaviours shown by some rats on the unstable plus maze may mimic the chronically hyper-aroused, persistent 'fight/flight' state exhibited by certain categories of anxiety patients, and hence provide a behavioural model of extreme anxiety.

Escape-related behaviours in an unstable, elevated and exposed environment. II. Long-term sensitization after repetitive electrical stimulation of the rodent midbrain defence system

The behavioural indices of anxiety/fear/panic range from freezing/inhibition of ongoing behaviour to active defence strategies i.e. fight/flight, depending on the proximity or intensity of the aversive stimulus. However, evidence suggests that when the initial stressor is sufficiently intense, the neural defence circuitry may undergo a long-term increase in sensitivity. Such sensitization has been evoked to explain the chronically hyper-aroused fight/flight state of patients suffering from extreme anxiety states. The purpose of this study was to establish whether direct activation of the rodent midbrain defence system could result in a similar long-term alteration of the animal's responses to subsequent stressful events. This was achieved by studying the behaviour of animals in a threatening environment after repetitive electrical stimulation of the superior colliculus. Animals were then placed on an unstable, elevated and exposed plus maze and their behaviour recorded. Testing was carried out regularly over 3 months. Stimulated animals reliably exhibited significantly increased levels of behaviours designed to escape the aversive conditions of the unstable plus maze. These included visual scanning, end-reaching, preparing to jump, and jumping off the apparatus. Unstimulated control animals, on the other hand, exhibited decreased levels of these behaviours post-stimulation. In contrast, the experimental animals' performance on standard anxiety tests did not differ from controls. These results demonstrate that repetitive tactile stimulation can produce a long-term change in reactions to threat, and is proposed as a functional model of extreme anxiety.

Role of cholecystokinin type B receptors in ultrasound induced behavior in rats

The cholecystokinin-tetrapeptide (CCK-4) can induce panic attacks in humans. The present study investigates the effects of CCK-4 and the CCK-B receptor antagonist L-365.260 on ultrasound induced defense behavior in the rat that may model the unconditioned aspects of panic behavior in man. CCK-4 (50 microg/kg) increased the defense response induced by ultrasound (95 dB) an effect prevented by pretreatment with L-365.260 (10 microg/kg). Compared with other antipanic/panicogenic drugs the effects of CCK-4 and L-365.260 were relatively small. In conclusion, drugs acting at the CCK-B receptor appear to have only a minor role in the modulation of an unconditioned aversive response.

Brain circuits in panic disorder

This paper reviews the pathophysiology of panic disorder (PD), within the context of newly described "fear circuitries," which have been well characterized in preclinical models. Substantial advances in the neurosciences have made it possible for clinical neuroscientists to refine our understanding of the pathophysiology of PD and the mechanisms of currently effective treatment. These advances have in turn helped generate testable hypotheses for future neurobiological and psychopharmacologic research. Perturbation of mutual modulation ("cross talk") between key brain transmitter systems (serotonin, norepinephrine, gamma-aminobutyric acid, corticotropin-releasing factor, and others) may underlie the pathogenesis of panic-anxiety. Restoration of normal homeostasis may be an important therapeutic component of antipanic therapy and may provide information about underlying neurocircuits. Neuroimaging, an important new tool, has already begun to bridge the gap between the preclinical and clinical neurosciences through confirmation of hypothesized dysfunction of the complex human prefrontal cortex and its subcortical components. In higher species, such as humans, dysfunction of cortical inhibition or excessive cortical activation of caudal limbic structures is postulated to lead to activation of the phylogenetically conserved amygdalofugal pathways. Consistent with probable subtypes of PD, overlapping theoretical models of panic neurocircuitries are proposed, including ventilatory dysregulation, which is coupled with neurovascular instability in a critical area of the panic neurocircuitry--the amygdalohippocampus. Neuroimaging appears a critical tool in guiding further elaboration of the interaction of cortical and subcortical components of the panic neurocircuitry, whereas challenge studies appear crucial in gathering further information regarding brain stem dysfunction.

Anxiolytic-like actions of BW 723C86 in the rat Vogel conflict test are 5-HT2B receptor mediated

The 5-HT2B receptor agonist, BW 723C86 (10, 30(mg/kg i.p. 30 min pre-test), increased the number of punishments accepted in a rat Vogel drinking conflict paradigm over 3 min, as did the benzodiazepine anxiolytics, chlordiazepoxide (2.5-10 mg/kg p.o. 1 h pre-test) and alprazolam (0.2-5 mg/kg p.o. 1 h pre-test), but not the 5-HT2C/2B receptor agonist, m-chlorophenylpiperazine (mCPP, 0.3-3 mg/kg i.p) or the 5-HT1A receptor agonist, buspirone (5-20 mg/kg p.o. 1 h pre-test). The effect of BW 723C86 was unlikely to be secondary to enhanced thirst, as BW 723C86 did not increase the time that rats with free access to water spent drinking, nor did it reduce sensitivity to shock in the apparatus. The anti-punishment effect of BW 723C86 was opposed by prior treatment with the 5-HT2/2B receptor antagonist, SB-206553 (10 and 20 mg/kg p.o. 1 h pre-test), and the selective 5-HT2B receptor antagonist, SB-215505 (1 and 3 mg/kg p.o. 1 h pre-test), but not by the selective 5-HT2C receptor antagonist, SB-242084 (5 mg/kg p.o.), or the 5-HT1A receptor antagonist, WAY 100635 (0.1 or 0.3 mg/kg s.c. 30 min pre-test). Thus, the anti-punishment action of BW 723C86 is likely to be 5-HT2B receptor mediated. This is consistent with previous reports that BW 723C86 exhibited anxiolytic-like properties in both the social interaction and Geller-Seifter conflict tests.

Clinical utility of the selective serotonin reuptake inhibitors in the spectrum of anxiety

The selective serotonin reuptake inhibitors (SSRIs) are now being employed in the treatment of the full spectrum of anxiety disorders. In comparative trials, the SSRIs are proving to be equal or superior in efficacy to traditional antianxiety medications. Due to their favorable side effect profile, safety, and tolerability, they are rapidly replacing older agents in the treatment of anxiety. Neuroanatomical pathways that may be important in the antianxiety effect of the SSRIs are outline and discussed, followed by a review of the clinical evidence supporting the efficacy of this class of medications in the treatment of anxiety disorders.

Anxiety and increased aggression as pacemakers of depression

The concept of a serotonin (5-HT)-related, anxiety and/or aggression-driven, stressor-precipitated depression is formulated and discussed. The serotonergic disturbances found in some depressed individuals, particularly those with lowered CSF 5-HIAA, are linked to the anxiety and aggression components of the depressive syndrome. In this type of depression, called 5-HT-related depression, dysregulation of anxiety and/or aggression is hypothesized to be primordial and mood-lowering is a derivative phenomenon. In other words, this is a group of anxiety/aggression-driven depressions. The author submits that the serotonergic impairment in certain types of depression is a trait phenomenon, i.e. it persists during remission. This disturbance makes the individual susceptible to perturbation of anxiety and aggression regulation. Anxiety and (overt or suppressed) anger are core constituents of the stress syndrome. Thus the serotonergic disturbance will induce a heightend sensitivity to stressful events, i.e. the latter will induce stress phenomena, including anxiety and anger, more readily than normal. The latter psychological features induce lowering of mood, and thus 'drive' the patient into a full-blown depression. Furthermore, it is predicted that anxiolytics and serenics (i.e. anti-aggressive drugs) that act via normalization of serotonergic circuits will exert an antidepressant effect in 5-HT related depression, in addition to their therapeutic actions in anxiety disorders and states of increased aggressiveness, respectively. The exact nature of the serotonergic impairment in 5-HT-related depression has yet to be elucidated.

Information processing in anxiety: a pilot study of the effect of manipulating 5-HT function

We gave the indirect 5-HT agonist, D-fenfluramine (30 mg), the 5 -HT antagonist methysergide (2 mg) and placebo to 11 patients with an anxiety disorder and 12 controls in a double-blind, balanced-order, cross-over design. Compared to controls, patients had significantly higher anxiety ratings, were slower in naming colours in computerized Stroop tests and showed greater interference for anxiety-related words. Patients tended to have a lower critical flicker fusion threshold (CFFT) than controls and in the subjects taken as a whole there was a significant inverse correlation between CFFT and Spielberger state (r=-0.54, p<0.01) and trait anxiety (r=-0.55, p<0.01). Neither drug had significant effects on anxiety ratings or on Stroop interference. D-Fenfluramine significantly increased CFFT (p < 0.02) and methysergide non-significantly reduced CFFT with no significant differences between patients and controls. 5-HT may be involved in lower-level visual information processing but we found no evidence for its direct involvement in the attentional bias for anxiety-related information in the emotional Stroop.

Antiaversive effects of 5HT2C receptor agonists and fluoxetine in a model of panic-like anxiety in rats

Dose-dependent increases in threshold for operant fear/escape responses of rats submitted to aversive stimulation of the dorsolateral periaqueductal gray (dPAG) were recorded following intraperitoneal injection of three chemically unrelated but selective 5HT2C receptor agonists (Ro 60-0175, Org 12962 and Ro 60-0332) and fluoxetine. The decreased sensitivity of rats to the acute panic-like aversion elicited by stimulation of this limbic periventricular region was detected at dosages devoid of impairing effects on the latencies needed for operant brain stimulation interruption. In this paradigm which has been validated as a simulation of acute anxiety with relevance to panic disorder, the selective activation of 5HT2C receptors by Ro 60-0175, Org 12962 or Ro 60-0332 induces effects analogous to those observed following benzodiazepine receptor activation by antipanic agents such as clonazepam or alprazolam or following non-selective and indirect 5HT receptor activation by fluoxetine. Potency and efficacy of 5HT2C receptor agonists were intermediate between those of clonazepam and fluoxetine, indicating authentic antiaversive properties and suggesting antipanic potential for these 5HT2C receptor agonists. In addition, these data suggest that the 5HT2C receptor subtype may play a major role in the therapeutic properties of selective serotonin reuptake inhibitors. It is also speculated that serotonin/benzodiazepine interactions existing in the brain may functionally involve the 5HT2C receptor subtypes and that the anxiogenic action reported under certain circumstances for 5HT mimetics are not mediated by 5HT2C receptor subtypes.

Behavioral effects of phenelzine in an experimental model for screening anxiolytic and anti-panic drugs: correlation with changes in monoamine-oxidase activity and monoamine levels

This study investigated the effects of acute and chronic (one daily i.p. injection for 14 days) treatments with the non-selective irreversible monoamine-oxidase (MAO) inhibitor phenelzine (10 and 30 mg/kg) on defensive behaviors of Swiss mice in the mouse defense test battery (MDTB) which has been designed for screening anxiolytic and anti-panic drugs. In the MDTB, subjects were confronted with a natural threat (a rat) and situations associated with this threat. MAO-A and MAO-B activities and levels of brain monoamines (serotonin (5-HT), dopamine (DA) and norepinephrine (NE)) and their deaminated metabolites were subsequently measured. Behavioral results showed that acute administration of phenelzine did not specifically modify defensive behaviors. By contrast, after chronic treatment, phenelzine produced a significant reduction in avoidance distance when the rat was approaching, an effect which is consistent with an anti-panic-like action. In addition, phenelzine displayed weak anxiolytic-like effects as it increased risk assessment responses when mice were constrained in one part of the apparatus facing the rat which remained at a constant distance. No other specific drug effect was observed. These behavioral changes were associated with a dramatic increase in 5-HT levels, in particular after chronic treatment, while levels of DA and NE increased only slightly. Importantly, no significant differences in DA and NE levels between acute and chronic regimens were observed. Levels of deaminated metabolites of monoamines were markedly decreased. Measurements of MAO activity revealed substantial reductions in both type A and B forms with a full inhibition of both forms being observed only after chronic treatment with phenelzine. These results suggest that the effects of phenelzine may be due mainly to its effects on the 5-HT system and presumably related to the full inhibition of MAO-A and/or MAO-B.

Serotonin and panic

BACKGROUND

Tricyclic (TCA) and monoamine oxidase inhibitor (MAOI) antidepressants are effective in the treatment of panic disorder. Two selective serotonin reuptake inhibitors (SSRIs) have also been licensed in the UK for this indication and studies with three other SSRIs have recently been completed. These provide further evidence for the role of serotonin in panic.

METHOD

Review of clinical, animal and laboratory studies.

RESULTS

SSRIs have been shown to be effective in the treatment of panic disorder. The reported rates of improvement of 60-70% of patients taking SSRIs are similar to those seen with TCAs and greater than placebo. Other serotonergic agents do not appear to be effective. Animal work and human studies including measures of 5-HT in plasma, cerebrospinal fluid and platelets, challenge paradigms and tryptophan depletion show that the relationship between 5-HT and anxiety is complex.

CONCLUSION

Clinical trials have shown that of all the serotonergic agents only the SSRIs are effective in panic disorder. They are as beneficial as the TCAs and seem to be better tolerated which may be particularly significant in view of the chronic nature of the condition. Serotonin plays a role in panic disorder and serotonergic dysfunction, however the results and evidence do not fit one theory alone. It is also likely that different brain regions and 5-HT receptors are involved in specific ways.

Psychoneuroendocrinology of anxiety disorders

This article focuses on neuroendocrine measures in anxiety disorders and their relationships to neurotransmitter and neuroendocrine function. In particular, the hypothalamic-pituitary-somatotropin and the hypothalamic-pituitary-adrenal (HPA) axes are emphasized, and a role for extrahypothalamic corticotropin releasing factor is proposed. Additional neuroactive hormones are also considered. A nonhuman primate model of anxiety is discussed in terms of its neuroendocrine relevance. And, throughout, a hypothetical functional-anatomic model for anxiety and panic is proposed using the findings of cognitive neuroscience fear research. Finally, an effort is made to synthesize existing psychoneuroendocrinologic data into a current conceptualization of the pathophysiology of anxiety disorders.

Regulation of contextual conditioning by the median raphe nucleus

The median raphe nucleus (MRN) has been suggested as the origin of a behavioral inhibition system that projects to the septum and hippocampus. Electrical stimulation of this mesencephalic area causes behavioral and autonomic manifestations characteristic of fear such as, freezing, defecation and micturition. In this study we extend these observations by analyzing the behavioral and autonomic responses of rats with lesions in the MRN submitted to a contextual conditioning paradigm. The animals underwent electrolytic or sham lesions of the median raphe nucleus. One day (acute) or 7 days (chronic) later they were tested in an experimental chamber where they received 10 foot-shocks (0.7 mA, 1 s with 20-s interval). The next day, sham and MRN-lesioned animals were tested again either in the same or in a different experimental chamber. During this, the duration of freezing, rearings, bouts of micturition and number of fecal boli were recorded. Sham-operated rats placed in the same chamber showed more freezing than rats exposed to a different context. This freezing behavior was clearly suppressed in rats with acute or chronic lesions in the MRN. MRN lesions also reduced the bouts of micturition and number of fecal boli. These rats showed a reduced number of rearings than sham-lesioned rats. This effect is probably the result of the displacement effect provoked by freezing since no significant differences in the number of rearings could be observed between these animals and the NMR-lesioned rats tested in an open field. This lesion produced higher horizontal locomotor activity in this test than the controls (sham-lesioned rats). These results point to the importance of the median raphe nucleus in the processing of fear conditioning with freezing being the most salient feature of it. Behavioral inhibition is also under control of MRN but its neural substrate seems to be dissociated from that of contextual fear.

Changes in plasma prolactin during SSRI treatment: evidence for a delayed increase in 5-HT neurotransmission

We studied the effect of the selective serotonin reuptake inhibitor (SSRI), paroxetine, on basal plasma prolactin concentrations in 11 healthy subjects. Subjects were tested before paroxetine, and after 1 and 3 weeks of treatment (20 mg daily). On each test occasion prolactin levels were sampled before and following administration of a placebo capsule, for a total of 4 h. After 3 weeks paroxetine treatment plasma prolactin levels were significantly higher than those seen either pre-treatment or after 1 week of treatment. In contrast, 1 week of paroxetine treatment did not significantly increase prolactin concentrations over pre-treatment values. Plasma concentrations of paroxetine did not differ between 1 and 3 weeks of treatment. The secretion of plasma prolactin is, in part, under the tonic regulation of serotonergic pathways and the present results therefore support animal experimental data suggesting that SSRIs produce a delayed increase in some aspects of brain serotonin neurotransmission.

Brain 5-HT neurotransmission during paroxetine treatment

BACKGROUND

Animal experimental studies suggest that repeated administration of selective serotonin reuptake inhibitors (SSRIs) produces complex adaptive changes in brain serotonin (5-HT) pathways. The effect of these adaptive changes on different aspects of brain 5-HT neurotransmission and their clinical consequences are not well understood.

METHOD

We studied the effect of repeated administration of the SSRI, paroxetine (20 mg daily), on the cortisol responses to the 5-HT precursor, 5-hydroxytryptophan (5-HTP), in healthy subjects and depressed patients.

RESULTS

In healthy subjects, following one week of paroxetine treatment there was a large increase in the cortisol response to 5-HTP. This increase had all but disappeared following 3 weeks treatment. In contrast, in depressed patients treated with paroxetine for 8 weeks, the cortisol response to 5-HTP was significantly increased.

CONCLUSIONS

SSRI treatment in depressed patients produces a persistent increase in the cortisol response to 5-HTP, a probable measure of neurotransmission at central 5-HT2 receptors. Potentiation of 5-HT2 neurotransmission is unlikely to account for the efficacy of SSRIs in major depression but might underlie their actions in obsessive-compulsive disorder and also perhaps certain of their adverse effects, notably sexual dysfunction.

The long term effects of childhood sexual abuse: rationale for and experience of pharmacotherapy with nefazodone

Studies estimate that 1 in 6 women and 1 in 10 men experience childhood sexual abuse (CSA). Whilst ill-health is not an inevitable consequence, approximately 20% of victims go on to have serious long-term pathology. This is reflected in an excess incidence of CSA survivors in problem populations, be they medical, forensic or psychiatric. Four groups of research studies suggest that PTSD-related mechanisms might be a major mediating factor in the development of symptoms: 1. There is high incidence of PTSD following sexual trauma; 2. Psychometric studies show evidence of impaired limbic functions; 3. There is evidence of neuroendocrine disturbances similar to those seen in war veterans with PTSD; 4. MRI studies show evidence of hippocampal atrophy. There is a growing database of studies showing that drugs that act as seroionergic modulators are effective in the treatment of PTSD. Drugs such as nefazodone which block 5HT2 receptors and inhibit the uptake of serotonin may be of particular value. Studies suggest that trauma-related issues are not always addressed in patients with a history of abuse. Given the problems posed, there is a need to review their treatment in the light of emergent knowledge. Further research is needed to validate these findings.

Serotonergic systems

Since its identification in neurons of the central nervous systems (CNS), serotonin (5-HT) has been implicated in the pathophysiology of psychiatric disorders, such as schizophrenia, depression, generalized anxiety disorder (GAD), panic disorder (PD), and obsessive-compulsive disorder (OCD). These findings have stimulated research on brain 5-HT pathways, especially during the last two decades as more selective drugs have been introduced into medical practice. This article reviews selected topics relevant for psychiatry.

Role of 5-HT2A and 5-HT2C receptor subtypes in the two types of fear generated by the elevated T-maze

To study the role of 5-HT2A and 5-HT2C receptor subtypes in anxiety, the behavioral effects of drugs that either block or stimulate these receptors were measured in an animal model of anxiety, the elevated T-maze. One arm of the maze is enclosed by walls and stands perpendicular to the two open arms. Inhibitory (passive) avoidance--representing learned fear--was measured by placing a rat at the end of the enclosed arm and recording the time to leave this arm with the four paws during three consecutive trials. After 30 s, the same animal was placed at the end of one of the open arms and the time to leave this arm with the four paws was recorded. This one-way escape response represents unconditioned fear. The I.P. injection of the preferential 5-HT2C receptor agonists mCPP and TFMPP (0.1-0.8 mg/kg), 25 min before the experimental session enhanced inhibitory avoidance. In contrast, the same drugs either tended to impair (mCPP) or significantly inhibited (TFMPP) one-way escape. The preferential 5-HT2A agonist DOI (0.03-0.3 mg/kg) did not change either inhibitory avoidance or one-way escape. Inhibitory avoidance was impaired by the selective 5-HT2C antagonists SB 200646A (3.0-30 mg/kg) and SDZ SER 082 (0.1-1.0 mg/kg), by the 5-HT2A antagonist SR 46349B (1.0-10.0 mg/kg), and by the mixed 5-HT(2A,2C) antagonist ritanserin (0.3-3.0 mg/kg). However, it was not affected by the selective 5-HT2A antagonist RP 62203 (0.25-4.0 mg/kg). All the 5-HT2 antagonists used were ineffective on one-way escape. Therefore, conditioned fear seems to be tonically facilitated through 5-HT2C receptor stimulation, although the 5-HT2A receptor may also participate in its regulation. Unconditioned fear might be phasically inhibited by 5-HT2C receptor activation.

Dual role of 5-HT in defense and anxiety

Pharmacological results obtained in animals tested in approach/avoidance conflict situations have led to the suggestion that 5-HT enhances anxiety by acting on forebrain structures. In contrast, results with intracerebral drug injection associated with aversive electrical brain stimulation indicate that 5-HT inhibits aversion in the dorsal periaqueductal gray (DPAG). To reconcile this evidence, it has been suggested that 5-HT may enhance conditioned fear in the amygdala while inhibiting innate fear in the DPAG. To test this hypothesis, we used three drug treatments known to increase the release of 5-HT from terminals of the dorsal raphe nucleus (DR): (1) intra-DR microinjection of the benzodiazepine inverse agonist FG 7142, (2) intra-DR microinjection of the excitatory amino acid kainic acid and (3) intraperitoneal injection of the 5-HT releaser and uptake blocker D-fenfluramine. All drug treatments enhanced inhibitory avoidance (learned fear) in the elevated T-maze, a new animal model of anxiety. Intra-raphe kainate and D-fenfluramine also decreased one-way escape (innate fear) in the T-maze. In contrast, reduction of 5-HT release by intra-DR injection of 8-OH-DPAT impaired inhibitory avoidance without affecting one-way escape. Overall, these results agree with the above hypothesis. Clinical implications are discussed, especially with regard to panic disorder.

Differentiation of anxiolytic and panicolytic drugs by effects on rat and mouse defense test batteries

The use of ethoexperimental techniques to elicit and maximize the full range of defensive behaviors of rats and mice enables a very precise analysis of the effects of drugs on these behavior patterns. Two rat defense test batteries (the fear/defense test battery or F/DTB and the anxiety/defense test battery or A/DTB) have provided evidence that anxiolytic drugs, even from different classes, produce a common pattern of changes in specific behaviors. A recently developed mouse defense test battery (MDTB) has enabled description of mouse defensive behaviors to a predator, for comparison to those of rats, and a series of studies of drug effects on the behaviors measured in the MDTB provides evidence of cross-species generality of anxiolytic drug effects, or lack of effect, on specific defensive behaviors. In addition, tests with panicogenic and panicolytic drugs in the MDTB indicate that these enhance and reduce, respectively, flight reactions, which generally are not altered by anxiolytic compounds. Thus, results from the MDTB, taken in conjunction with those of the two rat test batteries and other defense analyses in rats and mice, provide evidence that many defensive behaviors are similar across rodent species, while the differences obtained provide a consistent pattern across situations. Moreover, the defense test batteries may be used to differentiate the effects of drugs effective against generalized anxiety as opposed to panic, through effects on specific defensive behaviors.

Ritanserin facilitates anxiety in a simulated public-speaking paradigm

The effects of ritanserin, a 5-HT2A/2C (5-hydroxytryptamine) antagonist, have been investigated in simulated public speaking with healthy volunteers. The aim was to investigate the role of 5-HT in subjective experimental anxiety. There were three experimental groups each comprising four or five males and 11 females. Subjects received placebo, ritanserin 2.5 or 10 mg, p.o. They rated themselves using the Spielberger State-Trait Anxiety Inventory and visual analogue scales factored into anxiety, sedation and discontentment scores. Autonomic measures included skin conductance and heart rate. Subjects were told, 75 min after drug or placebo ingestion, without prior warning, to prepare a 4-min speech. Measures were taken before, during and after the speech. Ritanserin prolonged the anxiety induced by the procedure on the subjective ratings but had minimal effect on autonomic responses to the procedure. The result contrasts with an anxiolytic-like effect of ritanserin on aversively conditioned autonomic responses. The present finding is compatible with animal behavioural evidence that 5-HT has distinct and opposing roles in modulating conditioned and unconditioned anxiety.

Brain 5-HT function in obsessive-compulsive disorder. Prolactin responses to d-fenfluramine

BACKGROUND

Drugs that potentiate brain serotonin (5-HT) neurotransmission are effective in the treatment of obsessive-compulsive disorder (OCD), but it is unclear whether disturbances in brain 5-HT function play a role in the pathophysiology of OCD.

METHOD

We studied the prolactin response to the selective 5-HT releasing agent d-fenfluramine in 14 non-depressed, drug-free OCD patients, and 14 healthy controls matched for age and gender.

RESULTS

The prolactin response to d-fenfluramine was significantly increased in OCD patients compared with controls.

CONCLUSIONS

The disparate results of studies of 5-HT neuroendocrine function in OCD make it unlikely that disturbances of brain 5-HT function play a central role in the pathophysiology of OCD. Increased brain 5-HT neurotransmission in non-depressed OCD subjects may represent an adaptive neurobehavioural mechanism which can be amplified to therapeutic advantage by treatment with 5-HT potentiating drugs.

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.

5-HT 1B/D receptor antagonists

1. 5-Hydroxytryptamine-1B (5-HT 1B, formerly designated 5-HT 1D beta) and 5-hydroxy-tryptamine-1D (5-HT 1D, formerly designated 5-HT 1D alpha) receptors are distinct molecular entities that mediate serotonergic neurotransmission. Both are G-protein-coupled receptors without introns in their coding region, negatively coupled to adenylate cyclase; their precise function in human beings remains to be defined. In brain, they are highly enriched in the globus pallidus and the substantia nigra. 2. Presynaptic 5-HT 1B/D receptors take part in the control of the release not only of 5-HT itself, but also of other neurotransmitters-for example, acetylcholine, glutamate, dopamine, noradrenaline and gamma-aminobutyric acid. Selective blockade of central 5-HT 1B/D autoreceptors should facilitate 5-HT neurotransmission and may offer a novel approach to antidepressant therapy. Other 5-HT 1B/D receptors are located postsynaptically; those receptors may be supersensitive in the pathophysiology of obsessive-compulsive disorder and may be a potential target for its treatment. 3. Few if any ligands show selectivity for 5-HT 1B or 5-HT 1D receptors or both. Most pharmacological studies have been performed with nonselective antagonists-for example, metergoline, I-naphthylpiperazine, methiothepin, ketanserin and ritanserin. Recently, a novel series of benzanilides have been reported as the first examples of selective 5-HT 1B/D receptor antagonists. GR 127935, a representative compound of this series, displays mixed agonist-antagonist properties both in vitro and in vivo. It induces upon systemic administration in the guinea pig either an opposite (decrease) effect or a small increase (65%, 5 mg/kg) in the concentration of cortical extracellular 5-HT compared with fluoxetine (218%, 10 mg/kg). The importance of blockade of 5-HT 1B/D receptors in the raphé region, their possible interaction with 5-HT 1A receptors, and consequent inhibition of 5-HT release in terminal 5-HT 1B/D receptor-containing regions are discussed. 4. To find out whether the available so-called 5-HT 1B/D receptor antagonists are indeed antagonists and not partial agonists, efficacy was measured at recombinant human 5-HT 1B and 5-HT 1D receptor sites by using a [35S]-GTP gamma S binding assay to membrane preparations of stably transfected rat C6-glial cell lines. Metergoline and the selective 5-HT 1B/D receptor ligands GR 127935 as well as GR 125743 showed significant intrinsic activity (43% to 69%) at the 5-HT 1D receptor subtype, whereas the nonselective ligand 1-naphthylpiperazine yielded less (15% to 19%) intrinsic activity at both receptor subtypes. In contrast, the nonselective ligands methiothepin, ketanserin and ritanserin are inverse agonists because they displayed negative efficacy (-14% to -28%). Differential blockade of 5-HT 1B/D receptors by neutral antagonists and inverse agonists is discussed in relation to the 5-HT tone on 5-HT 1B/D receptors. 5. It can concluded that 5-HT 1B/D receptor ligands modulate 5-HT neurotransmission through a terminal 5-HT 1B/D receptor. Future work should be directed toward the identification of selective 5-HT 1B and 5-HT 1D receptor ligands that display either neutral antagonist or inverse agonist properties to evaluate the therapeutic potential of 5-HT 1B/D receptor blockade.

Paradoxical increase of exploratory behavior in the elevated plus-maze by rats exposed to two kinds of aversive stimuli

Albino rats were submitted to a 24-h period of social isolation (individual housing) combined with 0, 1, 2 or 3 twenty-four-hour periods of exposure to different vivaria (novelty) and tested in the elevated plus-maze. Results, reported as mean +/- SEM for N = 12, show that the time (in seconds) spent in the open arms by rats exposed to novelty for 0, 1, 2 and 3 days was 28.3 +/- 4.4, 31.6 +/- 3.2, 29.1 +/- 3.5 and 25.0 +/- 3.3, respectively, when grouped in the same vivarium; 29.6 +/- 2.7, 7.6 +/- 2.1, 9.6 +/- 4.4 and 28.5 +/- 3.7 when grouped in different vivaria; 2.9 +/- 1.1, 1.8 +/- 1.0, 2.7 +/- 1.1 and 0 +/- 0 when isolated in the same vivarium, and 2.6 +/- 1.1, 31.5 +/- 8.2, 24.8 +/- 4.2 and 0 +/- 0 when isolated in different vivaria. The number of entries into the open and closed arms followed a similar trend. This indicates that, separately, both exposure to novelty and isolation are aversive manipulations. Paradoxically, when novelty was combined with a concomitant 24-h period of social isolation prior to testing, the decrease in exploratory behavior caused by either of the two aversive manipulations alone was reverted. These results are indicative that less intense anxiety triggers mechanisms mediating less energetic behavior such as freezing, while higher levels trigger mechanisms mediating more vigorous action, such as flight/fight behavior, since the combination of two aversive situations resulted in more exploratory behavior than with either alone. They are also suggestive of habituation to the effects of novelty, since exposure to it for 3 days produced exploratory behavior similar to that of controls.

Involvement of the cholinergic system and periaqueductal gray matter in the modulation of tonic immobility in the guinea pig

Unilateral microinjection of carbachol (CCh, 1.0 microg/0.2 microl) into the ventrolateral periaqueductal gray matter (vPAG) increased the duration of tonic immobility (TI) episodes induced by postural inversion and by movement restriction maneuvers in adult male guinea pig (Cavia porcellus), while stimulation with the same drug at the same concentration into the dorsolateral and dorsomedial periaqueductal gray matter (dl/dmPAG) decreased the duration of TI. Pretreatment with atropine (7.6 microg/0.4 microl) showed that the action of CCh is mediated by muscarinic receptors in the ventrolateral PAG but not in the dorsomedial and dorsolateral regions. These data suggest that the PAG and the cholinergic system are involved in the modulation of TI episodes and that different regions of the guinea pig PAG play distinct roles in the organization of this behavior.

Anxiolytic-like actions of the selective 5-HT4 receptor antagonists SB 204070A and SB 207266A in rats

The highly selective 5-HT4 receptor antagonists, SB 204070A (0.001-0.1 mg/kg s.c., 30 min pretest) and SB 207266A (0.01, 1 and 10 mg/kg p.o., 1 hr pre-test), increased time spent in social interaction without affecting locomotor activity, in a rat 15 min social interaction test under high light, unfamiliar conditions. At 1 and 10 mg/kg s.c., SB 204070A was no longer active. These results are consistent with the profile expected of anxiolytic treatments in this procedure. In a rat 5 min elevated x-maze test, SB 204070A (0.01 and 1 mg/kg s.c., 30 min pre-test) significantly increased the percentage of time spent on the open arms. SB 204070A (0.01 mg/kg s.c.) and SB 207266A (1 mg/kg p.o., 1 hr pre-test) also increased percentage entries to the open arms. Neither compound affected locomotion at any dose tested in the procedure. The effects of both compounds in this procedure are also consistent with anxiolysis. Neither SB 204070A (0.1 or 1 mg/kg s.c., 30 min pre-test) nor SB 207266A (0.1 or 1 mg/kg p.o., 1 hr pre-test) affected either unpunished or punished responding, in a rat Geller-Seifter conflict model of anxiety. The maximal efficacy of both SB 204070A and SB 207266A in the rat social interaction test was similar to that of the benzodiazepine anxiolytic chlordiazepoxide (5 mg/kg s.c. or p.o.) used as a positive control, but was considerably less in the elevated x-maze procedure. The results suggest that 5-HT4 receptor antagonists may have modest anxiolytic-like actions in rats.

Animal models of anxiety: an ethological perspective

In the field of anxiety research, animal models are used as screening tools in the search for compounds with therapeutic potential and as simulations for research on mechanism underlying emotional behaviour. However, a solely pharmacological approach to the validation of such tests has resulted in distinct problems with their applicability to systems other than those involving the benzodiazepine/GABAA receptor complex. In this context, recent developments in our understanding of mammalian defensive behaviour have not only prompted the development of new models but also attempts to refine existing ones. The present review focuses on the application of ethological techniques to one of the most widely used animal models of anxiety, the elevated plus-maze paradigm. This fresh approach to an established test has revealed a hitherto unrecognized multidimensionality to plus-maze behaviour and, as it yields comprehensive behavioural profiles, has many advantages over conventional methodology. This assertion is supported by reference to recent work on the effects of diverse manipulations including psychosocial stress, benzodiazepines, GABA receptor ligands, neurosteroids, 5-HT1A receptor ligands, and panicolytic/panicogenic agents. On the basis of this review, it is suggested that other models of anxiety may well benefit from greater attention to behavioural detail.

Contrasting effects of systemic and intracerebral infusions of the 5-HT1A receptor agonist 8-OH-DPAT on spatial short-term working memory in rats

The present study compared the effects of systemic 8-OH-DPAT (0.05, 0.1 and 1.0 mg/kg) with intra-raphe and intra-hippocampal infusions of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (10, 30 100 ng) on delayed non-matching-to-position (DNMP) performance in rats. The highest dose of 8-OH-DPAT administered systemically impaired DNMP performance in a delay-independent manner, increased premature responding and increased response bias. Infusions of 8-OH-DPAT (100 ng) into the median raphe nucleus improved performance accuracy, independent of delay whilst having no effect on any other response measure. Infusions of 8-OH-DPAT into the dorsal raphe nucleus had no effect on performance at any dose tested. Infusions of 8-OH-DPAT into the dorsal hippocampus produced a small impairment in performance which was also independent of delay. However, this decrement in performance accuracy was not accompanied by any changes in other response measures. These findings demonstrate a dissociation between the effects of stimulation of pre- and post-synaptic 5-HT1A receptors on performance of a DNMP task although the changes in performance cannot be accounted for by changes in mnemonic function.

High intensity social conflict in the Swiss albino mouse induces analgesia modulated by 5-HT1A receptors

Social conflict between mice produces analgesia in the attacked mouse. Both the magnitude and type (opioid or nonopioid) of this analgesia have been related to attack intensity and strain of mouse. In the present study low intensity social conflict (7 bites) did not produce analgesia, whereas high intensity - 30 and 60 bites - interactions produced, respectively, short-lasting (5 min) and very short-lasting (1 min) analgesia in Swiss albino mice, when compared with nonaggressive interaction (0 bite). The 30 bites aggressive interaction induced analgesia (AIIA) was not affected by IP injection of either naloxone (5.0 and 7.5 mg/kg) or diazepam (0.5, 1.0, 2.0 and 4.0 mg/kg). However, this attack-induced analgesia was reduced after IP administration of the 5-HT1A agonists, gepirone (0.3 and 3.0 mg/kg) and BAY R 1531 (0.01 mg/kg). These results indicate that the analgesia induced by 30 bites social conflict in Swiss albino mice does not involve opioid and GABA-benzodiazepine (GABA-BZD) mechanisms. In addition, they suggest that high-intensity social conflict activates serotonergic pain modulatory systems that act through 5-HT1A receptors.

The pharmacotherapy of post traumatic stress disorder. A literature review and case report of treatment with nefazodone

Post traumatic stress disorder (PTSD) is a common and serious debilitating condition with a host of co-morbid disorders. Research has uncovered a number of structural and neurochemical abnormalities that provide exciting insights into the mind-body continuum under stress. Increasing efforts are being addressed to tailor medication according to a model based on the dysregulation of neurotransmitters. Controlled trials of treatment, whilst offering hope, are inadequate to permit the development of evidence-based protocols for the treatment of PTSD. Further psychopharmacological research is urgently required.

The effect of central and systemic injection of the 5-HT1A receptor agonist 8-OHDPAT and the 5-HT1A receptor antagonist WAY100635 on periaqueductal grey-induced defence behaviour

The effects of the selective 5-HT(1A) agonist, 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OHDPAT) and the selective 5-HT(1A) antagonist, N-[2-[4-(2-methoxyphenyl)-1-piperzinyl]ethyl]-N-(pyridinyl) cyclohexanecarboxamide trichloride (WAY100635) on periaqueductal grey (PAG)-stimulated defence behaviour were tested in the rat. Microinjection of the excitatory amino acid, D, L-homocysteic acid (DLH) into the dorsal region of the PAG produced overt aversive behaviour characteristic of the defence response, consisting of explosive motor behaviours which were quantified in terms of their duration and the number of arena revolutions and jumps made by the animal. Intra-PAG pre-treatment with 8-OHDPAT (3, 10 and 25 nmol in 250 nl) 10 min before DLH stimulation significantly attenuated the defence behaviour. This could be reversed by peripheral application of WAY100635 (0.1 mg/kg). In contrast, peripheral 8-OHDPAT (0.03, 0.1 and 0.3mg/kg) produced a significant potentiation of the DLH response which could also be blocked by peripheral WAY100635. When WAY100635 (10 nmol in 250 nl) alone was given into the PAG a significant increase in DLH induced behaviours was observed whereas peripherally applied WAY100635 (0.1 mg/kg) was without effect. These data support previous findings which indicate that serotonergic modulation of aversive behaviours such as defence can be mediated by 5-HT(1A) receptors. Furthermore there is evidence to indicate a differential involvement of pre- and postsynaptic 5-HT(1A) receptors.

The treatment of anxiety disorders: a legacy of William Sargant

In 1962 William Sargant and his colleagues described the therapeutic value of phenelzine, a monoamine oxidase inhibitor (MAOI), in chronic anxiety disorders and in the same year Klein and Fink reported the treatment of similar conditions with imipramine, a tricyclic antidepressant. Subsequent research has confirmed these findings and demonstrated the range of similar drugs that are effective in anxiety disorders. At the time of these original observations about the drug treatment of anxiety, there were no psychological treatments of proven value but in the intervening years much progress has been made in developing behavioural and cognitive procedures. The progress in determining the mode of action of these pharmacological and psychological treatments is reviewed and the implications of the findings are considered in relation to research into the causes of the anxiety disorders and to the treatment of patients.

Faulty cortisol/serotonin interplay. Psychopathological and biological characterisation of a new, hypothetical depression subtype (SeCA depression)

The hypothesis is proposed of a new subtype of depression named: stressor-precipitated, cortisol-induced, serotonin-related, anxiety/aggression-driven depression (SeCA depression). Biologically, these patients are characterized by impaired 5-HT synthesis and reduced 5-HT1A receptor sensitivity. Under normal conditions these functions proceed marginally; in times of stress they easily fail, due to sustained overproduction of cortisol. Psychopathologically this depression type shows the following characteristics: anxiety and aggression, not mood lowering, heralding a depressive episode; the personality structure shows 'character neurotic' impairments and tolerance for (certain) traumatic life events is low. As specific therapeutic agents selective 5-HT1A agonists and cortisol or CRH antagonists are proposed. Prophylactically, maintenance treatment with 5-HT1A agonists seems indicated as well as psychological interventions to increase the stressor threshold.

5-HT neuroendocrine function in major depression: prolactin and cortisol responses to D-fenfluramine

We studied the prolactin and cortisol responses to the 5-HT releasing agent, D-fenfluramine in 31 drug-free depressed patients and 29 healthy controls, using a double-blind, placebo-controlled design. There was no difference in either endocrine response between depressives and controls. Examining the sexes separately, or restricting comparisons to patients with melancholic depression, did not lead to any differences between the groups. Our findings suggest that in contrast to other 5-HT neuroendocrine probes such as L-tryptophan and clomipramine, the prolactin response to fenfluramine are not consistently blunted in depressed patients.

Eye movement desensitisation treatment utilises the positive visceral element of the investigatory reflex to inhibit the memories of post-traumatic stress disorder: a theoretical analysis

BACKGROUND

Eye movement desensitisation and reprocessing (EMDR) has attracted controversy and has led to publications covering a wide range of psychological problems treated by EMDR, in particular, post-traumatic stress disorder (PTSD). There is growing clinical evidence of the effectiveness of EMDR, but a lack of a convincing theoretical basis to explain its rapid effect.

METHOD

This paper argues that a combination of Pavlovian and Darwinian theory provide a theoretical explanation for the therapeutic effectiveness of EMDR.

RESULTS

We suggest that the investigatory component of the orienting reflex is an evolutionary development enabling organisms to assess their environment for both opportunities and threats. We propose that EMDR is rapidly effective because it is a clinical method of Pavlovian conditioning by which the positive visceral element of the investigatory reflex can be paired with clinically-induced noxious memories to remove their negative affect.

CONCLUSION

Compared with established forms of treatment for PTSD, EMDR is rapid, with resulting clinical and economic benefits. Our suggested theoretical basis for EMDR has implications for further explanatory research and for developments in EMDR treatment.

5-HT1A receptor expression is modulated by corticosteroid receptor agonists in primary rat hippocampal culture

The effects of corticosteroid receptor agonists on the expression of 5-HT1A receptor mRNA were measured in rat hippocampal cultures using in situ hybridization histochemistry. In our normal culture system, grown in serum and steroid-free media, moderate to heavy signal for 5-HT1A mRNA transcripts were detected in hippocampal neurons and glial cells. Aldosterone, a type I corticosteroid receptor agonist (10(-9) M), significantly reduced the expression of 5-HT1A mRNA both in neurons and glial fibrillary acidic protein (GFAP)-immunoreactive (IR) cells. The type II corticosteroid receptor agonist, Ru28362 (10(-8) M), also significantly decreased neuronal 5-HT1A mRNA expression. However, it was not as effective as aldosterone in reducing the label over GFAP-IR cells. These data indicate that corticosteroids may directly regulate the expression of hippocampal 5-HT1A receptors at the mRNA level in cultured hippocampal cells.

The 5-HT1A receptor antagonist (S)-UH-301 augments the increase in extracellular concentrations of 5-HT in the frontal cortex produced by both acute and chronic treatment with citalopram

In a recent study, utilizing single cell recording techniques, we have shown that administration of 5-HT1A receptor antagonists, e.g. (S)-UH-301, to rats concomitantly treated, acute or chronically, with the selective serotonin reuptake inhibitor (SSRI) citalopram significantly increases the activity of 5-hydroxytryptamine (5-HT) containing neurons in the dorsal raphe nucleus (DRN). Here we report correlative experiments using microdialysis in freely moving animals to measure extracellular levels of 5-HT and its metabolite 5-hydroxyindole acetic acid (5-HIAA) in the frontal cortex, a major projection area for DRN-5-HT neurons. Acute administration of (S)-UH-301 (2.5 mg/kg s.c.) or citalopram (2.0 mg/kg s.c.) increased 5-HT concentrations with a maximum of about 70% and 185%, respectively, above baseline. However, when (S)-UH-301 was administered 30 min before citalopram the maximal increase in 5-HT levels was approximately 400%. In rats chronically treated with citalopram (20 mg/kg/day i.p. for 14 days) basal 5-HT concentrations in the frontal cortex were significantly increased and 5-HIAA concentrations were decreased when measured 10-12 h, but not 18-20 h, after the last injection of citalopram, as compared to basal 5-HT and 5-HIAA concentrations in chronic saline-treated rats. When (S)-UH-301 (2.5 mg/kg s.c.) was administered 12 h, but not 20 h, after the last dose of citalopram it produced a significantly larger increase in extracellular concentrations of 5-HT than in control rats. However, in rats pretreated with a single, very high dose of citalopram, 20 mg/kg i.p., administration of (S)-UH-301 at 12 h after citalopram did not increase 5-HT levels. The augmentation by (S)-UH-301 of the increase in brain 5-HT output produced by acute administration of citalopram is probably due to antagonism of the citalopram induced feedback inhibition of 5-HT cells in the DRN, as previously suggested. However, the capacity of (S)-UH-301 to further increase the already elevated extracellular concentrations of 5-HT in brain in animals maintained on a chronic citalopram regimen, in which significant tolerance to the initial feedback inhibition of DRN-5-HT cells and developed, represents a novel finding. Generally, the reduced feedback inhibition of 5-HT neurons obtained with chronic citalopram treatment, and the associated elevation of brain 5-HT concentrations, may be related to functional desensitization of somatodendritic 5-HT1A autoreceptors in the DRN. This phenomenon may also largely explain the larger increase in 5-HT output produced by (S)-UH-301 in chronic citalopram treated animals as compared to its effect in control animals. Yet, a contributory factor may be a slight, remaining feedback inhibition of the 5-HT cells caused by residual citalopram at 12, but not 20 h after its last administration. Previous clinical studies suggest that addition of a 5-HT1A receptor antagonist to an SSRI in the treatment of depression may accelerate the onset of clinical effects. Moreover, in therapy-resistant cases maintained on SSRI treatment, addition of a 5-HT1A receptor antagonist may improve clinical efficacy. Since the therapeutic effect of SSRIs in depression has been found to be critically linked to the availability of 5-HT in brain, our experiments results support, in principle, both of the above clinically based notions.

Role of hippocampal 5-HT1A receptors on elevated plus maze exploration after a single restraint experience

Previous studies have shown that 2 h restraint stress induces deficits in open arm exploration of an elevated plus maze 24 h later. This effect was attenuated by a post-stress systemic injection of the 5-HT non-selective agonist, 5-methoxy-N,N-dimethyltryptamine (5-MeODMT). To verify a possible involvement of hippocampal 5-HT1A receptors in this effect, rats were stereotaxically implanted with canulae in the dorsal hippocampus. Seven days later they received bilateral microinjections of 5-MeODMT (20 nmol/0.5 microliter) or saline. No difference was found on exploration of an elevated plus maze 24 h later. However, when treatments were performed immediately after 2 h of restraint stress, the drug was able to increase open arm exploration 24 h later. This effect was antagonized by a previous microinjection of (+)WAY-100135 (40 nmol/0.5 microliter), a selective 5-HT1A antagonist. The results suggest that hippocampal 5-HT1A receptors may attenuate stress behavioral consequences.

Role of 5-HT in stress, anxiety, and depression

There are conflicting results on the function of 5-HT in anxiety and depression. To reconcile this evidence, Deakin and Graeff have suggested that the ascending 5-HT pathway that originates in the dorsal raphe nucleus (DRN) and innervates the amygdala and frontal cortex facilitates conditioned fear, while the DRN-periventricular pathway innervating the periventricular and periaqueductal gray matter inhibits inborn fight/flight reactions to impending danger, pain, or asphyxia. To study the role of the DRN 5-HT system in anxiety, we microinjected 8-OH-DPAT into the DRN to inhibit 5-HT release. This treatment impaired inhibitory avoidance (conditioned fear) without affecting one-way escape (unconditioned fear) in the elevated T-maze, a new animal model of anxiety. We also applied three drug treatments that increase 5-HT release from DRN terminals: 1) intra-DRN microinjection of the benzodiazepine inverse agonist FG 4172, 2) intra-DRN microinjection of the excitatory amino acid kainic acid, and 3) intraperitoneal injection of the 5-HT releaser and uptake blocker D-fenfluramine. All treatments enhanced inhibitory avoidance in T-maze. D-Fenfluramine and intra-DRN kainate also decreased one-way escape. In healthy volunteers, D-fenfluramine and the 5-HT agonist mCPP (mainly 5-HT2C) increased, while the antagonists ritanserin (5-HT2A/2C) and SR 46349B (5-HT2A) decreased skin conductance responses to an aversively conditioned stimulus (tone). In addition, D-fenfluramine decreased, whereas ritanserin increased subjective anxiety induced by simulated public speaking, thought to represent unconditioned anxiety. Overall, these results are compatible with the above hypothesis. Deakin and Graeff have suggested that the pathway connecting the median raphe nucleus (MRN) to the dorsal hippocampus promotes resistance to chronic, unavoidable stress. In the present study, we found that 24 h after electrolytic lesion of the rat MRN glandular gastric ulcers occurred, and the immune response to the mitogen concanavalin A was depressed. Seven days after the same lesion, the ulcerogenic effect of restraint was enhanced. Microinjection of 8-OH-DPAT, the nonselective agonist 5-MeO-DMT, or the 5-HT uptake inhibitor zimelidine into the dorsal hippocampus immediately after 2 h of restraint reversed the deficits of open arm exploration in the elevated plus-maze, measured 24 h after restraint. The effect of the two last drugs was antagonized by WAY-100135, a selective 5-HT1A receptor antagonist. These results are compatible with the hypothesis that the MRN-dorsal hippocampus 5-HT system attenuates stress by facilitation of hippocampal 5-HT1A-mediated neurotransmission. Clinical implications of these results are discussed, especially with regard to panic disorder and depression.

Predator-elicited flight responses in Swiss-Webster mice: an experimental model of panic attacks

1. The nosological status of panic disorder is still a matter of debate. Nevertheless, evidence is emerging that panic attacks have a different pattern of drug responsiveness from other forms of anxiety. 2. Several experimental animal models of panic attacks have been developed. These vary in the extent to which they meet criteria for face validity, predictive validity and construct validity, normally applied to such models. 3. In the present review, the authors examine the possibility that predator-elicited flight responses in Swiss-Webster mice might serve as an experimental model for the screening of panic-modulating drugs. 4. Drug effects on flight responses clearly indicate that this model has good predictive validity as panic-promoting agents increase flight reactions, while panicolytic drug challenge induces opposite effects. In addition, drugs devoid of any effect on panic attack, also do not alter flight behavior. 5. These findings strongly suggest that the model of predator-elicited flight responses in Swiss-Webster mice is useful for the investigation of panic-modulating drugs.

Sub-chronic cold stress reduces 5-HT1A receptor responsiveness in the old but not in the young rat

The inhibitory effect of the prototypical 5-hydroxytryptamine (5-HT)1A receptor agonist 8-hydroxy-2-(di-n-propyl amino)tetraline (8-OH-DPAT) on forskolin-stimulated adenylyl cyclase activity, has been examined as an index of the functional activity of 5-HT1A receptors in the hippocampus of young (3 months) and old (18 months) rats exposed during 24 h or 5 days to cold. In both young and old rats exposed to cold stress during 24 h, there was a reduction in the potency (EC50) and/or the maximal inhibitory effect (Emax) of 8-OH-DPAT in reducing forskolin-induced cAMP accumulation. The properties of the hippocampal 5-HT1A sites labelled by [3H]8-OH-DPAT were not affected by these stressful conditions. Moreover, while the sensitivity of 5-HT1A receptors to 8-OH-DPAT in young rats returned to control values after 5 days of cold exposure, old rats still exhibited a significant desensitization of 5-HT1A receptors as compared to naive animals. These results point out the capacity of young but not of old rats to adapt to the aversive effects of a subchronic stressor.

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.