Effect of 5-HT1A receptor agonists in two models of anxiety after dorsal raphe injection

5-HT1A receptor agonists: recent developments and controversial issues

During the last decade, serotonin (5-HT)1A receptors have been a major target for neurobiological research and drug development. 5-HT1A receptors have been cloned and a variety of selective agonists, such as the aminotetraline 8-OH-DPAT and the pyrimidinylpiperazine ipsapirone, have become available. Demonstrations of apparent intrinsic activity of these ligands at 5-HT1A receptors, however, depend highly on the particular assay system. This may be due to the possible existence of receptor subtypes and to assay (or brain region)-dependent differences in receptor reserve and the nature of receptor-effector coupling. Nevertheless, the apparent intrinsic activity of 8-OH-DPAT seems to be higher (although possibly not yet maximal) than that of the pyrimidinylpiperazines. In the brain, 5-HT1A receptors are located presynaptically as somatodendritic receptors on 5-HT neurons and postsynaptically in particular limbic and cortical regions. Although it is generally accepted that presynaptic 5-HT1A receptors control 5-HT neuronal activity, recent evidence suggests an additional role of postsynaptic 5-HT1A receptors in cortex as part of a negative feedback loop. Anxiolytic and antidepressive properties of selective 5-HT1A receptor agonists have now been confirmed by clinical studies. Although it is well established that the latter properties depend on the agonistic activity of these compounds, the optimal level of intrinsic activity is still a matter of debate and may be dependent on the clinical indication. Such compounds may also have antiaggressive effects, and possibly anticraving effects (manifested by their alcohol intake-reducing effects in dependent animals), but the specificity of these so-called anti-impulsivity effects is still controversial and not yet tested clinically. Anticataleptic, antiemetic and neuroprotective properties have been demonstrated in different species. Behavioral studies on the mechanisms underlying the anxiolytic and antidepressive effects have examined the relative contribution of pre- and postsynaptic 5-HT1A receptors by means of local cerebral application and lesion techniques. Most evidence points towards a critical involvement of presynaptic receptors in the anxiolytic effects of 5-HT1A receptor agonists (although a possible contribution of postsynaptic receptors cannot be excluded). With regard to the antidepressive properties, a case can be made for the reverse; i.e., a strong involvement of postsynaptic receptors and a questionable contribution of presynaptic receptors. However, as the therapeutic effects of those 5-HT1A receptor (partial) agonists which have been tested clinically require repeated administration, attention has been directed increasingly towards chronic studies.(ABSTRACT TRUNCATED AT 400 WORDS)

The anxiolytic effects of flesinoxan, a 5-HT1A receptor agonist, are not related to its neuroendocrine effects

The effects of flesinoxan, a selective 5-HT1A receptor agonist, were studied under basal non-stress conditions and in the shock-probe burying paradigm. Flesinoxan (1 and 3 mg/kg s.c.) significantly reduced burying and freezing behaviour, indicating clear anxiolytic properties. Under non-stress conditions, injection of 3 mg/kg flesinoxan significantly enhanced plasma corticosterone and glucose levels, whereas prolactin secretion was significantly enhanced after both 1 mg/kg and 3 mg/kg flesinoxan. Flesinoxan (1 and 3 mg/kg) did not suppress shock-probe stress-induced rises in plasma corticosterone and glucose levels. The enhanced plasma prolactin levels induced by flesinoxan were not further affected by shock-probe exposure. Our data show that the anxiolytic effects of flesinoxan in the shock-probe burying paradigm are not related to increases in plasma corticosterone and glucose levels.

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.

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.

Serotonin receptor binding in a colony model of chronic social stress

Male rats housed in mixed-sex groups quickly established dominance hierarchies in which subordinates appeared severely stressed. Subordinate rats had elevated basal corticosterone (CORT) levels relative to dominants and individually housed controls. Several subordinates had blunted CORT responses to a novel stressor, leading to the classification of subordinates as either stress-responsive or nonresponsive. Binding to 5-HT1A receptors was reduced in stress-responsive subordinates compared to controls throughout hippocampus and dentate gyrus. Decreased binding was observed in nonresponsive subordinates only in CA3 of hippocampus. In addition, 5-HT1A binding was decreased in CA1, CA3, and CA4 in dominants compared to controls. Binding to 5-HT2 receptors was increased in parietal cortex in both responsive and nonresponsive subordinates compared to controls. No changes were observed in binding to 5-HT1B receptors. These results are discussed in the context of regulation of the serotonergic system by stress and glucocorticoids and possible relevance to the pathophysiology of depression.

5-Hydroxytryptamine-interacting drugs in animal models of anxiety disorders: more than 30 years of research

An overview of the behavioral data arising from the vast literature concerning the involvement of 5-hydroxytryptamine (5-HT) neurotransmission in the regulation of anxiety is presented. More than 1300 experiments were carried out in this area and they provide evidence that: (1) results obtained in ethologically based animal models of anxiety with drugs stimulating 5-HT transmission are most consistent with the classic 5-HT hypothesis of anxiety in that they show an increase in animals' emotional reactivity; (2) no category of anti-anxiety models are selectively sensitive to the anxiolytic-like effects of drugs targetting 5-HT1A, 5-HT2A or 5-HT2C receptor subtypes; (3) anxiolytic-like effects of 5-HT3 receptor antagonists, in the great part, are revealed by models based on spontaneous behaviors. Taken together, these observations lead to the conclusion that different 5-HT mechanisms, mediated by different receptor subtypes, are involved in the genesis of anxiety.

Anxiolytic effect of the 5-HT1A compounds 8-hydroxy-2-(di-n-propylamino) tetralin and ipsapirone in the social interaction paradigm: evidence of a presynaptic action

This study analyses at which site, pre- or postsynaptic, the 5-HT1A ligands--8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and ipsapirone--induce their anxiolytic action. The experimental anxiety was assessed in the social interaction test. An anxiolytic action was observed after the systemic administration of 8-OH-DPAT (0.25 and 0.5 mg/kg) and ipsapirone (5 but not 10 mg/kg). In 5,7-dihydroxytryptamine (5,7-DHT, 150 micrograms/10 microliters) lesioned rats the anxiolytic effect of 8-OH-DPAT and ipsapirone was not observed, suggesting a presynaptic action of these drugs. When directly injected into the dorsal raphe nucleus 8-OH-DPAT (0.1 microgram/microliter) and ipsapirone (0.2 microgram/microliter), both compounds produce anxiolytic effects. At same doses, these drugs lacked an effect after their intrahippocampal infusion. All data strongly suggest that both drugs act presynaptically to reduce the anxiety levels in the social interaction paradigm.

The role of postsynaptic 5-HT1A receptors in the anticonflict effect of ipsapirone

In the present paper we have studied the anticonflict effect (in the Vogel test) of ipsapirone, a partial agonist of 5-hydroxytryptamine1A (5-HT1A) receptors, administered to the hippocampus of rats. In addition, a comparison of the effect of ipsapirone with the effect of other 5-HT1A receptor ligands (busipone, gepirone and 8-OH-DPAT) has been carried out. Finally, the interaction between ipsapirone and NAN-190 (an antagonist of 5-HT1A receptors and alpha 1-adrenoceptors) has also been examined. It has been found that ipsapirone injected intrahippocampally (i.hp.) in doses of 0.3, 1 and 3 micrograms (bilaterally) shows an anticonflict effect by increasing the number of punished licks by about 36, 151 and 109%, respectively. A similar effect has also been found after i.hp. injections of buspirone (0.3-3 micrograms), gepirone (3-30 micrograms) and 8-OH-DPAT (0.3-3 micrograms). We have also demonstrated that the anticonflict effect of ipsapirone injected i.hp. is antagonized by NAN-190 administered i.hp. (0.3 or 1 microgram) or intraperitoneally (i.p., 1 mg/kg). Furthermore, NAN-190 injected i.hp. (0.3 microgram) antagonizes the anticonflict effect of ipsapirone administered i.p. (5 mg/kg). At the same time, the anticonflict effects of ipsapirone are not affected by prazosin (0.3-1 microgram i.hp. or 0.5-1 mg/kg i.p.), a selective antagonist of alpha 1-adrenoceptors. Our results seem to indicate that the anticonflict effect of ipsapirone stems from stimulation of postsynaptic 5-HT1A receptors in the hippocampus.

The effect of serotonin depletion and intra-hippocampal midazolam on rat behavior in the Vogel conflict test

The effect of intra-hippocampal (dentate gyrus) microinjections of the benzodiazepine, midazolam (10 micrograms per site, bilaterally), was examined in the Vogel conflict test in serotonin depleted rats (pretreated with p-chlorophenylalanine). p-Chlorophenylalanine and midazolam, when given separately, significantly disinhibited rat conflict behavior. Midazolam retained its anticonflict potency in the p-chlorophenylalanine pretreated animals. These and other data indicate: (i) the specific involvement of limbic serotonin in emotional control, (ii) a dissociation of the effects of serotonin depletion and intra-hippocampal benzodiazepine on conflict behavior, and (iii) the important role of the hippocampus in processing emotional input.

The pharmacological properties of Y-23684, a benzodiazepine receptor partial agonist

1. The pharmacological properties of a benzodiazepine receptor (BZR) partial agonist, Y-23684 were investigated in comparison with those of diazepam, a conventional BZR full agonist. 2. Y-23684 and diazepam showed high and selective affinity for the BZR with Ki values of 41 and 5.8 nM, respectively. 3. In contrast to diazepam, variability was noted in the anticonvulsive potency of Y-23684 depending on convulsants (bicuculline, pentylenetetrazol and maximal electrical shock). Y-23684 produced the most potent protective effect against bicuculline in rats and mice with ED50S of 1.3 and 1.2 mg kg-1, respectively. 4. In rat conflict models (Geller-Seifter and water-lick tests), Y-23684 produced an antipunishment action at doses 2-4 times lower than diazepam. In contrast to diazepam, Y-23684 did not affect unpunished responding up to 50 mg kg-1 in the Geller-Seifter test. 5. In other rat models of anxiety (social interaction and elevated plus-maze tests), Y-23684 was as efficacious as and ten fold more potent than diazepam. In a mouse model of anxiety (exploration (light/dark box) test), Y-23684 was as efficacious and two fold less potent as diazepam. In these paradigms, Y-23684 showed a selective anxiolytic profile over a wide dose-range without loss of efficacy and sedative action. 6. The impairment of motor coordination (rotarod) and potentiation of CNS depressants (ethanol and hexobarbitone) by Y-23684 was much weaker than that of diazepam. 7. These results suggest that Y-23684 would be a potent and selective anxiolytic agent in man with less side-effects than conventional BZ-anxiolytics.

Contrasting behavioural effects of 8-OH DPAT in the dorsal raphé nucleus and ventral hippocampus

Administration of the 5-HT1A receptor agonist (+-)-8-Hydroxy-dipropylaminotetralin (8-OH DPAT, 50 ng) into the dorsal raphé nucleus (DRN) increased social interaction but did not change the motor activity of rats tested in high light, thus indicating an anxiolytic response. This effect was blocked by coadministration of the 5-HT1A antagonist, tertatolol (3 micrograms). In contrast, 8-OH DPAT (50 and 100 ng) was without effect on social interaction when administered to the DRN projection area in the ventral hippocampus, but did change locomotor activity. The effects depended on the light level and dose: thus when the rats were tested in low light, 50 ng increased locomotor activity, but in high light a decrease was found with 100 ng and also an induction of wet dog-shakes. Thus, our results support the importance of the somatodendritic autoreceptors in the DRN in alleviating anxiety, whereas the post-synaptic receptors in the ventral hippocampus play no role. They do, however, mediate changes in activity and the 5-HT syndrome.

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.

Neuronal circuits involved in the anxiolytic effects of the 5-HT1A receptor agonists 8-OH-DPAT ipsapirone and buspirone in the rat

In rats, the 5-HT1A receptor full agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and the 5-HT1A receptor partial agonists ipsapirone and buspirone dose dependently and completely inhibited shock-induced ultrasonic vocalization after systemic injection and after microinjection into the dorsal raphe nucleus, a brain region rich in somatodendritic 5-HT1A receptors. As compared with injection into the dorsal raphe nucleus, ipsapirone and 8-OH-DPAT were significantly less potent after microinjection into the lateral ventricle or the median raphe nucleus. Depletion of brain 5-HT (5-hydroxytryptamine) by means of 5,7-dihydroxytryptamine or parachlorophenylalanine inhibited ultrasonic vocalization. In lesioned rats, however, ipsapirone (i.p. or dorsal raphe nucleus) and 8-OH-DPAT (dorsal raphe nucleus) retained their ability to inhibit ultrasonic vocalization and, in non-lesioned rats, bilateral injection of ipsapirone, buspirone and 8-OH-DPAT into the dorsal hippocampus and the amygdala - two brain regions rich in postsynaptic 5-HT1A receptors - also inhibited ultrasonic vocalization. In a Geller-Seifter conflict test, i.p. and local injection of 8-OH-DPAT in the dorsal raphe nucleus and the hippocampus selectively enhanced punished responding. It is suggested that both presynaptic and (possibly to a lesser extent) postsynaptic 5-HT1A receptors are involved in the anxiolytic effects of ipsapirone, buspirone, and 8-OH-DPAT.

Modification of the anxiolytic effects of 5-HT1A agonists by shock intensity

Contradictory evidence exists concerning the anxiolytic effects of 5-HT1A agonists in the conflict test. In the present work, a modification of the Vogel conflict model was used to assess different doses of diazepam (0.1-5.6 mg/kg), ipsapirone (1.0-17.8 mg/kg), buspirone (1.7-17.8 mg/kg), and indorenate (0.56-17.8 mg/kg) in rats receiving two different electric shock intensities (0.16 and 0.32 mA). The results show that the three 5-HT1A agonists had a smaller anticonflict effect than diazepam. The anticonflict effect with each compound was of a greater magnitude at 0.16 mA intensity than at 0.32 mA. This study shows that, using different electric shock intensities, compounds produce a differential effect: the anticonflict effects were more pronounced with the lower electric shock intensity than with the higher intensity. The present results suggest that the use of different shock intensities can play distinct roles over the drug's effect in the conflict test.

Serotonergic innervation of the hippocampus and nucleus accumbens septi and the anxiolytic-like action of midazolam and 5-HT1A receptor agonists

An involvement of serotonergic innervation of the hippocampus (HP) and the nucleus accumbens septi (NAS) in anxiolytic activity of benzodiazepine midazolam and 5-HT1A receptor agonists was studied in two different animal models of anxiety. Injection of midazolam (10.0 and 20.0 micrograms) or 8-OH-DPAT (0.5 and 1.0 micrograms) into the hippocampus increased punished consumption of water in the Vogel conflict test. Buspirone given at 0.1, 0.5 and 1.0 microgram was ineffective in the Vogel test, while at 5.0 micrograms it enhanced shock-induced suppression of drinking. In the open-field test midazolam (0.01 and 0.1 microgram), 8-OH-DPAT (0.1, 0.5 and 1.0 microgram) and buspirone (2.5 and 5.0 micrograms) increased the number of entries into the central part of the open-field and the time spent in the central sector. Depletion of 5-HT had no influence on the anxiolytic-like effect in the open-field test of intrahippocampally-administered 8-OH-DPAT (0.5 microgram), but the drug tended to increase motor activity in lesioned animals. Midazolam and buspirone injected into the NAS did not have an anxiolytic effect in the Vogel test. A small increase in punished drinking was observed after 8-OH-DPAT (1.0 and 2.5 micrograms). Following intra-NAS injection, midazolam, 8-OH-DPAT and buspirone all failed to produce any marked anxiolytic-like effect in the open-field test. It appears that the hippocampus, rather than the NAS, is involved in mediating anxiolytic-like effects of 5-HT1A receptor agonists. Hippocampal postsynaptic 5-HT1A receptors may account for the anti-emotional influence of this group of drugs. The results indicate some similarities in the psychotropic profile of 5-HT1A receptor agonists and midazolam.

Sustained 5-hydroxytryptamine release-inhibitory and anxiolytic-like action of the partial 5-HT1A receptor agonist, buspirone, after prolonged chronic administration

The effect of prolonged administration of high doses of buspirone on its 5-hydroxytryptamine (5-HT) release-inhibitory and anxiolytic-like properties was investigated. The 5-HT release-inhibitory effect of a challenge dose of buspirone (0.5 mg/kg, s.c.) was identical in rats chronically treated with vehicle or buspirone (10 mg/kg, b.i.d. for 10 weeks), as estimated by in vivo microdialysis in the ventral hippocampus. In the same set of animals there was a significant anxiolytic-like effect in the elevated plus-maze after 5 weeks of treatment with buspirone. The results indicate that the functional capacity of 5-HT release-controlling 5-HT1A autoreceptors is retained upon chronic administration of buspirone, and that this effect may well be associated with the anxiolytic-like action of the compound.

Clomipramine increases firing rate in lateral septal neurons related with hippocampus

Long-term clomipramine administration (CMI: 2.5 mg/kg IP, 25 days) effects on 274 identified hippocampus-septal neurons was assessed in rats. Stimulation of CA 1/dentate areas produced a biphasic response in 61% of lateral septal neurons. A brief activation was followed by a period of suppression of firing. In CMI-treated rats, the neurons showing this biphasic response increased their firing rate about fourfold, while the suppression of firing shortened around threefold. Other septal neurons (36%), in which only the burst of activation appeared but not suppression of firing, did not change their firing rate even after long-term CMI. In conclusion, CMI produces an increase of firing rate in lateral septal neurons receiving inhibitory hippocampal influence which is likely to be mediated by a disinhibitory process.

Activation of serotonin1A receptors inhibits midbrain periaqueductal gray neurons of the rat

The midbrain periaqueductal gray (PAG) is involved in a variety of functions including pain modulation, vocalization, autonomic control, fear and anxiety. This area contains serotonin receptors, particularly 5-HT1A that are known to play a role in the above functions. The goals of this study were to characterize the effects of 8-OH-DPAT, a selective 5-HT1A agonist, on the firing characteristics and membrane properties of PAG neurons. Both in vivo and in vitro preparations were used. The effects of 8-OH-DPAT on baseline activity of 91 neurons were tested in the in vivo preparation. In 50/91 cells, 8-OH-DPAT produced a decrease in the firing rate that ranged between 21 and 98% (mean +/- S.E.M. decrease of 49 +/- 1.9%). This inhibitory effect was dose dependent and could be blocked by spiperone. In 10/91 cells, 8-OH-DPAT produced an increase in the firing rate that ranged between 13 and 290%, with mean increase of 83 +/- 7.4%. The baseline firing rate of the remaining 31 cells was not affected by 8-OH-DPAT. In the PAG slice preparation, the effects of 8-OH-DPAT on synaptic and membrane properties of 17 PAG neurons were tested using whole-cell voltage clamp-recording procedures. In 14 cells, application of 8-OH-DPAT produced hyperpolarization that ranged between 6 and 21 mV, with mean of 8.4 +/- 2.0 mV. This hyperpolarization was associated with a decrease in membrane impedance that ranged between 8 and 45%, with mean decrease of 21.6 +/- 4.5%. The remaining three neurons did not respond to 8-OH-DPAT.(ABSTRACT TRUNCATED AT 250 WORDS)

Stimulation of hippocampal 5-HT1A receptors causes amnesia and anxiolytic-like but not antidepressant-like effects in the rat

Administration of 2 and 5 but not 0.4 microgram/microliter 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) into the CA1 region of the dorsal hippocampus 10 min before the acquisition trial in a passive avoidance task significantly reduced retention latencies 24 h later. The effect of 5 micrograms 8-OH-DPAT on retention latencies was completely antagonized by 1 microgram/microliter spiroxatrine, a 5-HT1A receptor antagonist, infused into the dorsal hippocampus 5 min before 8-OH-DPAT. Administered 5 min after the acquisition trial, 5 micrograms/microliters 8-OH-DPAT had no effect on retention latencies 24 h later. Administration of 5 micrograms/microliters 8-OH-DPAT into the dorsal hippocampus did not significantly change the thresholds for responses to the same electrical stimuli used in the passive avoidance task and had no antidepressant-like effect in the forced swimming test. The dose of 5 micrograms/microliters 8-OH-DPAT administered into the dorsal hippocampus caused anxiolytic-like effects assessed by stress-induced deficit in open field locomotor activity. The results suggest that stimulation of 5-HT1A receptors in the dorsal hippocampus impairs rats' performance in a passive avoidance task by interfering with memory processes or by attenuating the emotional impact of the shock through an anxiolytic action.

Anxiolytic effects of serotonergic interventions in the shock-probe burying test and the elevated plus-maze test

Although serotonergic neural systems have been implicated in the control of anxiety for a number of years, evidence in favour of this role is controversial. The present experiments were designed to further characterize the putative role of serotonin (5-HT) in anxiety, using two pharmacologically validated animal models: the elevated plus-maze and the shock-probe burying tests. If the integrity of 5-HT neural systems is necessary for the expression of 'anxious' behaviors, then disruption of 5-HT systems should produce effects in the plus-maze and shock-probe tests that are similar to those of anxiolytic drugs. In the present experiments, serotonergic function was disrupted in rats, either by chemical depletion using the synthesis inhibitor p-CPA, by inhibitory autoreceptor activation using the selective 5-HT1A receptor ligand 8-OH-DPAT, or by electrolytic lesions of the serotonin-containing, dorsal raphe nucleus. p-CPA and dorsal raphe lesions produced robust anxiolytic effects in the elevated plus-maze and the shock-probe burying tests, whereas 8-OH-DPAT produced anxiolytic effects only in the shock-probe burying test, and 'anxiogenic' effects in the elevated plus-maze test. Although these results generally support the view that serotonin plays a role in the expression of 'anxious' behavior, the opposite effects of 8-OH-DPAT in the two behavioral paradigms suggest that the 5-HT1A receptor subtype exerts differential control over different types of experimental anxiety.