Stress-induced hyperthermia as a putative anxiety model

Anxiolytic-like activity of oxytocin in male mice: behavioral and autonomic evidence, therapeutic implications

RATIONALE

Oxytocin (OT) acts as a neuromodulator/neurotransmitter within the central nervous system (CNS) and regulates a diverse range of CNS functions. Notably, evidence from studies in females has revealed an important role for OT in regulating anxiety behavior.

OBJECTIVES

The objective of this study was to examine the effects of OT on both behavioral and autonomic parameters of the anxiety response in male mice using three pharmacologically validated preclinical models of anxiety: the four-plate test (FPT), elevated zero maze (EZM), and stress-induced hyperthermia (SIH).

RESULTS

In the FPT, both peripherally (3-30 mg/kg i.p.) and centrally (1-10 microg i.c.v.) administered OT produced dose-dependent increases in punished crossings, indicating an anxiolytic-like effect. The effects of centrally administered OT in the FPT were blocked with peripheral administration of a brain-penetrant OT receptor (OTR) antagonist WAY-162720 (30 mg/kg i.p.), and the effects of peripherally administered OT were blocked with central administration of a non-penetrant OTR antagonist L-371,257, suggesting OT acts centrally. In the EZM, centrally administered OT (0.1-1.0 microg, i.c.v.) produced significant increases in the percentage time spent in the open quadrants of the maze, comparable to alprazolam (0.5-1.0 microg, i.c.v.). In SIH, OT (1-10 mg/kg i.p.) dose-dependently attenuated stress-induced increases in core body temperature, comparable to the reference anxiolytic chlordiazepoxide (CDP) (10 mg/kg i.p.).

CONCLUSIONS

These results provide specific behavioral and autonomic evidence of anxiolytic-like effects for oxytocin in males and, together with previously reported observations in females, suggest the potential utility of OTR agonism as a therapeutically relevant mechanism of action for novel anxiolytics in both sexes.

Pharmacological characterization of stress-induced hyperthermia in DBA/2 mice using metabotropic and ionotropic glutamate receptor ligands

RATIONALE

Accumulating evidence suggests that drugs acting on the glutamatergic system may represent promising novel therapeutic targets for the treatment of anxiety disorders. The stress-induced hyperthermia paradigm has been used widely to model some of the physiological symptoms associated with anxiety disorders and has produced results that are predictive of clinical efficacy. We have modified this paradigm to measure the autonomic consequences of stress induced by the fear of predation in mice.

OBJECTIVE

To evaluate the efficacy of several classes of metabotropic and ionotropic glutamate receptor ligands, as well as known anxiolytics and psychotropic comparators, in attenuating predatory-stress-induced hyperthermia.

METHODS

Male DBA/2 mice were implanted with radiotelemetric transmitters in the peritoneal cavity to measure stress-related increases in core body temperature, following placement in a novel cage containing soiled rat shavings.

RESULTS

Clinically active compounds such as chlordiazepoxide (5-10 mg/kg), alprazolam (0.3-3 mg/kg), and buspirone (10-30 mg/kg) exhibited an anxiolytic profile. Assessment of glutamatergic agents indicated that the mGlu1 receptor antagonist LY456236 (10-30 mg/kg), mGlu5 receptor antagonist MPEP (10-30 mg/kg), mGlu2/3 receptor agonist LY354740 (3-10 mg/kg), mGlu2 receptor potentiator LY566332 (30 and 100 mg/kg), mGlu8 receptor agonist (S)-3,4-dicarboxyphenylglycine (30-60 mg/kg), competitive NMDA receptor antagonist LY235959 (1 mg/kg), AMPA receptor antagonist GYKI-52466 (10-20 mg/kg), and glycine transporter-1 (GlyT-1) inhibitor ALX-5407 (3-10 mg/kg) dose-dependently attenuated stress-induced hyperthermia. The AMPA receptor potentiator LY451646, iGlu5 kainate receptor antagonist LY382884, glycine(B) receptor partial agonist D: -cycloserine, and GlyT-1 inhibitor ORG-24461 were ineffective in this model.

CONCLUSION

Select metabotropic and ionotropic glutamate receptor ligands exhibited an anxiolytic profile, as measured by the attenuation of stress-induced hyperthermia, and may represent viable targets for the development of pharmacological treatments for anxiety-related disorders.

Clozapine increases cutaneous blood flow and reduces sympathetic cutaneous vasomotor alerting responses (SCVARs) in rats: comparison with effects of haloperidol

RATIONALE

Clozapine inhibits sympathetic outflow to the cutaneous vascular bed. Clozapine reverses hyperthermia and cutaneous vasoconstriction induced by 3,4-methylenedioxymethamphetamine (MDMA, Ecstasy) or by lipopolysaccharide (LPS). Clozapine also reverses cutaneous vasoconstriction elicited by exposure to cold. These actions distinguish clozapine from haloperidol. Clozapine could also inhibit sympathetic cutaneous vasomotor alerting responses (SCVARs), vasoconstrictor episodes that reflect emotional/psychological function, and this property might also distinguish clozapine from haloperidol.

OBJECTIVES

Experiments in rats determined whether clozapine and haloperidol inhibit SCVARs, and whether SR46349B (a 5HT2A receptor antagonist), 8-OH-DPAT (a 5-HT1A agonist), L741,626 (a dopamine D2 antagonist) or SCH23390 (a dopamine D1 antagonist) have clozapine-like effects on SCVARs.

METHODS

Mean level and pulse amplitude of the tail artery Doppler flow signal were recorded in conscious freely moving rats before and after alerting stimuli (e.g. tapping the cage), and expressed as a SCVAR index (fall to zero flow implies SCVAR index of 100%, no fall implies 0%).

RESULTS

Clozapine (0.0625-1.0 mg/kg, s.c.) dose-dependently increased resting tail blood flow. After 1 mg/kg, the SCVAR index was 18+/-1%, compared with 83+/-2% after vehicle. SR46349B (0.01-1.0 mg/kg) and 8-OH-DPAT (0.25 mg/kg) had similar but less potent effects on cutaneous blood flow and on SCVARs. Haloperidol (0.005-0.5 mg/kg) and L741,626 (1 mg/kg) had no or little effect on these variables. SCH23390 mildly inhibited SCVARs.

CONCLUSIONS

Clozapine, but not haloperidol, increases resting cutaneous blood flow and decreases SCVARs. Antagonism at 5-HT2A receptors and agonism at 5-HT1A receptors could contribute to these actions.

Neurobiology of mood, anxiety, and emotions as revealed by studies of a unique antidepressant: tianeptine

Recent studies have provided evidence that structural remodeling of certain brain regions is a feature of depressive illness, and the postulated underlying mechanisms contribute to the idea that there is more to antidepressant actions that can be explained exclusively by a monoaminergic hypothesis. This review summarizes recent neurobiological studies on the antidepressant, tianeptine (S-1574, [3-chloro-6-methyl-5,5-dioxo-6,11-dihydro-(c,f)-dibenzo-(1,2-thiazepine)-11-yl) amino]-7 heptanoic acid, sodium salt), a compound with structural similarities to the tricyclic antidepressant agents, the efficacy and good tolerance of which have been clearly established. These studies have revealed that the neurobiological properties of tianeptine involve the dynamic interplay between numerous neurotransmitter systems, as well as a critical role of structural and functional plasticity in the brain regions that permit the full expression of emotional learning. Although the story is far from complete, the schema underlying the effect of tianeptine on central plasticity is the most thoroughly studied of any antidepressants. Effects of tianeptine on neuronal excitability, neuroprotection, anxiety, and memory have also been found. Together with clinical data on the efficacy of tianeptine as an antidepressant, these actions offer insights into how compounds like tianeptine may be useful in the treatment of neurobiological features of depressive disorders.

The effects of angelica essential oil in three murine tests of anxiety

The effects of angelica essential oil in three assays predictive of anxiolytic activity in male mice were studied, with diazepam as a positive anxiolytic control. In the elevated plus-maze test, compared to the positive control diazepam, angelica essential oil (30.0 mg/kg, PO) had a modest anxiolytic-like effect (increased the percentage of open-arm time and reduced the percent protected head dips). In the light/dark test, angelica essential oil (30.0 mg/kg) prolonged the time spent in the light area without altering the locomotor activity of the animals. In the stress-induced hyperthermia test, 60 and 70 min after drug administration, rectal temperature was measured twice, angelica essential oil at the dose of 30.0 mg/kg inhibited stress-induced hyperthermia. Thus, these findings indicate that angelica essential oil, as does diazepam, exhibits an anxiolytic-like effect. Further studies will be required to assess the generality of the present findings to other species and behavioural paradigms.

Repeated sensory contact with aggressive mice rapidly leads to an anticipatory increase in core body temperature and physical activity that precedes the onset of aversive responding

The present study investigated whether the 'psychological threat' induced by sensory contact with an aggressive conspecific would be a sufficient factor in inducing behavioural and physiological disturbances. Repeated sensory contact with an aggressive mouse (social threat) in a partitioned cage was compared with repeated exposure to a novel partitioned cage in male NMRI mice. We first examined parameters of stress responsiveness (body weight, plasma corticosterone levels, frequency of self-grooming and defecation). The temperature and physical activity responses to stress were also recorded during and after the 4 weeks of stress using radiotelemetry. Finally, cognitivo-emotional performance was assessed after acute stress and 2 and 4 weeks of stress by measuring decision making, sequential alternation performance and behaviour in the elevated T-maze. Social threat had a greater impact than novel cage exposure on most parameters of stress responsiveness, although mice did not habituate to either stressor. Social threat rapidly led to an anticipatory rise in core body temperature and physical activity before the scheduled stress sessions. Such anticipation developed within the first week and persisted for 9 days after ending the stress procedure. Some memory impairment in the sequential alternation test was found in stressed mice, independent of the stressor. After 4 weeks of stress, inhibitory avoidance in the elevated T-maze was enhanced in socially stressed mice and reduced in novel cage mice. The sustained anticipation of stress in the social threat group preceded aversive responding. It remains to be established whether anticipation contributes to the development of aversive responses.

Stress-induced hyperthermia in the mouse: c-fos expression, corticosterone and temperature changes

In mammals, stress exposure is frequently associated with an elevated body temperature ['emotional fever', stress-induced hyperthermia (SIH)]. Rectal measurement of body core temperature of the mouse induces a rise of 1-1.5 degrees C over a 10- to 15-min time interval. This phenomenon has been exploited to design a specific test for measuring stress-induced hyperthermia: the singly-housed SIH paradigm in mice. In the present experiments, changes in body temperature and corticosterone levels were studied 10, 30, 60, 90 and 120 min after the first insertion of the rectal probe. In addition, changes in patterns of neural activation, as observed after immunostaining for Fos-immunoreactivity (Fos-IR), were studied in the brains of animals perfused at times 0, 60 or 120 min. Our results show that SIH and corticosterone levels have their peak values between 10 and 30 min and are no longer different from control values after 60 min. Patterns of Fos-IR have been studied in 11 brain areas, of which 2 brain areas (anterodorsal preoptic and periolivary nuclei) showed a continuing rise in Fos-IR after 60 and 120 min, while six nuclei, mostly hypothalamic and septal, showed a peak induction of Fos-IR after 60 min. In three brain areas, no consistent changes in Fos-IR could be observed. The authors conclude that the changes observed in the patterns of Fos-IR, after application of the singly-housed SIH-test in mice, reflect the effects of both the stressor application and the ensuing thermoregulatory responses. The role of each activated brain area in either one of these effects is discussed in view of data available from the literature.

Methamphetamine neurotoxicity in dopamine nerve endings of the striatum is associated with microglial activation

Methamphetamine intoxication causes long-lasting damage to dopamine nerve endings in the striatum. The mechanisms underlying this neurotoxicity are not known but oxidative stress has been implicated. Microglia are the major antigen-presenting cells in brain and when activated, they secrete an array of factors that cause neuronal damage. Surprisingly, very little work has been directed at the study of microglial activation as part of the methamphetamine neurotoxic cascade. We report here that methamphetamine activates microglia in a dose-related manner and along a time course that is coincident with dopamine nerve ending damage. Prevention of methamphetamine toxicity by maintaining treated mice at low ambient temperature prevents drug-induced microglial activation. MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine), which damages dopamine nerve endings and cell bodies, causes extensive microglial activation in striatum as well as in the substantia nigra. In contrast, methamphetamine causes neither microglial activation in the substantia nigra nor dopamine cell body damage. Dopamine transporter antagonists (cocaine, WIN 35,428 [(-)-2-beta-carbomethoxy-3-beta-(4-fluorophenyl)tropane 1,5-naphthalenedisulfonate], and nomifensine), selective D1 (SKF 82958 [(+/-)-6-chloro-7,8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrobromide]), D2 (quinpirole), or mixed D1/D2 receptor agonists (apomorphine) do not mimic the effect of methamphetamine on microglia. Hyperthermia, a prominent and dangerous clinical response to methamphetamine intoxication, was also ruled out as the cause of microglial activation. Together, these data suggest that microglial activation represents an early step in methamphetamine-induced neurotoxicity. Other neurochemical effects resulting from methamphetamine-induced overflow of DA into the synapse, but which are not neurotoxic, do not play a role in this response.

Anxiolytic-like effect of succinic acid in mice

The putative anxiolytic activity of succinic acid was examined in male mice by using a number of experimental paradigms of anxiety and compared with that of the known anxiolytic compound diazepam. Use of the elevated plus-maze test revealed that diazepam (1.0, 2.0 and 4.0 mg/kg, PO) or succinic acid (3.0 or 6.0 mg/kg, PO) increased the percentage of entries into open arms and of time spent on open arms. In novel food consumption test, succinic acid (3.0, 6.0, and 12.0 mg/kg, IP) caused significant increases in food intake during 5 min when compared with the vehicle. In the stress-induced hyperthermia test, 40 min after drug administration rectal temperature was measured, succinic acid at dose of 1.5 mg/kg, inhibited stress-induced hyperthermia. Thus, these findings indicated that, in contrast with diazepam, succinic acid exhibits anxiolytic-like effect.

5-Hydroxytryptamine 1A receptors inhibit cold-induced sympathetically mediated cutaneous vasoconstriction in rabbits

5-HT1A receptor agonists lower body temperature. We have investigated whether activation of 5-HT1A receptors inhibits cutaneous sympathetic discharge so that dilatation of the cutaneous vascular bed lowers body temperature by increasing heat transfer to the environment. We measured ear pinna blood flow in conscious rabbits (with chronically implanted Doppler ultrasound flow probes), and postganglionic sympathetic vasomotor nerve activity in anaesthetized rabbits. Recordings from conscious rabbits were made in a cage at 26 degrees C and the rabbit was then transferred to a cage at 10 degrees C. The ear pinna Doppler signal fell from 56 +/- 4 cm s-1 in the 26 degrees C cage to 4 +/- 1 cm s-1 (P < 0.0001, n = 24) after 30 min in the 10 degrees C cage, and body temperature increased from 38.8 +/- 0.2 to 39.0 +/- 0.2 degrees C (P < 0.01, n = 24). The 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT; 0.1 mg kg-1 I.V.) reversed the cold-induced fall in ear pinna blood flow (Doppler signal increased from 5 +/- 1 to 55 +/- 8 cm s-1, P < 0.001, n = 7) within 5 min when administered 30 min after transfer to the 10 degrees C cage, and prevented the fall in ear pinna blood flow when administered before the rabbit was transferred to the 10 degrees C cage. Body temperature decreased after administration of 8-OH-DPAT. These changes were abolished by the specific 5-HT1A antagonist WAY-100635 (0.1 mg kg-1 I.V.). In anaesthetized rabbits, 8-OH-DPAT (0.1 mg kg-1 I.V.) reduced resting postganglionic cutaneous sympathetic vasomotor discharge, and prevented the increase normally elicited by cooling the trunk. Our experiments constitute the first demonstration that activation of 5-HT1A receptors powerfully inhibits cold-induced increases in cutaneous sympathetic vasomotor discharge, thereby dilating the cutaneous vascular bed and increasing transfer of heat to the environment.

Characteristics of thermoregulatory and febrile responses in mice deficient in prostaglandin EP1 and EP3 receptors

Previous studies have disagreed about whether prostaglandin EP1 or EP3 receptors are critical for producing febrile responses. We therefore injected lipopolysaccharide (LPS) at a variety doses (1 microg kg(-1)-1 mg kg(-1)) intraperitoneally (i.p.) into wild-type (WT) mice and mice lacking the EP1 or the EP3 receptors and measured changes in core temperature (Tc) by using telemetry. In WT mice, i.p. injection of LPS at 10 microg kg(-1) increased Tc about 1 degrees C, peaking 2 h after injection. At 100 microg kg(-1), LPS increased Tc, peaking 5-8 h after injection. LPS at 1 mg kg(-1) decreased Tc, reaching a nadir at 5-8 h after injection. In EP1 receptor knockout (KO) mice injected with 10 microg kg(-1) LPS, only the initial (< 40 min) increase in Tc was lacking; with 100 microg kg(-1) LPS the mice showed no febrile response. In EP3 receptor KO mice, LPS decreased Tc in a dose- and time-dependent manner. Furthermore, in EP3 receptor KO mice subcutaneous injection of turpentine did not induce fever. Both EP1 and EP3 receptor KO mice showed a normal circadian cycle of Tc and brief hyperthermia following psychological stress (cage-exchange stress and buddy-removal stress). The present study suggests that both the EP1 and the EP3 receptors play a role in fever induced by systemic inflammation but neither EP receptor is involved in the circadian rise in Tc or psychological stress-induced hyperthermia in mice.

5-hydroxytryptamine(2A) receptors regulate sympathetic nerves constricting the cutaneous vascular bed in rabbits and rats

Hyperthermia induced by 3,4-methylenedioxymethamphetamine (MDMA) is partially due to sympathetically-mediated cutaneous vasoconstriction that impairs normal heat dissipation. MDMA acts by releasing monoamines, including 5-hydroxytryptamine (5-HT), but receptor mechanisms underlying MDMA-elicited hyperthermia and cutaneous vasoconstriction are not known. The specific 5-HT2A agonist (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) is a potent hallucinogen that also causes marked hyperthermia, suggesting the possibility that DOI, via stimulation of 5-HT2A receptors, might also cause sympathetically mediated cutaneous vasoconstriction. We tested this hypothesis in conscious unrestrained rabbits and rats. Blood flow was assessed by chronically implanted Doppler ultrasonic flow probes. Body temperature was measured by i.p. telemetric probes. We compared effects of DOI on cutaneous blood flow (ear pinna in rabbits, tail in rats) with effects on mesenteric blood flow and arterial pressure.Hyperthermia induced by DOI (5-100 microgram/kg i.v. in rabbits and 100 microgram/kg s.c. in rats) was preceded and accompanied by markedly reduced blood flow to the cutaneous bed, with no change in flow to the mesenteric bed. In rabbits, DOI (5 microgram/kg i.v.) did not affect arterial pressure or heart rate. DOI (100 microgram/kg i.v.) caused a moderate rise in arterial pressure. In rabbits, the 5-HT2A receptor antagonists ketanserin (0.3 mg/kg i.v.) and AC90179 (0.5 mg/kg i.v.) reversed the ear pinna vasoconstriction induced by DOI (5 microgram/kg i.v.). In rats, ketanserin (3 mg/kg s.c.) reversed tail vasoconstriction and hyperthermia induced by DOI (100 microgram/kg s.c.). In rabbits, the cutaneous vasoconstricting effect of DOI (5 microgram/kg i.v.) was substantially abolished in the ipsilateral ear pinna after interruption of preganglionic sympathetic nerve activity by unilateral section of the cervical sympathetic trunk. Thus hyperthermia evoked by direct stimulation of 5-HT2A receptors is associated with marked sympathetically mediated vasoconstriction, selective for the cutaneous bed. Impairment of the ability to dissipate heat following drug-induced stimulation of 5-HT2A receptors is likely to contribute to hyperthermia induced by MDMA and by hallucinogenic drugs such as LSD.

An exploratory factor analysis of the Tail Suspension Test in 12 inbred strains of mice and an F2 intercross

To explore the genetic dimensions of the stress response in rodents, we tested 12 inbred strains of mice and an F2 intercross (n=745) on the Tail Suspension Test (TST) and the Tail Suspension-Induced Hyperthermia (TSIH) paradigm. These selected 12 strains provide a representative sampling of the genetic heterogeneity of mousedom. An F2 intercross was derived from NMRI and 129S6 strains, which differ in their responses on the TST. Both inbred strains and F2 mice underwent a standardized protocol of automated TST with two sessions: (1) baseline and (2) imipramine TST. The duration of immobility and the body temperature after TST were recorded. The inbred strains were also tested in the Light-Dark Transition (LDT) test and in the Open Field Test (OFT), measuring the distance traveled, vertical movements, and center time as independent variables. The F2 mice were measured for core temperature after TST (TSIH). High intercorrelations among strain means were found for the LDT and OFT measures. Principal components analysis extracted four factors: "exploratory fear," body weight, imipramine response on immobility, and "stress reactivity." These dimensions were largely confirmed in the F2 population with one additional factor: imipramine response on TSIH. The results support a distinction between "stress reactivity" as measured by the TST and "exploratory fear" behavior as measured by the LDT and OFT.

Role of the brain heme oxygenase-carbon monoxide pathway in stress fever in rats

This study was aimed at testing the hypothesis that the brain heme oxygenase (HO)-carbon monoxide (CO) pathway plays a role in stress fever. To this end, the effect of the HO inhibitor, zinc deuteroporphyrin 2,4-bis glycol (ZnDPBG), on restraint-induced fever was tested. Intracerebroventricular ZnDPBG (200 nmol) did not affect the body core temperature of unrestrained rats, but markedly attenuated restraint-induced fever. However, at the same dose, intraperitoneal ZnDPBG did not affect the febrile response to restraint. Taken together, these results indicate that the brain HO-CO pathway plays a major role in the genesis of stress fever in rats.

Tail-suspension induced hyperthermia: a new measure of stress reactivity

The tail suspension test (TST), an antidepressant screening paradigm, uses the uncontrollable, inescapable stressor of tail suspension to elicit immobility. As hyperthermia occurs following numerous stressors, hyperthermia might exist following the TST. We tested whether tail suspension induced hyperthermia (TSIH) was a distinct variable for TST. Hyperthermia was measured by two methods: a rectal probe and a subcutaneously implanted microchip (ELAMS()). In outbred ICR male mice, TSIH was robustly demonstrated compared to control (No-TST) mice. TSIH peaked after TST and remained elevated at 120 min. Among five (129/SvEvTac, A/J, C57BL/6J, NMRI and ICR) strains examined for TSIH, significant strain variations were detected. NMRI showed the highest temperature rise (2.3 degrees C) and A/J mice showed the lowest (0.6 degrees C). Sex differences were found for the C57BL/6J and NMRI strains on TSIH. TSIH and duration of immobility were not significantly correlated (r=0.22, P=0.17) in outbred mice. Both duration of TST immobility and TSIH were measured when ICR male mice were administered diazepam, imipramine (a TCA antidepressant), venlafaxine (a SNRI antidepressant), sertraline and paroxetine (SSRI antidepressants), propranolol and nadolol (beta-adrenergic receptor blockers), CP-154,526 (a CRF(1) receptor antagonist), and indomethacin (a cyclo-oxygenase inhibitor). Diazepam dose-dependently increased immobility and decreased TSIH. Propranolol blocked TSIH, but nadolol had no effect. Antidepressants showed more complex patterns of effects with venlafaxine, sertraline, and paroxetine inhibiting TSIH. TSIH demonstrated inter-strain variability, sex differences and a distinct pharmacology, suggesting that TSIH provides an independent, robust physiologic parameter to supplement the TST paradigm. This TSIH method may prove useful for pharmacologic, transgenic, and mechanistic studies.

Stress-induced hyperthermia and anxiety: pharmacological validation

When mammals, including man, are confronted with a stressful event, their core body temperature rises, stress-induced hyperthermia. In mice, the stress-induced hyperthermia procedure has been developed to measure antistress or anxiolytic-like effects of psychoactive drugs. Group-housed and singly housed versions of the stress-induced hyperthermia generate comparable results. Because the number of animals needed to perform an experiment is much lower in the singly housed versus the group-housed procedure, the former is the test of choice for pharmacological testing. A typical stress-induced hyperthermia test starts with an injection 60 min before the first rectal temperature measurement (T(1)), followed by a second temperature measurement (T(2)) 10-15 min later. The difference DeltaT (=T(2)-T(1)) is the stress-induced hyperthermia. The procedure also measures the intrinsic activity of drugs on the basal body temperature and DeltaT is relatively independent from the intrinsic temperature effects of drugs. Anxiolytic drugs (benzodiazepines, 5-HT(1A) receptor agonists, alcohol) reduce DeltaT suggestive of anxiolytic-like effects. Because the parameter measured for anxiety in the stress-induced hyperthermia procedure is not dependent on locomotor activity, like in almost all other anxiety tests, the stress-induced hyperthermia procedure is an attractive addition to tests in the anxiety field. Because the stress-induced hyperthermia is also present with a comparable pharmacological profile in females, this procedure has a wide species and gender validity. The procedure was applied in various genetically modified mice [5-HT(1A) and 5-HT(1B) receptor knockout (KO) mice and corticotropin-releasing hormone overexpressing (CRH-OE) mice] to study phenotypic influences of the various mutations on aspects of anxiety. The stress-induced hyperthermia test in singly housed male and female mice appears a useful and extremely simple test to measure effects of drugs on certain aspects of anxiety or to help to determine phenotypic differences in mutant mice.

GABAA-benzodiazepine receptor complex ligands and stress-induced hyperthermia in singly housed mice

Stress-induced hyperthermia (SIH) in singly housed mice, in which the rectal temperature of a mouse is measured twice with a 10-min interval, enables to study the effects of a drug on the basal (T1) and on the stress-enhanced temperature (T2), 10 min later, using the rectal procedure as stressor. SIH (T2-T1) reflects a stress-induced phenomenon sensitive to stress- or anxiety-modifying effects of drugs. Several benzodiazepine agonists (diazepam, chlordiazepoxide, oxazepam and alprazolam) dose-dependently antagonized SIH either in NMRI mice from two different breeders or in BALB/c mice. No major differences in the sensitivity for any of the drugs tested were found between strains or between substrains from different breeders. The selective BZ1 receptor agonists alpidem and zolpidem only at relatively high doses antagonized SIH, whereas flumazenil, FG7142, pentylenetetrazol and phenobarbital did not affect SIH. Alcohol antagonized SIH, and the effects of diazepam could be antagonized by flumazenil. The findings that full BZ receptor agonists have anxiolytic-like effects in the singly housed SIH paradigm are comparable to those previously found in the group-housed version. The singly housed SIH is proposed as a simple and reliable screen for detecting anxiety-like properties of drugs that is valid in every mouse strain tested so far.

5-HT(1B) receptor knockout mice show no adaptive changes in 5-HT(1A) receptor function as measured telemetrically on body temperature and heart rate responses

Two presynaptic receptors play an important role in the regulation of serotonergic neurotransmission, i.e., the 5-HT(1A) and 5-HT(1B) receptor. The present study focuses on putative adaptive changes in the 5-HT(1A) receptor system in mice that lack 5-HT(1B) receptors (5-HT(1B) KO). 5-HT(1A) receptor sensitivity was assessed in vivo in two models of presynaptic 5-HT(1A) receptor activity: agonist-induced hypothermia and prevention of stress-induced hyperthermia. The effects of 5-HT(1A) receptor activation by flesinoxan (0.1-3.0 mg/kg s.c.) were determined telemetrically on body temperature and heart rate in 5-HT(1B) KO and wild-type (WT) mice. Flesinoxan induced hypothermia dose-dependently without affecting heart rate and prevented stress-induced hyperthermia and tachycardia equipotently in both genotypes. Specificity of these responses was confirmed by blockade with the selective 5-HT(1A) receptor antagonist WAY100635 (1.0 mg/kg s.c.). The importance of continuous sampling in freely moving subjects to improve appropriate characterization of mutants is discussed. 5-HT(1B) KO mice showed no shift in 5-HT(1A) receptor sensitivity compared to WT mice. This study found no indications for adaptive changes in presynaptic 5-HT(1A) receptor function in 5-HT(1B) KO mice as measured telemetrically on body temperature and heart rate responses.

Mechanisms and mediators of psychological stress-induced rise in core temperature

OBJECTIVE

Despite numerous case reports on "psychogenic fever," it remains uncertain how psychological stress raises core temperature and whether the rise in core temperature is a real fever or a hyperthermia. This article reviews studies on the psychological stress-induced rise in core temperature (PSRCT) in animals with the aim to facilitate studies on the mechanisms of so-called psychogenic fever in humans.

METHODS

To address this question, we reviewed the mechanisms and mediators of the PSRCT and classic conditioning of the fever response in animals.

RESULTS

The PSRCT is not due to the increased locomotor activity during stress, and the magnitude of the PSRCT is the same in warm and cold environments, indicating that it is a centrally regulated rise in temperature due to an elevated thermoregulatory "set point." The PSRCT caused by conventional psychological stress models, such as open-field stress, is attenuated by cyclooxygenase inhibitors, which block prostaglandin synthesis. On the other hand, the PSRCT elicited by an "anticipatory anxiety stress" is not inhibited by cyclooxygenase inhibitors but by benzodiazepines and serotonin Type 1A receptor agonists. The febrile response can be conditioned to neutral stimuli after paired presentation with unconditioned stimuli such as injection of lipopolysaccharide, a typical pyrogen.

CONCLUSIONS

Most findings indicate that the PSRCT is a fever, a rise in the thermoregulatory set point. The PSRCT may occur through prostaglandin E2-dependent mechanisms and prostaglandin E2-independent, 5-HT-mediated mechanisms. The febrile response can be conditioned. Thus, these mechanisms might be involved in psychogenic fever in humans.

Alarm pheromone enhances stress-induced hyperthermia in rats

Behavioral and physiological effects of alarm pheromones emanating from stressed conspecific animals were investigated. Experimentally naive male Wistar rats were exposed to the test chambers containing alarm pheromones, which had been released by other rats receiving foot shocks in the same chamber beforehand. Along with behavioral analysis, the heart rate (HR) and core body temperature (cBT) were measured simultaneously using a biotelemetory system. Exposure to the alarm pheromones increased freezing, sniffing and walking and decreased resting as compared with rats exposed to control odors. In addition, these pheromone-exposed animals showed consistent increases in body temperature, i.e., stress-induced hyperthermia. After exposure to the alarm substances, immunoreactivity to nuclear Fos protein in the mitral cell layer in the accessory olfactory bulb (AOB) also increased compared with the reaction to control odors. These results suggest that an alarm pheromone enhances stress responses of conspecific animals both behaviorally and physiologically, and that these effects are mediated via activation of the AOB.

Stress-induced hyperthermia in mice: effects of flesinoxan on heart rate and body temperature

Stress-induced hyperthermia in mice has predictive validity for anxiolytic properties of drugs. In this paradigm, 60 min after drug administration rectal temperature is measured, which causes hyperthermia of 1-1.5 degrees C (DeltaT) in about 10 min. Flesinoxan, a selective 5-HT(1A) receptor agonist with anxiolytic-like properties, causes hypothermia, which complicates interpretation of stress-induced hyperthermia. Therefore, we combined flesinoxan treatment and the stress paradigm with radiotelemetric measurement of body temperature and heart rate, which is also related to anxiety. Subjects were either undisturbed or injected with flesinoxan (0-0.1-0.3-1.0 and 3.0 mg/kg), with or without the stress paradigm. Flesinoxan (1.0 and 3.0 mg/kg) caused a relatively long-lasting hypothermia, but did not lower heart rate. The rectal temperature procedure caused hyperthermia and tachycardia. Flesinoxan reduced the stress-induced hyperthermia and the tachycardia evoked by the stress procedure. Continuous radiotelemetric measurement of heart rate, apart from body temperature, revealed that flesinoxan has anxiolytic-like properties in mice.

5-HT(3) receptor antagonists and anxiety; a preclinical and clinical review

The present paper reviews the evidence for anxiolytic activity of 5-HT(3) receptor antagonists in animal models of anxiety and in clinical trials in humans. Compared to the established anxiolytics (benzodiazepine receptor agonists and, to a lesser extent, 5-HT(1A) receptor agonists) 5-HT(3) receptor antagonists display a different anxiolytic profile. They are anxiolytic in a limited number of animal anxiety models. If active, they often are very potent and display bell-shaped dose response curves, whereas the ratio between therapeutic activity and side effects appears remarkably large. 5-HT(3) receptor antagonists remain active after chronic dosing and no indications for tolerance, dependence or rebound effects were found, which seems to make these drugs an attractive alternative to the benzodiazepines. However, the large body of animal data indicating a complete lack of psychotropic activity of 5-HT(3) receptor antagonists weakens the prediction of anxiolytic activity in these drugs. Human data are also controversial; some investigators have reported positive effects in anxiety disorders (panic disorder, GAD), others did not. It can be concluded that 5-HT(3) receptor antagonists do not represent a breakthrough in the treatment of various anxiety disorders, as initially suggested.

Disappearance of stress-induced hyperthermia following a low dose of X-irradiation: involvement of the vomeronasal system in the modulation of the radiation-induced effects

When the rectal temperatures of group-housed mice are measured sequentially, the temperature of the last mouse to be measured is higher than that of the first mouse. The hyperthermia effect observed in the last animal to be measured forms the basis of an experimental paradigm for studying the neurobiology of anticipatory anxiety. Stress-induced hyperthermia (SIH) was calculated as the difference (delta T) between the basal temperature (the averages of the first three mice) and the final temperature (the averages of the last three mice) when the temperatures of the 15 mice were measured sequentially, with a 2 min interval between each temperature measurement. The hyperthermia observed in the last animals measured was abolished by prior treatment with X-irradiation at the relatively low dose of 5-15 cGy. Prevention of the SIH response could be found when the irradiation was confined to the head region only, suggesting the importance of the brain in the radiation-induced effect. Relatively higher doses of 25 or 35 cGy failed to reduce the hyperthermia stress effect. Furthermore, the effect of X-irradiation was not observed following olfactory bulbectomy or resection of the vomeronasal tract. These results indicate that the disappearance of SIH response may only be found following irradiation at low dose levels. Furthermore, the results implicate the olfactory system in the radiation-induced anti-stress effect.

The 5-HT1A receptor agonist flesinoxan increases aversion in a model of panic-like anxiety in rats

Acute systemic administration of the selective serotonin (5-HT)1A receptor full agonist flesinoxan enhanced the sensitivity of rats to the panic-like aversion elicited by local stimulation of the dorsolateral periaqueductal grey (dPAG). This experimental paradigm in rats has previously been validated as a simulation of acute anxiety with particular relevance to panic disorder. The dose-dependent decrease in threshold for acute fear responses recorded in rats following intraperitoneal administration of flesinoxan (1-10 mg/kg) was similar to that induced by the panic precipitating agent yohimbine and opposite to the threshold increase induced by the antipanic drug alprazolam. The proaversive effect of flesinoxan observed in rats is consistent with the reported aggravation of the condition of panic patients following oral flesinoxan treatment. Thus, the model adequately detects drug-induced panicogenic-like properties. Data suggest that selective activation of 5-HT1A receptors (pre- and/or post-synaptic in brain and/or periphery) following systemic administration of 5-HT1A receptor full agonists exacerbates aversion in animals or patients with panic anxiety; activation of these receptor subtypes may probably mediate the panicogenic action reported under certain circumstances with non-selective 5-HT mimetics.

The 5-HT1A receptor and its ligands: structure and function

An overview is presented on progress made in research on 5-HT1A receptors and their ligands since their discovery in 1983. Molecular biology has offered new tools, for example cloned 5-HT1A receptors, their mutants and chimeras to study structure and function. Many compounds, belonging to different chemical classes, display high affinity and selectivity for 5-HT1A receptors. The majority of these compounds are agonists or partial agonists, full antagonists are still scarce. Agonists and partial agonists are active in various animal models of anxiety and depression. Partial receptor agonists have been proven to be effective in general anxiety disorder and depression in man. Potential therapeutic applications for 5-HT1A receptor antagonists are evaluated, for example, in cognition disorders.

Stress and emotionality: a multidimensional and genetic approach

The use of behavioural tests aiming to assess the psychological components of stress in animals has led to divergent and sometimes arbitrary interpretations of animal behaviour. This paper presents a critical evaluation of behavioural methods currently used to investigate stress and emotionality. One of its main goals is to demonstrate, through experimental evidence, that emotionality may no longer be seen as a unidimensional construct. Accordingly, following a discussion about concepts, we propose a multiple-testing approach, paralleled by factor analyses, as a tool to dissociate and study the different dimensions of emotionality. Within this multidimensional context, genetic studies (illustrated here by different rat models) are shown to be particularly useful to investigate the neurobiology of stress/emotionality. A genetic approach can be used (i) to broaden and dissect the variability of responses within and between populations and (ii) to search for the molecular bases (i.e. genes and gene products) which underlie such a variability.

Recurrent aggressive episodes entrain ultradian heart rate and core temperature rhythms

The present study was designed to investigate the effects of recurrent aggressive episodes on the synchrony of autonomic circadian and ultradian rhythms. Eight aggressive male rats were entrained to a reverse 12 h:12 h light-dark cycle and then implanted with telemetry senders to continuously monitor heart rate (HR) and core temperature (Tc). The amplitude and the time of the peak (acrophase) for each of the circadian and ultradian oscillations were quantified by nonlinear, least-squares, multioscillator cosinor analysis that included the first four harmonics of the circadian rhythm. After recovery from surgery, the 3- and 5-cycle/day ultradian rhythms of HR and Tc were the prominent ultradian components that were synchronized to the light-dark cycle. First, the resident males confronted a male intruder daily at lights-off (0800 hours) for a period of 3 weeks. Second, after a 3-week recovery period, 15 daily aggressive confrontations were scheduled, with the intruders being introduced at 1200 hours. During the course of the confrontations the amplitude of 3- and 5-cycle/day oscillations in HR and Tc decreased, whereas the hemicircadian (2 cycles/day) rhythm amplitude doubled with minor changes of the circadian amplitude. The hemicircadian acrophase coincided with the time of the confrontation most clearly, and this alignment lasted for more than 1 week after the last social confrontation, even in the absence of a reminder. We interpret the synchronization of the hemicircadian acrophases to the time point of social confrontations as anticipating the physiological demands of the aggressive encounters.

Anxiolytic effects of flesinoxan in the stress-induced hyperthermia paradigm in singly-housed mice are 5-HT1A receptor mediated

In the stress-induced hyperthermia paradigm in singly-housed male mice, two sequential rectal temperature measurements reveal the basal temperature (T1) and, 10 min later, an enhanced body temperature (T2), due to the stress of the first rectal measurement. The difference T2 - T1 (deltaT) is the stress-induced hyperthermia and putatively reflects a stress-induced anxiogenic response. The full 5-HT1A receptor agonist flesinoxan ((+)-enantiomer), its (-)-enantiomer and the racemic mixture reduced stress-induced hyperthermia effects, indicating putative anxiolytic properties. The ratio of their potencies to reduce stress-induced hyperthermia was similar to their potency in receptor binding affinities for 5-HT1A receptors, supporting that the anti-hyperthermia effects are mediated by the 5-HT1A receptor. This was further substantiated when the 5-HT1A receptor antagonists WAY 100635 ((N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclo-hexane carboxamine trihydrochloride) and DU 125530 (2-[4-[4-(7-chloro-2,3-dihydro-1,4-benzodioxin-5-yl)-1-piperazinyl ]butyl]-1,2-benzisothiazol-3(2H)-one-1,1-dioxide, monomesylate) both were able to antagonize the anti-stress-induced hyperthermia effects of flesinoxan. The stress-induced hyperthermia paradigm in singly-housed mice represents a simple and robust paradigm to measure putative anxiolytic effects of drugs.

Influence of 5-HT1A receptor antagonism on plus-maze behaviour in mice. II. WAY 100635, SDZ 216-525 and NAN-190

To understand further the role of 5-hydroxytryptamine receptor subtype 1A (5-HT1A) mechanisms in anxiety, the behavioural effects of 5-HT1A receptor antagonists with different selectivity and intrinsic activity were examined using an ethological version of the murine elevated plus-maze test. WAY 100635 (0.03-9.0 mg/kg) produced a behavioural profile indicative of an anxiolyticlike effect, with an apparent bell-shaped dose-response relationship and increases in nonexploratory behaviours at the largest dose tested. SDZ 216-525 exerted a dose-dependent antianxiety action at doses of 0.05-0.8 mg/kg, with some loss of activity at 3.2 mg/kg. In contrast, smaller doses of NAN-190 had a significant effect, whereas higher doses (2.5-10.0 mg/kg) decreased locomotor activity and other active behaviours, a profile similar to that produced by the alpha1-adrenoceptor antagonist prazosin (2.5 mg/kg), which also inhibited open arm activity. Findings are discussed in relation to 5-HT1A receptor and alpha1-adrenoceptor antagonism and corresponding neurochemical changes. The results of the present series support the view that 5-HT1A receptor antagonists have therapeutic potential in the management of anxiety.

Effects of 5-HT1A receptor ligands in a modified Geller-Seifter conflict model in the rat

In a modified Geller-Seifter conflict procedure, rats were trained to lever-press for food under a multiple variable interval-fixed ratio (VI30: food; FR10: food + shock) schedule of reinforcement. The ability to antagonize response suppression in the punished period is considered a good predictor for anxiolytic activity. Chlordiazepoxide and alprazolam increased punished responding. The 5-HT1A receptor agonists flesinoxan (R(+)-N-[2[4-(2,3-dihydro-2-2-hydroxymethyl-1,4-benzodioxin-5-yl)- 1-piperazinyl]ethyl]-4-fluorobenzoamide; 0.1-10.0 mg/kg) and 8-OH-DPAT (8-hydroxy-2-(di-n-propyl-amino)tetralin; 0.03-0.5 mg/kg) significantly increased punished responding, supporting a role of the 5-HT1A receptor in anxiety. 8-OH-DPAT and flesinoxan also reduced unpunished responding. The anxiolytic effects of 8-OH-DPAT and flesinoxan could only be antagonized with a high dose (1.0 and 3.0 mg/kg respectively) of the 5-HT1A receptor antagonist WAY-100635 (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohexanecarboxamide trihydrochloride). All doses of WAY-100635 antagonized the 5-HT1A-induced effects on unpunished responding. The dissimilarity in dose-response curve of WAY-100635 on punished and unpunished behaviour poses questions about the mediation of these effects.

Neuroendocrine effects of diazepam and flesinoxan in the stress-induced hyperthermia test in mice

In the stress-induced hyperthermia (SIH) paradigm in mice, both a benzodiazepine receptor agonist, diazepam, and a 5-HT1A receptor agonist, flesinoxan, reduced the stress-induced increase in rectal temperature. The SIH procedure itself enhanced plasma ACTH and corticosterone levels but not plasma glucose levels. Diazepam (3, 6, and 12 mg/kg p.o.) did neither affect basal plasma ACTH, corticosterone, or glucose levels, nor did it suppress the stress-induced rises in these parameters. Flesinoxan (1, 3, and 10 mg/kg p.o.) enhanced plasma ACTH and corticosterone concentrations under nonstress conditions but did not affect the stress-induced increases in ACTH and corticosterone secretion. No clear effects of flesinoxan on plasma glucose levels were found. Our results indicate that in mice the anxiolytic effects of diazepam and flesinoxan in the SIH paradigm are not paralleled by a blockade of stress-induced increases in plasma ACTH, corticosterone, and glucose levels.