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.

5-HT3 receptor ligands lack discriminative stimulus properties

The putative discriminative stimulus of the 5-HT3 receptor antagonists ondansetron and (DL)-11-[(2-methyl-1H-imidazol-1-yl)methyl]-4,5,6,7,10,11,12-octahydroazepinol[3,2,1-jk]-carbazol-12-one hydrochloride (DU122932), and of the 5-HT3 receptor agonists 2-methyl-5-HT and 3,4-dichlorophenylbiguanide (3,4DCPB) were investigated in a standard two-lever, food-reinforced drug-saline discrimination procedure with groups of rats (N= 10 per group). In three groups of rats after 80 sessions with training doses ranging from 0.1 to 4.0 mg/kg po, stimulus control by ondansetron, DU122932 and 2-methyl-5-HT was still absent. The same 30 animals thereafter rapidly learned to discriminate chlordiazepoxide (CDP) from vehicle. In three other groups of rats, stimulus control by CDP was first established. Then, the vehicle was gradually (from 0.1 to 2.0 mg/kg po) replaced by either ondansetron, DU122932 or 2-methyl-5-HT. Finally, the dose of CDP was gradually decreased. In all three groups, stimulus control disappeared. A seventh group was trained to discriminate 3,4DCPB (5.0 mg/kg po) from saline. When training was not successful, dose and route were changed but discrimination was not attained. It is concluded that in the rat, using the classical two lever discrimination procedure, the 5-HT3 receptor ligands ondansetron, DU122932, 2-methyl-5-HT and 3,4DCPB are incapable of producing an internal state that can act as a stimulus to control responding.

Discriminative stimulus properties of eltoprazine in the pigeon

Twelve pigeons were successfully (ED50 = 2.4 mg/kg p.o.) trained to discriminate the 5-HT(1A/B) receptor agonist eltoprazine (5.0 mg/kg p.o.) from its vehicle in a fixed-ratio (FR)30 two-key operant drug discrimination procedure. Tests for generalization and antagonism showed that 5-HT1A receptor agonists, such as 8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)tetralin) (66.7%), flesinoxan (72.7%), buspirone (58.3%), and ipsapirone (36.4%) only partially substituted for the eltoprazine cue. Compounds with mixed agonistic action at 5-HT1 receptors, completely (> or = 80%) [(eltoprazine; TFMPP (1-(3-trifluoromethylphenyl) piperazine (ED50 = 7.68 mg/kg) and RU 24969 (5-methoxy-3-(1,2,3,6-tetrahydropyridin-4-yl-1H-indole) (ED50 = 15.8 mg/kg)] substituted for eltoprazine; whereas m-CPP (1-(3-chlorophenyl)piperazine) did not. The selective 5-HT reuptake inhibitor fluvoxamine partially (44%) substituted for the eltoprazine cue. The 5-HT1A receptor antagonist NAN-190 (1-(2-methoxyphenyl)-4-[4-(2-phtalimido)butyl]piperazine) fully blocked the eltoprazine cue. Both (+/-)-pindolol and (+/-)-propranolol showed partial antagonism of the eltoprazine cue (66.7 and 50.0%, respectively). (+/-)-Pindolol also showed partial substitution (50%) for the eltoprazine cue, but NAN-190 and (+/-)propranolol did not. It is concluded that the discriminatory stimulus properties of eltoprazine in the pigeon are mediated by 5-HT1A and 5-HT1B receptors.

Neurochemical mediators of anxiety have inconsistent effects on hypothalamic self-stimulation in rats

We studied effects of anxiogenic and anxiolytic compounds on the electric self-stimulation of the medial fore-brain bundle in male rats to find out if there is a link between reward and anxiety-related behaviours. The cholecystokinin agonist, caerulein (25-100 micrograms/kg) and the 5-HT agonist 1-(3-chlorophenyl)piperazine (0.2-1 mg/kg) dose-dependently inhibited the electric self-stimulation. The 5-HT2A antagonist, ketanserin, at 2.5 mg/kg, increased the self-stimulation at high currents but not at threshold current. The 5-HT3 antagonist ondansetron (10 and 100 micrograms/kg). The alpha 1-adrenergic antagonist, prazosin (0.125 and 0.5 mg/kg), the beta-adrenergic antagonist, propranolol (5 and 10 mg/kg) and the alpha 2-adreno-receptor antagonist, atipamezole (4 mg/kg), did not affect the self-stimulation. Nor did the benzodiazepine agonist, diazepam (5-15 mg/kg), a benzodiazepine receptor antagonist flumazenil (at 10 and 25 mg/kg) or the inverse agonist of benzodiazepine receptors, N-methyl-beta-carboline-3-carboxamide (10 and 20 mg/kg), cause any substantial changes of the self-stimulation. We conclude that only two anxiolytic drugs (caerulein and 1-(3-chlorophenyl)piperazine) suppress the electric self-stimulation. These findings indicate that anxiogenicity as such is not able to weaken the hypothalamic electric self-stimulation. Anxiety and reward are apparently mediated through separate neural pathways.

Stress-induced hyperthermia as a putative anxiety model

In group-housed mice (ten per cage), mice removed last from their home cage always have higher rectal temperatures than mice removed first from this cage. Stress-induced hyperthermia is calculated as the difference (delta T) between the basal temperature (mouse number 1) and the end temperature (mouse number 10) when the temperature of the ten mice is sequentially measured using a 1-min interval between rectal measurements. Using this protocol, various drugs, belonging to different pharmacological classes, were tested in order to investigate their putative anxiolytic effect, measured as a decrease in delta T. Benzodiazepines (diazepam, alprazolam), alcohol, and some (flesinoxan, buspirone), but not all (ipsapirone) 5-HT1A receptor agonists had anxiolytic properties with this protocol. Clonidine (alpha 2-adrenoceptor agonist) and prazosine (alpha 1-adrenoceptor antagonist) had, but at high doses, some anxiolytic actions. Antidepressants (desipramine, fluvoxamine, nomifensine, tianeptine, amitriptyline, clomipramine, imipramine), serotonergic ligands (ondansetron, ketanserin, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), fenfluramine, metachlorophenylpiperazine (mCPP), eltoprazine) and various other drugs (phenobarbital, pentetrazol, haloperidol, apomorphine, amphetamine, (+)-N-[1-methyl-2-oxo-5-phenyl-2,3-dihydro-1H-1,4-benzodiazepin-3( R)-yl]- N'-(3-methylphenyl)urea (MSD 365260), dizocilpine and acetyl salicylic acid) had no anxiolytic activity. The stress-induced hyperthermia protocol used was unable to detect anxiogenic properties of drugs, probably due to a (physiological) ceiling in the maximal end temperature. The stress-induced hyperthermia protocol with mice can be used to measure anxiolytic properties of drugs and is a fast and robust model which does not need extensive training of animals.

New animal models of anxiety

In an attempt to develop new animal models of anxiety with face and predictive validity for the spectrum of human anxiety disorders, two new animal paradigms have been described, stress-induced hyperthermia (SIH) in mice and ultrasonic pup vocalizations (UV) in rats. In SIH mice develop enhanced body temperature in anticipation of an aversive event. This SIH can be antagonized by benzodiazepines, alcohol and 5-HT1A receptor agonists, but not by specific 5-HT reuptake inhibitors (SSRIs) or 5-HT3 receptor antagonists. When rat pups are separated from their mother and littermates they produce ultrasonic sounds, indicative of a separation distress. Benzodiazepines, 5-HT1A receptor agonists and SSRIs decrease this calling, whereas 5-HT3 receptor antagonists have no effect. Antidepressants in general do not decrease pup calling because in contrast to the SSRIs, noradrenergic uptake blockers enhance calling. These two animal models of anxiety can be added to the range of anxiety models and will be of help in predicting new putative anxiolytic drugs.