Changes in brain serotonin and 5-hydroxyindolacetic acid levels in the rabbit following tonic immobility

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.

Immobilization and restraint effects on pain reactions in animals

Acute physical restraint represents a potent stressor in several animal species and is accompanied by a complex pattern of hormonal responses and functional changes in the central nervous system. Repeated immobilization leads to partial blunting of the behavioral and hormonal responses, with transient modifications of neurotransmitter systems in the brain. Pain reactions, as investigated by different kinds of nociceptive tests, are usually attenuated both during and immediately following acute immobilization and the analgesic effect of opiate compounds potentiated; these behavioral alterations may be attributed at least in part to activation of an endogenous opioid system. In some species, restraint may induce a reflex immobility (animal hypnosis or tonic immobility) which is also characterized by suppression of pain reactions in rabbits, probably subserved by different mechanisms. Analysis of available data suggests that pain testing in unanesthetized, restrained animals may involve alterations of the animal's reactivity to noxious stimuli.

Tonic immobility and the serotonergic system in chickens: differential blockade of receptors by cyproheptadine and cinanserin

Low doses of serotonin potentiated immobility durations in 2 1/2-week-old chickens (Gallus gallus) while high doses attenuated the response. Cyproheptadine, a serotonergic antagonist, eliminated increases generated by low doses of serotonin and reduced durations in control subjects, but had no effect on the attenuation produced by high doses of the drug. Cinanserin, another serotonergic blocker, also eliminated the potentiation generated by low doses of serotonin and partially reduced the attenuation caused by high doses, but had no effect on immobility durations in control animals. Larger doses of cinanserin eliminated the attenuation produced by high doses of serotonin and increased durations in control subjects. Results are discussed in terms of the differential blockade of various serotonin receptors.

Interaction of tonic immobility and dexamethasone in the modulation of hippocampal 5-HT activity in rabbits

The immobility reaction (animal hypnosis) in the rabbit is related to basal levels of corticosterone and to modifications in 5-HT activity in the brain. In order to investigate the relationship between adrenocortical function, brain 5-HT activity and immobility response in rabbits, the susceptibility to the reaction and 5-HT and 5-HIAA levels in the hippocampus were studied in animals whose pituitary-adrenal function was altered by chronic DEX treatment. It was still possible to induce immobility in animals treated with DEX. The reduction in duration observed under the present testing schedule (two sessions of immobility separated by a one week interval) did not occur in the treated animals. There was a reduction in 5-HT turnover as a consequence of the immobility response and this confirms previous findings in other brain areas of the same species. In animals pretreated with DEX, so as to drastically reduce ACTH and corticosterone, not only was a reduction in 5-HT turnover prevented, but there was also an increase in 5-HT and 5-HIAA levels when immobility was induced. These results are discussed in terms of the influence of the pituitary-adrenal system in the neuroendocrine and behavioural aspects of the immobility reaction.

Central and peripheral effects of serotonin on the immobility response in chickens

The effects of low and high doses of serotonin on tonic immobility (TI) duration and susceptibility in 10- and 45-day-old chickens were examined. High doses of serotonin reduced the number of inductions required to produce TI, regardless of the subject's age. In contrast, low and high doses of serotonin produced biphasic increases and decreases in TI duration in 10-day-old birds, but there were no apparent effects on immobility duration by either dose of this drug in older chickens. These results are discussed in terms of the formation of the blood-brain barrier in domestic fowl and the differential peripheral versus central actions by serotonin on TI susceptibility and response duration.