Circadian rhythms of tonic immobility in the rat: Evidence of an endogenous mechanism

CHARLES W. HENNIG 0 1 WILLIAM P. DUNLAP 0 1 0 Requests for reprints should be sent to Charles W. Hennig, Department of Psychology, Tulane University , New Orleans, Louisiana 70118 1 Tulane University , New Orleans, Louisiana 70118 Cireadian rhythms of tonic immobility were found in male albino rats raised on a 12-h diurnal light eyde and tested at 6-h intervals. Durations of immobility were twiee as long at 2000 h as at 1400 h. These differenees persisted when rats were exposed to eonstant darkness for 10 days, but disappeared when rats were maintained in eonstant light for the same period. Sinee endogenous cireadian rhythms of certain monoamine levels persist in constant darkness yet disappear under constant light, it is suggested that cycles of tonic immobility in rats are also endogenous. When the diurnal pattern of tonic immobility duration is compared to that of various neurohumors, immobility duration appears to parallel melatonin production and to be opposite in phase to the cycle of serotonin levels. Duration of immobility increased over trials, although the number of inductions required to produce immobility decreased. This suggests that instrumental conditioning may modify the immobility response to some extent. - Many behaviors, in a number of species, run in cycles or rhythms that vary in length. Most animals show circadian rhythms in their patterns of feeding and drinking, vocalization, and gross motor activity (Marler & Hamilton, 1966). Presumably, evolution has selected circadian rhythms in common behaviors that approximate the important cycles in the physical environment, thus allowing maximum efficiency of time and resources (Halberg, 1960). Although the precise mechanisms underlying circadian rhythms have not yet been identified, a number of hypotheses exist that attempt to explain these phenomena. The two most common explanations are the endogenous and exogenous hypotheses (for a review, see Brown, Hastings, & Palmer, 1970). Proponents of the exogenous hypo thesis maintain that circadian cycles result from intracellular rhythms entrained to geo physical fluctuations (Wallace, 1973); common en training agents are thought to be light-dark cycle, day length, and temperature (Aschoff, 1962). The endogenous hypothesis maintains that these rhythms are under control of an innate biochemical "dock" that may be influenced by environmental changes but is not entirely dependent upon them (Wallace, 1973). Tonic immobi1ity (TI), also commonly referred to as animal hypnosis, is a behavior that has recently received much experimental attention. This response, observed in a variety of species, is produced by manual restraint and characterized by a prolonged state resembling paralysis. The immobility response appears to represent an innate fear reaction (Gallup, 1974; Gallup, Nash, Donegan, & McClure, 1971; Gallup, Rosen, & Brown, 1972), and a number of studies have demonstrated its involvement in simulated and actual predatory encounters (Gallup, 1973; Gallup, Nash, & Ellison, 1971; Sargeant & Eberhardt, 1975). In the context of a predatory theory of TI, one might except cyclic changes in the susceptibility of this behavior as a function of time of day, depending on such factors as when an animal is most active and consequently more subject to predation. Recently, several studies have examined the cir cadian rhythms of tonic immobility in a variety of species. Ternes (1977) has reported that duration of TI varied with time of day for testing in both a species of toad (Bujo marinus) and a species of tarantula (Cyrtopholis potoricae) , with the toads showing Iongest durations of TI around dawn and the tarantulas showing longest durations around midnight. Both species are nocturnal. Hennig and Dunlap (in press) have found longer durations of TI at night than during the day in two species of lizard tHemidactylus turcicus and Anolis carolinensisi, although the former is a nocturnal animal while the latter is diurnal. Piroch (1974) has examined cir cadian rhythms of immobility in young domestic chickens and found longer durations of TI at night than during the day in that species also. Similarities in the rhythmic cycles of tonic im mobility in these diverse species, comprising both nocturnal and diurnal animals, suggest that some explanation other than general activity cycles are in volved in these daily changes, perhaps of some as yet unknown biochemical nature. The existence of such cycles of TI in another nocturnal animal, the albino rat, was explored in the present study and the question of whether such cycles are endogenous or exogenous was examined. Since the biochemical cycles of the rat have been studied extensively, research using this species may also permit compari sons of cycles of TI with those of neurotransmitters whose involvement in the mediation of TI is suspected. The albino rat has been regarded as a poor subject for studies of TI (McGraw & Klemm, 1969, 1973; Ratner, 1967; Svorad, 1957; Teschke, Master, & Gallup, 1975); the present study documents method ology by which TI can be elicited reliably in rats and quantifies some behaviors of the rat during im mobilityepisodes. EXPERIMENT 1 The first experiment was designed to determine if there are cyclic rhythms for either duration of tonic immobility or the number of inductions required to produce TI, and to delineate other aspects of im mobility behavior. Method Subjects. Twenty-four experimentally naive male albino rats of the Charles River strain, 85-90days of age at the time of testing, were used as subjects. The rats were housed two to a cage until they reached 60 days of age, at which time they were moved to individual cages, Purina Rat Chow and water were avail able ad lib. A light/dark cycle, in which the light was provided by four sets of fluorescent ceiling fixtures and the dark by their absence, with the lights off between 1800 hand 0600 h, was in effect throughout the study. Apparatus. A large cardboard box was used to transport each rat individually to the sound-attenuated testing room, which was lighted by a single 15-W bulb in a lamp with a frosted glass cover. The animals were immobilized in a wooden trough, 30 cm long and 16 cm wide at the top, which sloped from 8 cm at the sides to 2 cm at the middle, and which prevented subjects from accidental Iy rolling on their sides and terminating immobility episodes prematurely. A stopwatch was used to time durations of im mobility, and leather gloveswere used to handle all animals. Procedure. At the start of testing, the subjects were divided into four equal groups of six rats each. The first group was tested for tonic immobility at 0800 h, the second group at 1400 h, the third at 2000 h, and the fourth at 0200 h. After a I-day interval, each group was tested at another of the four times of day. This procedure was repeated four times, with a l-day interval between each test period, such that each rat was tested at all four times of d (...truncated)