The effects of capsaicin on emotional responses to odors in the rat.

Capsaicin is described as disturbing the autonomic responses to stress-inducing environments. The effects of capsaicin (130 mg/kg in 2 series of subcutaneous injections) on emotionality responses were studied in 19 Sprague-Dawley male rats using the open-field test. Eleven rats treated with isotonic saline served as controls. Emotionality (E) measured before capsaicin treatment in the open-field ventilated with deodorized air was similar in the 2 groups of rats. Nine out of the 19 treated rats survived. Their E was significantly higher than that of there 10 rats that died from capsaicin. When a frightening odor (fox feces) was added to the open-field E increased in the controls but remained unchanged in the capsaicin-surviving rats. The ability to discriminate palatable food or female odor was similar in the two groups. The results suggest that; 1) Highly emotional rats survived subcutaneous capsaicin injections; 2) Reaction to an emotionality-inducing environment was decreased in the capsaicin-surviving rats; 3) Olfactory discrimination was not impaired by capsaicin treatment.

The effect of an intraperitoneal injection of capsaicin on the thermopreferendum in the frog (Rana esculenta).

In 7 frogs (Rana esculenta) weighing 70 to 180 g, thermopreferendum (Thp), measured by recording cutaneous temperature (Ts) in the animal placed in the warm end of an aqueous temperature gradient (0 degree C-40 degrees C), equalled 25 +/- 2 degrees C. After an intraperitoneal (IP) injection of 20 mg/kg of capsaicin, Thp was significantly decreased and equalled 3 +/- 1 degree C. The frogs then remained in the cold end of the gradient for 60 minutes. When the time of observation was extended to 3 hours, one frog died from hypothermia. Seven to 24 days after the capsaicin injection, Thp was still decreased in 4 surviving frogs (Thp = 15 +/- 2 degrees C). Capsaicin or isotonic saline solution injected in frogs maintained at 25 degrees C ambient temperature had no effect on Ts or on cloacal temperature. According to results previously obtained in homeothermic species, small doses of capsaicin activated heat-loss responses in the frog.

Hypothalamic thermosensitivity in capsaicin-desensitized rats.

In rats, we tested the hypothesis that capsaicin desensitization reduces hypothalamic warm thermosensitivity. We locally heated and cooled the hypothalamus using water-perfused thermodes while observing thermoregulatory variables. In untreated rats, a small dose of capsaicin had profound effects on thermoregulation. However, desensitizing rats to capsaicin had no effect on hypothalamic thermosensitivity for metabolic rate or changes in body temperature due to displacements of hypothalamic temperature. Contrary to current opinion, we conclude that capsaicin desensitization does not alter hypothalamic thermosensitivity to warm or cold.

The effects of long warm and cold ambient exposures on food intake water intake and body weight in the capsaicin desensitized rat.

Seventeen Sprague Dawley rats received, subcutaneously, 250 mg . kg-1 of capsaicin divided into 10 increasing doses (10-50 mg . kg-1) and administered on 7 successive days. Nine controls were treated with an isotonic saline solution using the same protocol. The rats spent, in succession, 5 weeks at 20 degrees C, 6 weeks at 33.5 degrees C, 6 weeks at 8 degrees C, 4 weeks at 30 degrees C and, finally, 5 weeks at 20 degrees C ambient temperature. Their mean food intake (FI), water intake (WI) and body weights were recorded daily. In the 2 groups of rats, FI was inversely related to ambient temperature. However, during the first few days of the exposures, FI in treated rats was greater than controls in the warm environment and less in the cold environment. In controls, WI increased linearly with ambient temperature in the warm environment. This relation was not found in treated rats: they drank less water than controls and lost body weight. During the first days at 8 degrees C ambient temperature, rectal temperature decreased in treated rats and two animals died. The results are similar to those described for rats with hypothalamic lesions. They may also be related to a peripheral effect of the drug.

Behavioral self warming and cooling of spinal canal by rats.

Rats with a chronic thermode implanted in their spinal canal could bar-press to warm or cool their spinal cord. With a "cold" lever, they cooled their spinal canal less in a cold environment than in a warm environment. With a "warm" lever they behaved in the same way, i.e., warmed their spinal canal more in a warm than in a cold environment. In a two-lever situation they pressed the cold and the warm levers alternatively in warm environment, but did not press either in cold environment. These results suggest that cold and warm spinal cord provided the rats with rewards of a different nature.

The effect of ambient temperature on rectal temperature, food intake and short term body weight in the capsaicin desensitized rat.

1. Subcutaneous injections of capsaicin (mean cumulative dose: 80.1 +/- 3.6 mg , kg-1) permanently reduced the capacity of rats to withstand a hot environment when deprived of water. With water available, hyperthermia was discrete or absent in capsaicinized rats in hot environment. 2. Desensitization was followed by a significant decrease in both food intake and body weight. Treated rats recovered normal body weight after 3 weeks. 3. In a hot environment, compared to controls, food intake was significantly increased in capsaicin desensitized rats which maintained their body weight. In cold environment, food intake was decreased in capsaicin desensitized rats which lost body weight. At normal ambient temperature, food intake and body weight were similar in the 2 groups. 4. Caloric intake adjustment at high and at low temperatures was therefore disturbed in capsaicin desensitized rats. It is concluded that hypothalamic thermodetectors implicated in both thermoregulation and food intake behaviour could be partially damaged by capsaicin.

Thermoregulatory behaviour after repetitive cooling of the preoptic area and of the spinal cord in the rat.

The thermoregulatory behaviour of 6 rats was studied during exposure to cold and warm ambient temperatures after either the preoptic area or the cervical spinal cord had been intermittently cooled for an average of 130 h. The precooled animals worked more for heat in cold environments and for cool air in a warm environment than the control animals. This behaviour, probably due to a decreased ability of the precooled animals to retain heat, suggested that the precooled animals were not fully adapted to cold. 1974) will learn to press a lever to modify their thermal environment, and this paper describes the thermoregulatory behaviour of rats after prolonged cooling of the preoptic area, and of the spinal cord. The thermoregulatory behaviour of cold exposed and cold adapted animals has already been studied in several species (Carlton and Marks, 1958; Laties and Weiss, 1960; Revusky, 1966; balwin and Ingram, 1967).

The effect of capsaïcin on temperature regulation of the rat.

1. Subcutaneous injection of capsaïcin (6-11 mg, or 21-66 mg cumulative), permanently reduced the capacity of rats to withstand a hot environment, as described by Jancsó-Gábor et al. (1970). 2. The treated rats thermoregulatory behaviour was not different from that of control rats, both in hot and cold environments. 3. Saliva secretion was decreased in a hot environment, and the weight of the submaxillary glands was reduced in capsaïcinized rats. 4. It is concluded that hyperthermia present in treated rats when subjected to a warm environment is not due to a disruption of sensu stricto temperature regulation, but rather to a decreased salivary secretion. Whether this decrease has a central or a peripheral origin is not known.

Dog behaviour as related to spinal cord temperature.

3 dogs could behaviourally modify their own spinal cord temperature (Tspin. cord). In a hot environment, 2 dogs did not cool their spinal cord, 1 dog warmed it. The higher the environmental temperature, the higher the chosen Tspin. cord. These results seem to imply that this latter dog tended, in warm environment, to behaviourally reduce: Ts greater than Tspin. cord (Ts mean skin temperature). Data obtained previously support this explanation.