Effect of bombesin on thermoregulation of the rabbit

Attainment of thermoregulation as affected by environmental factors

The review addresses the development of thermoregulation in poultry embryos as well as the effect of acute and chronic changes of environmental factors on this process and the incubation temperature being the foremost. In poultry, the early development of adaptive body functions, like the thermoregulatory system, is characterized by the following peculiarities. First, the development of peripheral as well as central nervous thermoregulatory mechanisms start during the prenatal ontogeny. However, their maturity is attained during early postnatal development. In the perinatal period, environmental factors have a high effect on development of temperature regulation. Second, acute changes in the environmental conditions induce as a rule first uncoordinated and immediately nonadaptive reactions. Later, the uncoordinated nonadaptive reactions change into coordinated (adaptive) reactions. Prenatal environmental influences may have a training effect on the postnatal efficiency of the thermo-regulatory system. Third, functional systems of the organism develop from an open loop system without feedback control into a closed system controlled by a feedback mechanism. During this critical period, the actual environment modulates the development of the respective physiological control systems for the entire life period, especially by changes in neuroorganization and expression of related effector genes. Knowledge on these mechanisms might be specifically used to generate long-term adaptation of the organism to the postnatal climatic conditions (perinatal epigenetic temperature adaptation). In poultry, perinatal epigenetic temperature adaptation was developed by changes in the incubation temperature. When a comparison is made in birds, which were incubated at 37.5 degrees C, a low incubation temperature induced postnatal cold adaptation, and warm incubation temperature induced postnatal heat adaptation. Perinatal epigenetic temperature adaptation exhibited changes in the neuronal thermosensitivity in the hypothalamus as well as in the peripheral thermoregulatory mechanisms. These alterations could be already found at the end of incubation. Further, temperature-experienced embryos have a lower c-fos expression than in the control after acute heat stress.

Influence of bombesin on neuronal hypothalamic thermosensitivity during the early postnatal period in the Muscovy duck (Cairina moschata)

The influence of bombesin (1 microg/0.1 ml artificial cerebrospinal fluid) on neuronal thermosensitivity of the preoptic area of the anterior hypothalamus in brain slices of 5- (n = 7 neurons) and 10-day-old (n = 36 neurons) Muscovy ducks (Cairina moschata) was investigated. Similar to adult mammals, most of the neurons investigated increased the firing rate (FR) after bombesin application. Changes in FR were not related to changes in thermal coefficient (TC). The neurons react to bombesin also under synaptic blockade. The bombesin-induced effect on TC (increase or decrease in nearly the same number of neurons, e.g. nine neurons increased and ten decreased TC in 10-day-old ducklings) in the postnatal bird neurons investigated was different from the results described in adult mammals, where the main reaction to bombesin was an increase of TC in warm-sensitive and temperature-insensitive-neurons and a transformation of temperature-insensitive-neurons into warm-sensitive ones. This may be related to the assumption that during early ontogeny, body functions react to exogenous and endogenous factors nonspecifically. It is to speculate, that later, probably at the end of embryonic development or during the early postnatal period, the reactivity of these functions changes qualitatively, so that the reaction of an individual function to different factors becomes specific (ultimately adaptive).

Hibernation triggers and cryogens: do they play a role in hibernation?

A survey of the literary evidence on cryogens and hibernation induction triggers is given and the results of experiments on the effect of hypothalamic or i.v. injections of opioids and plasma from hibernating European hamsters on body temperature control of rabbits are presented. Pharmacological doses of a delta opioid--DADLE (25 or 50 micrograms), when injected into the anterior hypothalamus, induce a small and short-lasting hypothermic effect in cold exposed rabbits, due to the downward shift of the temperature threshold for shivering. Lower doses (5 micrograms) are without effect, similarly as i.v. administrations (500 micrograms/kg) of this substance. Intrahypothalamic injections of met-enkephalin (0.1-1 microgram) induce a slight hyperthermia due to the shift of all thermoregulatory effectors to higher body temperatures. Intrahypothalamic injections of plasma from hibernating European hamsters do not influence the body temperature control in rabbits.

Effects of GABA agonists and antagonists on temperature-sensitive neurones in the rat hypothalamus

1. Extracellular recordings were obtained from 94 warm-sensitive, 6 cold-sensitive and 117 temperature-insensitive neurones in slices of the hypothalamic medial preoptic area of rats, to determine the effect of the GABAA agonist muscimol, the GABAA antagonist bicuculline, the GABAB agonist baclofen and the GABAB antagonist phaclofen on tonic activity and temperature sensitivity. 2. Muscimol and baclofen dose-dependently inhibited the tonic activity of 69% (36/52) and 97% (36/37) of the hypothalamic neurones, respectively, regardless of their type of thermosensitivity. In contrast, the GABAA antagonist bicuculline increased the tonic activity of the majority of neurones (58/83), while the GABAB antagonist phaclofen increased neuronal activity only in the high dose of 100 microM. 3. The temperature sensitivity of hypothalamic neurones was only changed by ligands of GABAB receptors, and this effect was restricted to warm-sensitive neurones. The temperature coefficient (TC) was significantly increased by the GABAB agonist baclofen (delta TC = 0.69 +/- 0.11 imp s-1 degree C-1, P < 0.01, n = 18). In contrast, the GABAB antagonist phaclofen (10 microM) decreased the temperature sensitivity (delta TC = -0.67 +/- 0.09 imp s-1 degree C-1, P < 0.01, n = 10) in doses which did not affect tonic activity. 4. The increase in temperature sensitivity due to the GABAB agonist baclofen was significantly enhanced by co-perfusion of the GABAA antagonist bicuculline, indicating an interaction of GABAA and GABAB receptor-mediated mechanisms with regard to neuronal thermosensitivity. 5. The results suggest that neurones in the medical preoptic area are subject to GABA-mediated tonic inhibition resulting in modulation of tonic activity and temperature sensitivity of warm-sensitive neurones possibly involved in the control of body temperature. The data support the hypothesis that the hypo- or hyperthermic action of an endogenous substance is related to its effect on the thermosensitivity rather than on tonic activity of hypothalamic neurones.

Mode of ACTH antipyretic action

The method of intestinal cooling was used to analyze the effect of centrally administered ACTH in microgram quantities on hypothalamic centers regulating activity of thermoregulatory outputs (cold thermogenesis--CT, peripheral vasomotor tone--PVMT, respiratory evaporative heat loss--REHL). ACTH, when injected into the supraoptic area of the anterior hypothalamus of normal rabbits, had no significant effect on body temperature control. Intrahypothalamic administration of ACTH during the early phase of the fever, induced by intravenous injection of exogenous pyrogen, evoked dissociation of temperature thresholds for cold and warm defence, shifting the threshold for induction of cold thermogenesis to lower central temperatures. The thermosensitivity of centers controlling cold thermogenesis was lowered and the maximal values of cold thermogenesis were depressed to about 30% of those in control rabbits. Central administration of ACTH in the late phase of the fever (120 min after IV injection of endotoxin) induced a smaller effect than in the early phase of the fever--the downward shift of the temperature threshold for cold thermogenesis was less evident and the thermosensitivity of the controller remained unchanged. The changes in activity of thermoregulatory centers that occurred after ACTH in febrile rabbits correspond to those observed in the late phase of the fever in ACTH-untreated rabbits. It is suggested therefore, that the presumed increase in ACTH production during fever might represent a negative feed-back mechanism contributing to the termination of the febrile state.

Thermoregulation of the rabbit during the late phase of endotoxin fever

In the late phase of the fever occurring 120 or more min after i.v. injection of endotoxin (1 microgram/kg) to female rabbits, marked shifts of thresholds for respiratory evaporative heat loss and for peripheral vasodilatation to higher body core temperatures were observed. In contrast, the threshold body core temperature for cold thermogenesis was shifted downwards. As a result, the interthreshold zone was widened. Within the body temperature range of 37.4 to 39.9 degrees C neither heat production or heat loss mechanisms were operant and the body temperature was determined mainly by passive heat transfer between the body and the environment. Outside this zone, the sensitivities of the heat and cold defence activities to changes in body core temperature appeared to be unchanged.