Participação da rafe bulbar na regulação da temperatura corporal em ave precoce

Abstract

Numerous evidences in rodents indicate that caudal brain regions, such as the raphe, are involved in the activation of production effectors (tremor-dependent and tremor-independent thermogenesis) and conservation (peripheral vasoconstriction) of thermal energy. On the other hand, in birds, which had an evolutionary trajectory for acquiring endothermy totally separate from mammals, this scenario of neural regulation of body temperature is not clear. Therefore, the present study aimed to investigate the participation of medullary raphe in the activation of thermoeffectors of cold (thermogenesis and peripheral vasoconstriction) and heat (dry and evaporative heat loss) in chicks in the first week of post-hatch life. For this, body temperature, oxygen consumption (thermogenic index), pulmonary ventilation (thermal tachypnea to heat or hyperpnea to cold) and dry heat loss index (from skin temperature) were measured in animals with inhibition and activation of rafe and subjected to cold (26°C), heat (36°C) or neutral environment (31°C). Inhibitions and activations were performed with local microinjection of GABA and NMDA glutamate receptor antagonists. And the results indicated the existence of glutamatergic and GABAergic excitatory influences on the medullary raphe of chicks modulating thermogenesis, and glutamatergic stimulation preventing tachypnea, without having any role in the defense responses to heat. Since the raphe is the main serotonergic region of the brain, the participation of serotonergic neurons in this region in the thermoeffector responses of chicks exposed to cold and heat was also verified. However, analysis of the activation of chick medullary raphe neurons showed that they are activated in the cold (increased expression of c-Fos) and are involved in thermogenesis, but do not appear to be those that synthesize serotonin, since a low percentage of neurons serotonergics expressed c-Fos and there was no effect of serotonergic neuron-specific injury on thermoeffector activity. In addition to serotonergic neurons, the medullary raphe of chicks was also shown to have GABAergic neurons, and the lesion of these neurons affected Tc, O2 consumption, and influenced the ventilatory pattern under cold and thermoneutral conditions. Thus, the results indicate that, unlike in mammals, the modulation of thermogenic and ventilatory responses to cold by medullary raphe in chicken chicks does not involve serotonergic neurons. Among other phenotypes of neurons possibly involved in this neural modulation, we evidence a role for GABAergic raphe neurons. Such results open a new perspective related to the neural regulation of body temperature in endothermic vertebrates.