Role of Medulary Raphe in the Control of Thermoeffectors of Precocious Birds

Abstract

In mammals, caudal brain regions, such as raphe nuclei, are involved in the modulation of thermoeffectors such as peripheral vasodilation/vasoconstriction and nonshivering and shivering thermogenesis. In birds, it is still completely unknown if the same scenario occurs. The aim of the present study was to verify the effect of GABAA and GLU inhibition in the medullary raphe on body temperature (Tb), oxygen consumption, ventilation and heat loss index (HLI) of precocious birds under neutral (31ºC) and cold (26ºC), using of one-week-old chicks as animal models. To this end, the Tb, oxygen consumption (index of thermogenesis), ventilation and heat loss index were measured before and after intra-raphe microinjections of the GABAA receptor antagonist, bicuculline (0.05 mM and 0.5 mM), and the NMDA receptor antagonist, AP5 (0.5mM and 5mM), or vehicle. Protocols were approved by the local Animal Care Committee (CEUA of FCAV/Unesp, nº 013907/17). Bicuculline (0.5 mM) at 31°C significantly decreased (P<0.05) Tb (vehicle: 41.2±0.1°C; Bicuculline 0.05mM: 40.9±0.1°C; Bicuculline 0.5mM: 40.6±0.2°C) and oxygen consumption (13.9±0.7% decrease) of the animals when compared to the vehicle group. Bicuculline (0.05 mM) at 26°C significantly reduced (P<0.05) Tb (vehicle: 41.4±0.1°C; Bicuculline 0.05mM: 40.6±0.1°C) and oxygen consumption (12.6±1.4% of decrease) of the animals when compared to the vehicle group. AP5 (5 mM) at 31°C significantly decreased (P<0.05) Tb (vehicle: 41.3±0.1°C; AP5 0.5mM: 40.9±0.1°C; AP5 5mM: 40.3±0.2°C) and oxygen consumption (14.7±0.6% decrease) of the animals when compared to the vehicle group. AP5 (0.5 mM) at 26°C significantly reduced (P<0.05) Tb (vehicle: 41.2±0.2°C; AP5 0.5mM: 40.7±0.1°C) and oxygen consumption (17.4±3.1% of decrease) of the animals when compared to the vehicle group. Regarding ventilation, only treatment with AP5 transiently increased breathing frequency during cold exposure (30.6±9.8% increase; P <0.05). At 31ºC, the highest doses of bicuculline and AP5 increased HLI at the beginning of the Tb reduction (P<0.05) and at 26°C there was no difference on HLI. Our results suggest that GABAA and NMDA receptors inhibition in raphe neurons show an inhibitory effect on Tb of chicks affecting thermogenesis under neutral (31ºC) and cold (25ºC) conditions.