Role of catecholaminergic neurons in the regulation of thermogenesis in brown adipose tissue

Catecholaminergic neurons of the A1, C1 and A7 cell populations project to the region of the raphe pallidus involved in the regulation of sympathetic outflow to brown adipose tissue (BAT), however the role of these neurons in the sympathetic regulation of brown adipose tissue has not yet been evaluated. As we have previously reported, in urethane/chloralose anesthetized rats the increase in BAT sympathetic nerve activity (SNA) evoked by cooling the core temperature to ~35.5°C was completely inhibited by stimulation of the area of the ventrolateral medulla containing the A1/C1 cell populations. Similarly, microinjection of the alpha2‐adrenergic receptor agonist, clonidine (60nl, 20mM) into the raphe pallidus also nearly completely reversed (94 ± 5% inhibition, n=8) the cold‐evoked activation of BAT SNA and decreased the temperature of BAT (T BAT ) by −2.5 ±0.2 °C. In contrast, stimulation of the area containing the A7 catecholaminergic cell population potentiated the cold‐evoked activation of BAT SNA (+1250 ± 80% of control above the cold‐evoked level, n=3), and increased T BAT by +1.9 ± 0.4 °C. These results suggest that different populations of catecholaminergic neurons with inputs to the raphe pallidus may have differential impacts on neurons regulating BAT SNA. The details of the neuroanatomical and neuropharmacological underpinnings of these responses and the physiological stimuli that drive these neurocircuits represent important ongoing avenues of research.