Presence of alpha-1 norepinephrinergic and GABA-A receptors on medial preoptic hypothalamus thermosensitive neurons and their role in integrating brainstem ascending reticular activating system inputs in thermoregulation in rats.

Thermal messages are relayed to the medial preoptic O-anterior hypothalamus (mPOAH) via the ascending reticular activating system (ARAS). According to previous findings that norepinephrine (NE)-ergic and GABA (gamma-amino butyric acid)-ergic inputs convey thermal information to the CNS, those neurotransmitters may be responsible for reciprocal correlation between body temperature and mPOAH warm-(WSNs) and cold-(CSNs) sensitive neuronal firing rates for thermoregulation. In this study on Wistar rats, we have characterized in vivo the role of alpha-1 NE-ergic and GABA-A receptors in the possible modulation of ARAS inputs to the thermosensitive neurons in the mPOAH. Nine WSNs, 7 CSNs and 19 thermo-insensitive neurons were recorded from mPOAH and effects of ARAS stimulation and iontophoretic application of prazosin as well as picrotoxin on those neurons were evaluated. The WSNs were excited by ARAS stimulation but inhibited by both prazosin and picrotoxin; whereas the CSNs were inhibited by ARAS stimulation and prazosin, but excited by picrotoxin. The NE excited the WSNs as well as the CSNs, while GABA had opposite effects on them, suggesting that NE and GABA interact in the mPOAH for thermoregulation. The findings unravel an intriguing possibility that in the mPOAH, GABA simultaneously acts on hetero-receptors located at pre-and post-synaptic sites, modulating the release of NE on the WSNs and CSNs for thermoregulation. Further, ARAS stimulation-induced similar excitatory and inhibitory responses of the WSNs and the CSNs support such converging inputs on these neurons for thermoregulation.