In vitro thermosensitivity of rat lateral parabrachial neurons

The lateral parabrachial (LPB) neurons play a pivotal role in the thermoregulatory afferent pathway by transmitting cutaneous thermosensory signals received from spinal neurons directly to the thermoregulatory command center, the preoptic area (POA). The present study was conducted to electrophysiologically characterize the local temperature responsiveness of rat LPB neurons in brain slices to evaluate their local thermosensitivity and permit comparison with thermosensitive neurons in POA and spinal cord slices under consistent experimental conditions. In current clamp, warm- and cold-sensitive neurons were recorded in LPBel, LPBc and LPBd, the three LPB subnuclei responsible for the transmission of cutaneous feedforward signals. Of the 92 spontaneously firing LPB neurons, 27% were warm sensitive, 10% were cold sensitive, and 63% were temperature insensitive, and the spontaneous firing rate of the warm-sensitive neurons was significantly greater than that of the temperature-insensitive neurons. These proportions and spontaneous activity are similar to results obtained in the POA and spinal cord. Furthermore, the thermosensitivity was also present in 38% of silent neurons evoked by injection of a small amount of depolarizing current. Warm-sensitive neurons in the LPB were similar in thermoresponsiveness to those in the POA and spinal cord. However, cold sensitivity in the LPB was distinct from that in the POA. The firing rate of most cold-sensitive neurons changed steeply at a relatively narrow band of temperature, and some of them were silent near thermoneutrality. The percentages of thermosensitive and insensitive neurons within the three LPB subnuclei were not significantly different, nor were the mean maximal thermal coefficients of the thermosensitive neurons. These results suggest that LPB have local thermosensory functions as POA and spinal cord, and might be an important extrahypothalamic "thermoregulator".