Thermally-induced activities of the mesencephalic reticulospinal and rubrospinal neurons in the rat.

Unit activities of 226 midbrain reticulospinal (mRfS) and non-mRfS neurons and 238 rubrospinal (RbS) and non-RbS neurons were investigated during changes in temperatures of midbrain (Tmb), preoptic and anterior hypothalamus (Thyp) and skin (Ts) in the urethane-anesthetized rat. Responsiveness to Tmb, Thyp and Ts were found in 43.5%, 41.6% and 51.5% of neurons of midbrain reticular formation (mRf), and in 35.2%, 32.7% and 17.6% of neurons of red nucleus (Rb). Higher incidence of responsiveness to remote temperatures was found among Tmb responsive neurons than Tmb unresponsive neurons in both mRf and Rb. The mRf contains significantly greater numbers of neurons having such multiple thermal responsiveness and also of neurons which were activated by falls in temperatures (cold-responsive neurons) than the Rb. These characteristics were more conspicuously seen among mRfS neurons, showing a high degree of convergence of cold signals from different sites of body. On the other hand, RbS neurons did not differ from non-RbS neurons regarding thermal characteristics and showed no particular combinations of responsiveness to temperatures of different sites. Microinjection of procaine and glutamate into the mRf just dorsolateral to the Rb, but not into the Rb, decreased and increased cold-induced increase in EMG activity and shivering without changes in cardiovascular and respiratory parameters and pilomotor activity. The results suggest that mRfS neurons are involved in the control of thermoregulatory muscle tone and shivering.