Loss and restoration of preoptic thermoreactiveness after lesions of the rostral raphe nuclei.

Preoptic neurons, extracellularly recorded in the rat's brain, were tested for their responses to thermal stimulation of the scrotal and abdominal skin before and after electrolytic lesions of about 1 mm3 in the area of the rostral raphe nuclei, nucleus raphe dorsalis and centralis (NRD/NRC). All analyzed neurons were of the switching type, i.e. they changed their firing rates to a higher or lower level when a threshold of the peripheral stimulation temperature was exceeded. When major parts of NRD or NRC were destroyed, the preoptic neurons no longer changed their firing rates after thermal stimulation, whereas transmission of noxious information in most cases was not impeded. Smaller lesions in NRD or NRC did not abolish the responses, but brought about essentially modified responses compared to those before the lesions. Lesions lateral to NRD or NRC had no effect. If the lesions were effective and the neurons could be observed for a longer period after the lesions, the response was restored in many cases. As the noxious response had often not been abolished and the lateral lesions were without any effect, it might be that the lesion effects and the restoration of responses involve short-term plasticity. However, temporary block of input to the neurons by unspecific effects cannot be excluded.