A study of the connections of nociceptive substantia nigra neurones.

(i) Of 226 spontaneously active neurones recorded from the substantia nigra (SN) of rats anaesthetised with halothane, 112 (50%) were found to be nociceptive. (ii) Nociceptive neurones were located in both the zona compacta (ZC) and zona reticulata (ZR) compartments of the SN. (iii) Using the collision-test technique it has been shown that approximately equal proportions of nociceptive and non-nociceptive SN neurones project to the thalamus. The majority of nigrostriatal neurones were found to be non-nociceptive. In contrast striatonigral and amygdalonigral afferents are more likely to influence nociceptive than non-nociceptive SN neurones. (iv) Experiments were performed to determine whether nociceptive SN neurones were dopaminergic. Examination of spontaneous firing rate, action potential configuration and responses to systemically administered dopaminomimetic agents of histologically identified SN neurones did not allow a reliable distinction to be made between dopaminergic and non-dopaminergic neurones. It was therefore not possible to resolve this question.