NOCICEPTIVE NEURONS IN RAT SUPERIOR COLLICULUS .2. EFFECTS OF LESIONSTO THE CONTRALATERAL DESCENDING OUTPUT PATHWAY ON NOCIFENSIVE BEHAVIORS

A wealth of evidence implicates the crossed descending projection from the superior colliculus (SC) in orientation and approach behaviours d irected towards novel, non-noxious stimuli. In our preceding paper, we identified a population of nociceptive neurones in the rat SC that ha ve axons that project to the contralateral brainstem via this output p athway. The purpose of the present study was, therefore, to evaluate t he prediction that the crossed descending projection of the SC is also involved in the control of orientation and approach movements of the head and mouth made during the localisation of persistent noxious stim uli. An independent-groups design was used to test the effects of inte rrupting the contralateral descending projection from the SC on the be havioural reactions elicited by noxious mechanical stimuli presented t o the tail and hindpaws. In different groups of animals, a microwire k nife was used to cut the contralateral descending fibres at two differ ent locations: (1) a sagittal cut at the level of the dorsal tegmental decussation; (2) a bilateral coronal cut of the predorsal bundle at t he level of the medial pontine reticular formation. Retrograde anatomi cal tracing techniques were then used to evaluate the effectiveness of the cuts and to assess possible involvement of non-collicular fibre s ystems in both lesioned and control animals. Additional behavioural pr ocedures were performed to test for general neurological status and re sponsiveness of animals to non-noxious stimuli. Anatomical tracing dat a indicated that the largest population of neurones with fibres severe d by both cuts were the cells-of-origin of the contralateral descendin g projection in the intermediate white layer of the SC. Behavioural re sults showed that significantly more animals in both lesion groups fai led to locate and bite a mechanical clip placed on the tail. Instead o f switching to motor behaviours to localise and remove noxious stimuli , they persisted with defensive reactions, which included freezing, vo calisation or forward and backward escape. In contrast, when the clip was placed on the hindpaws, it was successfully localised by most lesi oned and control animals; however, lesioned animals had reliably longe r latencies and spent less time in close contact with the clip. Consis tent with the established role of the contralateral descending project ion in non-noxious orientation, lesioned animals also showed orienting deficits to a range of non-noxious sensory stimuli. These data sugges t that, under certain behavioural circumstances, nociceptive informati on from the SC is integral to the elaboration of orienting and approac h movements of the head and mouth elicited by persistent noxious stimu li.