Analysis of nociceptive neurones in the rat superior colliculus using c-fos immunohistochemistry

The superior colliculus (SC) has an established role in the sensory guidance of motor commands required to orient an animal towards novel stimuli. In addition to the representations of visual, auditory, and somatosensory stimuli, the SC also contains a large population of nociceptive neurones. The purpose of the present investigation was to see if nociceptive neurones in the SC can be characterised with c-fos immunohistochemistry as a prelude to establishing anatomical connectivity with specific target regions in the brainstem. To ensure comparability with previous electrophysiological investigations, the present study was conducted in animals anaesthetised with urethane. A series of independent issue relating to basic aspects of experimental protocol were investigated. The principal findings were: (i) Despite minimising the exposure of animals to extraneous stimuli, basal levels of immunostaining were observed. (ii) Urethane anaesthesia induced an increase in Fos-like immunoreactivity (FLI) over the basal condition. (iii) No additional labelling was induced by non-noxious tactile stimulation of the hindpaw. (iv) Unilateral noxious mechanical stimulation elicited a reliable increase in FLI over all control conditions. (v) This increase in FLI was expressed bilaterally and restricted largely to the intermediate white layer. (vi) The induction of FLI was related to noxious stimulation intensity. (vii) No reliable differences in the spatial topography of FLI expression were observed when unilateral noxious mechanical stimulation was administered to the face or hind foot. (vii) A higher level of urethane anaesthesia had a generally suppressive effect on FLI expression. (ix) There were no differences in the distribution of FLI induced by noxious mechanical or noxious chemical stimulation. (x) The increase in FLI induced by noxious pinch was abolished by a naloxone reversible pre-treatment with morphine. These data confirm that c-fos immunohistochemistry can be used to characterise nociceptive cells in the rat superior colliculus, and generally complement recent electrophysiological data. The identification of nociceptive cells in the stratum album intermediale, the source of collicular input to regions of the contralateral brainstem involved in orienting, suggests the SC may play a significant role in the localisation of pain.