THE ANTERIOR PRETECTAL NUCLEUS - A PROPOSED ROLE IN SENSORY PROCESSING

Four nuclei of the pretectal complex, the olivary pretectal nucleus, t he medial pretectal nucleus, the nucleus of the optic tract and the po sterior pretectal nucleus, all have a demonstrated role in visual func tion. In contrast, the anterior pretectal nucleus (APtN) has no inputs from retina and has few outputs to visual accessory nuclei. The APtN has connections with areas associated with sensory functions and it ha s been suggested that this nucleus may have a role to play in somatose nsory processing. An increasing number of behavioural and electrophysi ological studies support this view. Brief low-intensity electrical or chemical stimulation of the APtN causes antinociception in the tail fl ick test in both unanaesthetised and anaesthetised animals. This inhib ition of the tail flick response is attenuated by naloxone, alpha-adre noceptor antagonists and muscarinic cholinergic receptor antagonists. Electrical stimulation of the APtN is similarly effective in the paw p ressure and formalin tests. APtN stimulation also causes a brief inhib ition of the tooth pulp-evoked jaw opening reflex. studies with [C-14] 2-deoxyglucose indicate that peripheral noxious stimuli will cause an increase in metabolic activity within the APtN. Animals with electrode s placed in the APtN will self-administer electrical stimulation and t his can reduce the aversive and autonomic effects of stimulating the v entromedial hypothalamus. Part of the antinociceptive effects of stimu lating the APtN are due to a descending inhibition of spinal dorsal ho rn projection neurones. Multireceptive neurones deep in the dorsal hor n are inhibited by APtN stimulation. In contrast, superficial projecti on neurones that respond to intense cutaneous stimuli are excited by A PtN stimulation. The APtN receives an excitatory input from low-thresh old afferents via the dorsal column pathway and a high-threshold excit atory drive from superficial cells projecting through the dorsolateral funiculus. The excitatory input from the dorsal columns may well part icipate in the long-term inhibition of spinal projection neurones evok ed by dorsal column stimulation. These ascending excitatory pathways m ay also be important to the long-term activation of descending inhibit ion from the APtN.