Afferent projections to the rostral anterior pretectal nucleus of the rat: a possible role in the processing of noxious stimuli.

The afferent inputs to the rostral pole of the anterior pretectal nucleus have been examined by utilizing the retrograde axonal transport of a fluorescent dye, Fast Blue. After unilateral injection of the dye into the rostral anterior prectectal nucleus, large numbers of labelled neuronal somata were found in the somatosensory cortex, the ventrolateral geniculate nucleus, the zona incerta, the superior colliculus, the deep mesencephalic nuclei, the pedunculopontine tegmental nucleus and the medial vestibular nucleus. In addition, the contralateral parabigeminal nucleus provided a major input to the rostral part of the anterior pretectal nucleus. Smaller and sparser collections of stained cell bodies could be found in the ventromedial hypothalamus, the posterior pretectal nucleus, the nucleus of the posterior commissure, the peripeduncular nucleus, the periaqueductal central gray, the contralateral anterior pretectal nucleus, and the locus coeruleus. Many of the inputs originated in areas associated with nociceptive pathways. The regional distribution of neurons projecting to the rostral pole of the anterior pretectal nucleus differs substantially from that of the cells innervating the anterior pretectal nucleus proper, i.e. its more caudal parts. It is concluded from this that the rostral pole constitutes a separate nucleus, anatomically distinct from the rest of the anterior pretectal nucleus and other cell groups in the pretectal complex. The demonstration that many of the afferents to the rostral anterior pretectal nucleus arise in regions involved in nociception supports recent electrophysiological and behavioural evidence that this brain area plays a role in the processing of noxious stimuli, rather than as a component in the pretectal control of visual system reflexes.