Descending projections from the superior colliculus in rat: a study using orthograde transport of wheatgerm-agglutinin conjugated horseradish peroxidase

Abstract

Despite extensive behavioural work on the rat superior colliculus, its descending efferent pathways have not been fully characterised with modern anatomical tract-tracing techniques. To investigate these pathways, wheatgerm-agglutinin conjugated with horseradish peroxidase (1%) was injected at various locations within the superior colliculus of hooded rats. Label judged to be transported orthogradely was plotted on coronal sections modified from the atlas of Paxinos and Watson (1982). Two major descending pathways were identified. (i) The bulk of the fibres in the ipsilateral descending pathway leave the superior colliculus ventrolaterally, and course around the lateral margin of the midbrain reticular formation. Caudally, projecting fibres leave the main bundle to innervate the cuneiform nucleus, and parts of the pontomedullary reticular formation. Terminal fields associated with the major bundle of fibres are found in an area medial to the brachium of the inferior colliculus; the parabigeminal nucleus and adjacent tegmentum; the ventrolateral midbrain reticular formation; and the lateral pontine nuclei. (ii) The fibres of the main contralateral descending pathway leave the superior colliculus ventromedially, to cross midline in the dorsal tegmental decussation. They immediately turn caudally to join the predorsal bundle, in which they run the length of the brainstem to reach the cervical spinal cord. Major terminal fields occur in nucleus reticularis tegmenti pontis; the pedunculopontine/parabrachial area; paramedian pontomedullary reticular formation; and inferior olive. In addition there is lighter labelling in many areas of the pontomedullary reticular formation and in the cervical spinal cord. There was also a much sparser contralateral descending projection that crossed midline in the tectal commissure, and sent terminals to the contralateral cuneiform area and adjoining regions. These results suggest that the distribution of the descending efferent pathways from the superior colliculus in rats is similar to those described in other species. The fact that the two major pathways project to quite different terminal areas, together with previous findings that they have separate cells of origin within the tectum, suggests that they may also be functionally distinct.

GABA in the Deep Layers of the Superior Colliculus/Mesencephalic Reticular Formation Mediates the Enhancement of Startle by the Dopamine D1Receptor Agonist SKF 82958 in Rats

“…A recent report from our laboratory has identified a heavy ipsilateral innervation from the deep SC/Me to the part of the PnC critical for startle (Meloni and Davis, 1999b). Similar pathways have been described before (Redgrave et al, 1987;Yasui et al, 1994) and are believed to mediate a wide range of motor-related behaviors (Dean et al, 1988b). Thus, we believe that this tectal-reticular pathway could provide a direct neural connection relaying effects in the deep SC/Me to the acoustic startle circuit to affect this behavior.…”

Stimulation of the lateral geniculate, superior colliculus, or visual cortex is sufficient for eyeblink conditioning in rats

“…Like the ventral LGN, the SC receives a projection from the cerebellum (interpositus nucleus) (Gonzalo-Ruiz and Leichnetz 1987). The superficial, intermediate, and deep layers of SC send direct projections to the dorsolateral and dorsomedial pontine nucleus (Kawamura and Brodal 1973;Redgrave et al 1987;Wells et al 1989). Lesions of the SC in the Koutalidis et al (1988) study produced a partial impairment in CR acquisition, demonstrating that the SC is not essential for eyeblink conditioning with a visual CS but probably contributes to acquisition.…”

“…The ventral LGN projects to the medial, and to a lesser extent, the lateral PN (Graybiel 1974;Wells et al 1989). The superficial, intermediate, and deep layers of SC send projections to both the dorsomedial and dorsolateral PN (Redgrave et al 1987;Wells et al 1989). The VCTX has a direct projection to the rostral and lateral portions of the PN (Glickstein et al 1972;Baker et al 1976;Mower et al 1980;Wells et al 1989).…”

Central mechanisms regulating coordinated cardiovascular and respiratory function during stress and arousal

“…Furthermore, the dlPAG (but not other PAG subregions) receives a direct input from the deep layers of the SC (129). At the same time, there are significant differences in the connections of the dlPAG and deep layers of SC; for example, there is a dense projection from the medial prefrontal cortex to the dlPAG but not to the adjacent SC (4) and a descending projection to the pontomedullary reticular formation from the deep layers of SC but not from the dlPAG (42,128). Furthermore, cardiovascular and respiratory responses evoked from the dlPAG, but not those evoked from the SC, are dependent on the activation of the DMH (48,77,114).…”