Indirect hypothalamo-cerebellar pathway? Demonstration of hypothalamic efferents to the lateral reticular nucleus.

Hypothalamic efferents to the lateral reticular nucleus (NRL) have been demonstrated in the cat by means of anterograde and retrograde axonal transport of the wheat germ agglutinin-horseradish peroxidase (WGA-HRP) complex. Pressure injections of the WGA-HRP complex into the hypothalamus resulted in anterograde labelling of branching terminal axons both in the NRL and in an adjacent X area, presumably the ventrolateral catecholaminergic cell group (A1). After microiontophoretical ejections of the WGA-HRP complex into the NRL from a ventral approach, retrogradely labelled neurons were found in the lateral, dorsal, posterior and anterior hypothalamic areas and in the tubero-mammillary, dorsomedial and periventricular nuclei. The projection is bilateral with a clear ipsilateral predominance and has its main origin in the lateral hypothalamic area. The locations of hypothalamic cells projecting to the NRL are somewhat different from those giving rise to hypothalamo-cerebellar and hypothalamo-spinal connections. The present demonstration of a hypothalamic input to one of the major precerebellar relay nuclei introduces a new possible indirect route through which the cerebellum may be influenced by the hypothalamus. The different indirect and direct hypothalamo-cerebellar pathways and their potential functional importance are discussed.