An investigation of a possible direct projection from the medial nucleus of the cerebellum to the paraventricular nucleus of the hypothalamus in the rat: a study using retrograde WGA-HRP and Fluoro-Gold tracing techniques.

Retrograde transport of lectin-conjugated horseradish peroxidase and Fluoro-Gold was used in an attempt to obtain data to confirm the existence, predicted from physiological studies, of a direct, monosynaptic projection from the medial nucleus of the cerebellum (MN) to the paraventricular nucleus of the hypothalamus (PVH) in the rat. Injections of these two tracers that included the PVH and surrounding diencephalic structures, or that in the case of Fluoro-Gold were localized to the PVH, resulted in retrograde neuronal labeling in widely separated nuclei known to project to the areas included in the injection sites. Thus, effective uptake and transport of both tracers occurred under the experimental conditions employed in this study. However, injections confined to the PVH and regions of the hypothalamus adjacent to it, or to the PVH alone, produced no retrograde neuronal labeling in the medial nucleus, indicating that the MN does not project directly to the PVH. Alternative explanations for the findings from physiological experiments were sought. The possibility that electrical stimulation of fibers of passage through the region of the MN might produce a monosynaptic response in the contralateral PVH was discarded, because retrogradely labeled neurons in nuclei such as the locus ceruleus and lateral parabrachial nucleus were distributed mainly ipsilateral to hypothalamic injection sites. However, tracer injections into the MN produced retrograde labeling of neurons in the same region of the lateral paragigantocellular nucleus (LPGi) in which labeled cells were found following tracers injections into the PVH. Axon collaterals of individual neurons in the LPGi might, therefore, project both to the MN and to the PVH. The possibility that such a circuit could, in the absence of a direct MN to PVH projection, provide the basis to explain the physiological findings is discussed.