Distribution of hypothalamic neurons projecting to the thoracic and sacral spinal segments in the cat

The distribution of hypothalamic neurons participating in hypothalamo-spinal projections (hypothalamo-spinal HSP neurons) to the thoracic and sacral segments was studied using the technique of retrograde axonal transport. Horseradish peroxidase (HRP) and nuclear yellow (NY) were injected into various thoracic and/or sacral spinal cord segments. The retrogradely labeled cells were distributed in a continuous crescent-shaped field in the posterior, dorsolateral and lateral regions of the hypothalamus: starting from the ventral tegmental area (VTA), through the posterior hypothalamic nucleus (PH), the supramammillary nucleus (SM), the dorsal- and lateral hypothalamic area (AH1) and LH), the dorsomedial nucleus (DM) as well as in the paraventricular nucleus (PV). Neurons in all of the above hypothalamic nuclei project as caudally as to the sacral segments. The projection is bilateral and the contralaterally projecting fibres cross the midline at or near their termination site. Projection to thoracic segments is mainly ipsilateral. HSP neurons projecting to upper thoracic and sacral segments showed different patterns of distribution. A sacral injection resulted in most labeled neurons in the SM and LH and less labeled neurons in the PV, than a thoracic injection of comparable size and locations. Experiments with two tracer substances suggested that some of HSP neurons had divergent axon collaterals terminating both in thoracic and sacral spinal segments.