Innervation of capillaries by local neurons in the cat hypothalamus: a light microscopic study with horseradish peroxidase.

ABSTRACT

The protein tracer horseradish peroxidase (HRP) has been used in an attempt to define the cell bodies of origin of "nonadrenergic" varicose axons which terminate on the walls of hypothalamic capillaries. Capillaries in this region are also known to receive direct axonal contacts from adrenergic neurons in the pontine locus ceruleus. Solutions of HRP were infused into the lateral ventricles of adult cats of either sex and permitted to circulate in the cerebrospinal fluid spaces for 10 min, 20 min, or 2 h. During these periods HRP entered the perivascular spaces around penetrating arterioles and spread into the surrounding extracellular spaces of the hypothalamus. Certain neurons in the periarteriolar neuropil were consistently labeled by the tracer after all three circulation periods. These cells, including all of their processes, could be visualized in detail. Most neurons, by contrast, did not accumulate HRP. The axons of some tracer-filled neurons terminated on the walls of capillaries in the immediate vicinity of the penetrating arteriole. The arrangement and distribution of these cells suggest that they may provide a substrate for local neural influences on the hypothalamic microcirculation.