Projections to the caudolateral medulla from the pons, midbrain, and diencephalon in the cat

Abstract

Previous research implicated neurons in the caudolateral medulla in the expression of estrous behaviors triggered by genital stimulation in the female cat. The present study identified descending pathways through which the activity of neurons in estrogen-concentrating cellular regions of the diencephalon and anterior brain stem could be transmitted to caudolateral medullary neurons. Cats received medullary injections of 50% horseradish peroxidase (HRP) in or around nucleus ambiguus. After 1 to 3 days, retrograde transport of HRP was demonstrated using tetramethyl benzidine as a chromogen. In the pons, labeled cells were most numerous in the ipsilateral parabrachial nuclei, the Kölliker-Fuse nucleus, and lateral tegmental field. In the midbrain, the central gray contained many labeled neurons bilaterally, especially at trochlear and caudal oculomotor nuclear levels. Labeled cells were also found in the midbrain reticular formation bilaterally and in the contralateral deep tectum and red nucleus. In the diencephalon, some labeled neurons were in lateral and periventricular hypothalamic regions, usually posteriorly, and many paraventricular nucleus neurons were labeled. The existence of central gray and deep tectal projections to the lateral medulla was also verified electrophysiologically by antidromic invasion. The substantial projection from the central gray to the caudolateral medulla provides a potential route for the activity of estrogen-concentrating neurons to be transmitted to cells involved in genitally triggered estrous responses because some central gray cells bind estrogen and the central gray also receives strong projections from hypothalamic estrogen-concentrating neuronal regions.