Ultrastructural localization and afferent sources of corticotropin‐releasing factor in the rat rostral ventrolateral medulla: Implications for central cardiovascular regulation

We investigated the ultrastructural localization, afferent sources, and arterial pressure effects of corticotropin‐releasing factor (CRF) in the nucleus reticularis rostroventrolateralis (RVL), a region of the ventrolateral medulla containing C1 adrenergic neurons and sympatho‐excitatory reticulospinal afferents to sympathetic preganglionic neurons. A polyclonal antibody, to CRF was localized in acrolein‐fixed sections through the rat RVL by the peroxidase–antiperoxidase (PAP) method. Light microscopy showed that 1–7 perikarya/30 μm section and numerous varicose processes contained CRF‐like immunoreactivity (CRF‐LI). By electron microscopy, CRF‐LI was most intensely localized to large (80–100 nm) dense‐core vesicles within numerous terminals and a few perikarya and large dendrites. Approximately half of the terminals containing CRF‐LI were in direct contact with unlabeled perikarya or dendrites; the remainder were in apposition to either unlabeled terminals or astrocytes. Most synaptic specializations were asymmetric synapses on small, unlabeled dendrites. To examine potential extrinsic sources of CRF‐containing terminals in the C1 area of the RVL, PAP immunocytochemical localization of CRF was combined with retrograde transport of wheat germ agglutinin‐conjugated horseradish peroxidase (WGA‐HRP). In all cases examined, a number of dually labeled neurons were found in the paraventricular nucleus (PVN) of the hypothalamus and a few dually labeled neurons were observed in the nuclei of the solitary tract; these labeled neurons were ipsilateral to the unilateral injection of WGA‐HRP into the C1 area. Fewer dually labeled perikarya were detected in the lateral hypothalamic area and the lateral parabrachial nuclei, ipsilateral to the WGA‐HRP injection. Additional physiological studies showed that bilateral microinjections of CRF into the C1 area of the RVL of urethane‐anesthetized rats elicited a dose‐related increase in arterial pressure. The results suggest that within the C1 area of the RVL, CRF released from terminals, arising predominantly from the PVN of the hypothalamus and probably from local neurons as well, may excite sympathoexcitatory reticulospinal neurons. © 1993 Wiley‐Liss, Inc.