An enkephalin-containing pathway from nucleus tractus solitarius to the pressor area of the rostral ventrolateral medulla of the rabbit.

A technique combining retrograde tracing of wheat germ-conjugated gold particles with immunocytochemical demonstration of enkephalin-containing neurons was used to study intramedullary enkephalin-containing pathways to the pressor area of the rostral ventrolateral medulla in rabbits. The rostral ventrolateral medulla represents a main source of bulbospinal sympathoexcitatory neurons, and is critical to the tonic and reflex control of blood pressure. Firstly, the distribution of enkephalin-positive neurons and terminal fibres in rabbit medulla were described, with special reference to a moderately dense terminal plexus in the rostral ventrolateral medulla. Then, retrograde tracing studies were conducted; the rostral ventrolateral medullary pressor region was first localized by injection of L-glutamate (25 nmol in 50 nl). Slow (30-min) injections of wheat germ-gold (1.00 microliter) were then made at the same coordinates, resulting in a restricted injection site corresponding to the C1 pressor area, verified by the presence of tyrosine hydroxylase- and neuropeptide Y-containing neurons. Transported gold was revealed by silver reduction, and enkephalin immunoreactive cells were revealed by immunocytochemistry. Enkephalin-positive gold-containing neurons were found primarily in the nucleus tractus solitarius, especially in the commissural and medial intermediate subnuclei. Cells in the nucleus tractus solitarius containing other transmitters (substance P. galanin, neuropeptide Y and catecholamines) did not show the same degree or pattern of double-labelling, suggesting that the transport was not due to non-specific silver reduction or spread from the pipette track. The potential importance of this endogenous intramedullary opiate system is discussed in terms of medullary control of the cardiovascular system. It is hypothesized that this opiate projection from the nucleus tractus solitarius to the rostral ventrolateral medulla could play an important modulatory function, influencing baroreceptor or other cardiopulmonary reflex pathways involved in the primary regulation of the cardiovascular system. Furthermore, this pathway could represent a central substrate underlying opiate effects on the cardiovascular system during such conditions as hemorrhagic shock, stress or opiate intoxication.