DIRECT PROJECTIONS FROM THE VENTROLATERAL MEDULLA-OBLONGATA TO THE LIMBIC FOREBRAIN - ANTEROGRADE AND RETROGRADE TRACT-TRACING STUDIES IN THE RAT

Neurons in the ventrolateral medulla oblongata, a brain region implica ted in central vasomotor regulation, have previously been reported to project to some forebrain limbic structures. The aim of the present st udy was (1) to describe the termination pattern of ventral medullary a fferents in forebrain limbic areas using anterograde tract tracing, an d (2) to determine the location and some morphological characteristics of the projection neurons using retrograde tract tracing from selecte d forebrain sites. Following ionophoretic microinjections of the anter ograde tract tracer Phaseolus vulgaris leucoagglutinin into the rostra l ventrolateral medulla, labelled afferents were observed in the hippo campus, entorhinal and retrosplenial cortices, dorsal septum, nucleus accumbens, and the: medial prefrontal cortex. Anterogradely labelled a xons, ascending from the caudal ventrolateral medulla, could be traced only to the rostral aspects of the investigated forebrain limbic stru ctures. Here, the main target of the ascending projection was in the v entral septum. However, labelled terminals were also present in the nu cleus accumbens, the dorsolateral septum, and in the infralimbic corte x. The density of the ventrolateral medullary projections into all exa mined forebrain areas was low. The location of the cells in the ventra l medulla oblongata which give rise to direct forebrain projections wa s examined using retrograde tract tracing with wheat germ agglutinin c onjugated horseradish peroxidase (WGA-HRP). Following WGA-HRP injectio ns into the septo-accumbens region, retrogradely labelled cells were p resent in both the rostral and caudal ventrolateral medulla. When the tract tracer injection was restricted to the Ventral region of the sep tal complex, the labelled cells were concentrated in the caudal aspect s of the ventrolateral medulla (and the nucleus of the solitary tract) . Following tracer injections into the anterior cingulate cortex or th e hippocampus or the entorhinal cortex, retrogradely labelled cells in the medulla oblongata were predominantly in the rostral ventrolateral medulla. As a first attempt to reveal the chemical nature of the proj ection cells, the contribution of tyrosine hydroxylase-immunoreactive cells to the innervation of the septo-accumbens area was also investig ated: tyrosine hydroxylase-immunoreactive cells of both the caudal ven trolateral medulla and the nucleus of the solitary tract were found to contribute to the innervation of the septo-accumbens area. The distri bution of retrogradely labelled cells as well as the termination patte rn of the anterogradely labelled terminals indicated that the innervat ion of the various forebrain limbic areas arises from cells, diffusely distributed in the rostral and/or the caudal ventrolateral medulla ob longata. Considering the important role of the ventrolateral medulla o blongata in autonomic coordination, it is proposed that direct project ions from the ventral medulla oblongata to limbic forebrain structures might contribute to the coordination of behavioural states and cardio vascular performance. (C) 1994 Wiley-Liss, Inc.