Donald J. Reis

Clonidine, an antihypertensive drug, binds to alpha 2-adrenergic and imidazoline receptors. The endogenous ligand for imidazoline receptors may be a clonidine-displacing substance, a small molecule isolated from bovine brain. This clonidine-displacing substance was purified and determined by mass spectroscopy to be agmatine (decarboxylated arginine), heretofore not detected in brain. Agmatine binds to alpha 2-adrenergic and imidazoline receptors and stimulates release of catecholamines from adrenal chromaffin cells. Its biosynthetic enzyme, arginine decarboxylase, is present in brain. Agmatine, locally synthesized, is an endogenous agonist at imidazoline receptors, a noncatecholamine ligand at alpha 2-adrenergic receptors and may act as a neurotransmitter.

Anterograde, retrograde, and combined axonal transport methods were used to describe the descending efferent projections of a region of rostral ventrolateral medullary reticular formation important in cardiovascular control. We have termed this region, which contains C1 adrenaline-synthesizing neurons, the nucleus reticularis rostroventrolateralis (RVL). Efferent projections from the RVL innervate all segmental levels of the thoracic intermediolateral and intermediomedial columns as shown using retrograde transport of lectin-conjugated horseradish peroxidase (HRP) or fast blue dye, and anterograde transport of either HRP or labeled amino acids. The projection is highly specific in that there are no projections to thoracic dorsal or ventral horns. This innervation corresponds to the distribution of preganglionic sympathetic neurons in the intermediolateral column. In particular, terminals surround neurons projecting to the adrenal medulla, as demonstrated by combined anterograde and retrograde transport methods at the light level. Terminals containing phenylethanolamine-N-methyl transferase (PNMT) were mapped using immunocytochemical techniques. PNMT-labeled terminals were present at all levels of thoracic intermediolateral column, in a distribution similar to that of the descending projections from the RVL. We have previously shown using double label techniques (Ross et al., '81-'83), that many of the spinal projections of the RVL originate from C1 neurons. These data support our suggestion that certain bulbospinal neurons within the RVL, in particular the C1 neurons, are crucial for tonic vasomotor control.

Although the auditory cortex is believed to be the principal efferent target of the medial geniculate body (MG), our recent behavioral studies indicate that in rats the conditioned coupling of emotional responses to an acoustic stimulus is mediated by subcortical projections of the MG. In the present study we have therefore used WGA-HRP as an anterograde and retrograde axonal marker to (1) define the full range of subcortical efferent projections of the MG; (2) identify the cells of origin within the MG of each projection; and (3) determine whether the subregions of the MG that project to subcortical areas receive inputs from acoustic relay nuclei of the mid-brain, particularly the inferior colliculus. The rat MG was first parcelled into three major cytoarchitectural areas: the ventral, medial, and dorsal divisions. The suprageniculate nucleus, located within the body of the MG just dorsal to the medial division, was also identified. Efferent projections of the MG were determined by combined anterograde and retrograde tracing methods. Injections of WGA-HRP in the MG produced anterograde transport to cortex and several subcortical areas, including the posterior caudate-putamen and amygdala, the ventromedial nucleus of the hypothalamus, and the subparafascicular thalamic nucleus. The cells of origin of the subcortical projections were then mapped retrogradely after injections in the anterogradely labeled areas. Injections in the caudate-putamen or amygdala retrogradely labeled the medial division of the MG and the suprageniculate nucleus, as well as several adjacent areas of the posterior thalamus surrounding the MG. In contrast, injections in the ventromedial nucleus of the hypothalamus or the subparafascicular thalamic nucleus only produced labeling in the areas surrounding MG. Afferents to MG from the inferior colliculus were then identified. The central nucleus of the inferior colliculus, the main lemniscal acoustic relay nucleus in the midbrain, was found to project to the ventral and medial divisions of the MG. In contrast, the dorsal cortex and external nucleus of the inferior colliculus project to each division of the MG and to several additional nuclei in adjacent areas of the posterior thalamus. These data demonstrate that the medial division of MG, the suprageniculate nucleus, and immediately adjacent areas of the posterior thalamus provide a direct linkage between auditory neurons in the inferior colliculus and subcortical areas of the forebrain and thereby support the view that thalamic sensory nuclei relay afferent signals to subcortical as well as cortical areas.

Projections from the nucleus tractus solitarii (NTS) to autonomic control regions of the ventrolateral medulla, particularly the nucleus reticularis rostroventrolateralis (RVL), which serves as a tonic vasomotor center, were analyzed in rat by anterograde, retrograde, and combined axonal transport techniques. Autonomic portions of the NTS, including its commissural, dorsal, intermediate, interstitial, ventral, and ventrolateral subnuclei directly project to RVL as well as to other regions of the ventrolateral medulla. The projections are organized topographically. Rostrally, a small cluster of neurons in the intermediate third of NTS, the subnucleus centralis, and neurons in proximity to the solitary tract selectively innervate neurons in the retrofacial nucleus and nucleus ambiguus. Neurons generally located in more caudal and lateral sites in the NTS innervate the caudal ventrolateral medulla (CVL). The RVL, CVL, and nucleus retroambiguus are interconnected. A combined retrograde and anterograde transport technique was developed so as to prove that projections from the NTS to the ventrolateral medulla specifically innervate the region of RVL containing neurons projecting to the thoracic spinal cord or the region of the nucleus containing vagal preganglionic neurons. When the retrograde tracer, fast blue, was injected into the thoracic spinal cord, and wheat germ agglutinin-conjugate horseradish peroxidase (HRP) was injected into the NTS, anterogradely labeled terminals from the NTS surrounded the retrogradely labeled neurons in the RVL and in the nucleus retroambiguus in the caudal medulla. Among the bulbospinal neurons in the RVL innervated by the NTS were adrenaline-synthesizing neurons of the C1 group. When fast blue was applied to the cervical vagus, and HRP was injected into the NTS, anterogradely labeled terminals from the NTS surrounded retrogradely labeled neurons in the rostral dorsal motor nucleus of the vagus, the region of the nucleus ambiguus, the retrofacial nucleus, and the dorsal portion of the RVL, a region previously shown to contain cardiac vagal preganglionic neurons. This combined anterograde and retrograde transport technique provides a useful method for tracing disynaptic connections in the brain. These data suggest that the RVL is part of a complex of visceral output regions in the ventrolateral medulla, all of which receive afferent projections from autonomic portions of the NTS. Bulbospinal neurons in the RVL, in particular the C1 adrenaline neurons, may provide a portion of the anatomic substrate of the baroreceptor and other visceral reflexes.