DISINHIBITION OF THE ROSTRAL VENTRAL MEDULLA INCREASES BLOOD-PRESSUREAND FOS EXPRESSION IN BULBOSPINAL NEURONS

The GABA agonist muscimol, injected into the depressor area of the cau dal ventrolateral medulla, increased blood pressure and increased the expression of the immediate early gene c-fos in the rostral ventral me dulla (RVM) of the rat. The number of Fos-immunoreactive (Fos-IR) neur ons seen in the RVM was increased 3-fold after muscimol compared to Fo s-IR after vehicle treatment. In the rostral aspect of the RVM approxi mately half of the Fos-IR neurons were identified as spinally projecti ng after the injection of the retrograde tracer cholera toxin B subuni t into the upper thoracic spinal cord. These bulbospinal Fos-IR neuron s were identified in the lateral aspects of the RVM, in the area where baroreceptor-sensitive neurons have been identified in electrophysiol ogical studies, and also in more medial areas of the RVM. Fos-IR neuro ns were also identified in the intermediolateral cell column of the th oracic spinal cord after muscimol injection, but were rarely observed in this area after vehicle treatment. This study demonstrates the func tional connectivity of the caudal and rostral areas of the medulla obl ongata and the spinal cord, supporting the view that the caudal ventro lateral medulla contains neurons that provide a tonic inhibitory contr ol over neurons in the RVM and that, in turn, the spinally projecting neurons in the RVM provide an excitatory input to the spinal cord symp athetic preganglionic neurons. The results show that the withdrawal of an inhibitory input is a sufficient stimulus to increase the expressi on of c-fos and increase neuronal activity in the RVM. c-fos expressio n also increases in the neurons that receive an excitatory input from these RVM neurons.