EXCITATORY INNERVATION OF CAUDAL HYPOGLOSSAL NUCLEUS FROM NUCLEUS-RETICULARIS GIGANTOCELLULARIS IN THE RAT

We examined the possible innervation of the caudal hypoglossal nucleus by the nucleus reticularis gigantocellularis of the medulla oblongata , based on single-neuron recording and retrograde tracing experiments in Sprague-Dawley rats. Under pentobarbital sodium (50 mg/kg, i.p.) an esthesia, electrical stimulation of the caudal portion of the nucleus reticularis gigantocellularis with repetitive 0.5-ms rectangular pulse s increased (46 of 51 neurons) the basal discharge frequency of sponta neously active cells, or evoked spike activity in silent, hypoglossal neurons located at the level of the obex. This excitatory effect was r elated to the intensity (25-100 mu A) and/or frequency (0.5-20 Hz) of the stimulating pulses to the nucleus reticularis gigantocellularis. P erikaryal activation of neurons by microinjection of L-glutamate (0.5 nmol, 25 nl) into the caudal portion of the nucleus reticularis gigant ocellularis similarly produced an excitatory action on eight of 14 hyp oglossal neurons. Retrogradely labeled neurons were found bilaterally within the confines of the nucleus reticularis gigantocellularis follo wing unilateral microinjection of wheatgerm agglutinin-conjugated hors eradish peroxidase or Fast Blue into the corresponding hypoglossal rec ording sites. Furthermore, the distribution of labeled neurons in the nucleus reticularis gigantocellularis substantially overlapped with th e loci of electrical or chemical stimulation. These complementary elec trophysiological and neuroanatomical results support the conclusion th at an excitatory link exists between the nucleus reticularis gigantoce llularis and at least the caudal portion of the hypoglossal nucleus in the rat.