Abstract
Repetitive electrical stimulation to the midbrain periaqueductal gray (PAG) and the ventrolateral pons (pontine call site : PCS) reported to evoke natural sounding vocalization in cats. The purpose of the present study was to investigate the descending pathways for vocalization from PAG to the lower brainstem. By means of the electrode which could be used for both microstimulation and microinjection, an anterograde tracer, Phaseolus vulgaris leucoagglutinin (PHA-L), was injected at the site of the ventrolateral PAG where vocalization was induced by electrical stimulation.
At the level of caudal superior colliculus, most labelled fibers were sparsely distributed, and they passed laterally through the dorsal part of the midbrain tegmentum. At the level of the inferior colliculus, they could be traced ipsilaterally to the ventral area along the medial lemniscus. They consisted of a narrow bundle in the dorsolateral part of pontine tegmental reticular nucleus. At the level of medulla, they appeared to be concentrated in a discrete area ventrolateral to the inferior olive and dorsal to the pyramidal tract, and their terminals were found in the vicinity of retrofacial nucleus (RFN) and retroambigual nucleus (NRA).
The bundle of the descending fibers in the ventrolateral pons seemed to correspond to the PCS. RFN and NRA where the terminals of these descending fibers were distributed included the respiratory neurons. The projections from PAG to RFN and NRA would be essential for vocalization by modulating respiratory movements.
