Substance P in the hypoglossal nucleus of the rat

The distribution of substance P (SP)-containing synaptic terminals in the hypoglossal nucleus (XII) of adult rats was examined by retrograde peroxidase labelling and immunocytochemistry. From the location of peroxidase injections into the tongue and of labelled neurones in the ventral lamina of XII, motor neurones that supply intrinsic vertical, longitudinal and transverse fibres as well as the extrinsic muscle genioglossus appear to have been labelled. SP-containing terminals were found making contact, and sometimes dual synapses, with unlabelled neuronal dendrites but not with retrogradely labelled somata or dendrites. These findings suggest that SP terminals may contact dendrites of interneurones or of neurones supplying other extrinsic muscles located in the anterior part of the tongue. Dual SP- containing synapses between XII motor neurones may be the means by which tongue muscle fibres are recruited and their function synchronized.