Histochemical characterisation of trigeminal neurons that innervate monkey extraocular muscles

Sensory trigeminal innervation is a consistent feature of extraocular muscles across species, in spite of a variable occurrence of muscle spindles. We studied the histochemical properties of trigeminal ganglion (TG) cells projecting to the extraocular eye muscles to obtain more information about their function. In monkey TG neurons were retrogradely filled by tracer injections (cholera toxin subunit B; wheat-germ agglutinin) into the belly or myotendinous junction of eye muscles; one conjunctival injection served as a control. Retrogradely labelled TG neurons were processed for the presence of parvalbumin (PV), substance P (SP), or nitric oxide synthase (NOS) by double-immunofluorescence. The results indicate that approximately 10% of trigeminal afferents to all parts of the eye muscle are PV-positive, whereas around 20% are SP-positive. Twice as many SP-positive TG projection neurons were counted after a conjunctival tracer injection, presumably relaying nociceptive signals. A surprisingly large population of NOS-positive TG cells (30%) was found only after distal tracer injections. Up to now none of these TG cell groups could be related to the palisade endings located at the myotendinous junction.