Chemical Phenotypes of Muscle and Cutaneous Afferent Neurons in the Rat Trigeminal Ganglion

Retrograde labeling was combined with cytochemistry to investigate phenotypic differences in primary afferent neurons relaying sensory information from deep and superficial craniofacial tissues. Calcitonin gene-related peptide (CGRP), substance P (SP), somatostatin (SOM) immunoreactivity and isolectin IB4, and cholera toxin B (ChTB) binding were examined for trigeminal masticatory muscle and cutaneous afferent neurons. Somata labeled from muscle were larger than cutaneous afferent neurons. Muscle afferent neurons exhibited positive staining as follows: 22% CGRP, 5% SP, 0% SOM; 18% ChTB, 5% IB4. The somata of CGRP- and SP-positive muscle afferent neurons were smaller than that of the overall muscle afferent population. Size differences were not detected between IB4- or ChTB-binding muscle afferent neurons and the total muscle afferent population. The following distribution was found for cutaneous afferent neurons: 26% CGRP, 7% SP, 1% SOM, 26% ChTB, 44% IB4. Cutaneous afferent neurons positive for SP were smaller, while ChTB-binding cutaneous afferents were larger than the overall cutaneous afferent population. No size differences were found between cutaneous CGRP-, SOM-, or IB4-positive neurons and the total cutaneous afferent population. Target-specific differences exist for SOM and IB4. The percentage of cutaneous afferent neurons positive for SOM and IB4 exceeds that for SOM- or IB4-positive muscle afferents. The number of retrogradely labeled neurons never differed between sexes. The percentage of retrogradely labeled muscle afferent neurons that were CGRP-positive was greater in males than females. These data indicate the presence of phenotypic, target, and sex differences in trigeminal ganglion primary afferent neurons.