Correlation of neuronal size and peptide immunoreactivity in the guinea-pig trigeminal ganglion

Frequency and size of guinea-pig trigeminal neurones immunoreactive with antisera to alpha-neo-endorphin-(alpha-neo-END), dynorphin A- (DYN), vasoactive intestinal polypeptide- (VIP), somatostatin- (SOM), and substance P- (SP) are reported. Co-localisation of the various peptides to the same ganglion cells was investigated immunocytochemically in consecutive 7-micron thick paraffin sections. According to their size, all peptide-immunoreactive neurones belong to the class of "small" ganglion cells. Within this cell group, SP-immunoreactive neurones appear to be the largest, followed by SOM-, VIP-, alpha-neo-END- and DYN-immunoreactive ganglion cells. The observed differences in size are statistically significant with the exception of that between alpha-neo-END and DYN. This finding correlates well with the observed co-occurrence of the two immunoreactive peptides. All alpha-neo-END-immunoreactive perikarya are also reactive to VIP antisera. These neurons are significantly smaller than those containing VIP-immunoreactivity exclusively. Ganglion cells displaying co-existence of alpha-neo-END- and SP-immunoreactivity or VIP- and SP-immunoreactivity are found too infrequently to allow morphometric analysis. Some non-immunoreactive ganglion cells are shown to be approached by dense baskets of VIP-, alpha-neo-END- or SP-immunoreactive varicose fibres, indicating the presence of intraganglionic modulation sites. The combination of immunohistochemistry and morphometry presented in this study allows the differentiation of diverse populations of primary afferent neurones exhibiting peptide immunoreactivity, most likely reflecting their involvement in different central and peripheral reflex arcs and sensory modalities.