Combined retrograde tracing and enzyme/immunohistochemistry of trigeminal ganglion cell bodies innervating tooth pulps in the rat.

Rat trigeminal neurons innervating tooth pulps were retrogradely labelled with fluorogold and analysed enzyme- and immunohistochemically for their content of substance P, calcitonin gene-related peptide, fluoride-resistant acid phosphatase, GM 1 ganglioside, carbonic anhydrase and neurofilament protein. The data showed that both small, medium-sized and large trigeminal neurons were labelled after fluorogold deposition in maxillary molar pulps, with a majority of the cells being medium-sized and large. Less than 2% of the pulpal neurons showed substance P-like immunoreactivity. Fifty-six per cent of the pulpal nerve cells were calcitonin gene-related peptide-positive. These cells were small, medium-sized and large. Only 1% of the fluorogold-labelled cells contained fluoride-resistant acid phosphatase enzyme activity. This paralleled the finding that the pulpal neurons were unstained by Griffonia simplicifolia isolectin I-B4, a plant lectin which preferentially binds to fluoride-resistant acid phosphatase-positive cells. Choleragenoid-like immunoreactivity, which identifies cells with the GM 1 ganglioside receptor, was found in 70% of the fluorogold-labelled pulpal neurons. Approximately 80% of the fluorogold-labelled cells showed RT 97-immunoreactivity. RT 97 labels neurofilament protein and is present in large light primary sensory neurons. No pulpal neurons appeared to contain carbonic anhydrase, as judged from both enzyme- and immunocytochemical observations. The findings suggest that, in the rat, trigeminal tooth pulp neurons, which according to the classical view are nociceptive, form a heterogeneous group of neurons with a minority of small cells which may contain calcitonin gene-related peptide but rarely either substance P or fluoride-resistant acid phosphatase. However, the vast majority of pulpal nerve cells appear to have sizes and cytochemical characteristics which are not generally associated with nociceptive primary sensory neurons.