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Cytochemical characteristics of neurons in the trigeminal mesencephalic nucleus of hatchling chicks

The goal of the present study was to identify cytochemical markers characteristic of muscle afferents in hatchling chicks. To this end, we stained neurons in the trigeminal mesencephalic nucleus with a variety of markers that label subsets of neurons in avian dorsal root ganglia. We found that trigeminal mesencephalic neurons are surprisingly heterogeneous in their cytochemical make‐up, expressing, to varying degrees, substance P, cholecystokinin, carbonic anhydrase, calbindin D‐28k, parvalbumin, and S‐100β. Calbindin D28k and S‐100β appeared to be expressed equally in medial and lateral divisions of the trigeminal mesencephalic nucleus. In contrast, substance P‐ and cholecystokinin‐immunoreactive neurons were more abundant in the medial division, whereas carbonic anhydrase activity and parvalbumin immunoreactivity were stronger in the lateral division. We were unable to detect met‐enkephalin, neuropeptide Y, calcitonin gene‐related peptide, vasoactive intestinal peptide, somatostatin, γ‐aminobutyric acid, or tyrosine hydroxylase in the trigeminal mesencephalic nucleus. Moreover, these neurons did not appear to bind the lectin Dolichos biflorus agglutinin. The heterogeneity of expression of markers among trigeminal mesencephalic nucleus neurons, especially between neurons in the medial and lateral divisions, suggests that these neurons are functionally diverse.