Neuron numbers in the sensory trigeminal nuclei of the rat: A GABA- and glycine-immunocytochemical and stereological analysis.

The volume, total neuron number, and number of GABA- and glycine-expressing neurons in the sensory trigeminal nuclei of the adult rat were estimated by stereological methods. The mean volume is 1.38+/-0.13 mm3 (mean+/-SD) for the principal nucleus (Vp), 1.59+/-0.06 for the n. oralis (Vo), 2.63+/-0.34 for the n. interpolaris (Vip), and 3.73+/-0.11 for the n. caudalis (Vc). The total neuron numbers are 31,900+/-2,200 (Vp), 21,100+/-3,300 (Vo), 61,600+/-8,300 (Vip), and 159,100+/-25,300 (Vc). Immunoreactive (-ir) neurons were classified as strongly stained or weakly stained, depending on qualitative criteria, cross-checked by a densitometric analysis. GABA-ir cells are most abundant in Vc, in an increasing rostrocaudal gradient within the nucleus. Lower densities are found in Vip and Vp. The mean total number of strongly labeled GABA-ir neurons ranges between 1,800 in Vp to 7,800 in Vip and 22,900 in Vc, and varies notably between subjects. Glycine-ir neurons are more numerous and display more homogeneous densities in all nuclei. Strongly labeled Gly-ir cells predominate in all nuclei, their total number ranging between 9,400 in Vp to 24,300 in Vip and 34,200 in Vc. A substantial fraction of immunolabeled neurons in all nuclei coexpress GABA and glycine. In general, all neurons strongly immunoreactive for GABA are small, while weakly GABA-ir cells which coexpress Gly are larger. In Vc, one-third of all neurons are immunoreactive: 16.6% of them are single-labeled for GABA and 31.6% are single-labeled for glycine. The remaining 51.8% express GABA and glycine in different combinations, with those showing strong double labeling accounting for 22.6%.