A comparison of the postnatal development of muscle-spindle and periodontal-ligament neurons in the mesencephalic trigeminal nucleus of the rat.

The trigeminal mesencephalic nucleus (Vmes) is known to include primary afferent neurons of jaw muscle spindles (MS neurons) and periodontal ligament receptors (PL neurons). The aim of this study was to clarify the postnatal development of Vmes neurons by comparing MS neurons with PL neurons using horseradish peroxidase labeling. We measured somal diameter and somal shape of MS and PL neurons in rats from postnatal day (P)7 to P70. No significant changes were seen between postnatal day P7 and P70 in somal diameter or somal shape of MS neurons. Conversely, PL neurons showed a larger somal diameter at P7 than at P14, and in terms of somal profile, multipolar neurons comprised 0% at P7, but 4.8% at P14 and 16.9% at P70. These findings suggest that PL neurons develop with the eruption of teeth, taking into account the fact that tooth eruption occurs from around P14 in rats. Conversely, the lack of postnatal changes in MS neurons is due to the fact that these neurons have been active since the embryonic period, as swallowing starts in utero.

Postnatal changes in 5HT and NK1 receptors in rat trigeminal motor nucleus and surroundings.

Our previous study showed that developmental changes to serotonin and substance P coexist in the trigeminal motor nucleus (Vmo), dorsolateral subnucleus (Vmo.dl), ventromedial subnucleus (Vmo.vm) and the area within 300 microm surrounding Vmo (SVmo). This occurred in rats from embryonic day (E)19 to postnatal day (P)70, with density of these terminals peaking at P7 in these three areas. The present study examined postnatal development of serotonergic 1A receptor (5HT1A receptor) and substance P receptor (NK1 receptor) expression in Vmo and SVmo in rats from E19 to P70. No significant changes in percentages of 5HT1A and NK1 receptor-expressing cells were seen between E19 and P70 by immunohistochemical study or in situ hybridization. In a real-time PCR study, quantities of 5HT1A and NK1 receptor expression peaked at P7, indicating that expression of these receptors was maximized in each neuronal cell body in the Vmo and SVmo at P7. This result corresponds with postnatal changes in serotonin/substance P-coexisting terminals found in our previous study. Furthermore, 5HT1A and NK1 receptors displayed very similar patterns of expression, which may support the hypothesis that potentiation of serotonin and substance P are involved in excitability regulating trigeminal motor functions, including mastication and breathing.