Proprioceptive afferents survive in the masseter muscle of trkC knockout mice

Peripheral innervation patterns of proprioceptive afferents from dorsal roo t ganglia and the mesencephalic trigeminal nucleus were assessed in trkC-de ficient mice using immunohistochemistry for protein gene product 9.5 and pa rvalbumin. In trkC knockout mice, spinal proprioceptive afferents were comp letely absent in the limb skeletal muscles, M. biceps femoris and M. gastro cnemius, as previously reported. In these same animals, however, propriocep tive afferents from mesencephalic trigeminal nucleus innervated masseter mu scles and formed primary endings of muscle spindles. Three wild-type mice a veraged 35.7 spindle profiles (range: 31-41), six heterozygotes averaged 32 .3 spindles (range: 27-41), and four homozygotes averaged 32.8 spindles (ra nge: 26-42). Parvalbumin and Nissl staining of the brain stem showed approx imately 50% surviving mesencephalic trigeminal sensory neurons in trkC-defi cient mice. TrkC -/- mice (n = 5) had 309.4 +/- 15.9 mesencephalic trigemin al sensory cells versus 616.5 +/- 26.3 the sensory cells in trkC +/+ mice ( n = 4). These data indicate that while mesencephalic trigeminal sensory neurons are significantly reduced in number by trkC deletion, they are not completely absent. Furthermore, unlike their spinal counterparts, trigeminal proprioce ptive afferents survive and give rise to stretch receptor complexes in mass eter muscles of trkC knockout mice. This indicates that spinal and mesencep halic trigeminal proprioceptive afferents have different neurotrophin-suppo rting system during survival and differentiation. It is likely that one or more other neurotrophin receptors expressed in mesencephalic trigeminal pro prioceptive neurons of trkC knockout mice compensate for the lack of normal neurotrophin-3 signaling through trkC. (C) 1999 IBRO. Published by Elsevie r Science Ltd.