Neurotrophins Cause a New Sensation

and trkC null mutant mice display loss of large diameter parvalbumin-positive neurons, loss of spinal projections from muscle afferents, and severe movement abnormalities. These features are consistent with the hypothesis that TrkC neurons convey proprioceptive information (Ernfors et An important organizational principle of the developing al., 1994b; Fariñ as et al., 1994; Klein et al., 1994). In peripheral nervous system is the reliance of neurons on the periphery, proprioceptive axons convey information specific members of the NGF family of neurotrophins from stretch and tension receptors (muscle spindles and for survival. The primary sensory neurons of the dorsal Golgi tendon organs) in skeletal muscle. NT-3 mRNA is root ganglion (DRG) have become a favored group for diffusely expressed in muscle during development but exploring neurotrophin function owing to both their ac-becomes restricted to muscle spindles, a target end cessibility for in vivo and in vitro studies and the potential organ of proprioceptive afferents, postnatally (Schecter-importance of neurotrophins in regulating sensory func-son and Bothwell, 1992; Copray and Brouwer, 1994). tions, particularly pain. Neurotrophin studies have bene-The most puzzling groups of DRG neurons related to fited from the wealth of information available about the neurotrophin dependence have been the low threshold physiological properties, peptide expression, cytoskele-cutaneous mechanoreceptive neurons that convey in-tal markers, and connectivity of DRG neurons (see Scott, formation from receptor organs in the dermis such as 1992). This information has allowed an initial character-hair follicles, Pacinian corpuscles, Ruffini corpuscles, ization of sensory neuron loss in settings where neuro-Meissners corpuscles, and Merkel cells. These DRG trophin or neurotrophin receptor function has been neurons have small-and medium-sized somas with my-blocked by antibodies or gene targeting (for review, see elinated axons and spinal projections that terminate in Snider, 1994). To date, however, the neurotrophin de-the deep dorsal horn (see Brown, 1981). To date, how-pendence of several classes of DRG neurons has re-ever, no good histochemical markers have been de-mained obscure. In this issue of Neuron, Airaksinen et scribed that specifically identify these populations. It al. characterize the neurotrophin requirements of two was predicted that low threshold mechanoreceptive classes of cutaneous mechanoreceptive neurons. Fur-neurons would require brain-derived neurotrophic factor ther, these investigators present evidence for previously (BDNF) for survival since the BDNF receptor, TrkB, is unappreciated complexities of neurotrophin regulation, expressed by a subpopulation of small-to medium-even in this relatively simple region of the nervous sized DRG neurons (Ernfors et al. Recent studies of neurotrophin …