A ROLE FOR L-TYPE CALCIUM CHANNELS IN DEVELOPMENTAL REGULATION OF TRANSMITTER PHENOTYPE IN PRIMARY SENSORY NEURONS

To examine the influence of activity-dependent cues on differ entiatio n of primary afferent neurons, we investigated the short- and long-ter m effects of depolarization and calcium influx on expression of transm itter traits in sensory ganglion cell cultures. We focused on expressi on of tyrosine hydroxylase (TH), a marker for dopaminergic neurons, in developing petrosal ganglion (PG), nodose ganglion, and dorsal root g anglion neurons grown in the presence or absence of depolarizing conce ntrations of KCl. Exposure to 40 mM KCl increased the proportion of TH -immunoreactive neurons in all three ganglia in a developmentally regu lated manner that corresponded to the temporal pattern of dopaminergic expression in vivo. PG neurons, for example, were most responsive to elevated KCl on embryonic day 16.5 (E16.5), the age at which the dopam inergic phenotype is first detectable in vivo. However, KCl was relati vely ineffective at increasing TH expression in neonatal PG, indicatin g a critical period for induction of this phenotype by depolarization. Detailed analysis of TH induction in PG neurons demonstrated that, al though N-type calcium channels carried the majority of the high voltag e-activated calcium current, only L-type calcium channel blockade inhi bited the effect of elevated KCl. Further studies revealed that after removal of high KCl, neurons remained sensitized to subsequent stimula tion for >1 week. Specifically, cultures exposed to KCl beginning on E 16.5 (the conditioning stimulus), then returned to control medium, and subsequently re-exposed to elevated KCl after 9 d (the test stimulus) contained fourfold more TH-positive neurons than did cultures exposed to the test stimulus alone. Moreover, blockade of L-type calcium chan nels during the conditioning stimulus completely abolished long-term p otentiation of the TH response to elevated KCl. These findings demonst rate a novel role for L-type calcium channels in activity-dependent pl asticity of transmitter expression in sensory neurons and indicate tha t exposure to depolarizing stimuli during early development may alter neuronal response properties at later ages.