Neurotrophic Regulation of Calcium Channels by the Peptide Neurotransmitter Luteinizing Hormone Releasing Hormone

Abstract

We exploited the simple organization of bullfrog paravertebral sympathetic ganglia (BFSG) to test whether the neurotransmitter peptide luteinizing hormone releasing hormone (LHRH), which generates the late slow EPSP, could also exert long-term neurotrophic control of ion channel expression. Whole-cell recordings from B-cells in BFSG showed that removal of all of the sources of ganglionic LHRH for 10 d by cutting preganglionic C-fibers in vivo caused a 28% reduction in Ca2+ current density. When BFSG B-neurons were dissociated from adult bullfrogs and maintained in a defined-medium, neuron-enriched, low-density, serum-free culture, the ICa density was increased by 49% after 6-7 d in the presence of 0.45 microm LHRH. This increase was not associated with alterations in the voltage dependence of Ca2+ current activation or inactivation and reflected a selective increase in N-type Ca2+ channel current. The increase in ICa density induced by LHRH was blocked by the transcription inhibitor actinomycin D. These results suggest that chronic exposure to a neurotransmitter that acts through G-protein-coupled receptors exerts long-term control of ion channel expression in a fully differentiated, adult sympathetic neuron in vitro or in vivo.

Differential Neurotrophic Regulation of Sodium and Calcium Channels in an Adult Sympathetic Neuron

“…Although LHRH exclusively increases I Ca,N with no effect on activation or inactivation kinetics (Ford et al 2003a), the effects of NGF are more complex and involve an increase in I Ca,L , decreased inactivation of total I Ca , and an increase in I Ca,N (Lei et al 1997). Mechanisms additional to ras-MAPK may be involved in NGF effects on I Ca,L and inactivation processes.…”

“…We have shown previously that this LHRH-induced increase in I Ba does not reflect a change in the voltage dependence of activation or inactivation. It does, however, reflect a selective increase in I Ca,N in a transcription-dependent manner (Ford et al 2003a). Exposure to LHRH may therefore induce synthesis or affect the trafficking of N-type Ca 2 ϩ channels.…”

“…For example, sustained activation of ERK is required for the induction of axonal growth (Huang and Reichardt 2003). It is therefore possible that LHRH may induce a transient activation of ERK that may signal the surface expression of new Ca 2ϩ channels via a transcription-dependent process (Ford et al 2003a). We therefore further characterized the time course of ERK phosphorylation and found that it was maximal after 10 min of LHRH treatment and returned to basal activation levels after 6 h of exposure to the peptide (Fig.…”

“…Although "molecular" approaches such as RNA interference (Holen and Mobbs 2004) or introduction of dominant negative or constitutively active forms of ras (Fitzgerald 2000) might be considered, the lack of information on the Rana catesbeiana genome and the fact that suitable reagents are not readily available makes this approach difficult or infeasible. Since this study is a continuation of an extensive series of physiological studies (Ford et al 2003a;Jassar et al 1993;Lei et al 1997Lei et al , 1998Lei et al , 2001Petrov et al 2001) on Rana catesbeina, it is not appropriate to change to a mammalian (Fitzgerald 2000;Fitzgerald and Dolphin 1997) or cell line system even though they are clearly more amenable to molecular biological approaches (Black et al 2003).…”

“…We have also demonstrated that luteinizing hormone releasing hormone (LHRH), a neurotransmitter released from preganglionic C-fibers in BFSG (Jan et al 1979(Jan et al , 1980, is capable of regulating functional expression of Ca 2ϩ channels (Ford et al 2003a). Gonadotrophin receptors can couple via G q/11 , G s , or G i to various downstream effectors including ras-MAPK (Naor et al 1998;Sim et al 1995).…”