Nerve growth factor affects Ca²⁺ currents via the p75 receptor to enhance prolactin mRNA levels in GH₃ rat pituitary cells

Abstract

In clonal pituitary GH₃ cells, spontaneous action potentials drive the opening of Ca_v1 (L-type) channels, leading to Ca²⁺ transients that are coupled to prolactin gene transcription. Nerve growth factor (NGF) has been shown to stimulate prolactin synthesis by GH₃ cells, but the underlying mechanisms are unknown. Here we studied whether NGF influences prolactin gene expression and Ca²⁺ currents. By using RT-PCR, NGF (50 ng ml⁻¹) was found to augment prolactin mRNA levels by ∼80% when applied to GH₃ cells for 3 days. A parallel change in the prolactin content was detected by Western blotting. Both NGF-induced responses were mimicked by an agonist (Bay K 8644) and prevented by a blocker (nimodipine) of L-type channels. In whole-cell patch-clamp experiments, NGF enhanced the L-type Ca²⁺ current by ∼2-fold within 60 min. This effect reversed quickly upon growth factor withdrawal, but was maintained for days in the continued presence of NGF. In addition, chronic treatment (≥ 24 h) with NGF amplified the T-type current, which flows through Ca_v3 channels and is thought to support pacemaking activity. Thus, NGF probably increases the amount of Ca²⁺ that enters per action potential and may also induce a late increase in spike frequency. MC192, a specific antibody for the p75 neurotrophin receptor, but not tyrosine kinase inhibitors (K252a and lavendustin A), blocked the effects of NGF on Ca²⁺ currents. Overall, the results indicate that NGF activates the p75 receptor to cause a prolonged increase in Ca²⁺ influx through L-type channels, which in turn up-regulates the prolactin mRNA.