Cooperative Effects of Ciliary Neurotrophic Factor and Norepinephrine on Tyrosine Hydroxylase Expression in Cultured Rat Locus Coeruleus Neurons

Abstract

Ciliary neurotrophic factor (CNTF) was found to promote the expression of tyrosine hydroxylase (TH) immunoreactivity by cultured noradrenergic neurons from the locus coeruleus (LC) of E18 rat fetuses, but only in the concomitant presence of norepinephrine (NE), their own neurotransmitter. The number of TH-positive cells in LC cultures was shown to decrease by 65% within 3 days and by 75% after 6 days. Treatment with 10 TU/ml human recombinant CNTF together with 1 μ M NE was able to fully maintain the initial number of TH-positive neurons for 3 days. This effect, however, was no longer seen after 6 days of continuous exposure. A 24-hr treatment with CNTF/NE was capable of completely restoring the initial number of TH-positive cells, even if its addition was delayed for 2 days. Moreover, when its addition was delayed for 5 days, CNTF/NE restored ∼80% of the TH-positive neurons that were initially present. These results suggest that the disappearance of TH-positive neurons in LC cultures is not due to their death, but rather to the reduced expression of TH and that the simultaneous exposure to CNTF and NE upregulates TH. Effects on TH-positive cell number were not evoked by CNTF or NE alone. The CNTF/NE effect was dependent on protein synthesis, but was only partially inhibited by RNA synthesis inhibitors, suggesting that both transcription from preexisting mRNA and synthesis of new RNA were stimulated. The effect of CNTF/NE was mediated by α 2-adrenoceptors, since it was blocked by α 2-antagonists and since α 2-agonists were able to substitute for NE. Our results suggest a novel mechanism of regulation of the phenotype of the noradrenergic LC neuron, involving the collaborative influences of CNTF and norepinephrine, their own neurotransmitter.