Modulation of cholinergic marker expression by nerve growth factor in dorsal root ganglia

The presence of cholinergic markers in sensory ganglia has suggested a possible functional role of acetylcholine both as a cofactor of morphogenesis in embryonic life and in sensory transduction during adult life. Acetylcholine, in fact, is able to excite cutaneous nociceptors and to modulate noxious stimuli. Nerve growth factor (NGF) overexpression induces the survival of nociceptive neurons, the expression of their specific markers, and hyperalgesia. On the other hand, NGF modulate the levels of cholinergic markers in several area of nervous system. Considering these observations, the present work aims to investigate whether NGF is able also to control the expression of cholinergic markers in chick sensory neurons in culture. We selected three developmental stages (E8, E12, and E18) representative of different phases of chick embryo development and performed observations on culture in which NGF was omitted at the plating time, withdrawn after the initial 24 hr of culture or maintained for 48 hr. In the experimental protocol devised, NGF did not significantly affect cell survival. At E12 a 48 hr treatment with NGF causes a significant but limited increase in acetylcholinesterase activity; activity increase was not observed when NGF was removed after 24 hr. No changes in acetylcholinesterase activity were observed at E8 and E18 stages. NGF appears to be more effective in the modulation of choline acetyltransferase activity. At E12, in fact, about a doubling of enzyme activity was measured after 24 or 48 hr of treatment with NGF. A response was also found at E18, when a 50% increase in choline acetyltransferase activity was observed just after 24 hr treatment. The behavior of muscarinic receptors in response to NGF differs compared to the two cholinergic enzymes. At E8 and E12 a profound increase in muscarinic receptor expression was observed. Conversely, at E18 NGF produces a 50% reduction of receptors. Considering these observations and the demonstrated role of muscarinic receptors in the desensitization of nociceptors, the reduction of muscarinic receptors in DRG after NGF treatment is in agreement with the proposed algogenic action of NGF in the skin.