DIFFERENTIAL RESPONSE OF NEUTRAL ENDOPEPTIDASE 24.11 (ENKEPHALINASE),AND CHOLINERGIC AND OPIOIDERGIC MARKERS TO HYPOGLOSSAL AXOTOMY

Neutral endopeptidase 24.11 (NEP; ''enkephalinase'') may inactivate a number of centrally active neuropeptides including the enkephalins and substance P. In most areas of the central nervous sysem, the cell typ es which express NEP activity are not known. The hypoglossal nucleus ( N.XII) was selected as a model system to characterize the cytochemical localization of NEP. The effect of hypoglossal nerve axotomy upon the distribution of NEP activity in the hypoglossal nucleus was compared to the effect upon cholinergic markers, the mu opiate receptor, and th e enkephalins. By use of a fluorescence histochemical method, NEP was localized at all levels of N.XII to the soma and proximal processes of the majority of the apparent motor neurons in the nucleus. Fluorescen t double-labeling studies revealed the presence of numerous enkephalin ergic varicosities which localized to the neuropil surrounding NEP-sta ined motor neurons. To determine whether NEP was synthesized by these motor neurons, 18 rats received a unilteral transection of the hypoglo ssal nerve. A pronounced decrease in NEP staining in N.XII was observe d on the operated side as early as 3 days following axotomy. This decr ease persisted at all levels of the nucleus for about 5 weeks. By 7 we eks, the staining between the control and operated sides was indisting uishable. By contrast, there was no apparent change in the density or distribution of enkephalin8immunoreactive varicosities in five animals examined 6 to 32 days following axotomy. In contrast to the opiate sy stem, the cholinergic enzymes choline acetyltransferase, acetylcholine sterase, and pseudocholinesterase showed a coordinate decrease in moto r neuron8associated staining on the operated side of N.XII at 3, 6, an d 11 days following axotomy which paralleled the decrease in NEP stain ing. By contrast, the lysosomal enzyme marker, acid phosphatase, showe d a pronounced increase in staining on the operated side. The results of this study are consistent with the synthesis of NEP by cholinergic N.XII motor neurons and indicates that the enkephalins and NEP in N.XI I are closely associated, but derive from separate neuronal population s. The widespread overlap in the distribution of NEP-stained motor neu rons and enkephalinergic varicosities in N.XII provides additional ana tomical support for a potential role for NEP in the inactivation of ce ntrally active enkephalins. The apparently stable levels of the enkeph alins and mu-opiate receptor afer axotomy, in contrast to the coordina te decrease in cholinergic enzyme staining, suggest a differential reg ulation of the opiate and cholinergic systems in response to axotomy. (C) 1994 Wiley-Liss, Inc.