Detection of neutral endopeptidase (NEP, enkephalinase, E.C.3.4.24.11) in relation to dopaminergic and gonadoliberinergic nerve endings in the median eminence of the male rat: a double labeling ultrastructural study.

ABSTRACT

The existence of neutral endopeptidase (Enkephalinase, NEP, E.C.3.4.24.11) in membranes of nerve endings in the rat median eminence suggests that some neuropeptides have paracrine and/or autocrine actions in this region. In vitro, neutral endopeptidase is capable of hydrolysing a variety of regulatory peptides but in vivo, many works indicate that in the central nervous system this enzyme is highly implicated in the biological inactivation of enkephalins and tachykinins. In addition there is evidence that NEP is also involved in the inactivation of neurotensin in vivo. The modulation of the release of gonadotrophin releasing hormone (GnRH) is one of the documented actions of enkephalins within the median eminence. However, it is at present unclear whether enkephalins act on dopamine endings, on GnRH endings or on both. As the technical parameters and particularly the tissue fixation used to detect neutral endopeptidase are compatible with immunocytochemical detection of GnRH and tyrosine-hydroxylase (the rate limiting enzyme in the synthesis of catecholamines), two double immunolabelings were realised at the ultrastructural level to determine if GnRH and dopamine nerve endings have the enzyme inserted within their plasma membrane. Our study shows the presence of neutral endopeptidase on tyrosine-hydroxylase-immunoreactive nerve endings while presence of the enzyme on GnRH-immunoreactive nerve endings is not demonstrated. Consequently, our results provide morphological arguments for possibilities of paracrine and/or autocrine actions by neuropeptides inactivated by neutral endopeptidase on tuberoinfundibular dopaminergic nerve endings. Conversely, action of the same peptides on GnRH boutons seems more unlikely.