This chapter discusses the identification and distribution of enkephalins and endorphins. The two main methods of detecting and quantitating enkephalins and endorphins used so far are radioimmunoassay and bioassay. In addition, opioid receptor binding assays were originally utilized to detect general opioid activity. An area of the brain in which the localization of enkephalin seems to tie in well with certain effects of narcotics is the hypothalamus. Immunohistochemistry reveals that both enkephalin-containing terminals and cell bodies are localized within the hypothalamus. In particular, enkephalinergic terminals are localized in the external layer of the median eminence. Quantitative radioimmunoassay data have been reported by two groups for β-endorphin, both using the same antibody. Concentrations of β-endorphin and β-lipotropin in the pituitary are extremely high. The main localization of this peptide by radioimmunoassay appears to be the neurohypophysis/pars intermedia area rather than in the adenohypophysis. β-endorphin can also be degraded by an enzyme in synaptosomal membranes. The products formed depend on the pH at which the experiment is performed and also on whether bacitracin is present.
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