Osmotic regulation of methionine enkephalin in the posterior pituitary of the rat.

Methionine (Met) enkephalin is detectable in rat hypothalamic and neurointermediate lobe (NIL) tissue extracts using a specific radioimmunoassay. Reversed-phase HPLC revealed that only the pentapeptide form was present in the extracts. Total amounts of Met-enkephalin in extracts containing the median eminence (ME), supraoptic nucleus (SON), paraventricular nucleus (PVN), or NIL from control animals were 9.7 +/- 1.6, 0.9 +/- 0.2, 25.2 +/- 6.8 and 14.8 +/- 1.2 pmol respectively (means +/- S.E.M., n = 6). In animals given 340 mmol NaCl/l to drink for 5 days, no significant changes occurred in Met-enkephalin content in the SON or ME, but significant decreases were observed in the NIL and PVN (9.8 +/- 0.8 and 13.6 +/- 1.7 pmol, respectively). Amounts of Met-enkephalin in these tissues were further decreased after 12 days of 340 mmol NaCl/l (3.4 +/- 0.4 and 6.0 +/- 0.6 pmol). These data demonstrate that enkephalin immunoreactivity in the NIL is principally in the form of the Met-enkephalin pentapeptide, and that this peptide is released in response to increased plasma osmolality. The concomitant changes in the PVN and NIL tissue content suggest that the PVN is the source of NIL Met-enkephalin.