Postnatal development of beta-endorphin immunoreactivity in the medulla oblongata of rat

Beta-endorphin (B-END) like immunoreactivity (i.r.) levels were measured by radioimmunoassay in the medulla oblongata of developing rats on postnatal ages P1-P42 at 7 day intervals, and in adult rats. From P1 to P42, B-END i.r. increased from 77.0 +/- 1.3 fm to 900.0 +/- 21.6 fm per medulla region (Mean +/- S.E.M.). Adult levels of B-END i.r. were 852.0 +/- 17.0 fm per medulla region. When B-END i.r. was determined per unit protein during this developmental period, a statistically significant change in levels was noted. B-END i.r. dropped from P1 to P7, and then increased from P7 to P14 (P less than 0.01). From P14 through adult, levels did not change significantly. Despite a "drop-out" in the observed immunostaining of B-END neurons in caudal medulla (perikarya in the nucleus tractus solitarius) at P21, radioimmunoassayable levels of this peptide remained constant from P21 through adult per unit protein.