On the distribution of [met5]- and [leu5]-enkephalins in the brain of the rat, guinea-pig and calf.

1 By means of a highly sensitive radioimmunoassay, the content of [met5]-enkephalin (ME) and [leu5]-enkephalin (LE) was measured in various regions of the rat, guinea-pig and calf brain. Provisions were made to differentiate ME from LE by the use of cyanogen bromide (CNBr) to destroy methionine, the carboxy terminal amino acid in the ME sequence. This allows for correction of possible errors due to the cross-reactivity of the ME to the LE antiserum. Evidence is presented to demonstrate that the specific radioimmunoassay combined with the CNBr technique is a valid method to measure LE and ME content in crude tissue extracts. 2 In all the species studied, enkephalins appeared to be highly concentrated in the striatum and hypothalamus while very low amounts were found in the cerebe-lum and hippocampus. 3 Although the ratio between ME and LE content varied from area to area, the ME content in every region of the rat, guinea-pig and calf brain was more than 4 fold greater than that of LE.