Abstract
Methionine enkephalin (Met-Enk) is considered to function as a neurotransmitter or neuromodulator. It is interesting and important to study the Met-Enk levels in the CNS and other tissues during stress. In this paper we examined changes of Met-Enk and catecholamine induced in the rat adrenal gland, corpus striatum and pituitary by cold water stress. Met-Enk was measured by RIA, and catecholamine was assayed by HPLC-ECD. After 30 min and 3 hr of cold water stress, Met-Enk in an acid extract of the adrenal gland was increased to 2 times that of the control. However, after 15 hr stress was Met-Enk almost the same as that of the control. Changes of Met-Enk in the striatum and pituitary had a different pattern. Met-Enk increase in both tissues depended on the duration and severity of the stress. Met-Enk increased continuously with time of cold water stress, and when the stress was changed from that due to cold water to that due to ice-cold water, Met-enk contents increased further in both tissues. To find the mechanism of different responsivilty to severe stress in the adrenal gland and striatum, the processing of Met-Enk in both tissues after 15 hr cold water stress was studied. After acid extracts were chromatographed on Sephadex G-75 column, Met-Enk in each fraction was measured following trysin and carboxypeptidase B treatment. In the striatum Met-Enk immunoreactivity was found in the high molecular weight fraction, in the middle size fraction, and in the small molecular size fraction. This suggests universal synthesis and processing of Met-Enk in the striatum under this kind of stress. In the adrenal gland, high Met-Enk immunoreactivity (MEI) was observed in the high molecular weight fraction, but the MEI in the middle size and low molecular weight fractions was very low, suggesting the inhibition of Met-Enk processing from its precursor. Epinephrine and norepinephrine contents in adrenal gland were not changed by stress tolerance, but tyrosine hydroxylase activity and dopamine concentration were markedly increased.
