Galanin inhibits insulin and corticosterone release after injection into the PVN

The neuropeptide galanin (GAL, 1 microgram/0.3 microliters) was injected into the paraventricular nucleus (PVN) at two different times of the 12:12 h light/dark cycle, namely 2 h before the dark ('pre-dark') and before the light ('pre-light') periods. Blood samples were collected 15 min after injection and examined for serum levels of insulin (INS), corticosterone (CORT) and glucose (GLUC). Results indicate that PVN GAL injection in the pre-dark period strongly inhibits both CORT and INS release but has no effect on GLUC levels. These hormone changes, however, were not detected in the blood samples collected in the pre-light period. At this time, baseline levels of CORT were significantly lower than in the pre-dark period, while INS and GLUC levels were generally similar at both time periods. These results indicate that the effects of PVN GAL on CORT and INS secretion are inhibitory in nature and are temporally linked to the diurnal cycle.