Effects of acute and chronic caffeine intake on intravenous self-administration of morphine in two rat strains.

Effects of acute and chronic caffeine intake on the level and pattern of morphine self-administration behavior in WAG/G and Fisher-344 rats were studied. Both acute and chronic caffeine intake decreased morphine self-administration only in WAG/G rats, which attested to increased sensitivity of these rats to reinforcing effects of morphine. Possible relationship between the observed changes and increased anxiety in rats receiving caffeine is discussed.