Lysine deficient diet and lysine replacement affect food directed operant behavior

As a test for specific hunger for amino acids with the goal of identifying brain regions which might mediate motivation to alleviate L-lysine (Lys) deficiency, rats were trained to lever press to receive complete diet. Rats were given Lys deficient (Lys-def) diet ad lib. Intraperitoneal (IP) injection of Lys, 2 h before the test session, inhibited lever pressing. Lys injected animals pressed at nearly the same low rate as did animals fed ad lib complete diet, although the response of saline injected animals was also partially decreased. Next, osmotic minipumps were implanted IP and animals given Lys-def diet ad lib. Chronic Lys infusion also strongly inhibited lever pressing. Rats allowed ad lib access to Lys to drink drank significantly more Lys than when given complete diet. Lys chronically infused by minipump into the lateral hypothalamus, also inhibited pressing by rats given Lys-def diet. Therefore, animals lacking Lys in the diet will work to receive complete diet, but replacement of Lys by drinking, chronic IP infusion, or directly into the lateral hypothalamic area inhibits bar pressing behavior.