Endogenous opioid dependence after intermittent use of glucose, sodium chloride, and monosodium glutamate solutions

In 2002, Colantuoni et al described the possibility of dependency in rats after intermittent, excessive consumption of a 25% glucose solution over a one‐week period. We hypothesized that the intermittent consumption of any tasty solution can lead to endogenous opioid dependency. Another aim was to determine whether dependency is connected to the taste of the consumed substance or with its physiological significance. Rats were maintained on chow and cyclic glucose, NaCl, or monosodium glutamate (MSG) solution for 8 days. On day 9, after a 12‐hr deprivation period and administration of intraperitoneally (IP) naloxone, the general withdrawal index was calculated as the sum of teeth chattering, head shaking, forepaw tremors, and wet dog shakes. Motor activity was also documented. After the intermittent consumption of any tasty solution, rats were found to demonstrate signs of endogenous opioid dependence. The development of dependence was not related to taste preferences or the amount of solution consumed. Intermittent use of essential substance with a pleasant taste, as glucose and sodium chloride, leads to the rapid development of endogenous opioid dependence. Withdrawal syndrome following the consumption of MSG solution is associated with the presence of sodium ions in the MSG molecule.