Tetrahydro-beta-carboline micro-injected into the hippocampus induces an anxiety-like state in the rat

Guide cannulae for bilateral micro-injection were implanted stereotaxically in the rat to rest just dorsal to the hippocampus. Following recovery, 1,2,3,4-tetrahydro-beta-carboline (THBC) hydrochloride in a concentration of 10 or 50 ng was infused bilaterally into the animal's hippocampus in a volume of 3.0 microliter. In the control condition, the artificial cerebrospinal fluid (CSF) vehicle was micro-injected into the hippocampus and a sham injection was made prior to the CSF or THBC infusion. The behavioral response of the rat was examined subsequently in an open-field chamber, in terms of the number of grid squares crossed, duration of grooming time and instances of freezing-immobilization during the test interval of 7.5 min. Other behaviors recorded included the appearance of tail rigidity and the number of fecal boluses excreted. The intra-hippocampal infusion of the 10 ng dose of beta-carboline reduced the motor activity of the rat whereas the higher dose of THBC increased the duration of the freezing-immobilization. THBC failed to alter significantly the grooming activity of rats or their rate of defecation. Following repeated micro-injections of 50 ng of THBC, the duration of freezing-immobilization gradually decreased, but the response itself remained essentially intact. These results suggest that the well-known anxiogenic action of certain of the beta-carboline class of aldehyde adducts may be mediated in part by neurons in the hippocampus, or the constituent pathways of this limbic system structure, or both.