Further evidence that histamine in hippocampus affects the exploratory behavior in the rat.

The effect of histamine (HA) and 3-methyl-histamine (3-MHA) in the hippocampus on hole-board behavior was studied. Male rats were microinjected stereotaxically into the hippocampus with 1 microliter of saline solution containing 9, 45 or 90 nMol of HA or 3-MHA. Five min later, all rats were tested for 5 min in a hole-board and locomotor, rearing, grooming and head-dipping activities were measured by direct observation. HA inhibited rearing and grooming at all the doses used but it affected locomotor activity only at 45 and 90 nMol doses. There was no effect on head-dipping behavior. 3-MHA instead increased locomotor and head-dipping activities at all the doses used and it did not change rearing and grooming. The present results give a further support for a physiological participation of HA in the hippocampus.