Effects of histamine microinjections into the hippocampus on open-field behavior in rats

Increasing evidence supports the concept that histamine would subserve neurotransmitter functions in the brain. In hippocampus, some cases of histamine receptors coupled to adenylate cyclase has been described. In our laboratory we were interested in studying the possible physiological role of these histamine receptors and behavior. Unilateral cannulae were stereotaxically implanted in the caudal hippocampus in adult male rats. Forty-eight hours later, after an acute microinjection of histamine (1 microgram), 3-methyl-histamine (1 microgram) or saline (1 microliter), the following behavioral parameters were measured by an observer in an open-field: locomotor, rearing, sniffing, grooming, social sniffing and holding activities. It was observed that histamine inhibited locomotor and rearing activities; whereas, 3-methyl-histamine induced an increase in almost all behaviors. Results support the concept of a probable function of histamine in exploratory behavior in the rat.