Histamine sensitive sites in hippocampus: their probable role on prolactin release in male rats

The effect of histamine (HA), 3-methyl-histamine (3MHA), HA antagonists or drugs interfering with HA synthesis, microinjected into the hippocampus (HPC), on prolactin (PRL) secretion were studied in rats. Three experiments were performed. In Experiment 1, increasing doses of HA or 3-MHA (9-90 nmol) were microinjected stereotaxically into the ventral HPC of adult male rats. In Experiment 2, 135 nmol of pyrilamine (PYR, an H1-HA-antagonist) or ranitidine (RAN, an H2-HA-antagonist) were administered locally into the ventral HPC. Fifteen min later, the rats were microinjected again with 45 nmol of HA. In Experiment 3, rats were microinjected with different doses of HA-antagonists or with 20 nmol of alpha-fluormethyl-histidine (FMH, an inhibitor of the enzyme of HA synthesis) and later subjected to an immobilization stress of 15 min duration. In all cases, the PRL plasma concentrations were measured in blood samples taken at different time intervals (0-120 min) after the last brain injection. Results showed that HA applied locally in ventral HPC induced an increase in PRL levels which was statistically significant from saline-injected rats between 5-30 min after the HPC stimulation. On the contrary, local applications of 3-MHA did not change significantly the PRL blood levels (Experiment 1). Only PYR did block partially the PRL response due to HA in basal conditions. RAN in these later conditions had no effect (Experiment 2). When animals were subjected to stress neither PYR nor RAN, alone or in combination, locally applied were able to block the PRL increase due to stress. Only FMH blunted significantly the hormone response.(ABSTRACT TRUNCATED AT 250 WORDS)