Brain histamine is essential for anti-stress effect of chewing

Objective: Our recent studies suggest that chewing counteracts stress-attenuation of the hippocampal memory process. The activation of the brain's histaminergic system by mastication is one of the potent mechanisms underlying this anti-stress effect of chewing, since neuronal histamine facilitates the hippocampus-dependent learning and memory process. To address this, we studied the role brain histamine plays in the long-term potentiation (LTP) of neurons from rats that had experienced spontaneous chewing during stress exposure.

Methods: We administrated pyrilamine (histamine receptor-1 blocker) into the tail vein of male SD rats. We assigned half the rats to a control group (CT) and half to an experimental group (stress and chewing group; SC). We returned the control rats to their home cages immediately after administering the drug and without exposing them to stress. Fifteen minutes after administering pyrilamine to the experimental rats, we subjected them to restraint stress for 30 min, during which time they spontaneously chewed a wooden stick. We prepared hippocampal slices of these rats 24h after exposure to stress and recorded the LTP of field excitatory postsynaptic potential of neurons from the hippocampus.

Results: The mean amplitudes of LTP were 160±15% and 205±3% of baseline in group SC and group CT, respectively. In a previous experiment, we have already established a mean value of LTP for neurons from rats that experienced the same period of stress with chewing but had not received pyrilamine. The amplitude of LTP for neurons from rats in group SC was about 70% of the value we had measured in that experiment (190±8% of baseline).

Conclusion: Pyrilamine prevented the ameliorative effect of chewing on stress-attenuated LTP. This result suggests that brain histamine plays an essential role in the process by which chewing relieves stress. MEXT supported this work with an Open Research Center subsidy (H18).