OBJECTIVETo examine the effect of acetylcholine (ACh) on the electric activities of pain-excitation neurons (PEN) and pain-inhibitation neurons (PIN) in the hippocampal CA1 area of normal rats or morphinistic rats, and to explore the role of ACh in regulation of pain perception in CA1 area under normal condition and morphine addiction.

METHODSThe trains of electric impulses applied to sciatic nerve were set as noxious stimulation. The discharges of PEN and PIN in the CA1 area were recorded extracellularly by glass microelectrode. We observed the influence of intracerebroventricular (i.c.v.) injection of ACh and atropine on the noxious stimulation-evoked activities of PEN and PIN in the CA1 area.

RESULTSNoxious stimulation enhanced the electric activity of PEN and depressed that of PIN in the CA1 area of both normal and addiction rats. In normal rats, ACh decrease the pain-evoked discharge frequency of PEN, while increased the frequency of PIN. These effects reached the peak value at 4 min after injection of ACh. In morphinistic rats, ACh also inhibited the PEN electric activity and potentialized the PIN electric activity, but the maximum effect appeared at 6 min after administration. The ACh-induced responses were significantly blocked by muscarinic receptor antagonist atropine.

CONCLUSIONCholinergic neurons and muscarinic receptors in the hippocampal CA1 area are involved in the processing of nociceptive information and they may play an analgesia role in pain modulation. Morphine addiction attenuated the sensitivity of pain-related neurons to the noxious information.