Dopamine regulates pain perception in some areas of the central nervous system. Previously, we have confirmed that
dopamine potentiated the electric activities of the evoked discharges of
pain-excited neurons (PENs) and inhibited those of
pain-inhibited neurons (PINs) in the parafascicular nucleus (Pfn) of normal rats. The mechanism of action of
dopamine on
pain-related neurons in the Pfn of
morphine-dependent rat is still unknown. The present study aimed to determine the effects of
dopamine and its receptor antagonist
droperidol on the
pain-evoked responses of the PEN and PIN in the Pfn of
morphine-dependent rats, and to compare the effects between the
morphine-dependent rat and the normal rat. The trains of electric impulses applied to the sciatic nerve were used as noxious stimulation. The discharges of PEN or PIN in the Pfn were recorded by using a glass
microelectrode. The results showed that intra-Pfn microinjection of
dopamine decreased the frequency of noxious stimulation-induced discharges of PEN and increased the frequency of PIN. The intra-Pfn administration of
droperidol produced an opposite effect. These results demonstrated that
dopamine is involved in nociceptive modulation in the
morphine-dependent rat, the responses to noxious stimulation between normal rat and
morphine-dependent rat are completely opposite. The effect of
dopamine is through the
dopamine D(2) receptor of PENs and PINs in Pfn. The results suggest that the
dopamine system of the Pfn may become a therapeutic target for
analgesia and the treatment of
morphine dependence.