Influence of morphine microinjected into head of caudate nucleus on electric activities of nociceptive neurons in parafascicular nucleus of rat thalamus.

Morphine (8 micrograms/microliter) was injected into the head of the caudate nucleus (CN). The pain-discharges of the pain-excitation neurons (PEN) in the parafascicular nucleus (Pf) were inhibited, ie, decreasing in frequency of pain-discharges and lengthening in latent period of pain-discharges. The inhibitory effect of the pain-inhibition neurons (PIN) in the Pf induced by noxious peripheral stimulation were relieved, ie, increasing the firing rate and shortening the inhibitory duration. The opiate receptor antagonist naloxone (0.75 mg/kg, ip) blocked the above effects of morphine. These results suggest that the intracaudate opioid peptide system play an important role in the modulation of pain information in the Pf of thalamus.