The parafascicular nucleus (PFN) of thalamus, as a supraspinal structure, has an important role in processing of nociceptive information. In addition, μ-
opioid receptor contributes to supraspinal modulation of nociception. In the present study, the effects of microinjection of
naloxone (a non-specific
opioid-receptor antagonist) and
naloxonazine (a specific μ-
opioid receptor antagonist) were investigated on
morphine-induced antinociception in a rat model of acute trigeminal
pain. Right and left sides of PFN of thalamus were implanted with two guide cannulas. Acute trigeminal
pain was induced by local corneal surface application of hypertonic saline and the number of eye wipes as a
pain index was recorded for 30 sec. Microinjection of
morphine at doses of 1, 2 and 4 μg per site significantly (p < 0.05) decreased the number of eye wipes. Alone microinjection of
naloxone (4 μg per site) and
naloxonazine (1 and 2 μg per site) significantly (p < 0.05) increased corneal
pain severity. Prior microinjection of
naloxone (2 and 4 μg per site) and
naloxonazine (1 and 2 μg per site) significantly (p < 0.05) prevented the antinociceptive effect induced by
morphine (4 μg per site). All the above-mentioned chemicals did not alter locomotor behavior in an open-field test. The results of the present study showed an antinociceptive effect of
morphine at the PFN level of thalamus.
Mu-opioid receptor of the PFN of thalamus may be involved in
morphine-induced antinociception.