Morphine applied to the thalamic nucleus submedius produces a naloxone reversible antinociceptive effect in the rat

Abstract

Our previous studies have indicated that the thalamic nucleus submedius (Sm) is involved in nociceptive modulation and plays an important role in an endogenous analgesic system (a feedback loop) consisting of spinal cord – Sm – ventrolateral orbital cortex (VLO) – periaqueductal gray (PAG) – spinal cord. To further investigate the neurotransmitter and receptor mechanisms in this nociceptive modulatory pathway, we tested the effects of microinjection of morphine and naloxone into the Sm on the rat tail flick (TF) reflex. A unilateral microinjection of morphine (8.0 mM, 0.5 μl) into the Sm significantly depressed the TF reflex, whereas a unilateral microinjection of naloxone (5.0 mM, 0.5 μl) into the Sm facilitated the TF reflex. Five minutes after morphine application into Sm, injection of naloxone in this nucleus markedly reversed the inhibition evoked by applying morphine in Sm. These findings suggest that the endogenous opioid peptides may be involved in the antinociceptive effects evoked by activation of the Sm–VLO–PAG pathway which depressed the nociceptive inputs at the spinal level via the brainstem descending inhibitory system, and exert a tonic descending influence.