Effect of spinal morphine after long-term potentiation of wide dynamic range neurones in the rat

Background: Studies have shown that long-term increase in the excitability of single wide dynamic range neurones in the spinal dorsal horn of rats may be induced after tetanic stimulation to the sciatic nerve. This sensory ev ent is possibly an lit vivo counterpart of long-term potentiation, describe d in the brain. This study investigated whether this phenomenon occurs in t he halothane-anesthetized rat and whether the antinociceptive effects of sp inally administered morphine are altered when tested on the enhanced activi ty. Methods: Single unit extracellular recordings were made in three different groups of halothane-anesthetized rats (n = 6 in each group). In group 1, th e evoked neuronal responses of wide dynamic range neurones by a single elec trical stimulus to the peripheral nerve were recorded every 4 min, for 1 h before (baseline) and for 3 h after brief high-frequency conditioning stimu lation of the sciatic nerve. in group 2, morphine was applied onto the spin al cord after long-term potentiation had been established. Increasing conce ntrations of morphine mere added until the C fiber-evoked responses were ab olished; this was followed by naloxone reversal. In group 3, the same proto col as in group 2 was used except a waiting period substituted for the elec trical conditioning. Results: The C fiber-evoked responses were significantly increased (P < 0.0 01) after conditioning compared with baseline and those in control animals. Further, significantly higher concentrations of morphine (P = 0.008) were needed to abolish the C fiber-evoked responses in tetanized animals than in control animals. Naloxone reversed the effects of morphine to the predrug potentiated baseline in group 2, showing that opioids do not block the main tenance of spinal long-term potentiation. Conclusions: Long-term potentiation of C fiber-evoked responses also can be induced in. halothane-anesthetized rats, and morphine seems to have less p otency during such conditions. These data suggest that long-term potentiati on-like mechanisms may underlie some forms of hyperalgesia associated with a reduced effect of morphine.