Alterations in neuronal excitability and the potency of spinal mu, delta and kappa opioids after carrageenan-induced inflammation.

These electrophysiological results show that the development of inflammation following peripheral injection of carrageenan into the paw is accompanied by alterations in the magnitude of the C-fibre evoked response of multireceptive dorsal horn neurones. The evoked response of the dorsal horn cells was found to either increase or decrease in the 3 h following the carrageenan injection, and the direction of this change was related to the degree of wind-up exhibited by the cell. Regardless of whether a cell was facilitated or inhibited by carrageenan, mu, delta and kappa opioids applied topically onto the spinal cord (equivalent to an intrathecal injection) exhibited increased antinociceptive potency.

Intrathecal opioids, potency and lipophilicity.

To determine the relationship between potency and lipophilicity for intrathecal opioids, morphine, normorphine, pethidine and methadone were studied in an electrophysiological model in the rat. Dose-response curves were constructed for the opioid effects on C fibre evoked activity of dorsal horn nociceptive neurones following intrathecal application of each opioid, and the ED50 values were correlated with lipid solubility. A significant inverse correlation was found (P = 0.

Subcutaneous formalin-induced activity of dorsal horn neurones in the rat: differential response to an intrathecal opiate administered pre or post formalin.

Many studies of pain and nociception use short-lasting acute stimuli which may have limited relevance to prolonged or chronic pain states. Using extracellular single-unit recording in the dorsal horn of the rat lumbar spinal cord the present study examines the response of neurones to a long-lasting nociceptive stimulus, i.e.

Electrophysiological studies on the effects of intrathecal morphine on nociceptive neurones in the rat dorsal horn.

We have studied the effects of intrathecal morphine on the responses of 38 dorsal horn neurones in the intact rat under halothane anaesthesia to A and C fibre electrical stimulation and to natural stimuli applied to their receptive fields. Morphine selectively reduced the C fibre and pinch evoked activity in a dose-dependent naloxone-reversible manner with an ED50 of 7 nmoles. The 'wind-up' of neurones to repetitive stimulation was little altered except with the highest doses (50-150 nmoles) tested.