Clonidine and the hyper-responsiveness of dorsal horn neurones following morphine withdrawal in the spinal cat

The effect of spinal ibuprofen on opioid withdrawal in the rat

This study examines the effect of spinal ibuprofen on the behavioral manifestations associated with the opioid abstinence syndrome. Rats (n = 8 per group) were infused for 5 days with morphine and then pretreated with a spinal bolus dose of ibuprofen before systemic naloxone antagonism (300 microg). Groups included ibuprofen S(+) 1. 36, 13.6, and 136 nmol, and ibuprofen R(-) 136 nmol. A separate group of saline-infused rats was given ibuprofen S(+) 136 nmol before naloxone antagonism. Ibuprofen S(+), but not R(-), dose-dependently and stereospecifically blocked opioid withdrawal hyperalgesia but did not significantly alter other signs of the opioid abstinence syndrome. We conclude that hyperalgesia associated with opioid withdrawal can be blocked by spinally administered ibuprofen, and suggest that there may be a role for spinal prostaglandins in the enhancement of nociception observed in association with the opioid abstinence syndrome.

IMPLICATIONS

This study shows that spinal ibuprofen blocks opioid withdrawal hyperalgesia in the rat in a stereospecific fashion, implicating the likely release of spinal prostaglandins during withdrawal and their possible role as neuromodulators in the enhancement of nociception that accompanies this phenomenon.

Lack of effect of microinjection of noradrenaline or medetomidine on stimulus-evoked release of substance P in the spinal cord of the cat: a study with antibody microprobes

1. Experiments were performed on barbiturate anaesthetized, spinalized cats to investigate the effect of microinjected noradrenaline or medetomidine on the release of immunoreactive substance P in the dorsal spinal cord following peripheral nerve stimulation. The presence of immunoreactive substance P was assessed with microprobes bearing C-terminus-directed antibodies to substance P. 2. Noradrenaline or medetomidine were microinjected into the grey matter of the spinal cord, near microprobe insertion sites, at depths of 2.5, 2.0, 1.5 and 1.0 mm below the spinal cord surface with volumes of approximately 0.125 microliters and a concentration of 10(-3) M. 3. In the untreated spinal cord, electrical stimulation of the ipsilateral tibial nerve (suprathreshold for C-fibres) elicited release of immunoreactive substance P which was centred in and around lamina II. Neither noradrenaline nor medetomidine administration in the manner described produced significant alterations in this pattern of nerve stimulus-evoked release. 4. In agreement with recent ultrastructural studies these results do not support a control of substance P release by catecholamines released from sites near to the central terminals of small diameter primary afferent fibres.

Intraspinal release of substance P and calcitonin gene-related peptide during opiate dependence and withdrawal

The antibody microprobe technique was used to study the release of immunoreactive substance P and immunoreactive calcitonin gene-related peptide within the lower lumbar spinal cord of anaesthetized spinalized cats pretreated twice daily for 3.5 days with increasing doses of morphine hydrochloride (2-20 mg/kg, i.p.). Both peptides were released in the region of the substantia gelatinosa during noxious cutaneous thermal stimulation or high-intensity electrical stimulation of a hind limb nerve. Intravenous administration of naloxone increased the nociceptive excitation of lumbar dorsal horn neurons, but did not alter the evoked release of immunoreactive substance P or immunoreactive calcitonin gene-related peptide in the superficial gray matter dorsal to these neurons. In addition, the release of both peptides was not significantly different to that detected under similar experimental conditions in opioid-naive cats. The results suggest that alterations in neuropeptide release from the central terminals of nociceptive primary afferent neurons do not occur during states of opiate dependence and withdrawal, and thus do not contribute to the characteristic signs of these phenomena in dependent animals.

Inhibition of nociceptive responses of dorsal horn neurones in the cat spinal cord by ohmefentanyl, a preferring mu opioid receptor agonist [corrected]

An action of ohmefentanyl (OMF), a novel preferring mu opioid receptor agonist, on nociceptive response of the dorsal horn neurone, has been studied in the spinal cat. OMF (0.5-2 micrograms/kg, i.v.) produced a potent naloxone-reversible inhibition of firing of dorsal horn neurones in response to noxious heating of glabrous hindpaw or to impulses in C primary afferents. When ejected into the substantia gelatinosa, OMF produced a relatively selective inhibition of C response, but not A response to excitation of myelinated afferent fibres, of the dorsal horn neurone. An interaction between ohmefentanyl and mu opioid receptors is discussed.

Primary sensory neurones and naloxone-precipitated morphine withdrawal

1. The C-fibre-evoked depressor reflex following i.a. injection of capsaicin and the wiping movements following chemical irritation of the cornea by capsaicin were both found to be augmented in the naloxone-precipitated morphine withdrawal phase. 2. The in vitro capsaicin-evoked release of substance P from central terminals of C-fibre afferents in the spinal cord was decreased in morphine-treated rats. Following naloxone, the release in the morphine-treated group was as large as in the control group. 3. The C-fibre mediated plasma extravasation in the rat paw after naloxone was found to be the same in morphine-treated and control rats. 4. It is concluded that C-fibre-evoked reflex reactions are augmented during morphine withdrawal, but that the mechanisms responsible are either located postsynaptically to the primary sensory neurone or further centrally.

Cellular and molecular mechanisms of drug dependence

The molecular and cellular actions of three classes of abused drugs--opiates, psychostimulants, and ethanol--are reviewed in the context of behavioral studies of drug dependence. The immediate effects of drugs are compared to those observed after long-term exposure. A neurobiological basis for drug dependence is proposed from the linkage between the cellular and behavioral effects of these drugs.