Effects of systemic naloxone upon ventrobasal thalamus neuronal responses in arthritic rats

Antagonist treatment of opioid withdrawal translational low dose approach

Although antagonist drugs are receiving increasing attention in the treatment of opioid withdrawal, the mechanisms of interaction of opiate agonists and antagonists remain largely to be investigated. We focused on the effects of very low quantities of opiate antagonists, following the clinical indication of their potential utility in detoxification. Upon reviewing the evidence on the administration of small doses of naloxone and naltrexone in the presence of agonist drugs, the effects of low-dose naltrexone during opiate administration and withdrawal are described. The application of a translational methodology allowed completing the clinical design with behavioral and cellular information obtained from a specifically developed animal model. The initial results indicate that low doses of naltrexone may help reducing the manifestation of opioid withdrawal, offer suggestions for further investigations and confirm the utility of a translational research approach to the clinical neurobiology of drug addiction.

Single neurone studies of opioid tolerance and dependence at the ventrobasal thalamic level in an experimental model of clinical pain, the arthritic rat

The aim of this electrophysiological study was to investigate the effects of an acute injection of morphine (1 mg/kg i.v.) or the opioid antagonist naloxone (0.6-2 mg/kg i.v.) on thalamic ventrobasal (VB) neuronal activities recorded in arthritic rats rendered tolerant/dependent by pretreatment with relatively low doses of morphine. Recordings were performed in animals immobilized by i.v. injections of gallamine triethiodide (Flaxedil) and artificially ventilated under a moderate gaseous anesthesia (mixture of one-third O2, two-thirds N2O, 0.5-0.6% halothane). This level of anesthesia, as checked by the electrocorticogram, was stable and appeared sufficiently deep, since no sign of suffering or stress could be detected. The efficacy of morphine on VB neuronal responses induced by mild stimulation of the joints was greatly reduced in morphine-pretreated arthritic rats, compared to naive animals (mean neuronal inhibition of 35 vs 85%, respectively). This indicates that the tolerance phenomena observed in behavioral studies are reflected at the VB level, on neurons involved in pain processes. In addition, naloxone (0.6, 1 and 2 mg/kg i.v.) induced a dramatic increase in the evoked (52, 88 and 93%) and spontaneous (64, 211 and 292%) VB neuronal activities recorded in morphine-pretreated arthritic rats, while these activities were not significantly altered in naive arthritic rats. The time-courses of the modifications induced by naloxone in morphine-pretreated arthritic animals were similar to those of the naloxone-precipitated morphine withdrawal observed in freely moving rats. These findings may represent the neuronal correlate at the VB level of the withdrawal response and/or the hyperalgesia induced in tolerant arthritic rats by high doses of naloxone.

[Effects of intravenous naloxone on the secondary effects and analgesia after epidural injection of fentanyl]

On demand intravenous naloxone reverses respiratory depression following epidural morphine but does not have any effect on analgesia. This study aimed to assess the action of a preventive naloxone infusion on the side-effects and analgesia induced by epidural fentanyl. Sixteen patients were studied. Three had isolated uncomplicated flail chest. The thirteen others had undergone thoracotomy, and were included in the protocol at least 6 h after extubation. All patients had two epidural injections, they received an intravenous infusion of either 10 micrograms.kg-1.h-1 naloxone after a 400 micrograms bolus (group F + N) or 5% dextrose (group F), which was randomly allocated. In group F, but not in group F + N PaCO2 increased from the 15th min to the 4th, and sedation occurred from the 15th min to the 6th h. A significant and similar pain relief was noted in both groups. Duration of analgesia was not statistically different in the two groups. This preventive action of intravenous naloxone on the supraspinal adverse effects of epidural fentanyl was not accompanied by a reduction in analgesia. This could lead to widespread use of this analgesic technique.

Chronic pain in the rat: selective alterations in CNS and pituitary pools of dynorphin as compared to vasopressin

Inoculation of rats at the tail-base with Mycobacterium led to arthritic swelling and inflammation of all four limbs. Immunoreactive (ir)-dynorphin (DYN) increased in anterior but not neurointermediate pituitary. In the brain, only thalamus showed a rise and, in spinal cord, a large elevation was seen. Ir-vasopressin (VP) was not affected in these tissues but increased in midbrain. These effects might reflect a role of DYN in the control of chronic pain. In addition, they support a differential modulation of DYN as compared to VP extrinsic to the hypothalamic-neurohypophyseal axis.

Further evidence for a strong depressive effect of low doses of morphine on VB thalamic neuronal responses (a study on arthritic rats)

The effects of various i.v. morphine doses (30, 100 and 1000 micrograms/kg) were studied upon unitary ventral basal (VB) neuronal responses elicited by joint stimulation in 24 Freund's adjuvant induced arthritic rats. The depressive effect of morphine was significantly dose-related and generally naloxone-reversible; however there were sometimes some difficulties to reverse morphine effect with the lowest dose of naloxone (10 micrograms/kg). The effect of morphine was not significantly different from that obtained in normal rats upon responses of specific nociceptive VB neurons. Although these results do not explain enhancement of morphine analgesia in arthritic rats by comparison with normal rats, they do confirm efficiency of low doses of morphine upon VB neuronal responses elicited by stimuli which induce nociceptive reaction in freely moving animals.