Endogenous opioids and pain: status of human studies and new treatment concepts

Biochemical aspects of chronic pain and its relationship to treatment

This review presents an overview of the neurotransmitters and neuromodulators involved in acute and chronic pain. Although there is little evidence that the neuronal pathways differ in the two types of pain, it is clear that different transmitters or receptor types are involved in hyperalgesia and chronic pain. While most attention has been focussed on spinal processes, it is apparent that some types of chronic pain have both a peripheral and a supraspinal component. The presently available drugs are probably adequate for acute pain, but the treatment of chronic pain may need to be tailored to the individual patient.

Influence of neuropeptides on the processes of higher nervous activity in primates. The action of cyclic analogs of enkephalins on the behavior of the lower monkeys

The effect of two cyclic enkephalin analogs (IVS-43 and IVS-46) on the group and individual behavior of rhesus macaques and hamadryas baboons has been studied. A pronounced influence of IVS-46 on competitive and operant goal-directed behavior of the monkeys was identified.

Combined use of cerebral spinal fluid drainage and naloxone reduces the risk of paraplegia in thoracoabdominal aneurysm repair

PURPOSE

This report summarizes our experience with the use of cerebral spinal fluid drainage (CSFD) and naloxone for prevention of postoperative neurologic deficit (paraplegia or paraparesis).

METHODS

We reviewed 110 consecutive patients with 86 thoracoabdominal aneurysms and 24 thoracic aneurysms. The status of 47 patients (43%) was acute (rupture or dissection), and the status of 52 (47%) was Crawford type I or II. None of the patients had intercostal artery reimplantation. There were two patient groups for analysis of neurologic deficit risk. Group A (61 patients) received naloxone and CSFD, and group B (49 patients) did not.

RESULTS

One deficit occurred in group A and 11 deficits occurred in group B (p = 0.001). By multiple logistic regression analysis, the variables acute status, Crawford type II, or group B classification were significant factors for deficit risk. Use of the same logistic regression analysis on the subgroup of 47 patients with acute aneurysms and 33 patients with Crawford type 2 aneurysms confirmed the protective effect of combined CSFD and naloxone (group A) and that clinical presentation and extent of aorta replaced are the primary risk factors for development of deficit. To test this conclusion we developed a highly predictive model (correlation coefficient 0.997 with 16 series of thoracoabdominal aneurysms) for neurologic deficit. We applied our data to this model. Group B had the predicted number of deficits, and group A had substantially fewer deficits than predicted.

CONCLUSIONS

We conclude that the combined use of CSFD and naloxone offers significant protection from neurologic deficits in patients undergoing thoracoabdominal and thoracic aortic replacement.

Effects of morphine and D-Ala2-D-Leu5-enkephalin in the seizure-susceptible El mouse

Opioid agonists were used to investigate the modulation of seizures in the seizure-susceptible El mouse. Morphine and D-Ala2-D-Leu5-enkephalin (DADLE) were injected subcutaneously or intracisternally as prototypic agonists for mu and delta opioid receptors. Systemic or intracisternal injection of both morphine and DADLE decreased the incidence of seizures and the seizure score in El mice in a dose-dependent manner. The anticonvulsant effects of morphine and DADLE were reversed by naloxone (2 mg/kg, s.c.). This implies that opioid agonists have anticonvulsant properties which are mediated by mu and delta opioid receptors. In conclusion, a deficit in endogenous opioid peptides, which act as anticonvulsants may play a significant role in the etiology or pathophysiology of seizures in the El mouse.

Analgesic activity of a novel bivalent opioid peptide compared to morphine via different routes of administration

A novel bivalent opioid tetrapeptide, biphalin (Tyr-D-Ala-Gly-Phe-NH)2, was synthesized based on structure-activity relationships. The analgesic activity of biphalin was assessed in comparison to morphine in rats. Drugs were administered subcutaneously (s.c.), intravenously (i.v.) and intrathecally (i.t.). Tail flick and tail pinch were used as tests for analgesia. Biphalin s.c. showed negligible analgesic activity, but when given i.v. produced significant analgesia, although less potent than morphine via this route. In contrast, intrathecal biphalin was more potent than morphine. These results indicate that biphalin has intrinsic activity that is compromised by enzymatic degradation or redistribution in the periphery, properties that may render it useful in exploring analgesic actions of locally applied opioids in the periphery without the likelihood of unwanted central effects.