Endogenous oploid peptides stimulate post-exercise insulin response to glucose in rats

Low-intensity exercise reverses chronic muscle pain in the rat in a naloxone-dependent manner

OBJECTIVE

To determine the effects of low-intensity exercise on chronic muscle pain and potential activation of the endogenous opioid system.

DESIGN

Randomized placebo-controlled trial.

SETTING

Animal laboratory.

ANIMALS

Sixty-three male Sprague-Dawley rats.

INTERVENTIONS

Rats performed a low-intensity exercise protocol for 5 consecutive days after the induction of chronic muscle pain. In a separate experiment, naloxone or saline was administered systemically before 5 low-intensity exercise sessions.

MAIN OUTCOME MEASURE

Mechanical hyperalgesia was measured using von Frey filaments to determine the mechanical withdrawal threshold.

RESULTS

Low-intensity exercise increased mechanical withdrawal threshold in the chronic muscle pain model. Naloxone attenuated the antihyperalgesic effects of low-intensity exercise.

CONCLUSIONS

Low-intensity exercise reversed mechanical hyperalgesia in the chronic muscle pain model through activation of the endogenous opioid system.

Involvement of serotonin in the hypoglycemic response to 2Hz electroacupuncture of zusanli acupoint (ST36) in rats

In our previous studies, an insulin-dependent hypoglycemic effect produced by electroacupuncture (EA) was shown to be mediated by endogenous opioid peptides (EOP). In the present study, we applied 2 Hz EA to both zusanli acupoints (ST36) in the test group for 30 min, and to a nonacupoint area in the control group for 30 min to compare the acupoint specific character in the hypoglycemic effect of EA. Assays of plasma beta-endorphin and insulin levels were performed by ELISA kits. The insulin-dependent mechanism of the hypoglycemic effect was also investigated in streptozotocin (STZ)-induced diabetic rats. The mediation of EOP and the role of mu-opioid receptor were examined by naloxone and mu-opioid receptor knockout mice (MOR-KOM). The serotonin depletion was carried out by injecting (i.p.) p-chlorophenylalanine (PCPA); two low doses of serotonin were also injected (i.v.) to analyze the direct effect on plasma glucose levels. The hypoglycemic effect of EA was much greater in rats stimulated at ST36 than in rats receiving the same stimulation at the nonacupoint area. The plasma levels of insulin and beta-endorphin were also significantly elevated after stimulation of both zusanli acupoints, but remained unchanged following stimulation at the nonacupoint area. There was no sharp hypoglycemic response to 2 Hz EA at zusanli acupoint of STZ-induced diabetic rats. However, the hypoglycemic effect of this EA was not totally blocked by the sufficient dose of naloxone (1 mg/kg, i.v.). Additionally, 2 Hz EA at ST36 also showed a sharp decrease in plasma glucose levels of MOR-KOM. Pretreatment with PCPA did not reproduce hypoglycemic response to 2 Hz EA in naloxone-treated rats and MOR-KOM mice. Furthermore, injection of serotonin decreased the plasma glucose levels significantly. Therefore, we suggest that serotonin also involved in the hypoglycemic action of 2 Hz EA at both zusanli acupoints of normal rats.

Multiple sources of endogenous opioid peptide involved in the hypoglycemic response to 15 Hz electroacupuncture at the Zhongwan acupoint in rats

A decrease in plasma glucose levels was observed in rats which received electroacupuncture (EA) stimulation at the Zhongwan acupoint. In the present study, the role of the adrenal gland in this hypoglycemic response to EA at high frequency (15 Hz) was investigated on adrenalectomized (ADX) normal rats. There was a sharper decrease in plasma glucose by EA stimulation in the fasting ADX group than in the fasting sham-operated group. Naloxone blocked this hypoglycemic response to EA stimulation in rats which received ADX. Stimulation of EA failed to elicit an increase in plasma beta-endorphin and insulin levels in ADX rats. Similar results were observed in sham and ADX mice. EA stimulation of ADX mice can reduce plasma glucose levels. Furthermore, naloxone abolished the hypoglycemic response to EA stimulation in mice. Such a hypoglycemic response to EA stimulation was also observed in micro-opioid receptor knockout mice (MOR-KOM). Mediation by another opioid peptide should also be considered in future experiments. We conclude that multiple sources of endogenous opioid peptide participated in the lowering of plasma glucose in rats induced by EA stimulation at higher frequency (15 Hz) at the Zhongwan acupoint. Increase in beta-endorphin levels from the adrenal gland enhances the secretion of insulin, there by reducing plasma glucose levels, and is partially involved in this EA stimulation.

Release of beta-endorphin from adrenal gland to lower plasma glucose by the electroacupuncture at Zhongwan acupoint in rats

We found that electroacupuncture (EA) at the Zhongwan acupoint released beta-endorphin to induce an insulin-dependent hypoglycemia in rats. The present study investigated the source of beta-endorphin. EA at 2 Hz for 30 min in rats decreased plasma glucose that could be abolished by naloxone. A similar effect of EA was also observed in wild-type mice but disappeared in mu-opioid receptor knockout mice. Mediation of the mu(1)-opioid receptor is considered from the blockade of response to EA by naloxonazine in rats. Otherwise, adrenalectomy abolished not only the hypoglycemic response to EA in rat and mouse but also the increase of plasma beta-endorphin and insulin by EA in rats. In conclusion, the increase of plasma beta-endorphin by EA at 2 Hz is mainly from the adrenal gland.

Endogenous opiates: 1996

This paper is the nineteenth installment of our annual review of research concerning the opiate system. It summarizes papers published during 1996 reporting the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress, tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.