Opioid peptides (endorphins) in pituitary and brain

Effect of Electroacupuncture at ST36 on Gastric-Related Neurons in Spinal Dorsal Horn and Nucleus Tractus Solitarius

The aim of this study was to observe the effect of electroacupuncture (EA) at the ST36 acupoint on the firing rate of gastric-related neurons in the spinal dorsal horn (SDH) and nucleus tractus solitarius (NTS). There were different effects of gastric distention in SDH and NTS in 46 male Sprague-Dawley rats. In 10 excitatory neurons in SDH, most of the neurons were inhibited by homolateral EA. The firing rates decreased significantly (P < 0.05) in 10 excitatory gastric-related neurons in NTS; the firing rates of 6 neurons were further excited by homolateral EA, with a significant increase of the firing rates (P < 0.05); all inhibitory gastric-related neurons in NTS were excited by EA. The inhibition rate of homolateral EA was significantly increased in comparison with contralateral EA in gastric-related neurons of SDH (P < 0.05). There was no significant difference between homolateral and contralateral EA in gastric-related neurons of NTS. EA at ST36 changes the firing rate of gastric-related neurons in SDH and NTS. However, there are some differences in responsive mode in these neurons. The existence of these differences could be one of the physiological foundations of diversity and complexity in EA effects.

Gastrointestinal motility disorders and acupuncture

During the last decades, numerous studies have been performed to investigate the effects and mechanisms of acupuncture or electroacupuncture (EA) on gastrointestinal motility and patients with functional gastrointestinal diseases. A PubMed search was performed on this topic and all available studies published in English have been reviewed and evaluated. This review is organized based on the gastrointestinal organ (from the esophagus to the colon), components of gastrointestinal motility and the functional diseases related to specific motility disorders. It was found that the effects of acupuncture or EA on gastrointestinal motility were fairly consistent and the major acupuncture points used in these studies were ST36 and PC6. Gastric motility has been mostly studied, whereas much less information is available on the effect of EA on small and large intestinal motility or related disorders. A number of clinical studies have been published, investigating the therapeutic effects of EA on a number of functional gastrointestinal diseases, such as gastroesophageal reflux, functional dyspepsia and irritable bowel syndrome. However, the findings of these clinical studies were inconclusive. In summary, acupuncture or EA is able to alter gastrointestinal motility functions and improve gastrointestinal motility disorders. However, more studies are needed to establish the therapeutic roles of EA in treating functional gastrointestinal diseases.

Neurobiological mechanisms of the onset of puberty in primates

An increase in pulsatile release of LHRH is essential for the onset of puberty. However, the mechanism controlling the pubertal increase in LHRH release is still unclear. In primates the LHRH neurosecretory system is already active during the neonatal period but subsequently enters a dormant state in the juvenile/prepubertal period. Neither gonadal steroid hormones nor the absence of facilitatory neuronal inputs to LHRH neurons is responsible for the low levels of LHRH release before the onset of puberty in primates. Recent studies suggest that during the prepubertal period an inhibitory neuronal system suppresses LHRH release and that during the subsequent maturation of the hypothalamus this prepubertal inhibition is removed, allowing the adult pattern of pulsatile LHRH release. In fact, y-aminobutyric acid (GABA) appears to be an inhibitory neurotransmitter responsible for restricting LHRH release before the onset of puberty in female rhesus monkeys. In addition, it appears that the reduction in tonic GABA inhibition allows an increase in the release of glutamate as well as other neurotransmitters, which contributes to the increase in pubertal LHRH release. In this review, developmental changes in several neurotransmitter systems controlling pulsatile LHRH release are extensively reviewed.

Physiological relevance and time course of a tonic endogenous opioid modulation of nociceptive messages, based on the effects of naloxone in a rat model of localized hyperalgesic inflammation

In a rat model of localized hyperalgesic inflammation induced by intraplantar injection of carrageenin, the effect of a relatively high dose of naloxone (1 mg/kg i.v.) was investigated using the measure of the vocalization threshold as a nociceptive test, on both the inflamed and non-inflamed paws. The effects of the drug were determined at two different periods after the intraplantar injection of carrageenin, in the same group of rats. We showed that 4 h after carrageenin (a few hours after the onset of the inflammatory process), naloxone induced a significant further decrease in the vocalization threshold induced by pressure on either paw, suggesting that naloxone had reduced a tonically active inhibitory system involving endogenous opioid peptides. Twenty-four hours after carrageenin, a consistent hyperalgesic effect of naloxone was observable only in rats which had recovered from their carrageenin-induced hyperalgesia. A significant negative correlation between the behavioral effect of naloxone and the degree of hyperalgesia determined for each animal was observed. This suggests that the tonic inhibition exerted by the endogenous opioids was particularly effective in rats which recovered from their initial hyperalgesia. By contrast, these opioid controls could have been weaker in those rats which remained hyperalgesic.

The bidirectional dose-dependent effect of systemic naloxone is also related to the intensity and duration of pain-related disorders: a study in a rat model of peripheral mononeuropathy

In an experimental model of mononeuropathy in the rat, created by 4 ligatures around the sciatic nerve, i.v. naloxone 1 week after surgery induces bidirectional effects (antinociceptive effects at very low doses, hyperalgesic effects with high doses). Using the same nociceptive test (vocalization thresholds to paw pressure), the activity of the same doses of naloxone (3 micrograms/kg, and 1 mg/kg) was investigated 2 weeks after sciatic ligation, when the behavioural pain-related disorders are at a maximum. Three micrograms/kg naloxone produced a significant antinociceptive effect on the lesioned and non-lesioned paw, which was clearly related to the degree as well as to the duration of pain-related signs in the rat. By contrast, the high dose of naloxone did not induce a mean significant effect when tested on either paw; however, it elicited a potent hyperalgesic effect in those rats which had recovered from hyperalgesia at this 2 week time point after the sciatic injury.

Behavioural evidence for a bidirectional effect of systemic naloxone in a model of experimental neuropathy in the rat

In animal models of inflammatory pain, we have demonstrated that the opioid antagonist naloxone induces a paradoxical analgesic effect at very low systemic doses, and a hyperalgesic effect at high doses. We have therefore proposed, that opioid systems are modified in these animals with persistent pain. The aim of the present study was to investigate the activity of naloxone through another model of pain in the rat due to a peripheral neuropathy of the sciatic nerve. The neuropathy was created by 4 ligatures around the sciatic nerve. We analyzed the effects of i.v. naloxone (3 and 10 micrograms/kg, 1 mg/kg) on the vocalization thresholds to paw pressure 8 days after the sciatic ligation. Three and 10 micrograms/kg naloxone produced a significant paradoxical antinociceptive effect on responses from the affected paw (with a mean increase of about 50 and 30% of the preinjection values, respectively) and also from the non-affected paw, although the effect was less potent. By contrast, 1 mg/kg naloxone elicited a significant hyperalgesia on responses from the affected and non-affected paw. The effects of the microdoses, but not those of the high dose, were clearly related to the vocalization thresholds measured for each rat just before injection. This study clearly shows that naloxone induces bidirectional effects in a rat model of neuropathic pain, which contradicts the current statement that neuropathic pain is opioid-resistant. The present results also suggest that these effects are not related to inflammatory processes, and may be due to modifications of opioid systems in these animals with persistent pain.

Cross-tolerance between analgesic low doses of morphine and naloxone in arthritic rats

The effects of acute injections of naloxone (3-3000 micrograms/kg i.v.) and morphine (100-1000 micrograms/kg i.v.) on the vocalization threshold induced by pressure on the paw were analyzed in adjuvant-induced arthritic rats pretreated either with naloxone or with morphine administered at low doses (9 micrograms/kg s.c. and 3000 micrograms/kg s.c., respectively) over 4 consecutive days. In naloxone-pretreated arthritic rats, the paradoxical analgesic effect of low doses of naloxone was almost abolished, and the potent analgesic effects of low doses of morphine were also strongly and dose-dependently reduced. In morphine-pretreated arthritic animals, the analgesic effect of low doses of naloxone was significantly attenuated. These results attest that a cross-tolerance with low analgesic doses of morphine and naloxone can be demonstrated in these chronic suffering animals. By contrast, in rats pretreated either with naloxone or morphine, the hyperalgesic effect of naloxone produced by higher doses persisted and even was unmasked for doses which were analgesic before the pretreatment. These data emphasize the involvement of opiate receptors different in their sensitivity and/or their functions in the two opposite effects of naloxone. They also suggest that opiate receptors and endorphinergic systems differ in normal animals and animals which experience persistent pain.

Clonidine and naltrexone in the outpatient treatment of heroin withdrawal

Clonidine hydrochloride (an alpha-2 adrenergic agonist) and naltrexone hydrochloride (an opioid antagonist), given in combination, provided a safe and effective treatment of abrupt opioid withdrawal over 5 days in an outpatient/day setting. Before starting the clonidine, a naloxone challenge test was used to verify and quantify opioid dependence, and the naloxone challenge test score was then used to determine initial medication doses. Initial naloxone challenge test scores predicted subsequent patient discomfort during the 5-day clonidine-naltrexone protocol. Twelve of 14 (86%) heroin users successfully withdrew from opioids and simultaneously initiated naltrexone maintenance.

Analgesia produced by low doses of the opiate antagonist naloxone in arthritic rats is reduced in morphine-tolerant animals

In a model of experimental chronic pain (adjuvant-induced arthritic rats), low doses of the opiate antagonist naloxone produced a profound analgesia. Maximum analgesia was seen with 3 micrograms/kg (i.v.). In contrast, hyperalgesia was obtained with much higher doses (1-3 mg/kg, i.v.). The hyperalgesic effects were not affected in arthritic animals rendered tolerant to morphine, but the paradoxical analgesic effects were significantly reduced. This decrease suggests that naloxone analgesia involves interaction with opiate receptors and that the operation of endorphinergic systems differs in normal animals and animals which experience persistent pain.

Increase in the threshold of a nociceptive test induced by naloxone in morphine-tolerant rats

The effects of various doses of naloxone (3-1000 micrograms/kg i.v.) on the vocalization threshold elicited by pressure on the paw were evaluated in rats chronically treated with high doses of morphine. In addition to the well known precipitation of withdrawal induced by naloxone, an unexpected dose-related increase in the vocalization threshold was observed.

Development of new agents and techniques for intravenous analgesia

The search for new anesthetic and analgesic drugs will be aided by analysis of the structures of chemical compounds which variously stimulate the five proposed opioid receptors or the benzodiazepine receptors. The mechanism of interaction between the benzodiazepines, opioids, and the neuroleptics will be further elucidated. These studies will result in discovery of drugs with fewer side-effects and more specific clinical effects.

Inquiry into endorphinergic feedback mechanisms during the development of opiate tolerance/dependence

Met-enkephalin and beta-endorphin levels were determined in the pituitary and brain of rats after treatment for several weeks with either agonists of high receptor affinity, such as levorphanol and etorphine, or with the narcotic antagnoist naloxone. Long-term activation of opiate receptors failed to change the endorphin levels in restricted areas of brain and pituitary, although a high degree of tolerance/dependence is apparent in those animals. Chronic blockade of opiate receptors by naloxone also fails to affect endorphin levels in the pituitary, but selectively increases metenkephalin levels in the striatum. The present data do not support the notion of negative feedback mechanisms to regulate endorphingergic functions during the development of opiate tolerance/dependence.

Neurons of the periaqueductal gray matter as revealed by Golgi study

Neurons of the periaqueductal gray matter of the normal adult cat are classified into seven general types, Ia, Ib, II, IIIa, IIIb, IIIc, and IIId, based on the Golgi-Cox impregnated materials. Types Ia and Ib are spindle shaped bipolar neurons with one straight dendritic process forming varicosities and short stemmed spines. Type Ia is small in size and Ib is larger. The axon of each neuron emerges from another pole and projects beyond the PAG region. Occasionally it may share the origin with a dendrite or a tuft of dendrites. Type II, triangular shaped, had an apical dendrite that traverses a long distance within the PAG and an axon emerging from the basal portion and projecting beyond the PAG. Type IIIa, b, c, and d are pleomorphic multipolar neurons. Type IIIa has a rhomboid-shaped soma and dichotomically branching dendrites. Type IIIb has a spheroidal soma and short axons that terminate within the PAG. Type IIIc has a piriform soma and spiny dendrites that ramify perfusely and an axon which terminates within the PAG. Type IIId has the largest soma of all these neurons and the structure resembles an undifferentiated motor neuron of the CNS. Axons of the types IIIa and IIId are projecting in nature. Type Ia is found exclusively in the area immediately surrounding the aqueduct, the nucleus medialis. Types IIIc and IIId are found exclusively in the lateral region of the PAG which corresponds to the nucleus lateralis while the remaining cell types are found mostly in nuclei lateralis and dorsalis.

Opiate receptors: some anatomical and physiological aspects

It is possible to localize opiate receptors by light microscopic autoradiography using techniques developed for the autoradiographic localization of small diffusible molecules. The receptors are found in the neuropil of anatomical areas associated with various physiologic functions known to be altered by opiates. In some areas, opiate receptors are associated with the axons of small diameter fibers. Many kinds of studies, especially electrophysiologic studies, provide evidence that receptors revealed in these studies have a physiological function. Electrophysiological and immunohistochemical studies of the enkephalins provide results which are compatible with the notion that the enkephalins are the endogenous substrates of the opiate receptors.

Lipotropin, melanotropin and endorphin: in vivo catabolism and entry into cerebrospinal fluid

Anesthetized rabbits were given intravenous injections of either beta-lipotropin (beta-LPH), beta-melanotropin (beta-MSH) or beta-endorphin. The postinjection concentrations of these peptides in plasma and cerebrospinal fluid (CSF) were measured by radioimmunoassay (RIA). The plasma disappearance half-times were 13.7 min for beta-LPH, 5.1 min for beta-MSH, and 4.8 min for beta-endorphin. Circulating beta-LPH is cleaved to peptides tentatively identified as gamma-LPH and beta-endorphin. Each of these peptides appeared in the CSF within 2 min postinjection. The maximum CSF to plasma ratios were 0.08 for beta-LPH, 1.48 for beta-MSH,and 0.23 for beta-endorphin.