Peptides as neurotransmitters: focus on the enkephalins and endorphins

A protein kinase inhibitor, H-7, blocks naloxone-precipitated changes in dopamine and its metabolites in the brains of opioid-dependent rats

The influence of an inhibitor of cAMP-dependent protein kinase and protein kinase C, H-7 [1-(5-isoquinolinesulfonyl)-2-methylpiperazine], on naloxone (an opioid receptor antagonist)-precipitated withdrawal signs and changes in levels of dopamine (DA) and its metabolites in morphine- or butorphanol-dependent rats was investigated. Animals were infused continuously with morphine (a mu-opioid receptor agonist) or butorphanol (a mu/delta/kappa mixed opioid receptor agonist) for 3 days. Naloxone precipitated withdrawal syndrome and decreased the levels of DA in the cortex, striatum, and midbrain; 3, 4-dihydroxyphenylacetic acid (DOPAC) in the cortex, striatum, limbic areas, and midbrain; and homovanilic acid (HVA) in the striatum, limbic areas, and midbrain regions. In animals rendered dependent on butorphanol, the results obtained were similar to those of morphine-dependent rats except for the changes in DOPAC levels. Concomitant infusion of H-7 and opioid blocked both the expression of withdrawal signs and the decreases in DA, DOPAC, and HVA levels in a dose-dependent manner. These results suggest that the enhancement of cAMP-dependent protein kinase and/or protein kinase C activity accompanying the increase of DA neuron activity during continuous infusion of opioids leads to an abrupt reduction in levels of DA and its metabolites precipitated by naloxone, which is intimately involved in the expression of physical dependence on opioids.

Effects of 5-HT2 receptors blockade on fear-induced analgesia elicited by electrical stimulation of the deep layers of the superior colliculus and dorsal periaqueductal gray

The deep layers of the superior colliculus (DLSC) and the dorsal periaqueductal gray matter (DPAG) have been implicated in the control of defensive-like behaviors. Electrical and chemical stimulation of these structures elicits fear and escape behaviour, expressed by immobility (freezing) and wild running, followed by jumps and rapid rotations. There is evidence that the neural substrates responsible for defensive behavior in this level of the midbrain tectum (MT) may also be responsible for fear-induced analgesia. This study was aimed at examining the characteristics of the analgesia that follows the defense-oriented reactions induced by electrical midbrain tectum stimulation at freezing and escape thresholds. The animals were submitted to the tail-flick test, following the induction of the defense behavioral responses. The obtained results show that the antinociception that follows the freezing and escape behaviors were not antagonized by MT microinjections of the opioid antagonist naltrexone. These results emphasize previous data showing the non-opioid nature of this analgesia. On the other hand, the fear-induced analgesia was inhibited by microinjections of the serotonergic blockers, methysergide and ketanserin in the MT. Since methysergide is a non-specific antagonist of 5-HT receptors and ketanserin acts with a high degree of specificity at 5-HT2 receptors the present results suggest that activation of 5-HT2 receptors may be implicated in the antinociception induced by midbrain tectum stimulation.

Mechanisms of chronic pain

Chronic pain differs from acute pain in that it serves no useful function, causes suffering, limits activities of daily living, and increases costs of healthcare payments, disability, and litigation fees. Pain perception begins with activation of peripheral nociceptors and conduction through myelinated A delta and unmyelinated C fibers to the dorsal root ganglion. From here, signals travel via the spinothalamic tract to the thalamus and the somatosensory cortex. Modulation of sensory input (i.e., pain) occurs at many levels. Nociceptors are also neuroeffectors, and transmission can be modulated by their cell bodies, which secrete inflammatory mediators, neuropeptides, or other pain-producing substances. Descending pathways from the hypothalamus, which has opioid-sensitive receptors and is stimulated by arousal and emotional stress, can transmit signals to the dorsal horn that modulate ascending nociceptive transmissions. Modulation to alter the perception of pain also can occur at higher centers (e.g., frontal cortex, midbrain, medulla) by opioids, anti-inflammatory agents, as well as antagonists and agonists of neurotransmitters. This article will review our current knowledge of the mechanisms involved in (1) the transduction of tissue injury or disease signals (nociception and nociceptive receptors); (2) the transmission of signals rostrally to the thalamus and higher nervous system centers (involving perception of the quality, location, and intensity of noxious signals); and (3) the modulation of ascending sensory messages at all levels (periphery, spinal cord, and higher centers).

Naloxone-precipitated changes in biogenic amines and their metabolites in various brain regions of butorphanol-dependent rats

Influence of a naloxone (an opioid receptor antagonist) challenge (5 mg/kg, IP) on levels of biogenic amines and their metabolites in various brain regions of rats infused continuously with butorphanol (a mu/delta/kappa mixed opioid receptor agonist; 26 nmol/microliter/h) or morphine (a mu-opioid receptor agonist; 26 nmol/microliter/h) was investigated using high-performance liquid chromatography with electrochemical detection (HPLC-ED). Naloxone precipitated a withdrawal syndrome and decreased the levels of: dopamine (DA) in the cortex and striatum, 3,4-dihydroxyphenylacetic acid (DOPAC) in the striatum, homovanilic acid (HVA) in the striatum, limbic, midbrain, and pons/medulla regions in butorphanol-dependent rats. However, the levels of norepinephrine (NE), serotonin (5-hydroxytryptamine; 5-HT), and 5-hydroxyindoleacetic acid (5-HIAA) in the regions studied were not affected by naloxone-precipitated withdrawal. In addition, naloxone increased the HVA/DA ratio in the cortex, while this ratio was reduced in the limbic, midbrain, and pons/medulla. The reduction of 5-HIAA/5-HT ratio was also detected in the limbic area. In the animals rendered dependent on morphine, the results obtained were similar to those of butorphanol-dependent rats except for changes of 5-HIAA levels in some brain regions. These results suggest that an alteration of dopaminergic neuron activity following a reduction of DA and its metabolites in specific brain regions (e.g., striatum, limbic, midbrain, and pons/medulla) play an important role in the expression of the opioid withdrawal syndrome.

Effects of diltiazem, a Ca2+ channel blocker, on naloxone-precipitated changes in dopamine and its metabolites in the brains of opioid-dependent rats

The effects of diltiazem, an L-type Ca2+ channel blocker, on naloxone (an opioid receptor antagonist)-precipitated withdrawal signs and changes in extracellular levels of dopamine (DA) and its metabolites in various brain regions of morphine (a mu-opioid receptor agonist) or butorphanol (a mu/delta/kappa mixed opioid receptor agonist) dependent rats were investigated using high performance liquid chromatography fitted with an electrochemical detector (HPLC-ED). Rats were rendered opioid-dependent by continuous intracerebroventricular (i.c.v.) infusion with morphine (26 nmol/microliters per h) or butorphanol (26 nmol/microliters per h) for 3 days. The expression of physical dependence produced by these opioids, as evaluated by naloxone (5 mg/kg. i.p.)-precipitated withdrawal signs, was reduced by concomitant infusion of diltiazem (10 and 100 nmol/microliters per h). Under the same condition, naloxone decreased the levels of: DA in the cortex, striatum, and midbrain; 3,4-dihydroxyphenylacetic acid (DOPAC) in the cortex, striatum, limbic areas, and midbrain: and homovanilic acid (HVA) in the striatum, limbic areas, and midbrain regions. In animals rendered dependent on butorphanol, the results obtained were similar to those of morphine-dependent rats except for the changes in DOPAC levels. Furthermore, concomitant infusion of diltiazem and opioids blocked the decreases in levels of DA, DOPAC, and HVA in a dose-dependent manner. These results suggest that the augmentation of intracellular Ca2+ mediated through L-type Ca2+ channels during continuous opioid infusion results in a decrease in extracellular levels of DA and its metabolites in some specific regions, which are intimately involved in the expression of withdrawal syndrome precipitated by naloxone.

Simultaneous measurement of biogenic amines and their metabolites in rat brain regions after acute administration of and abrupt withdrawal from butorphanol or morphine

Changes in levels of biogenic amines and metabolites were measured using high performance liquid chromatography fitted with an electrochemical detection in various rat brain regions after acute administration of and abrupt withdrawal from continuous intracerebroventricular infusion of butorphanol (a mu/delta/kappa mixed opioid receptor agonist) or morphine (a mu-opioid receptor agonist). A single dose of butorphanol (26 nmol/5 microliters) or morphine (26 nmol/5 microliters) increased levels of 3,4-dihydroxyphenylacetic acid in the striatum and limbic region and of homovanilic acid in the cortex, striatum, and limbic region. In animals which had been infused with butorphanol (26 nmol/microliters/hr) or morphine (26 nmol/microliters/hr) for 3 days, an increase in dopamine turnover was observed. The levels of 3,4-dihydroxyphenylacetic acid was decreased and that of homovanilic acid was increased in the striatum, limbic region, and midbrain immediately after termination of opioid infusion. Both dopamine metabolites (in these areas) were decreased at 2 and 6 hr after butorphanol or morphine withdrawal. Changes in norepinephrine, serotonin, and 5-hydroxyindoleacetic acid levels in some brain regions were observed in the morphine-, but not in butorphanol-dependent rats. These data suggest that the increase and the decrease in dopaminergic activity, but not noradrenergic and serotonergic neurons, in the some brain regions are closely associated with the production of antinociception of and the expression of withdrawal syndrome from butorphanol and morphine, respectively.

Simultaneous changes in hypothalamic catecholamine levels and plasma corticosterone concentration in the rat after acute morphine and during tolerance

The effects of morphine on plasma corticosterone and hypothalamic noradrenaline (NA) and dopamine (DA) content were studied in naive and in morphine-tolerant rats. Acutely administered morphine (30 mg/kg i.p.) significantly increased the plasma levels of corticosterone and significantly reduced the hypothalamic NA and DA content. In chronically morphine-treated rats (subcutaneously implanted with pellets for 7 days), a challenge dose of morphine (30 mg/kg intraperitoneally (i.p.)) did not modify the plasma corticosterone levels and inhibited the morphine-induced decreases in hypothalamic NA and DA content. These results suggest that: (1) In naive rats, the morphine-induced activation of hypothalamus-pituitary-adrenocortical (HPA) axis is mediated by catecholaminergic neurons in the hypothalamus; (2) In tolerant rats morphine did not modify the plasma corticosterone concentrations, presumably by attenuating hypothalamic noradrenergic and dopaminergic activity. (3) Hypothalamic catecholamines have a role in regulating the HPA axis during morphine tolerance.

Enkephalin inhibits presynaptically the contractility of urinary tract smooth muscle

The influence of methionine-enkephalin and leucine-enkephalin on human detrusor and on pig detrusor, trigone, bladder neck and urethral smooth muscle was investigated in vitro. The enkephalins inhibited the smooth muscle contractility presynaptically. Methionine-enkephalin was about 40% more potent than leucine-enkephalin. Phentolamine-resistant contractions were less inhibited by the enkephalins than atropine-resistant contractions and contractions that were not blocked. The inhibitory nerve responses of the trigone, bladder neck and urethra were unaffected.

The ontogeny of seizures induced by leucine-enkephalin and beta-endorphin

Rats ranging in postnatal age from 6 hours to 28 days were implanted with cortical and depth electrodes as well as an indwelling cannula in the lateral ventricle. We then administered varying amounts of the opiate peptides leucine-enkephalin and beta-endorphin intracerebroventricularly with continuous electroencephalographic monitoring. Leucine-enkephalin produced electrical seizure activity in rats as young as 2 days. beta-Endorphin administration was associated with seizures at the fifth postnatal day, with a high incidence of apnea resulting in death in animals as young as 6 hours. An adult seizure response to beta-endorphin and leucine-enkephalin was seen at 15 and 28 days of age, respectively. Naloxone blocked the seizure produced by these opiate peptides in all age groups. The data indicate that the opiate peptides are potent epileptogenic compounds in developing brain, that seizures induced by leucine-enkephalin differ from those caused by beta-endorphin, and that petit mal-like seizure activity can be an adult response in the rodent.

Is intracellular acetylcholinesterase involved in the processing of peptide neurotransmitters?

The evidence that acetylcholinesterase may be a peptidase with a role in the processing of the precursors of some neuropeptides is briefly summarized. The proposition that the rate of processing and not the synthesis of the precursor may be the limiting step in the provision of neuropeptides to the nerve terminal will also be evaluated.

Evidence against involvement of beta-endorphin in thermoregulation in the cat

In the cat naloxone has little, if any, effect on temperature under usual laboratory conditions and does not reduce febrile responses to leukocytic pyrogen. Hence, endogenous opioid peptides that are antagonized by naloxone are not essential for induction of fever or for maintenance of normal temperature in the absence of appreciable thermal stress. The purpose of this study was to assess the contribution of such endogenous opioids to thermoregulation in cats exposed to more severe thermal and non-thermal stresses. Changes in temperature of unanesthetized cats were determined after third cerebral ventricular injections of large doses (100, 250 micrograms) of naloxone or saline vehicle. Naloxone had no appreciable effect on the temperature of cats acutely exposed to hot (34 degrees C) or cold (4 degrees C) environments, either before or after tolerance to morphine had been induced by progressively greater daily or twice-daily intraventricular doses of 10-70 micrograms morphine sulfate. Naloxone also did not significantly affect the temperature of cats subjected to neck-restraint or forced to stand on a small platform if they were to avoid contact with ice water. These results provide no indication that an endogenous opioid peptide, such as beta-endorphin, that is antagonized by naloxone contributes appreciably to thermoregulation in cats. They do not rule out the possibility that endogenous opioids, such as Met-enkephalin, that are not readily antagonized by naloxone are important for normal thermoregulation.

Lack of morphine-induced hyperactivity in C57BL/6 mice following striatal kainic acid lesions

Bilateral injection of kainic acid (0.15 micrograms/0.3 microliters) into the striatum (caudatus/putamen) of C57BL/6 mice prevented stimulation of locomotor activity by morphine (20 mg/kg, i.p.). This effect was specific to morphine since mice with the same lesion did not show any impairment of amphetamine (2 mg/kg)-induced locomotor hyperactivity. Histological inspections showed neuron damage also in the nucleus accumbens, while hippocampus was not damaged by kainic acid. Moreover, mice with kainic acid lesions in the hippocampus were more stimulated by morphine, compared with the morphine-injected sham lesion group. The results, which suggest the existence of non-catecholaminergic mediations in the locomotor effects of morphine, are discussed in terms of opioid systems in the brain.