Drug reinforcement studied by the use of place conditioning in rat

Rats display a preference for an environment in which they previously received morphine. The present report provides behavioral and pharmacological data for this simple model of reinforcement produced by opiates and describes an aversion in rats for an environment in which they previously received naloxone. Preferences were produced with intravenous (i.v.) morphine sulfate at doses of 0.08-15 mg/kg and durations of the pairing between environment and morphine of 10 min to 1.5 h. Preferences were also seen with other opiate agonists (etorphine-HCl and levorphanol-tartrate), another route of drug administration (subcutaneous), and after 1-4 administrations of morphine. Cocaine-HCl (i.v.), a non-narcotic drug, known to be self-administered by humans, also produced a place preference. Lithium chloride (i.v.), an agent found to be a punishing stimulus in other situations, produced a place aversion. There was no appreciable preference for an environment paired with dextrorphan-tartrate and naloxone-HCl (2 mg/kg, i.p.) blocked the production of the preference produced by i.v. morphine. In contrast to the effect produced by morphine, aversions were produced with (-)-naloxone-HCl alone at doses of 0.1-45 mg/kg (i.v.). The aversion was not produced at (+)-naloxone. Implantation of rats with a 75 mg morphine pellet 3 days prior to place conditioning potentiated the aversive effect of naloxone. It was concluded that place conditioning produced by morphine and naloxone is mediated by specific opiate receptors and that stimulating and decreasing activity of the endogenous opioid peptide system with systemically administered drugs is positively reinforcing and aversive, respectively. The discussion emphasizes application of the simple and sensitive place conditioning model to drug reinforcement research, including analyses of reinforcement produced by microinjection of opiates into the brain.     

Evidence for opiate-activated NMDA processes masking opiate analgesia in rats

The acute interaction between opioid receptors and N-methyl-D-aspartate (NMDA) receptors on nociception was examined in rats using tail-flick and paw-pressure vocalisation tests. When injected at various times (1 to 6 h) after morphine (5 to 20 mg/kg, i.v.) or fentanyl (4x40 microgram/kg, i.v.), the opioid receptor antagonist naloxone (1 mg/kg, s.c.) not only abolished the opiate-induced increase in nociceptive threshold, but also reduced it below the basal value (hyperalgesia). The noncompetitive NMDA receptor antagonist MK-801 (0.15 or 0.30 mg/kg, s.c.) prevented the naloxone-precipitated hyperalgesia and enhanced the antinociceptive effects of morphine (7.5 mg/kg, i.v.) and fentanyl (4x40 microgram/kg, i.v.). These results indicate that the antinociceptive effects of morphine and fentanyl, two opiate analgesics widely used in humans in the management of pain, are blunted by concomitant NMDA-dependent opposing effects which are only revealed when the predominant antinociceptive effect is sharply blocked by naloxone. This study provides new rationale for beneficial adjunction of NMDA receptor antagonists with opiates for relieving pain by preventing pain facilitatory processes triggered by opiate treatment per se.     

Enkephalinase-inhibitors modulate immune responses.

Exogenous opioid peptides, and enkephalins in particular, modulate a variety of immune performances in vivo and in vitro. In this study, the immunomodulatory role of endogenous opioids was investigated by means of central and peripheral administration of four peptidase inhibitors in the rat. Animals sensitized with sheep red blood cells (SRBC) were daily treated intraperitoneally (i.p.) with 0.2 mg/kg or 1 mg/kg of each inhibitor, or intracerebroventricularly (i.c.v.) with 0.02 mg/kg and 0.2 mg/kg of bestatin, des-tyrosine-methionine-enkephalin, (Des-Tyr1)Met-Enk; and actinonin, and 0.005 and 0.5 mg/kg of N-Carboxymethyl-phenilalanine-leucine,(N-C)Phe-Leu. Controls were injected i.p. and i.c.v. with saline. The results revealed that in animals treated i.p. with 0.2 mg/kg of bestatin and (N-C)Phe-Leu potentiated the plaque-forming cell (PFC) response and hemagglutinin production. In contrast, these immune responses were suppressed by 1 mg/kg. On the other hand, i.p. doses of 0.2 and 1 mg/kg of actinonin and (Des-Tyr1)Met-Enk potentiated humoral immune responses. When given i.c.v., all of the inhibitors used exerted clear dose-dependent immunomodulatory effects, i.e. increase in the PFC response and hemagglutinin production when given at lower doses (0.005-0.02 mg/kg), and decrease when injected with higher doses (0.2-0.5 mg/kg). These effects of enkephalin-related peptidase inhibitors, applied i.p. and i.c.v., suggest the involvement of endogenous enkephalins in immune mechanisms.     

Change in Intracellular Cyclic Nucleotide Content Accompanies Relaxation of the Isolated Canine Internal Anal Sphincter

Changes in cyclic nucleotide content arc associated with relaxation of lower esophageal sphincter (LES) smooth muscle. Although agents that increase cyclic AMP (cAMP) or cyclic CMP (cGMP) can relax the LES, relaxation produced by activation of enteric neurons is associated with, an increase in intracellular cGMP. To evaluate these changes in another sphincteric area of the gut, ice determined the effects of electrical field stimulation (EFS) (65 V, I ms) and of several relaxant agents that alter cyclic nucleotide content in isolated strips of canine internal anal sphincter. Each strip was contracted with norepinephrine (3 μM). During maximum contraction, tissues were relaxed by EFS or by addition of relaxant agents. EFS produced, a frequency-related relaxation, accompanied by a significant, increase, in cGMP; however, cAMP increased only slightly at the maximum frequency tested (8 Hz). Both EFS-induced relaxation and increased cGMP content were blocked 1 μM tetrodotoxin. Cumulative addition of sodium nitroprusside, forskolin, 8-bromoguanosine 3',5'-cylic monophosphate, or 8-bromoadenosine 3',5'-cyclic monophosphate relaxed the internal anal sphincter in a concentration-dependent manner. Sodium nitroprusside-induced relaxations were accompanied by concentration-dependent increases in cGMP content, whereas forskolin-induced relaxations were accompanied by concentration-dependent increases in cAMP content. In conclusion, both cAMP and cCMP appear to be intracellular mediators of relaxation in the canine internal anal sphincter. In addition, relaxation produced by activation of intrinsic inhibitory neurons is associated with an elevation of intracellular cGMP. Together with our previous findings in the LES, these results suggest that cCMP may be the intracellular mediator of neuronally induced, relaxation of gut sphincteric regions.     

Blockade of long-term potentiation by phencyclidine and sigma opiates in the hippocampus in vivo and in vitro

Long-term potentiation (LTP) of synaptic transmission in the rat hippocampus in vivo and in vitro, was studied using field potentials. Pretreatment with phencyclidine (PCP) or 'sigma' opiates blocked LTP in vivo while mu and kappa opiates and the antagonist naloxone were ineffective. Scopolamine (20 mg/kg i.p.) neither prevented LTP nor antagonized the LTP-blocking effect of PCP. In vitro, PCP up to 100 microM did not alter synaptic activation of CA1 pyramidal cells by stratum radiatum stimulation but blocked LTP in a dose-dependent manner (ED50: 3 microM). The sigma opiate, cyclazocine, also prevented the induction of LTP in vitro while morphine and procaine were ineffective.     

Different mechanisms for dopaminergic excitation induced by opiates and cannabinoids in the rat midbrain

1. The mechanism underlying morphine and cannabinoid-induced excitation of meso-accumbens and nigro-striatal dopaminergic neurons was investigated by extracellular single unit recording techniques coupled with antidromic activation from the nucleus accumbens and striatum respectively, in unanesthetized rats. 2. The intravenous administration of cumulative doses (1-4 mg/kg) of morphine, dose-dependently increased the firing rate of dopaminergic neurons projecting to the nucleus accumbens and neostriatum, while the same doses inhibited the activity of pars reticulata neurons of the substantia nigra. Both effects were antagonized by naloxone (0.1 mg/kg i.v.) but not by the selective CB1 receptor antagonist SR 141716A (1 mg/kg i.v.). 3. The intravenous administration of cumulative doses (0.125-0.5 mg/kg) of delta9-tetrahydrocannabinol (delta9-THC) also increased the firing rate of meso-accumbens and nigro-striatal dopaminergic neurons; this effect was antagonized by SR 141716A (1 mg/kg i.v.), but not by naloxone. 4. Furthermore, nor delta9-THC up to a dose of 1 mg/kg, maximally effective in stimulating dopamine neurons, neither SR 141716A (1 mg/kg i.v.) at a dose able to reverse the stimulatory effect of delta9, THC on dopamine cells, did alter the activity of SNr neurons. 5. The results indicate that morphine and delta9-THC activate dopaminergic neurons through distinct receptor-mediated mechanisms; morphine may act by removing the inhibitory input from substantia nigra pars reticulata neurons (an effect mediated by mu-opioid receptors). Alternatively, the delta9-THC-induced excitation of dopaminergic neurons seems to be mediated by CB1 cannabinoid receptors, while neither mu-opioid receptors nor substantia nigra pars reticulata neurons are involved.     

Enkephalins in the inferior mesenteric ganglion of the cat and in the area of the lower digestive tract innervated by this ganglion: Quantification by radio-immunoassay and characterization by high pressure liquid chromatography

Met-enkephalin, Leu-enkephalin and Met-enkephalin-Arg-Gly-Leu were quantified and characterized in the cat inferior mesenteric ganglion and in the area of the lower digestive tract innervated by this ganglion, including the proximal colon, distal colon and internal anal sphincter. In the structures studied, the concentrations of enkephalins expressed as femtomole/mg of wet tissue ranged from 66 to 160 with Met-enkephalin, from 15 to 45 with Leu-enkephalin and from 2 to 12 for Met-enkephalin-arg-gly-leu. In the lower digestive tract, the Met- and Leu-enkephalin content decreased from the proximal colon to the internal anal sphincter. The Met-enkephalin versus Leu-enkephalin ratio of the structures investigated were as follows: inferior mesenteric ganglion 3.2, proximal colon 4.4, distal colon 5, internal and sphincter 4.5. In individual samples of all the structures assayed the results of high pressure liquid chromatography (HPLC) analysis pointed to the presence of authentic Met- and Leu-enkephalin. HPLC analysis could not be carried out on Met-enkephalin-Arg-Gly-Leu due to the very low concentrations of this peptide in all the structures assayed. Our results, combined with those of previous immunohistochemical and physiological studies, support the idea that enkephalins are involved in the nervous control of the motility of the lower digestive tract.     

INVOLVEMENT OF NMDA RECEPTORS IN MORPHINE STATE–DEPENDENT LEARNING IN MICE

In the present study, the effects of intracerebroventricular (i.c.v.) injection of NMDA receptor agonist and antagonist on impairment of memory formation and the state-dependent learning by morphine have been investigated in mice. Pretraining administration of morphine (5 mg/kg; s.c.) decreased the learning of one-trial passive avoidance task. Pretest administration of morphine (5 mg/kg) induced state-dependent learning acquired under pretraining morphine influence. Pretest administration of NMDA receptor agonist, L-glutamate (0.00001 and 0.0001 and 0.001 μg/mouse, i.c.v.) following pretraining saline treatment did not affect retention. Amnesia induced by pretraining morphine was significantly reversed by pretest administration of L-glutamate (0.0001 and 0.001 μg/mouse, i.c.v.). Pretest administration of noncompetitive NMDA receptor antagonist, MK-801 (0.5, 1, and 2 μg/mouse, i.c.v.) significantly impaired memory formation. Amnesia induced by pretraining morphine was increased by pretest administration of MK-801 (2 μg/mouse, i.c.v.). Pretest coadministration of L-glutamate (0.0001 and 0.001 μg/mouse, i.c.v.) or MK-801 (0.5, 1, and 2 μg/mouse, i.c.v.) with morphine (5 mg/kg, s.c.) increased and decreased morphine state-dependent learning, respectively. The results suggest that NMDA receptors are involved in morphine state–dependent learning in mice.     

The response of the 5-hydroxyindole oxidation current to noxious stimuli in the spinal cord of anesthetized rats: modification by morphine.

The effects of cutaneous noxious heating and of systemic morphine on serotonergic activity in the spinal cord were examined in anesthetized rats. An oxidation current of 5-hydroxyindole signal was seen at 280-300 mV with differential normal pulse voltammetry. Noxious heat stimuli produced a mean signal increase over control values of 15.5 +/- 3.4% at 52 degrees C, and 7.2 +/- 5.5% at 45 degrees C. These increases lasted for 5-10 min. Non-noxious stimuli (37 degrees C) did not affect the 5-hydroxyindole signal. Morphine (0.5, 2.0 and 5.0 mg/kg, i.p.) in the absence of cutaneous stimulation did not change the signal significantly. Systemic morphine alone did not significantly modify the 5-hydroxytryptamine (5-HT) metabolism, as observed in in vivo voltammetry, in the spinal cord of anesthetized rat. However, a low dose of morphine (0.5 mg/kg, i.p.) attenuated the increase in the signal modified by noxious stimuli, and high doses (2.0 or 5.0 mg/kg, i.p.) enhanced it. Both effects of morphine were antagonized by naloxone (0.5 mg/kg, i.v.). It is likely that morphine with noxious stimuli modify the sensitivity of serotonergic descending inhibitory system. It is concluded that noxious heating of the skin increases the 5-HT metabolism in the spinal cord of anesthetized rats and that systemic administration of morphine modulates this 5-HT metabolism.     

In vivo electrochemical evidence that the tricyclic antidepressant femoxetine potentiates the morphine-induced increase in 5-HT metabolism in the medullary dorsal horn of freely moving rats

Acute administration of tricyclic antidepressants (TCAs) is known to potentiate morphine antinociception. At the medullary dorsal horn (MDH) level systemic morphine has been shown to increase serotonin (5-HT) metabolism as measured by in vivo electrochemistry in freely moving rats. Using similar electrochemical detection of 5-hydroxyindole (peak '3') within the MDH, the present study investigated the effect of the specific 5-HT uptake inhibitor femoxetine on peak 3 and the effects of this TCA on changes in 5-HT metabolism induced by morphine. Acutely administered femoxetine (40 mg/kg i.p.) (i) induced a small but significant increase in peak 3 and (ii) strongly potentiated the effect of morphine (10 mg/kg i.p.) on 5-HT metabolism, this potentiation being opiate specific since simultaneous injection of naloxone (1 mg/kg i.p.) abolished the effect of morphine. These findings provide an in vivo neurochemical basis for the potentiation of morphine antinociception by TCAs. They further emphasize the importance of 5-HT bulbospinal descending pathways in morphine antinociception.     

Depression by morphine-6-glucuronide of nociceptive activity in rat thalamus neurons: comparison with morphine

To assess the contribution of the active metabolite of morphine, morphine-6-glucuronide (M6G), to the analgesic effect of systemically administered morphine, experiments were carried out on rats under urethane anesthesia in which nociceptive activity was evoked by electrical stimulation of afferent C fibers in the sural nerve and recorded from single neurons in the ventrobasal complex of the thalamus. Intravenous (i.v.) injections of morphine completely blocked the activity at doses of 500 and 1000 μg/kg, the ED,, being 44 μg/kg. M6G administered by i.v. injection reduced the evoked nociceptive activity only by about 40% at 80 and 160 μg/kg, the ED₅₀ being 6 μg/kg. After intrathecal (i.t.) injection, morphine produced maximum depression of 55% of the control activity at 20 μg the ED₅₀ is 18 μg. M6G injected i.t. produced maximum depression of 40% at doses ranging from 0.2 to 10 μg. The ED₅₀ of M6G i.t. is below 0.2 μg. The effects of morphine and M6G were reversed by naloxone (200 μg/kg i.v.). The results show that M6G is more potent than morphine, regardless of the route of administration, while morphine is more effective when injected i.v. Due to the low efficacy of M6G, it seems unlikely that this glucuronide contributes substantially to the analgesic effect of morphine when renal function is normal. The results also make evident that the maximum effect of morphine results from an action at spinal and supraspinal sites.     

Influence of morphine and cyclazocine on the cortical epileptic foci in rabbits

The effects of morphine, cyclazocine and naloxone on penicillin- and strychnine-induced epileptic foci were studied in rabbits. The intracortical injection of penicillin (75, 150 and 300 units) elicited isolated spikes followed by repeated ictal events. The application of strychnine (0.062 and 0.125%) over the cortical surface of one side induced appearance of ipsilateral spiking spreading to the contralateral cortex.

Administration of morphine (0.25–0.75 mg/kg i.v.) or cyclazocine (0.05–3.0 mg/kg i.v.) inhibited the occurrence or the duration of the EEG and motor manifestations induced by penicillin (75 and 150 units) and strychnine (0.062 and 0.125%), while it did not influence the effect of 300 units of penicillin. High doses of morphine (up to 10 mg/kg i.v.) failed to affect the epileptic responses to penicillin and strychnine and at the same time significantly reduced the pO₂ in arterial blood.

Naloxone per se potentiated the effects of the lower doses of penicillin and strychnine. Only at very high doses (20 mg/kg i.v.) displayed a weak antagonism towards the anticonvulsant effect of the two opiates. A full antagonism is only observed towards the effect of cyclazocine (2 mg/kg i.v.) administered after penicillin.

Present data provide additional evidence of the heterogeneity of regulations by opioids of convulsive phenomena. One can hypothesize that the anticonvulsant effect of the two opiate agonists is mediated by naloxone-insensitive opiate receptors, while the proconvulsant-convulsant effect of naloxone might be related to an inhibition of GABA and glycine-mediated transmission.     

Electrophysiological correlates of presynaptic opiate receptor activation: reduction in norepinephrine-mediated inhibition from the locus coeruleus

Inhibitory responses of rat cerebellar Purkinje cells to locus coeruleus (LC) stimulation and iontophoresis of norepinephrine (NE) were examined before and after administration of morphine to determine whether the inhibitory modulation of NE release by opiates results in a functional impairment in noradrenergic synaptic action. Administration of morphine systemically (0.2-1.2 mg/kg, i.v.) or by iontophoresis reduced inhibitions in Purkinje firing elicited by LC stimulation without affecting depressions in activity induced by postsynaptic applications of NE. This antagonistic effect of morphine on LC-induced inhibition was reversed or prevented by naloxone and mimicked by administration of levorphanol but not dextrorphan. Morphine increased the excitatory response of Purkinje cells to monosynaptic input from the parallel fibers, whereas it blocked gamma-aminobutyric acid-induced inhibitions in firing via a non-opiate receptor-mediated mechanism. These results demonstrate that morphine interferes with synaptic inhibition derived from the LC and suggest that this may occur via activation of presynaptic opiate receptors residing on noradrenergic nerve terminals.     

Influence of morphine and cylazocine on the cortical epileptic foci in rabbits

The effects of morphine, cyclazocine and naloxone on penicillin- and strychnine-induced epileptic foci were studied in rabbits. The intracortical injection of penicillin (75, 150 and 300 units) elicited isolated spikes followed by repeated ictal events. The application of strychnine (0.062 and 0.125%) over the cortical surface of one side induced appearance of ipsilateral spiking spreading to the contralateral cortex.Administration of morphine (0.25–0.75 mg/kg i.v.) or cyclazocine (0.05–3.0 mg/kg i.v.) inhibited the occurrence or the duration of the EEG and motor manifestations induced by penicillin (75 and 150 units) and strychnine (0.062 and 0.125%), while it did not influence the effect of 300 units of penicillin. High doses of morphine (up to 10 mg/kg i.v.) failed to affect the epileptic responses to penicillin and strychnine and at the same time significantly reduced the pO₂ in arterial blood.Naloxone per se potentiated the effects of the lower doses of penicillin and strychnine. Only at very high doses (20 mg/kg i.v.) displayed a weak antagonism towards the anticonvulsant effect of the two opiates. A full antagonism is only observed towards the effect of cyclazocine (2 mg/kg i.v.) administered after penicillin.Present data provide additional evidence of the heterogeneity of regulations by opioids of convulsive phenomena. One can hypothesize that the anticonvulsant effect of the two opiate agonists is mediated by naloxone-insensitive opiate receptors, while the proconvulsant-convulsant effect of naloxone might be related to an inhibition of GABA and glycine-mediated transmission.     

Anxiolytic-like activity of the non-selective galanin receptor agonist, galnon

Galanin's influence on monoaminergic neurotransmission, together with its discrete CNS distribution in corticolimbic brain areas, points to a potential role for this neuropeptide in mediating anxiety- and depression-like responses. To evaluate this hypothesis, the non-selective galanin receptor agonist, galnon, was tested in multiple preclinical models of anxiolytic- and antidepressive-like activity. Acute administration of galnon (0.03-1mg/kg, i.p.) dose-dependently increased punished crossings in the four plate test, with magnitude similar to the effects of the endogenous ligand, galanin (0.1-1.0 microg, i.c.v.). Moreover, the effects of galnon and galanin were blocked by central administration of the non-selective galanin receptor antagonist, M35 (10 microg, i.c.v.). Interestingly, the benzodiazepine receptor antagonist, flumazenil (1mg/kg, i.p.), reversed galnon's effect in the four plate test, implicating GABAergic neurotransmission as a potential mechanism underlying this anxiolytic-like response. In the elevated zero maze, galnon (0.3-3.0mg/kg, i.p.) and galanin (0.03-0.3 microg, i.c.v.) increased the time spent in the open arms, while in the stress-induced hyperthermia model, galnon (0.3-30 mg/kg, i.p.) attenuated stress-induced changes in body temperature. Consistent with these anxiolytic-like effects, in vivo microdialysis showed that acute galnon (3mg/kg, i.p.) treatment preferentially elevated levels of GABA in the rat amygdala, a brain area linked to fear and anxiety behaviors. In contrast to the effects in anxiety models, neither galnon (1-5.6 mg/kg, i.p.) nor galanin (0.3-3.0 microg, i.c.v.) demonstrated antidepressant-like effects in the mouse tail suspension test. Galnon (1-10mg/kg, i.p.) also failed to reduce immobility time in the rat forced swim test. In vitro, galnon and galanin showed affinity for human galanin receptors expressed in Bowes melanoma cells (K(i)=5.5 microM and 0.2 nM, respectively). Galanin displayed high affinity and functional potency for membranes expressing rat GALR1 receptors (K(i)=0.85 nM; EC(50)=0.6 nM), while galnon (10 microM) failed to displace radiolabeled galanin or inhibit cAMP production in the same GALR1 cell line. Galnon (10 microM) showed affinity for NPY1, NK2, M5, and somatostatin receptors but no affinity for galanin receptors expressed in rat hippocampal membranes. Taken together, the present series of studies demonstrate novel effects of galnon in various preclinical models of anxiety and highlight the galaninergic system as a novel therapeutic target for the treatment of anxiety-related disorders. Moreover, these data indicate rodent GALR1 receptors do not mediate galnon's in vivo activity.     

Dependency on the brain function of arginine vasopressin system of the development to and recovery from analgesic tolerance to morphine.

Concomitant intracerebroventricular (i.c.v.) injection of anti-arginine vasopressin (AVP) antiserum dose-dependently suppressed the development of analgesic tolerance to daily morphine, 10 mg/kg, s.c., in mice. This suppressive effect of the antiserum was reduced by incubating the antiserum with AVP in vitro, before i.c.v. injection, suggesting that the antiserum inactivates brain AVP to result in the suppression of the development of tolerance in vivo. Similar to the antiserum, both AVP V1 and V2 antagonists given i.c.v., 10 ng and 20 ng/mouse, respectively, suppressed the development of morphine tolerance. Meanwhile, the administration of antiserum dose-dependently recovered morphine analgesia in morphine-tolerant mice and a complete recovery of analgesia was observed at the highest dose of antiserum following the second injection, and the effect of antiserum was maintained for 3 days after its withdrawal. Likewise, 10-100 ng/mouse of AVP V1 receptor antagonist given i.c.v. recovered morphine analgesia partially but significantly in a dose-dependent manner; however, AVP V2 receptor antagonist at the same doses partially recovered analgesic effect but the effect was neither significant nor dose-dependent. These findings suggest that the tolerance developed to morphine can be reversible when disturbing the function of brain AVP, but in addition to the different mechanisms of antiserum, V1 and V2 receptor antagonists, the V1 receptor-mediated mechanism may be more closely concerned in this phenomenon.     

In vivo voltammetric studies of the effect of morphine on the serotonergic system in the cat spinal cord

The purpose of the present paper is to investigate the effect of morphine on the serotonergic system through an analysis of electrochemical signals in the spinal cord of the anesthetized cat. Electrochemical signals showed a peak of 5-hydroxyindole at 280-300 mV, and were highest at a depth of 1500-2500 microns in the dorsal horn of the spinal cord. Electrical stimulation of the nucleus raphe magnus (NRM) produced an increase in the electrochemical signal. Five-hydroxytryptophan (25 mg/kg, i.v.) also enhanced the signal remarkably. Both systemic administration of morphine (1-2 mg/kg, i.v.) and microinjection (5 and 10 micrograms/microliter) of morphine into the NRM increased the signal. In conclusion, it is suggested that morphine-induced enhancement of the peak of 5-hydroxyindole contributes to the activity of the serotonergic system in the cat spinal cord.     

Suppression of acute and chronic opioid withdrawal by a selective soluble guanylyl cyclase inhibitor

Previous studies have shown that activation of N-methyl-D-aspartate (NMDA) receptors and formation of nitric oxide (NO) contributes to the hyperactivity of locus coeruleus (LC) noradrenergic neurons and behavioural symptoms seen during opioid withdrawal. However, the role of soluble guanylyl cyclase (sGC), the 'physiological' target of NO, in this phenomenon is unclear. In this study, the effect of 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), a highly selective sGC inhibitor, on the naloxone-precipitated morphine withdrawal was examined using differential normal pulse voltammetry (DNPV) to measure LC activity, in vivo microdialysis to measure glutamate/aspartate release response, and behavioural assessment to evaluate withdrawal symptoms. In halothane-anaesthetized rats, acute intracerebroventricular (i.c.v.) morphine (10 microg) reduced the catecholamine oxidation current (CA.OC) (54.5+/-4.9% of baseline). Naloxone (2 mg/kg, i.v.) reversed this action of morphine and produced a rebound increase in CA.OC (136.1+/-6.0% of baseline), representing acute morphine withdrawal. Administration of ODQ (200 nmol, i.c.v.) blocked this response without affecting acute morphine action. In animals chronically treated with morphine (15 microg/microl/h, i.c.v., 5 days), naloxone significantly increased both the CA.OC signal (270.0+/-19.6% of baseline) and the release of L-glu (193+/-30.4%) and L-asp (221.5+/-28.4%) above baseline. These responses were attenuated in animals pretreated with ODQ. In unanaesthetized chronic morphine dependent rats, ODQ treatment suppressed the signs of withdrawal precipitated by naloxone (10 mg/kg). Taken together, the results of this study suggest that sGC plays an intermediary role in the genesis of LC neuronal hyperactivity and behavioural signs of morphine withdrawal.     

Effects of morphine and naloxone on hippocampal CA3 field potentials following systemic administration in the freely-moving rat

The effect of IV morphine, 2, 6 and 15 mg/kg, on hilar-evoked CA3 field potentials was studied to determine if this area would be more sensitive to mu-type opiate agonists than the CA1 or dentate regions. In addition, the effect of IV naloxone, 2 and 25 mg/kg, on the same responses was studied to determine if endogenous opiates reported to be present in the mossy fibers are released by electrical stimulation of this pathway. Neither morphine nor naloxone had an effect on CA3 field potentials at any dose used. The CA1 region of the hippocampus is the area most sensitive to morphine, and this effect of morphine correlates best, anatomically, with the localization of mu-receptors identified by the binding of dihydromorphine. Physiological release of endogenous opiates from the hippocampus remains to be shown.     

Pentobarbital-like electroencephalographic rubral rhythm induced by ketamine in rabbits

1. The effects caused by the dissociative anaesthetic drugs (PCP, KT), the sigmaopiates (SKF 10.047, cyclazocine), and the mixed excitatory amino acid antagonist CCP on the electrical activity of the red nucleus in rabbits were compared with PB. 2. Ketamine (10 mg/kg, i.v.) induced the appearance of a pentobarbital-like EEG synusoidal rhythm characterized by an increase in amplitude and a decrease in frequency of the basal electrical activity at the red nucleus level. 3. Both pentobarbital and ketamine induced rhythms were blocked by the GABA-antagonist pentylenetetrazol at the subconvulsant dose of 10 mg/kg, i.v. 4. Phencyclidine, SKF 10.047, cyclazocine, and the mixed excitatory amino acid antagonist CCP failed to affect the basal electrical activity of the red nucleus. 5. These data indicate an interaction of ketamine on the GABA neurotransmission at the level of cerebello-rubral pathways which the other PCP/sigma opiates did not present.