Possible involvement of the locus coeruleus in inhibition by prostanoid EP(3) receptor-selective agonists of morphine withdrawal syndrome in rats

We examined the mechanism of the inhibitory effect of prostanoid EP₃ receptor agonists on naloxone-precipitated withdrawal syndrome in morphine-dependent rats. Rats were rendered morphine dependent by subcutaneous (s.c.) implantation of two pellets containing 75 mg morphine for 5 days. Morphine withdrawal syndrome was precipitated by i.p. injection of naloxone (3 mg/kg). Intracerebroventricular (i.c.v.) administration of (±)-15-hydroxy-9-oxo-16-phenoxy-17,18,19,20-tetranorprost-13-trans-enoic acid (M&B28,767: prostanoid EP₃ receptor agonist) or sulprostone (prostanoid EP₁/EP₃ receptor agonist) significantly suppressed many withdrawal signs. Northern blotting and in situ hybridization studies revealed that i.c.v. administration of M&B28,767 (1 pg/rat) attenuated the elevation of c-fos mRNA during naloxone-precipitated withdrawal in many brain regions, including the cerebral cortex, thalamus, hypothalamus and locus coeruleus. Double in situ hybridization analysis revealed that in the locus coeruleus most of the tyrosine hydroxylase mRNA-positive neurons expressed μ-opioid receptor mRNA and more than half of these neurons were positive for prostanoid EP₃ receptor mRNA. These results indicate that the suppression by prostanoid EP₃ receptor agonists of naloxone-precipitated morphine withdrawal syndrome can be attributed to the inhibition of neuronal activity in several brain regions, including the locus coeruleus, the largest source of central noradrenergic neurons.     

Opposed effects of locus coeruleus and substantia nigra lesions on social behavior in rat colonies

Rats were observed in enriched colony environments following radio-frequency lesions of the locus coeruleus (LC), zona compacta of substantia nigra (SN, or control operations. LC-lesioned animals were initially inactive, stayed in the burrows, and fell when climbing ropes and ramps. SN-lesioned rats were opposite to LC animals in many ways. They were hyperactive, had minimal motor disturbances, and were hyperaggressive. SN animals self-isolated and were not social-groomers, whereas LC rats socially-groomed and mounted other animals more than controls. These results provide evidence that the dopaminergic nigrostriatal pathway and the noradrenergic fibers innervating the cerebral cortex and limbic forebrain exert opposed effects upon behavior.     

Response of rat globus pallidus neurons to microiontophoretically applied μ and κ opioid receptor agonists

The effects of the microiontophoretic application of dynorphin A-(1–13) (DYN 13) and the benzomorphans ethylketocyclazocine (EKC), bremazocine and MRZ 2549, (κ) opioid agonists, and of morphine and morphiceptin, (μ) opioid agonists, were compared on spontaneous or glutamate-evoked discharge of globus pallidus (GP) neurons in rat. Our results demonstrate that μ and κ opioid agonists are able to depress the excitability of pallidal neurons, possibly by interacting with μ and κ opioid receptor subtypes, respectively. In addition, the μ agonists and dynorphin A-(1–13), but not the benzomorphans, enhanced the excitability of a number of pallidal neurons. We have proposed a presynaptic site as the basis for this opioid-induced excitation, possibly also mediated by a μ opioid receptor. The selectivity of dynorphin A-(1–13) for benzomorphan κ opioid receptors in the rat GP appears to be low and dynorphin A-(1–13) may elicit effects that are different from those produced by the benzomorphan κ agonists by virtue of its ability to interact with other opioid receptor subtypes, for example μ opioid receptors.     

Inhibition by ethanol of excitatory amino acid receptors and nicotinic acetylcholine receptors at rat locus coeruleus neurons

The frequency of spontaneous action potentials of locus coeruleus (LC) neurons was recorded extracellularly in pontine slices of the rat brain. Ethanol (1–100 mM) elevated the firing rate in most neurons; this effect was concentration-dependent. (S)--amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA; 0.03–1 M), kainate ( 0.1–3 M), N-methyl-D-aspartate (NMDA; 1–30 M), substance P 0.01–1 M, nicotine 0.1–10 M and ,-methylene ATP ,-meATP; 0.3–30 M, all increased the firing. Application of ethanol g10–100 mM to the superfusion medium for 10 min, reproducibly and concentration-dependently inhibited the facilitatory effect of NMDA g10 M. However, the inhibitory effect of ethanol (100 mM) decreased during a 30-min superfusion period and after the washout of ethanol the sensitivity of LC neurons to NMDA (10 M) tended to overshoot above their initial level. Although NMDA was more potent in the absence than in the presence of external Mg²⁺, ethanol (100 mM) continued to depress the facilitatory effect of a low concentration of NMDA (EM) in a Mg²⁺-free medium. By contrast, in a medium containing normal Mg²⁺, ethanol (100 mM) failed to significantly interfere with the increase in firing rate induced by a high concentration of NMDA (30 M). The effects of kainate (0.5 M), AMPA (0.3 M) and nicotine (1 M) were also depressed by ethanol (100 M), while the effects of substance P (0.03 M) and ,-meATP (30 M) were not changed. In conclusion, ethanol selectively counteracts the opening of cationic channels caused by excitatory amino acid (EAA) receptor agonists and nicotinic acetylcholine receptor agonists. During a longer lasting incubation with ethanol, the inhibition of the NMDA-induced excitatory effect declines, indicating the development of tolerance. Correspondence to: P. Illes at the above address     

Possible involvement of the locus coeruleus in inhibition by prostanoid EP3 receptor-selective agonists of morphine withdrawal syndrome in rats

We examined the mechanism of the inhibitory effect of prostanoid EP₃ receptor agonists on naloxone-precipitated withdrawal syndrome in morphine-dependent rats. Rats were rendered morphine dependent by subcutaneous (s.c.) implantation of two pellets containing 75 mg morphine for 5 days. Morphine withdrawal syndrome was precipitated by i.p. injection of naloxone (3 mg/kg). Intracerebroventricular (i.c.v.) administration of (±)-15α-hydroxy-9-oxo-16-phenoxy-17,18,19,20-tetranorprost-13-trans-enoic acid (M&B28,767: prostanoid EP₃ receptor agonist) or sulprostone (prostanoid EP₁/EP₃ receptor agonist) significantly suppressed many withdrawal signs. Northern blotting and in situ hybridization studies revealed that i.c.v. administration of M&B28,767 (1 pg/rat) attenuated the elevation of c-fos mRNA during naloxone-precipitated withdrawal in many brain regions, including the cerebral cortex, thalamus, hypothalamus and locus coeruleus. Double in situ hybridization analysis revealed that in the locus coeruleus most of the tyrosine hydroxylase mRNA-positive neurons expressed μ-opioid receptor mRNA and more than half of these neurons were positive for prostanoid EP₃ receptor mRNA. These results indicate that the suppression by prostanoid EP₃ receptor agonists of naloxone-precipitated morphine withdrawal syndrome can be attributed to the inhibition of neuronal activity in several brain regions, including the locus coeruleus, the largest source of central noradrenergic neurons.     

The release of dopamine from nerve terminals and dendrites of nigrostriatal neurons induced by excitatory amino acids in the conscious rat

Summary The possible localization of excitatory amino acid (EAA) receptors on dopaminergic neurons was studied by microdialysis in conscious male rats. Varying concentrations of 3 specific EAA agonists, N-methyl-D-aspartate (NMDA), kainate and amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA), were infused into the striatum or into the substantia nigra, and the extracellular dopamine (DA) was recorded by the same probe. All 3 compounds induced a dose-dependent increase in both striatal and nigral extracellular DA. Kainate and AMPA were more potent than NMDA. Nigral DA release was stimulated by lower concentrations of kainate and AMPA than striatal DA release.The effects of two concentrations of NMDA and kainate on the release of DA were analyzed in terms of tetrodotoxin (TTX) dependency and sensitivity to ibotenic acid-induced striatal lesion. It appeared that NMDA and kainate stimulated DA release by 3 different mechanisms. The first mechanism is seen at low concentrations of kainate, it fulfills the criteria for a functional receptor-interaction: it is TTX-sensitive and independent of the ibotenic acid lesion. The second mechanism was observed when relatively low concentrations of NMDA stimulate the release of DA; in this effect postsynaptic structures are involved. The third mechanism lacks specificity as it is seen after high concentrations of kainate as well as of NMDA. The latter mechanism is TTX-independent and is probably of a toxic nature. Finally NMDA and kainate were infused into the nigra, whereas DA was recorded with a second probe implanted into the striatum. Kainate and NMDA induced an increase of striatal DA, but kainate was about 100 times more potent in this model than NMDA.The present data therefore support localization of kainate and (probably) AMPA-receptors on nigrostriatal dopaminergic neurons. The receptors on the somatoden-dritic sites were observed to be more sensitive than those on the nerve terminals. Send offprint requests to B. H. C. Westerink at the above address     

Antinociceptive evaluation of 14β-(bromoacetamido)- 7,8-dihydro-N(cyclopropylmethyl)-normorphinone in mice

This study evaluated the supraspinal opioid effects of 14β-(bromoacetamido)-7,8-dihydro-N(cyclopropylmethyl)-normorphinone (N-CPM-H₂BAMO) in the mouse acetic acid-induced writhing and tail-flick assays. In the writhing test, N-CPM-H₂BAMO produced a time- and dose-dependent antinociception after i.c.v. administration, with a 50% antinociceptive response being obtained with 0.28 (0.19–0.39) nmol when given 10 min before testing. The antinociceptive effect of N-CPM-H₂BAMO was antagonized in a dose-dependent manner by the κ-selective opioid receptor antagonist, nor-binaltorphimine. In the mouse tail-flick assay, N-CPM-H₂BAMO failed to produce any antinociception after i.c.v. administration. N-CPM-H₂BAMO produced a dose-dependent antagonism of morphine-induced antinociception but not antinociception induced by the δ-opioid receptor agonist [D-Pen²,D-Pen⁵]enkephalin. Nor-binaltorphimine (0.3 nmol) at dose that completely antagonized N-CPM-H₂BAMO-induced antinociception in the writhing assay did not prevent the antagonistic effect of N-CPM-H₂BAMO on morphine-induced antinociception. Therefore, these data indicate that N-CPM-H₂BAMO produces antinociception by acting at supraspinal κ-opioid receptors in the writhing assay, and also acts as a μ-opioid receptor antagonist.     

Selective effects of [D-Ser(O-t-butyl),Leu]enkephalyl-Thr and [D-Ser(O-t-butyl),Leu]enkephalyl-Thr(O-t-butyl), two new enkephalin analogues, on neurotransmitter release and adenylate cyclase in rat brain slices

The selectivity and potency of two new enkephalin-derived δ-opioid receptor agonists, DSTBULET ([D-Ser²(O-t-butyl), Leu⁵]enkephalyl-Thr⁶) and BUBU ([D-Ser²(O-t-butyl),Leu⁵]enkephalyl-Thr⁶(O-t-butyl)) were determined with functional tests in vitro of μ-, δ-, and κ-opioid receptor activation in the rat brain. Both peptides concentration dependently (1 nM-1 μM) inhibited the release of radiolabeled acetylcholine (ACh) from striatal slices (pD₂ 7.6–7.9), an effect exclusively mediated by δ-opioid receptor activation. Fentanyl isothiocyanate (FIT), an irreversible δ-antagonist, completely blocked the inhibitory effects of DSTBULET and BUBU. Up to a concentration of 1 μM, the peptides did not affect striatal [³H]dopamine (DA) release nor cortical [³H]noradrenaline (NA) release, processes which are known to be inhibited by opioids activating κ and μ-receptors, respectively. Furthermore, both DSTBULET and BUBU caused a strong inhibition (pD₂ 8.2–8.3) of D-1 dopamine receptor-stimulated cyclic AMP efflux from striatal slices, an effect known to be mediated by μ- and/or δ-opioid receptor activation. However, the peptides were without effect when D-1 and D-2 dopamine receptors were stimulated simultaneously, a situation in which only μ-agonists are able to inhibit the resulting cAMP efflux. In conclusion, DSTBULET and BUBU appear to display a high selectivity and potency toward functional δ-opioid receptors in the brain.