[D-Pen2-D-Pen5]enkephalin, a delta opioid agonist, given intracerebroventricularly in the mouse produces antinociception through medication of spinal GABA receptors

Association analysis of GABA receptor subunit genes on 5q33 with heroin dependence in a Chinese male population

GABAA receptor subunit genes clustered on 5q33 play a role in the development of alcoholism and methamphetamine use disorder without psychosis. The present study explored the possible contribution of the same subunit genes to the development of heroin dependence. Single nucleotide polymorphisms (SNPs) of the GABAA receptor subunits GABRB2, GABRA6, GABRA1, and GABRG2 were examined in 178 male Han Chinese heroin-dependent and 170 male control subjects. A significant difference in allele frequency for the SNP rs211014 in the GABAAgamma2 receptor subunit gene between cases and controls was identified (P = 0.015). A possible mechanism for the involvement of the GABA receptor subunit genes on 5q33 in the development of heroin dependence is discussed.

Neuroanatomical approaches of the tectum-reticular pathways and immunohistochemical evidence for serotonin-positive perikarya on neuronal substrates of the superior colliculus and periaqueductal gray matter involved in the elaboration of the defensive behavior and fear-induced analgesia

Deep layers of the superior colliculus, the dorsal periaqueductal gray matter and the inferior colliculus are midbrain structures involved in the generation of defensive behavior and fear-induced anti-nociception. Local injections of the GABA(A) antagonist bicuculline into these structures have been used to produce this defense reaction. Serotonin is thought to be the main neurotransmitter to modulate such defense reaction in mammals. This study is the first attempt to employ immunohistochemical techniques to locate serotonergic cells in the same midbrain sites from where defense reaction is evoked by chemical stimulation with bicuculline. The blockade of GABA(A) receptors in the neural substrates of the dorsal mesencephalon was followed by vigorous defensive reactions and increased nociceptive thresholds. Light microscopy immunocytochemistry with streptavidin method was used for the localization of the putative cells of defensive behavior with antibodies to serotonin in the rat's midbrain. Neurons positive to serotonin were found in the midbrain sites where defensive reactions were evoked by microinjection of bicuculline. Serotonin was localized to somata and projections of the neural networks of the mesencephalic tectum. Immunohistochemical studies showed that the sites in which neuronal perikarya positive to serotonin were identified in intermediate and deep layers of the superior colliculus, and in the dorsal and ventral columns of the periaqueductal gray matter are the same which were activated during the generation of defense behaviors, such as alertness, freezing, and escape reactions, induced by bicuculline. These findings support the contention that serotonin and GABAergic neurons may act in concert in the modulation of defense reaction in the midbrain tectum. Our neuroanatomical findings indicate a direct neural pathway connecting the dorsal midbrain and monoaminergic nuclei of the descending pain inhibitory system, with profuse synaptic terminals mainly in the pontine reticular formation, gigantocellularis nucleus, and nucleus raphe magnus. The midbrain tectum-gigantocellularis complex and midbrain tectum-nucleus raphe magnus neural pathways may provide an alternative output allowing the organization of the fear-induced anti-nociception by mesencephalic networks.

Spinal pharmacology of antinociception produced by microinjection of mu or delta opioid receptor agonists in the ventromedial medulla of the rat

This study examined the role of spinal GABAergic, serotoninergic and alpha(2) adrenergic receptors in the antinociception produced by the microinjection of equi-antinociceptive doses of selective opioid receptor agonists in the nucleus raphe magnus (NRM) or the nucleus reticularis gigantocellularis pars alpha (NGCpalpha) of the rat. Rats were pretreated with intrathecal administration of either the GABA(A) receptor antagonist bicuculline, the GABA(B) receptor antagonist CGP35348, the serotonin(1/2) receptor antagonist methysergide, the alpha(2) adrenergic receptor antagonist yohimbine or saline. Ten minutes later, either the delta(1) opioid receptor agonist [D-Pen(2,5)]enkephalin (DPDPE), delta(2) opioid receptor agonist [D-Ala(2),Glu(4)]deltorphin (DELT) or mu opioid receptor agonist [D-Ala(2),NMePhe(4),Gly-ol(5)]enkephalin (DAMGO) was microinjected into the NRM, NGCpalpha or sites in the medulla outside these two regions. The increase in tail-flick latency produced by microinjection of DPDPE into the NRM or NGCpalpha was antagonized by intrathecal pretreatment with either methysergide or yohimbine. Intrathecal pretreatment with CGP35348 antagonized the antinociception produced by microinjection of DPDPE in the NRM, whereas bicuculline antagonized the antinociception produced by microinjection of DPDPE in the NGCpalpha. The increase in tail-flick latency produced by microinjection of DELT into the NGCpalpha, but not the NRM was antagonized by intrathecal pretreatment with yohimbine or CGP35348. Intrathecal pretreatment with methysergide or bicuculline did not antagonize the antinociception produced by microinjection of DELT into either the NRM or the NGCpalpha. The increase in tail-flick latency produced by microinjection of DAMGO in the NRM was antagonized by intrathecal pretreatment with methysergide or CGP35348, but not by bicuculline or yohimbine. Taken together, these results support the hypothesis that the antinociception produced by activation of delta(1), delta(2) or mu opioid receptors in the rostral ventromedial medulla is mediated by different neural substrates.

Methadone and heroin antinociception: predominant delta-opioid-receptor responses in methadone-tolerant mice

Antinociceptive tail flick responses to heroin and 6-monoacetylmorphine mediated in the brain by mu-opioid receptor are switched by morphine pellet implantation to delta1- and delta2-opioid-receptors mediation, respectively. Present results showed that the mu-receptor response (inhibited by beta-funaltrexamine) to methadone was changed by morphine pellet implantation to delta1 (inhibited by 7-benzylidenenaltrexone)- and delta2 (inhibited by naltriben)-opioid-receptor responses. Methadone pellet implantation likewise changed mediation from mu- to delta-opioid receptors for heroin and methadone but not for morphine (beta-funaltrexamine continued to inhibit). Methadone mu action in the brain was linked through a descending system to activate spinal serotonin receptors (inhibited by methysergide), but this link was gone in the methadone-pellet-implanted group. In the latter group, the new delta1- and delta2-receptor responses were mediated by spinal GABAA (inhibited by bicuculline) and GABAB (inhibited by 2-hydroxysaclofen) receptors. These shifts in neuronal systems meant that mu receptors on a given neuron were not changed into delta receptors. Preliminary results showed that delta-agonist action for methadone was prevented from appearing by MK801, a NMDA-receptor antagonist, and did not occur in 129S6/SvEv mice which lack NMDA responsiveness. Could methadone maintenance treatment in humans uncover delta-agonist actions?

Morphine tolerance in mice changes response of heroin from mu to delta opioid receptors

Heroin produced antinociception in the tail flick test through mu receptors in the brain of ICR and CD-1 mice, a response inhibited by 3-O-methylnaltrexone. Tolerance to morphine was produced by subcutaneous morphine pellet implantation. By the third day, the heroin response was produced through delta opioid receptors. The response was inhibited by simultaneous intracerebroventricular (i.c. v.) administration of naltrindole, a delta opioid receptor antagonist. More specifically, delta1 rather than delta2 receptors were involved because 7-benzylidenenaltrexone, a delta1 receptor antagonist, inhibited but naltriben, a delta2 antagonist, did not. Also, antinociception produced by i.c.v. heroin was inhibited by intrathecal administration of bicuculline and picrotoxin consistent with the concept that delta1 receptors in the brain mediated the antinociceptive response through descending neuronal pathways to the spinal cord to activate GABAA and GABAB receptors rather than spinal alpha2-adrenergic and serotonergic receptors activated originally by the mu agonist action in naive mice. The mu response of 6-monoacetylmorphine, a metabolite of heroin, was changed by morphine pellet implantation to a delta2 response (inhibited by naltriben but not 7-benzylidenenaltrexone). The agonist action of morphine in these morphine-tolerant mice remained mu. Thus, the opioid receptor selectivity of heroin and 6-monoacetylmorphine in the brain is changed by production of tolerance to morphine. Such a change explains how morphine tolerant mice are not cross-tolerant to heroin.

Differential potentiative effects of GABA receptor agonists in the production of antinociception induced by morphine and beta-endorphin administered intrathecally in the mouse

The effect of muscimol or baclofen injected intrathecally (i.t.) on the inhibition of the tail-flick response induced by morphine and beta-endorphin administered i.t. was studied in ICR mice. The i.t. injection of muscimol (100 ng) or baclofen (10 ng) alone did not affect the basal inhibition of the tail-flick response. Morphine (0.2 microg) and beta-endorphin (0.1 microg) caused only slight inhibition of the tail-flick response. Baclofen, but not muscimol, injected i.t. enhanced the inhibition of the tail-flick response induced by i.t. administered morphine. Both muscimol and baclofen injected i.t. significantly enhanced i.t. injected beta-endorphin-induced inhibition of the tail-flick response. Our results suggest that the GABA(B), but not GABA(A), receptors located in the spinal cord appear to be involved in enhancing the inhibition of the tail-flick response induced by morphine administered spinally. In addition, both GABA(A) and GABA(B) receptors are involved in enhancing the inhibition of the tail-flick response induced by beta-endorphin administered i.t.

Sensory thresholds and the antinociceptive effects of GABA receptor agonists in mice lacking the beta3 subunit of the GABA(A) receptor

A line of mice was recently created in which the gabrb3 gene, which encodes the beta3 subunit of the GABA(A) receptor, was inactivated by gene-targeting. The existence of mice with a significantly reduced population of GABA(A) receptors in the CNS enabled an investigation of the role of GABA and GABA(A) receptors in nociception. The present study examined the sensory thresholds of these mice, as well as the antinociceptive effects of subcutaneously or intrathecally administered GABA(A) and GABA(B) receptor agonists. Homozygous null (beta3-/-) mice displayed enhanced responsiveness to low-intensity thermal stimuli in the tail-flick and hot-plate test compared to C57BL/6J and 129/SvJ progenitor strain mice, and their wild-type (beta3+/+) and heterozygous (beta3+/-) littermates. The beta3-/- mice also exhibited enhanced responsiveness to innocuous tactile stimuli compared to C57BL/6J, 129/SvJ and to their beta3+/+ littermates as assessed by von Frey filaments. The presence of thermal hyperalgesia and tactile allodynia in beta3-/- mice is consistent with a loss of inhibition mediated by presynaptic and postsynaptic GABA(A) receptors in the spinal cord. As expected, subcutaneous administration of the GABA(A) receptor agonist 4,5,6,7-tetrahydroisoxazolo-(5,4-c)pyridin-3-ol did not produce antinociception in beta3-/- mice, whereas it produced a dose-dependent increase in hot-plate latency in C57BL/6J, 129/SvJ, beta3+/+ and beta3+/- mice. However, the antinociceptive effect of the GABA(B) receptor agonist baclofen in the tail-flick and hot-plate tests was also reduced in beta3-/- mice compared to the progenitor strains, beta3+/+ or beta3+/- mice after either subcutaneous or intrathecal administration. This finding was unexpected and suggests that a reduction in GABA(A) receptors can affect the production of antinociception by other analgesic drugs as well.

Selective interaction of homophtalazine derivatives with morphine

Homophtalazines show specific binding sites in the nigrostriatal system and to find their target of action the interactions between these derivatives, nerisopam and girisopam, and chlorpromazine, chlordiazepoxide and morphine were assessed. The compounds did not influence the chlorpromazine induced decrease in motility and catalepsy, nor did they alter the antiaggressive and anticonvulsive action of chlordiazepoxide. However, nerisopam and girisopam augmented the agonist potency of morphine to induce catalepsy or analgesia; they also altered the opioid antagonist potency of naloxone. The naloxone-induced decrease in sucrose consumption in drinking water was augmented by nerisopam and girisopam. It is suggested that a possible target of action of homophtalazines is the opioid signal transduction.

Effects of GABA receptor antagonists injected spinally on antinociception induced by opioids administered supraspinally in mice

The present study was designed to investigate the modulatory effects of blockade of spinal GABAA and GABAB receptors on antinociception induced by supraspinally administered mu- and epsilon-opioid receptor agonists. The effects of intrathecal (i.t.) injections with GABAA and GABAB receptor antagonists, SR 95531 [2-(3-carboxypropyl)-3-amino-6-(4-mehylphenyl)pyridazinium bromide] and 5-aminovaleric acid, respectively, on the antinociception induced by morphine (a mu-opioid receptor agonist) and beta-endorphin (an epsilon-opioid receptor agonist) injected intracerebroventricularly (i.c.v.) were studied. Antinociception was assayed using the tail-flick test. The i.t. injection of SR 95531 (0.04-0.16 nmol) and 5-aminovaleric acid (32.5-130 nmol), administered alone did not affect the latencies of the tail-flick response, but selectively antagonized the inhibition of the tail-flick response induced by muscimol (a GABAA receptor agonist) and baclofen (a GABAB receptor agonist), respectively. The i.t. injection of SR 95531 attenuated dose-dependently the inhibition of the tail-flick response induced by i.c.v. administered morphine, without affecting the i.c.v. administered beta-endorphin-induced response. 5-Aminovaleric acid attenuated dose-dependently the inhibition of the tail-flick response induced by beta-endorphin, without affecting the response to i.c.v. administered morphine. Our results indicate that GABAA but not GABAB receptors located at the spinal cord appears to be involved in the antinociception induced by morphine administered supraspinally whereas GABAB but not GABAA receptors located at the spinal cord may be involved in the antinociception induced by supraspinally administered beta-endorphin, supporting further the hypothesis that morphine and beta-endorphin administered supraspinally produce their antinociception via the activation of different descending pain inhibitory systems.

Supraspinal delta 2 opioid agonist analgesia in Swiss-Webster mice involves spinal GABAA receptors

The tail-flick response is a spinal reflex that can be modulated by administration of antinociceptive agents supraspinally through activation of descending systems and involvement of the action of neurotransmitters in the spinal cord. Descending noradrenergic and serotonergic systems are involved in morphine (and other mu opioid receptor agonists)-induced antinociception. These descending systems, however, are not involved in supraspinal delta opioid receptor agonist-induced antinociception. Recently, a descending system mediated by spinal gamma-aminobutyric acid (GABA) A and B receptors has been demonstrated to be involved in the antinociceptive action of delta 1 opioid receptor agonists ([D-Pen2,5]enkephalin in ICR mice and [D-Pen2,5]enkephalin and heroin in Swiss-Webster mice). In the present study, the involvement of spinal GABAA receptors in the antinociceptive action of supraspinal delta 2 opioid receptor agonists, [D-Ser2]-Leu-enkephalin-Thr and 6-monoacetylmorphine, action was demonstrated. The intrathecal administration of GABAA receptor antagonists, bicuculline and picrotoxin, inhibited the antinociceptive action of both [D-Ser2]-Leu-enkephalin-Thr and 6-monoacetylmorphine given intracerebroventricularly. The intrathecal administration of 2-hydroxysaclofen, a GABAB receptor antagonist, had no effect. These studies suggest that supraspinal delta 2, like delta 1, opioid receptor action involves spinal GABAA receptors, but delta 2, unlike delta 1, action does not involve GABAB receptors. Thus, the supraspinal delta 1 agonist action (heroin, DPDPE) and the delta 2 agonist action (6MAM, DSLET) can be further differentiated by the selectivity of the spinal GABA receptors involved in Swiss-Webster mice.

Spinal GABA receptors mediate brain delta opioid analgesia in Swiss Webster mice

Morphine and heroin act on supraspinal mu-opioid receptors in ICR mice to activate descending noradrenergic and serotonergic systems to inhibit the tail flick response. Antinociception induced by supraspinal [D-Pen2,5]-enkephalin (DPDPE, delta agonist) involves a descending system mediated by spinal gamma-aminobutyric acid, GABAA and GABAB, receptors. Because in Swiss Webster mice the receptor selectivity of heroin changes to delta whereas morphine remains mu, the purpose of the present study was to determine whether this delta action of heroin was mediated spinally by GABAA and GABAB receptors. Bicuculline (GABAA receptor antagonist) and picrotoxin (chloride ion channel blocker) given intrathecally produced rightward shifts in the dose-response curves of DPDPE and heroin given intracerebroventricularly. Thus, spinal GABAA receptors were involved. Intrathecal administration of 2-hydroxysaclofen (GABAB receptor antagonist) also shifted the dose-response curves to the right. Thus, the antinociception produced by heroin, like DPDPE, by activation of delta receptors in the brain of Swiss Webster mice involved both GABAA and the GABAB receptors in the spinal cord.