Non-stereospecific excitatory actions of morphine may be due to GABA-A receptor blockade

Dopamine and serotonin release in dorsal striatum and nucleus accumbens is differentially modulated by morphine in DBA/2J and C57BL/6J mice

Numerous studies have demonstrated that genetic factors significantly influence opioid ability to induce behavioral modification in mice. This differential sensitivity has been extensively studied, particularly in the DBA/2J and C57BL/6J strains. In the present study, using the "in vivo" microdialysis technique in these strains, we investigated the effect of morphine administration on the extracellular levels of dopamine (DA), serotonin (5-HT), and their metabolites in the nucleus accumbens and dorsal striatum--areas thought to be involved in morphine-induced locomotor hyperactivity. In the nucleus accumbens, morphine (20 mg/kg) significantly increased extracellular levels of DA in both strains. However, in dorsal striatum the morphine-induced increase of extracellular DA was lower in DBA/2J mice than in C57BL/6J. Moreover, morphine significantly stimulated 5-HT and 5-hydroxyindolacetic acid (5-HIAA) release both in nucleus accumbens and dorsal striatum of C57BL/6J mice, whereas it decreased 5-HT release without modifying 5-HIAA levels in DBA/2J mice. These results suggest that the different behavioral and biochemical responses to acute morphine described in these two strains could be mediated by different sensitivity of both the dopaminergic and the serotonergic systems.

Activation of brain stem nuclei by improgan, a non-opioid analgesic

Improgan is a compound developed from histamine antagonists which shows the pre-clinical profile of a highly effective, non-opioid analgesic when administered into the rodent CNS. Pharmacological studies suggest that improgan activates descending pain-relieving circuits, but the brain and spinal sites of action of this drug have not been previously studied. Presently, the effects of intracerebral and intrathecal microinjections of improgan were evaluated on thermal nociceptive responses in rats. Improgan produced large, dose- and time-related reductions in nociceptive responses following administration into the ventrolateral periaqueductal gray (PAG), the dorsal PAG, and the rostral ventromedial medulla (RVM). The drug had no measurable effects after injections into the caudate nucleus, basolateral amygdala, hippocampus, ventromedial hypothalamus, superior colliculi, ventrolateral medulla, or the spinal subarachnoid space. Inactivation of the RVM by muscimol microinjections completely attenuated antincociceptive responses produced by intraventricular improgan. These findings, taken with earlier results, show that, like opioids and cannabinoids, improgan acts in the PAG and RVM to activate descending analgesic systems. Unlike these other analgesics, improgan does not act in the spinal cord or in CNS areas outside of the brain stem.

Place aversion induced by blockade of mu or activation of kappa opioid receptors in the dorsal periaqueductal gray matter

Neural circuits in the dorsal periaqueductal grey matter (DPAG) play an important role in the integration of defensive behaviour. As considerable numbers of mu and kappa opioid receptors have been found in this region, we studied the effects of morphine, [3H]-[H-D-Phe-Cys-Tyr- D-Trp-Orn-Thr-Pen-Thr-NH2] (CTOP), a selective peptide antagonist for mu opioid receptors, U-50488H, a specific agonist for kappa opioid receptors, and nor-binaltorphimine (nor-BNI), a long-lasting selective antagonist for kappa opioid receptors, injected into the DPAG of rats submitted to the corral method, a conditioned place preference test. The behavioural testing apparatus was a circular open field consisting of four uniform quadrants that were equally preferred by the rats prior to drug treatments. For conditioning, rats received drug injections on three consecutive days and were placed into their assigned quadrant. Injection of 40 nmol of morphine into the DPAG produced place aversion effects, with reduced time spent in the drug-paired quadrant on the testing day. These place aversion effects were not inhibited by previous DPAG microinjection of CTOP (1 nmol) but were significantly reduced by prior systemic injections of nor-BNI (2 mg / kg). Microinjection of CTOP alone produced a clear decrease in the time spent in the treatment quadrant, whereas nor-BNI alone did not. Similarly, microinjection into the DPAG of the kappa agonist U-50488H (10 nmol) mimicked the effects of morphine, also producing place aversion for the drug-paired quadrant. These findings suggest that blockade of mu opioid receptors or activation of kappa opioid receptors in the DPAG may produce conditioned place aversion.

Blockade of mu- and activation of kappa-opioid receptors in the dorsal periaqueductal gray matter produce defensive behavior in rats tested in the elevated plus-maze

We studied the effects of morphine injected into the dorsal periaqueductal gray using conventional and novel ethological measures of the behavior of rats submitted to the elevated plus-maze test. Morphine (20 and 40 nmol) applied into the dorsal periaqueductal gray produced dose dependent aversive effects with reduced entries and time spent in the open arms. Freezing behavior was the most prominent novel ethological measure produced by microinjections of these doses of morphine. These pro-aversive effects were not inhibited by previous dorsal periaqueductal gray microinjection of [D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH(2)](CTOP) (1 nmol), a selective peptide antagonist for mu-opioid receptors. On one hand, microinjection of CTOP produced a dose dependent increase in scanning and stretched attended postures, by its own. On the other hand, the aversive effects of morphine into the dorsal periaqueductal gray microinjections were significantly reduced by systemic administration of nor-binaltorphimine, an opioid receptor antagonist with a tardive and selective action at kappa-opioid receptors. These findings suggest that mechanisms mediated by mu-opioid receptors in the dorsal periaqueductal gray may be involved in the control of risk assessment behavior. On the other hand, the pro-aversive effects produced by microinjections of morphine into the dorsal periaqueductal gray are probably mediated by kappa-opioid receptors.

Effects of Ca2+ channel blockers on apomorphine, bromocriptine and morphine-induced locomotor activity in mice

The effects of L-type voltage-dependent Ca2+ channel blockers on apomorphine, bromocriptine and morphine-induced changes in locomotor activity were examined in mice. Apomorphine (4 mg/kg) and morphine (20 mg/kg) produced locomotor stimulation. Bromocriptine (8 mg/kg) produced a biphasic effect on motor behaviour, an early depressant phase, followed by locomotor stimulation. Amlodipine (2.5 mg/kg), nicardipine (10 mg/kg), diltiazem (10 mg/kg) and verapamil (10 mg/kg), which by itself did not affect locomotor activity, inhibited the stimulant phase of bromocriptine without altering the depressant phase, while they did not affect apomorphine- and morphine-induced locomotor stimulation. Apomorphine, bromocriptine and morphine-induced locomotor stimulation was decreased by SCH 23390 (R-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepine-7- ol) (0.05 mg/kg) or haloperidol (0.1 mg/kg). These results indicate that L-type voltage-dependent Ca2+ channels are involved in the motor stimulant effect of bromocriptine, but not in apomorphine- and morphine-induced locomotor stimulation. The effects of Ca2+ channel blockers on the dopaminergic system appears not to be directly related to dopamine receptor blockade.

Interaction between lead acetate and morphine on antinociception in mice by formalin test

1. In this study, the effects of lead acetate on two types of pain (nociception and inflammation) induced by formalin and its interactions with opioid system and morphine-induced analgesia were examined. Male albino mice weighing 22-27 g were used in the experiments. 2. To study nociception, the formalin test was selected. Morphine was administered subcutaneously 30 min before formalin injection. Lead acetate treatment was administered 90 min before any injection. Comparisons between groups were made by analysis of variance and then by Newman-Keuls test. Differences with P < or = 0.05 was considered statistically significant. 3. Different doses of morphine induced antinociception in both phases of the formalin test. Lead acetate induced non-dose-dependent nociception in the early phase and dose-dependent analgesia in the late phase. 4. Pretreatment with lead acetate antagonized the effect of morphine in the early phase. In the other hand, the effect of lead acetate in the early phase was reduced by morphine and its effect eliminated in the late phase. 5. It is concluded that lead can modulate pain response and interact with morphine-induced antinociception. Additional research to find the mechanisms of these effects are suggested.

Clozapine and some other antipsychotic drugs may preferentially block the same subset of GABA(A) receptors

Selective blockade of a subset of GABA(A) receptors may be involved in the antipsychotic effects of Clozapine and several other antipsychotic drugs. Seven antipsychotic drugs, and 11 drugs classified as antidepressants that only partially reverse the inhibitory effect of 1 microM GABA on [35S]TBPS binding, do not yield additive reversal when tested pairwise with Clozapine, which also only partially reverses the inhibitory effect of GABA. This suggests that all of these antipsychotic/antidepressant drugs may block a common subset of GABA(A) receptors. DMCM and Ro 5-4864 are also partial reversers of GABA's inhibitory effect, but they yield additive reversals when tested pairwise with the antipsychotic/antidepressant drugs, and also with each other, suggesting that DMCM, Ro 5-4864, and the antipsychotic drugs define three heterogeneous subsets of GABA(A) receptors, with variable overlap, depending on the drug. Several potent ligands for benzodiazepine binding sites can block the GABA inhibitory effects of DMCM and Ro 5-4864, but with different patterns: the ligands generally blocked DMCM less potently, but more completely than Ro 5-4864. Ro 5-4864 was not blocked by Flumazenil or CGS-8216, ligands that potently blocked DMCM. Nine additional antipsychotic/antidepressant drugs, as well as Clozapine, and 7 "classical" GABA(A) receptor blockers, all of which reversed GABA nearly completely, when tested at lower concentrations that only reverse approximately 20-35%, yielded almost complete additivity when tested pairwise with DMCM or Ro 54864. Another convulsant benzodiazepine, KW-1937, a positional isomer of Brotizolam, fully reverses the inhibitory effect of 1 microM GABA. At a lower concentration yielding about 50% reversal, KW-1937 is completely additive with DMCM, but entirely nonadditive with Ro 5-4864. The 50% reversal obtained with KW-1937 was potently blocked by Triazolam, but with a plateau similar to that obtained with Ro 5-4864. The results with KW- 1937 suggest that its 50% reversal largely corresponds to the reversal obtained with Ro 5-4864, and that virtually all of the [35S]TBPS binding sites inhibited by 1 microM GABA are coupled to benzodiazepine binding sites. The fraction of GABA(A) receptors preferentially blocked by all the antipsychotic/antidepressant drugs, roughly 25% of the [35S]TBPS binding sites inhibited with 1 microM GABA, are sensitive to KW-1937, but not to DMCM or to Ro 5-4864.

Antinociceptive interaction between alprazolam and opioids

Alprazolam is a triazolobenzodiazepine, with a potent anxiolytic action and a short half-life. Alprazolam analgesia was measured, using the radiant heat tailflick assay in mice, which was administered alone or in combination with opioids. Intrathecally administered alprazolam produced a dose-response increase in the tailflick latency with an ED50 of 34 microg (19.4-72.5, 95% CL). There were almost no effects after intracerebroventricular injections. Naloxone almost completely abolished the analgesia response mediated by alprazolam. This sensitivity to naloxone indicates that at least some of the analgesic effects of alprazolam are mediated by an opioid mechanism of action. When administered together with various antagonists of opioid receptor subtypes, we found that the mu antagonists, but not the delta and kappa1 subtypes inhibited alprazolam analgesia significantly. No effect was found when alprazolam was coadministrated with kappa3 opioid agonists. In addition, we found a supra-additivity (synergistic) increase in analgesia when alprazolam was given with morphine. Competition binding assays show the highest affinity of alprazolam to the mu1 subtype. In summary, we conclude that alprazolam mediates its analgesic effect, most probably via an mu opiate mechanism of action.

Reduction in firing rate of substantia nigra pars reticulata neurons by valproate: influence of different types of anesthesia in rats

Nondopaminergic, presumably GABAergic neurons in the substantia nigra pars reticulata (SNR) are thought to function as a gating mechanism for seizure propagation. Systemic administration of anticonvulsant doses of the antiepileptic drug valproate (VPA) has previously been reported to inhibit the firing of nondopaminergic SNR neurons in anesthetized but not in awake, paralyzed and locally anesthetized rats, suggesting that the findings in anesthetized rats were due to an interaction between VPA and the general anesthetic used. In the present study, we determined the influence of different anesthetic measures on the effect of an anticonvulsant dose of VPA (100 mg/kg) on extracellularly recorded spontaneous single unit activity of nondopaminergic SNR neurons in rats. Rats were anesthetized by continuous infusion of the general anesthetic chloral hydrate, the dissociative anesthetic ketamine or the narcotic opioid fentanyl, or were only locally anesthetized and paralyzed. VPA significantly reduced SNR firing in all groups with a time course that matched its anticonvulsant time course in rodents. However, VPA's inhibitory effect on SNR firing was significantly less marked under anesthesia with chloral hydrate than in any of the other groups, indicating that this anesthetic suppresses the action of VPA, which may be related to an interaction with GABA-related processes in the SNR. The closest approximation to the effect of VPA in awake rats was obtained under anesthesia with ketamine, while VPA's inhibitory action on SNR neuronal firing seemed to be enhanced in the fentanyl group, which exhibited the highest baseline firing rates of all groups. Determination of VPA in the SN showed that the difference in VPA's inhibitory effect on SNR neurons was not secondary to differences in local drug concentrations. The data demonstrate that VPA is capable of significantly slowing the spontaneous activity of nondopaminergic SNR neurons, but that the magnitude of this effect depends on the anesthetic measures used. In view of the presumed role of SNR neurons in seizure propagation and the finding that VPA consistently inhibits these neurons at an anticonvulsant dose, the present data suggest that suppression of spontaneous SNR neuronal firing may be an important mechanism through which VPA exerts its anticonvulsant properties.

Effects of microinjections of mu and kappa receptor agonists into the dorsal periaqueductal gray of rats submitted to the plus maze test

Several lines of evidence have shown that aversive states are under the influence of opioid mechanisms in the dorsal periaqueductal gray (DPAG). In order to characterize the type of opioid receptors involved in these effects in this work we injected DAMGO and U50,488H, mu and kappa selective agonists, respectively, directly in this structure. Rats implanted with chemitrode in the DPAG were submitted to the elevated plus maze test for 5 min. The effects of DAMGO (0.1-1 nmol/0.2 microliter) and U50,488H (1-10 nmol/0.2 microliter) following administration into DPAG were studied. Low doses of DAMGO (0.1 and 0.3 nmol) caused dose-dependent increases in the number of entries and time spent in the open arms while an overall deficit in the exploratory activity was produced by the higher dose used (1.0 nmol). Clear aversive effects were observed following the administration of U50,488H in the DPAG. The antiaversive effects of 0.3 nmol DAMGO were inhibited by the intraperitoneal administration of the mu receptor antagonist naltrexone (2.0 mg/kg, IP) whereas the aversive effects of 5.0 nmol U50,488H were antagonized by the selective kappa receptor antagonist nor-binaltorphimine (1.0 mg/kg, IP). It is suggested that activation of mu receptors inhibit and kappa receptors enhance the neural substrate of aversion in the DPAG.

Distinct patterns of activated neurons throughout the rat midbrain periaqueductal gray induced by chemical stimulation within its subdivisions

This study provides a map of those neurons in the midbrain periaqueductal gray which are activated by chemical stimulation within different subdivisions of the periaqueductal gray. In pentobarbital anesthetized rats, the expression of the c-FOS protein was detected by immunocytochemistry and was used as a marker of neuronal activity. Microinjections of the gamma-aminobutyric acid (GABAA) receptor antagonist bicuculline (200 pmol in 50 nl) were used to increase selectively the firing rate of neurons originating from the injection site. The pattern of c-FOS immunoreactivity was highly specific for different injection sites. Dorsal injections were characterized by an extensive distribution of c-FOS immunoreactivity along the entire rostrocaudal extent of the periaqueductal gray, while ventral injections produced a much more restricted labeling. Following injection into the dorsal subdivision of the rostral periaqueductal gray, c-FOS immunoreactivity was present bilaterally in the dorsal and dorsolateral subdivisions of the rostral periaqueductal gray and was found in all subdivisions of the caudal periaqueductal gray. Dorsolateral injections at the level of the oculomotor nuclei produced strictly ipsilateral labeling in the dorsal and dorsolateral periaqueductal gray at the level of injection and throughout the ipsilateral half of the periaqueductal gray at more caudal levels. Stimulation in the ventrolateral periaqueductal gray induced FOS in the ventrolateral periaqueductal gray and the adjoining reticular formation. At rostral levels c-FOS immunoreactivity was also seen in the lateral periaqueductal gray but was absent caudal to the injection site. The identified patterns of activity in the periaqueductal gray provide a new basis for the interpretation of the diverse functional consequences of stimulation at periaqueductal gray sites.

Naloxone blocks the antianxiety but not the motor effects of benzodiazepines and pentobarbital: experimental studies and literature review

The role of opioid systems in the anticonflict effect of chlordiazepoxide, diazepam and pentobarbital was evaluated with a modified Vogel procedure. First, morphine, ineffective by itself, was combined with subeffective or marginally effective doses of the benzodiazepines in order to detect possible potentiation. However, the combined treatment reduced licking in the Vogel procedure as well as in a licking test where no shock was administered. Several doses of the benzodiazepines and pentobarbital were then administered in combination with several doses of the opiate antagonist naloxone. A dose-dependent inhibition of anticonflict effect was obtained. In an additional experiment, it was shown that naloxone blocked the effects of diazepam in the elevated plus-maze procedure. Motor deficiencies, as evaluated with a rotarod test, produced by the benzodiazepines and pentobarbital could not be antagonized by naloxone. It is concluded that opioids are important for the anticonflict but not for the motor effects of these drugs. An analysis of published studies concerning the interaction of opioids and benzodiazepines in several procedures supposed to reflect anxiolytic effects shows that the inhibition obtained with naloxone is reliable and not procedure specific. The mechanisms by which opiate antagonists produce this inhibition of anticonflict activity are not known. It is tentatively suggested that opioid activation associated with stress may be a necessary component of anxiolysis.

In vitro electrophysiological characterization of midbrain periaqueductal gray neurons in female rats: responses to GABA- and Met-enkephalin-related agents

Met-Enkephalin, which can be induced by estrogen in the ventromedial nucleus of hypothalamus (VMH), has been proposed to help mediate estrogenic action on lordosis behavior by acting on midbrain periaqueductal gray (PAG) neurons. Also, in the PAG, GABA may locally regulate the levels of lordosis behavior through GABAA receptors. Therefore, we examined the effects of both Met-enkephalin and GABA-related agents on neuronal activity of PAG neurons in slices. Overall, 72.6% of the PAG neurons were inhibited by GABA and 60.9% of GABA-responsive neurons were also excited by the GABAA receptor antagonist, bicuculline methiodide (BMI), suggesting that many of GABA-responsive PAG neurons are tonically inhibited by GABAergic neurons through GABAA receptors. Dorsal PAG neurons were more responsive to BMI than ventral PAG neurons. Moreover, in the middle part of the dorsal PAG, where prominent inhibitory behavioral effects of BMI have been reported, BMI excited 94% of GABA-responsive PAG neurons from estrogen-treated animals, significantly more than observed in ovariectomized control (50%). The most frequent action of Met-enkephalin on PAG neurons was inhibitory (38 out of 149 recorded neurons) although it excited 12 neurons. A dose-dependent increase of inhibitory action of enkephalin was found in the estrogen-primed group but not in the ovariectomized control group while higher doses of enkephalin failed to excite any more neurons in both groups. Most frequently (90%), enkephalin inhibited the same neurons as those on which GABA had the inhibitory effects. Conversely, these neurons composed about 50% of the entire GABA-responsive PAG neurons. Moreover, 76% of neurons inhibited by enkephalin were found to be tonically inhibited by endogenous GABA through GABAA receptors. It is argued, therefore, that increased enkephalinergic influences from the VMH to the PAG in estrogen-treated females could participate in the PAG neuronal control of lordosis by acting on the same neurons as are innervated by intrinsic GABAergic neurons. Since GABAA agonists actually facilitate lordosis in the PAG, these PAG neurons inhibited by both GABA and enkephalin may themselves facilitate behaviors which are antagonistic to lordosis, such as defensive behaviors.

Enkephalinase inhibition and hippocampal excitatory effects of exogenous and endogenous opioids

1. The relationships between the in vivo and in vitro epileptogenic effects of opioids or enkephalins and the electrophysiological activity of inhibitors of endogenous enkephalinase were analyzed. 2. The functional effects of the inhibition of the endogenous enkephalinase has been compared with the role of the endogenous opioid peptidergic system in the control of neuronal excitability.

Neural substrate of defensive behavior in the midbrain tectum

It has been shown that the gradual increase in the intensity of electrical stimulation of the dorsal periaqueductal gray (DPAG), deep layers of the superior colliculus (DLSC) and inferior colliculus of rats induces, in a progressive manner, characteristic aversive responses such as arousal, freezing, and escape behavior. The DPAG-DLSC together with the periventricular gray substance of the diencephalon, amygdala and the inferior colliculus, constitute the neural substrate of aversion in the brain. In general, the behavioral responses induced by midbrain tectum stimulation are accompanied by increases in the mean arterial blood pressure, heart rate, and respiration. Both the behavioral and autonomic consequences of electrical stimulation of the mesencephalic tectum have been shown to be attenuated by minor tranquilizers, probably through enhancement of GABAergic neurotransmission. Besides GABAergic mechanisms several lines of evidence have clearly implicated opioid, serotonergic, and excitatory amino acids-mediated mechanisms in the control of the neural substrates commanding defensive behavior in the brain aversive system.

Effect of morphine-induced catalepsy, lethality, and analgesia by a benzodiazepine receptor agonist midazolam in the rat

Previously we have shown that intrathecal administration of midazolam can increase or decrease morphine-induced antinociception, depending upon relative concentration of these drugs by modulating spinal opioid receptors, and it also can inhibit morphine-induced tolerance and dependence in the rat. Now we report that midazolam also influences catalepsy, lethality, and analgesia induced by morphine in the rat. In the acute treatment, animals were first treated with saline or midazolam (0.03 to 30.0 mg/kg, b.wt., IP), and 30 min later with a second injection of saline or morphine (1.0 to 100.0 mg/kg, b.wt., SC). The catalepsy was measured 60 min after the second injection and lethality was checked after 24 h. Midazolam injection increased the morphine-induced catalepsy and lethality. In the chronic treatment, animals were injected with two injections daily for 11 days. The first injection consisted of saline or midazolam (0.03 to 3.0 mg/kg, b.wt., IP), and 30 min later with a second injection of saline or morphine (10.0 mg/kg, b.wt., IP) was given. Lethality, antinociception, and body weight were measured. Chronic morphine treatment also increased lethality in a dose-dependent manner. Chronic treatment with midazolam and morphine increased the antinociception on day 11, as measured in the tail-flick and hot-plate tests. Midazolam administration also prevented the morphine-induced weight loss. These results suggest a strong interaction between midazolam and morphine in altering catalepsy, lethality, and analgesia in rat.

Inhibition of clonic seizure-like excitatory effects induced by intrathecal morphine using two NMDA receptor antagonists: MK-801 and ACEA-1011

Microinjection of high doses of morphine into the spinal lumbar intrathecal (i.t.) space of mice produces dose-dependent clonic seizure-like excitatory effects. Naloxone, an opioid antagonist (10 mg/kg, i.p.), injected 5 min prior to i.t. morphine, did not reverse the seizure-like motor effects, suggesting that these effects of morphine are not mediated through opioid receptors. Systemic administration of MK-801, a non-competitive NMDA receptor antagonist (0.01-0.10 mg/kg, i.p.), or ACEA-1011, a novel NMDA receptor/glycine site antagonist (0.5-20.0 mg/kg, i.p.), attenuated the clonic seizure-like excitatory effects induced by i.t. morphine in a dose-dependent manner. Sensorimotor performance of the mice was evaluated using the rotarod test. Although both compounds (MK-801 and ACEA-1011) impaired the sensorimotor performance of mice in a dose-dependent fashion, there was no impairment of motor performance at doses employed to block the excitatory effects induced by i.t. morphine. These data suggest that NMDA receptors play a pivotal role in the clonic seizure-like behaviors induced by i.t. morphine.

Mechanisms of defense in the inferior colliculus

The inferior colliculus (IC) is a well known relay station for auditory pathways in the brainstem. In the present review we are suggesting that aversive states are also generated and elaborated in the inferior colliculus and that this structure may be part of a brain system commanding defensive behavior. The evidences presented in this review have been obtained from experiments carried out with the combined use of intracerebral microinjections and of electrical stimulation of the inferior colliculus. This electrical stimulation caused a behavioral activation together with autonomic reactions usually observed as part of the defense reaction. NMDA--an excitatory amino acid--, or bicuculline--a GABAA antagonist--injected into the IC mimicked the effects of its electrical stimulation. The IC electrical stimulation showed clear aversive properties as rats submitted to a switch-off paradigm quickly learned to interrupt it. Systemic administration as well as IC microinjections of the anxiolytic compound midazolam caused dose-dependent increases in the latency and reductions in the frequency of switch-off responses to the inferior colliculus electrical stimulation. Similar results were obtained following microinjections into this brainstem structure of the GABAA agonist muscimol. These results suggest that neural substrates responsible for defensive behavior in the inferior colliculus may be depressed by benzodiazepines as part of the anxiolytic action of these compounds. This anti-aversive action may be produced by the enhancement of GABAA mechanisms. Serotonergic mechanisms seem also to be involved in the modulation of these aversive states as IC microinjections of zimelidine, a 5-HT uptake blocker, caused a significant inhibition of the switch-off responses in the shuttle-box.(ABSTRACT TRUNCATED AT 250 WORDS)

Benzodiazepine receptor binding following chronic treatment with naloxone, morphine and met-enkephalin in normal rats

The effects of chronic administration of naloxone, morphine and met-enkephalin on benzodiazepine (BDZ) receptor binding in rat brain were determined 2 and 50 days after treatments were accomplished. Two days after naloxone treatment (75 micrograms/h s.c. for 14 days), enhanced BDZ receptor binding was observed in cingulate, frontal, piriform, entorhinal and sensorimotor cortices; amygdala complex, hippocampus, substantia nigra and central gray. Two days after morphine treatment (20 mg/kg i.p. daily for 6 days), increased BDZ receptor binding was detected in cingulate, frontal, piriform, entorhinal and sensorimotor cortices; amygdala complex, hippocampus and substantia nigra. Two days after met-enkephalin treatment (10 micrograms/h i.c.v. for 6 days) enhanced BDZ receptor binding was shown only in sensorimotor cortex. No significant changes were observed 50 days after the treatments were completed. These data indicate an important interaction between GABAergic and opioid peptide systems.

Fentanyl antagonizes diazepam on carotid sinus baroreflex control of circulation in rabbits

To investigate the effects of a combination of fentanyl and diazepam on carotid sinus baroreflex in conscious rabbits, we examined the responses of mean systemic arterial pressure (MAP), heart rate (HR) and total peripheral resistance (TPR) to bilateral carotid occlusion (BCO). Seven rabbits were given 0.5 mg.kg(-1) of diazepam i.v. followed by 10 mg.kg(-1) of fentanyl i.v. at 5 min intervals (group 1), and the drugs were given in the reverse order to 5 other rabbits (group 2). BCO was repeated in conscious state (control) and after each drug injection. MAP responses did not differ from control response in either group when both drugs were given. In group 1, however, diazepam decreased HR response to 71.4% of control, and increased TPR response by 36%. Fentanyl administration reversed diazepam-induced changes in BCO responses to the control level. In group 2, fentanyl decreased TPR response to 61.6% of control and increased HR response by 41.5%. Administration of diazepam following fentanyl restored HR and TPR responses to control levels. Carotid sinus baroreflex gain was 3.1 +/- 0.4 (mean +/- SEM) in control and 3.1 +/- 0.4 after administration of both drugs in 12 rabbits. The results suggest that a sedative dose of either fentanyl or diazepam antagonizes the other drug's action on the carotid sinus baroreflex. The combination of fentanyl and diazepam has little influence on carotid sinus baroreflex control of the circulation in rabbits.

Aversive and antiaversive effects of morphine in the dorsal periaqueductal gray of rats submitted to the elevated plus-maze test

The dorsal periaqueductal gray (DPAG) is a well-known region for processing defensive behavior in the brainstem. Rats implanted with cannulae in the DPAG were submitted to the elevated plus-maze test for 5 min. The effects of morphine following systemic (0.1-1.0 mg/kg) or DPAG administration (5-30 nmol) were compared with the benzodiazepine compound midazolam injected similarly (1-10 mg/kg, IP, and 10-80 nM, DPAG). Morphine and midazolam caused dose-dependent increases in the number of entries and time spent in the open arms. A systemic injection of naloxone in doses that block mu-opioid receptors reversed the effects of centrally administered morphine. Higher doses of morphine (70 nmol) induced a non-naloxone-reversible "fearful" hyperreactivity. It is suggested that low doses of morphine inhibit the neural substrate of aversion in the DPAG, probably through activation of mu-receptors, and that microinjections of higher doses of morphine cause proaversive actions not mediated by these opioid receptors.

Opioid mediation of the antiaversive and hyperalgesic actions of bradykinin injected into the dorsal periaqueductal gray of the rat

Reported evidence indicates that the dorsal region of the periaqueductal gray matter (PAG) is involved in the modulation of both pain and aversion, and that opioid mechanisms, among others, participate in their modulation. Since many central actions of bradykinin (BK) have been shown to be similar to those of morphine, the present was undertaken to measure the effects of microinjection of BK into the PAG on the thresholds of aversive electrical stimulation of the same brain area and of dental pulp electrical stimulation. Bradykinin, injected into the dorsal PAG, induced a dose-dependent increase in the aversive threshold, an effect similar to that reported by others for morphine. Also, as reported for morphine, the antiaversive effect of BK was antagonized by naloxone injected intraperitoneally. Whereas subcutaneously administered morphine induced marked analgesia, intra-PAG administration of BK caused a small but significant hyperalgesia. Similarly, morphine injected into the dorsal PAG tended to cause hyperalgesia instead of analgesia. Furthermore, the hyperalgesic effect of BK also appears to involve opioid mechanisms since it was blocked by naloxone. As in previously reported studies, intracerebroventricularly injected BK raised the pain threshold. These results indicate that BK mobilizes opioid mechanisms in the dorsal PAG that inhibit aversion but not pain.

The GABAA receptor complex in relation to epilepsy. Reversal of [3H]TBOB inhibition: a prediction of proconvulsive properties?

[3H]-t-Butylbicycloorthobenzoate ([3H]TBOB), a convulsant, is known to label a binding site on the GABAA receptor complex. Bicuculline methochloride (bicuculline MCl), folic acid, pentazocine, naloxone, ethyl-beta-carboline-3-carboxylate (beta CCE) and Ro 5-4864 have (pro)convulsive properties in vivo. In the present study, we determined the extent to which these compounds modify the binding of [3H]TBOB in the presence of IC50 amounts of GABA (5 microM) or diazepam (50 microM). We found that the GABA antagonist bicuculline MCl reversed the inhibitory effect of GABA on [3H]TBOB binding completely, as was expected. Folic acid, pentazocine and naloxone also reversed the inhibitory effect of GABA on [3H]TBOB binding. This finding is compatible with the view that the proconvulsive effects of these compounds can be credited to a reduction of GABAergic action at the GABAA receptor complex. We suggest that the reversal of GABA's inhibition of [3H]TBOB binding is a sufficient (but not a necessary) condition to predict proconvulsive (side) effects of drugs. beta CCE and Ro 5-4864 modified [3H]TBOB binding in the presence of GABA in a biphasic fashion. A unique relation between beta CCE, Ro 5-4864 and the GABAA complex might exist. Bicuculline MCl reversed the inhibitory effect of diazepam on [3H]TBOB binding only partly. beta CCE did not reverse the inhibitory effect of diazepam on [3H]TBOB binding, neither did Ro 5-4864. The presence of a GABA-independent interaction between a low affinity benzodiazepine recognition site and the TBOB site is proposed.

Defense reaction elicited by microinjection of kainic acid into the medial hypothalamus of the rat: antagonism by a GABAA receptor agonist

Electrical stimulation of either the midbrain central gray or the medial hypothalamus induces a defense reaction in the rat, characterized mainly by increased locomotion, rearing, and leaping. However, microinjection of the excitatory amino acid glutamate was effective only in the former region. Because excitatory amino acids do not depolarize axons of passage, it was suggested that the hypothalamus is devoid of soma/dendrites of neurons commanding the defense reaction. In the present study, we show that a subtoxic dose (60 pmol) of another excitatory amino acid, kainic acid, injected into the medial hypothalamus significantly enhanced locomotion and rearing of Wistar rats systematically observed in an open field. Similar behavioral changes have been reported following microinjection of drugs impairing GABAergic neurotransmission. Local pretreatment with the GABAA receptor agonist THIP (2 nmol) blocked the effect of kainic acid. Therefore, the medial hypothalamus of the rat seems to contain a population of neuronal cell bodies commanding the defense reaction, which is activated by excitatory amino acids and tonically inhibited by GABAergic fibers.

Wild running elicited by microinjections of bicuculline or morphine into the inferior colliculus of rats: lack of effect of periaqueductal gray lesions

Bicuculline methiodide, a GABAA receptor antagonist, or a high dose of morphine was injected at the same site within the inferior colliculus (IC) of rats. Both drugs elicited the same behavioral activity (wild running). However, the time course and magnitude of the effects of the two drugs differed. Since the behavioral activation elicited was reminiscent of what was found with microinjections of bicuculline methiodide or morphine into the periaqueductal gray (PAG), we lesioned the PAG in another group of rats. It was found that extensive lesions of the PAG including those extending to the medial part of the superior colliculus did not significantly reduce the wild running.

Naloxone enhances epileptogenic and behavioral effects of pentazocine in rats

Eight groups of six rats were either injected with saline, pentazocine, naloxone, or a combination of pentazocine and naloxone. Studied were the effects on EEG and behavior. It was found that pentazocine induced epileptic seizures in a dose-dependent fashion. In addition, similar behavioral changes were present after all three doses of pentazocine. High doses of naloxone did not cause epilepsy and affected behavior only slightly. Seizures induced by pentazocine were not antagonized by the opiate antagonist naloxone, but were facilitated after the combination of a noneffective dose of pentazocine and a noneffective dose of naloxone. In addition, exploratory behavior was facilitated by the combination of pentazocine and naloxone. It seems that both an opiate and a nonopiate system are involved in this type of epilepsy and in this type of behavior.

Calcium channel blockers: effect on morphine-induced hypermotility

Acute morphine treatment has been shown to cause a uniform calcium depletion in various brain regions and to evoke hypermotility in mice. On the other hand, it has been reported previously that calcium channel blockers reduce the behavioral stimulation induced by different methods in mice, and it is known that these drugs increase the morphine analgesia and reduce the abstinence syndrome. The effect of calcium channel blockers, nifedipine and diltiazem, on the morphine- and amphetamine-induced hypermotility were evaluated. Mice activity was measured with photocell motility meters. The results show that neither nifedipine nor diltiazem decrease significantly the motility in control and amphetamine-treated mice; however, when they were administered to morphine-treated mice the hypermotility was significantly reduced. The mechanism responsible for this interference is still unknown.

Effects of microiontophoretically-applied morphine on the Purkinje cell in the cerebellum of the cat

The effects of microiontophoretically-applied and pneumatically-applied morphine on the spontaneous discharge of Purkinje cells in the cerebellum of the anesthetized cat were examined. Microiontophoretic application of morphine produced both inhibitory and excitatory responses of the Purkinje cells. Pneumatic application of morphine produced similar effects to those of microiontophoresis. Both types of application of morphine induced dose-dependent responses. Excitatory responses were antagonized by naloxone (opiate antagonist), but inhibitory responses were not affected by naloxone, propranolol (beta-receptor antagonist) or methysergide (serotonin antagonist). Bicuculline and picrotoxin, GABA antagonists, abolished completely the morphine-induced inhibitory response. These results suggest that morphine-induced excitation is connected with the opiate system and that inhibition is related to the GABAergic system, in the cerebellum of the cat.

Excitatory amino acids: role in morphine excitation in rat periaqueductal gray

Morphine was previously found to elicit an explosive excitatory behavior following its injection at a high dose in the rat periaqueductal gray (PAG). This non-naloxone reversible excitatory action of morphine was mimicked by the GABAA receptor antagonist, bicuculline, suggesting that morphine excitation was due in part to GABAA receptor blockade. In this paper, we report that injections of the excitatory amino acid (EAA) analogues, N-methyl-D-aspartate (NMDA), quisqualate (Q) or kainate (K) in the rat PAG resulted in similar (but not identical) behaviors. The excitatory actions of morphine or of NMDA (but not Q or K) were blocked or attenuated by the NMDA receptor antagonist, 2-amino-7-phosphonoheptanoate. These results show that both GABAA receptors as well as receptors for the EAAs may contribute to the excitatory actions of morphine in the PAG, and suggest that GABA may normally function to counterbalance a tonic excitatory influence of the EAAs.

Two groups of amino acids interact with GABA-A receptors coupled to t-[35S]butylbicyclophosphorothionate binding sites: possible involvement with seizures associated with hereditary amino acidemias

Seven L-amino acids (Trp, Arg, Lys, Met, Ile, Val, and Phe) partially (28-81%) reversed the inhibitory action of 1 microM gamma-aminobutyric acid (GABA) on t-[35S]butylbicyclophosphorothionate ([35S]TBPS) binding to rat brain membranes, with EC50 values ranging from 5 to 120 mM. D-Trp, D-Arg, D-Lys, D-Met, D-Val, and D-Phe were approximately equipotent with their L-isomers. Tyramine, phenethylamine, and tryptamine, the decarboxylation products of the aromatic amino acids (Tyr, Phe, and Trp, respectively), reversed the inhibitory action of 1 microM GABA on [35S]TBPS binding more potently than the parent amino acids (EC50 values = 1.5-3.0 mM). Human hereditary amino acidemias involving Arg, Lys, Ile, Val, and Phe are associated with seizures, and these amino acids and/or their metabolites may block GABA-A receptors. Five other L-amino acids (ornithine, His, Glu, Pro, and Ala) as well as Gly and beta-Ala inhibited [35S]TBPS binding with IC50 values ranging from 0.1 to 37 mM, and these inhibitions were reversed by the GABA-A receptor blocker R 5135 in all cases. The inhibitory effects of L-ornithine, L-Ala, L-Glu, and L-Pro were stereospecific, because the corresponding D-isomers were considerably less inhibitory. L-His, D-His, and L-Glu gave incomplete (plateau) inhibitions. Human hereditary amino acidemias involving L-ornithine, His, Pro, Gly, and beta-Ala are also associated with seizures, and we speculate that these GABA-mimetic amino acids may desensitize GABA-A receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation by morphine of aversive-like behavior induced by GABAergic blockade in periaqueductal gray or medial hypothalamus

Pretreatment with "analgesic" doses (15 nmoles) of morphine injected either into the periaqueductal gray (PAG) or into the medial hypothalamus (MH) were found to modulate flight behavior elicited by bicuculline injected into the same brain sites. When injected into the MH, morphine always suppressed bicuculline-induced flight, while PAG injections paradoxically either suppressed or facilitated the behavioral effects produced by bicuculline. Whenever a facilitation of the bicuculline-induced effects had been observed following pretreatment with 15 nmoles of morphine into the PAG, the infusion of lower doses (6 nmoles) did no longer induce facilitation but clear suppression. In those animals that had shown suppression of the aversive-like effects of bicuculline following the same 15 nmoles pretreatment, infusion of higher doses (24 nmoles) of morphine into the PAG still produced the same kind of suppression. And yet, when injected into the PAG, very high doses of morphine (50 nmoles) were found to induce, by themselves, flight behavior known as explosive motor behavior. In contrast, such high doses of morphine never induced comparable explosive motor behavior when injected into the MH. These data can be explained by the involvement of different types of receptors in the neural mechanisms subserving and controlling the generation of aversion in periventricular brain regions (PAG and MH).