Convulsant and anticonvulsant effects of opioids: relationship to GABA-mediated transmission

Anticonvulsant Effects of Thalidomide on Pentylenetetrazole-Induced Seizure in Mice: A Role for Opioidergic and Nitrergic Transmissions

Although accumulating evidence indicates that the immunomodulatory medication thalidomide exerts anticonvulsant properties, the mechanisms underlying such effects of thalidomide are still unknown. Our previous preclinical study suggested that nitric oxide (NO) signaling may be involved in the anticonvulsant effects of thalidomide in a mouse model of clonic seizure. Additionally, several studies have shown a modulatory interaction between thalidomide and opioids in opioids intolerance, nociception and neuropathic pain. However, it is unclear whether opioidergic transmission or its interaction with NO signaling is involved in the anticonvulsant effects of thalidomide. Given the fact that both opioidergic and nitrergic transmissions have bimodal modulatory effects on seizure thresholds, in the present study we explored the involvement of these signaling pathways in the possible anticonvulsant effects of thalidomide on the pentylenetetrazole (PTZ)-induced clonic seizure in mice. Our data showed that acute administration of thalidomide (5-50 mg/kg, i.p., 30 min prior PTZ injection) dose-dependently elevated PTZ-induced clonic seizure thresholds. Acute administration of low doses (0.5-3 mg/kg, i.p., 60 min prior PTZ) of morphine exerted anticonvulsant effects (P < 0.001), whereas higher doses (15-60 mg/kg, 60 min prior PTZ) had proconvulsant effects (P < 0.01). Acute administration of a non-effective anticonvulsant dose of morphine (0.25 mg/kg) prior non-effective dose of thalidomide (5 mg/kg) exerted a robust (P < 0.01) anticonvulsant effect. Administration of a non-effective proconvulsant dose of morphine (7.5 mg/kg) prior thalidomide (5 mg/kg) didn't affect clonic seizure thresholds. Acute administration of a non-effective dose of the opioid receptor antagonist naltrexone (1 mg/kg, i.p.) significantly prevented anticonvulsant effects of thalidomide (10 mg/kg, i.p.). Pretreatment with non-effective dose of the NO precursor L-arginine (60 mg/kg, i.p.) significantly (P < 0.01) reduced the anticonvulsant effects of combined low doses of morphine (0.25 mg/kg) and thalidomide (5 mg/kg). Conversely, pretreatment with non-effective doses of either non-selective (L-NAME, 5 mg/kg, i.p.) or selective neuronal (7-nitroindazole, 30 mg/kg, i.p.) NO synthase (NOS) inhibitors significantly augmented the anticonvulsant effects of combined low doses of thalidomide and morphine, whereas the inducible NOS inhibitor aminoguanidine (100 mg/kg, i.p.) did not exert such effect. Our results indicate that opioidergic transmission and its interaction with neuronal NO signaling may contribute to the anti-seizure activity of thalidomide in the mice PTZ model of clonic seizure.

Effects of naloxone on motion sickness in cats alone and with broad spectrum antiemetics

Doses of naloxone far below those which elicit emesis increase the sensitivity to motion sickness. In order to evaluate the possible interaction with broad spectrum antiemetics, low doses of naloxone were tested alone and in combination with 8-hydroxy-2-(di-n-propylamine)tetralin (DPAT), fentanyl and the NK1 antagonist CP-99994. A modified autonomic symptom rating scale was unaffected by any drug and thus considered of little value. Fentanyl and NK1 antagonists decreased the duration of the retch/vomit sequence. Naloxone alone and in combination with each of the drugs increased the duration of retching/vomiting. Naloxone also increased the number of vomiting sequences. The results are interpreted in terms of possible site(s) of action of the antiemetic drugs.

Administration of lithium and magnesium chloride inhibited tolerance to the anticonvulsant effect of morphine on pentylenetetrazole-induced seizures in mice

Although morphine has an anticonvulsant effect in several animal models of seizures, its potential clinical application in epilepsy may be hindered by its adverse effects like opioid tolerance. The present study evaluated the development of tolerance to the anticonvulsant effect of morphine in a model of clonic seizures induced with pentylenetetrazole (PTZ) in male Swiss mice. We also examined whether administration of either lithium chloride (LiCl) or magnesium chloride (MgCl(2)) was able to prevent the probable tolerance. Our data demonstrated that the anticonvulsant effect of a potent dose of morphine (1mg/kg) was abolished in chronic morphine-treated mice (mice administered the same dose of morphine intraperitoneally twice daily for 4 days). Four days of pretreatment with low and noneffective doses of MgCl(2) (2 and 5mg/kg) and LiCl (5mg/kg) inhibited the development of tolerance to the anticonvulsant effect of morphine (1mg/kg, ip). Moreover, a single acute injection of the aforementioned agents at the same doses reversed the expression of tolerance to the anticonvulsant effects of morphine (1mg/kg, ip). Chronic 17-day treatment with LiCl (600 mg/L in drinking water) also inhibited the development of tolerance to the anticonvulsant effects of 1mg/kg morphine. These results demonstrate that the anticonvulsant effect of morphine is subject to tolerance after repeated administration. Both development and expression of tolerance are inhibited by either LiCl or MgCl(2). As both LiCl and MgCl(2) can modulate the function of N-methyl-d-aspartate (NMDA) receptors, we discuss how NMDA receptor functioning might be involved in the effects of LiCl and MgCl(2) on the development of tolerance to the anticonvulsant effect of morphine.

Role of ATP-sensitive potassium channels in the biphasic effects of morphine on pentylenetetrazole-induced seizure threshold in mice

Although several studies have indicated that the opioid receptor agonist morphine exerts biphasic effects on clonic seizure threshold, as yet little is known of the underlying mechanisms in this effect. In the present study, using the specific ATP-sensitive K(+) (K(ATP)) channel blocker glibenclamide and the specific K(ATP) channel opener cromakalim, the possible involvement of K(ATP) channels in the effects of morphine on pentylenetetrazole (PTZ)-induced seizure threshold in mice was investigated. Acute administration of lower doses of morphine (1, 3 and 7.5 mg/kg, i.p.) increased and higher doses of morphine (30 and 60 mg/kg, i.p.) decreased the PTZ-induced seizure threshold. Glibenclamide (2.5-5 mg/kg) increased the PTZ-induced seizure threshold. Non-effective dose of cromakalim (0.1 microg/kg) inhibited anticonvulsant effect of glibenclamide (5 mg/kg). Acute administration of non-effective dose of glibenclamide (1 mg/kg) interestingly inhibited both anticonvulsant and pro-convulsant effects of morphine and this effect was significantly reversed by cromakalim (0.1 microg/kg). These results support the involvement of K(ATP) channels in the modulation of seizure threshold by morphine.

Ultra-low dose naltrexone potentiates the anticonvulsant effect of low dose morphine on clonic seizures

Significant potentiation of analgesic effects of opioids can be achieved through selective blockade of their stimulatory effects on intracellular signaling pathways by ultra-low doses of opioid receptor antagonists. However, the generality and specificity of this interaction is not well understood. The bimodal modulation of pentylenetetrazole-induced seizure threshold by opioids provide a model to assess the potential usefulness of this approach in seizure disorders and to examine the differential mechanisms involved in opioid anti- (morphine at 0.5-3 mg/kg) versus pro-convulsant (20-100 mg/kg) effects. Systemic administration of ultra-low doses of naltrexone (100 fg/kg-10 ng/kg) significantly potentiated the anticonvulsant effect of morphine at 0.5 mg/kg while higher degrees of opioid receptor antagonism blocked this effect. Moreover, inhibition of opioid-induced excitatory signaling by naltrexone (1 ng/kg) unmasked a strong anticonvulsant effect for very low doses of morphine (1 ng/kg-100 microg/kg), suggesting that a presumed inhibitory component of opioid receptor signaling can exert strong seizure-protective effects even at very low levels of opioid receptor activation. However, ultra-low dose naltrexone could not increase the maximal anticonvulsant effect of morphine (1-3 mg/kg), possibly due to a ceiling effect. The proconvulsant effects of morphine on seizure threshold were minimally altered by ultra-low doses of naltrexone while being completely blocked by a higher dose (1 mg/kg) of the antagonist. The present data suggest that ultra-low doses of opioid receptor antagonists may provide a potent strategy to modulate seizure susceptibility, especially in conjunction with very low doses of opioids.

The interaction of cannabinoids and opioids on pentylenetetrazole-induced seizure threshold in mice

Cannabinoid and opioid receptor agonists show functional interactions in a number of their physiological effects. Regarding the seizure-modulating properties of both classes of receptors, the present study examined the possibility of a functional interaction between these receptors. We used acute systemic administration of cannabinoid selective CB(1) receptor agonist (ACPA) and antagonist (AM251) and opioid receptor agonist (morphine) and antagonists (naltrexone and norbinaltorphimine) in a model of clonic seizure induced by pentylenetetrazole (PTZ). Acute administration of ACPA (1.5-2 mg/kg) increased the PTZ-induced seizure threshold. In contrast, AM251 (0.5-2 mg/kg) dose-dependently decreased the seizure threshold. Low dose of AM251 (0.5 mg/kg), which did not alter seizure threshold by itself, reversed the anticonvulsant effect of ACPA (2 mg/kg), showing a CB(1) receptor-mediated mechanism. Naltrexone (1 or 10 mg/kg) but not specific kappa-opioid receptor antagonist norbinaltorphimine (5 mg/kg) completely reversed the anticonvulsant effect of ACPA (2 mg/kg). Moreover, the combination of the lower doses of AM251 (0.5 mg/kg) and naltrexone (0.3 mg/kg) had an additive effect in blocking the anticonvulsant effect of ACPA. In accordance with previous reports, morphine exerted biphasic effects on clonic seizure threshold with anticonvulsant effect at lower (0.5-1 mg/kg) and proconvulsant effect at a higher (30 mg/kg) doses. The pretreatment with AM251 blocked the anticonvulsant effect of morphine at 1 mg/kg, while pretreatment with ACPA (1 mg/kg) potentiated the anticonvulsant effect of morphine at 0.5 mg/kg. The proconvulsant effect of morphine at 30 mg/kg was also inhibited by AM251 (2 mg/kg). A similar interaction between cannabinoids and opioids was also detected on their anticonvulsant effects against the generalized tonic-clonic model of seizure. In conclusion, cannabinoids and opioids show functional interactions on modulation of seizure susceptibility.

The role of alpha2-adrenoceptors in the modulatory effects of morphine on seizure susceptibility in mice

PURPOSE

To evaluate the effect of the alpha2-adrenoceptor agonist clonidine and the antagonist yohimbine on the dual modulation of seizure susceptibility induced by morphine and the anticonvulsant effect of acute stress in mice.

METHODS

The thresholds for the clonic seizures induced after intravenous administration of pentylenetetrazole (PTZ) or bicuculline were assessed in mice weighing 23-30 g. Acute stress was induced by restraining mice for 2 h in a restrainer.

RESULTS

Morphine at lower doses (0.5, 1, and 3 mg/kg) increased and, at higher doses (15, 30, and 75 mg/kg), decreased the seizure threshold. Pretreatment with clonidine (0.001-0.1 mg/kg) inhibited the anticonvulsant effect of morphine, while potentiating its proconvulsant effect. Conversely, yohimbine (0.5-2 mg/kg) potentiated the anticonvulsant effect of morphine but inhibited its proconvulsant effects. Acute stress induced an anticonvulsant effect that was reversible by naloxone (1 mg/kg) or clonidine (0.05-0.1 mg/kg) or a combination of their lower doses (0.3 and 0.01 mg/kg, respectively), while being potentiated by yohimbine (1 mg/kg).

CONCLUSIONS

alpha2-Adrenoceptors play a dual role in the anticonvulsant effects of morphine. The activation of these receptors also can decrease the anticonvulsant effect of acute restraint stress in mice.

The involvement of endogenous opioids and nitricoxidergic pathway in the anticonvulsant effects of foot-shock stress in mice

The involvement of endogenous opioids and nitric oxide (NO) in the anticonvulsant effects of stress against pentylenetetrazole (PTZ)- or electroconvulsive shock-induced seizures was assessed in mice. The prolonged and intermittent foot-shock stress, which induced opioid-mediated analgesia, had significant protective effects against both seizure types which was reversible by naloxone (0.3, 1 or 2 mg/kg), while brief and continuous foot-shock did not alter the seizure susceptibility. Pre-treatment with non-specific nitric oxide synthase (NOS) inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME, 1, 2, 5, 10 or 30 mg/kg), but not with specific inducible NOS (iNOS) inhibitor, aminoguanidine (50 or 100 mg/kg), blocked the stress-induced anticonvulsant effects. The lower doses of naloxone (0.3 mg/kg) and L-NAME (2 mg/kg) showed additive effects in blocking the stress-induced anticonvulsant properties. L-arginine at a per se non-effective dose of 20 mg/kg potentiated the stress-induced anticonvulsant properties, an effect which was inhibited by L-NAME but not by aminoguanidine. Furthermore, a low dose of morphine (0.5 mg/kg) showed potentiation with stress in increasing PTZ seizure threshold. This potentiation was reversed by either naloxone or L-NAME at low doses but not by aminoguanidine. Taken together, these results show that NO synthesis, through constitutive but not iNOS, is involved in opioid-dependent stress-induced anticonvulsant effects against electrical and PTZ-induced convulsions.

The role of nitric oxide in anticonvulsant and proconvulsant effects of morphine in mice

Acute subcutaneous administration of lower doses of morphine (0.5, 1 and 3 mg/kg) increase the threshold of seizures induced by pentylenetetrazole (PTZ) in mice, whereas higher doses of morphine (15, 30 and 60 mg/kg) have proconvulsant effects. The effect of systemic administration of nitric oxide synthase (NOS) inhibitors N(G)-nitro-L-arginine methyl ester (L-NAME) and N(G)-nitro-L-arginine (L-NNA) and nitric oxide synthase (NOS) L-arginine on biphasic effect of morphine was investigated. Acute administration of both L-NAME (1, 3 and 10 mg/kg) and L-NNA (1 and 10 mg/kg) as well as chronic pretreatment with L-NAME (1 and 10 mg/kg, 4 days) dose-dependently inhibited both the anticonvulsant and proconvulsant effects of morphine (1 and 30 mg/kg, respectively). The inhibition was complete for anticonvulsant effect while partial for proconvulsant effect. L-arginine at doses that did not affect seizure threshold per se (acute, 30 and 60 mg/kg; chronic, 60 mg/kg) potentiated both anticonvulsant and proconvulsant properties of less potent doses of morphine (0.5 and 15 mg/kg, respectively). The L-arginine induced potentiation of both phases of morphine effect was blocked by L-NAME (0.5-30 mg/kg). Moreover, low and per se non-effective doses of naloxone (0.1 mg/kg) and L-NAME (0.3, 0.5 or 1 mg/kg) showed additive effects in inhibiting both phases of morphine effects. These results support the involvement of L-arginine/nitric oxide pathway in the modulation of seizure threshold by morphine.

Aspartic acid in the arcuate nucleus attenuates the depressive effects of naloxone on ventilation

Ventilation, oxygen consumption, the ventilatory equivalent for oxygen, and ventilatory responses to hypoxia and to hypercapnia were evaluated in conscious male rats who received each of four treatments: (1) microinjection of artificial cerebrospinal fluid (aCSF) into the arcuate nucleus and subcutaneously saline (CS); (2) aspartic acid into the arcuate nucleus and saline subcutaneously (AS); (3) aCSF into the arcuate nucleus and naloxone subcutaneously (CN); and (4) aspartic acid into the arcuate nucleus and naloxone subcutaneously (AN). Rats treated with CN exhibited a depression of ventilation, ventilatory equivalent, ventilatory response to hypercapnia, and tidal volume response to hypoxia and to hypercapnia. AS had no effect on any parameters. Administration of both aspartic acid and naloxone attenuated all the effects of CN except the depression of minute ventilation in response to hypercapnia. Therefore the naloxone (a mu opioid receptor antagonist) induced a depression of ventilation that was attenuated by aspartic acid acting on N-methyl-D-aspartic acid receptors in the arcuate nucleus.

Medial septal injection of naloxone elevates acetylcholine release in the hippocampus and induces behavioral seizures in rats

The effects of injections of naloxone, a universal opioid receptor antagonist, into the medial septal nucleus on hippocampal acetylcholine (ACh) release and behavior were investigated in freely moving rats by means of the microdialysis method. The injection of naloxone (2, 10 and 20 micrograms) produced a marked increase in hippocampal ACh release in a dose-dependent manner. These effects of naloxone were reversed by the post-injection of [D-Ala2, N-Me-Phe4, Gly-ol]-enkephalin (DAGO; 10 micrograms), an opioid mu receptor agonist. Furthermore, basal release of hippocampal ACh was significantly reduced by the injection of DAGO alone. It was also found that rats given an injection of naloxone showed an increase in motor activity and occasionally exhibited behavioral seizures. These effects of naloxone were also reversed by the post-injection of DAGO. The present results suggest that endogenous opioids ionically inhibit the activity of septo-hippocampal cholinergic neurons via mediation of mu opioid receptors in the medial septal nucleus. They also suggest that endogenous opioids modulate the incidence of seizures, at least in part, through opioid mu receptors in the medial septal nucleus.

Cholecystokinin potentiates morphine anticonvulsant action through both CCK-A and CCK-B receptors

Recent studies have suggested that cholecystokinin may have a role in modulating the effects of the endogenous opioid system in physiological functions such as thermoregulation and pain control. However, the possible interaction of cholecystokinin and morphine in epileptogenesis is unknown. We studied the effect of subcutaneous morphine and intracerebroventricularly administered cholecystokinin octapeptide sulphate ester and receptor antagonists CCK-A (MK 329) and CCK-B (L 365,260) on seizures provoked by maximal electroshock in male Sprague-Dawley rats. Seizures were induced through electrode-gel-coated ear clip electrodes by a high voltage, high internal resistance constant current generator, 30 minutes after morphine administration and 10 minutes after cholecystokinin-8-SE, CCK-A and CCK-B infusion. Morphine decreased the length of the tonic component of the seizure and cholecystokinin potentiated this decrease. Cholecystokinin antagonists blocked the effects of both cholecystokinin and morphine. The results suggest that cholecystokinin acts as an endogenous agonist with opioids in the regulation of seizure susceptibility through both CCK-A and B receptors and may be responsible for part of the anticonvulsant action of morphine.

Dermorphin-induced hyperexcitability in hippocampal CA3 and CA1 in vitro

Dermorphin, a specific mu 1-opioid receptor agonist, has been studied for its effects on the physiology of the rat hippocampal slice. Population responses in CA3 to threshold levels of stimulation from both the Schaffer collaterals and mossy fibers were markedly enhanced in the presence of 50-100 nM dermorphin, while CA1 responses to threshold Schaffer collateral stimulation were less effected. Responses at higher stimulus levels than threshold were negligibly responsive to dermorphin, although at 500 nM dermorphin all responses became epileptiform and in some slices spontaneous bursting erupted. [L-Ala2]Dermorphin, a biologically inactive dermorphin analogue, did not increase response amplitudes nor evoke epilepsy in the slice. 5 microM naloxone blocked the effect of dermorphin on Schaffer collateral and mossy fiber-evoked responses, though less effectively in the latter case. These data provide in vitro evidence to support in vivo observations that excessive mu-opioid receptor activation can be proconvulsant in the hippocampus, but that normally the receptors may function as facilitatory modulators of responses in the threshold range.

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.

Interaction of GABAergic and beta-noradrenergic drugs in the regulation of memory storage

These experiments examined the interaction of drugs affecting noradrenergic and GABAergic systems, administered post-training, in influencing retention of an inhibitory avoidance response. Male CD1 mice (23-28 g) were trained in an inhibitory avoidance task, given immediate post-training ip injections of saline or GABAergic and adrenergic drugs administered either alone or concurrently. Retention was tested 48 h later. In agreement with extensive previous evidence, the GABAergic antagonist bicuculline (0.3, 1.0, or 3.0 mg/kg) produced dose-dependent (inverted-U) enhancement of retention and the GABAergic agonist muscimol (1.0 mg/kg) impaired retention. The retention-enhancing effects of bicuculline were blocked by concurrent administration of the beta-nor-adrenoceptor antagonist propranolol (2.0 mg/kg). Also in agreement with previous evidence, the beta-adrenoceptor agonist clenbuterol (0.030, 0.100, or 0.300 mg/kg, ip) produced dose-dependent (inverted-U) enhancement of retention. Clenbuterol also blocked the retention-impairing effects of muscimol (1.0 mg/kg). In addition, propranolol (2.0 mg/kg) potentiated the retention impairing effects of muscimol (1.0 or 3.0 mg/kg, ip). These findings support the view that GABAergic systems modulate memory through an interaction with beta-noradrenergic mechanisms.

Neuromodulatory systems and memory storage: role of the amygdala

This article reviews findings of research examining the interaction of peripheral adrenergic systems with cholinergic, opioid peptidergic and GABAergic systems in modulating memory storage. It is well established that retention is enhanced by posttraining systemic or intra-amygdala injections of adrenergic agonists, opiate antagonists and GABAergic antagonists. These influences appear to be mediated by activation of NE receptors within the amygdala, as intra-amygdala injections of beta-adrenergic antagonists block the memory-modulating effects of hormones and drugs affecting these systems. Furthermore, these influences also appear to involve, at a subsequent step, activation of a cholinergic system: atropine blocks the memory-enhancing effects of adrenergic agonists and opiate and GABAergic antagonists and oxotremorine attenuate the memory-impairing effects of opiate agonists and GABAergic agonists. These findings suggest that the amygdala integrates the memory-modulating effects of neuromodulatory systems activated by learning experiences.

Interaction of beta-endorphin and GABAergic drugs in the regulation of memory storage

These experiments examined the interaction of beta-endorphin and GABAergic drugs, administered post-training, in influencing retention of an inhibitory avoidance response. Male CD1 mice were trained in an inhibitory avoidance task, given immediate post-training ip injections and tested 24 h later for retention. beta-Endorphin (0.5, 1.0, or 2.0 micrograms/kg) and muscimol (0.5, 1.0, or 2.0 mg/kg) produced dose-dependent impairment of retention; picrotoxin (0.25, 0.5, or 1.0 mg/kg) and bicuculline (0.1, 0.25, or 0.5 mg/kg) produced dose-dependent enhancement of retention. A low subeffective dose of muscimol (0.5 mg/kg) potentiated the retention-impairing effect of beta-endorphin (1.0 microgram/kg). In addition, concurrent administration of low, subeffective doses of bicuculline (0.1 mg/kg) or picrotoxin (0.25 mg/kg) attenuated the retention-impairing effects of beta-endorphin (1.0 microgram/kg). The findings are consistent with previous evidence indicating that beta-endorphin influences memory through an interaction with GABAergic mechanisms.

Inhibitory influence of morphinans on ictal and interictal EEG changes induced by cortical application of penicillin in rabbits: a comparative study with NMDA antagonists and pentobarbitone

The effects of dextrorphan (DX) and dextromethorphan (DM) were tested using the electroencephalogram (EEG) and behavioral effects induced by topical cortical application of penicillin in rabbits. For comparison, the influence of the NMDA antagonists, dizocilpine (MK 801) and 3-((+-(-)2-carboxypiperazine-4-yl)propyl-1-phosphonic acid (CPP), and of pentobarbitone was investigated. Intracortical injection of 500 IU of penicillin produced an EEG spiking followed by a repeated generalization of the electrical and behavioral symptoms. Within a few minutes, DX (5-15 mg/kg, IV) or pentobarbitone (5-10 mg/kg, IV) reduced dose dependently and significantly (p less than 0.01) the interictal and ictal EEG and behavioral effects elicited by cortical injection of 500 IU of penicillin. Higher doses of pentobarbitone (20 mg/kg, IV) but not of DX (20 mg/kg, IV) completely blocked the ictal behavioral and EEG effects elicited by cortical injection of 500 IU of penicillin. Within a few minutes, MK 801 (0.1-0.2 mg/kg, IV) or CPP (10-20 mg/kg, IV) reduced dose dependently and significantly (p less than 0.01) the ictal EEG and behavioral effects elicited by cortical injection of 500 IU of penicillin, while they did not affect the penicillin-induced interictal EEG changes. Higher doses of MK 801 (0.3 mg/kg, IV) completely blocked the ictal behavioral and EEG effects elicited by cortical injection of 500 IU of penicillin. Within a few minutes, DM (10-20 mg/kg, IV) blocked the behavioral effects, but failed to affect either the interictal or the ictal EEG effects induced by cortical injection of 500 IU of penicillin.(ABSTRACT TRUNCATED AT 250 WORDS)

Increases in cerebral blood flow in rat hippocampus after medial septal injection of naloxone

BACKGROUND AND PURPOSE

In a previous study, we occasionally found that the rat given naloxone in the preoptic region develops behavioral seizures. In view of knowledge that the forebrain including the medial septal nucleus provides cholinergic projections to the hippocampal formation, the present study examined the effects of naloxone injected into the medial septal nucleus on the local blood flow in the hippocampus.

METHODS

A polyurethane-coated platinum electrode with a 1-mm bare tip for measurement of blood flow and a guide cannula made of stainless steel tube for naloxone injection were implanted chronically into the brain. The cerebral blood flow was measured by the hydrogen clearance method in freely moving rats.

RESULTS

The injection of 50 micrograms naloxone caused a significant increase in hippocampal blood flow, with its peak at 20 minutes. Twenty micrograms naloxone caused a similar increase, but 10 micrograms caused only a slight increase that peaked at 30 minutes, suggesting a dose-response of naloxone effect. Hippocampal blood flow was not changed after the injection of saline into the medial septal nucleus and after the injection of naloxone into the caudate nucleus.

CONCLUSIONS

Taken together with previous findings, the results suggest that endogenous opioids exert a decreasing effect on the local blood flow in the hippocampus, probably mediated by the magnocellular cholinergic neurons projecting to the hippocampus.

Effects of cold-restraint and swim stress on convulsions induced by pentylenetetrazol and electroshock: influence of naloxone pretreatment

The influence of two stressogenic conditions, restraint at 4 degrees C for 30 min (cold-restraint stress; CRS) or swimming at 20 degrees C for 3 min (swim stress; SS), on nociception and on convulsions triggered by different agents was assessed in mice. In saline-pretreated mice CRS and SS caused analgesia (hot-plate test, 56 degrees C), delayed the onset of convulsions induced by pentylenetetrazol (PTZ, 100 mg/kg, IP) and aggravated convulsions elicited by maximal transcorneal electroshock (150 mA pulses at 60 Hz for 0.2 s). Pretreatment with naloxone (10 mg/kg, SC, 30 min prior to testing), which did not affect the responsiveness of nonstressed mice to the hot plate or to the convulsant treatments, attenuated the development of analgesia following CRS, but not SS, and further prolonged the latency to onset of PTZ-induced convulsions in both stressed groups. Thus the extent to which CRS and SS can each delay the onset of PTZ-triggered convulsion appears to be limited by activation of a proconvulsant opioid system. In contrast, naloxone pretreatment did not modify the effects of CRS or SS on the severity of electroshock-induced seizures. In conclusion, CRS and SS can each, simultaneously, exert anticonvulsant and proconvulsant influences on responsiveness to PTZ and electroshock, respectively. Also, both forms of stress can activate an opioid system modulating the onset of PTZ-induced seizures, which is distinct from that controlling nociception. These findings, together with those of other stress, convulsions and opioid systems, which depends on the characteristics of the stressogenic condition, species, convulsant agent and parameter considered.

Intracerebroventricular administration of kappa-agonists induces convulsions in mice

Intracerebroventricular (ICV) administration of kappa-agonists (PD 117302, U-50488H and U-69593) induced convulsions in a dose-related manner in mice. The dose at which 50% of animals convulsed (CD50) was in nmol ranges for all opioids. Among the opioids used, PD 117302 was the most potent convulsant. ICV administration of either vehicle alone or U-53445E, a non-kappa-opioid (+) enantiomer of U-50488H did not induce convulsions. The convulsive response of kappa-agonists was differentially susceptible for antagonism by naloxone and/or MR 2266. Collectively, these findings support the view that convulsions induced by kappa-agonists in mice involve stereospecific opioid receptor mechanisms. Furthermore, the convulsant effect of kappa-agonists could not be modified by pretreatment with MK-801, ketamine, muscimol or baclofen. It is concluded that kappa-opioid but not NMDA or GABA receptor mechanisms are involved in convulsions induced by kappa-agonists. These results are the first experimental evidence implicating stereospecific kappa-receptor mechanisms in opioid-induced convulsions in mice.

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.

An in vitro study on the hippocampal epileptogenic properties of enkephalins and enkephalinase inhibitors in rats

1. The effects of enkephalins and enkephalinase inhibitors were studied in CA1 area in rat hippocampal slices. 2. The data demonstrate a prevalent involvement of mu opiate receptors in the epileptogenic properties of enkephalins. 3. A potentiation of the mu opiate receptor-mediated epileptogenic response by enkephalinase inhibitors has been shown. 4. The results also show an inability to affect basal CA1 field potentials by inhibition of endogenous endopeptidase.

Naltrexone potentiates 4-aminopyridine seizures in the rat

The effects of a pharmacological blockade of the mu opiate receptors on the manifestation of tonic-clonic seizures were investigated in freely moving animals. 4-aminopyridine, a specific blocker of the neuronal K+ channels was used to produce generalized convulsions. After pretreatment of adult rats with 1 mg/kg naltrexone HCl, 3, 5, 7, 9, 14 mg/kg 4-aminopyridine was injected intraperitoneally, and the latencies of the symptoms generated by 4-aminopyridine were measured. Naltrexone HCl decreased these latencies and enhanced the seizures significantly. The experiments provided further evidence for the existence of a tonic anticonvulsant opioid system in the brain.

Effects of naloxone and naltrexone on memory consolidation in CD1 mice: involvement of GABAergic mechanisms

The involvement of GABAergic mechanisms in the effects exerted by the opioid antagonists naloxone and naltrexone on memory consolidation was investigated in CD1 mice tested in a one-trial inhibitory avoidance task. In a first group of experiments posttraining administration of naloxone (2.0 and 4.0 but not 1.0 mg/kg) and naltrexone (0.5 and 1.0 but not 0.25 mg/kg), as well as those of the GABA-antagonists picrotoxin (0.5 and 1.0 but not 0.25 mg/kg) and bicuculline (0.25 and 0.5 but not 0.1 mg/kg) enhanced, whereas those of the GABA-agonist muscimol (1.0 and 2.0 but not 0.5 mg/kg) impaired retention on a 24-hr test. In a second group of experiments, picrotoxin, or bicuculline, administration enhanced, while muscimol treatment attenuated the effects of naloxone and naltrexone on retention. The results suggest that naloxone and naltrexone may influence memory consolidation in CD1 mice by interacting with the GABAergic system.

Neuropharmacology of several beta-carboline derivatives and their 9-acetylated esters. In vivo versus in vitro studies in the rabbit

The effects of 3-methoxycarbonyl- (beta-CCM, Ia), 3-ethoxycarbonyl- (beta-CCE, Ic), 3-propoxycarbonyl- (PrCC, Ie), 3-N-methylcarboxamido- (FG-7142, Ig) beta-carboline and 2-acetyl-3-methoxycarbonyl-1,2-dihydro-beta-carboline (IIa) as well as of their corresponding 9-acetyl derivatives (Ib, Id, If, Ih and IIb) have been studied in rabbits. In addition, the effects of 6,7-dimethoxy-4-ethyl-3-methoxycarbonyl-beta-carboline (DMCM) have also been studied. In in vitro studies, these drugs compete with 3H-diazepam to benzodiazepine (BDZ) receptor in membrane preparations from brain cortex. The values of IC50 are in the nanomolar range without significant differences between the acetyl derivatives and their congeners only compound If shows a 10-fold decrease of the binding capacity in respect to its congener Ie. In the presence of 10(-5) M GABA, a decrease in the binding capacity for DMCM, Ia, Ic and Ig and an increase for If are observed. In vivo studies show that DMCM, Ia, Ib, IIa and IIb elicit three dose-dependent stages of electrocortical changes (trains of slow waves, trains of spike-and-wave complexes and "grand-mal" seizures). Compounds Ic, Id and Ig elicit only the first two stages. Compound Ih elicits only the first stage. While compound Ie does not affect the EEG pattern, its 9-acetyl derivative If induces changes (cortical spindles and disruption of the hippocampal theta waves) characteristic of agonist ligands of BDZ receptor. These findings confirm that the efficacy of compounds DMCM, Ia, Ic, Id, Ig and Ih as inverse agonists of BDZ receptor in the EEG paradigm parallels the reduction of their apparent binding affinity in the presence of GABA. The 9-acetylated compounds may be more inverse agonist in vivo than predicted from the in vitro findings, due to hydrolysis in the plasma.

Endogenous opioids and post-ictal increase in seizure threshold in Mongolian gerbils

In gerbils, the convulsive thresholds both for electroconvulsions and for pentetrazole-induced convulsions were increased when determined 15 min after a convulsion elicited by an air blast to the back of the animals. Contrary to a recently offered hypothesis, this sign of post-ictal depression could not be reversed by pretreatment with the morphine antagonists naloxone and naltrexone. This result speaks against a mediation of post-ictal depression by endogenous opioids.

Convulsant action of morphine, [D-Ala2, D-Leu5]-enkephalin and naloxone in the rat amygdala: electroencephalographic, morphological and behavioural sequelae

Morphine hydrochloride (25-200 nmol), [D-Ala2, D-Leu5]enkephalin (10-200 nmol) and naloxone hydrochloride (100-1000 nmol) were injected unilaterally into the rat amygdala and the following electrographic, behavioural and neuropathological responses were studied. Microinjections of low doses of morphine (25-50 nmol) resulted in behavioural alterations characterized by staring, gustatory automatisms and wet shakes, whereas higher doses additionally produced motor limbic seizures and status epilepticus. The first changes in the electroencephalogram appeared in the amygdala immediately after the administration of morphine and rapidly spread to hippocampal and cortical areas. Electrographic alterations consisted of high voltage fast activity, spiking, bursts of polyspiking, electrographic seizures and periods of postictal depression. Neuropathological analysis of frontal forebrain sections by means of light microscopy revealed widespread, seizure-related damage confined to amygdala, olfactory cortex, thalamus, hippocampal formation, neocortex and substantia nigra. Pretreatment of animals with naloxone, 2-20 mg/kg s.c., as well as simultaneous microinjection of the non-convulsant dose of naloxone, 100 nmol, with morphine, 100 nmol, into the amygdala failed to block the development of convulsant activity and seizure-related brain damage produced by the opiate. In contrast, diazepam, 10 mg/kg i.p., when administered prior to the microinjection of morphine into the amygdala, abolished the epileptogenic effects of the drug. [D-Ala2, D-Leu5]Enkephalin, 10-200 nmol, elicited electrographic and behavioural responses similar to those seen after low doses of morphine, when administered into the amygdala. High voltage fast activity, single spikes, bursts of polyspiking, electrographic seizures and periods of postictal depression were seen in the electroencephalogram, but no behavioural signs of motor limbic seizures could be detected. The only behavioural correlates of epileptiform electrographic activity were wet shakes, myoclonic head twiches and gustatory automatisms. The examination of frontal forebrain sections from rats receiving [D-Ala2, D-Leu5]enkephalin revealed no morphological changes. Pretreatment of rats with either naloxone, 2 mg/kg, or diazepam, 10 mg/kg, blocked the development of behavioural and electrographic sequelae of the peptide. Naloxone, 100-1000 nmol, when microinjected into the amygdala, produced electrographic, behavioural and morphological alterations resembling those seen after high doses of morphine.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of opiates on the parameters of seizures in rats with full amygdaloid-kindled convulsions

The effect on the parameters of seizures of opiates, administered in doses used clinically for analgesic effects, was studied in rats with full amygdaloid-kindled seizures. The largest dose of fentanyl studied (100 micrograms/kg) had a pronounced inhibitory effect on kindled seizures: severity of seizures, duration of seizures and duration of afterdischarge were significantly reduced to 36, 40 and 37% of controls, respectively, and the latency of seizures was significantly increased to 168% of untreated animals. The largest dose of pentazocine (16 mg/kg) also significantly inhibited the duration of seizures and duration of afterdischarge. Morphine (1-4 mg/kg) and meperidine (4-16 mg/kg) had a tendency to inhibit the duration of seizure and afterdischarges but did not significantly affect any of the measured parameters of seizures. Fentanyl, meperidine and pentazocine resulted in a lowering, whereas morphine caused a slight elevation, of the threshold for initiation of kindled seizures. The data suggest that fentanyl, in relatively small doses, may cause an inhibition of the intensity of behavioural and electrographic seizures but, paradoxically, an increased sensitivity to induction of seizures in rats with full amygdaloid-kindled seizures.

Electroencephalographic investigations in rabbits of drugs acting at GABA-benzodiazepine-barbiturate/picrotoxin receptors complex

This paper describes the EEG profiles, observed in rabbits, of drugs which affect GABA synaptic activity at GBB complex. Drugs which enhance GABA synaptic activity induce sedation associated with EEG synchronization. However, muscimol, THIP, GHB and baclofen induce signs of CNS stimulation (light tremors of the forelimbs, chewing, light nystagmus and hyperpnea) associated with EEG spikes. Signs of light stimulation (chewing and jerks of the head) also occur after BDZs and barbiturates, and are associated with the presence of 12-24 and 20-25 Hz waves, respectively. Drugs which reduce GABA synaptic activity (bicuculline, inverse BDZ agonists, PTZ, picrotoxin and Ro 5-3663) induce three dose-dependent stages of EEG changes: trains of slow waves, trains of spike-and-wave complexes and paroxysmal activity in the rostral encephalic structures without apparent changes of the electrical activity in the spinal cord. The first two stages are associated with a behavioral state of alert and the third stage with tonico-clonic convulsions. Among the inverse BDZ agonists, DMCM and beta-CCM elicit all three stages, whereas FG 7142 and beta-CCE induce only the first two and CGS 8216 only the first. The BDZ antagonists Ro 15-1788 and Ro 15-3505 (0.2-30 mg/kg IV) do not significantly affect the EEG pattern. However, they selectively inhibit the effects of diazepam and of the inverse BDZ agonists. In both cases, the inhibition is observed with doses as low as 0.2 mg/kg IV and leads to an EEG desynchronization. The possible involvement of the modifications of GABA synaptic activity in the etiology of both petit mal and grand mal epilepsies is discussed.

Endogenous anticonvulsant substance in rat cerebrospinal fluid after a generalized seizure

Cerebrospinal fluid taken from rats subjected to electroshock-induced seizures and injected into the cerebral ventricles of rats that had not been shocked increased the seizure threshold of the recipients. The anticonvulsant activity of the donor cerebrospinal fluid was antagonized by opioid antagonists and enhanced by peptidase inhibitors. These results suggest the existence of an endogenous anticonvulsant substance in rat cerebrospinal fluid, possibly opioid in nature, which is activated as a consequence of a seizure and which may play a critical role in postseizure inhibition.

Neuropeptides: a role as endogenous mediators or modulators of epileptic phenomena

As more small peptidergic components of the central nervous system are isolated, their role in disease states is being investigated. Several of these neuropeptides, especially the opioidlike peptides, adrenocorticotropic hormone, and some hypothalamic releasing factors, have been found to alter neuronal excitability. This finding has led to the proposal that these peptides may play a role in the pathogenesis of the epilepsies. We tested this hypothesis in a genetic model of epilepsy. At nontoxic doses, several exogenously administered peptides had anticonvulsant properties, while others were proconvulsant. The most potent anticonvulsant was the opioidlike peptide beta-endorphin. Its effect was similar to that of the opioid alkaloids. Using the potent antagonist naloxone hydrochloride to block possible endogenous opioid-like peptides, we found no effects on seizures in naive animals. Naloxone did alter postictal events, however, by partially blocking the postictal refractoriness to further seizures. We speculate that one possible role for the endogenous opioid peptides may be to limit the spread of seizures or to modulate postictal susceptibility to further seizures. Naloxone was effective in this model only after stressful situations occurred that modified the seizures and presumably induced a release of endogenous opioidlike peptides. Support for this hypothesis from other epilepsy models is discussed. Other peptidergic systems may also be active in various epileptic models, and the current understanding of their roles is reviewed.