Differential epileptogenic potentials of selective mu and delta opiate receptor agonists

Receptor expression and signaling properties in the brain, and structural ligand motifs that contribute to delta opioid receptor agonist-induced seizures

The δ opioid receptor (δOR) is a therapeutic target for the treatment of various neurological disorders, such as migraines, chronic pain, alcohol use, and mood disorders. Relative to μ opioid receptor agonists, δOR agonists show lower abuse liability and may be potentially safer analgesic alternatives. However, currently no δOR agonists are approved for clinical use. A small number of δOR agonists reached Phase II trials, but ultimately failed to progress due to lack of efficacy. One side effect of δOR agonism that remains poorly understood is the ability of δOR agonists to produce seizures. The lack of a clear mechanism of action is partly driven by the fact that δOR agonists range in their propensity to induce seizure behavior, with multiple δOR agonists reportedly not causing seizures. There is a significant gap in our current understanding of why certain δOR agonists are more likely to induce seizures, and what signal-transduction pathway and/or brain area is engaged to produce these seizures. In this review we provide a comprehensive overview of the current state of knowledge of δOR agonist-mediated seizures. The review was structured to highlight which agonists produce seizures, which brain regions have been implicated and which signaling mediators have been examined in this behavior. Our hope is that this review will spur future studies that are carefully designed and aimed to solve the question why certain δOR agonists are seizurogenic. Obtaining such insight may expedite the development of novel δOR clinical candidates without the risk of inducing seizures. This article is part of the Special Issue on "Opioid-induced changes in addiction and pain circuits".

Exploration of beta-arrestin isoform signaling pathways in delta opioid receptor agonist-induced convulsions

The δ-opioid receptor (δOR) has been considered as a therapeutic target in multiple neurological and neuropsychiatric disorders particularly as δOR agonists are deemed safer alternatives relative to the more abuse-liable µ-opioid receptor drugs. Clinical development of δOR agonists, however, has been challenging in part due to the seizure-inducing effects of certain δOR agonists. Especially agonists that resemble the δOR-selective agonist SNC80 have well-established convulsive activity. Close inspection suggests that many of those seizurogenic δOR agonists efficaciously recruit β-arrestin, yet surprisingly, SNC80 displays enhanced seizure activity in β-arrestin 1 knockout mice. This finding led us to hypothesize that perhaps β-arrestin 1 is protective against, whereas β-arrestin 2 is detrimental for δOR-agonist-induced seizures. To investigate our hypothesis, we characterized three different δOR agonists (SNC80, ADL5859, ARM390) in cellular assays and in vivo in wild-type and β-arrestin 1 and β-arrestin 2 knockout mice for seizure activity. We also investigated downstream kinases associated with β-arrestin-dependent signal transduction. We discovered that δOR agonist-induced seizure activity strongly and positively correlates with β-arrestin 2 efficacy for the agonist, but that indirect inhibition of ERK activation using the MEK inhibitor SL327 did not inhibit seizure potency and duration. Inhibition of the PI3K/AKT/mTOR signaling with honokiol but not PQR530, attenuated SNC80 seizure duration in β-arrestin 1 knockout, but honokiol did not reduce SNC80-induced seizures in wild-type mice. Ultimately, our results indicate that β-arrestin 2 is correlated with δOR agonist-induced seizure intensity, but that global β-arrestin 1 knockout mice are a poor model system to investigate their mechanism of action.

A selective delta opioid receptor agonist SNC80, but not KNT-127, induced tremor-like behaviors via hippocampal glutamatergic system in mice

Although delta opioid receptors (DOP) are now known to play a major role in modulating chronic pain and controlling emotional processes, unfortunately, some DOP agonists, such as SNC80, reportedly produced convulsive-like behaviors manifesting as tremor-like behaviors in a preclinical study. Therefore, these induced convulsions limit the progress of the clinical development of DOP agonists. However, mechanisms underlying DOP-induced convulsant activity remain unclarified. Thus, the study aimed to elucidate mechanisms that could cause tremor-like behaviors of SNC80. These drugs were microinjected into the ventral hippocampus CA3 (vCA3), amygdala (AMY), and insular cortex (IC) of mice. In addition, we examined the extracellular glutamate levels after DOP agonist local treatment. Microinjection of SNC80 into the vCA3 increased the number of tremor-like behaviors and extracellular glutamate levels but did not cause tremor-like behaviors in mice when microinjected into IC and AMY. Pretreatment with α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainite receptor antagonist CNQX into vCA3 totally inhibited the SNC80-induced increases in tremor-like behaviors. In contrast, another DOP agonist, KNT-127, did not cause tremor-like behaviors in any of the tested brain areas. Further, the extracellular glutamate levels in the hippocampus were significantly lower in the KNT-127-treated mice than in the SNC80-treated mice. Our results showed that the administration of SNC80, but not KNT-127, into vCA3 induced tremor-like behaviors by activating glutamatergic neurons in mice. We propose that KNT-127 should be further studied clinically as a DOP agonist that is expected to have a low risk for convulsions than those resulting in antinociceptive and antidepressant effects.

NOP receptor agonist attenuates nitroglycerin-induced migraine-like symptoms in mice

Migraine is an extraordinarily prevalent and disabling headache disorder that affects one billion people worldwide. Throbbing pain is one of several migraine symptoms including sensitivity to light (photophobia), sometimes to sounds, smell and touch. The basic mechanisms underlying migraine remain inadequately understood, and current treatments (with triptans being the primary standard of care) are not well tolerated by some patients. NOP (Nociceptin OPioid) receptors, the fourth member of the opioid receptor family, are expressed in the brain and periphery with particularly high expression known to be in trigeminal ganglia (TG). The aim of our study was to further explore the involvement of the NOP receptor system in migraine. To this end, we used immunohistochemistry to examine NOP receptor distribution in TG and trigeminal nucleus caudalus (TNC) in mice, including colocalization with specific cellular markers, and used nitroglycerin (NTG) models of migraine to assess the influence of the selective NOP receptor agonist, Ro 64-6198, on NTG-induced pain (sensitivity of paw and head using von Frey filaments) and photophobia in mice. Our immunohistochemical studies with NOP-eGFP knock-in mice indicate that NOP receptors are on the majority of neurons in the TG and are also very highly expressed in the TNC. In addition, Ro 64-6198 can dose dependently block NTG-induced paw and head allodynia, an effect that is blocked by the NOP antagonist, SB-612111. Moreover, Ro 64-6198, can decrease NTG-induced light sensitivity in mice. These results suggest that NOP receptor agonists should be futher explored as treatment for migraine symptoms. This article is part of the special issue on Neuropeptides.

The mu-opioid receptor-selective peptide antagonists, antanal-1 and antanal-2, produce anticonvulsant effects in mice

The activation of the mu-opioid receptors (MOR) in the central nervous system has a proconvulsant effect and seizures are a common side effect of high doses of short acting opioids, like morphine or fentanyl. However, the correct assessment of the role of MOR blockade in the initiation and propagation of epilepsy was hampered by the lack of potent and selective MOR antagonists. In this study we aimed at characterizing the effect of MOR blockade on the seizure threshold in mice using recently developed selective antagonists antanal-1 and antanal-2 and a classical MOR antagonist, β-funaltrexamine (β-FNA). The effect of the centrally administered MOR antagonists was characterized in the maximal electroshock seizure threshold (MEST), the 6 Hz psychomotor seizure threshold and the intravenous pentylenetetrazole (PTZ) seizure threshold test in mice. The acute effect of the studied compounds on skeletal muscular strength in mice was quantified in the grip-strength test. Antanal-1 and antanal-2 (30 and 50 nmol/mouse, i.c.v.), but not β-FNA significantly increased the seizure threshold in the MEST test in mice. In the 6-Hz test, all tested MOR antagonists significantly increased the psychomotor seizure threshold and the most potent anticonvulsant effect was observed for antanal-2 (2, 10 and 30 nmol/mouse, i.c.v.). The i.c.v. administration of β-FNA (10 and 30 nmol/mouse, i.c.v.), antanal-1 and antanal-2 (both 30, 50 and 100 nmol/mouse, i.c.v.) did not produce any significant effect on PTZ seizure threshold, the generalized clonus or the forelimbs tonus. All tested compounds did not affect muscle strength, as determined in the grip strength test. Our study demonstrated that the novel MOR-selective antagonists antanal-1 and antanal-2 displayed a potent and dose-dependent anticonvulsant action involving non-GABA-ergic, but some other pathways and mechanisms in animal models of epileptic seizures. We suggest that antanals are promising drug templates for future therapeutics, which may be used in the treatment of epilepsy in humans.

Delta-opioid receptor activation prolongs respiratory motor output during oxygen-glucose deprivation in neonatal rat spinal cord in vitro

Delta opioid receptor (DOR) activation protects the adult mammalian brain during oxygen-glucose deprivation (OGD), but it is not known whether neonatal spinal motor circuits are also protected. Also, it is unclear whether the timing of spinal DOR activation relative to spinal OGD is important for neuroprotection. Thus, a split-bath in vitro neonatal rat brainstem/spinal cord preparation was used to record spontaneous respiratory motor output from cervical (C4-C5) and thoracic (T5-T6) ventral spinal roots while exposing only the spinal cord to OGD solution (0 mM glucose, bubbled with 95% N(2)/5% CO(2)) or DOR agonist drugs (DADLE, DPDPE). Spinal OGD solution application caused respiratory motor output frequency and amplitude to decrease until all activity was abolished (i.e. end-point times) after 25.9±1.4 min (cervical) and 25.2±1.4 min (thoracic). Spinal DOR activation via DPDPE (1.0 μM) prior-to and during spinal OGD increased cervical and thoracic end-point times to 35-48 min. Spinal DADLE or DPDPE (1.0 μM) application 15 min following spinal OGD onset increased cervical and thoracic end-point times to 36-45 min. Brief spinal DPDPE (1.0 μM) application for 10 min at 25 min before spinal OGD onset increased cervical and thoracic end-point times to 41-46 min. Overall, the selective DOR agonist, DPDPE, was more effective at increasing end-point times than DADLE. Naltrindole (DOR antagonist; 10 μM) pretreatment blocked DPDPE-dependent increase in end-point times, suggesting that DOR activation was required. Spinal naloxone (1.0 μM) application before and during spinal OGD also increased end-point times to 31-33 min, but end-point times were not altered by Mu opioid receptor (MOR) activation or DOR activation/MOR blockade, indicating that there are complex interactions between OGD and opioid signaling pathways. These data suggest DOR activation before, during, and after spinal OGD protects central motor networks and may provide neuroprotection during unpredictable perinatal ischemic events.

The convulsive and electroencephalographic changes produced by nonpeptidic delta-opioid agonists in rats: comparison with pentylenetetrazol

delta-Opioid agonists produce convulsions and antidepressant-like effects in rats. It has been suggested that the antidepressant-like effects are produced through a convulsant mechanism of action either through overt convulsions or nonconvulsive seizures. This study evaluated the convulsive and seizurogenic effects of nonpeptidic delta-opioid agonists at doses that previously were reported to produce antidepressant-like effects. In addition, delta-opioid agonist-induced electroencephalographic (EEG) and behavioral changes were compared with those produced by the chemical convulsant pentylenetetrazol (PTZ). For these studies, EEG changes were recorded using a telemetry system before and after injections of the delta-opioid agonists [(+)-4-[(alphaR)-alpha-[(2S,5R)-2,5-dimethyl-4-(2-propenyl)-1-piperazinyl]-(3-methoxyphenyl)methyl]-N,N-diethylbenz (SNC80) and [(+)-4-[alpha(R)-alpha-[(2S,5R)-2,5-dimethyl-4-(2-propenyl)-1-piperazinyl]-(3-hydroxyphenyl)methyl]-N,N-diethylbenzamide [(+)-BW373U86]. Acute administration of nonpeptidic delta-opioid agonists produced bilateral ictal and paroxysmal spike and/or sharp wave discharges. delta-Opioid agonists produced brief changes in EEG recordings, and tolerance rapidly developed to these effects; however, PTZ produced longer-lasting EEG changes that were exacerbated after repeated administration. Studies with antiepileptic drugs demonstrated that compounds used to treat absence epilepsy blocked the convulsive effects of nonpeptidic delta-opioid agonists. Overall, these data suggest that delta-opioid agonist-induced EEG changes are not required for the antidepressant-like effects of these compounds and that neural circuitry involved in absence epilepsy may be related to delta-opioid agonist-induced convulsions. In terms of therapeutic development, these data suggest that it may be possible to develop delta-opioid agonists devoid of convulsive properties.

Behavioral and neurobiological effects of the enkephalinase inhibitor RB101 relative to its antidepressant effects

Nonpeptidic delta-opioid receptor agonists produce antidepressant-like effects in rodents, and compounds that inhibit the breakdown of endogenous opioid peptides have antidepressant-like effects in animal models. In this study, the behavioral effects of the enkephalinase inhibitor, RB101 (N-[(R, S)-2-benzyl-3-[(S)(2-amino-4-methyl-thio)-butyldithio]-1-oxopropyl]-l-phenylalanine benzyl ester), were examined. Specifically, the effects of RB101 on convulsive activity, locomotor activity, and antidepressant-like effects in the forced swim test were studied in Sprague-Dawley rats, and the opioid receptor types mediating these effects were examined by antagonist studies. In addition, the effects of RB101 on brain-derived neurotrophic factor (BDNF) mRNA expression were evaluated in relation to its antidepressant effects. RB101 produced delta-opioid receptor-mediated antidepressant effects (32 mg/kg i.v. and 100 mg/kg i.p.) and increased locomotor activity (32 mg/kg i.v.) in rats. RB101 did not produce convulsions or seizures and did not alter BDNF mRNA expression. In conclusion, RB101 has the potential to produce antidepressant effects without convulsions.

Peptidic delta opioid receptor agonists produce antidepressant-like effects in the forced swim test and regulate BDNF mRNA expression in rats

Systemically active, nonpeptidic delta opioid receptor agonists have been shown to produce antidepressant and anxiolytic effects in animal models in rodents. In addition, delta agonists have been shown to increase expression of brain-derived neurotrophic factor (BDNF) mRNA, an effect of some antidepressants, which may be important for the clinical efficacy of antidepressant drugs. The present study examined whether a variety of peptidic delta agonists, DPDPE, JOM-13, a systemically active derivative of DPDPE, deltorphin II, and H-Dmt-Tic-NH-CH2-Bid could produce convulsions and antidepressant-like effects in the forced swim test. In addition, some of these compounds were examined for their influence on BDNF mRNA expression. All four agonists dose-dependently decreased immobility in the forced swim test, indicating an antidepressant-like effect. Only JOM-13 produced convulsions at doses required for antidepressant-like effects. In addition, DPDPE increased BDNF mRNA expression, as measured by in situ hybridization, in the frontal cortex. The antidepressant-like effect of the agonists in the forced swim test and the increase in BDNF mRNA expression produced by DPDPE were blocked by the delta antagonist naltrindole. Therefore, activation of the delta receptor by centrally administered peptidic agonists and intravenously administered JOM-13 produces behavioral antidepressant-like effects without producing convulsions, and some peptidic agonists can increase BDNF mRNA expression, however, not as consistently as the systemically active nonpeptidic agonists.

Differential behavioral tolerance to the delta-opioid agonist SNC80 ([(+)-4-[(alphaR)-alpha-[(2S,5R)-2,5-dimethyl-4-(2-propenyl)-1-piperazinyl]-(3-methoxyphenyl)methyl]-N,N-diethylbenzamide) in Sprague-Dawley rats

Nonpeptidic delta-opioid agonists produce a number of behaviors, such as antidepressant-like effects, locomotor stimulation, antinociception, and convulsions. To consider this class of compounds as potential therapeutics for humans, the effects of delta-opioid agonists after repeated administration must be evaluated. Therefore, the present study investigated the effects of repeated delta-opioid agonist, SNC80 ([(+)-4-[(alphaR)-alpha-[(2S,5R)-2,5-dimethyl-4-(2-propenyl)-1-piperazinyl]-(3-methoxyphenyl)-methyl]-N,N-diethylbenzamide), administration on its antidepressant-like effects in the forced swim test, locomotor activity, and convulsions in male Sprague-Dawley rats. Tolerance developed rapidly to the convulsive and locomotor-stimulating effects of SNC80 but not to the antidepressant-like effects. In addition, tolerance was evaluated at the level of the receptor-G protein interaction by measuring 5'-O-(3-[35S]thio)triphosphate binding in brains from rats that were pretreated with SNC80. With various exposure durations to SNC80, some brain regions demonstrated tolerance at different times, suggesting that adaptations in the delta-opioid system may occur during agonist exposure. Overall, the lack of observable tolerance to the antidepressant-like effects of SNC80 indicates that this class of compounds has potential as a novel antidepressant therapy.

Comparison of peptidic and nonpeptidic delta-opioid agonists on guanosine 5'-O-(3-[35S]thio)triphosphate ([35S]GTPgammaS) binding in brain slices from Sprague-Dawley rats

Previous studies have demonstrated that peptidic and nonpeptidic delta-opioid receptor agonists have different effects depending on the measure. For example, nonpeptidic delta-opioid agonists, but not peptidic agonists, produce convulsions in rats, and in vitro studies suggested that peptidic and nonpeptidic delta-opioid agonists might have differential mechanisms of receptor downregulation. The present study evaluated potential differences between peptidic and nonpeptidic delta-opioid agonists in their ability to activate G proteins using guanosine 5'-O-(3-[(35)S]thio)triphosphate ([(35)S]GTPgammaS) autoradiography experiments in rat brain slices. The peptidic agonist [d-Pen(2),d-Pen(5)]-enkephalin and the nonpeptidic agonist (+)BW373U86 [(+)-4-[alpha(R)-alpha-[(2S,5R)-2,5-dimethyl-4-(2-propenyl)-1-piperazinyl]-(3-hydroxyphenyl)methyl]-N,N-diethylbenzamide] demonstrated concentration-dependent increases in [(35)S]GTPgammaS binding that were attenuated by the delta-opioid antagonist naltrindole. (+)BW373U86 was more potent and efficacious than the peptidic agonist, and this difference remained consistent across brain regions where significant stimulation was observed. In addition, multiple delta-opioid compounds were evaluated for their agonist activity in this assay. These data suggested that differences between peptidic and nonpeptidic delta-opioid agonists in behavioral studies were most likely caused by differences in agonist efficacy. Finally, these data also revealed that [(35)S]GTPgammaS autoradiography could be used to compare efficacy differences among agonists across various brain regions in rat brain slices.

Behavioral effects of delta-opioid receptor agonists: potential antidepressants?

The development of selective delta-opioid receptor agonists has revealed some very intriguing behavioral properties. delta-Opioid agonists have antinociceptive, seizuregenic and convulsive properties. A number of studies have identified a novel behavioral effect of delta-opioid-receptor agonists, implicating a role for the delta-opioid receptor in depression. Early clinical experiments demonstrated that exogenously administered opioid peptides had antidepressant activity in human patients. Also, enkephalinase inhibitors, which prevent the degradation of endogenous enkephalins, produced antidepressant-like effects mediated through the delta-opioid receptor in animal models of depression. More recently, the selective non-peptidic delta-opioid agonists SNC80 and (+)BW373U86 demonstrated antidepressant-like activity in the forced swim assay in rats. These studies propose that the delta-opioid receptor may provide a new therapeutic target for treating human depression.

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.

Anticonvulsant effects of U-54494A and U-50488H in genetically epilepsy-prone rats and DBA/2 mice: a possible involvement of glycine/NMDA receptor complex

1. The effects of U-54494A and U-50488H on convulsions produced by sound have been studied in genetically epilepsy-prone DBA/2 mice and genetically epilepsy-prone rats. 2. Both compounds showed a dose-dependent anticonvulsant activity. U-54494A was less potent as an anticonvulsant than U-50488H in genetically epilepsy-prone rats and elicited a similar potency to that of U-50488H in DBA/2 mice when administered intracerebroventricularly or intraperitoneally. 3. Similar sedative and hypothermic effects were observed after the highest dose of U-54494A and U-50488H in DBA/2 mice. U-50488H seems to exhibit a greater sedative effect and to affect the rotarod test in rats much more than U-54494A. U-54494A elicited a better therapeutic index than U-50488H. 4. The anticonvulsant properties of both compounds are antagonized by high doses of naloxone and nor-binaltorphimine, a selective kappa-opioid antagonist. 5. The effects of U-50488H and U-54494A in DBA/2 mice were also antagonized by the glycine/NMDA receptor antagonist D-serine. 6. The present results suggest a possible interaction between kappa-opioid and the glycine/NMDA receptors during epileptic phenomena.

Behavioural effects of two dipeptides L-phenyl alanyl-L-arginine (Phe-L-Arg) and L-phenyl alanyl-D-arginine (Phe-D-Arg) after intracerebroventricular or intrathecal injections in mice

L-phenylalanyl-L-arginine (Phe-L-Arg) and L-phenylalanyl-D-arginine (Phe-D-Arg), dissolved in a physiological saline were injected intrathecally into the lateral brain ventricle, and their behavioural as well as analgesic effects in mice were determined. It was found that a 100 nM dose of either peptide intracerebroventricularly (i.c.v.) injected induced an increase in the locomotor activity and a convulsive effect, whereas the same dose if injected produced a significant analgesic effect. It has been concluded that Phe-L-Arg and Phe-D-Arg have significant but different central effects in mice.

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.

Involvement of multiple opiate receptors in opioid kindling

D-Tyr-Ser-Gly-Phe-Leu-Thr (DSLET), beta-endorphin, morphiceptin and morphine were microinjected at 48-h intervals into the amygdala or hippocampus of awake rats in an attempt to identify the opiate receptor types involved in opioid kindling. DSLET, beta-endorphin, morphiceptin and morphine were injected into the lateral ventricle to assess the possibility of kindling seizures by this route. The delta-receptor agonist DSLET effectively kindled convulsions when microinjected into amygdala or ventral hippocampus. The convulsions were suppressed or strongly attenuated by ICI 174,864, a specific antagonist of the delta-receptor, microinjected into the same brain site, but were not affected by ICI 174,864 administered peripherally. When microinjected into amygdala or hippocampus, beta-endorphin and morphiceptin also kindled convulsions, which were antagonized by naloxone but not by ICI 174,864. Morphine evoked EEG epileptiform activity but did not kindle convulsions from limbic brain sites. DSLET occasionally evoked epileptiform spiking and submaximal convulsions when injected into ventricle, and morphiceptin evoked epileptiform spiking only, but tolerance to these effects occurred after repetition of the injections. Thus, convulsions can be kindled by activation of either mu-, delta- or epsilon-receptors when opioids are injected directly into limbic tissue. However, the ability of these compounds to kindle seizures is markedly reduced when they are administered into ventricle. The striking differences between the present results and previous results obtained by peripheral or intraventricular administration of opioid peptides suggest that the route of administration, among other variables, is a crucial factor in assessing the epileptogenic properties of opioid peptides.

Phelorphan, an inhibitor of enzymes involved in the biodegradation of enkephalins, affected the withdrawal symptoms in chronic morphine-dependent rats

Intracerebroventricular administration of phelorphan (158 nmol/2 microliters), a blocker of dipeptidylaminopeptidase (enkephalinase B) and other enzymes involved in the enkephalin biodegradation, inhibited in chronic morphine-dependent rats, the occurrence of some of the naloxone-precipitated withdrawal symptoms. This effect of phelorphan was compared with an equimolar dose of the dipeptidyl-carboxypeptidase inhibitor (enkephalinase A), thiorphan. The results indicate that both drugs decrease some of the naloxone-precipitated withdrawal symptoms (writhing, digging, head hiding, chewing, diarrhoea and Straub tail), while others were potentiated (penile licking) or unaltered (wet dog shakes, grooming and rearing). In addition, phelorphan compared with the controls or thiorphan, pretreated animals, increased the frequency of paw tremor, head shakes, scratching, erection and ejaculation, but other symptoms were decreased (stretching) or unaltered (teeth chattering). The results are discussed in light of the differences in permeability and specificity of the two enkephalinase inhibitors. Furthermore, these data support the hypothesis that the use of enkephalinase inhibitors might be a promising way for the attenuation of the severity of the withdrawal syndrome.

Inhibition of enkephalinase activity attenuates naloxone-precipitated withdrawal symptoms

In this study we examined the effects of the enkephalinase inhibitor, thiorphan, on the naloxone-precipitated withdrawal syndrome in chronic morphine dependent rats. Intracerebroventricular administration of thiorphan (40 micrograms/2 microliter) in morphine dependent rats, inhibited the severity of the naloxone-precipitated abstinential syndrome. Administration of thiorphan (20 micrograms/0.5 microliter) in the periaqueductal grey matter of morphine dependent rats, in addition to explosive motor behaviour and ipsilateral rotation, also significantly suppressed most of the naloxone-precipitated withdrawal symptoms. It is suggested that a decreased biotransformation of endogenous opioid peptides might replace the relative shortage of morphine during withdrawal in opiate addicted subjects and attenuate the abstinence symptoms.

Studies on the effects of intrathalamically injected DADL and morphine on nociceptive thresholds and electroencephalographic activity: a thalamic delta receptor syndrome

Bilateral microinjections of DADL (D-Ala2-D-Leu5-enkephalin) and morphine were carried out in rats in a systematic fashion at histologically identified medial and lateral thalamic sites. DADL produced a dose-dependent (1.5-15.0 nmol), naloxone-reversible (1 mg/kg, i.p.) increase in the hot-plate (HP), tail-flick (TF) and catalepsy (CAT) response latencies with a predominance of activity occurring at lateral as opposed to medial thalamic sites. These effects were seen within 5 min of microinjection. At a significant number of sites, DADL precipitated convulsive seizure activity. Equimolar doses of morphine had a negligible effect on nociceptive indices and were not productive of seizures even at sites where DADL was found to be active. To further examine seizure activity, rats were prepared with bilateral frontal cortical electrodes and microinjected also at medial and lateral thalamic sites with equimolar doses of DADL and morphine (15 nmol). DADL was found to produce electrographically defined seizures unaccompanied by convulsive motor behavior (cataleptic seizures), as well as convulsive seizures. All animals in this group exhibiting analgesia and catalepsy had electrographic evidence of a seizure with markedly abnormal EEG tracings showing postictal spiking and changes in baseline frequency and amplitude. These seizures appeared to be naloxone-reversible. Morphine on the other hand was not productive of seizures, but did produce changes in electroencephalographic activity including spindle bursting, high-voltage slow-frequency activity as well as spiking. As noted, these changes were not associated with any effects on nociceptive measures.

Opiate-induced seizures: a study of mu and delta specific mechanisms

Two groups of experiments were conducted to determine if morphine- and enkephalin-induced seizures are specifically mediated by the mu and delta receptor, respectively. In the first experiments, designed to assess the ontogeny of mu- or delta-seizures, rats from 6 h to 85 days of age received implanted cortical and depth electrodes as well as an indwelling cannula in the lateral ventricle. Various amounts of the mu-receptor agonists, morphine and morphiceptin, and the delta agonists, D-Ala2-D-Leu5-enkephalin (DADL) and Tyr-D-Ser-Gly-Phe-Leu-Thr (DSLET), were then administered intracerebroventricularly (icv) with continuous EEG monitoring. The second experiments entailed use of the nonspecific opiate antagonist, naloxone, as well as the specific delta antagonist, ICI 154,129, against seizures induced by icv-administered morphine, morphiceptin, DADL, or DSLET. Both morphine and morphiceptin produced electrical seizure activity in rats as young as 5 days after birth. The drugs produced similar seizure activity in terms of electrical morphology, observed behavior, ontogeny, threshold dose, and reversibility with small doses of naloxone. In the pharmacologic experiments, icv naloxone blocked all opiate-induced seizures. ICI 154,129 blocked DSLET seizure, had little effect on enkephalin or DADL seizures, and no effect on morphine or morphiceptin seizures. These data indicate that DSLET seizures are delta-specific but that all other opiate-induced seizures studied may involve multiple opiate receptor-mediated mechanisms.

Differential electrographic patterns for specific mu- and delta-opioid peptides in rats

Cortical electroencephalographic (EEG) recordings were performed on rats after i.v. administration of morphine and specific mu- and delta-opioid peptides. DAGO (Tyr-D X Ala-Gly-N X Me X Phe-Gly-ol), the mu-selective peptide, produced repetitive paroxysmal discharges organized in a pattern analogous to that seen in tonic clonic seizures at doses which produced analgesia while DTLET (Tyr-D X Thr-Gly-Phe-Leu-Thr), the delta-selective peptide, produced 'petit-mal'-like seizures at doses which caused neither analgesia nor catatonia. It is suggested that the delta receptor is preferentially implicated in the epileptogenic spectrum of opioids.

Regional cerebral blood flow during enkephalin-induced seizures in the rat

Blood flow, determined by the radioactive microsphere technique during epileptiform seizures induced by [D-Ser2,Leu5]enkephalyl-Thr (DSLET), a specific delta-opioid receptor agonist, was examined in different areas of the brain of the rat at various time intervals. An increase in blood flow to the hippocampus and brain stem was observed 2.5 min after administration of DSLET into the left lateral ventricle. An additional increase in flow occurred in the striatum and cerebellum 2.5 min later (5 min after the injection), at which time both the neural and vascular effects of the drug were most marked. Ten minutes after the administration of the drug, cerebral blood flow in all regions except the hippocampus, returned to the respective baseline values. Since the time-course and the magnitude of functional activity and blood flow in the hippocampus showed a good correlation, it is suggested that this region of the brain may play an essential role in triggering and maintaining the seizure phenomena induced by enkephalin.

The anticonvulsant effects of DADLE are primarily mediated by activation of delta opioid receptors: interactions between delta and mu receptor antagonists

Dose-response comparisons of the ability of the selective delta antagonist ICI 154,129 (12.5-50 nmol), the nonselective antagonist naloxone (29-290 nmol), and the irreversible selective mu antagonist beta-fNA (1.3-21 nmol) to alter the threshold response to DADLE or etorphine was studied in the rat flurothyl seizure test. DADLE (35 nmol, i.c.v.) and etorphine (122 nmol/kg, s.c.) both caused increases in seizure threshold which were differentially antagonized by pretreatment (i.c.v.) with the respective antagonists. For DADLE, only ICI 154,129 and naloxone produced a dose-related blockade of the increase in seizure threshold, with ICI 154,129 being more potent than naloxone. In contrast, the anticonvulsant action of etorphine was not antagonized by ICI 154,129 (50 nmol), but was blocked by a low dose of naloxone (29 nmol) or beta-fNA (21 nmol). In addition, prior occupancy of mu-sites with beta-fNA (21 nmol) significantly diminished the abilities of either ICI 154, 129 (50 nmol) or naloxone (290 nmol) to antagonize the anticonvulsant action of DADLE. The results of this study demonstrated that the effects of DADLE to increase seizure threshold in the rat were primarily mediated by activation of a delta-opioid receptor system. Furthermore, evidence has been provided for a functional interaction between delta and mu receptors in the opioid regulation of seizure threshold.

Epileptiform effects of met-enkephalin, beta-endorphin and morphine: kindling of generalized seizures and potentiation of epileptiform effects by handling

Repeated spaced injection of small amounts of beta-endorphin or Met-enkephalin into the hippocampus or posterior amygdala of the rat led to the development of kindled generalized convulsions. Similar injection of morphine into the hippocampus or anterior amygdala resulted in epileptiform spiking followed by tolerance. The epileptiform spiking and convulsive behavior varied in a dose-related manner. Naloxone blocked or greatly attenuated the electrographic seizure and convulsive behavior. Prior kindling with beta-endorphin or Met-enkephalin significantly facilitated electrical kindling of the amygdala. Handling or conspecific threat potentiated the epileptiform spiking and convulsive behavior in some cases. The results indicate that the epileptogenic response to intracerebrally applied opioid peptides is site-specific within the rat brain, and they support the idea that endogenous opioid mechanisms may play a role in convulsive seizures. They also suggest a possible opiate-based mechanism for the stress-induced exacerbation of seizures.

Metabolic rate in different rat brain areas during seizures induced by a specific delta opiate receptor agonist

The glucose utilization during specific delta opiate agonist-induced epileptiform phenomena, determined by the [14C]2-deoxyglucose technique (2-DG), was examined in various rat brain areas at different time intervals. The peak in EEG spiking response and the most intensive 2-DG uptake occurred 5 min after intraventricular (i.v.t.) administration of the delta opiate receptor agonist. The most pronounced 2-DG uptake at this time interval can be observed in the subiculum, including the CA1 hippocampal area, frontal cortex and central amygdala. A general decrease of glucose consumption, compared to control values, is observed after 10 min, in all regions, with exception of the subiculum. Since functional activity and 2-DG uptake are correlated, we suggest that the subiculum and/or CA1 area, are probably the brain regions most involved in the enkephalin-induced epileptic phenomena.

Enkephalinase inhibition antagonizes the increased susceptibility to seizure induced by REM sleep deprivation

In order to elucidate the relationship between REM sleep and the enkephalinergic system, the effects of REM sleep deprivation (REMSD), stress and the enkephalinase inhibitor phosphoramidon on handling-induced convulsions were studied in mice. REMSD, stress and phosphoramidon (25-500 micrograms icv) increased the frequency of handling-induced convulsions (HIC) in normal mice. However, only in the last two groups were HIC antagonized by naloxone (1 mg/kg IP). In REMSD mice, phosphoramidon decreased the frequency of HIC, this effect being abolished by naloxone. The increase of neuronal excitability during REMSD is suggested to be associated with an insufficiency of the enkephalinergic system.