Amygdala kindling modifies extracellular opioid peptide content in rat hippocampus measured by microdialysis

The Kappa Opioid Receptor System in Temporal Lobe Epilepsy

Temporal lobe epilepsy is considered to be one of the most common and severe forms of focal epilepsies. Patients frequently develop cognitive deficits and emotional blunting along progression of the disease. The high incidence of refractoriness to antiepileptic drugs and a frequent lack of admissibility to surgery pose an unmet medical challenge. In the urgent quest for novel treatment strategies, neuropeptides and their receptors are interesting candidates. However, their therapeutic potential has not yet been fully exploited. This chapter focuses on the functional role of the dynorphins (Dyns) and the kappa opioid receptor (KOR) system in temporal lobe epilepsy and the hippocampus.Genetic polymorphisms in the prepro-dynorphin (pDyn) gene causing lower levels of Dyns in humans and pDyn gene knockout in mice increase the risk to develop epilepsy. This suggests a role of Dyns and KOR as modulators of neuronal excitability. Indeed, KOR agonists induce inhibition of presynaptic neurotransmitter release, as well as postsynaptic hyperpolarization in glutamatergic neurons, both producing anticonvulsant effects.The development of new approaches to modulate the complex KOR signalling cascade (e.g. biased agonism and gene therapy) opens up new exciting therapeutic opportunities with regard to seizure control and epilepsy. Potential adverse side effects of KOR agonists may be minimized through functional selectivity or locally restricted treatment. Preclinical data suggest a high potential of such approaches to control seizures.

Generalized seizures evoked by nucleus accumbens stimulation induced an opiate-mediated suppression of psychosis relevant behaviors

We have previously demonstrated that kindling of the nucleus accumbens (NAc) induced psychosis relevant behaviors only after one, but not after five, stage-5 seizures, suggesting that five stage-5 NAc-evoked seizures antagonized psychosis relevant behaviors in rats. We hypothesized that brain opioid receptors are responsible for seizure-induced reduction of psychosis relevant behaviors in NAc kindled rats. Rats received NAc kindling until a stage-4 seizure was induced, after which naloxone, a non-specific opioid receptor antagonist, at dose of 1 or 10 mg/kg i.p., or saline (0.3 mL) i.p., was injected 15 min before each kindled seizure. Duration of afterdischarge (AD) was not significantly different among naloxone- and saline-treated groups. However, duration of postictal behavioral depression induced by a stage-5 seizure was significantly shorter in 10 mg/kg naloxone-treated than saline-treated rats, for long (>36 s) AD duration. When tested 3-4 days after five stage-5 seizures, 10 mg/kg naloxone-treated rats, as compared to saline-treated rats, showed a statistically significant loss of gating of hippocampal auditory evoked potentials, and significant reduction of startle response amplitude, but non-significant differences in prepulse inhibition and methamphetamine-induced locomotion. It is inferred that stage-5 seizures, by releasing endogenous opiates, contribute to postictal behavioral depression, and some long-term seizure-induced antipsychotic effects.

On-Column Dimethylation with Capillary Liquid Chromatography-Tandem Mass Spectrometry for Online Determination of Neuropeptides in Rat Brain Microdialysate

We have developed a method for online collection and quantitation of neuropeptides in rat brain microdialysates using on-column dimethylation with capillary liquid chromatography-tandem mass spectrometry (cLC-MS2). This method addresses a number of the challenges of quantifying neuropeptides with cLC-MS. It is also a completely automated and robust method for the preparation of stable isotope labeled-peptide internal standards to correct for matrix effects and thus ensure accurate quantitation. Originally developed for tissue-derived proteomics samples ( Raijmakers et al. Mol. Cell. Proteomics 2008 , 7 , 1755 - 1762 ), the efficacy of on-column dimethylation for native peptides in microdialysate has not been demonstrated until now. We have modified the process to make it more amenable to the time scale of microdialysis sampling and to reduce the accumulation of nonvolatile contaminants on the column and, thus, loss of sensitivity. By decreasing labeling time, we have a temporal resolution of 1 h from sample loading to elution and our peptide detection limits are in the low pM range for 5 μL injections of microdialysate. We have demonstrated the effectiveness of this method by quantifying basal and potassium stimulated concentrations of the neuropeptides leu-enkephalin and met-enkephalin in the rat hippocampus. To our knowledge, this is the first report of quantitation of these peptides in the hippocampus using MS.

The Opioid System in Temporal Lobe Epilepsy: Functional Role and Therapeutic Potential

Temporal lobe epilepsy is considered to be one of the most common and severe forms of focal epilepsies. Patients often develop cognitive deficits and emotional blunting along the progression of the disease. The high incidence of resistance to antiepileptic drugs and a frequent lack of admissibility to surgery poses an unmet medical challenge. In the urgent quest of novel treatment strategies, neuropeptides are interesting candidates, however, their therapeutic potential has not yet been exploited. This review focuses on the functional role of the endogenous opioid system with respect to temporal lobe epilepsy, specifically in the hippocampus. The role of dynorphins and kappa opioid receptors (KOPr) as modulators of neuronal excitability is well understood: both the reduced release of glutamate as well of postsynaptic hyperpolarization were shown in glutamatergic neurons. In line with this, low levels of dynorphin in humans and mice increase the risk of epilepsy development. The role of enkephalins is not understood so well. On one hand, some agonists of the delta opioid receptors (DOPr) display pro-convulsant properties probably through inhibition of GABAergic interneurons. On the other hand, enkephalins play a neuro-protective role under hypoxic or anoxic conditions, most probably through positive effects on mitochondrial function. Despite the supposed absence of endorphins in the hippocampus, exogenous activation of the mu opioid receptors (MOPr) induces pro-convulsant effects. Recently-expanded knowledge of the complex ways opioid receptors ligands elicit their effects (including biased agonism, mixed binding, and opioid receptor heteromers), opens up exciting new therapeutic potentials with regards to seizures and epilepsy. Potential adverse side effects of KOPr agonists may be minimized through functional selectivity. Preclinical data suggest a high potential of such compounds to control seizures, with a strong predictive validity toward human patients. The discovery of DOPr-agonists without proconvulsant potential stimulates the research on the therapeutic use of neuroprotective potential of the enkephalin/DOPr system.

Dynamic measurement of extracellular opioid activity: status quo, challenges, and significance in rewarded behaviors

Opioid peptides are the endogenous ligands of opioid receptors, which are also the molecular target of naturally occurring and synthetic opiates, such as morphine and heroin. Since their discovery in the 1970s, opioid peptides, which are found widely throughout the central nervous system and the periphery, have been intensely studied because of their involvement in pain and pleasure. Over the years, our understanding of opioid peptides has widened to cover a multitude of functions, including learning and memory, affective state, gastrointestinal transit, feeding, immune function, and metabolism. Unsurprisingly, aberrant opioid activity is implicated in numerous pathologies, including drug addiction, overeating, pain, depression, and obesity. To date, virtually all preclinical and clinical studies aimed at understanding the function of endogenous opioids have relied upon manipulating endogenous opioid fluxes using opioid receptor ligands or genetic manipulations of opioid receptors and endogenous opioids. Difficulties in directly monitoring endogenous opioid fluxes, particularly in the central nervous system, have presented a major obstacle to fully understanding endogenous opioid function. This review summarizes these challenges and offers suggestions for future goals while focusing on the neurobiology of reward, specifically drawing attention to studies that have succeeded in dynamically measuring opioid peptides.

Neuropeptides as targets for the development of anticonvulsant drugs

Epilepsy is a common neurological disorder characterized by recurrent seizures. These seizures are due to abnormal excessive and synchronous neuronal activity in the brain caused by a disruption of the delicate balance between excitation and inhibition. Neuropeptides can contribute to such misbalance by modulating the effect of classical excitatory and inhibitory neurotransmitters. In this review, we discuss 21 different neuropeptides that have been linked to seizure disorders. These neuropeptides show an aberrant expression and/or release in animal seizure models and/or epilepsy patients. Many of these endogenous peptides, like adrenocorticotropic hormone, angiotensin, cholecystokinin, cortistatin, dynorphin, galanin, ghrelin, neuropeptide Y, neurotensin, somatostatin, and thyrotropin-releasing hormone, are able to suppress seizures in the brain. Other neuropeptides, such as arginine-vasopressine peptide, corticotropin-releasing hormone, enkephalin, β-endorphin, pituitary adenylate cyclase-activating polypeptide, and tachykinins have proconvulsive properties. For oxytocin and melanin-concentrating hormone both pro- and anticonvulsive effects have been reported, and this seems to be dose or time dependent. All these neuropeptides and their receptors are interesting targets for the development of new antiepileptic drugs. Other neuropeptides such as nesfatin-1 and vasoactive intestinal peptide have been less studied in this field; however, as nesfatin-1 levels change over the course of epilepsy, this can be considered as an interesting marker to diagnose patients who have suffered a recent epileptic seizure.

Activation of κ opioid receptors increases intrinsic excitability of dentate gyrus granule cells

The dentate gyrus of the hippocampus is thought to control information flow into the rest of the hippocampus. Under pathological conditions, such as epilepsy, this protective feature is circumvented and uninhibited activity flows throughout the hippocampus. Many factors can modulate excitability of the dentate gyrus and ultimately, the hippocampus. It is therefore of critical importance to understand the mechanisms involved in regulating excitability in the dentate gyrus. Dynorphin, the endogenous ligand for the kappa (κ) opioid receptor (KOR), is thought to be involved in neuromodulation in the dentate gyrus. Both dynorphin and its receptor are widely expressed in the dentate gyrus and have been implicated in epilepsy and other complex behaviours such as stress-induced deficits in learning and stress-induced depression-like behaviours. Administration of KOR agonists can prevent both the behavioural and electroencephalographic measures of seizures in several different models of epilepsy. Antagonism of the KORs also prevents stress-induced behaviours. This evidence suggests the KORs as possible therapeutic targets for various pathological conditions. In addition, KOR agonists prevent the induction of LTP. Although there are several mechanisms through which dynorphin could mediate these effects, no studies to date investigated the effects of KOR activation on intrinsic membrane properties and cell excitability. We used whole-cell, patch-clamp recordings from acute mouse hippocampus slices to investigate the effect of KOR activation on dentate gyrus granule cell excitability. The agonist U69,593 (U6, 1 μM) resulted in a lower spike threshold, a decreased latency to first spike, an increased spike half-width, and an overall increase in spike number with current injections ranging from 15 to 45 pA. There was also a reduction in the interspike interval (ISI) both early and late in the spike train, with no change in membrane potential or input resistance. Preincubation of the slice with the selective KOR antagonist, nor-binalthorphimine (BNI, 1 μM) inhibited the effect of U6 on the latency to first spike and spike half-width suggesting that these effects are mediated through KORs. The inclusion of GDP-βS (1 mM) in the recording pipette prevented all of the U6 effects, suggesting that all effects are mediated via a G-protein-dependent mechanism. Inclusion of the A-type K+ current blocker, 4-aminopyridine (4-AP, 5 mM) in the pipette also antagonised the effects of U6. Kv4.2 is one of the channel α subunits thought to be responsible for carrying the A-type K+ current. Incubation of hippocampus slices with U6 resulted in a decrease in the Kv4.2 subunit protein at the cell surface. These results are consistent with an increase in cell excitability in response to KOR activation and may reflect new possibilities for additional opioid functions.

Mu opioid receptor mRNA expression, binding, and functional coupling to G-proteins in human epileptic hippocampus

Mu opioid receptors (MOR) are known to be involved in seizure activity. The main goal of the present study was to characterize the MOR mRNA expression, binding, as well as G protein activation mediated by these receptors in epileptic hippocampus of patients with pharmacoresistant mesial temporal lobe epilepsy (TLE). In contrast with autopsy samples, hippocampus obtained from patients with mesial TLE demonstrated enhanced MOR mRNA expression (116%). Saturation binding experiments revealed significantly higher (60%) B(max) values for the mesial TLE group, whereas the K(d) values were not statistically different. Although mesial TLE group demonstrated high levels of basal binding for the G proteins (136%), DAMGO-stimulated [(35)S]GTPγS binding did not demonstrate significant alterations. In conclusion, our present data provide strong evidence that the epileptic hippocampus of patients with pharmacoresistant mesial TLE presents significant alterations in MOR. Such changes may represent adaptive mechanisms to compensate for other as yet unknown alterations.

Review of recent advances in analytical techniques for the determination of neurotransmitters

Methods and advances for monitoring neurotransmitters in vivo or for tissue analysis of neurotransmitters over the last five years are reviewed. The review is organized primarily by neurotransmitter type. Transmitter and related compounds may be monitored by either in vivo sampling coupled to analytical methods or implanted sensors. Sampling is primarily performed using microdialysis, but low-flow push-pull perfusion may offer advantages of spatial resolution while minimizing the tissue disruption associated with higher flow rates. Analytical techniques coupled to these sampling methods include liquid chromatography, capillary electrophoresis, enzyme assays, sensors, and mass spectrometry. Methods for the detection of amino acid, monoamine, neuropeptide, acetylcholine, nucleoside, and soluble gas neurotransmitters have been developed and improved upon. Advances in the speed and sensitivity of these methods have enabled improvements in temporal resolution and increased the number of compounds detectable. Similar advances have enabled improved detection at tissue samples, with a substantial emphasis on single cell and other small samples. Sensors provide excellent temporal and spatial resolution for in vivo monitoring. Advances in application to catecholamines, indoleamines, and amino acids have been prominent. Improvements in stability, sensitivity, and selectivity of the sensors have been of paramount interest.

30 years of dynorphins--new insights on their functions in neuropsychiatric diseases

Since the first description of their opioid properties three decades ago, dynorphins have increasingly been thought to play a regulatory role in numerous functional pathways of the brain. Dynorphins are members of the opioid peptide family and preferentially bind to kappa opioid receptors. In line with their localization in the hippocampus, amygdala, hypothalamus, striatum and spinal cord, their functions are related to learning and memory, emotional control, stress response and pain. Pathophysiological mechanisms that may involve dynorphins/kappa opioid receptors include epilepsy, addiction, depression and schizophrenia. Most of these functions were proposed in the 1980s and 1990s following histochemical, pharmacological and electrophysiological experiments using kappa receptor-specific or general opioid receptor agonists and antagonists in animal models. However, at that time, we had little information on the functional relevance of endogenous dynorphins. This was mainly due to the complexity of the opioid system. Besides actions of peptides from all three classical opioid precursors (proenkephalin, prodynorphin, proopiomelanocortin) on the three classical opioid receptors (delta, mu and kappa), dynorphins were also shown to exert non-opioid effects mainly through direct effects on NMDA receptors. Moreover, discrepancies between the distribution of opioid receptor binding sites and dynorphin immunoreactivity contributed to the difficulties in interpretation. In recent years, the generation of prodynorphin- and opioid receptor-deficient mice has provided the tools to investigate open questions on network effects of endogenous dynorphins. This article examines the physiological, pathophysiological and pharmacological implications of dynorphins in the light of new insights in part obtained from genetically modified animals.

Stage-specific modulation of neprilysin and aminopeptidase N in the limbic system during kindling progression

Aminopeptidase N (APN) and neprilysin (NEP) inactivate neuropeptides released into the brain extracellular fluid. We previously showed that the expression of pyroglutamyl peptidase II (PPII), the TRH degrading ecto-enzyme, is regulated in rat brain by amygdaline kindling, a paradigm that activates neuronal pathways in the limbic system increasing the expression of several neuropeptides including TRH and opioids. To understand the specificity of this phenomenon, we studied APN and NEP expression in brains of partially or fully kindled rats (stage II and V), sacrificed 6 h after last stimulus, compared with sham-operated animals. In situ hybridization analysis of NEP mRNA levels showed decreased expression at stage II in CA1, CA2, olfactory tubercle and medial mammillary nucleus, and increased at stage V in CA1 and CA2 cells. These changes were specific for the ipsilateral side. APN mRNA levels, semi-quantified by RT-PCR, were decreased at stage II and increased at stage V, in frontal cortex-olfactory tubercle, and hippocampus. NEP and APN enzymatic activities, determined by fluorometric assays, followed similar variations to their respective mRNA levels. The coordinated changes (in some regions) of NEP and APN expression were opposite to those previously observed for PPII mRNA and activity levels in limbic regions. These results demonstrate that expression of ectopeptidases can be regulated when peptide neurons are activated and, that regulation is enzyme-, region-, and stage-specific.

Levels of vancomycin in the cerebral interstitial fluid after severe head injury

OBJECTIVE

To evaluate the concentrations of vancomycin in the cerebral interstitial fluid after intravenous administration by multiple boli.

DESIGN

Prospective non randomized study

SETTING

University hospital general ICU.

PATIENTS

Four patients undergone to craniotomy for evacuation of cerebral posttraumatic hemorrhage, who developed pneumonia 4-7 days from the injury

METHODS

Two microdialysis catheters were placed in each patient: one in the edematous brain surrounding the focal lesion and one in the subcutaneous tissue of abdomen. Levels of vancomycin were measured in serum and in the microdialysates samples.

RESULTS

Plasmatic concentration of 10-15 microg/ml in the trough level was obtained after four administrations of vancomycin. Levels of vancomycin in the subcutaneous tissue was above minimum inhibitory concentrations in all patients after the second administration. Mean serum/brain ratio was 8%. Cerebral interstitial concentration of vancomycin was never above minimum inhibitory concentrations; its maximum value was 1.2 microg/ml.

CONCLUSIONS

In edematous brain close to a posttraumatic hemorrhage the levels of vancomycin do not differ from that in healthy subjects. At these plasmatic concentrations cerebral interstitial levels of vancomycin were insufficient while subcutaneous interstitial levels were effective for clinical use.

Kainate seizures increase nociceptin/orphanin FQ release in the rat hippocampus and thalamus: a microdialysis study

The neuropeptide nociceptin/orphanin FQ (N/OFQ) has been suggested to play a facilitatory role in kainate seizure expression. Furthermore, mRNA levels for the N/OFQ precursor are increased following kainate seizures, while its receptor (NOP) density is decreased. These data suggest increased N/OFQ release. To obtain direct evidence that this is the case, we have developed a microdialysis technique, coupled with a sensitive radioimmunoassay, that allows measurement of N/OFQ release from the hippocampus and thalamus of awake, freely moving animals. In both these brain areas, the spontaneous N/OFQ efflux decreased by approximately 50% and 65% when Ca2+ was omitted and when tetrodotoxin was added to the perfusion medium, respectively. Perfusion of the dialysis probe with high K+ increased N/OFQ release (approximately threefold) in a Ca2+-dependent and tetrodotoxin-sensitive manner. Kainate seizures caused a twofold increase in N/OFQ release followed, within 3 h, by a return to baseline levels. Approximately 5 h after kainate, a late increase in N/OFQ release was observed. On the following day, when animals were having only low grade seizures, N/OFQ release was not significantly different from normal. These phenomena were observed with similar patterns in the hippocampus and in the thalamus. The present data indicate that acute limbic seizures are associated with increased N/OFQ release, which may prime the molecular changes described above, i.e. cause down-regulation of NOP receptors and activation of N/OFQ biosynthesis.

Involvement of the neuropeptide nociceptin/orphanin FQ in kainate seizures

The neuropeptide nociceptin/orphanin FQ (N/OFQ) has been shown to modulate neuronal excitability and neurotransmitter release. Previous studies indicate that the mRNA levels for the N/OFQ precursor (proN/OFQ) are increased after seizures. However, it is unclear whether N/OFQ plays a role in seizure expression. Therefore, (1) we analyzed proN/OFQ mRNA levels and NOP (the N/OFQ receptor) mRNA levels and receptor density in the kainate model of epilepsy, using Northern blot analysis, in situ hybridization, and receptor binding assay, and (2) we examined susceptibility to kainate seizure in mice treated with 1-[(3R, 4R)-1-cyclooctylmethyl-3-hydroxymethyl-4-piperidyl]-3-ethyl-1, 3-dihydro-benzimidazol-2-one (J-113397), a selective NOP receptor antagonist, and in proN/OFQ knock-out mice. After kainate administration, increased proN/OFQ gene expression was observed in the reticular nucleus of the thalamus and in the medial nucleus of the amygdala. In contrast, NOP mRNA levels and receptor density decreased in the amygdala, hippocampus, thalamus, and cortex. Mice treated with the NOP receptor antagonist J-113397 displayed reduced susceptibility to kainate-induced seizures (i.e., significant reduction of behavioral seizure scores). N/OFQ knock-out mice were less susceptible to kainate seizures compared with their wild-type littermates, in that lethality was reduced, latency to generalized seizure onset was prolonged, and behavioral seizure scores decreased. Intracerebroventricular administration of N/OFQ prevented reduced susceptibility to kainate seizures in N/OFQ knock-out mice. These data indicate that acute limbic seizures are associated with increased N/OFQ release in selected areas, causing downregulation of NOP receptors and activation of N/OFQ biosynthesis, and support the notion that the N/OFQ-NOP system plays a facilitatory role in kainate seizure expression.

Effect of transient hippocampal inhibition on amygdaloid kindled seizures and amygdaloid kindling rate

In this study the effect of transient inhibition of the CA1 region of the dorsal hippocampus by lidocaine on amygdala kindling rate and amygdaloid kindled seizures was investigated. In experiment 1, rats were divided into four groups. In group 1, animals were implanted only with a tripolar electrode into the amygdala but in groups 2-4, two guide cannulae were also implanted into the CA1 regions of the dorsal hippocampi. Animals were stimulated daily to be kindled. In groups 3 and 4, saline or 2% lidocaine (1 microl/2 min) was also injected respectively into the hippocampus, 5 min before each stimulation. Results obtained showed that amygdala kindling rate and the number of stimulations to receive from stage 4 to stage 5 seizure were significantly increased in group 4. In experiment 2, lidocaine (1% and 2%) was infused (1 microl/2 min) into the hippocampus of amygdala kindled rats bilaterally and animals were stimulated at 5, 15 and 30 min after drug injection. Twenty four h before lidocaine injection, saline was also infused (1 microl/2 min) into the hippocampus as control. Obtained results showed that afterdischarge duration was reduced 5 min after lidocaine (1% and 2%) injection. Stage 5 seizure duration was also decreased 5 and 15 min after 2% lidocaine. Thus, it may be suggested that in amygdala kindling, activation of the hippocampal CA1 region has a role in seizure acquisition and seizure severity so that inhibition of this region results in decreasing of seizure severity and retards amygdala kindling rate.

Analytical considerations for microdialysis sampling

Adaptations in microdialysis probe designs have made it possible to obtain samples from the extracellular fluid of a variety of tissues with high temporal resolution. The resulting small volume samples, often with low concentration of the analyte(s) of interest, present a particular challenge to the analytical system. Rapid separations can be coupled on-line with microdialysis to provide near real-time data. By combining microdialysis sampling with a liquid chromatographic or capillary electrophoretic separation and a highly sensitive detection method, a separation-based sensor can be developed. Such sensors have been applied to the investigation of drug entities as well as to study endogenous analytes.

Subregional pathology of the amygdala complex and entorhinal region in surgical specimens from patients with pharmacoresistant temporal lobe epilepsy

The hippocampus, amygdala complex, and entorhinal region represent anatomically linked limbic structures of the mesiotemporal lobe. Chronic seizures and mnestic deficits in patients with pharmacoresistant mesial temporal lobe epilepsy (TLE) appear to correlate with distinct patterns of histopathological alterations in these areas. The complex anatomical organization of the amygdala and entorhinal region, however, render a detailed neuropathological evaluation of surgical specimens difficult. In this study, we present a combined cytoarchitectonical, pigmentarchitectonical, myelinarchitectonical, and immunohistochemical reconstruction of the amygdala, entorhinal region, and hippocampus from surgical TLE specimens (n = 20) in order to analyze their regional and cellular patterns of pathology. Anterior mesiotemporal lobes dissected in different spatial planes were obtained from 4 autopsy control patients and used for the characterization of neuroanatomical landmarks. Lateral, basal, and granular subnuclei of the amygdala were consistently identified in the surgical specimens. Major histopathological alterations included neuronal cell loss as revealed by extracellular lipofuscin accumulation, glial satellitosis, as well as cellular and fibrillary gliosis. The regional distribution of neuropathological changes varied considerably between different subnuclei but the lateral nucleus was more often involved than basal and granular nuclei. These amygdala nuclei appeared to be more severely affected compared to the adjacent entorhinal region. In addition, patients presenting with secondary generalized tonic-clonic seizures showed significantly more damage in mesiotemporal structures. Pathological alterations in the amygdala and entorhinal region were found to be associated with Ammon's horn sclerosis in most but not all cases. Our findings reveal the amygdala as a major target for epilepsy-associated neuronal cell damage. Significant variations in the lesional pattern among patients with chronic TLE would also be compatible with different spreading pathways of epileptogenic activity within the mesial temporal lobe.

Anticonvulsant and proconvulsant effects of tramadol, its enantiomers and its M1 metabolite in the rat kindling model of epilepsy

1. The centrally acting analgesic tramadol has recently been reported to cause seizures at re-commended dosages in patients, whereas animal experiments had indicated that seizures only occur in high, toxic doses. Tramadol has a dual mechanism of action that includes weak agonistic effects at the mu-opioid receptor as well as inhibition of monoamine (serotonin, norepinephrine) re-uptake. Its major (M1) metabolite mono-O:-desmethyltramadol, which is rapidly formed in vivo, has a markedly higher affinity for mu receptors and may thus contribute to the effects of the parent compound. Furthermore, the pharmacological effects of tramadol appear to be related to the different, but complementary and interactive pharmacologies of its enantiomers. In the present study, we evaluated (+/-)-tramadol, its enantiomers, and its M1 metabolite ((+)-enantiomer) in the amygdala kindling model of epilepsy in rats. Adverse effects determined in kindled rats were compared to those in nonkindled rats. 2. At doses within the analgesic range, (+/-)-tramadol and its enantiomers induced anticonvulsant effects in kindled rats. However, at only slightly higher doses seizures occurred. With (+/-)-tramadol, generalized seizures were observed at 30 mg kg(-1) in most kindled but not in nonkindled rats. The (-)-enantiomer induced myoclonic seizures at 30 mg kg(-1) in most kindled but not in nonkindled rats, although myoclonic seizure activity was observed in some nonkindled rats at 10 or 20 mg kg(-1). Seizures were also observed after the (+)-enantiomer and the (+)-enantiomer of the M1 metabolite, but experiments with higher doses of these compounds were limited by marked respiratory depression. 3. The data demonstrate that kindling enhances the susceptibility of rats to convulsant adverse effects of tramadol and its enantiomers, indicating that a preexisting lowered seizure threshold increases the risk of tramadol-induced seizures.

Can a generalized kindling seizure induce a reward state?

The postictal behavioral depression (PBD), characterized by behavioral immobility and unresponsiveness to environmental stimuli, observed after a stage 5 kindling seizure is opioid dependent. Morphine injection prolongs while naloxone and naltrexone (opioid antagonists) reduce or eliminate PBD. Opioids have clear rewarding actions that can be easily detected by place preference conditioning (PPC). In the present study, we evaluated if the opioid release after a stage 5 kindling seizure that produces PBD could induce PPC. Male rats were kindled in the medial preoptic area (MPOA), the amygdala (AMG) or insular cortex (IC). After kindling was established their initial preference in a three-compartment chamber was determined. During conditioning, subjects received a standard kindling stimuli that evoked a stage 5 seizure. At the end of the after discharge and during the PBD the animals were placed in the non-preferred chamber for 30 min. On alternate days they were placed without stimulation in the preferred chamber. At the end of conditioning the kindled groups showed a clear change of preference. This change of preference was completely blocked by injection of naloxone. These results suggest that opioid release after a stage 5 kindling seizure can induce a positive affect of sufficient intensity and duration to induce conditioning.

Opioid peptide systems following a subconvulsant dose of pentylenetetrazol in rats

The development of epilepsy and a progressive increase in susceptibility to seizures may involve changes in inhibitory and excitatory systems from the beginning of the process. The present study was focused to analyze the opioid peptide changes induced by a chemical sub-convulsant stimulation. Experiments were carried out to determine opioid peptide release, mu receptor binding and proenkephalin expression in rat brain, as well as nociceptive responses, following the administration of a sub-convulsant dose of pentylenetetrazol (PTZ) (30 mg/kg, i.p.). Membrane binding experiments revealed reduced number of mu binding sites (Bmax) in cortex and amygdala, but not in striatum and hippocampus, an effect that was evident only 24 h, but not 28 days, after PTZ treatment. In situ hybridization experiments suggested a significant enhancement of proenkephalin mRNA expression in specific brain regions 24 h after PTZ treatment. Microdialysis combined with a universal opioid peptide radioimmunoassay revealed extracellular opioid peptide levels to be elevated in the amygdala (137%) 90 min after PTZ administration. Evaluation of nociceptive responses using the Randall-Selitto test showed an analgesic effect short term (30-90 min) after PTZ injection. Collectively, these data provide evidence for a significant activation of opioid peptide systems as a consequence of the administration of a sub-convulsant dose of PTZ. These neurochemical changes may play an important role in the progression of epileptogenesis.

Gabapentin modifies extracellular opioid peptide content in amygdala: a microdialysis study

Opioid peptide release was monitored in the amygdala and hippocampus of freely moving rats following a single oral administration of gabapentin using microdialysis. Extracellular opioid peptide levels were elevated above basal levels in the amygdala within the first 60 (54%) and 90 min (68%) after gabapentin administration. Levels returned to basal conditions 120 min following the treatment. No significant changes were detected in the hippocampus. The majority of immunoreactive material recovered from the amygdala following gabapentin administration was identified as Leu-enkephalin and Met-enkephalin by high performance liquid chromatography (HPLC) analysis. It is proposed that the enhanced opioid peptide release in the amygdala induced by gabapentin might be involved with the antiepileptic effects as well as with some adverse events produced by this drug.

Opioid peptide pharmacology and immunocytochemistry in an animal model of self-sustaining status epilepticus

In a model of self-sustaining status epilepticus induced in rats by 30 min intermittent stimulation of the perforant path through chronically implanted electrodes, a decrease in dynorphin-like immunoreactivity in the dentate gyrus and CA3 was observed 3 h and 24 h after the induction of status epilepticus. Enkephalin-like immunoreactivity decreased 3 h but not 24 h after perforant path stimulation. Injection into the hilus of the dentate gyrus 10 min prior to stimulation of the kappa-receptor agonist dynorphin-A(1-13), the delta-receptor antagonists ICI-174864 and naltrindole, as well as i.p. injection of naloxone prevented the development of status epilepticus. Perihilar administration of the delta-agonist [D-Ser2]Leu-enkephalin-Thr6 or the kappa-antagonist nor-Binaltorphimine, but not of the mu-agonist [D-Ala2,N-Me-Phe4,Gly-ol5]-Enkephalin, facilitated the establishment of self-sustaining status epilepticus. Injection into the hilus of dynorphin-A(1-13) after the end of perforant path stimulation, stopped established status epilepticus, while administration of naloxone, naltrindole and ICI-174864 were ineffective. We conclude that kappa-opioids in the hippocampus counteract initiation and maintenance of status epilepticus, while delta-opioids promote initiation, but not maintenance of seizure activity. These data are important for the understanding the mechanisms which underlie initiation and maintenance of status epilepticus and for the development of new approaches for its effective management.

The effect of pentylenetetrazol kindling on synaptic mechanisms of interacting glutamatergic and opioid system in the hippocampus of rats

Endogenous opioids modulate processes of central excitability such as long-term potentiation and electrical kindling. Little is known about the neurochemical alterations in the interaction of the glutamatergic and opioid system in the development of pentylenetetrazol (PTZ) kindling in rats. Therefore, in the present study we investigated glutamate, DAMGO and naltrindole receptor binding, receptor protein expression by Western blot and ex vivo glutamate transmitter release in PTZ kindled rats. The specific 3H-DAMGO and -naltrindole binding to hippocampal membranes displayed no significant changes in kindled rats compared to controls. In contrast, the 3H-l-glutamate binding was significantly enhanced after completion of PTZ kindling. The expression of receptor protein for glutamate as well as the naloxone- and naltrindole-induced 3H-d-aspartate release from hippocampal slices did not alter in any case as a consequence of PTZ kindling. The PTZ induced enhancement of the glutamate binding sites in the hippocampus was downregulated to control level by natrindole treatment of rats prior to each PTZ application. Furthermore, naltrindole pretreatment of rats significantly inhibited the development of seizure susceptibility. In contrast, naloxone was not able to alter the seizure activity induced by PTZ as well as the transmitter receptor binding. The results are discussed in the light of a modulating role of delta-opioid receptors in PTZ kindling.

Endogenous opiates: 1997

This paper is the twentieth installment of our annual review of research concerning the opiate system. It summarizes papers published during 1997 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; eating and drinking; alcohol; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurologic disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunologic responses; and other behaviors.