Hippocampal kindling-induced after-discharge and behavioural depression: immediate and long-term attenuation by opiate antagonists

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.

Behaviors induced or disrupted by complex partial seizures

We reviewed the neural mechanisms underlying some postictal behaviors that are induced or disrupted by temporal lobe seizures in humans and animals. It is proposed that the psychomotor behaviors and automatisms induced by temporal lobe seizures are mediated by the nucleus accumbens. A non-convulsive hippocampal afterdischarge in rats induced an increase in locomotor activity, which was suppressed by the injection of dopamine D(2) receptor antagonist in the nucleus accumbens, and blocked by inactivation of the medial septum. In contrast, a convulsive hippocampal or amygdala seizure induced behavioral hypoactivity, perhaps by the spread of the seizure into the frontal cortex and opiate-mediated postictal depression. Mechanisms underlying postictal psychosis, memory disruption and other long-term behavioral alterations after temporal lobe seizures, are discussed. In conclusion, many of the changes of postictal behaviors observed after temporal lobe seizures in humans may be found in animals, and the basis of the behavioral change may be explained as a change in neural processing in the temporal lobe and the connecting subcortical structures.

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.

Kindling of hippocampal field CA1 impairs spatial learning and retention in the Morris water maze

We used two procedures to assess the spatial learning and memory of rats in the Morris water maze task subsequent to kindling of hippocampal field CA1: (1) seizures were kindled with stimulation of CA1 prior to training in the water maze (acquisition); and (2) maze training was imposed until performance stabilized, seizures were kindled with stimulation of CA1, and then performance in the maze was reassessed (retention). In both conditions, behavioral testing occurred 24 h after the last kindled seizure. When the effects of CA1 kindling on acquisition were tested, we found that kindling of generalized seizures with stimulation of field CA1 (kindling), but not kindling of non-convulsive or partial seizures (partial kindling), produced deficits in the water maze. When the effects of CA1 kindling on retention were tested, however, we found that kindling of either partial or generalized seizures produced deficits in the water maze. The results suggest that the processing of spatial information is vulnerable to the long-lasting changes in neural excitability associated with kindling.

Opioid peptides content in the rat brain during the ictal phase and after pentylenetetrazol-kindled rats

We determine the opioid peptide content in the rat brain during the ictal phase and postictal depression after pentylenetetrazol kindling rats. Radioimmunoassays with highly specific antisera risen for Met-enkephalin, Leu-enkephalin and octapeptide, were carried out during the ictal phase, and 15, 30 and 60 min after seizures. We always found an initial IR-Met-enkephalin decrease during the postictal depression content, followed by a reduction in IR-Leu-enkephalin and IR-octapeptide tissular concentration. We suggest a functional and differential release of the opioid peptides, during the postictal depression time-course.

Characterization of mu opioid receptor binding during amygdala kindling in rats and effects of chronic naloxone pretreatment: an autoradiographic study

Using in vitro autoradiography, mu receptor binding in rat brain was characterized at different amygdala kindling stages and in amygdaloid kindled animals pretreated chronically with naloxone. Male Sprague-Dawley rats implanted with bipolar electrodes in the right amygdala received one of the following pretreatments s.c. for 14 days via osmotic minipumps: normal saline solution, 0.5 microliters/h, or naloxone HCl, 75 micrograms/h. Two days after treatments were accomplished animals were stimulated daily. Our data showed different patterns of mu receptor binding during the normal kindling process: during stage II-III, pronounced binding increase was detected in cingulate, temporal and entorhinal cortices, anterior amygdala, caudate putamen, thalamic nuclei, ventrolateral and dorsolateral portions of central gray, substantia nigra pars compacta and pars reticulata. Twenty-four hours after the last stage V kindled seizure, enhanced binding was observed in cingulate and frontoparietal cortices, anterior amygdala, caudate putamen and ventromedial thalamic nucleus. Twenty-eight days after the last stage V kindled seizure, binding augmentation was noticed in cingulate and frontoparietal cortices, whereas decreased binding was detected in amygdala complex, substantia nigra pars reticulata, piriform, perirhinal, parietal, temporal and entorhinal cortices. Mu receptor binding in kindled rats chronically pretreated with naloxone was significantly higher in several structures when compared with control and normal kindled groups. Our data indicate different regional selective patterns of mu receptor binding during amygdala kindling which may depend on epileptogenesis and long-term changes induced by this process. In addition, even higher mu receptor binding results from chronic naloxone administration prior to kindling.

Sexual behavior enhances postictal behavioral depression in kindled rats: opioid involvement

Past research has demonstrated brain opioid and GABA release in response to ejaculation. In the present study we evaluated the potential role of these neurotransmitters in the postictal behavioral depression (PBD), after-discharge (AD) duration, and seizure intensity in rats kindled in the medial preoptic area (MPOA) and amygdala (AMG). The PBD, the AD duration and the seizure intensity were measured after a standard kindling stimulus and after a standard kindling stimulus applied 2 min after ejaculation. The PBD was significantly increased when the animals were stimulated 2 min after ejaculation. This increase was found in MPOA- but not in AMG-kindled rats. Ejaculation had no effect on AD duration or seizure intensity. Naloxone administration before the initiation of sexual behavior completely blocked the increase in PBD in MPOA-kindled rats. It is suggested, by indirect evidence, that opioid release during sexual behavior is added to the release associated with kindled seizures, increasing the duration of the PBD. Since sexual behavior lacked effect on AD duration or seizure intensity, no evidence could be found suggesting that functionally relevant amounts of GABA are released during this behavior.

Effects of chronic naloxone pretreatment on amygdaloid kindling in rats

Effects of chronic naloxone pretreatment (75 or 270 micrograms/h for 14 days) on the development of amygdaloid kindling in rats were evaluated. The acquisition of seizure activity was modified in the naloxone pretreated animals, depending on the nucleus stimulated: facilitation of stages IV and V occurred in 37%, variability of electrographic and behavioral responses to electrical stimulation during the kindling development in 33%, and facilitation of stages IV and V followed by long periods of seizure suppression in 29%. Enhancement of postictal seizure suppression during a recycling paradigm was observed in all the naloxone pretreated rats. It was concluded that the chronic administration of naloxone (known to induce opioid binding upregulation and supersensitivity), in association with the enduring changes in opioid mechanisms provoked by kindled seizures, were responsible for the facilitation and suppression of epileptic activity. These findings support bidirectional modulatory effects of opioid peptides on epileptic seizures as well as the view that epileptic seizures can induce enduring alterations in opioid mechanisms.

Endogenous opiates: 1988

This paper is the eleventh installment in our annual review of the research during the past year involving the endogenous opiate system. It is concerned with nonanalgesic and behavioral studies of the opiate peptides that were published during 1988. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic functions; mental illness; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical activity; locomotor activity; sex, pregnancy, and development; immunology and cancer; and other behavior.

Deep prepyriform cortex kindling differentially alters the levels of prodynorphin mRNA in rat hippocampus and striatum

The effect of deep prepyriform cortex (DPC) kindling on the levels of prodynorphin mRNA (DYN mRNA) in rat hippocampus and striatum was examined under two different stimulation paradigms. Electrical stimulations were delivered to rats twice per day (slow kindling) or once every hour (fast kindling) until two consecutive stage 5 kindled seizures occurred. Animals were decapitated 24 h after reaching the second stage 5 seizure, and DYN mRNA levels in the brain were determined by RNA blot analysis. In the slow kindling model, the level of DYN mRNA in the hippocampus was reduced by 57%, whereas the level of striatal DYN mRNA was increased by 34% compared to sham-operated controls. Fast kindling induced a similar decrease in the DYN mRNA level in the hippocampus, but did not alter that in the striatum. These results, together with the previous report that kindling decreased dynorphin A(1-8) level in the hippocampus, suggest that electrical kindling decreases the biosynthesis of dynorphin peptides in the hippocampus and, in the slow DPC kindling model, also increases the gene expression of dynorphin in the striatum.