Precipitous induction of audiogenic kindling by activation of adenylyl cyclase in the amygdala

Audiogenic kindling and secondary subcortico-cortical epileptogenesis: Behavioral correlates and electrographic features

Human epilepsy is usually considered to result from cortical pathology, but animal studies show that the cortex may be secondarily involved in epileptogenesis, and cortical seizures may be triggered by extracortical mechanisms. In the audiogenic kindling model, recurrent subcortical (brainstem-driven) seizures induce secondary epileptic activation of the cortex. The present review focuses on behavioral and electrographic features of the subcortico-cortical epileptogenesis: (1) behavioral expressions of traditional and mild paradigms of audiogenic kindling produced by full-blown (generalized) and minimal (focal) audiogenic seizures, respectively; (2) electrographic manifestations of secondary epileptic activation of the cortex - cortical epileptic discharge and cortical spreading depression; and (3) persistent individual asymmetry of minimal audiogenic seizures and secondary cortical events produced by their repetition. The characteristics of audiogenic kindling suggest that this model represents a unique experimental approach to studying cortical epileptogenesis and network aspects of epilepsy. This article is part of a Special Issue entitled "Genetic and Reflex Epilepsies, Audiogenic Seizures and Strains: From Experimental Models to the Clinic".

Alcohol withdrawal in epileptic rats - Effects on postictal depression, respiration, and death

Patients with epilepsy are at risk of sudden unexpected death in epilepsy (SUDEP). The most common series of events in witnessed cases of SUDEP is a generalized convulsive seizure followed by terminal apnea. Risk factors for SUDEP include prolonged postictal depression (PID), as well as alcohol abuse. The present study examined these issues in a genetic epilepsy model that exhibits generalized convulsive audiogenic seizures (AGSz) but rarely exhibits seizure-induced death, the genetically epilepsy-prone rats (GEPR-9s). We evaluated the effect of ethanol withdrawal (ETX) in GEPR-9s on respiration patterns, duration of PID, and the incidence of seizure-induced death. Audiogenic seizures were induced in GEPR-9s and in normal Sprague-Dawley rats, which were subjected to a 4-day binge ethanol protocol, 18-24h after the last ethanol dose. Following the tonic seizures, all GEPR-9s exhibited PID, characterized by loss of the righting reflex and respiratory distress (RD), which were absent during ETX seizures in the normal rats. During ETX, GEPR-9s exhibited significant increases in the duration of PID and RD, compared with vehicle-treated GEPR-9s. A significant increase in the incidence of death following seizure in GEPR-9s subjected to ETX was observed, compared with that in vehicle-treated GEPR-9s and normal rats subjected to ETX. Death in GEPR-9s was preceded by prolonged seizures because, in part, of the emergence of post-tonic generalized clonus. These results indicate that ETX induced significant increases in the duration of PID and RD, which contributed to the greater incidence of mortality in GEPR-9s compared with that in vehicle-treated GEPR-9s and normal rats. These experiments observed an elevated risk of sudden death associated with alcohol withdrawal in a genetic epilepsy model that had previously been identified as a risk factor in human SUDEP.

Alcohol Withdrawal Increases Protein Kinase A Activity in the Rat Inferior Colliculus

BACKGROUND

Cyclic AMP-dependent protein kinase A (PKA) signaling is a key target for the action of alcohol and may therefore play a role in the pathophysiology of alcohol withdrawal seizures (AWSs). Here, we investigated the role of PKA activity with respect to increased seizure susceptibility in rats that were subjected to alcohol withdrawal.

METHODS

Adult male Sprague Dawley rats received 3 daily doses of ethanol (EtOH) (or vehicle) for 4 consecutive days. Rats were then tested for susceptibility to acoustically evoked AWSs 3, 24, and 48 hours after the last alcohol dose. In separate experiments, the inferior colliculus (IC) was collected at these same time points from rats subjected to alcohol withdrawal and control rats following alcohol withdrawal. PKA activity, catalytic Cα (PKA_Cα ) protein, regulatory RIIα (PKA_RIIα ) protein, and RIIβ (PKA_RIIβ ) protein were measured in the IC. Lastly, in situ pharmacological studies were performed to evaluate whether inhibiting PKA activity in the IC suppressed AWSs.

RESULTS

In the EtOH-treated group, AWSs were observed at the 24-hour time point, but not at the 3-hour or 48-hour time points. In the IC, PKA activity was significantly higher both 3 hours (i.e., before AWS susceptibility) and 24 hours after the last alcohol dose (when AWS susceptibility peaked) than in control rats. Consistent with these findings, protein levels of the PKA_Cα subunit were significantly increased in the IC both 3 and 24 hours after the last alcohol dose. Lastly, in situ inhibition of PKA activity within the IC suppressed AWSs.

CONCLUSIONS

The increase in PKA activity and PKA_Cα protein expression in the IC preceded the occurrence of AWSs, and inhibiting PKA activity within the IC suppressed acoustically evoked AWSs. Together, these findings suggest that altered PKA activity plays a key role in the pathogenesis of AWSs.

Long-term disease-modifying effect of the endocannabinoid agonist WIN55,212-2 in a rat model of audiogenic epilepsy

BACKGROUND

Modulation of the endocannabinoid (eCB) transmission is a promising approach to treating epilepsy. Animal models can be used to investigate this approach. Krushinsky-Molodkina (KM) rats have, genetically, audiogenic epilepsy. Moreover, in these animals, repeated induction of audiogenic seizures results in a progressive prolongation of the seizures, known as audiogenic kindling.

METHODS

The present study evaluated, in these KM rats, acute and long-term effects of a single dose of 4 mg/kg of the cannabinoid-receptor agonist WIN55,212-2.

RESULTS

Administration of the single dose of WIN55,212-2 one hour before the 4th seizure delayed the kindling process by two weeks, without any acute effect on the audiogenic seizures.

CONCLUSIONS

This result suggests that short-term potentiation of the eCB system might modify the epileptogenic disease process in patients with a progressive course of epilepsy.

The amygdala to periaqueductal gray pathway: plastic changes induced by audiogenic kindling and reversal by gabapentin

Repeated, periodic induction of AGS (AGS kindling) in GEPR-9s increases seizure duration and induces an additional generalized clonus phase [post-tonic clonus (PTC)], which involves expansion of the localized brainstem AGS network to the amygdala. The pathway between central amygdala (CeA) and ventrolateral periaqueductal gray (vlPAG) is implicated in several disorders, including pain and anxiety. This pathway is also implicated in the network of audiogenic seizures (AGS) in genetically epilepsy-prone rats (GEPR-9s). We examined AGS kindling-induced changes in vlPAG extracellular action potentials evoked by electrical stimuli in CeA in awake, behaving GEPR-9s, using chronically-implanted stimulation electrodes in CeA and microwire recording electrodes in vlPAG. The effect of gabapentin, an anticonvulsant drug that is also effective in pain and anxiety disorders, on the CeA to vlPAG pathway in AGS-kindled GEPR-9s was also evaluated. Electrical stimulation in CeA evoked consistent, short latency and intensity-dependent vlPAG neuronal firing increases. However, in AGS-kindled GEPR-9s these responses showed a precipitous firing increase with increasing stimulus intensity, as compared to non-kindled GEPR-9s. Gabapentin (50mg/kg, i.p.) significantly reduced vlPAG neuronal responses to CeA stimulation to pre-AGS-kindled levels and reversibly blocked PTC in AGS-kindled GEPR-9s. These data suggest that the amygdala to vlPAG pathway may be critical in mediating the emergence of PTC during AGS kindling. The ability of gabapentin to suppress this pathway may be important for its anticonvulsant effects in AGS-kindled GEPR-9s, and this effect may contribute to gabapentin's effectiveness in anxiety and pain in which the amygdala to PAG pathway is also implicated.

Pro-epileptic effects of the cannabinoid receptor antagonist SR141716 in a model of audiogenic epilepsy

Endocannabinoid system and its CB1 receptors are suggested to provide endogeneous protection against seizures. The present study examines whether CB1 receptors contribute to resistance to seizures and kindling epileptogenesis in a model of audiogenic epilepsy. Three groups of Wistar rats were used: rats unsusceptible to audiogenic seizures, rats with acquired resistance to audiogenic seizures and rats with reproducible audiogenic running seizures. Chronic treatment with the CB1 receptor antagonist SR141716 (5 daily dosing of 30mg/kg) did not change innate resistance to audiogenic seizures in non-epileptic rats but reverted acquired seizure resistance in rats which lost their epileptic sensitivity with repeated testing. In the latter rats, audiogenic running seizures reappeared for at least two weeks after the end of treatment. In rats with reproducible seizure response, acutely, SR lengthened audiogenic seizures due to prolongation or appearance, de novo, of post-running limbic clonus without any effect on running seizure per se. This limbic component mimicked audiogenic kindling and indicated propagation of sound-induced brainstem seizure to the limbic forebrain. After chronic SR administration the incidence of the limbic clonus remained to be increased for at least two weeks. The present study supports the hypothesis about a role of CB1 receptors in endogeneous anticonvulsive mechanisms of the brain.