Molecular Substrates And Neuronal Network Subserving Audiogenic Kindling In Genetically Epilepsy-Prone Rats

Author

Srinivasan Tupal, Southern Illinois University CarbondaleFollow

Date of Award

1-1-2009

Degree Name

Doctor of Philosophy

Department

Pharmacology

First Advisor

Faingold,Carl

Abstract

Genetically epilepsy-prone rats of the severe seizure strain (GEPR-9s) are a model of generalized tonic-clonic seizures. GEPR-9s exhibit inherited susceptibility to audiogenic seizures (AGS) in response to intense acoustic stimulation. Several lines of evidence indicate that the neuronal network of AGS is primarily localized in the brainstem in GEPR-9s. The audiogenic seizure kindling (AGS kindling) process involves periodically repeated induction of AGS. In GEPR-9s induction of 14 or more AGS over 1-2 week period leads to a gradual increase in seizure duration, cortical epileptiform EEG, and the emergence of post-tonic clonus (PTC), which are persistent features of subsequent AGS. AGS kindling results in the expansion of the requisite brainstem seizure network to recruit forebrain areas, particularly the amygdala, into the expanded network. The auditory pathway projects acoustic information from the primary auditory pathway to the lateral nucleus of amygdala (LA). The LA is important in mediating the changes in seizure behavior in AGS kindled GEPR-9s, which involves NMDA receptor activation. The persistence of AGS kindling appears to be dependent on the molecular mechanisms initiated by NMDA receptor activation in LA, which may involve adenylyl cyclase (AC). Therefore, I studied the ability of AC activation in LA to mimic AGS kindling in GEPR-9s. We performed a one-time focal bilateral microinjection of an AC activator, MPB forskolin {7-Deacetyl-7-[O-(N-methylpiperazino)-γ-butyryl] - forskolin dihydrochloride} (25-100 pmol/side) into the LA of non-kindled GEPR-9s. The one-time microinjection of MPB forskolin in GEPR-9s, across all the doses at 24 hr, precipitously induced an AGS kindling-like effect, which involved significant increases in seizure duration and long-lasting susceptibility to AGS that culminated in PTC. To further examine the role of AC in the emergence of PTC we performed bilateral focal microinjections of an AC inhibitor, SQ 22,356 (0.25-0.5 nmol/side), in AGS kindled GEPR-9s. Microinjection of SQ 22,536 significantly reduced PTC in AGS kindled GEPR-9s without blocking AGS. These data suggest that AC activation in the LA is a critical step in the NMDA receptor activation-dependent molecular cascade, mediating the appearance and persistence of AGS that culminate in PTC, which is induced by AGS kindling. The LA projects to the major amygdala output nucleus, the central nucleus of amygdala (CeA), which in turn has major projections to the periaqueductal gray (PAG). Previous studies have demonstrated the existence of projections from the CeA to the ventrolateral nucleus of periaqueductal gray (PAG). In GEPR-9s, the PAG, is known to play a critical role in the propagation of AGS. Therefore, extracellular single unit action potentials of PAG neurons were studied in awake behaving GEPR-9s in response to electrical stimulation in the CeA, acoustic stimulation, and during AGS. In response to electrical stimulation in the CeA a significantly greater increase in neuronal firing in PAG was seen in AGS kindled GEPR-9s as compared to non-kindled GEPR-9s. PAG neurons in AGS kindled GEPR-9s showed a precipitous increase in responses to CeA stimulation. The PAG neurons also displayed significantly increased responsiveness to acoustic stimulation as a result of AGS kindling in GEPR-9s. These PAG neurons displayed tonic pattern of activity during wild running and tonic hind-limb extension phases of AGS in both non-kindled and AGS kindled GEPR-9s. The tonic firing pattern in PAG transformed into a burst firing pattern coinciding temporally with the PTC behavior in kindled GEPR-9s that is absent in non-kindled GEPR-9s. The neuronal firing pattern in PAG is similar to that previously observed in the LA. Burst patterns of neuronal activity are known to be associated with increased synaptic efficiency and are indicative of neuroplasticity associated with AGS kindling. Gabapentin is an anticonvulsant drug that is also effective in treating non-epileptic conditions such as anxiety disorders, ethanol withdrawal and neuropathic pain. The pathway connecting CeA to PAG has been implicated in the networks subserving these conditions. Gabapentin reversed the increase in single unit firing seen in PAG of AGS kindled GEPR-9s in response to electrical stimulation in CeA at a dose (50 mg/kg) that significantly reduced the incidence of PTC during AGS. Therefore, the effectiveness of gabapentin in the treatment of the above-mentioned CNS conditions may also be due, in part, to its effect on the pathway from CeA to PAG. Focal bilateral microinjection of NMDA (2.5 nmol/side) into the PAG of non-kindled GEPR-9s did not induce susceptibility to AGS, culminating in PTC. Since previous studies found that microinjection of NMDA into the LA initiated transient susceptibility to PTC during AGS in non-kindled GEPR-9s, the present data suggest that, unlike the LA, neuroplasticity in the PAG is not likely to mediate AGS kindling in GEPR-9s. Taken together, the results of the present studies suggest that activation of AC within the molecular cascade initiated by NMDA receptor-activation in the LA may be an important epileptogenic mechanism that subserves AGS kindling in GEPR-9s. The PAG, which undergoes changes associated with AGS kindling, may serve as the brainstem site for re-entry of epileptic activity from the amygdala in kindled GEPR-9s. However, the amygdala may be more greatly involved than the PAG in the neuroplastic mechanisms subserving AGS kindling.