NEUROETHOLOGICAL AND MORPHOLOGICAL (NEO-TIMM STAINING) CORRELATES OF LIMBIC RECRUITMENT DURING THE DEVELOPMENT OF AUDIOGENIC KINDLING IN SEIZURE-SUSCEPTIBLE WISTAR RATS

Acute audiogenic seizures are a model of generalized tonic-clonic seiz ures, induced by high intensity acoustic stimulation in genetically su sceptible rodents, The neural substrates are sensory motor brainstem n uclei. Recruitment of forebrain structures takes places upon repetitio n of acoustically evoked seizures. The term audiogenic kindling means forebrain kindling evoked by repeated brainstem seizures and has been described in several strains of genetically epilepsy-prone rats. Thus, the present work was conducted in order to test the hypothesis that a udiogenic kindling recruits the forebrain, which may be behaviorally e valuated and associated with morphological changes as well. The behavi oral sequences observed during the development of audiogenic kindling were assessed by neuroethological methods (cluster analysis), with the ETHOMATIC program. Seizure severity indexes (brainstem and limbic sei zures) and latencies of wild running and tonic-clonic seizures were me asured to quantify seizure evolution. Densitometric analysis of Neo-Ti mm staining was used for assessing morphological changes associated wi th audiogenic kindling. In group I, 11 resistant (R) and 16 susceptibl e (S) animals were stimulated (120 dB) 21 times, and allowed a 10 day recovery period prior to retesting. In group II, 22 R and 20 S were st imulated 60 times, and allowed a 2 month recovery period prior to rete sting. Repetition of the acoustic stimulation in group I and group II susceptible animals led to a progressive and statistically significant attenuation of the behaviors associated with brainstem seizures and a concomitant increased expression of the behaviors associated with lim bic seizures. After either a 10 day (group I) or 2 month (group II) re covery period, acoustic stimulation preferentially evoked brainstem-as sociated behaviors and seizures rather than limbic ones in the audioge nic susceptible animals, although in some animals overlapped brainstem and limbic seizures were detected. Latencies for the wild running and tonic seizures after acoustic stimulation significantly increased dur ing audiogenic kindling for both group I and group II susceptible anim als. The quantitative ethological evaluation in both group I and group II, illustrated by flowcharts, showed the evolution of the kindling i nstallation by the presence of limbic seizure clusters, competing in t ime with the original tonic-clonic clusters. Expression of limbic seiz ures by group I animals, after acoustic stimulation, was not associate d with changes in the messy fiber Neo-Timm staining pattern of these a nimals. In group II however, Neo-Timm staining revealed messy fiber sp routing in the ventral hippocampus (but not in the dorsal), and a sign ificant change in the optical density of amygdaloid nuclei and perirhi nal cortex in susceptible animals as compared to resistant ones. In co nclusion, audiogenic kindling effectively recruits forebrain structure s, responsible for the appearance of limbic seizures. It is possible t hat the paradigm used in group I was subthreshold for the development of clear-cut synaptic reorganization in the hippocampal messy fiber sy stem, since the behavioral patterns reverted ten days after the last s eizure induction. In group II, however, an increased number of evoked seizures and a more prolonged time after the last chronic seizure show ed structural re-arrangements in amygdala, perirhinal cortex and hippo campus, associated with permanence in terms of behavioral data (lack o f regression of limbic seizures to control values).