Mary Ellen Kelly

Kindling modulates the IL-1β system, TNF-α, TGF-β1, and neuropeptide mRNAs in specific brain regions

Cytokines and neuropeptides may be involved in seizure-associated processes. Following amygdala kindling in rats, we determined alterations of IL-1β, IL-1 receptor antagonist (IL-1Ra), IL-1 receptor type I (IL-1RI), IL-1 receptor accessory proteins (IL-1R AcPs) I and II, TNF-α, TGF-β1, neuropeptide Y (NPY), glycoprotein 130 (gp 130) and pro-opiomelanocortin (POMC) mRNA levels in the parietal, prefrontal and piriform cortices, amygdala, hippocampus and hypothalamus. Messenger RNAs expression in all brain regions was determined 2 h or 3 weeks following the last generalized convulsive seizure triggered from the ipsilateral kindled amygdala. The same brain region sample was used to assay for changes of all mRNA components. The results show that the 2 h-kindled group exhibited a significant up-regulation of IL-1β, IL-1RI, TNF-α and TGF-β1 mRNAs in all three cortical brain regions, amygdala and hippocampus. The largest up-regulation occurred in the prefrontal cortex (about 30-fold induction for IL-1β and TNF-α mRNAs). IL-1R AcP I and II mRNA levels were also up-regulated in the cortical regions. No changes in IL-1β, IL-1RI or TNF-α mRNA levels occurred in the 3 week-kindled group. NPY mRNA levels increased in the hippocampus, prefrontal and piriform cortices in the 2 h-kindled group, while IL-1Ra, gp 130, or POMC mRNA levels did not change in any group. The overall profile of mRNA changes shows specificity of transcriptional modulation induced by amygdala kindling. The data support a role of cytokines and NPY in the adaptive mechanisms associated with generalized seizure activity, with implications for neuroprotection, neuronal dysfunction and vulnerability associated with epileptic activity.

Efferent projections of the anterior perirhinal cortex in the rat.

Because convulsive seizures develop very rapidly from kindling sites in the anterior perirhinal cortex, we studied perirhinal efferents by using the anterograde tracer Phaseolus vulgaris leucoagglutinin (PhAL). PhAL injections into the anterior perirhinal cortex labelled a prominent network of fibers within the frontal cortex that was most dense within layers I and II and layer VI. As individual PhAL injection sites within the perirhinal cortex were restricted to one or two adjacent laminae, we were able to determine that layer V was the main source of the perirhinofrontal projection. This was confirmed by frontal cortex injections of the retrograde tracer Fluorogold (FG).

FAST and SLOW amygdala kindling rat strains: comparison of amygdala, hippocampal, piriform and perirhinal cortex kindling

In our companion paper, we selectively bred offspring of a Long Evans Hooded and Wistar rat cross for either fast or slow rates of amygdala kindling (Racine et al., 1999. Development of kindling-prone and kindling resistant rats: Selective breeding and electrophysiological studies, Epilepsy Res. 35, 183-195). Within 10 generations, there was no overlap in the distribution of kindling rates between these newly developed FAST and SLOW kindling strains. In the present report, we compared the local excitability, kindling rates, and convulsion profiles of kindling sites in either the amygdala, dorsal hippocampus, piriform cortex or perirhinal cortex in the two strains. Local excitability, measured as the local afterdischarge (AD) threshold and its duration, showed varied effects between structures and strains. Before kindling, the AD threshold was lower in the FAST than the SLOW rats in the hippocampus, piriform and perirhinal cortices, but not the amygdala (the selection structure). Also, the duration of the AD threshold duration was significantly longer in the FAST than in the SLOW rats in all structures, except the CA1 hippocampus. Most of these differences were maintained after kindling. Kindling itself was significantly faster in the FAST compared with the SLOW rats in all structures; however, the different structural kindling rates showed proportional differences between strains that were about five times different in the amygdala compared with only about two times different in the hippocampus. This suggested a selection bias for the amygdala and its networks. As in other rat strains, the fastest kindling rates were seen in the perirhinal cortex followed by the piriform cortex, amygdala and hippocampus in both FAST and SLOW rats. Other important differences between strains and structures occurred in the stage-5 convulsion profiles, including latency to forelimb clonus, clonus duration and duration of associated local afterdischarges. The differences in kindling profiles between strains and structures were discussed with respect to possible underlying mechanisms, significance for epileptogenesis, and impact on other normal behaviours.

Kindling in the perirhinal cortex.

In vitro experiments have indicated that the perirhinal cortex is highly excitable and its relationship to the basolateral amygdala and piriform cortex is altered by previous amygdala or dorsal hippocampal kindling. As a result, we felt it was important to assess the excitability of the perirhinal cortex in vivo by comparing its kindling profile to that of the basal amygdala, piriform cortex or dorsal hippocampus. We observed that the after-discharge (AD) threshold of the perirhinal cortex was higher than the other 3 structures but the AD duration was not different. Subsequently, the perirhinal cortex kindled more rapidly than the other 3 structures, and with extremely short latencies to onset of forelimb clonus. With the view that synchronized discharge in the perirhinal-piriform area provides the critical trigger for limbic kindled convulsions, the relationship of kindling rate and convulsion latencies and durations between the 4 structures was discussed.

Perirhinal cortex involvement in limbic kindled seizures

Investigations into the anatomical substrate of temporal lobe epilepsy have yielded a number of important observations regarding the involvement of the piriform and perirhinal cortical areas in temporal lobe seizure propagation. Although early reports indirectly suggested that the circuits of the piriform cortex might act as a critical conduit for limbic seizure discharges to access motor systems, recent reports more strongly implicate the perirhinal cortex in this process. In the following report, we provide a brief summary of the earlier work involving the piriform cortex and its potential involvement in kindled limbic seizures. This is followed then by the results of several recent in vivo and in vitro electrophysiological studies that ascribe a critical importance for the perirhinal cortex in convulsive limbic seizures. Finally, since our anatomical studies indicated that the perirhinal cortex densely innervates the frontal motor cortex, we examined the involvement of this latter region in amygdala kindled seizures using the reversible functional lesion of cortical spreading depression. Based on these findings we suggest that the circuits of the perirhinal cortex may be important in the amplification and distribution of temporal lobe seizure discharges, providing access to structures that are capable of driving a convulsive response.

Differential sensitivity of various temporal lobe structures in the rat to kindling and status epilepticus induction

Using focal brain stimulation (kindling), discrete seizures can be triggered from many neuroanatomic sites with varying degrees of facility. From several of these sites, protracted seizures or status epilepticus (SE) also can be triggered. To date, no comparison has been made between different brain sites in their sensitivity both to kindling and to SE development. In this report, we have compared the kindling profiles of three amygdala nuclei, namely the basal (BL), central (CE), and medial (ME) nuclei, to the adjacent piriform (PIR) and perirhinal (PRH) cortices. In addition, three weeks following kindling, the susceptibility of each kindled site to status epilepticus (SE) was assessed by exposing the site to 60 min of electrical stimulation. We observed that (a) during the course of daily kindling, the afterdischarge threshold dropped progressively and significantly in all structures, (b) the rate of kindling in the PRH and PIR cortices and the CE amygdala was significantly faster than either the BL or ME amygdala, (c) when discrete convulsions were triggered, the latency to forelimb clonus in the PRH cortex and CE amygdala was significantly shorter than the other three structures, and (d) despite being slower to kindle than most other sites, stimulation of the BL nucleus most readily triggered SE. The kindling data suggest that discharges triggered from the PRH and CE more readily access motor systems supporting limbic convulsions than discharges triggered from the BL, ME nuclei or the PIR cortex. On the other hand, the SE data indicate that the mechanisms and circuits associated with the development of discrete kindled seizures are not identical to those associated with the induction of limbic SE.

The Parahippocampal Cortices and Kindling

Abstract: The piriform and perirhinal cortices are parahippocampal structures with strong connections to limbic structures, including the amygdala and hippocampus, as well as other parahippocampal structures such as the entorhinal cortex. In this paper, we present results, based on anatomical, physiological, and kindling studies, that suggest that the perirhinal and piriform cortices might be very important in the secondary generalization of limbic seizures, particularly those with convulsive expression. These kindling data further suggest that the progressive lowering of afterdischarge thresholds in the parahippocampal structures, due to insult and/or genetic predisposition, might provide the neural basis for the clinical presentation of temporal lobe epilepsy.

Secondary generalization of hippocampal kindled seizures in rats: examining the role of the piriform cortex.

A primary feature of epilepsy is the potential for focal seizures to recruit distant structures and generalize into convulsions. Key to understanding generalization is to identify critical structures facilitating the transition from focal to generalized seizures. In kindling, development of a primary site leads progressively to secondarily generalized convulsions. In addition, subsequent kindling of a secondary site results in rapid kindling from that site, presumably because of its facilitated access to the primary kindled network. Here, we investigated the role of the piriform cortex in convulsive generalization from a secondary site kindled in the hippocampus after primary site amygdala kindling. In a necessarily complicated design, rats initially experienced forebrain commissurotomy to lateralize the experiment to one hemisphere. Then the amygdala was kindled and, 3 weeks later, it was electrically-triggered into status epilepticus, which destroyed the ipsilateral piriform cortex. This experience occurred several days before secondary site kindling of the dorsal hippocampus. In rats with complete piriform cortex loss, there was no disruption in kindling or convulsive seizure expression from the hippocampus. However, when damage also involved parts of the perirhinal, insular and entorhinal cortices, convulsive expression was blocked. Although other evidence suggests that piriform lesions affect generalization of primary site kindling, the present study shows that they do not alter secondary site kindling in the dorsal hippocampus. The additional involvement of parahippocampal cortical areas in convulsive expression suggests an important functional association between these cortical regions and the hippocampus in seizure propagation and clinical expression.

Is the Pyriform Cortex Important for Limbic Kindling

Interest in the contribution of the pyriform cortex to complex partial seizures is not new. In the 1890s Hughlings Jackson and colleagues (9, 10) described a lesion limited to the human uncus, the homologue of the rodent pyriform cortex (2), which they believed initiated ‘uncinate fits’. The development of elaborate behavioral symptoms during the uncinate seizure was presumed to be a result of seizure spread beyond this area, perhaps to the frontal cortex via the uncinate fasciculus (25). Occasionally these spontaneous uncinate seizures developed secondarily into full generalized convulsions, an outcome frequently observed after electrical stimulation of the uncus (21). Thus, it seems that provocation of the uncus is able to directly trigger, or gain access to mechanisms necessary to trigger, secondarily generalized convulsions.

Dorsal hippocampal kindling produces long-lasting changes in the origin of spontaneous discharges in the piriform versus perirhinal cortex in vitro

In an in vitro slice preparation of the amygdala-piriform-perirhinal cortex (A-P area), it was shown previously (McIntyre, D.C., Plant, J. R., 1993. Long-lasting changes in the origin of spontaneous discharges from amygdala-kindled rats: piriform vs. perirhinal cortex in vitro, Brain Res. 624, 268-276) that the infrequent spontaneous field potentials that initially originated in or near the perirhinal (PRh) cortex of slices from control rats began instead in the piriform (Pir) cortex of amygdala-kindled rats. This change in onset was only observed in the A-P area ipsilateral to the kindled amygdala. In the present experiment, we determined whether similar changes in activity were evident following kindling from a different limbic site, the dorsal hippocampus (DH). Kindling of the DH resulted in changes in the origin of the spontaneous discharges in the A-P area similar to amygdala kindling but, importantly, the changes involved both hemispheres. In addition, the origin of spontaneous discharges in slices from partial kindled rats (those that received as many hippocampal afterdischarges as the fully kindled rats but had not developed generalized convulsive responses) initially were similar to control tissue, but, during 0 Mg(2+) perfusion, changed more quickly than control tissue to mimic the profile of generalized kindled rats. The enduring changes in A-P area excitability caused by previous generalized kindling highlights the importance of the A-P area in convulsive generalization of limbic-kindled seizures.