Regional cerebral blood flow and kainic acid-induced focal limbic seizures in cats

Caffeine exacerbates seizure-induced death via postictal hypoxia

Sudden unexpected death in epilepsy (SUDEP) is the leading epilepsy-related cause of premature mortality in people with intractable epilepsy, who are 27 times more likely to die than the general population. Impairment of the central control of breathing following a seizure has been identified as a putative cause of death, but the mechanisms underlying this seizure-induced breathing failure are largely unknown. Our laboratory has advanced a vascular theory of postictal behavioural dysfunction, including SUDEP. We have recently reported that seizure-induced death occurs after seizures invade brainstem breathing centres which then leads to local hypoxia causing breathing failure and death. Here we investigated the effects of caffeine and two adenosine receptors in two models of seizure-induced death. We recorded local oxygen levels in brainstem breathing centres as well as time to cessation of breathing and cardiac activity relative to seizure activity. The administration of the non-selective A1/A2A antagonist caffeine or the selective A1 agonist N6-cyclopentyladenosine reveals a detrimental effect on postictal hypoxia, providing support for caffeine modulating cerebral vasculature leading to brainstem hypoxia and cessation of breathing. Conversely, A2A activation with CGS-21680 was found to increase the lifespan of mice in both our models of seizure-induced death.

Role of endothelial nitric oxide synthase in cerebral blood flow changes during kainate seizures: A genetic approach using knockout mice

The role of endothelial nitric oxide (NO) in the cerebrovascular response to partial seizures was investigated in mice deleted for the endothelial NO synthase gene (eNOS-/-) and in their paired wild-type (WT) congeners. Local cerebral blood flow (LCBF, quantitative [14C]iodoantipyrine method) was measured 3-6 h after unilateral kainate (KA) injection in the dorsal hippocampus; controls received saline. In WT mice, KA seizures induced a 22 to 50% LCBF increase restricted to the ipsilateral hippocampus, while significant LCBF decreases (15-33%) were noticed in 22% of the contralateral areas, i.e., the parietal cortex, amygdala and three basal ganglia areas, compared to saline-injected WT mice. In eNOS-/- mice, no LCBF increases were recorded within the epileptic focus and generalized contralateral LCBF decreases (22-46%) were noticed in 2/3 of the brain areas, compared to saline-injected eNOS-/- mice. Thus, endothelial NO is the mediator of the cerebrovascular response within the epileptic focus and participates in the maintenance of LCBF in distant areas.

Role of nitric oxide in cerebral blood flow changes during kainate seizures in mice: genetic and pharmacological approaches

The role of neuronal nitric oxide (NO) in the cerebrovascular response to partial seizures induced by intrahippocampal injection of kainate (KA) was investigated in mice deleted for the neuronal NO synthase gene (nNOS-/-) and in wild-type controls (WT). A second group of WT mice received the nNOS inhibitor, 7-nitroindazole (WT-7NI). Local cerebral blood flow (LCBF) was measured using the quantitative (14)C-iodoantipyrine method. Within the epileptic focus, all three groups of seizing mice (WT, WT-7NI, and nNOS-/-) showed significant 26-88% LCBF increases in ipsilateral hippocampus, compared to saline-injected mice. Contralaterally to the epileptic focus, KA seizures induced a 21-47% LCBF decreases in hippocampus and limbic cortex of WT mice and in most contralateral brain structures of nNOS-/- mice, while WT-7NI mice showed no contralateral CBF change. Neuronal NO appears to be not involved in the cerebrovascular response within the epileptic focus, but may rather have a role in the maintenance of distant LCBF regulation during seizures.

Intracerebral temperature alterations associated with focal seizures

Because focal seizures produce an increase in local cerebral metabolism and blood flow, we wanted to determine whether they might lead to changes in brain temperature. We induced focal neocortical seizures by microinjection of 4-aminopyridine (4-AP) into the rat motor cortex. The temperature on the dura immediately over the injection site, or 8 mm away, was measured with a thermocouple and in some experiments relative blood flow was monitored with a laser Doppler probe. In animals that did not receive 4-AP, brain and rectal temperature remained fairly constant at 33.5 and 37.2 degrees C, respectively, over a 2 h monitoring period. In animals treated with 4-AP, brain temperature over the seizure focus rose an average of 0.3 degrees C, within a few seconds of seizure onset, while rectal temperature remained constant. The seizure-induced temperature rise was preceded by an increase in cortical blood flow. The temperature, but not blood flow, was also elevated 8 mm away from the seizure focus. When blood flow was increased independently of neuronal activity, by elevating pCO(2), brain temperature also rose by about 0.3 degrees C. Focal seizures in anesthetized rats produce a small, but statistically significant increase in local brain temperature, as a result of increased blood flow that brings brain temperature closer to body temperature. In humans, seizures could actually cause a reduction in brain temperature, because brain temperature is normally higher than body temperature.

Effect of theophylline and trimethobenzamide when given during kainate-induced status epilepticus: an improved histopathologic rat model of human hippocampal sclerosis

PURPOSE

The most common pathology in temporal lobe epilepsy (TLE) is hippocampal sclerosis. It is controversial whether status epilepticus (SE) or prolonged seizures plus secondary cerebral injuries are pathogenic mechanisms of hippocampal sclerosis. This study addressed this question in rat models of TLE.

METHODS

Hippocampal neuron densities and supragranular mossy fiber sprouting were determined in adult rats subjected to systemic kainate-induced SE (KA-only) and KA-induced SE followed 75 minutes later by theophylline (KA/Theo) or trimethobenzamide (KA/Tri). These drugs probably decrease seizure-induced cerebral hyperemia or hypertension.

RESULTS

Compared with controls and KA-only rats, KA/Tri and KA/Theo rats showed decreased CA3b and CA1 neuron densities (i.e., greater Sommer's sector injury). In addition, KA/Tri rats showed that increased trimethobenzamide dosages were associated with decreased hilar, CA3c, CA3b, CA1, and subiculum neuron densities. There were no significant differences in supragranular mossy fiber sprouting between KA-only, KA/Tri, and KA/Theo rats.

CONCLUSIONS

Pharmacologic manipulations during KA-induced SE are associated with differences in hippocampal pathology, especially in Sommer's sector, and the final pattern of damage and axon sprouting shows histopathologic similarities to that in patients with hippocampal sclerosis. Our findings support the hypothesis that secondary physiologic insults during SE that are likely to decrease seizure-induced cerebral hyperemia and hypertension may generate greater hippocampal neuronal injury compared with SE alone, and this may be a pathogenic mechanism of human hippocampal sclerosis in patients with TLE.

Propagation of hippocampal seizure activity arising from the hippocampus: a local cerebral blood flow study

We studied the propagation of spike discharges and the changes in local cerebral blood flow (LCBF) by means of the electroencephalogram (EEG) and autoradiographs to elucidate the underlying mechanism and the propagation pathway in rats with limbic seizures. Seizures were induced by the unilateral microinjection of kainic acid (KA) into the dorsal hippocampus during which behavioral, EEG and LCBF changes were documented. Overall, the results indicate an initial spreading to the contralateral hippocampus from the ipsilateral hippocampus, followed by activation of the ipsilateral amygdala, other limbic structures, striatum and sensorimotor cortex. Finally, seizure activity spreads to the ipsilateral globus pallidus, substantia nigra, subthalamic nucleus, thalamus, septum and parietal cortex. The activation of the contralateral hippocampus in the KA model studied here occurs earlier than in the acute amygdaloid seizure model. Elucidation of the mechanisms underlying this difference in the propagation pathway will require the application in parallel of techniques examining these and other aspects of seizure activity.

Evaluation of local cerebral glucose utilization and the permeability of the blood-brain barrier in the genetically epilepsy-prone rat

The genetically epileptic-prone rat (GEPR) is a valuable model for the study of gene-linked abnormalities involved in epilepsy. In comparison with normal Sprague-Dawley controls, we found, in GEPRs, a marked depression in local cerebral glucose utilization, widespread throughout the brain. This depression was accompanied by a significant increase of blood-brain barrier permeability and a reduction in regional blood volume. Finally GEPRs showed lower plasma levels of total triiodothyronine than normal controls. One can speculate that alterations in cerebral metabolism and microvascular regulation and thyroid hormone imbalance may be gene-linked factors involved in seizure susceptibility.

Blood-brain barrier dysfunctions following systemic injection of kainic acid in the rat

Changes in blood-brain barrier (BBB) permeability and cerebral metabolic activity following intravenous injection of kainic acid (KA; 6, 12 mg/Kg) in rats were assessed by calculating respectively a blood-to-brain transfer constant (Ki) for [14C]alpha-aminoisobutyric acid and local cerebral glucose utilization (LCGU) values, at different times (1 h, or acute seizures phase, and 48 h, or chronic pathology phase) after the induction of seizures. A significant increase in the local permeability of the BBB was observed 1 h after the injection of KA 6 mg/Kg (eliciting no significant changes in cerebral metabolic activity, except within the frontal cortex and the hippocampus) and 12 mg/Kg (which induced a marked and widespread enhancement of LCGU). On the contrary, during the pathology phase, persistent regional increases in Ki values were evidenced in rats treated with the lowest dose of the convulsant, but not in rats injected with KA 12 mg/Kg (a dose able to cause extensive neuronal damage). Thus one can speculate that: 1) KA-induced regional changes in the permeability of the BBB are not correlated with changes in neuronal activity; 2) opening of the BBB is not reliably associated with neuronal injury.

A serial study of regional cerebral blood flow before and after hemispherectomy in a child

Hemimegalencephaly (HME) is a severe unilateral brain malformation the prognosis of which may be improved by hemispherectomy. HME also provides a unique opportunity to compare normal and pathological hemispheric function in the same patient. We performed a serial functional cerebral imaging study in a child suffering from a neuropathologically confirmed left HME. He was hemispherectomized at 11 months because of intractable epilepsy; this led to cessation of seizures and dramatic psychomotor improvement. Regional cerebral blood flow (rCBF) was studied at 1, 7, 10, 12 and 25 months with single photon computed tomography (SPECT) using 133-Xenon and with simultaneous EEG recording. At one month of age SPECT was performed ictally. During left EEG discharges, rCBF was 40% higher on the left hemisphere than on the right, even in occipital and frontal regions, usually immature at this age. A crossed cerebellar hyperperfusion was also found. At 7 and 10 months, SPECT was performed interictally; rCBF was 45% lower in the left hemisphere than in the right. During follow-up, global and regional CBF values showed normal levels and normal maturation in the right hemisphere except for a mild and transient decrease observed one month after hemispherectomy. SPECT provides an additional procedure for studying hemispheric function in vivo. Serial SPECT imaging may be useful for the preoperative and postoperative evaluation in unilateral cerebral malformation.

Surgical treatment of experimental limbic status epilepticus

Limbic status epilepticus was induced by means of a KA microinjection into unilateral amygdala, and the focus extirpation (amygdalotomy) was made in order to examine whether the status was suppressed or not. The amygdalotomy was effective when the status was mild and the focus was circumscribed to the amygdala. However, the surgery was no more effective when a severe limbic status was induced and a secondary epileptogenic focus was established. Within 8 hours after induction of the limbic status, neuronal cell damage was observed in the pyramidal cell layer of the hippocampus.