Changes in Baroreflex Responses of Kindled Rats

Cerebral and systemic hemodynamic effect of recurring seizures

The increase in neuronal activity induced by a single seizure is supported by a rise in the cerebral blood flow and tissue oxygenation, a mechanism called neurovascular coupling (NVC). Whether cerebral and systemic hemodynamics are able to match neuronal activity during recurring seizures is unclear, as data from rodent models are at odds with human studies. In order to clarify this issue, we used an invasive brain and systemic monitoring to study the effects of chemically induced non-convulsive seizures in sheep. Despite an increase in neuronal activity as seizures repeat (Spearman’s ρ coefficient 0.31, P < 0.001), ictal variations of cerebral blood flow remained stable while it progressively increased in the inter-ictal intervals (ρ = 0.06, P = 0.44 and ρ = 0.22; P = 0.008). We also observed a progressive reduction in the inter-ictal brain tissue oxygenation (ρ =  − 0.18; P = 0.04), suggesting that NVC was unable to compensate for the metabolic demand of these closely repeating seizures. At the systemic level, there was a progressive reduction in blood pressure and a progressive rise in cardiac output (ρ =  − 0.22; P = 0.01 and ρ = 0.22; P = 0.01, respectively), suggesting seizure-induced autonomic dysfunction.

Changes in autonomic control of the cardiovascular system in the Wistar audiogenic rat (WAR) strain

We evaluated autonomic cardiovascular modulation and baroreflex control of heart rate (HR) in a particular epileptic rat strain, Wistar audiogenic rats (WARs). We studied spontaneous baroreflex sensitivity as well as reflex changes in HR evoked by phenylephrine/nitroprusside-induced changes in arterial pressure (AP). Atropine and propranolol were used to measure cardiac autonomic tone. AP and pulse interval (PI) variability analysis were performed in the time and frequency domains (FFT spectral analysis) to evaluate cardiovascular sympatovagal modulation in WARs. AP and HR were higher in WARs (109±2 mm Hg and 366±9 bpm) than in Wistar control rats (101±2 mm Hg and 326±10 bpm). The power of the low-frequency band of both AP and PI spectra, a marker of sympathetic modulation, was higher in WARs than in Wistar control rats. The high-frequency power of the PI spectra in normalized units, which is linked to cardiac vagal modulation, was lower in WARs. Both WARs and Wistar control rats had similar vagal tone (91±13 bpm vs 94±11 bpm, respectively), but sympathetic tone was higher in WARs (30±4 bpm vs 14±4 bpm). No differences were detected in the gain of evoked (1.32±0.1 ms/mm Hg vs 1.35±0.2 ms/mm Hg) or spontaneous (1.34±0.2 ms/mm Hg vs 2.04±0.2 ms/mm Hg) baroreflex sensitivity. The higher AP and HR and the autonomic imbalance (sympathetic predominance) in WARs might be associated with an increased risk of life-threatening cardiovascular events in this strain.

Status epilepticus produces chronic alterations in cardiac sympathovagal balance

PURPOSE

Status epilepticus (SE) activates the autonomic nervous system, increasing sympathetic nervous system control of cardiac function during seizure activity. However, lasting effects of SE on autonomic regulation of the heart, which may contribute to mortality following seizure activity, are unknown. Therefore, autonomic control of cardiac function was assessed following SE.

METHODS

Using Sprague-Dawley rats after 1-2 weeks of recovery from lithium-pilocarpine-induced SE or control procedures, we tested overall sympathovagal control of the heart, the individual contributions of the sympathetic and parasympathetic components of the autonomic nervous system, and baroreflex sensitivity.

RESULTS

SE induced a chronic shift in sympathovagal balance toward sympathetic dominance resulting from decreased parasympathetic activity. Baroreflex sensitivity to increased blood pressure was also decreased, likely resulting from diminished vagal activation.

DISCUSSION

Chronic alterations in autonomic regulation of cardiac function, characterized by increased sympathetic dominance, occur following SE and likely contribute to subsequent increased cardiac risk and mortality.

Effects of bilateral adrenalectomy on systemic kainate-induced activation of the nucleus of the solitary tract. Regulation of blood pressure and local neurotransmitters

Glutamatergic transmission through metabotropic and ionotropic receptors, including kainate receptors, plays an important role in the nucleus of the solitary tract (NTS) functions. Glutamate system may interact with several other neurotransmitter systems which might also be influenced by steroid hormones. In the present study we analyzed the ability of systemic kainate to stimulate rat NTS neurons, which was evaluated by c-Fos as a marker of neuronal activation, and also to change the levels of NTS neurotransmitters such as GABA, NPY, CGRP, GAL, NT and NO by means of quantitative immunohistichemistry combined with image analysis. The analysis was also performed in adrenalectomized and kainate stimulated rats in order to evaluate a possible role of adrenal hormones on NTS neurotransmission. Male Wistar rats (3 month-old) were used in the present study. A group of 15 rats was submitted either to bilateral adrenalectomy or sham operation. Forty-eight hours after the surgeries, adrenalectomized rats received a single intraperitoneal injection of kainate (12 mg/kg) and the sham-operated rats were injected either with saline or kainate and sacrificed 8 hours later. The same experimental design was applied in a group of rats in order to register the arterial blood pressure. Systemic kainate decreased the basal values of mean arterial blood pressure (35%) and heart rate (22%) of sham-operated rats, reduction that were maintained in adrenalectomized rats. Kainate triggered a marked elevation of c-Fos positive neurons in the NTS which was 54% counteracted by adrenalectomy. The kainate activated NTS showed changes in the immunoreactive levels of GABA (143% of elevation) and NPY (36% of decrease), which were not modified by previous ablation of adrenal glands. Modulation in the levels of CGRP, GAL and NT immunoreactivities were only observed after kainate in the adrenalectomized rats. Treatments did not alter NOS labeling. It is possible that modulatory function among neurotransmitter systems in the NTS might be influenced by steroid hormones and the implications for central regulation of blood pressure or other visceral regulatory mechanisms control should be further investigated.