Cardiovascular effects of intracerebroventricular bicuculline in rats with absence seizures

Blockade of α2-adrenergic or metabotropic glutamate receptors induces glutamate release in the locus coeruleus to activate descending inhibition in rats with chronic neuropathic hypersensitivity

Locus coeruleus (LC)-spinal noradrenergic projections are important to endogenous analgesic mechanisms and can be activated by local glutamate signaling in the LC. The current study examined the local glutamatergic, GABAergic, and noradrenergic influences on glutamate release in the LC and noradrenergic descending inhibition in rats 6 weeks after spinal nerve ligation (SNL). Intra-LC injection of the α2 adrenoceptor antagonist idazoxan or the group 2 metabotropic glutamate receptor (mGluR) antagonist (RS)-α-Methyl-4-tetrazolylphenylglycine (MTPG) increased withdrawal thresholds in SNL animals and this was reversed by the blockade of α-amino-3-hydroxy-5-methyl- 4-isoxazolepropionic acid (AMPA) receptors in the LC or α2-adrenoceptors in the spinal cord, but not in normal animals. Neither blockade of GABA-A nor GABA-B receptors in the LC affected withdrawal thresholds in normal and SNL animals. Intra-LC perfusion of idazoxan increased extracellular glutamate in the LC in SNL animals but not in normal animals. Intra-LC perfusion of MTPG increased extracellular glutamate in the LC in both normal and SNL animals. These results suggest that local noradrenaline and glutamate tonically inhibit glutamate release in the LC after peripheral nerve injury and this may contribute to reduced descending inhibition in response to noxious input during chronic neuropathic pain.

Cardiovascular and behavioral effects produced by administration of liposome-entrapped GABA into the rat central nervous system

Liposomes are nanosystems that allow a sustained release of entrapped substances. Gamma-aminobutyric acid (GABA) is the most prevalent inhibitory neurotransmitter of the central nervous system (CNS). We developed a liposomal formulation of GABA for application in long-term CNS functional studies. Two days after liposome-entrapped GABA was injected intracerebroventricularly (ICV), Wistar rats were submitted to the following evaluations: (1) changes in mean arterial pressure (MAP), heart rate (HR) and renal sympathetic nerve activity (RSNA) to ICV injection of bicuculline methiodide (BMI) in anesthetized rats; (2) changes in cardiovascular reactivity to air jet stress in conscious rats; and (3) anxiety-like behavior in conscious rats. GABA and saline-containing pegylated liposomes were prepared with a mean diameter of 200 nm. Rats with implanted cannulas targeted to lateral cerebral ventricle (n = 5-8/group) received either GABA solution (GS), empty liposomes (EL) or GABA-containing liposomes (GL). Following (48 h) central microinjection (2 μL, 0.09 M and 99 g/L) of liposomes, animals were submitted to the different protocols. Animals that received GL demonstrated attenuated response of RSNA to BMI microinjection (GS 48 ± 9, EL 43 ± 9, GL 11 ± 8%; P < 0.05), blunted tachycardia in the stress trial (ΔHR: GS 115 ± 14, EL 117 ± 10, GL 74 ± 9 bpm; P<0.05) and spent more time in the open arms of elevated plus maze (EL 6 ± 2 vs. GL 18 ± 5%; P = 0.028) compared with GS and EL groups. These results indicate that liposome-entrapped GABA can be a potential tool for exploring the chronic effects of GABA in specific regions and pathways of the central nervous system.

Extracellular hypothalamic gamma-aminobutyric acid (GABA) and L-glutamic acid concentrations in response to bicuculline in a genetic absence epilepsy rat model

The posterior part of the hypothalamus plays a vital role in the homeostatic processes of the internal environment, including blood pressure and heart rate regulation, by means of gamma-aminobutyric acid (GABA)ergic and glutamatergic neurotransmission. In this study we measured the extracellular levels of GABA and L-glutamic acid in the dorsomedial hypothalamic nucleus (DMH) and posterior hypothalamus (PH), following intracerebroventricular (i.c.v.) administration of bicuculline, a GABA(A)-receptor antagonist, in genetic absence epileptic rats from Strasbourg (GAERS), where heart rate, blood pressure, and EEG recordings were also collected simultaneously. The i.c.v. injection of bicuculline (0.3 nmol) produced no response in non-epileptic Wistar rats but caused an increase in mean arterial pressure in GAERS (P<0.01). Microdialysis experiments showed that L-glutamic acid increased in the DMH in GAERS after bicuculline administration (P<0.01). Additionally, extracellular GABA concentration decreased in the PH (P<0.05). Bicuculline suppressed the spike-and-wave discharges, the characteristic sign of absence seizures. All these results suggest that the bicuculline-induced blood pressure response is accompanied by changes in L-glutamic acid levels in the DMH and GABA levels in the PH, indicating a bicuculline hypersensitivity in the DMH and PH of GAERS that may make the GAERS display an altered mode of central cardiovascular regulation. These results suggest that the circuits affected in GAERS are not only restricted to the regions responsible for seizure generation but also present in the hypothalamus.

Enhancement of spike and wave discharges by microinjection of bicuculline into the reticular nucleus of rats with absence epilepsy

The aim of this study was to demonstrate the effect of administration of gamma-aminobutyric acid (GABA)(A) receptor antagonist, bicuculline, into the reticular nucleus of the thalamus (nRt) on spike and wave discharges (SWD) and cardiovascular regulation in conscious rats with genetic absence epilepsy. Rats were instrumented with guide cannulas for drug injection and extradural electrodes for electroencephalogram recording. After a 1 week recovery period, iliac arterial catheters were inserted for direct measurement of blood pressure and heart rate. Administration of bicuculline into the nRt produced increases in spontaneous SWD and failed to alter blood pressure and heart rate. These data suggest that GABA(A) receptors located within the nRt are involved in the incidence of SWD, whereas they do not seem to be involved in cardiovascular regulation of rats with genetic absence epilepsy.

Cardiovascular regulation through hypothalamic GABA(A) receptors in a genetic absence epilepsy model in rat

PURPOSE

Gamma-aminobutyric acid (GABA) plays a vital role in both central cardiovascular homeostasis and pathogenesis of epilepsy. Epilepsy affects autonomic nervous system functions. In this study, we aimed to clarify the role of GABAA receptors in hypothalamic cardiovascular regulation in a genetically determined animal model of absence epilepsy.

METHODS

Nonepileptic Wistar rats and genetic absence epilepsy rats from Strasbourg (GAERS) were instrumented with a guide cannula for drug injection and extradural electrodes for EEG recording. After a recovery period, iliac arterial catheters were inserted for direct measurement of mean arterial pressure and heart rate. Bicuculline, a GABA(A)-receptor antagonist, was injected into the dorsomedial (DMH) or posterior (PH) hypothalamic nuclei of nonepileptic control rats or GAERS. Blood pressure, heart rate, and EEG recordings were performed in conscious unrestrained animals.

RESULTS

Bicuculline injections into the hypothalamus produced increases in blood pressure and heart rate of both control rats and GAERS. The DMH group of GAERS showed a twofold increase in the blood pressure and the heart rate compared with those of control rats. Pressor responses to bicuculline, when microinjected into the PH, were similar in the nonepileptic animals and GAERS. Conversely, the amplitude of tachycardic responses to the administration of bicuculline into the PH was significantly higher in GAERS compared with those of control rats.

CONCLUSIONS

The bicuculline-induced increases in blood pressure and heart rate were more prominent when given in the DMH of GAERS. These results indicate an increased GABA(A) receptor-mediated cardiovascular response through the DMH in conscious rats with absence epilepsy.

Effects of adenosine administration on spike-wave discharge frequency in genetically epileptic rats

1. In the present study, the effects of the administration of adenosine on absence seizures were investigated in the Wistar Albino Glaxo/Rijswijk (WAG/Rij) strain of rats, which are an adequate model for human absence epilepsy. 2. After baseline electroencephalogram (EEG) recording, adenosine was injected intraperitoneally (i.p.) at doses of 6, 30 and 120 mg/kg and EEG recordings were continued for 1 h postinjection. In addition, to enable evaluation of the contribution of peripheral factors to the occurrence of spike-wave discharges (SWD) after adenosine injection, arterial blood pressure and rectal temperature were also recorded. 3. Injection of adenosine induced a dose-dependent increase in the appearance of SWD. The number and total duration of SWD were found to be significantly increased after 30 and 120 mg/kg adenosine (P < 0.05). Adenosine also induced a fall in both blood pressure and body temperature in all experimental groups. 4. These results show that peripheral administration of adenosine promotes absence seizures. It is likely that the lowering of blood pressure and body temperature and the activation of sensory afferents after adenosine injection may contribute to the facilitation of SWD observed in the present study.