cgp-56999a and Seizures

In a previous study, we reported a rat model of early-life limbic seizures which resulted in a loss of GABA(B) receptor inhibition in the hippocampus. Since gating of auditory evoked potentials in the hippocampus (auditory gating) requires GABA(B) receptors and spatial behaviors depend on the hippocampus, we hypothesize that rats with early-life limbic seizures manifest deficits of auditory gating and spatial behaviors. Seizure rats were given a single injection of GABA(B) receptor antagonist CGP56999A (1-1.2 mg/kg i.p.) on postnatal day (PND) 15, which induced multiple limbic seizures in 8h; control rats were given saline injection. When tested at 3-9 weeks after seizure/control treatment, seizure as compared to control rats showed no difference in finding a hidden platform in the water maze, but were deficient in learning and maintaining consecutive criterion performance in the 8-arm radial arm maze. Auditory gating, as measured by paired-click (conditioning followed by test click) average auditory evoked potentials in the hippocampus, revealed a significant difference between seizure rats and controls. Seizure as compared to control rats showed an increased ratio of the test to conditioning click response as adolescents (50 days old) or adults (70 days old). Heterosynaptic electric paired-pulse depression of hippocampal population excitatory postsynaptic potential in freely moving rats, a measure of hippocampal GABA(B)-receptor mediated inhibition, was decreased in seizure as compared to control rats. Seizure as compared to control rats showed increased locomotor activity in a novel open field for the first 10 min, and decreased activity at 15-60 min. However, auditory prepulse inhibition, a measure of sensorimotor gating, revealed no difference between seizure and control rats. In conclusion, early-life limbic seizures induced a long-lasting deficit in auditory gating, likely caused by GABA(B) receptor-mediated inhibition loss in the hippocampus. Auditory gating loss is a symptom of schizophrenia, and thus GABA(B) receptor inhibition loss in the hippocampus provides a mechanism linking early-life seizures to a psychiatric symptom.

Topics: Acoustic Stimulation; Animals; Electric Stimulation; GABA Antagonists; Hippocampus; Hyperkinesis; Injections; Injections, Intraventricular; Limbic Encephalitis; Limbic System; Male; Maze Learning; Motor Activity; Phosphinic Acids; Rats; Rats, Long-Evans; Receptors, GABA-B; Reflex, Startle; Seizures; Sensory Gating

Consequences of seizures in the developing brain are not completely understood. The aim of this study was to investigate the long-term alterations of synaptic transmission and seizure susceptibility in the hippocampus after early-life seizures induced by systemic injection of a GABA(B)-receptor antagonist CGP56999A in immature rats. Experimental rats were injected with CGP56999A 1-1.5mg/kg intraperitoneally (i.p.) on postnatal day 15, while controls were injected with saline i.p. Seizures induced by CGP56999A originated mostly from the hippocampus and amygdala, and were associated with no mortality. Thirty days after seizures, laminar field potentials were recorded in the hippocampus in urethane-anesthetized rats by 16-channel silicon probes and analyzed as current source density. As compared to early-life saline-injected rats, early-life CGP56999A-induced seizure rats showed a significant decrease in paired-pulse inhibition of population spikes at 150-400 ms interpulse intervals (IPIs) in CA1, following CA3 stimulation, and at 400 ms IPI in the dentate gyrus, following medial perforant path stimulation. In a separate experiment, adolescent rats that experienced CGP56999A-induced early-life seizures showed a robust facilitation of hippocampal kindling, as compared to saline controls. In conclusion, seizures induced by GABA(B)-receptor blockade in immature rats resulted in a long-lasting loss of GABA(B)-receptor mediated paired-pulse inhibition in CA1 and dentate gyrus, which may contribute to the increase of seizure susceptibility in the hippocampus.

Topics: Algorithms; Animals; Behavior, Animal; Dentate Gyrus; Disease Progression; Electrodes, Implanted; Electroencephalography; Electrophysiology; GABA Antagonists; GABA-B Receptor Antagonists; Hippocampus; Injections, Intraventricular; Kindling, Neurologic; Phosphinic Acids; Rats; Seizures

In Wistar rats with spontaneous non-convulsive absence epilepsy, absence seizures were dose dependently suppressed by intraperitoneal administration of the GABAB receptor antagonists CGP 36742, 50-400 mg/kg, and CGP 56999, 0.25-0.75 mg/kg, and by bilateral microinjections of the same compounds into the lateral nuclei of the thalamus. In rats susceptible to audiogenic seizures, intraperitoneal administration of both GABAB receptor antagonists, at doses which suppressed absence seizures, facilitated the elicitation of sound-induced tonic seizures. In non-epileptic control rats, intraperitoneal injections of higher doses of CGP 36742 (800-2400 mg/kg) and CGP 56999 (3-6 mg/kg) induced delayed clonic convulsions, which were suppressed by pretreatment with baclofen. c-Fos protein was expressed after GABAB receptor antagonist-induced seizures in the cortex, hippocampus, amygdala, perirhinal and piriform cortex. Intra-cortical and hippocampal microinfusion of both GABAB receptor antagonists produced focal seizures. In conclusion, GABAB receptor antagonists suppress non-convulsive absence seizures by blocking thalamic GABAB receptors, while they induce convulsions in cortical and limbic structures.

Topics: Animals; Brain; Epilepsy, Absence; GABA Antagonists; GABA-B Receptor Antagonists; Male; Organophosphorus Compounds; Phosphinic Acids; Proto-Oncogene Proteins c-fos; Rats; Seizures

CGP 56999A ([3-[1-(R)-[(3-cyclohexylmethyl)hydroxyphosphinyl]-2-(S)- hydroxy-propyl] amino]ethyl]-benzoic acid) is a potent GABAB receptor antagonist showing much more pronounced convulsant features in mice than do other previously studied GABAB receptor antagonists. The goal of this study was to elucidate the physiological mechanisms underlying this effect. In mice a dose of 0.6 mg/kg intraperitoneal (i.p.) CGP 56999A elicited behavioral activation and stereotypy with periods of intensive scratching and grooming. At 1 mg/kg i.p. most mice displayed myoclonic seizure-like episodes lasting several min. Pretreatment with the lower dose of 0.6 mg/kg i.p. also induced seizures after treatment with a subthreshold dose of pentylenetetrazole (40 mg/kg i.p.). In rats a dose of 3 mg/kg CGP 56999A (i.p.) induced convulsions of tonic-clonic nature. Intracellular sharp microelectrode recordings from rat cortical neurons in slices revealed no paroxysmal actions of CGP 56999A (10 microM). Similar to other GABAB receptor antagonists, CGP 56999A suppressed the late inhibitory postsynaptic potential (i.p.s.p.), but had no effect on the excitatory postsynaptic potential (e.p.s.p.) in the cortex. In cortical slices exposed to picrotoxin (10 microM), the compound evoked pronounced, spontaneous and intense epileptiform discharges. In conclusion, these findings demonstrated that the convulsive feature of the potent GABAB receptor antagonist, CGP 56999A, may be due to suppression of the late i.p.s.p., which becomes apparent in the intact brain only, whereas this action remains undetected in untreated brain slices. This remarkable discrepancy between in vitro and in vivo may be a consequence either of disruption of neuronal circuits during slice preparation or of the pronounced hyperpolarization of pyramidal neurons, at least in the case of cortical slice preparations.

Topics: Animals; Baclofen; Cerebral Cortex; Convulsants; Dose-Response Relationship, Drug; Drug Interactions; GABA Antagonists; GABA-B Receptor Antagonists; In Vitro Techniques; Injections, Intraperitoneal; Male; Mice; Neurons; Pentylenetetrazole; Phosphinic Acids; Picrotoxin; Pyramidal Cells; Rats; Rats, Sprague-Dawley; Receptors, GABA-B; Seizures; Stereotyped Behavior