A method for the continuous, long term superfusion of the cerebral cortex of unanaesthetized, unrestrained rats

Actions of a tremorgenic mycotoxin on amino acid transmitter release in vivo

The tremorgenic mycotoxin verruculogen was administered directly into the brain of freely moving rats by the use of cannula systems that superfused either the cortical surface or the lateral ventricular space. The tremor produced by these CNS routes was compared with that produced by i.p. administration of the toxin or the dried mycelium of the fungus that synthesizes the verruculogen. The nature and degree of tremor produced by the central vs peripheral routes suggest that the site of action of verruculogen is not immediately adjacent to the cannula sites in the brain. Measures of the amino acids in the superfusates collected during the verruculogen-induced tremor showed an increase in the excitatory neurotransmitters, glutamate and aspartate in superfusates from the lateral ventricle but not in superfusates from the cortical surface. The differential effect on transmitter release suggests that a subcortical action of verruculogen is responsible for its tremorgenic activity.

Effect of beta-bungarotoxin on the release of endogenous amino aids from the sensorimotor cortex

beta-Bungarotoxin, a snake neurotoxin purified from the venom of Bungarus multicinctus, caused a significant increase in the in vivo release of glutamate from the superfused sensorimotor cortex of awake animals. A smaller effect on GABA release was observed, but no change was detected in the release of six other amino acids measured. The effects on glutamate and GABA release were entirely blocked by tetrodotoxin (1 micrometer) and were reversible when the cortical tissue was washed with saline.

Release of amino acids from chronic epileptic and subepileptic foci in vivo

Perforated discs of cobalt or nickel were implanted onto the sensorimotor strip of the cerebral cortex of rats. Co discs gave rise to myoclonic contralateral limb-jerks in a manner similar to Co powder lesions. In contrast, Ni-implanted animals were never observed to show overt limb-jerks. On superfusion with an in vivo cannula system, both types of lesion released increased amounts of glutamate, valine and glycine as compared with unlesioned tissue. However, the enhancement of release from epileptogenic Co lesions was approximately 3-fold greater than that from non-epileptogenic lesions. In addition, epileptogenic lesions showed a marked reduction in glutamine release compared to controls, while non-epileptogenic lesions did not. The possible involvement of local disorders of the glutamate/glutamine system in the generation of cobalt focus hyperactivity is discussed.

The action of gamma-vinyl-GABA and gamma-acetylenic-GABA on the resting and stimulated release of GABA in vivo

The effects of gamma-vinyl-GABA (4-amino-hex-5-enoic acid, RMI 71754) and gamma-acetylenic-GABA (4-amino-hex-5-ynoic acid, RMI 71645), the selective and irreversible inhibitors of GABA-transaminase (E.C. 2.6.1.19), on the resting and stimulated release of GABA and other amino acids from sensorimotor cortex of rats was studied using a superfusion technique. Administration of a single dose of gamma-vinyl-GABA (1500 mg/kg i.p.) caused the appearance of GABA in superfusate. This reached a maximal peak size after 50 min and remained high for 3 h. gamma-Vinyl-GABA was also released and reached a maximum value after 20 min and decreased to low levels during 3 h. There was no direct parallelism between the time courses of release of GABA and gamma-vinyl-GABA. Also, the amount of GABA released was much larger when the drug was administered by i.p. injection than when applied directly to the cortex surface through the cannula at 100 microM. In the presence of gamma-vinyl-GABA, stimulation of the brachial plexus contralateral to the superfusion cannula increased GABA and glutamate release, but was without effect on gamma-vinyl-GABA itself or any other amino acid. In the presence of gamma-acetylenic-GABA the contralateral stimulation increased significantly only the rate of release of glutamate and the combined peaks of GABA and gamma-acetylenic-GABA. These changes were not observed to follow stimulation of the ipsilateral plexus. Stimulation of either plexus had no detectable action on the rates of release of GABA from the visual cortex. The changes in rates of release of GABA and glutamate due to stimuli were reversible, returning to control levels after cessation of the stimulation.

Peripheral sensory stimulation and the release of transmitter amino acids in vivo from specific regions of cerebral cortex

The effects of sensory stimulation on the release of amino acids from sensorimotor and visual cortex have been studied using a superfusion technique. Electrical stimulation of the brachial plexus contralateral to the superfusion cannula increased significantly the release of glutamate and glutamine from the sensorimotor cortex of anaesthetized rats. No clear effect was observed with the other amino acids. Stimulation of the ipsilateral plexus had no effect on glutamate and glutamine release. In unanaesthetized animals stimulation of the contralateral brachial plexus raised the levels of all the amino acids in sensorimotor cortex superfusate. Weak photic stimulation of the eyes of dark-adapted rats increased glutamate release from the visual cortex but caused no significant change in the release of other amino acids. All evoked increases in amino acids release were reversible at the cessation of the stimuli.

Peripheral sensory stimulation and the release of transmitter amino acids in vivo from specific regions of cerebral cortex

The effects of sensory stimulation on the release of amino acids from sensori-motor and visual cortices have been studied using a superfusion technique. Electrical stimulation of the brachial plexus contra-lateral to the superfusion cannula increased significantly the release of glutamate and glutamine from the sensori-motor cortex of anesthetized rats. No clear effect was observed with the other amino acids. Stimulation of the ipsi-lateral plexus had no effect on glutamate and glutamine release. In unanesthetized animals, stimulation of the contra-lateral brachial plexus raised the levels of all the amino acids in sensori-motor cortex superfusate. Weak photic stimulation of the eyes of dark-adapted rats increased glutamate release from the visual cortex but caused no significant change in the release of other amino acids. All evoked increases in amino acid release were reversible at the cessation of the stimuli.