Interactions of gamma-aminobutyric acid and noradrenaline in the high pressure neurological syndrome.

The effects on the high pressure neurological syndrome (HPNS) of reducing brain noradrenaline (NA) levels were studied in adult rats. The onset of tremors and convulsions, which occur as pressure is increased, were used as endpoints for assessing the onset and severity of the HPNS. Neonatal treatment with 6-hydroxydopamine (6-OHDA; 100 mg kg-1 i.p. alternate days from birth for 2 weeks) which depleted brain NA, produced no change in the HPNS as assessed by the appearance of tremors and convulsions. A second series of NA-depleted rats and equivalent controls were treated with a GABA agonist, muscimol, 0.1 microgram intracerebroventricularly. Subsequently the rats were exposed to pressure and the onset and severity of the HPNS was assessed by observation of tremors and convulsions. A combination of NA depletion and intracerebroventricular injection of muscimol significantly raised the onset pressures for tremors and convulsions, i.e. delayed the appearance of the HPNS. These results are consistent with the HPNS being associated with a disturbance in the balance of two or more neurotransmitter systems, rather than simply an increase or reduction in levels of a single transmitter.