Rat brain catecholamine release at 1, 10, 20, and 100 ATA heliox, nitrox, and trimix.

The pattern of neurotransmitter release in the rat caudate and hypothalamus was studied following exposure to pressure in combination with various gas mixtures. The caudate was selected for its control of extrapyramidal motor output. The hypothalamus was studied because of its involvement in autonomic responses. The neurotransmitters chosen for study were norepinephrine (NE) and dopamine (DA). Change of neurotransmitter concentration from control values and neurotransmitter turnover rate were chosen as measures of neuronal activity in the selected brain regions. Exposure to 100 ATA heliox resulted in a significant (P less than 0.05) increase in hypothalamic DA concentrations; however, NE concentrations remained relatively unchanged. One hundred ATA heliox produced the opposite effect on caudate DA concentrations; these values dropped to 1% of chamber control values. High partial pressures of nitrogen as seen with 100 ATA trimix and 20 ATA nitrox resulted in significant decreases in both DA and NE concentrations (both P less than 0.05) in the hypothalamus. These same exposures also produced significant decreases in DA concentrations in the caudate (P less than 0.05). Regional (caudate vs. hypothalamus) changes in neurotransmitter release in response to different pressures and/or gas mixtures may offer explanations for the physiologic and neurologic changes observed during nitrogen narcosis and high pressure nervous syndrome.