Effect of propranolol on central neurotransmitter release in Wistar rats analysed by brain microdialysis.

1. The effect of propranolol on amino acid neurotransmitter release in the rostral ventrolateral medulla (RVLM) was examined in urethane-anaesthetized male Wistar rats. 2. Amino acids released in the RVLM in response to intravenous administration of propranolol (0.4 mg/kg per min; n = 6) or nitroglycerin (0.02 mg/kg per min; n = 5) were determined by the brain microdialysis method. 3. Amino acids in dialysates were analysed by high-performance liquid chromatography and were quantified by ultraviolet absorbance. 4. Administration of both intravenous propranolol and nitroglycerin significantly decreased arterial blood pressure. Heart rate was decreased only by propranolol. 5. The reduction in arterial blood pressure produced by intravenous propranolol was accompanied by a decrease in the release of the excitatory amino acid glutamate in the RVLM. 6. The reduction in arterial blood pressure following intravenous nitroglycerin was not accompanied by a release of glutamate. 7. There were no significant changes in the levels of other amino acids (glycine, taurine, GABA) following either propranolol or nitroglycerin. 8. The decrease in glutamate release in the RVLM may account, in part, for the central depressor mechanism of propranolol.