Neural and cardiac activities are altered by injection of picomoles of glutamate into the nucleus ambiguus of the rat

A quantitative evaluation of the thresholds of changes in the firing rate/pattern and depolarizing block of the neuron and the bradycardiac response by pressure microinjection of 10 mM glutamate (Glu) into the region of the nucleus ambiguus (NA) of the ventral medulla was performed in anesthetized rats. A change in neuronal activity was shown with injection of about 2 pmol of Glu. A depolarizing block of single-unit activity could be observed at 2.9 +/- 0.3 nl (approximately 30 pmol, n = 22). Maximal bradycardiac response (-50 +/- 5%) was elicited with 4.4 +/- 0.7 nl (approximately 50 pmol, n = 10), which is significantly smaller than the ranges used in previous studies. Based on these results, a safe and effective use of 10 mM Glu to induce neuronal or physiological response should be in the range of a few nanoliters and less than 100 pmol, especially for the NA.