H+-sensitivity and pattern of discharge of neurons in the chemosensitive areas of the ventral medulla oblongata of rats in vitro.

Effect of H+ on neuronal activity in medullary chemosensitive structures was analyzed in brain slices of the rat in vitro. Spontaneous discharges of spikes were recorded from two populations of neurons in the ventral surface layer of the medulla oblongata. Neurons located in the rostro-lateral part of the hypoglossal nerve root (area II) fired irregular phasic and/or continuous tonic discharges. More rostral and medial to it, a population of 'H+-sensitive' tonic neurons was found previously (area I). The phasic activity of neurons in area II with variable bursting and silent periods (0.1--120 s) was either increased or decreased by H+. Some neurons firing tonically in area II started to show phasic burst under low pH conditions. After cutting the slice between area I and II, the number of neurons firing phasically in area II was reduced, and the activity of area II neurons was mostly depressed by H+. An increase in activity of area I neurons by H+ was kept intact even after section. The results suggest that neurons in area I play an important role not only in the excitatory response to H+ of area II neurons but also in the initiation of phasic neuronal discharges in area II.