Effect of H+ on spontaneous neuronal activity in the surface layer of the rat medulla oblongata in vitro.

The effect of changing extracellular pH (pHe) on the spontaneous activity of neurons in brain slices taken from the ventral layer of the rat medulla oblongata was compared to the response of neurons in dorsal slices. In the ventral medulla, more than 50% of the neurons were excited by H+. These neurons were found just lateral to the pyramidal tract between the root of the hypoglossal nerve and the trapezoid body. In the dorsal medulla, low pHe caused an inhibition of activity in most neurons, although a few were excited. The fact that H+ elicted excitation predominantly in the ventral medullary substrate to respond to pHe changes. Depression of synaptic transmission within the neuronal network in the slice by reducing the [Ca2+]e and increasing the [Mg2+]e altered the nature of responses of neurons to H+: In the ventral medulla, the majority of neurons were inhibited by H+, whereas in the dorsal medulla more than 50% of neurons were excited. Therefore, "specificity" of the ventral medullary neurons seemed to be dependent upon intact synaptic connections. A possible role of acetylcholine-acetylcholinesterase system in the response of ventral medullary neurons to H+ is discussed.