Chemosensitivity of non-respiratory rat CNS neurons in tissue culture

Neurons from many brainstem nuclei involved in respiratory control increase their firing rate in response to acidosis in vitro, suggesting that they a re central chemoreceptors. This property has been considered to be either u nique to neurons involved in respiratory control, or at least very unusual for non-respiratory neurons. However, recordings of intrinsic pH responses of neurons have not been made from enough non-respiratory regions of the CN S to be certain this assumption is true. Here, we have quantified changes i n firing rate of neurons cultured from the hippocampus (n = 43), neocortex (n = 33), and cerebellum (n = 29) in response to changes in CO2 between 3% and 9% (pH approximate to 7.6-7.2) after blockade of glutamatergic and GABA ergic transmission. The responses of neurons from these three regions were similar, with a subset of neurons (12% of the total 105) inhibited by acido sis, decreasing their firing rate to a mean of 70% of control in response t o a decrease in pH of 0.2. Some neurons (5% of total) were stimulated by ac idosis, with an increase in firing rate to a mean of 175% of control in res ponse to a decrease in pH of 0.2. We previously quantified chemosensitivity of neurons from the medullary raphe using the same methods [W. Wang, J.H. Pizzonia, G.B. Richerson, Chemosensitivity of rat medullary raphe neurones in primary tissue culture, J. Physiol., 511 (1998) 433-450]. Compared to th ese non-respiratory neurons, more raphe neurons were stimulated by acidosis (22%), and the average response was greater (to 300% of control) in respon se to the same stimulus. Thus, over a physiologically relevant pH range, st imulation by acidosis occurs in a significant percentage of neurons not inv olved in respiratory chemoreception. However, the degree of chemosensitivit y of these neurons was less than medullary raphe neurons under the same con ditions. Chemosensitivity is not an all-or-none neuronal property, and the degree of chemosensitivity may be relevant to the role neurons play in sens ing pH in vivo. (C) 2000 Elsevier Science B.V. All rights reserved.