Chemosensitivity of rat medullary raphe neurones in primary tissue culture

The respiratory control system is one of the most responsive neuronal systems to acid/base changes. The system-level response is presumed to be due to a high degree of chemosensitivity at the cellular level of specialized brainstem neurones. Early work had suggested that these central respiratory chemoreceptors are located exclusively within the ventrolateral medulla (VLM) (Mitchell et al. 1963; Schlaefke et al. 1970); however, in vitro experiments have identified neurones that increase their firing rate when exposed to acidosis within many brainstem nuclei linked to cardiorespiratory control, including the VLM (Fukuda et al. 1980; Jarolimek et al. 1990; Neubauer et al. 1991; Richerson, 1995), nucleus tractus solitarius (NTS) (Dean et al. 1990), hypothalamus (Dillon & Waldrop, 1992), locus coeruleus (Pineda & Aghajanian, 1997), medullary raphe (Richerson, 1995) and nucleus ambiguus (NA) (Rigatto et al. 1992). This wide distribution of chemosensitive neurones has been interpreted as suggesting that central respiratory chemoreception is a distributed property of the respiratory network. This possibility is also supported by data acquired using in vivo approaches (Sato et al. 1992; Coates et al. 1993; Bernard et al. 1996). In most cases, the sensitivity of chemosensitive neurones studied in vitro within respiratory nuclei has been less than anticipated based on the degree of sensitivity of the respiratory system as a whole. In some cases, CO2 changes needed to elicit a response have been outside the range expected to occur in vivo. In other cases, changes in firing rate in response to respiratory acidosis have been relatively small. It is possible that chemosensitive neurones with a low degree of sensitivity are central chemoreceptors. However, another alternative is that neurones within many brainstem respiratory regions respond to large acid/base changes, but neurones in only a limited number of regions have large responses to the small changes in CO2 that would typically occur under physiological conditions. If this is the case, all ‘chemoreceptors’ may not play an equal role in respiratory chemoreception. In most previous in vitro studies identifying neurones responsive to acid/base changes, chemosensitivity has been considered as an all-or-none neuronal property. The magnitude of the stimuli, and the criteria used to define neurones as chemosensitive have been variable. Thus, it remains unclear whether there are differences in the degree of chemosensitivity in different putative chemoreceptor regions. Without more quantitative information about the response of putative chemoreceptor neurones from different respiratory regions to acid/base changes within the normal physiological range, it will be difficult to determine what possible role chemosensitive neurones recorded in vitro play in respiratory chemoreception in vivo. In this report, the degree of chemosensitivity to physiologically relevant acid/base changes was quantified for neurones from the medullary raphe, a putative chemoreceptor region. To accomplish this goal, a primary dissociated monolayer cell culture preparation was developed. It was found that raphe neurones in tissue culture had baseline electrophysiological properties similar to those seen in brain slices and in vivo. This approach allowed quantification of the response to acid/base disturbances and further examination of the mechanisms of chemosensitivity in raphe neurones. A preliminary report of this work has previously been published (Richerson & Wang, 1997).