Response to CO2 of neurons in the rostral ventral medulla in vitro

Abstract

1. It has been hypothesized that CO2-sensitive neurons are located in the rostral ventral medulla. To demonstrate this at the cellular level, perforated patch-clamp recordings were made from rat medullary slices in vitro. The effect of respiratory acidosis/alkalosis on the electrophysiologic properties of neurons was studied by recording membrane potential while changing the CO2 of the bath solution and allowing pH to vary. 2. At baseline, most neurons in the rostral ventrolateral medulla (VLM) and rostral medullary raphe spontaneously fired repetitively at a regular rate (3.3 +/- 2.5 Hz, mean +/- SD) with a linear interspike ramp depolarization (n = 102 of 135). Spontaneous firing continued after synaptic blockade with high-magnesium, low-calcium solution (n = 14 of 15). Spontaneous firing of calcium spikes continued in tetrodotoxin (TTX; n = 13 of 13), but was blocked by TTX and cadmium (n = 4 of 4). 3. The effect of respiratory acidosis/alkalosis on neurons was examined by changing the CO2 of the bicarbonate-buffered bath solution within the range of 3-9%. Most neurons studied (n = 74 of 105) did not change their firing rate in response to this stimulus; however, some neurons were stimulated (n = 16) and other neurons were inhibited (n = 15) by increases in CO2. 4. In many CO2-stimulated neurons, the increase in firing rate caused by an increase in CO2 was associated with an increase in slope of the linear interspike ramp depolarization, whereas in many CO2-inhibited neurons the opposite occurred, i.e., an increase in CO2 resulted in a decrease in slope of the ramp depolarization. These changes occurred without a change in the level of afterhyperpolarization or spike threshold. 5. Whole cell patch-clamp recording invariably resulted in loss of spontaneous and stimulated repetitive firing over 10-40 min despite good resting potential, input resistance, and amplitude of single depolarization-evoked spikes. CO2 produced no change in membrane potential in neurons after rundown of repetitive firing. The loss of repetitive firing and CO2 sensitivity with whole cell recording required the use of perforated-patch recordings of membrane potential or cell-attached-patch recordings of spike transients to accurately study the baseline electrophysiologic properties and CO2 sensitivity of rostral medullary neurons. 6. Neuronal location was determined before each recording using direct visualization of living slices, and after some recordings using biocytin staining. CO2-stimulated and CO2-inhibited neurons were both found to have cell bodies in the rostral VLM, an area thought to contain central respiratory chemoreceptors.(ABSTRACT TRUNCATED AT 400 WORDS)