GFP-expressing locus ceruleus neurons from Prp57 transgenic mice exhibit CO2/H+ responses in primary cell culture

The locus ceruleus (LC) contains neurons that increase their firing rate (FR) in vitro when exposed to elevated CO2/H+ and have been proposed to influence the respiratory network to make compensatory adjustments in ventilation. Prp57 transgenic mice express green fluorescent protein (GFP) in the LC and were used to isolate, culture, and target LC neurons for electrophysiological recording. We hypothesized that GFP-LC neurons would exhibit CO2/H+ chemosensitivity under primary culture conditions, evidenced as a change in FR. This is the first study to quantify CO2/H+ responses in LC neuron FR in cell culture. Neurons were continuously bathed with solutions containing antagonists of glutamate and GABA receptors, and the acid-base status was changed from control (5% CO2; pH similar to 7.4) to hypercapnic acidosis (9% CO2; pH similar to 7.2) and hypocapnic alkalosis (3% CO2; pH similar to 7.6). FR was quantified during perforated patch current clamp recordings. Approximately 86% of GFP-LC neurons were stimulated, and similar to 14% were insensitive to changes in CO2/H+. The magnitude of the response of these neurons depended on the baseline FR, ranging from 155.9 +/- 6% when FR started at 2.95 +/- 0.49 Hz to 381 +/- 55.6% when FR started at 1.32 +/- 0.31 Hz. These results demonstrate that cultured LC neurons from Prp57 transgenic mice retain functional sensing molecules necessary for CO2/H+ responses. Prp57 transgenic mice will serve as a valuable model to delineate mechanisms involved in CO2/H+ responsiveness in catecholaminergic neurons.