ACID-SENSITIVITY AND ULTRASTRUCTURE OF THE RETROTRAPEZOID NUCLEUS IN PHOX2B-EGFP TRANSGENIC MICE

The retrotrapezoid nucleus (RTN) contains non-cholinergic non-catecholaminergic glutamatergic neurons that express the transcription factor Phox2b (chemically coded or “cc” RTN neurons). These cells regulate breathing and may be central chemoreceptors. Here we explore their ultrastructure and their acid-sensitivity using two novel BAC eGFP-Phox2b transgenic mice (B/G, GENSAT JX99) in which respectively 36% and 100% of the cc RTN neurons express the transgene in complete or partial anatomical isolation from other populations of eGFP neurons. All but one eGFP-labeled RTN neuron recorded in these mice were acid-activated in slices. These cells contained VGLUT2 mRNA and 50% contained preprogalanin mRNA (determined by single-cell PCR in the B/G mouse). Two neuronal subgroups were revealed which differed in discharge rate at pH 7.3 (Type I ~2; Type II ~4 Hz) and the degree of alkalization that silenced the cells (Type I: 7.4 – 7.6; Type II: 7.8 – 8.0). Medial to the RTN, C1 neurons recorded in a tyrosine-hydroxylase-GFP mouse were pH-insensitive between pH 6.9 – 7.5. Ultrastructural studies demonstrated that eGFP-labeled RTN neurons were surrounded by numerous capillaries and were often in direct contact with glial cells, pericytes and the basement membrane of capillaries. Terminals contacting large proximal eGFP-dendrites formed mainly symmetric, likely inhibitory, synapses. Terminals on more distal eGFP-dendrites formed preferentially asymmetric, presumably excitatory, synapses. In sum, C1 cells are pH-insensitive whereas cc RTN neurons are uniformly acid-sensitive. The RTN neurons receive inhibitory and excitatory synaptic inputs and may have unfettered biochemical interactions with glial cells and the local microvasculature.