Anatomical and electrotonic coupling in developing genioglossal motoneurons of the rat.

Dye-, tracer- and electrotonic coupling were studied independently in genioglossal (GG) motoneurons using intracellular recordings in in vitro brainstem slices from rats postnatal ages 1-30 days. The subpopulation of GG motoneurons were retrogradely labeled after an injection of dextran-rhodamine into the posterior tongue. Dye-coupling was studied with Lucifer yellow injected into 55 motoneurons and tracer-coupling with neurobiotin injected into 89 presumptive GG motoneurons. Of the motoneurons injected with Lucifer yellow, only 6 of 41 cells (16.2%) exhibited dye-coupling; all occurred in animals less than 9 days old. In all but one instance, dye-coupling was restricted to only one other cell. No evidence of dye-coupling was found in the 14 cells injected in animals older than 8 days. Tracer-coupling (neurobiotin) was demonstrated in 12 of 30 cells (40%) from animals 1-2 days old and in 6 of 21 cells (28.6%) from animals 3-8 days old. Of the remaining 38 cells from animals 10 days of age and older, only one cell was found to be tracer-coupled. Cells injected with neurobiotin were coupled to an average of two other cells. Electrotonic coupling, as demonstrated with a short latency depolarization (SLD) in response to stimulation of hypoglossal axons, was found in developing GG motoneurons. These SLDs were revealed in 17 of 40 GG motoneurons (42.5%) examined in 1-8-day-old animals. There were no SLDs recorded in the 10 cells examined from animals of 10 days and older. The significance of coupling relative to patency of the newborn upper airways is discussed.