In the Caudal Pontine Reticular Nucleus, Kv1.1 Expression Is Soma Size Dependent and Invariant During Postnatal Development.

The acoustic startle reaction is a rapid behavioral response to an unexpected auditory stimulus. In mammals, this reaction is based on an archaic reflex arch connecting auditory inputs via the sensor-motor interface in the caudal pontine reticular nucleus (PnC) of the reticular formation with motor output. The neuronal population in the PnC is heterogeneous, containing an interspersed set of giant neurons that represent the cellular substrate of the sensorimotor interface. To probe whether the heterogeneous cell population can be divided into distinct subpopulations based on somatic morphometry or potassium channel expression, we quantitatively analyzed immunofluorescence labeling in the PnC and compared this between three mammals. In gerbil, mouse, and Etruscan shrew, soma size and roundness showed a continuum over all analyzed cells. Somatic Kv1.1 labeling intensity continuously increased with increasing soma size. Overall, no subpopulations based on somatic morphometric parameters and Kv1.1 expression were observed, suggesting that the PnC is composed of neurons displaying a continuum of soma sizes. Moreover, in mice, neurons of the MNTB but not PnC showed a postnatal developmentally regulated Kv1.1 expression, contrasting this sensorimotor interface with the sensory nuclei of the auditory brainstem.