Physiological and morphological properties of Dbx1‐derived respiratory neurons in the pre‐Bötzinger complex of neonatal mice
Authors: Picardo Maria Cristina D. , Weragalaarachchi Krishanthi T. H. , Akins Victoria T. , Del Negro Christopher A. 
Published: March 2013

Key points

The transcription factor Dbx1 gives rise to putatively respiratory rhythm‐generating neurons in the pre‐Bötzinger complex. Comparative analysis of Dbx1‐derived (Dbx1+) and non‐Dbx1‐ derived (Dbx1−) neurons can help elucidate the cellular bases of respiratory rhythm generation.

In vitro, Dbx1+ neurons activate earlier in the respiratory cycle, discharge larger magnitude inspiratory bursts and exhibit a lower rheobase compared with Dbx1− neurons.

The Dbx1+ neurons tend to express the intrinsic currents IA (transient outward A‐current) and Ih (hyperpolarization‐activated current) in diametric opposition, which may facilitate temporal summation of excitatory synaptic inputs, whereas the Dbx1− neurons show no significant pattern of expression regarding IA and Ih.

The Dbx1+ neurons exhibit smooth, spineless dendrites that project in the transverse plane, whereas the Dbx1− neurons are confined to the transverse plane to a lesser extent and sometimes exhibit spines.

The properties of Dbx1+ neurons that may contribute to respiratory rhythmogenesis include a high level of excitability linked to ongoing network activity and dendritic properties that may facilitate synaptic integration.

Abstract  Breathing in mammals depends on an inspiratory‐related rhythm that is generated by glutamatergic neurons in the pre‐Bötzinger complex (preBötC) of the lower brainstem. A substantial subset of putative rhythm‐generating preBötC neurons derive from a single genetic line that expresses the transcription factor Dbx1, but the cellular mechanisms of rhythmogenesis remain incompletely understood. To elucidate these mechanisms, we carried out a comparative analysis of Dbx1‐expressing neurons (Dbx1+) and non‐Dbx1‐derived (Dbx1−) neurons in the preBötC. Whole‐cell recordings in rhythmically active newborn mouse slice preparations showed that Dbx1+ neurons activate earlier in the respiratory cycle and discharge greater magnitude inspiratory bursts compared with Dbx1− neurons. Furthermore, Dbx1+ neurons required less input current to discharge spikes (rheobase) in the context of network activity. The expression of intrinsic membrane properties indicative of A‐current (IA) and hyperpolarization‐activated current (Ih) tended to be mutually exclusive in Dbx1+ neurons. In contrast, there was no such relationship in the expression of currents IA and Ih in Dbx1− neurons. Confocal imaging and digital morphological reconstruction of recorded neurons revealed dendritic spines on Dbx1− neurons, but Dbx1+ neurons were spineless. The morphology of Dbx1+ neurons was largely confined to the transverse plane, whereas Dbx1− neurons projected dendrites to a greater extent in the parasagittal plane. The putative rhythmogenic nature of Dbx1+ neurons may be attributable, in part, to a higher level of intrinsic excitability in the context of network synaptic activity. Furthermore, Dbx1+ neuronal morphology may facilitate temporal summation and integration of local synaptic inputs from other Dbx1+ neurons, taking place largely in the dendrites, which could be important for initiating and maintaining bursts and synchronizing activity during the inspiratory phase.