Probes for CRE

Recent developments in intersectional strategies have greatly advanced our ability to precisely target brain cell types based on unique co-expression patterns. To accelerate the application of intersectional genetics, we perform a brain-wide characterization of 13 Flp and tTA mouse driver lines and selected seven for further analysis based on expression of vesicular neurotransmitter transporters. Using selective Cre driver lines, we created more than 10 Cre/tTA combinational lines for cell type targeting and circuit analysis. We then used VGLUT-Cre/VGAT-Flp combinational lines to identify and map 30 brain regions containing neurons that co-express vesicular glutamate and gamma-aminobutyric acid (GABA) transporters, followed by tracing their projections with intersectional viral vectors. Focusing on the lateral habenula (LHb) as a target, we identified glutamatergic, GABAergic, or co-glutamatergic/GABAergic innervations from ∼40 brain regions. These data provide an important resource for the future application of intersectional strategies and expand our understanding of the neuronal subtypes in the brain.

Rhythmic inspiratory activity is generated in the preBötzinger complex (preBötC), a neuronal network located bilaterally in the ventrolateral medulla. Cholinergic neurotransmission affects respiratory rhythmogenic neurons and inhibitory glycinergic neurons in the preBötC. Acetylcholine has been extensively investigated given that cholinergic fibers and receptors are present and functional in the preBötC, are important in sleep/wake cycling, and modulate inspiratory frequency through its action on preBötC neurons. Despite its role in modulating inspiratory rhythm, the source of acetylcholine input to the preBötC is not known. In the present study, we used retrograde and anterograde viral tracing approaches in transgenic mice expressing Cre-recombinase driven by the choline acetyltransferase promoter to identify the source of cholinergic inputs to the preBötC. Surprisingly, we observed very few, if any, cholinergic projections originating from the laterodorsal and pedunculopontine tegmental nuclei (LDT/PPT), two main cholinergic, state-dependent systems long hypothesized as the main source of cholinergic inputs to the preBötC. On the contrary, we identified glutamatergic and GABAergic/glycinergic neurons in the PPT/LDT that send projections to the preBötC. Although these neurons contribute minimally to the direct cholinergic modulation of preBötC neurons, they could be involved in state-dependent regulation of breathing. Our data also suggest that the source of cholinergic inputs to the preBötC appears to originate from cholinergic neurons in neighboring regions of the medulla, the intermediate reticular formation, the lateral paragigantocellularis, and the nucleus of the solitary tract.