Brainstem nitrergic innervation of the mouse visual thalamus

Ascending sensory information flowing through the visual thalamus is dynamically regulated by a number of modulatory influences. An important part of this ascending modulation is a cholinergic projection from the parabrachial region of the brainstem (PBR). In addition to containing cholinergic fibers, this projection has been identified in some species as the exclusive extrinsic source of fibers containing the neuronal form of nitric oxide synthase (nNOS). In this study, we examined the nitrergic innervation to the adult mouse visual thalamus. Retrograde tract-tracing with fluorescent microspheres was combined with nNOS and choline acetyltransferase (ChAT) immunocytochemistry, and nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) histochemistry to identify the source of nitrergic innervation. Double-labeling revealed that only two regions of the mouse brain contained nitrergic neurons that projected to the visual thalamus: the pedunculopontine tegmentum and, to a lesser extent, the lateral dorsal tegmentum (LDT). Though the LDT projected heavily to the mouse visual thalamus, very few of the retrogradely labeled neurons in that region colocalized NADPH-d. These observations suggest that the parabrachial brainstem region is the primary source of nitrergic fibers in the mouse visual thalamus, similar to that found in cat and monkey. Such similarity suggests that the presence of nNOS in presynaptic terminal fields of the visual thalamus is an important conserved property of thalamic physiology and that the mouse is a valid model for studies of nNOS functions in the brain.