Triggering genetically-expressed transneuronal tracers by peripheral axotomy reveals convergent and segregated sensory neuron-spinal cord connectivity

• Published in: Neuroscience Vol. 163; no. 4; pp. 1220 - 1232
• Main Authors: Bráz, J.M, Basbaum, A.I
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier Ltd 10.11.2009; Elsevier
• Subjects: Animals; Axotomy; Biological and medical sciences; dorsal root ganglion; Fundamental and applied biological sciences. Psychology; Genetic Engineering - methods; Green Fluorescent Proteins - genetics; Green Fluorescent Proteins - metabolism; Integrases - genetics; Mice; Mice, Transgenic; Nerve Fibers, Myelinated - metabolism; Nerve Fibers, Unmyelinated - metabolism; Neural Pathways - metabolism; Neurology; Neuronal Tract-Tracers - metabolism; pain; Peptide Fragments - genetics; Peptide Fragments - metabolism; Peripheral Nerve Injuries; Protein Kinase C - metabolism; Sciatic Nerve - injuries; Sensory Receptor Cells - metabolism; spinal cord; Spinal Cord - metabolism; Synapses - metabolism; Tetanus Toxin - genetics; Tetanus Toxin - metabolism; tracing; TRPV Cation Channels - metabolism; Vertebrates: nervous system and sense organs; WGA; Wheat Germ Agglutinins - genetics; Wheat Germ Agglutinins - metabolism; FG; fusion protein of the green fluorescent protein and the C fragment of tetanus toxin; neurofilament 200; PV; neuropeptide Y; trigeminal ganglion; CNS; fluorogold; lateral spinal nucleus; GFP-TTC; parabrachial nucleus; gamma isoform of protein kinase C; NK1; 1.8 subtype of voltage-gated sodium channel; neurokinin 1 receptor; NPY; spinal cord; wheat germ agglutinin; substance P; PBN; SP; pain; tracing; activation transcription factor 3; isolectin B4; LSN; gamma-aminobutyric acid; N52; axotomy; peripherin; TRPV1; Cre recombinase; PKCγ; central nervous system; WGA; TG; parvalbumin; dorsal root ganglion; Cre; DRG; GABA; IB4; Per; TRPV1 subtype of transient receptor potential; ATF3; Nav1.8; Axotomy; Spinal cord; Pain; Central nervous system; Sensory neuron; Spinal ganglion
• ISSN: 0306-4522; 1873-7544
• DOI: 10.1016/j.neuroscience.2009.07.051

Abstract To better understand the mechanisms through which non-painful and painful stimuli evoke behavior, new resources to dissect the complex circuits engaged by subsets of primary afferent neurons are required. This is especially true to understand the consequences of injury, when reorganization of central nervous system circuits likely contributes to the persistence of pain. Here we describe a transgenic mouse line (ZWX) in which there is Cre-recombinase-dependent expression of a transneuronal tracer, wheat germ agglutinin (WGA), in primary somatic or visceral afferent neurons, but only after transection of their peripheral axons. The latter requirement allows for both regional and temporal control of tracer expression, even in the adult. Using a variety of Cre lines to target WGA transport to subpopulations of sensory neurons, here we demonstrate the extent to which myelinated and unmyelinated “pain” fibers (nociceptors) engage different spinal cord circuits. We found significant convergence (i.e., manifest as WGA-transneuronal labeling) of unmyelinated afferents, including the TRPV1-expressing subset, and myelinated afferents to NK1-receptor–expressing neurons of lamina I. By contrast, PKCγ interneurons of inner lamina II only receive a myelinated afferent input. This differential distribution of WGA labeling in the spinal cord indicates that myelinated and unmyelinated sensory neurons target different and spatially segregated populations of postsynaptic neurons. On the other hand, we show that neurons of deeper laminae (III–V) receive direct (i.e., monosynaptic) inputs from myelinated afferents and polysynaptic input from unmyelinated afferents. Taken together, our results indicate that peripheral sensory information is transmitted to the central nervous system both through segregated and convergent pathways.