Changes in the axon terminals of primary afferents from a single vibrissa in the rat trigeminal nuclei after active touch deprivation or exposure to an enriched environment

Abstract

Lasting modifications of sensory input induce structural and functional changes in the brain, but the involvement of primary sensory neurons in this plasticity has been practically ignored. Here, we examine qualitatively and quantitatively the central axonal terminations of a population of trigeminal ganglion neurons, whose peripheral axons innervate a single mystacial vibrissa. Vibrissa follicles are heavily innervated by myelinated and unmyelinated fibers that exit the follicle mainly through a single deep vibrissal nerve. We made intraneural injections of a mixture of cholera-toxin B (CTB) and isolectin B4, tracers for myelinated and unmyelinated fibers, respectively, in three groups of young adult rats: controls, animals subjected to chronic haptic touch deprivation by unilateral whisker trimming, and rats exposed for 2 months to environmental enrichment. The regional and laminar pattern of terminal arborizations in the trigeminal nuclei of the brain stem did not show gross changes after sensory input modification. However, there were significant and widespread increases in the number and size of CTB-labeled varicosities in the enriched condition, and a prominent expansion in both parameters in laminae III-IV of the caudal division of the spinal nucleus in the whisker trimming condition. No obvious changes were detected in IB4-labeled terminals in laminae I-II. These results show that a prolonged exposure to changes in sensory input without any neural damage is capable of inducing structural changes in terminals of primary afferents in mature animals, and highlight the importance of peripheral structures as the presumed earliest players in sensory experience-dependent plasticity.

The greater occipital nerve and its spinal and brainstem afferent projections: A stereological and tract‐tracing study in the rat

“…The same antibodies against CTB and IB4 were used in a previous study from our laboratory (Fernández‐Montoya et al, ), and the results were comparable to those presented here.…”

“…The procedure used here to inject a CTB‐IB4 mixture into a sensory nerve was successfully tried before (Fernández‐Montoya et al, ). However, in that study the target nerve was smaller and consisted of a single fascicle, whereas the GON often appeared divided into two or more fascicles (Figure ), which could raise a concern regarding the barrier imposed by the perineurium, if intact, to tracer uptake.…”

“…One group of animals (Group 1, n = 13) received an intraneural injection in the GON (Figures and ) of a mixture of 1% cholera toxin B (CTB, Sigma‐Aldrich), 2% isolectin IB4 from Griffonia simplicifolia (IB4, Vector Laboratories) and traces of the supravital dye light green (Sigma‐Aldrich), using a glass micropipette (A‐M Systems, Sequim, WA; 20–30 μm tip outer diameter) coupled to a 10 μL Hamilton syringe. As previously described (Fernández‐Montoya, Martin, Negredo & Avendano, ) a narrow pliable metal plate was carefully placed under the exposed nerve and 1–1.5 μL of the tracer mixture were injected in 3–4 pulses over 30 minutes. Finally, the region was rinsed with saline and the skin was sutured with a 2/0 silk suture.…”

“…In two cases of Group 1, alternate sections were processed free‐floating using the same immunofluorescence protocol. The remaining cases were processed free‐floating using the avidin–biotin–peroxidase method (ABC), with diaminobenzidine (DAB) as a chromogen as previously described (Fernández‐Montoya et al, ). Briefly, after endogenous peroxidase inactivation and preincubation in a blocking solution containing 2% Triton X‐100, alternate series were incubated with either goat anti‐IB4 (1:1000; Vector Laboratories) or rabbit anti‐CTB (1:500; Sigma Aldrich) overnight at 4°C under agitation.…”

“…The volume occupied by labeled axons and their varicosities was estimated by point counting, applying the Cavalieri principle (Gundersen et al, ) on 40 μm‐thick IB4‐ or CTB‐immunolabeled sections at fixed intervals ( d = 160 μm), as previously reported (Fernández‐Montoya et al, ). With the integrated stereological equipment described in the previous section, and using a planopochromatic 20x dry objective (0.75 NA, Olympus UPLSAPO), two quadratic lattices of points of different density were superimposed on the immunolabeled regions.…”