Selective C-fiber deafferentation of the spinal dorsal horn prevents lesion- induced transganglionic transport of choleragenoid to the substantia gelatinosa in the rat q Ga ´bor Jancso ´ a, * , Pe ´ter Sa ´ntha a , Csaba Szigeti a,b , Ma ´ria Dux a a Department of Physiology, University of Szeged, Do ´m te ´r 10, H-6720 Szeged, Hungary b Department of Zoology and Cell Biology, University of Szeged, Egyetem u. 2, H-6722 Szeged, Hungary Abstract The effect of neonatal capsaicin treatment, producing selective elimination of almost all unmyelinated C-fiber sensory axons, was studied on lesion-induced transganglionic labelling of the substantia gelatinosa of the spinal cord by choleragenoid. In both control and capsaicin- pretreated rats, the injection of choleragenoid-horseradish peroxidase conjugate into the intact sciatic nerves resulted in intense labelling only of the deeper layers of the spinal dorsal horn. In the control but not the capsaicin-pretreated rats, the injection of the tracer into sciatic nerves transected 2 weeks previously produced an intense homogeneous labelling of the substantia gelatinosa. It is concluded that the uptake and axonal transport of choleragenoid by capsaicin-sensitive C-fiber afferents may be accounted for by the lesion-induced transganglionic labelling of the substantia gelatinosa, rather than by A-fiber sprouting. q 2003 Elsevier Ireland Ltd. All rights reserved. Keywords: Choleratoxin B subunit; Capsaicin; Sprouting; Pain; Nerve transection; Plasticity; Sensory ganglion The injection of choleragenoid, the B subunit of choler- atoxin (CTB), or its conjugates into chronically injured, but not into intact nerves results in an intense transganglionic labelling of the substantia gelatinosa of the spinal dorsal horn. Choleragenoid has been regarded as a specific marker for myelinated primary afferent fibers which do not normally terminate in the substantia gelatinosa. Accord- ingly, these findings were interpreted in terms of a sprouting response of myelinated, A-fiber primary afferents entering the substantia gelatinosa ventrally [19,20]. Invasion of the substantia gelatinosa by mechanoreceptive myelinated afferents has been suggested to contribute significantly to neuropathic pain developing after peripheral nerve lesions. This view has been widely accepted, and the presumed sprouting of myelinated primary afferents has been demon- strated in a number of experimental settings involving different types of nerve injuries [7,9,12,13,19,20]. However, recent findings have cast doubt on the sprouting hypothesis of injured myelinated afferent fibers. For instance, an analysis of the size-frequency distribution histograms of choleragenoid-labelled sensory ganglion cells has revealed a substantial increase in the proportion of small neurons after peripheral nerve transection [6,18]. Most of these small neurons belong in the capsaicin-sensitive small cell population of dorsal root ganglion neurons [6,16]. Immu- nohistochemical studies have demonstrated the co-localiz- ation of specific markers of injured unmyelinated primary afferents, e.g. vasoactive intestinal polypeptide and galanin, with choleragenoid in small sensory ganglion neurons and their central terminations [1,17]. Studies making use of the selective neurotoxic effect of capsaicin on the C-fiber primary sensory neurons have indicated that unmyelinated primary afferents may play a significant role in the mechanism of lesion-induced choleragenoid labelling of the substantia gelatinosa [5,6]. More importantly, electron microscopic histochemical studies have demonstrated that a large population of unmyelinated dorsal root axons relating to an injured, but not an intact peripheral nerve transport choleragenoid [16]. These findings afforded direct evidence for the notion that choleragenoid labelling of the substantia gelatinosa after nerve injury may be accounted for by an uptake and transganglionic transport of choleragenoid by the C-fiber afferents rather than by A-fiber sprouting. The present study was initiated in an attempt to furnish further evidence of the critical role of capsaicin-sensitive primary afferents in the mechanism of this phenomenon. An experimental approach was utilized that resulted in a Neuroscience Letters 361 (2004) 204–207 www.elsevier.com/locate/neulet 0304-3940/03/$ - see front matter q 2003 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.neulet.2003.12.063 q Dedicated to Professor Manfred Zimmermann on the occassion of his 70th birthday. * Corresponding author. Tel.: þ 36-62-545-099/544-577; fax: þ 36-62- 545-842. E-mail address: jancso@phys.szote.u-szeged.hu (G. Jancso ´).