Parasympathetic nerve fibers invade the upper dermis following sensory denervation of the rat lower lip skin

Abstract

The sympathetic division of the autonomic nervous system is known to play a role in the genesis of neuropathic pain. In the skin of the rat lower lip (hairy skin), sympathetic and parasympathetic fibers normally innervate the same blood vessels in the lower dermis but do not occur in the upper dermis. However, we have shown that sympathetic fiber migration into the upper dermis occurs following mental nerve lesions (Ruocco et al. [2000] J. Comp. Neurol. 422:287-296). As sensory denervation has a dramatic effect on sympathetic fiber innervation patterns in the rat lower lip skin, we decided to investigate the possible changes in the other autonomic fiber type in the skin-the parasympathetic fiber. Sensory denervation of the rat lower lip was achieved by bilateral transection of the mental nerve, and animals were allowed to recover for 1-8 weeks. Lower lip tissue was processed for double-labeling light microscopic immunocytochemistry (ICC), using antibodies against substance P (SP), which labels a subpopulation of peptidergic sensory fibers, and against the vesicular acetycholine transporter (VAChT), as a marker for parasympathetic fibers. In sham-operated rats, SP-immunoreactive (IR) sensory fibers were found in the epidermis and upper and lower dermal regions, whereas VAChT-IR fibers were confined to the lower dermis. Mental nerve lesions induced the gradual disappearance of SP-IR fibers from all skin layers accompanied by the progressive migration of VAChT-IR fibers into the upper dermis. Cholinergic fiber migration was evident by the second week post surgery, and the ectopic innervation of the upper dermis by these fibers persisted even at the last time point studied (8 weeks) when SP-IR fibers have completely regrown. VAChT-IR fibers were observed in the upper dermis, well above the opening of the sebaceous glands into the hair follicles. These results show that considerable changes occur in the innervation patterns of parasympathetic fibers following mental nerve lesions.

Sympathetic sprouting and changes in nociceptive sensory innervation in the glabrous skin of the rat hind paw following partial peripheral nerve injury

“…In other experimental models, parasympathetic innervation of the skin of the lower lip was determined to be limited to blood vessels in the lower dermis. After mental nerve transection, vesicular acetylcholine transporter immunoreactive (VAChT-IR) fibres were observed in the upper dermis, weIl above the opening of the sebaceous glands into the hair follic1es (Ramien et al, 2004). We found that in the glabrous skin of the rat hindpaw, parasympathetic innervation was lacking and immunoreactivity for V AChT was limited to fibres of sympathetic origin at sweat glands (Yen and Ribeiro-da-Silva, unpublished observations).…”

Autonomic fibre sprouting and changes in nociceptive sensory innervation in the rat lower lip skin following chronic constriction injury

“…They were then treated with 1% sodium borohydride in PBS and washed extensively in PBS for 60 min. For blocking non‐specific staining, tissue was placed in 10% normal goat serum (NGS) for 60 min and then incubated for 48 h at 4 °C in one of three primary antibodies: mouse monoclonal anti‐dopamine‐β‐hydroxylase (anti‐DBH) [1 : 5; PharmMingen, Canada (Mazzoni et al ., 1991); used to label sympathetic fibres]; a rabbit polyclonal serum against the vesicular acetycholine transporter (VAChT) [1 : 10 000 (Gilmor et al ., 1996) to label parasympathetic fibres] (for a detailed discussion concerning the specificity of this antibody and why we chose VAChT as a marker for cholinergic fibres, see Ramien et al ., 2004); and a rabbit polyclonal anti‐CGRP (1 : 2000; Sigma) as a marker of sensory peptidergic afferent fibres. This antibody was raised against rat CGRP and does not cross‐react with any other peptide except human CGRP and rat and human β‐CGRP (data supplied by Sigma).…”

“…In the past, we have found that following complete bilateral transection of the mental nerve (MN), which in rat is a purely sensory nerve of the trigeminal system, both sympathetic and parasympathetic fibres sprout into the sensory denervated upper dermis of rat lower lip skin from which they are normally absent (Ruocco et al ., 2000; Ramien et al ., 2004). The implications of sympathetic sprouting and the ensuing abnormal connectivity between sympathetic and sensory fibres has been well documented as mentioned above; however, less is understood regarding the novel interactions between parasympathetic and sensory neurons.…”

Role of Parasympathetic Nerves and Muscarinic Receptors in Allergy and Asthma

“…Although evidence for parasympathetic nervous function in the skin is limited to the lower lip, cholinergic nerves are capable of migrating into shallower layers of injured skin. Following sensory denervation, cholinergic (vesicular choline acetyltransferase positive) nerves were shown to migrate into the upper dermis past their normal depth and persist for at least 8 weeks [120, 121]. Whether these sprouting nerves are cholinergic sympathetic or parasympathetic nerves remain unknown.…”