Short communicationThe co-expression of ASIC3 with calcitonin gene-related peptide and parvalbumin in the rat trigeminal ganglion
Introduction
ASIC3 belongs to the family of acid-sensing ion channels [4]. The mRNA is expressed by sensory ganglia, brain and many internal tissues including lung and testis [1], [2]. Immunohistochemical methods have revealed that ASIC3 was localized to small neurons in the dorsal root ganglion [18]. Therefore, this channel is thought to play a role in nociception by functioning as a sensor of tissue acidosis [14], [17]. However, little is known about the distribution of ASIC3 in the trigeminal ganglion (TG).
Calcitonin gene-related peptide (CGRP) has been recognized to be a marker for small to medium-sized neurons in the TG [12], [15], [16]. These neurons supply the peripheral receptive fields with free nerve endings [11], [15], [16]. Immunoelectron microscopy revealed that CGRP-immunoreactive (IR) neurons have unmyelinated or finely myelinated axons [11]. Therefore, CGRP-IR neurons are considered to be associated with nociceptive transmission [13]. On the other hand, parvalbumin (PV) is mostly localized to large neurons in the TG [3], [7], [8], [9], [10]. Our previous studies have demonstrated that PV-IR TG neurons supply the tooth pulp with large myelinated axons [5], [6]. Because the tooth pulp has been considered to be innervated exclusively by nociceptive afferents, PV-containing TG neurons may participate in nociception of oro-facial structures.
In this study, we examine the co-expression of ASIC3 with CGRP or PV in TG neurons which innervate the tooth pulp and facial skin.
Section snippets
Materials and methods
Five TGs were obtained from four male Sprague–Dawley rats (200–300 g). Rats were anesthetized with ether to the level at which respiration was markedly suppressed, and transvascularly perfused with 50 ml of saline followed by 500 ml of 4% formaldehyde in 0.1 M phosphate buffer (pH 7.4). The materials were dissected, frozen-sectioned at 12 μm, and thaw-mounted on gelatin-coated glass slides.
For simultaneous visualization of ASIC3 with CGRP or PV, a double immunofluorescence method was used. The
The co-expression of ASIC3 with CGRP or PV in the TG
The TG contained many ASIC3-, CGRP- and PV-IR neurons (Fig. 1B and D); 22.8% (225/987) of TG neurons were immunoreactive for ASIC3. These neurons were of various sizes (range=43–1768 μm2, mean±S.D.=651±356 μm2). As described previously, CGRP-IR neurons were small to medium-sized whereas PV-IR neurons were mostly large [7], [15], [16]. These neurons were scattered throughout the TG. Our double immunofluorescence method revealed the co-expression of ASIC3 with CGRP or PV in the TG (Fig. 1A–D);
Discussion
The present study demonstrated that the TG contained abundant ASIC3-IR neurons; 23% of TG neurons were immunoreactive for ASIC3. These neurons were of various sizes. Our double immunofluorescence method also revealed that ASIC3-IR neurons co-expressed CGRP- or PV-IR. The axons of CGRP-IR sensory neurons are unmyelinated or finely myelinated [11], [13], [16], whereas those of PV-IR ones are large and myelinated [6]. Therefore, both myelinated and unmyelinated neurons probably contain ASIC3 in
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