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RESEARCH REPORT |
1 Division of Endodontics, University of Minnesota School of Dentistry;
2 Department of Endodontics, UTHSCSA School of Dentistry, Mail Code 7892, 7703 Floyd Curl Drive, San Antonio, TX 78229-3900; and
3 Department of Oral Medicine, University of Washington School of Dentistry;
*corresponding author, Hargreaves{at}UTHSCSA.edu
| ABSTRACT |
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KEY WORDS: dental pulp superfusion CGRP capsaicin norepinephrine albuterol ICI 118 551 atenolol epinephrine
| INTRODUCTION |
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- and ß-adrenoceptors. Dental pulp tissue expresses both
- and ß-adrenoceptors and, thus, is responsive to catecholamine administration (Tønder and Naess, 1978; Okabe et al., 1989; Liu et al., 1990; for review, see Suda and Ikeda, 2002). Previous studies have demonstrated that post-ganglionic sympathetic fibers and capsaicin-sensitive trigeminal afferent fibers have opposing effects on pulpal blood flow (PBF) in several species (Tønder and Naess, 1978; Kim and Dörscher-Kim, 1990; Olgart, 1992; Andrew and Matthews, 1996; Kim et al., 1996; Berggreen and Heyeraas, 2000). In addition, results from whole-animal studies measuring vascular parameters of PBF have led to the hypothesis that sympathetic fibers act in part by inhibition of capsaicin-sensitive trigeminal peptidergic afferent fibers (Kerezoudis et al., 1993a,b). Studies on cultured dorsal root ganglion neurons indicate that certain small-diameter sensory neurons may express the ß-adrenoceptor (Khasar et al., 1999). However, to date, no study has directly tested whether activation of ß-adrenergic receptors inhibits neuropeptide release from certain peptidergic sensory neurons innervating dental pulp.
In this study, using release of iCGRP from isolated superfused bovine dental pulp slices as a dependent measure, we used a biochemical approach to determine whether activation of ß-adrenergic receptors by exogenous adrenergic agonists inhibits exocytosis from trigeminal peptidergic afferent neurons. CGRP in dental pulp originates from trigeminal sensory neurons and thus represents a selective marker of certain peptidergic sensory neurons (Wakisaka et al., 1987), and the use of capsaicin as the test stimulus selectively targets the nociceptive subset thereof. In this report, we provide evidence that activation of ß2-adrenoceptor exogenous adrenergic agonists, but not ß1-adrenoceptors, inhibits capsaicin-evoked exocytosis of iCGRP from isolated neuronal terminals in bovine dental pulp.
| METHODS |
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Samples were collected over seven-minute periods after a 60-minute wash-out period. All chemicals and test drugs were purchased from Sigma Chemical Co. (St. Louis, MO, USA). Levels of iCGRP were measured by radioimmunoassay with an immunomagnetic separation method as previously described (Richardson et al., 1998). Separate standard curves were prepared with all test drugs to verify lack of interference with the assay. The minimum detection limit was 1-3 fmol/tube.
The data were analyzed by one-way ANOVA followed by Duncans multiple-range test for determination of differences between groups. A Students t test was conducted when two groups were compared. A difference was accepted as significant if the probability that it occurred due to chance alone was less than 5% (p < 0.05). Release data were normalized by calculation of the % increase over baseline rates of iCGRP release according to the formula 100 x (peak release - baseline)/(baseline). This transformation reduced intra-experimental variability due to differences among animals. Data are presented as mean + SEM.
| RESULTS |
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| DISCUSSION |
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1-,
2-, and ß1-adrenoceptors (Hoffman et al., 1996); therefore, we first considered that this catecholamine-mediated inhibitory effect could be due, at least in part, to the stimulation of ß2-adrenoceptors. This hypothesis was confirmed when we demonstrated that the norepinephrine effect could be significantly reduced by pre-treatment with ICI 118,551 (a selective ß2-adrenoceptor antagonist), but not by pre-treatment with atenolol (a selective ß1-adrenoceptor antagonist). Moreover, the administration of albuterol, a selective ß2-adrenoceptor agonist, significantly blocked capsaicin-evoked release of iCGRP. Collectively, these studies demonstrate that, in dental pulp, activation of ß2-adrenoceptors, but not ß1-adrenoceptors, is significantly able to reduce exocytosis of neuropeptides from capsaicin-sensitive nociceptors innervating this tissue. Although the biochemical method used in this study is useful for characterizing pharmacological mechanisms of action, it is not suitable for identifying the cell type that expresses the ß2-adrenoceptor mediating this response. At least two hypotheses can be advanced for the cell type expressing the ß2-adrenoceptor. First, the ß-adrenoceptor could be expressed on pulpal sympathetic fibers, where its activation could lead to enhanced release of endogenous norepinephrine or other neurotransmitter substances (Lundberg et al., 1990) that could then have a direct inhibitory action on sensory terminals. We view this possibility as unlikely, since the inhibitory effect observed after application of exogenous norepinephrine was blocked by pre-treatment with a ß2-adrenoceptor antagonist, a result not predicted by this hypothesis. Second, it is possible that this ß2-mediated inhibitory effect is due to activation of ß2-adrenoceptors expressed by capsaicin-sensitive trigeminal peptidergic neurons that innervate dental pulp, which likely express the vanilloid receptor type 1 (VR1), a receptor activated by capsaicin (Caterina et al., 1997). Thus, we hypothesize that capsaicin-sensitive trigeminal neurons innervating dental pulp may express ß2-adrenoceptors. To our knowledge, no study has yet evaluated this hypothesis. However, it has been reported that norepinephrine actives ß-adrenoceptors expressed in normal (uninjured) small-diameter sensory neurons, many of which are known to express the VR1 receptor (Abdulla and Smith, 1997; Caterina et al., 1997). In addition, neonatal capsaicin treatment, a procedure that destroys capsaicin-sensitive fibers, also reduced ß-adrenoceptor binding sites in the spinal cord (Patterson and Hanley 1987). Taken together, these data are consistent with the hypothesis that capsaicin-sensitive trigeminal afferent fibers express the ß2-adrenoceptor.
There are at least three implications from the present data. First, we have previously reported that activation of pulpal sympathetic fibers inhibits capsaicin-evoked release of neuropeptides from dental pulp (Hargreaves et al., 2003). This effect was predominantly due to activation of
-adrenoceptors, since approximately 80% of this sympathetic effect was blocked by application of
-adrenergic antagonists. We do not know if endogenous norepinephrine reaches sufficient concentrations to activate the ß2-adrenoceptors described in this present report. However, at least under the present experimental conditions, activation of ß2-adrenoceptors can account for only up to 20% of sympathetic fiber modulation of capsaicin-sensitive nociceptors.
Second, the demonstration of ß2-adrenoceptor regulation of iCGRP exocytosis may partially explain the prior observation that systemic administration of isoproterenol, a non-specific ß1/ß2-adrenoceptor agonist, evoked a biphasic effect on PBF (Tønder, 1976; Liu et al., 1990; Kim, 1985; Okabe et al., 1989). Thus, it is possible that the isoproterenol-mediated reduction in PBF could be due to inhibition of CGRP release from trigeminal peptidergic afferent fibers [as well as the previously advanced hypothesis of stealing of blood flow by adjacent tissues (Tønder, 1976)].
The third implication from the present study derives from the observation that capsaicin selectively activates a large subpopulation of nociceptors (Caterina et al., 1997; Caterina and Julius, 2001). If capsaicin-sensitive nociceptors express the ß2-adrenoceptor, then it is possible that ß2-adrenoceptor agonists may have clinical utility as peripherally acting therapeutics for dental pain and inflammation. In support of this hypothesis, most (Brochet et al., 1986; Nakamura and Lico, 1986; Nichol et al., 1990; Peng et al., 1992; Mico et al., 1997) but not all (Khasar et al., 1999) studies have reported that ß-adrenoceptor agonists, including albuterol, have significant antinociceptive and antihyperalgesic effects in various pain models. Although some of these effects are known to be centrally mediated, the present results are consistent with an additional peripheral site of ß2-adrenoceptor regulation via capsaicin-sensitive peptidergic nociceptive afferent fibers innervating dental pulp.
| ACKNOWLEDGMENTS |
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Received August 7, 2002; Last revision November 21, 2002; Accepted January 16, 2003
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