Incomplete suppressive effects of acetylsalicylic acid on the heat sensitization of canine testicular polymodal receptor activities

Sensory and signaling mechanisms of bradykinin, eicosanoids, platelet-activating factor, and nitric oxide in peripheral nociceptors

Peripheral mediators can contribute to the development and maintenance of inflammatory and neuropathic pain and its concomitants (hyperalgesia and allodynia) via two mechanisms. Activation or excitation by these substances of nociceptive nerve endings or fibers implicates generation of action potentials which then travel to the central nervous system and may induce pain sensation. Sensitization of nociceptors refers to their increased responsiveness to either thermal, mechanical, or chemical stimuli that may be translated to corresponding hyperalgesias. This review aims to give an account of the excitatory and sensitizing actions of inflammatory mediators including bradykinin, prostaglandins, thromboxanes, leukotrienes, platelet-activating factor, and nitric oxide on nociceptive primary afferent neurons. Manifestations, receptor molecules, and intracellular signaling mechanisms of the effects of these mediators are discussed in detail. With regard to signaling, most data reported have been obtained from transfected nonneuronal cells and somata of cultured sensory neurons as these structures are more accessible to direct study of sensory and signal transduction. The peripheral processes of sensory neurons, where painful stimuli actually affect the nociceptors in vivo, show marked differences with respect to biophysics, ultrastructure, and equipment with receptors and ion channels compared with cellular models. Therefore, an effort was made to highlight signaling mechanisms for which supporting data from molecular, cellular, and behavioral models are consistent with findings that reflect properties of peripheral nociceptive nerve endings. Identified molecular elements of these signaling pathways may serve as validated targets for development of novel types of analgesic drugs.

Bradykinin-induced nociceptor sensitization to heat is mediated by cyclooxygenase products in isolated rat skin

Bradykinin can excite C-polymodal nociceptors and sensitize them to heat and it can also enhance prostaglandin synthesis, but it is unclear whether these effects are causally related. The role of cyclooxygenase products was investigated using two enantiomers of the cyclooxygenase inhibitor flurbiprofen of which S(+)- is more potent than R(-)-flurbiprofen. Single-unit activity was recorded from mechano-heat-sensitive, polymodal C-fibers in the isolated rat skin-saphenous nerve preparation. Bradykinin pretreatment (10 microM, 5 min) induced a 219 +/- 26% increase in the number of spikes evoked by noxious heat stimulation and a drop in the heat threshold by 5.2 +/- 0.6 degrees C in a fully reproducible manner. S(+)-flurbiprofen (1 microM) abolished the bradykinin-induced heat sensitization but did not alter the unconditioned heat response itself. Under R(-)-flurbiprofen (1 microM) bradykinin still induced a significant heat sensitization which was reduced by 33 +/- 21% (P = 0.11) of its previous extent; this effect may be due to the limited purity of the enantiomer preparation or to a cyclooxygenase-independent action of flurbiprofen. The heat sensitization suppressed by S(+)-flurbiprofen could be significantly restored (to 43 +/- 12%) by addition of PGE(2) plus PGI(2) (10 microM both) to bradykinin. Neither S(+)- nor R(-)-flurbiprofen had an influence on the magnitude of the excitatory effect of bradykinin. It is concluded that (i) cyclooxygenase products are the main mediators of nociceptor sensitization to heat following bradykinin treatment in the isolated rat skin; (ii) PGE(2)/I(2) are essential but perhaps not the only relevant cyclooxygenase products involved and (iii) neither S(+)- nor R(-)-flurbiprofen inhibit the unconditioned noxious heat response and the excitatory bradykinin response of the polymodal C-nociceptors.

Primitivism and plasticity of pain--implication of polymodal receptors

Bio-warning and defense mechanisms play the most fundamental roles in living organisms. From an evolutionary point of view, nociceptive systems are very primitive and are richly provided with humoral signaling mechanisms of aboriginal humoral defense systems, as reflected in the primitive nature of the polymodal receptor, a poorly differentiated sensory receptor signaling nociceptive information. Recent advances in studies on pain have made it possible to explain neural mechanisms of pain systems under physiological conditions and reveal that there is a large gap between physiological and pathological pains. Protracted nociceptive inputs under pathological conditions induce plastic, either functional or structural, alterations in the nociceptive pathways. These plastic changes lead to crosstalk among the neural networks, including circuits related to motor, autonomic, or psychological functions. These plastic changes, once established, persist even after the original pain sources disappear in a memory-like fashion. Thus, it is revealed that chronic pain cannot be treated by blocking pain pathways, which is effective against acute pain, but require treatment from a multidisciplinary perspective.

Stable analogues of cyclic AMP but not cyclic GMP sensitize unmyelinated primary afferents in rat skin to heat stimulation but not to inflammatory mediators, in vitro

The aim of this investigation was to evaluate the role played by cyclic nucleotides in the transduction of inflammatory pain and hyperalgesia. Unmyelinated afferents (n = 79) were exposed to stable analogues of cyclic AMP and cyclic GMP, to inflammatory mediators and to Methylene Blue, an inhibitor of guanylyl cyclase. Analogues of cyclic AMP at a concentration of 1 mM (n = 9) but not 10 microM (n = 16) sensitized nociceptor responses to noxious heat and enhanced interstimulus activity. In addition. mechanical thresholds were moderately, but significantly lowered after superfusion of the cyclic AMP analogue (1 mM). Addition of 10 microM cyclic AMP analogue to a mixture of excitatory inflammatory mediators (serotonin, histamine, bradykinin and prostaglandin E2, 10 microM each) did not further increase nociceptor activity (n = 15), in contrast to a previous report that cAMP sensitized bradykinin responses. Cyclic GMP analogues (10 microM, 1 mM) did not alter heat sensitivity or mechanical thresholds of polymodal C-fibres, nor did they enhance the ongoing activity that resulted from repeated heat stimulation. After inhibition of guanylyl cyclase with Methylene Blue, cyclic GMP analogues (1-10 microM) did not alter nociceptor responses evoked by application of the mixture of inflammatory mediators. The findings indicate that polymodal nociceptor sensitization and excitation is independent of cyclic GMP. Cyclic AMP can obviously contribute to the increased heat sensitivity of inflamed tissue, whereas cyclic GMP might be of importance in the recruitment of "silent" nociceptors.