Figure 7.

Schematic diagram of proposed mechanism of reciprocal interaction between IP₃ and ryanodine receptors in the induction of hyperalgesic priming. Activation of PKCε (in males, only) by the priming stimulus in the peripheral terminal of an IB4⁺ nociceptor (right side of figure) triggers a cascade of events that ultimately produces hyperalgesic priming (Aley et al., 2000; Joseph et al., 2003; Joseph and Levine, 2010; Ferrari et al., 2013b), which includes calcium release from ER and requires both ryanodine (RyR) and IP₃ (IP₃R) receptors, activation of αCaMKII, and cytoplasmic polyadenylation element binding protein (CPEB) (Ferrari et al., 2013b). Direct activation of RyR and IP₃R also produces priming, in males and females. We hypothesize that release of calcium ions (Ca²⁺) from ER stores, causing a local increase in [Ca²⁺]_i (dashed and magnified boxes, middle) (Ehrlich et al., 1994; MacKrill, 1999, 2012) is a key factor required for the induction of priming. The suggested interaction between RyR and IP₃R, by Ca²⁺ released through these ionotropic receptors, is shown in the magnified box. Once outside the ER, Ca²⁺ can activate nearby RyR and IP₃R, increasing [Ca²⁺]_i even more, via CICR, to reach a level high enough to trigger mechanisms, such as calmodulin and αCaMKII (Shakiryanova et al., 2007, 2011; Wong et al., 2009), leading to induction of priming (Ferrari et al., 2013b). The ER calcium pump, SERCA, transports the Ca²⁺ released into the cytosol, back to stores, to refill them, restricting Ca²⁺ signals spatially and temporally, a “loop” mechanism that has been associated with the development of neuroplasticity (Pal et al., 2001; Scullin and Partridge, 2010; Kato et al., 2012; Komin et al., 2015; Yasuda and Mukai, 2015). Additional Ca²⁺ buffering in the cytosol limits the fast diffusion of Ca²⁺, supporting its local accumulation. Thus, the whole system of RyR and IP₃R can be triggered by administration of IP₃ or ryanodine. When the concentration of agonists reaches a certain level, the effective doses able to induce priming (Ferrari et al., 2016) (Fig. 2a), the local cascade of propagating CICR becomes self-supported. We believe this “small” (in the scale of the whole cell), but long-lasting, change in Ca²⁺ homeostasis is a crucial factor for the development of hyperalgesic priming. In contrast, when the Ca²⁺ signal produced is strong but of short duration, for example, that produced by calcium ionophore, due to the bell-shaped sensitivity of RyR and IP₃R for [Ca²⁺]_i, the production of the self-supported long-lasting local [Ca²⁺]_i increase is prevented, and hyperalgesic priming does not develop. Although the direct interaction between the receptors is important to generate amplification, we consider Ca²⁺ a key mediator of the reported reciprocal modulation of RyR and IP₃R, and the amount of Ca²⁺ initially released is the most important factor in the mechanism, regardless of which receptor is stimulated. This constitutes our paradigm of “the common (shared) amplifier,” postulating the amplification of Ca²⁺ signals produced by either input (RyR or IP₃R), which explains the similarity in the difference between males and females in the sensitivity to be primed by either IP₃ or ryanodine. Although this amplification system is present in both males and females, it is less potent in males (a 100,000-fold difference). In regard of the higher sensitivity of females to be primed by ryanodine or IP₃, the action of estrogen on EsRα (left) potentiates the amplification system, likely by sensitizing the ER receptors, as suggested by previous reports: modulation of RyR by estrogen-PKA signaling (Lin et al., 2007) or potentiation of IP₃R by estrogen through metabotropic glutamate receptor 1, phospholipase C, and generation of IP₃ (Tabatadze et al., 2015) (left, large arrow). This latter pathway could also result in tonic sensitization of IP₃R in females, due to an increased basal IP₃ levels. As shown in Figures 4 and 6, tonic activation of EsR in cultured male nociceptors, by incubation in the presence of β-estradiol, also potentiates their sensitivity to both ryanodine and IP₃. This finding suggests that the sensitizing machinery of EsR is also present in males, but probably not active enough due to a lower level (or lack) of estrogen. Nevertheless, there is still a possibility that the EsRα-dependent sensitization in females (as well as in males, when activated) is indirect, due to the suppression of some blocking pathway active in males and females when EsRα is not stimulated. Although little is known about endogenous competitive antagonists shared by RyR and IP₃R, according to our “shared amplification system paradigm,” suppression of either RyR and IP₃R by their antagonists, including endogenous (Lenzen and Rustenbeck, 1991; Uehara et al., 1996; Vervliet et al., 2015), could indeed reduce the sensitivity of the whole system (as shown in Figs. 1, 2b). Whether this mechanism is involved in the observed difference between males and females in our study remains to be a determined.