In rats, intrathecal (i.t.) administration of glutamate, NMDA, or PGE2 induces sensitization of mechanically stimulated nociceptive C-fibers (i.e., hypernociception) in the hind paw, characterized by the expression of Nav1.8 sodium channels (1–3). Notably, hypernociception induced by i.t. administration of PGE2 is NMDA-dependent and is similar to that induced by intraplantar (i.pl.) PGE2 injection into the hind paw, in both time course and magnitude (2). In addition, it has been suggested that glutamate released at the spinal level can act retrogradely on presynaptic NMDA receptors expressed on primary nociceptive neuron (PNN) terminals in the dorsal horn, leading to maintenance of the hind paw mechanical hypernociception (1). Thus, it is conceivable that PNN hypernociception is a phenomenon involving the whole neuron, an idea supported by the fact that i.pl. injection of morphine or dipyrone inhibits mechanical hypernociception of the hind paw induced by NMDA, glutamate, or PGE2 injected i.t. (that is, in proximity to terminals at the opposite extremity of the PNNs) (1, 3).
The opioids, a class of strong centrally acting analgesics, induce several supraspinal side effects, including respiratory depression and addiction, which restrict their clinical usefulness. To avoid these problems, i.t. or epidural administration of opioids is widely used as a clinical alternative. The discovery that opioids can also exert a peripheral-local analgesic effect in inflamed tissue (4) and the development of opioids devoid of central side effects (4–6) have enabled new approaches to the clinical treatment of inflammation (7). It is essential to understand that opioids only display peripherally mediated antinociceptive activity when PNNs are already sensitized, thus explaining why opioids have an enhanced efficacy against inflammatory pain.
In fact, sensitization of the PNN (nociceptors) is a common component of inflammatory response. In 1979, we proposed that PNN sensitivity was fine-tuned by a “yin–yang” mechanism, whereby increases in neuronal cAMP and Ca2+ promote hypernociception, whereas stimulation of the cGMP signaling pathway blocks ongoing sensitization (8).
Nowadays, the “yin” side of our updated model assumes that PGE2 sensitizes C-fibers via activation of a protein kinase A- (PKA) and a protein kinase C- (PKC) mediated pathway (9–11) to modulate the voltage-sensitive tetrodotoxin-resistant sodium (3, 10, 12) and/or potassium currents (13–15), thereby facilitating the induction and conduction of PNN action potentials. At the time of the initial proposal, we suggested that morphine counteracted hypernociception directly by inhibition of PGE2-sensitive adenylyl cyclase (16), allowing us to discover the peripheral effect of opioids (4). The “yang” side of our hypothesis stated that increasing intraneuronal cGMP levels would antagonize ongoing inflammatory hypernociception (8). When the NO signaling pathway was discovered (17–20) and their pharmacological inhibitors were made accessible, it was demonstrated that the analgesic effects of i.pl. injections of acetylcholine, morphine, dipyrone, and other peripherally acting analgesics results from the stimulation of the NO signaling pathway (21–23).
In fact, the antagonism of mechanical hypernociception induced by i.t. injections of NMDA or PGE2 by morphine or dipyrone applied peripherally, i.e., at a site anatomically and neuronally distant from the i.t space, was called “teleantagonism.” The objective of the present investigation was to determine whether this teleantagonism phenomenon is specific for inhibitors of the NO signaling pathway during peripheral analgesia or is a general property of PNNs.
Teleantagonism was investigated by comparing the effects of pairs of test substances when both were injected locally into the same site (i.t. plus i.t. or i.pl. plus i.pl.) with those observed when each substance in the pair was delivered to opposite ends (i.e., distal and proximal terminals) of the PNNs (i.pl. plus i.t. or i.t. plus i.pl.) (see Fig. 1). In the present series of experiments, the intensity of hypernociception was quantified by our modification of the Randall–Selitto behavior test (see Materials and Methods).
Fig. 1.
Diagrammatic illustration of a PNN showing the sites of i.pl. or i.t., between L4 and L5, injections. For teleantagonism, the injections of agonists or enzyme inhibitors were administered to opposite neuronal sites. Note also that drugs delivered to the ...
We sought, initially, to assess the contribution of the NO signaling pathway to the inhibitory effect of i.t.-administered opioids on hind-paw hypernociception induced by i.pl.-injected PGE2. Given that NO is a gas and can readily diffuse throughout the neuron, we examined the effect of L-NMMA [a nonselective inhibitor of NO synthase (NOS)] or ODQ [an inhibitor of soluble guanylyl cyclase (sGC)], administered either i.pl. or i.t., against antihypernociception induced by opioid agonists given by the i.t. or i.pl. route, respectively. Indeed, the antihypernociceptive effect of either i.t. or i.pl. opioid agonists was teleantagonized by L-NMMA or ODQ. To further explore this neuronal pharmacodynamic property, we investigated whether opioid receptor (OR) antagonists (naloxone, the μ-OR antagonist cyprodime and the κ-OR antagonist norBNI) displayed teleantagonism against their antinociceptive agonists. In addition, we investigated whether there is teleantagonism by the nonselective inhibitor of cyclooxygenase (COX) indomethacin or by antagonists of prostaglandin EP1/EP2 or dopamine D1/D5 receptors against the hypernociception induced by IL-1β, PGE2, or dopamine, respectively. Finally we assessed whether 3H-labeled naloxone could diffuse throughout the length of the PNN nerves to reach the spinal cord or the sciatic nerve and blood, respectively, during the 3-h period of our behavioral experiments.