Effect of centrally administered prostaglandin D2 and some prostaglandin synthesis inhibitors on carrageenan-induced paw oedema in rats

Acetaminophen Metabolite N-Acylphenolamine Induces Analgesia via Transient Receptor Potential Vanilloid 1 Receptors Expressed on the Primary Afferent Terminals of C-fibers in the Spinal Dorsal Horn

BACKGROUND

The widely used analgesic acetaminophen is metabolized to N-acylphenolamine, which induces analgesia by acting directly on transient receptor potential vanilloid 1 or cannabinoid 1 receptors in the brain. Although these receptors are also abundant in the spinal cord, no previous studies have reported analgesic effects of acetaminophen or N-acylphenolamine mediated by the spinal cord dorsal horn. We hypothesized that clinical doses of acetaminophen induce analgesia via these spinal mechanisms.

METHODS

We assessed our hypothesis in a rat model using behavioral measures. We also used in vivo and in vitro whole cell patch-clamp recordings of dorsal horn neurons to assess excitatory synaptic transmission.

RESULTS

Intravenous acetaminophen decreased peripheral pinch-induced excitatory responses in the dorsal horn (53.1 ± 20.7% of control; n = 10; P < 0.01), while direct application of acetaminophen to the dorsal horn did not reduce these responses. Direct application of N-acylphenolamine decreased the amplitudes of monosynaptic excitatory postsynaptic currents evoked by C-fiber stimulation (control, 462.5 ± 197.5 pA; N-acylphenolamine, 272.5 ± 134.5 pA; n = 10; P = 0.022) but not those evoked by stimulation of Aδ-fibers. These phenomena were mediated by transient receptor potential vanilloid 1 receptors, but not cannabinoid 1 receptors. The analgesic effects of acetaminophen and N-acylphenolamine were stronger in rats experiencing an inflammatory pain model compared to naïve rats.

CONCLUSIONS

Our results suggest that the acetaminophen metabolite N-acylphenolamine induces analgesia directly via transient receptor potential vanilloid 1 receptors expressed on central terminals of C-fibers in the spinal dorsal horn and leads to conduction block, shunt currents, and desensitization of these fibers.

Mechanism of action of paracetamol

Paracetamol (acetaminophen) is generally considered to be a weak inhibitor of the synthesis of prostaglandins (PGs). However, the in vivo effects of paracetamol are similar to those of the selective cyclooxygenase-2 (COX-2) inhibitors. Paracetamol also decreases PG concentrations in vivo, but, unlike the selective COX-2 inhibitors, paracetamol does not suppress the inflammation of rheumatoid arthritis. It does, however, decrease swelling after oral surgery in humans and suppresses inflammation in rats and mice. Paracetamol is a weak inhibitor of PG synthesis of COX-1 and COX-2 in broken cell systems, but, by contrast, therapeutic concentrations of paracetamol inhibit PG synthesis in intact cells in vitro when the levels of the substrate arachidonic acid are low (less than about 5 mumol/L). When the levels of arachidonic acid are low, PGs are synthesized largely by COX-2 in cells that contain both COX-1 and COX-2. Thus, the apparent selectivity of paracetamol may be due to inhibition of COX-2-dependent pathways that are proceeding at low rates. This hypothesis is consistent with the similar pharmacological effects of paracetamol and the selective COX-2 inhibitors. COX-3, a splice variant of COX-1, has been suggested to be the site of action of paracetamol, but genomic and kinetic analysis indicates that this selective interaction is unlikely to be clinically relevant. There is considerable evidence that the analgesic effect of paracetamol is central and is due to activation of descending serotonergic pathways, but its primary site of action may still be inhibition of PG synthesis. The action of paracetamol at a molecular level is unclear but could be related to the production of reactive metabolites by the peroxidase function of COX-2, which could deplete glutathione, a cofactor of enzymes such as PGE synthase.

Central muscarinic receptor subtypes and carrageenin-induced paw oedema in rats

In an earlier report from this laboratory, it was demonstrated that the central cholinergic system exerted a modulatory pro-inflammatory effect on carrageenin-induced paw inflammation in rats. In this study, the role of the central muscarinic receptors in cholinergic modulation of peripheral inflammation was investigated by using several M1 and M2 receptor agonists and antagonists. The M1 receptor agonists aceclidine and arecholine and the nonspecific muscarinic receptor agonist oxotremorine augmented carrageenin oedema, an effect attenuated by the M1 receptor antagonist scopolamine. Physostigmine behaved like a M1 receptor agonist at all dose levels. However, the other M2 receptor agonist, carbachol, produced a dose-dependent dual effect, with lower doses attenuating the oedema and higher doses augmenting the inflammation. While the former action appeared to be due to M2 receptor stimulation, because it was blocked by AF-DX 116--a M2 receptor antagonist, the latter action appeared to be induced by M1 receptor stimulation, because it was inhibited by scopolamine. The pro-inflammatory effect of the M2 receptor antagonists AF-DX 116 and gallamine appeared to be induced by enhanced neuronal release of acetylcholine, because the effects were not evident following pretreatment with hemicholinium, which attenuates synthesis of the amine. Muscarinic receptor binding studies with (3H)-QNB indicated that the corpus striatum has substantially higher population of M1 receptors compared with the cerebellum. In the corpus striatum, (3H)-QNB binding indicated initial up-regulation followed by down-regulation of M1 receptors during peak inflammation, which appeared to persist even after a decrease in the inflammation. In contrast, the M1 receptors in the cerebellum appeared to be down-regulated very transiently during the early phase of the inflammation. While these receptor alterations may be due to the inflammation, it is equally possible that they represent changes induced by the stress of pain and inflammation induced by carrageenin.