The aim of the present investigation was to describe and validate an electronic mechanical test for quantification of the intensity of inflammatory nociception in mice. The electronic pressure-meter test consists of inducing the animal hindpaw flexion reflex by poking the plantar region with a polypropylene pipette tip adapted to a hand-held force transducer. This method was compared to the classical von Frey filaments test in which pressure intensity is automatically recorded after the nociceptive hindpaw flexion reflex. The electronic pressure-meter and the von Frey filaments were used to detect time versus treatment interactions of carrageenininduced hypernociception. In two separate experiments, the electronic pressure-meter was more sensitive than the von Frey filaments for the detection of the increase in nociception (hypernociception) induced by small doses of carrageenin (30 µg). The electronic pressure-meter detected the antinociceptive effect of nonsteroidal drugs in a dose-dependent manner. Indomethacin administered intraperitoneally (1.8-15 mg/kg) or intraplantarly (30-300 µg/ paw) prevented the hypersensitive effect of carrageenin (100 µg/ paw). The electronic pressure-meter also detected the hypernociceptive effect of prostaglandin E 2 (PGE 2 ; 10-100 ng) in a dosedependent manner. The hypernociceptive effect of PGE 2 (100 ng) was blocked by dipyrone (160 and 320 µg/paw) but not by intraplantar administration of indomethacin (300 µg/paw). The present results validate the use of the electronic pressure-meter as more sensitive than the von Frey filaments in mice. Furthermore, it is an objective and quantitative nociceptive test for the evaluation of the peripheral antinociceptive effect of anti-inflammatory analgesic drugs, which inhibit prostaglandin synthesis (indomethacin) or directly block the ongoing hypernociception (dipyrone).

The objective of the present investigation was to compare the sensitivity of an electronic nociceptive mechanical paw test with classical mechanical tests to quantify the intensity variation of inflammatory nociception. The electronic pressure-meter test consists of inducing the hindpaw flexion reflex by poking the plantar region with a polypropylene pipette tip adapted to a hand-held force transducer. This method was compared with the classical von Frey filaments test and with the rat paw constant pressure test, a modification of the Randall and Selitto test developed by our group. When comparing the three methods, the electronic pressure-meter and the rat paw constant pressure test, but not the von Frey filaments test, detected time vs treatment interactions in prostaglandin E 2 (PGE 2 )-induced hypernociception. Both methods also detected the PGE 2 -induced hypernociception in dose-(50-400 ng/ paw) and time-(1-4 h) dependent manners, and time vs treatment interactions induced by carrageenin (25-400 µg/paw). Furthermore, the electronic pressure-meter test was more sensitive at early times, whereas the constant pressure test was more sensitive at later times. Moreover, the electronic pressure-meter test detected the dose-dependent antinociceptive effect of local indomethacin (30-300 µg/paw) and dipyrone (80-320 µg/paw) on carrageenin-(200 µg/paw) and PGE 2 -(100 ng/paw) induced hypernociception, respectively, and also detected the ineffectiveness of indomethacin (300 µg) on the effect of PGE 2 . Our results show that the electronic pressure-meter provides a sensitive, objective and quantitative mechanical nociceptive test that could be useful to characterize new nociceptive inflammatory mediators and also to evaluate new peripheral analgesic substances. Correspondence

1 Nitric oxide has been described either as pronociceptive or antinociceptive. In this investigation, using an electronic pressure-metre, the intradermal and the subcutaneous effects of prostaglandin E 2 (PGE 2 ) and agents that mimic or inhibit the arginine/NO/cGMP pathway were compared. 2 The hypernociceptive effect of the intradermal injection of PGE 2 (100 ng) was immediate, peaking within 15 -30 min and returning to basal values in 45 -60 min. The subcutaneous injection of PGE 2 induced a hypernociception with a delayed peak (3 h) plateauing for 4 -6 h. 3 Intradermal administration of 3-morpholino-sydnonimine-hydrochloride (SIN-1) enhanced, while its subcutaneous administration inhibited, subcutaneous hypernociception induced by PGE 2 . This inhibition was prevented by ODQ (8 mg) but not by NG-monomethyl-l-arginine (l-NMMA) (50 mg). 4 Intradermal but not subcutaneous administration of l-arginine (1 -100 mg), SIN-1 (1 -100 mg) and dibutyrylguanosine 3 0 :5 0 -cyclic monophosphate (db cGMP) (0.1 -100 mg) induced an early (15 -30 min) dose-dependent hypernociceptive effect. Intradermal pretreatment with NG-monomethyl-larginine (l-NMMA; 50 mg) inhibited the hypernociception induced by l-Arg (10 mg), but not that induced by SIN-1 (10 mg) or db cGMP (10 mg). 5 Intradermal injection of ODQ (8 mg) antagonized the hypernociception induced by l-arginine and SIN-1, but not that induced by db cGMP. 6 Considering (a) the different time course of intradermal and subcutaneous PGE 2 -induced hypernociception, (b) the opposite nociceptive effect of intradermal and subcutaneous administration of SIN-1 (db cGMP) as well as the arginine/NO/cGMP pathway, the existence of different subsets of nociceptive primary sensory neurons in which the arginine/NO/cGMP pathway plays opposing roles is suggested. This hypothesis would explain the apparent contradictory observations described in the literature.

. Recently, a new spinal mechanism of nociceptor sensitization, retrograde sensitization of the primary sensory neuron, was proposed (8). Induction and maintenance of inflammatory nociceptor sensitization (hypernociception) were shown to depend on spinal cord presynaptic NMDA receptors. In the present study, we have investigated the importance of sodium tetrodotoxin-resistant (TTX-R Na ϩ ) channels, which are characteristic of peripheral nociceptive small C fibers, for the development of hypernociception induced by intrathecal (i.t.) administration of NMDA.Intrathecal administration of prostaglandin E 2 (PGE 2 ), glutamate, and NMDA causes a bilateral long-lasting mechanical paw hypernociception (up to 6 h; refs. 9 and 10). It was previously shown that i.t. induced bilateral hypernociception was ipsilaterally inhibited by local s.c. injection of morphine or dipyrone. The doses of morphine and dipyrone used had no antinociceptive effect on the contralateral paw hypernociception (8, 9). In contrast with inhibitors of prostaglandin synthesis, which prevent the development of nociceptor sensitization, local administration of morphine or dipyrone directly antagonized ongoing hypernociception (10, 11). Peripheral ipsilateral blockade of mechanical hypernociception induced by i.t.-administered mediators constitutes a simple and straightforward behavioral test to show retrograde sensitization of primary sensory neurons.In the present investigation, mechanical hypernociception was measured with an electronic version of the von Frey hair test, in which the force that evokes a behavioral withdrawal is automatically recorded by an electronic pressure meter. Bilateral paw hypernociception was induced by i.t. administration of NMDA or PGE 2 . Pretreatment of the paws with dipyrone or morphine, at doses shown previously to cause antinociception only in the injected paws, was used to show the sensitization of the primary sensory neurons.Several studies have reported that inflammatory stimuli or mediators can significantly increase TTX-R Na ϩ channel NaV1.8 (SNS͞PN3) mRNA expression and enlarge the amplitude of TTX-R Na ϩ channel currents in dorsal root ganglia (DRG;. TTX-R Na ϩ channels are characteristically associated with fine primary sensory fibers (15, 16). By using antisense oligodeoxynucleotides (ODNs) to selectively ''knock down'' the expression of NaV1.8 (a specific and molecularly distinct TTX-R Na channel), it was shown that NaV1.8 present in primary afferent nociceptors contributes to peripheral sensitization induced by local PGE 2 (15,17). In the present study we reduced the expression of NaV1.8 mRNA by successive i.t. injections of specific antisense ODNs and compared the intensity of paw hypernociception induced by i.t. administration of NMDA with control animals (treated with mismatch ODNs followed by i.t. administration of NMDA). Materials and MethodsAnimals. The experiments were performed on 180-to 200-g male Wistar rats housed in an animal care facility of the University of São Paulo and taken to the testing ...