Possible involvement of the EP2 receptor subtype in PGE2-induced enhancement of the heat response of nociceptors

Prostaglandin E2 augments bradykinin- and heat-induced discharges of polymodal receptors as studied in vitro preparations. Our previous study revealed the involvement of the EP3 receptor subtype in the PGE2 induced enhancement of the BK response [Brain Res. 632 (1993) 321-324]. The agonist for EP2 (butaprost; 10(-8) M) significantly augmented heat responses, but did not augment the BK responses at concentrations from 10(-8) to 10(-5) M; however, the agonist for EP3 (M&B28767) or EP1 (17-phenyl-trinor-PGE2) at 10(-7) M did not affect the heat responses. These findings indicate the involvement of the EP2 receptor subtype in the augmenting effect of PGE2 on heat responses.

Histamine-induced sensitization of the heat response of canine visceral polymodal receptors

In elucidating the possible involvement of histamine (His) in hyperalgesia to heat, the effects of His on the heat response of testicular polymodal receptors were studied in vitro in canine testis-spermatic nerve preparations. His (100 and 1000 microM) induced discharges > 0.5 impulses/s (high-responders) in about one third of units tested. His (> or = 10 microM) facilitated the heat response irrespective of whether His alone induced substantial excitation, and the magnitude of facilitation at 100 microM did not differ between high- and low-responders (His-induced discharges < or = 0.5 impulses/s). Conduction velocity of high-responders at 100 microM (2.2 +/- 0.9 m/s, n = 5) was slower on average than that of low-responders (12.9 +/- 1.6 m/s, n = 9). The sensitizing effect of His reported in this paper suggests that His plays a role in hyperalgesia.

Forskolin does not augment the bradykinin response of canine visceral polymodal receptors in vitro

In clarifying the possible involvement of cyclic AMP (cAMP) in prostaglandin (PG) E2-induced sensitization of the bradykinin response of canine testicular polymodal receptors, the effects of forskolin, an activator of adenylyl cyclase, were studied in the presence and absence of acetylsalicylic acid (ASA, 550 microM) which blocks PG production. Forskolin (10 microM) seldom induced discharges in polymodal receptors. An unexpected outcome of this study was that forskolin induced no facilitation of the bradykinin (0.1 microM) response, both in the absence and presence of ASA. A slight yet significant suppression of the bradykinin response was instead observed in the absence of ASA. These results suggest that intracellular cAMP may be related with PG E2-induced sensitization of the bradykinin response through its decrease.

Involvement of EP3 subtype of prostaglandin E receptors in PGE2-induced enhancement of the bradykinin response of nociceptors

Prostaglandin E2 augments bradykinin-induced discharges of polymodal receptors as studied in vitro preparations. The antagonist and agonists for three subtypes of EP receptors were used to determine which subtype is involved in this phenomenon. The agonist for EP3 (M&B28767) simulated the PGE2-induced effect but not for EP2 (butaprost). The antagonist for EP1 (AH6809) did not suppress the effect. These findings indicate the involvement of the EP3 receptor subtype in the effect.

Augmenting effects of cyclic AMP on the heat response of canine testicular polymodal receptors

It has long been thought that the augmenting effects of E series prostaglandins (PG) are mediated by cyclic AMP (cAMP). In clarifying the validity of this hypothesis, we studied the effects of increases in intracellular cAMP on the heat response of testicular polymodal receptors. Recordings were obtained from testis-spermatic nerve preparations excised from dogs anesthetized with pentobarbital (30 mg/kg, i.v.). Induction of increases in intracellular cAMP by either forskolin, an adenylyl cyclase activator, or a mixture of dibutyryl cAMP (10(-4) M) and 3-isobutyl-1-methyl xanthine (10(-4) M) significantly and reversibly augmented the heat response. These results support the notion that cAMP is involved in the augmenting effect of PG E2 on the heat response of nociceptors.

Prostaglandin E2-induced sensitization of the heat response of canine visceral polymodal receptors in vitro

The sensitizing effect of prostaglandin (PG) E2 on the heat response of testicular polymodal receptors was studied in vitro by recording single polymodal receptor activities from the testis-spermatic nerve preparations excised from deeply anesthetized dogs. PG E2 at 10(-7) M had no significant influence on the heat response. PG E2 at 10(-6) and 10(-5) M did not induce substantial increases in discharge rates, yet it significantly augmented the heat response. The concentration necessary for augmenting the heat response is thus 100 times greater than that (10(-8) M) previously reported for bradykinin [Naunyn-Schmiedebergs Arch. Pharmacol., 344 (1991) 368-376].

Spinal termination patterns of canine identified A-delta and C spermatic polymodal receptors traced by intracellular labeling with Phaseolus vulgaris-leucoagglutinin

The spinal projection patterns of spermatic polymodal receptors were studied by intracellular labeling of functionally identified canine dorsal root ganglion (DRG) neurons with Phaseolus vulgaris-leucoagglutinin (PHA-L). The processes of 2 C-fiber and 1 A-delta-fiber spermatic polymodal receptor neurons were labeled well enough to trace their central terminations. The labeled C-fiber DRG neurons were of medium size (mean diameter 36.8 and 40.7 microns). On entering the spinal cord, axons of the C-polyclonal receptors divided into rostral and caudal main branches that extended over 3 spinal segments (20 and 25 mm, respectively), and issued a total of 16 and 15 collaterals, respectively. The majority of collaterals ran in or along the lateral surface, but both neurons had 1 or 2 collaterals or terminal branches running through the middle, or along the medial surface of the dorsal horn. Terminal swellings and en passant enlargements were observed mainly in laminae I, V, and VII. Some C-fiber terminations appeared in lamina II and the adjacent lateral column. The A-delta polymodal receptor had a termination pattern similar to that of the C-fiber units with the exception of a shorter distance over which its 13 identified collaterals were issued (10 mm), and continuation of the rostral main branch into Lissauer's tract (traced for 3.6 mm) after all branches appeared. Two terminal branches were found running just above the central canal in another A-delta neuron in which termination could be only partially traced.(ABSTRACT TRUNCATED AT 250 WORDS)

Strong heat stimulation sensitizes the heat response as well as the bradykinin response of visceral polymodal receptors

1. Hyperalgesia after thermal injury is a common phenomenon, but its mechanism is poorly understood. As a step toward understanding the mechanism for heat sensitization, we searched conditions that would induce sensitization consistently and studied changes in polymodal receptors in their responses to heat (45 and 48 degrees C) and to bradykinin. Experiments were conducted in vitro, with the use of testis-superior spermatic nerve preparations excised from anesthetized dogs. 2. Stimulation at 55 degrees C for 30 s induced clear augmentation of the responses to stimulations at 45 and 48 degrees C when tested within 10 min in 22 out of 24 testicular polymodal units. In the remaining two units, weak or delayed augmenting tendencies were observed. The response to stimulation at 45 degrees C increased from 0.28 +/- 0.08 to 2.99 +/- 0.46 (SE) imp/s, whereas at 48 degrees C it increased from 1.08 +/- 0.16 to 4.60 +/- 0.63 imp/s (P < 0.001, n = 24). A low-level ongoing discharge appeared in about one-half of the units studied. 3. Despite frequent rinsing of the receptive field, augmentation of the heat response observed after stimulation at 55 degrees C continued for up to approximately 3 h. 4. The response to bradykinin (94 nM) was also potentiated: the net mean discharge rate (NMDR) during a 1-min application period of bradykinin as well as the total number of impulses induced by one bradykinin application significantly increased from 0.91 +/- 0.27 to 1.97 +/- 0.32 imp/s and from 213.9 +/- 42.3 to 433.5 +/- 58.7 impulses, respectively (P < 0.001, n = 15).(ABSTRACT TRUNCATED AT 250 WORDS)

Sensitizing effects of bradykinin on the heat responses of the visceral nociceptor

1. Whether bradykinin (BK), which is known as an endogenous pain-producing substance, induces augmentation of the discharges of polymodal receptors evoked by heat stimulation was investigated in in vitro canine testis-superior spermatic nerve preparations. 2. The heat response was significantly augmented by pretreatments with BK at concentrations greater than 0.094 nM, whereas BK induced significant increases in the mean discharge rates at concentrations above 9.4 nM. Both effects increased in a concentration-dependent manner. The augmenting effect of BK on the heat response diminished within 10 min after application of BK, regardless of concentration. 3. When 9.4 nM BK was applied in a mixture with 940 nM NPC349, a B2 receptor antagonist, the averaged mean discharge rate evoked by BK and the averaged augmenting effect were both significantly suppressed compared with those induced by BK given alone. 4. The augmenting effect of BK on the heat response of polymodal receptors could be observed even in the absence of BK-evoked discharges per se in several cases in which low concentrations of BK or BK plus B2 antagonists were given. 5. These findings suggest that the augmenting effects of BK on the heat response depend on B2 receptor-mediated intracellular processes acting in parallel to, but not directly on, the impulse-generation mechanism of the heat response of the polymodal receptor.

Comparison of the effects of prostaglandins E2 and I2 on testicular nociceptor activities studied in vitro

Effects of exogenous prostaglandin (PG) E2 and PGI2 on testicular polymodal receptor activities were compared in in vitro recordings of single- or multi-fiber discharges from canine testis-spermatic nerve preparations. PGI2 up to 1.4 x 10(-6) mol/l (cumulative method) or 1.0 x 10(-5) mol/l (non-cumulative method) excited only weakly some of the receptors, and similar observations were made with PGE2. Both PGs applied cumulatively or non-cumulatively at concentrations above 1.4 x 10(-8) mol/l augmented the response to bradykinin (9.4 x 10(-8) mol/l) in more than half of the cases tested. The augmenting effect of PGE2 lasted longer than that of PGI2 both with the cumulative and the non-cumulative method. The degree of augmentation tended to increase dependent on concentration, but some cases showed no further increase or rather a decrease in augmentation by PGs at a ten times higher concentration, especially when PGs were applied cumulatively. A second challenge by PG after a short interval (2 min) did not induce augmentation. These phenomena were considered to be tachyphylaxis to PGs. Cross-tachyphylaxis to PGE2 and PGI2 was also observed. There was not much difference in excitatory and augmenting potencies between these two PGs, but there was a clear difference in the concentrations of the PGs necessary to induce excitation of polymodal receptors and to facilitate their response to bradykinin.

The effects of bradykinin agonists and antagonists on visceral polymodal receptor activities

The endogenous algesic agent bradykinin (BK) is a consistent stimulant for the polymodal receptor, a type of nociceptor. Two types of BK receptor, B1 and B2, have been proposed in smooth muscles by Regoli. The type of BK receptor mediating the BK response of the polymodal receptor was studied using 3 BK analogs, des-Arg9-BK (a B1 agonist), des-Arg9-[Leu8]-BK (a B1 antagonist), and [Thi5,8, D-Phe7]-BK (a B2 antagonist). Single- and multi-fiber activities from testicular polymodal receptors were recorded in vitro using testis-spermatic nerve preparations excised from dogs anesthetized with pentobarbital (30 mg/kg, i.v.). Neither des-Arg9-BK, des-Arg9-[Leu8]-BK, nor [Thi5,8,D-Phe7]-BK induced discharges in nociceptors at concentrations up to 9.4 x 10(-6) M. Des-Arg9-[Leu8]-BK (up to 9.4 x 10(-6) M) did not suppress responses to BK (9.4 x 10(-8 approximately -9) M), whereas [Thi5,8,D-Phe7]-BK (above 2.8 x 10(-7) M) suppressed the BK response in a concentration-dependent manner and shifted the concentration-response curve of BK to the right. It was ascertained that [Thi5,8,D-Phe7]-BK had no effect on responses to noxious heat and high K+ solution. These results suggest that the BK receptor mediating the nociceptor response to BK is of the B2 type.

Facilitatory effects of opioids on the discharges of visceral nociceptors

Effects of opioids on the activity of visceral nociceptors were tested using the in vitro testis-spermatic nerve preparations excised from deeply anesthetized dogs. Morphine, DADLE and dynorphin (10 microM) elicited discharges of polymodal receptors in approximately 1/3 of the tested cases. The incidence of the excitatory response of morphine increased at higher concentrations. The excitatory responses were quite variable among preparations and showed a strong tendency for tachyphylaxis. Similar increases in discharges were elicited when morphine was applied during the steady state of the response evoked by bradykinin (BK) or BK mixed with prostaglandin E2. Pretreatment of morphine for 5 min significantly augmented the subsequent BK responses for 30 min or more. Naloxone per se induced neither excitation nor augmentation of the subsequent BK response, however it reversed the augmenting effect of morphine on BK response. In contrast with previous reports proposing peripheral analgesic effects of opioids, suppressive effects on nociceptors were never observed in the present experiment. Peripheral effects of opioids were discussed.

Effects of ionic calcium on the responses of canine testicular polymodal receptors to algesic substances

1. To explore possible mechanisms of the responses to algesic substances (bradykinin, hypertonic saline, and high K+ solution) of polymodal receptors in the canine testis, the Ca2+ concentration was varied in vitro. 2. After 1 min in Ca2+-free media, the responses to both high K+ solution (60 mM K+) and hypertonic saline (0.6 M Na+) were significantly augmented and tended to increase further with time; return to normal Ca2+ concentration quickly reversed these changes. These augmenting effects were blocked by the substitution of Mg2+ for Ca2+. The excitation produced by 60 mM K+ was decreased by increasing Ca2+ in a concentration-dependent fashion. 3. Reducing the bath concentration of K+ decreased responses evoked by 9 X 10(-8) M bradykinin (BK), whereas increased K+ concentration had the opposite effect. 4. The excitatory effects of BK were significantly suppressed in extracellular Ca2+-free condition. The suppression was not affected by the addition of Mg2+. Prostaglandin E2, which has been known to be released by BK and to augment the BK response, failed to restore the suppressed response by either preapplication (2.8 X 10(-7) M) or simultaneous application in high concentration (1.4 X 10(-5) M). 5. On the basis of these observations, it was postulated that Ca2+ concentration-dependent changes of the responses to 60 mM K+ and 0.6 M Na+ results from Ca2+-dependent "membrane surface potential" changes. The suppressed response to BK by Ca2+ depletion may be explained by the intervention of Ca2+-dependent processes other than PG production.

Respiratory changes induced by activation of testicular afferents in dogs

Reportedly, more than 90% of the testicular afferents of the dog are of the polymodal type. The possible involvement of these afferents in modulation of respiration was studied using anesthetized dogs, which had been vagotomized and with both common carotid arteries ligated. Electrical stimulation of the superior spermatic nerve at an intensity of 1/5 Tc (Tc: threshold intensity for C-fiber activation) induced no substantial changes in respiration, while above 1/2 Tc it induced an increase in minute expiratory volume (VE), or a decrease followed by an increase in VE. The pattern of respiratory change was converted from the former to the latter by increasing the stimulation frequency while maintaining the same intensity. Mechanical stimulation of the testis through a stimulator with a tip of 1 cm in diameter caused an increase of VE at 200 g and a decrease followed by an increase of VE above 500 g. Intra-arterial injection of bradykinin (3 x 10(-6) M), hypertonic saline (1.5 M) and high K+ solution (180 mM) to the spermatic artery induced similar respiratory changes. Pneumograms showed a shift in end-expiratory position even at stimulus intensities without significant VE changes. Phrenic nerve recordings from some artificially ventilated dogs, revealed an prolongation of the first expiratory phase followed by augmentation of phrenic activity, mostly an increase in respiratory rate. Comparison between afferent activities and reflex respiratory changes suggests that above described two types of respiratory changes are brought about by the activities of testicular polymodal receptors.

Observations on electrophysiologically characterized receptive fields of thin testicular afferent axons: a preliminary note on the analysis of fine structural specializations of polymodal receptors

The sparse distribution of thin, principally unmyelinated sensory axons confined largely to the planar tunica vasculosa of the testis provides a suitable model for examining the fine structure of electrophysiologically characterized nerve fiber terminals. The marked sites of polymodal receptors of canine testis using the in vitro preparation devised by Kumazawa et al. (1987) were examined in serial sections traced to the terminal with the electron microscope, revealing the first micrographs of a characterized polymodal receptor ending. The inferred role of these terminals in nociception, their organelle content, and the problems encountered in interpreting our initial findings are considered in the context of the variety of morphological patterns and functional roles of thin sensory axons.

Effects of prostaglandins and other putative chemical intermediaries on the activity of canine testicular polymodal receptors studied in vitro

An in vitro testis-superior spermatic nerve preparation was used to evaluate the effects of chemical agents applied in the bathing solution. Both directly evoked discharges and responses to algesic solutions [bradykinin (BK) 9 X 10(-8) M, hypertonic saline 616 mM and high K+ solution 60 mM] of polymodal receptors were studied. Prostaglandin (PG)-E2 (1.4 X 10(-6)-1.4 X 10(-5) M) and serotonin (5-HT) (1.1 X 10(-6) to 1.4 X 10(-4) M) had only a weak excitatory effect. However, test responses to algesic substances were regularly greatly increased by PG-E2, -I2 and 5-HT. Concentrations of PG-E2 of 1.4 X 10(-8) M or greater augmented BK responses; higher concentrations and/or longer applications were needed to enhance responses to algesic salt solutions. Effective concentrations for the PGs and 5-HT were near those reported for inflamed tissues and exudate. Aspirin (ASA) (5.5 X 10(-4) M or greater, for more than 4 min) suppressed the responses to BK but not those evoked by hypertonic saline. The ASA effect on the BK response was largely restored by an addition of PG-E2. Substance P also had a weak excitatory effect on some polymodal receptors, but no significant enhancement of the response to BK was noted. These results further support a role of polymodal receptors in transmitting nociceptive information, of inflammatory origin.

Response properties of polymodal receptors studied using in vitro testis superior spermatic nerve preparations of dogs

Characteristics of the polymodal receptor were studied using in vitro testis superior spermatic nerve preparations excised from anesthetized dogs. They were in most aspects similar to those reported previously using in vivo preparations. The majority (90%) of the tested polymodal units had small myelinated nerve fibers; the rest had nonmyelinated fibers. The mean mechanical threshold as determined by von Frey hairs was 17.5 g/mm2 (n = 476). There was a tendency for a unit with a higher conduction velocity to have a lower mechanical threshold. Bradykinin and hypertonic saline consistently caused a dose-dependent increase in discharge rate of these units; high K+ solution was also found to be a consistent stimulant. The responses of C-fiber receptors were not significantly different from those of A-delta-fibers. Heat stimulation up to 50 degrees C evoked discharges in 99 out of the total 103 units tested. The mean threshold temperature was 44.4 degrees C for the first trial. In 19 units in which the same heat stimulation was tested after an interval of 10 min, 10 units showed sensitization; 3 units were deactivated; and no clear difference was observed in the rest. No unit responded with a substantial increase in discharge rate to cold stimuli of 20 degrees C or less. A small temperature rise of 2 degrees C from the normal surface temperature of the testis (34 degrees C) significantly increased the response to hypertonic saline (616 mM) (2.41 +/- 0.22 impulses/s at 34 degrees C to 3.23 +/- 0.44 impulses/s at 36 degrees C) and to bradykinin (9 X 10(-8) M) (1.95 +/- 0.35 impulses/s at 34 degrees C to 2.85 +/- 0.19 impulses/s at 36 degrees C). The majority of the units recorded from the superior spermatic nerve in this experiment were most probably of polymodal receptor type, although the heat response was tested in a limited number of units. A very small number of a different type of receptor was discovered: rapidly adapting mechano-receptors, which responded almost exclusively to mechanical stimulation and were especially sensitive to a light mechanical stimulus moving across the receptive fields. The response properties of receptors studied in vitro remained practically unchanged during the experiments of several hours. The present experiments have shown that this preparation is suitable for systematic investigations, especially of the effects of chemical agents, on the polymodal receptor, which plays important roles in nociceptive functions.

Thermally potentiated responses to algesic substances of visceral nociceptors

Using spermatic nerve preparations from testis superior, the effects of temperature rise on chemical responses were studied in vitro, within both subthreshold and suprathreshold ranges for testicular polymodal receptors, one type of visceral nociceptor. In the range of temperature subthreshold for polymodal receptors, the responses to algesic substances tested were greater at higher temperatures. The mean discharge rates induced by various concentrations of bradykinin (BK, 9 X 10(-9)-9 X 10(-6) M), were significantly greater at 36 degrees C than at 30 degrees C. Responses to hypertonic saline, tested in the temperature range, 34-43 degrees C, showed similar temperature-dependent increases, and Q10 values between 2.3 and 4.2. In the units exhibiting 'heat sensitization' with repeated testing of suprathreshold temperature rises, the response to hypertonic saline (616 mM) at 34 degrees C also increased. Although calor in the inflamed tissues is in itself not high enough to cause excitation of 'pain receptors,' temperature-dependent augmentation of chemical responses of the polymodal receptor might partly explain peripheral hyperalgesic behaviour observable in inflamed tissue on the basis of sensory receptor activity.

Abnormal activity of polymodal receptors induced by clioquinol (5-chloro-7-iodo-8-hydroxyquinoline)

To know possible involvement of primary afferents in paresthesia of subacute myelo-optico-neuropathy (SMON), influence on polymodal receptor, a type of nociceptor, of its causal agent clioquinol (5-chloro-7-iodo-8-hydroxyquinoline) was tested using in vitro testis-superior spermatic nerve preparations of dogs. Exposure to clioquinol (greater than 1 microM) induced an abnormal bursting activity in polymodal receptors, and thereafter the receptors were sensitized to an algesic stimulus (hypertonic saline) and gained a new sensitivity to cold, while pure mechanoreceptors were not influenced. On the other hand, clioquinol glucuronide (100 microM), a detoxicated form, had no such effects. These results are consistent with several clinical observations of sensory aberrations, suggesting possible involvement of the abnormal activities of polymodal receptors in SMON paresthesia.