Per Kristian Eide

The Role of 5-Hydroxytryptamine (5-HT) Receptor Subtypes and Plasticity in the 5-HT Systems in the Regulation of Nociceptive Sensitivity

This review shows that the role of 5–hydroxytryptamine (5–HT) in the regulation of nociception depends on the 5–HT receptor subtypes involved and on long-term functional changes in the 5–HT receptors. Stimulation of the 5–HT 1 receptors, as well as of the 5–HT 2 and 5–HT 3 receptors, may reduce nociceptive sensitivity. In addition, activation of 5–HT 2 and 5–HT 3 receptors may also enhance nociceptive sensitivity. Up- or down-regulation of the 5–HT receptors may result in long-lasting changes, plasticity, in the 5–HT systems. Lesioning of 5–HT neurons induces denervation supersensitivity to 5–HT, and prolonged stimulation of 5–HT receptors may produce subsensitivity to 5–HT. In the spinal cord denervation supersensitivity to 5–HT may depend on reduced release of substance P (SP). An increase in the release of SP, on the other hand, may reduce the effects of 5–HT receptor activation. Long-term treatment with antidepressants which are used in clinical pain therapy appears to up-regulate the 5–HT 1 receptors and to down-regulate the 5–HT 2 receptors.

Different role of 5-HT1A and 5-HT2 receptors in spinal cord in the control of nociceptive responsiveness.

The effects of the 5-hydroxytryptamine type-2 (5-HT2) receptor agonist (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) and the 5-HT1A agonist (+)-8-hydroxy-2-(di-n-propylamino)-tetralin [(+)-8-OH-DPAT] on nociceptive responsiveness were compared in mice. Intrathecal administration of DOI (5-20 micrograms) produced a dose-dependent behavioural syndrome, consisting of biting or licking, directed towards the caudal part of the body and reciprocal hindlimb scratching. However, (+)-8-OH-DPAT (5-20 micrograms) did not produce the biting and scratching behaviour. The response to DOI (20 micrograms) was reversed by treatment with the substance P receptor antagonist, [D-Arg1, D-Trp7,9, Leu11]-SP (Spantide) (5 micrograms). The tail-flick reflex was markedly depressed 5-20 min after administration of (+)-8-OH-DPAT; DOI did not change the tail-flick reflex after 5 min but significantly inhibited the reflex response 10-20 min after injection. The data show that stimulation of 5-HT2 receptors, but not 5-HT1A receptors, induced a behavioural syndrome, which may reflect activation of nociceptive pathways. The tail-flick reflex was more markedly inhibited by stimulation of 5-HT1A than 5-HT2 receptors. Accordingly, 5-HT2 and 5-HT1A receptors seem to have a different function in the modulation of nociceptive responsiveness in the mouse.

Apparent hyperalgesia after lesions of the descending serotonergic pathways is due to increased tail skin temperature

&NA; It has been suggested that the descending serotonergic pathways exercise a tonic inhibition on nociception in the spinal cord. In this study 5,6‐dihydroxytryptamine (5,6‐DHT, 20 &mgr;g base) injected intrathecally in rats reduced spinal serotonin concentration to 3.5% of control levels without significantly affecting spinal noradrenaline. The lesion reduced the mean tail‐flick latency by approximately 35% and increased the mean tail skin temperature by approximately 3.5°C; both parameters gradually returned to normal values within 2–3 weeks. Both in controls and in lesioned animals there was a highly significant negative correlation between tail skin temperature and tail‐flick latency. Multiple regression analysis showed that the effect of lesioning on tail‐flick latency was non‐significant when the effect of skin temperature was taken into account. Thus the reduced tail‐flick latency after lesions of descending serotonergic pathways, usually interpreted as increased nociception, may be due to changes in tail skin temperature.

The role of spinal cord 5-HT1A and 5-HT1B receptors in the modulation of a spinal nociceptive reflex

The role of the 5-hydroxytryptamine (5-HT) receptor subtypes in the spinal cord in the regulation of nociception is unknown. This study examined whether administration of different 5-HT1 receptor agonists into the spinal subarachnoid space of mice modulates the nociceptive tail-flick reflex, and whether effects on the tail-flick reflex involve changes in tail skin temperature. The tail-flick latencies (the time needed to evoke the tail-flick reflex by noxious radiant heat) were significantly increased after intrathecal (i. th.) injection of 5-HT (10-20 micrograms), the 5-HT1A/5-HT1B receptor agonist 5-methoxy-N,N-dimethyltryptamine (5-MeODMT, 10-20 micrograms), the selective 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT, 20 micrograms) and after i.th. injection of 1(m-chlorophenyl)piperazine (mCPP, 5-20 micrograms) and 5-methoxy-3(1,2,3,6-tetrahydropyridin-4-yl)-1H-indole (RU 24969, 5-20 micrograms) which have high affinity for the 5-HT1B receptors. None of the 5-HT1 receptor agonists had the ability to change the tail skin temperature. The results show that in the mouse i.th. injection of both 5-HT1A and 5-HT1B receptor agonists has the ability to inhibit the tail-flick reflex without interfering with the tail skin temperature.

5-HT depletion with 5,7-DHT, PCA and PCPA in mice: differential effects on the sensitivity to 5-MeODMT, 8-OH-DPAT and 5-HTP as measured by two nociceptive tests.

Depletion of 5-hydroxytryptamine (5-HT) in mice was produced by intracerebroventricular injection of 5,7-dihydroxytryptamine (5,7-DHT, 80 micrograms) or by systemic injections of p-chloroamphetamine (PCA, 3 X 40 or 4 X 40 mg/kg), p-chlorophenylalanine (PCPA, 5 X 400 or 14 X 400 mg/kg) or combined PCA (3 X 40 mg/kg) + PCPA (11 X 400 mg/kg). Neither of the pretreatments altered nociception in the increasing temperature hot-plate test, whereas hyperalgesia was demonstrated in 5,7-DHT lesioned animals in the tail-flick test. 5,7-DHT-pretreatment enhanced the antinociceptive effect of the 5-HT agonists 5-methoxy-N,N-dimethyltryptamine (5-MeODMT), 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and 5-hydroxytryptophan (5-HTP). This effect was observed after 2, 5 and 8 days in the tail-flick test and after 5 and 8 days in the hot-plate test. However, pretreatment with PCPA or PCA failed to alter the antinociception elicited by the 5-HT agonists, although a tendency towards enhancement of antinociception was found after combined treatment with PCA and PCPA. It is suggested that the injection of 5,7-DHT induces denervation supersensitivity of post-synaptic 5-HT receptors. The lack of such supersensitivity after PCPA-pretreatment which induces similar 5-HT depletion to 5,7-DHT, may suggest that other factors than the absence of 5-HT may contribute to the development of denervation supersensitivity. Alternatively, the three 5-HT depleting agents may produce a qualitatively different reduction of 5-HT.

Effects of serotonin receptor antagonists and agonists on the tail-flick response in mice involve altered tail-skin temperature.

Tail-flick latency and tail-skin temperature were measured in mice after administration of serotonin (5-HT) receptor antagonists (metergoline and metitepin) and agonists [5-methoxy-N,N-dimethyltryptamine (5-MeODMT) and 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT)]. Metergoline (4 mg/kg) and metitepin (0.5 mg/kg) reduced the tail-flick latencies and increased the tail-skin temperatures, but the effect on the tail-flick latencies was non-significant when the effect of temperature was taken into account. Both 5-MeODMT (3 mg/kg) and 8-OH-DPAT (1 mg/kg) reduced the tail-skin temperature but only 5-MeODMT increased the tail-flick latencies. The effect of 5-MeODMT on tail-flick latencies was still highly significant when the effect of temperature was taken into account. The results show that the apparent hyperalgesia elicited by 5-HT receptor antagonists in the tail-flick test in the mouse is secondary to increased tail-skin temperature and not due to increased nociceptive sensitivity. The antinociceptive effect of 5-MeODMT in the tail-flick test can, however, not be explained by effects of temperature.

Interactions between serotonin and substance P in the spinal regulation of nociception

Interactions between 5-hydroxytryptamine (5-HT) and substance P (SP) in the mouse spinal cord were investigated using the tail-flick test and the behavioral response evoked by intrathecal (i.th.) SP or i.th. 5-HT. I.th. injection of 5-HT (20 micrograms) or the 5-HT1 receptor agonists (+)-8-hydroxy-2-(di-n-propylamino)tetralin ((+)-8-OH-DPAT) (20 micrograms) or 5-methoxy-3(1,2,3,6-tetrahydropyridine-4-yl)-1H-indole (RU 24969) (20 micrograms) markedly inhibited the tail-flick reflex. The effect of these compounds was reduced when SP (5 micrograms) was given i.th. 55 min, or 55 and 45 min before the agonists. The tail-flick latencies recorded 5 min before injection of a 5-HT receptor agonist were similar in animals treated with SP or vehicle. The changes in the tail-flick test were not due to changes in tail skin temperature since only minimal differences in the skin temperature were recorded between the groups injected with SP or vehicle. I.th. injection of SP (10 ng) or 5-HT (2 micrograms) produced a similar behavioral response consisting of biting, licking and scratching of the caudal part of the body, indicative of nociceptive stimulation. The responses both to i.th. SP and 5-HT were reduced after i.th. application of the SP receptor antagonist [D-Arg1,D-Trp7.9,Leu11]-SP (Spantide) (5 micrograms), as well as 5 min after i.th. injection of the 5-HT receptor antagonist metergoline (4 micrograms). The data may indicate functional interactions between SP and 5-HT in the mouse spinal cord, which may take place in neurons involved in the processing of nociception.

The apparent hyperalgesic effect of a serotonin antagonist in the tail flick test is mainly due to increased tail skin temperature

It has been suggested that reduced activity in raphe-spinal serotonergic systems induces hyperalgesia. In rats, the serotonin antagonist metergoline (0.5 mg/kg intraperitoneally) reduced tail flick latency by 0.92 sec (p less than 0.001) and increased tail skin temperature by 2.4 degrees C (p less than 0.001) when measured 50 min after injection. Multiple regression analysis with tail flick latency as dependent variable and tail skin temperature and metergoline/vehicle as independent variables revealed a highly significant effect of tail temperature on tail flick latency. The increase of tail skin temperature explained a reduction of tail flick latency of 0.64 of the 0.92 sec observed [B = -0.267 +/- 0.034, t(37)= -7.75, p less than 0.0001]. When the effect on tail skin temperature was taken into account, metergoline reduced tail flick latency by 0.28 sec [B = -0.284 +/- 0.114, t(37) = -2.50, p less than 0.05]. Metergoline (0.5 and 2.0 mg/kg) did not significantly alter plantar paw skin temperature or the response temperature in the increasing temperature hot plate test. Thus, the observed effect of metergoline on tail flick latency is primarily due to an effect on tail skin temperature. The possibility exists that the remaining effect of metergoline may be due to inadequate correction for the skin temperature change, and it is concluded that the study provide no clear evidence for a tonic inhibition of nociception by serotonergic systems.

The tail-flick test needs to be improved.

The tail-flick test is one of the most frequently used tests of nociception in animals. Recently it has been shown that the results of this test are greatly influenced by the tail temperature [1,8]. For instance, any manipulation that influences thermoregulation and tail blood flow may change the results of this test in a critical way. Lesions of the descending noradrenergic or serotonergic systems [2,9], transection of the spinal cord [3], administration of serotonin or noradrenaline receptor agonists or antagonists [4,11], tricyclic antidepressants [7], stress and the handling of the animals, as well as moderate changes in the ambient temperature [7,9] have been shown to affect the tail temperature and the tail-flick latency significantly. A vast number of studies using the tail-flick test are published every year, most of which do not take the possible role of tail blood flow and tail temperature changes into consideration. Our letter was prompted by the paper by Lee et al. [6]. They found a reduced tail-flick latency in hyperglycemic rats treated with alloxan 2 weeks earlier. Insulin treatment normalized the tail-flick latency as well as the blood glucose level in these alloxandiabetic rats. The authors conclude that elevated blood glucose levels contribute to a decrease in pain threshold. The authors should consider an alternative explanation: the possibility that the hyperglycemic rats may have an increase in tail skin temperature, and that this temperature is normalized by the insulin treatment. It is not possible to know which explanation is the correct one without measuring the temperature and the tail-flick latency in the same session. We want to make Lee et al. as well as other researchers aware of this problem, but also point out that a simple method for recording of the tail temperature and the correction of the tail-flick latency for the temperature change has been described [lo]. Alternatively, preheating of the tail to a certain temperature before measuring the tail-flick latency, or a feedback control of the heat stimulus [5], may be used. In our opinion, the investigation of this nociceptive reflex may be very useful for certain purposes in basic pain research. The test should not be abandoned, but improved.

Acute and chronic treatment with selective serotonin uptake inhibitors in mice: effects on nociceptive sensitivity and response to 5-methoxy, N,N-dimethyltryptamine

&NA; The tail‐flick and increasing temperature hot‐plate tests were employed to study the effects of acute or chronic treatment with zimelidine, alaproclate or chlorimipramine on nociception and response to 5‐methoxy‐N,N‐dimethyltryptamine (5‐MeODMT) in mice. A single dose of the serotonin (5‐HT) uptake inhibitors produced antinociception in the hot‐plate test but not in the tail‐flick test. After chronic administration, reduced tail‐flick latencies were demonstrated 24, 48, 72 and 144 h after withdrawal of zimelidine treatment, 48 h after withdrawal of alaproclate and 48 and 96 h after withdrawal of chlorimipramine treatment. The hot‐plate response temperatures were slightly lowered after chronic zimelidine treatment but not after treatment with alaproclate or chlorimipramine. The response to 5‐MeODMT was not altered by a single dose of the 5‐HT uptake inhibitors, however, after withdrawal of chronic treatment this response was increased in the tail‐flick test but not in the hot‐plate test. It was concluded that acute and chronic treatment with 5‐HT uptake inhibitors modulate nociception differently, and that chronic treatment induces supersensitivity of spinal postsynaptic 5‐HT receptors. Different modulation of different 5‐HT receptor subpopulations by these compounds may possibly contribute to the test‐dependent results.