Noradrenergic-serotonergic interactions and nociception in the rat

Noradrenergic and serotonergic mediation of the locomotor and antinociceptive effects of clonidine in infant and adult rats

This experiment examined the necessity for intact noradrenergic and serotonergic function for the locomotor and nociceptive effects of clonidine in 10- and 100-day-old rats. Newborn rats were administered systemically 6-hydroxydopamine (100 micrograms/g; 12 and 24 hours after birth) to deplete norepinephrine (NE), and at 10 or 100 days they were injected with para-chlorophenylalanine (300 mg/kg PCPA; 5 and 24 hours before testing) to deplete serotonin (5-HT). They were then tested for the locomotor and analgesic effects of one of various clonidine doses (0, 10, 100 or 1000 micrograms/kg). Clonidine enhanced locomotion at 10 days. This effect was potentiated by NE depletion and reduced by 5-HT depletion. Clonidine reduced locomotion at 100 days, and again this was augmented by NE depletion but reduced by 5-HT depletion. NE depletion did not have an enduring effect on clonidine antinociception whereas 5-HT depletion reduced it at both ages. It is concluded that the locomotor effects of clonidine in both infant and adult rats, despite reversing with maturation, reflect its agonist action at postsynaptic alpha2 adrenoceptors. The results also add to the accumulating evidence for an early maturing and behaviorally relevant serotonergic system(s).

Adaptive changes in alpha-2 adrenoceptor mediated responses: analgesia, hypothermia and hypoactivity

The acute effects of the alpha-2 adrenoceptor agonists, clonidine and guanfacine, upon antinociception, hypothermia and motor activity were compared under conditions of receptor antagonism, denervation, and chronic administration of a tricyclic antidepressant compound. The analgesic actions of clonidine and guanfacine were antagonised by idazoxan, an alpha-2 receptor antagonist, but potentiated by pretreatment with the noradrenaline neurotoxin DSP4, and attenuated by chronic treatment with desipramine (DMI). Clonidine- and guanfacine-induced hypothermia was antagonised by idazoxan, potentiated by prior treatment with DSP4 and attenuated by chronic administration with DMI. Both clonidine and guanfacine produced decreases in motor activity that were attenuated by idazoxan but unaffected by prior DSP-4 treatment. Chronic DMI administration also attenuated clonidine-induced hypoactivity but potentiated guanfacine-induced hypoactivity. These diverse results describe both similar and differential adaptive mechanisms modulating the functional effect of alpha-2 receptor systems in the central nervous system.

Mode of antinociceptive action of flupirtine in the rat

1. Flupirtine is a novel, centrally acting, non-opioid analgesic agent. The present investigation was undertaken to ascertain which neuronal systems might be responsible for its antinociceptive effect in rodents. The antinociceptive responses to the test compounds were examined in the tail-flick test. 2. The selective destruction of noradrenergic pathways by 6-hydroxydopamine considerably reduced the flupirtine-induced inhibition of nociceptive responses but not the clonidine-induced antinociception which was significantly enhanced. Depletion of spinal 5-hydroxytryptaminergic pathways by pretreatment with 5,7-dihydroxytryptamine failed to affect the action of flupirtine and clonidine. 3. The depletion of neurotransmitters by reserpine totally abolished the antinociceptive action of flupirtine. By contrast, clonidine-induced inhibition of nociceptive responses remained unchanged. 4. Inhibition of the synthesis of noradrenaline by alpha-methyl-L-p-tyrosine attenuated the antinociception induced by flupirtine. In contrast, inhibition of the synthesis of 5-hydroxytryptamine by (+/-)-6-fluorotryptophan did not influence the antinociceptive activity of flupirtine. 5. Inhibition of noradrenaline uptake by imipramine led to a significant augmentation of flupirtine-induced antinociception. 6. Selective antagonists at alpha-adrenoceptors significantly decreased the antinociceptive action of flupirtine. Antinociception induced by clonidine was significantly diminished by idazoxan but not by prazosin. 7. The 5-hydroxytryptamine (5-HT) antagonist, ketanserin diminished the antinociceptive activity of flupirtine, probably due to its additional alpha 1-adrenoceptor antagonist activity. The antinociceptive effect of clonidine was not influenced by ketanserin. 8. Cholinoceptor antagonists such as mecamylamine and pirenzepine did not alter the antinociceptive action of flupirtine. Flupirtine-induced antinociception also remained unchanged after pretreatment with haloperidol. 9. Flupirtine has no pharmacologically relevant affinity for alpha 1-, alpha 2-adrenoceptors, 5-HT1- and 5-HT2-receptors as shown in direct binding studies. 10. The present results indicate that the antinociceptive action induced by flupirtine depends on the descending noradrenergic pain-modulating system.

Dorsal raphe stimulation, 5-HT and morphine microiontophoresis effects on noxious and nonnoxious identified neurons in the medial thalamus of the rat

In single cell experiments, the characterization of the responses of medial thalamic neurons to noxious and nonnoxious stimulation was made to examine the effects of two substances involved in pain, morphine and 5-HT, and the action of one pain suppressor mechanism, dorsal raphe stimulation. Single cell activity was recorded in urethane anesthetized rats. Tail pinch and tail immersion in hot water were used as nociceptive stimuli. Skin strokes, air puffs and hair brushing were used as nonnociceptive stimuli. Morphine, 5-HT microiontophoresis and dorsal raphe stimulation were performed in all the recorded units. Fifty-eight percent from 61 medial thalamic recorded units responded both to noxious and nonnoxious stimulation; whereas only 18% and 24.6% of the units responded exclusively to noxious and nonnoxious stimulation, respectively. The noxious responding units were located in the most posterior portions of the medial thalamus. Dorsal raphe stimulation and 5-HT ejection prevented the excitation elicited by noxious input. Morphine ejection prevented both the noxious and nonnoxious input in medial thalamus, in a different population as compared to dorsal raphe stimulation or 5-HT ejection. These findings support the existence of a pain ascending mechanism mediated by an opioid-serotonergic interaction in the medial thalamus of the rat.

Involvement of spinal monoaminergic pathways in antinociception produced by substance P and neurotensin in rodents

The antinociceptive effects of substance P and of neurotensin have been determined in rodents after depletion of serotonin (5-HT) or noradrenaline (NA) in the spinal cord. The antinociceptive effect of substance P, given intraventricularly, in rats and mice was blocked after depletion of 5-HT in the spinal cord with the neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) or with the inhibitor of the synthesis of 5-HT, p-chlorophenylalanine (PCPA), but not after depletion of NA in the spinal cord with the neurotoxin 6-hydroxydopamine (6-OHDA). Conversely, the antinociceptive effect of neurotensin in mice was blocked after lesion of spinal NA pathways with 6-OHDA. When 5-HT spinal pathways of mice were lesioned with 5,7-DHT, neurotensin-induced antinociception was blocked 7 but not 15 days after the lesion. p-Chlorophenylalanine failed to prevent this effect of neurotensin. The results suggest that the antinociceptive effect of substance P depends on the integrity of spinal 5-HT neurones, whereas that of neurotensin depends on spinal NA neurones and, only to a limited extent, on 5-HT neurones. It seems that different descending systems are involved in the antinociception elicited by these two neuropeptides.

Serotonin and depression: old problems and new data

1. Several lines of evidence implicating some dysfunction or alteration to brain serotonergic systems in depressive states are summarized. 2. Some aspects of the interactions between the 5-HT and catecholamine pathways in the action of antidepressant drugs are examined. 3. Finally, the important role of NA modulation of serotonergic activity in the action of antidepressants is suggested.

Interactions between noradrenergic and cholinergic mechanisms involved in spinal nociceptive processing

Antinociceptive effects have been demonstrated after systemic and spinal administration of the adrenoceptor agonist clonidine and cholinomimetic drugs in animals and human. The present investigation was undertaken in rats to study the possible interactions between spinal noradrenergic and cholinergic mechanisms in modulating the reaction to nociceptive stimuli. Using the tail immersion test, an additive antinociceptive effect was found between intrathecal (IT) clonidine (10 micrograms) and physostigmine (15 micrograms, IT). The effect of clonidine was attenuated by atropine (15 micrograms, IT). Physostigmine (15 micrograms, IT) antinociception, which was of short duration was abolished by atropine (15 micrograms, IT) and attenuated by phentolamine (20 micrograms, IT). Neostigmine (5 micrograms, IT) produced a prolonged antinociceptive response. In animals pretreated with 6-hydroxydopamine IT, leading to a selective depletion of spinal cord noradrenaline, physostigmine (15 micrograms, IT) was ineffective in altering the nociceptive test response. Neither clonidine, nor physostigmine produced changes in latency times in the hot plate test (58 degrees C) in the doses employed. In conclusion, a clear-cut interaction exists between spinal noradrenergic and cholinergic systems for antinociception. To explain the interactions, several possible mechanisms may be considered, including cholinomimetic effects produced by clonidine, and the presence of muscarinic receptors in the dorsal horn of the spinal cord.

Norepinephrine-stimulated inositol phospholipid breakdown in the rat cerebral cortex following serotoninergic lesion

Norepinephrine (NE)-stimulated inositol phospholipid hydrolysis ("PI breakdown") in rat cerebral cortical miniprisms was used as a measure of alpha 1-adrenoceptor function following serotonin and/or NE depletion. The use of ascorbic acid to prevent autooxidation of the NE during the PI breakdown assay was found to be warranted. Treatment of rats with 5,7-dihydroxytryptamine and DSP4 produced selective depletions of serotonin (79-95%) and NE (69-85%), respectively, in cortical and hippocampal brain regions. The degree of cortical NE-stimulated PI breakdown in the lesioned animals was not significantly different from that in the control animals, suggesting that under the conditions used, serotonin and NE depletion do not lead to a changed sensitivity of alpha 1-adrenoceptors coupled to PI breakdown in the rat cortex.

Intrathecal 5-hydroxytryptamine and electrical stimulation of the nucleus raphe magnus in rats both reduce the antinociceptive potency of intrathecally administered noradrenaline

The antinociceptive potency of noradrenaline (NA), as assessed by suppression of the spinal nocifensive tail flick and paw withdrawal reflexes was examined. The tail flick latency (TFL) was increased to the cut-off value for a period of approximately 120 min following the intrathecal microinjection of a standard 15 nmol dose of NA. A similar intrathecal dose of NA produced an increase in threshold to elicit the paw withdrawal reflex for a period of approximately 20 min. When preceded by a standard intrathecal microinjection of 260 nmol 5-hydroxytryptamine, the antinociceptive potency of NA was significantly reduced as reflected by both the tail flick and paw withdrawal tests. In addition, electrical stimulation of the posterior raphe complex immediately before and during the NA-induced increase in TFL, significantly reduced the antinociceptive potency of NA. It is concluded that spinal tryptaminergic activity can reduced the duration of the antinociceptive action of intrathecally applied NA.

Intrathecal noradrenaline restores 5-methoxy-N,N-dimethyltryptamine induced antinociception abolished by intrathecal 6-hydroxydopamine

Intrathecal administration of 6-hydroxydopamine (6-OHDA) abolished the antinociceptive effects of acute administration of 5-methoxy-N,N-dimethyltryptamine (5-MeODMT, 1 mg/kg, s.c.) in the hot-plate, tail-flick and shock titration tests of nociception. The antinociceptive effects of 5-MeODMT, abolished by the prior intrathecal 6-OHDA treatment, were restored by intrathecal administration (2 or 1 microgram) of noradrenaline (NA), immediately prior to 5-MeODMT, in all three tests of nociception. Biochemical analysis confirmed severe NA depletions (95 percent loss) in the lumbar and thoracic regions of the spinal and much lesser dopamine depletions (25-35 percent loss). Intrathecal 5,7-dihydroxytryptamine (5,7-DHT) attenuated 5-MeODMT induced antinociception in the tail-flick test and combined NA + 5-MeODMT induced antinociception in the hot-plate and tail-flick tests. Intrathecal administration of 5,7-DHT caused a severe depletion of 5-hydroxytryptamine in the lumbar region of the spinal cord. The present findings demonstrate further the modulatory role of NA upon serotonergic systems in nociception and indicate the necessity of NA availability for induction of 5-MeODMT analgesia.

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

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.

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.

Interdependence of spinal adenosinergic, serotonergic and noradrenergic systems mediating antinociception

The present investigations examined possible interdependence among serotonergic, noradrenergic and adenosinergic pathways as spinal antinociceptive systems. ED50 values for antinociception induced by intrathecal injections of noradrenergic, serotonergic or adenosinergic agonists in mice were determined. These results were compared to ED50 values determined when an antagonist or a sub-antinociceptive dose of a second agonist was coadministered i.t. with agonists. Interactions observed when serotonergic and adenosinergic agents were coadministered suggest that antinociception induced by serotonin (i.t.) is mediated, in part, via serotonin-stimulated release of adenosine. The mechanism by which norepinephrine administered i.t. induced antinociception, however, appeared to be independent of serotonergic and adenosinergic pathways.

Increased antinociception by alpha-adrenoceptor drugs after spinal cord noradrenaline depletion

Animals depleted of the bulbospinal NA fiber tracts have been reported to be supersensitive to antinociceptive effects of intrathecally administered noradrenaline (NA) in vivo. In the present investigation, the antinociceptive effects were determined after systemic or intrathecal injections of noradrenergic agents. NA and the selective alpha 2-adrenoceptor agonists guanfacine and clonidine were used. NA depletion was performed by treatment neonatally with 6-hydroxydopamine (6-OHDA), or in adult animals by intrathecal 6-OHDA administration or systemic N-2-chloroethyl-N-ethyl-2-bromobenzylamine hydrochloride (DSP4). The neurotoxins were found to cause a severe depletion of spinal NA without affecting dopamine (DA) or 5-hydroxytryptamine (5-HT) levels. The antinociceptive effects of intrathecal injection of NA, clonidine and guanfacine were more strongly enhanced in the depleted than in the control rats. It was also found that clonidine and guanfacine given systemically had a stronger effect in depleted than in control animals. In conclusion, depletion of descending NA pathways induces functional supersensitivity both to intrathecally administered NA and to the selective alpha 2-adrenoceptor agonists clonidine and guanfacine. It was also found that systemically administered clonidine and guanfacine had a stronger effect in NA-depleted than in control animals.

Enhancement of clonidine-induced analgesia by lesions induced with spinal and intracerebroventricular administration of 5,7-dihydroxytryptamine

The role of serotonin (5-HT) in analgesia induced by clonidine was examined by determining the effect of intraspinal (i.s.) and intracerebroventricular (i.c.v.) injections of 5,7-dihydroxytryptamine (5,7-DHT) on analgesia produced by clonidine in the tail-flick and hot plate tests. Depletion of amines was verified by high performance liquid chromatography analysis. Intraspinal injections of 5,7-DHT potentiated the action of clonidine in both tests for analgesia and caused depletion of 5-HT in the spinal cord. Intracerebroventricularly injected 5,7-DHT also increased the action of clonidine and depleted 5-HT in brain as well as in the spinal cord. In the groups given intracerebroventricular injections, there appeared to be a biphasic increase in the action of the clonidine. Significant hyperalgesia from pretreatment with neurotoxin was observed only on a limited number of occasions. The present results indicate that 5-HT mechanisms in the CNS are important mediators of the analgesic action of clonidine. Interactions between clonidine and 5-HT systems at both spinal and supraspinal sites are considered.

5-Methoxy-N,N-dimethyltryptamine-induced analgesia is blocked by alpha-adrenoceptor antagonists in rats

The effects of the alpha-adrenoceptor antagonists prazosin, phentolamine and yohimbine upon 5-methoxy-N,N-dimethyltryptamine (5-MeODMT)-induced analgesia were tested in the hot-plate, tail-flick and shock-titration tests of nociception with rats. Intrathecally injected yohimbine and phentolamine blocked or attenuated the analgesia produced by systemic administration of 5-MeODMT in all three nociceptive tests. Intrathecally administered prazosin attenuated the analgesic effects of 5-MeODMT in the hot-plate and tail-flick tests, but not in the shock titration test. Intrathecal yohimbine showed a dose-related lowering of pain thresholds in saline and 5-MeODMT-treated animals. Phentolamine and prazosin produced normal dose-related curves in the hot-plate test and biphasic effects in the shock titration and tail-flick tests. These results demonstrate a functional interaction between alpha 2-adrenoceptors and 5-HT agonist-induced analgesia at a spinal level in rats.

5-HT agonist induced analgesia modulated by central but not peripheral noradrenaline depletion in rats

The antinociceptive effect elicited by the 5-hydroxytryptamine (5-HT) agonist 5-methoxy-N,N-dimethyltryptamine (5-MeODMT) was reversed or blocked in animals which had previously sustained severe spinal noradrenaline (NA) depletion via either systemic N-2-chlorethyl-N-ethyl-2-bromobenzylamine hydrochloride (DSP 4), neonatal 6-hydroxydopamine (neon. 6-OHDA), or intrathecal 6-OHDA treatment. Biochemical analysis of the lumbar spinal cord samples confirmed severe central NA depletions. Animals were tested with nondamaging heat pain (tail-flick test, hot-plate test) and electric footshock titration to determine the amount of antinociception or nociception. Peripheral NA depletion following intravenous (i.v.) 6-OHDA injection to adult rats had no effect on the antinociception induced by 5-MeODMT, but did cause severe NA depletions in the left heart atrium. These results suggest a modulatory effect of central and not peripheral noradrenergic system upon 5-HT agonist induced analgesia, and also give evidence that this effect is spinally mediated.

Chronic treatment with antidepressant drugs and the analgesia induced by 5-methoxy-N,N-dimethyltryptamine: attenuation by desipramine

The effect of chronic and acute oral or intraperitoneal treatment with the antidepressant drugs, desipramine, amitriptyline, alaproclate and iprindole, upon pain thresholds in the tail flick, hot plate and shock titration tests of nociception in saline- and 5-MeODMT-treated rats was studied. Chronic desipramine treatment increased the pre-test tail flick latencies. In the saline-treated rats, chronic oral desipramine treatment increased tail flick latencies, whereas chronic oral amitriptyline treatment decreased tail flick latencies. In 5-MeODMT-treated rats, chronic oral desipramine treatment attenuated the effects of 5-MeODMT (1 mg/kg) in all three tests of nociception, whereas chronic amitriptyline caused a potentiation in the tail flick and hot plate tests. Chronic oral iprindole treatment attenuated 5-MeODMT-induced analgesia in the hot plate test. Chronic intraperitoneal desipramine treatment attenuated 5-MeODMT analgesia in the tail flick and shock titration tests. In a different chronic treatment experiment, oral desipramine treatment attenuated 5-MeODMT analgesia in the tail flick test and zimeldine did for both the tail flick and hot plate tests, whereas mianserin potentiated 5-MeODMT-induced analgesia in both the tail flick and hot plate tests. In the saline-treated rats, acute treatment with all four drugs, desipramine, amitriptyline, iprindole and alaproclate, elevated the shock thresholds, whereas in 5-MeODMT-treated rats, desipramine and amitriptyline elevated shock thresholds. Two main conclusions can be drawn: chronic desipramine caused a quite consistent attenuation of 5-MeODMT-induced analgesia and the effects of acute treatment differed strongly from that of the chronic treatment. The effects of chronic administration with these antidepressants were compared with other findings using different measures of behavioural and receptor function.

Spinal and locus coeruleus noradrenergic lesions abolish the analgesic effects of 5-methoxy-N,N-dimethyltryptamine

Two experiments were performed on Sprague-Dawley rats to study the effects of noradrenaline and 5-hydroxytryptamine depletion upon the antinociceptive effects of acute 5-methoxy-N,N-dimethyltryptamine (5-MeODMT) administration. 6-Hydroxydopamine-induced lesions following microinjections to either the locus coeruleus or the spinal cord (lumbar) abolished completely 5-MeODMT-induced analgesia in the tail-flick, hot-plate, and shock titration tests whereas 5,7-dihydroxytryptamine-induced lesions of the nucleus raphe magnus and the lumbar spinal cord attenuated 5-MeODMT analgesia in the tail-flick and shock titration tests. Thus, the experiments serve to demonstrate an important interaction between descending noradrenergic and serotonergic pathways, possibly at a spinal locus.

Spinal noradrenergic neurotransmission and the analgesia induced by brief footshock

Antinociception induced by brief footshock as well as by 5-methoxy-N,N-dimethyltryptamine was antagonized by lesions of the descending bulbospinal noradrenergic (NA) pathways by intrathecal injections of 6-hydroxydopamine. The alpha 2-adrenoceptor antagonist, yohimbine, injected intrathecally also blocked both types of nociceptive effects in the tail-flick and hot-plate tests. 5-Methoxy-N,N-dimethyltryptamine (1 mg/kg) potentiated shock-induced antinociception and this potentiation was also antagonized by decreased NA neurotransmission. These findings suggest an important role for spinal NA innervation, and possibly alpha 2-adrenoceptors in antinociception induced by brief footshock and serotonergic receptor stimulation induced analgesia in rats.

Noradrenaline depletion increases noradrenaline-induced antinociception in mice

Mice were treated with N-2-chloroethyl-N-ethyl-2-bromobenzylamine hydrochloride (DSP4), which causes severe noradrenaline (NA) depletions in brain regions and the spinal cord, or vehicle i.p. They were tested 14 days later for antinociception induced by intrathecal injections of different doses of NA. A potentiation of the NA effect upon pain sensitivity was observed, with both an increase in the magnitude and duration of the antinociceptive responses. Upon biochemical analysis of spinal cords, it was found that DSP4-treated mice had a 80% depletion of NA, whereas dopamine and 5-hydroxytryptamine were unaffected. Radioligand binding of [3H]clonidine in membranes prepared from spinal cord, showed no differences in density of alpha 2-adrenoceptors, but the affinity had been increased, probably explaining the supersensitivity.