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.

Behavior of mother rats in conflict tests sensitive to antianxiety agents.

Previous studies of freezing and open-field activity have demonstrated that lactating rats are less fearful or less anxious than nonpregnant ones. The purpose of this investigation was to observe the behavior of mother rats in conflict tests, which are frequently used in studies on the neurobiology of anxiety. In the punished drinking test, in which licking from a water spout is punished by electric shocks, mothers (observed on Day 1 postpartum following 24 hr of water deprivation) were found to drink more than virgins. Mothers (Day 1 postpartum) also consumed more food than controls in an unfamiliar open field. In contrast, no difference between mothers (Day 5 postpartum) and virgins was present in the exploration of an electrified shock probe. The largest maternal anticonflict effects in the drinking and feeding tests were recorded when the females were tested with their pups. Increased punished drinking was also observed in virgin rats treated with the anxiolytic benzodiazepine midazolam. Water-deprived virgins and mothers did not differ in the shock titration test, a result suggesting that diminished pain reactivity was unlikely to account for the increased punished drinking in mothers. Moreover, females in late pregnancy, which are hypoalgesic (Gintzler, 1980), did not lick more than virgins in the punished drinking test. Following 24 hr of water deprivation, unpunished drinking was higher in lactating females than in virgins, so the increased acceptance of punishment by mothers might have been due to their being more thirsty than virgins.(ABSTRACT TRUNCATED AT 250 WORDS)

Noradrenergic and serotonergic involvement in brief shock-induced analgesia in rats.

Four experiments were performed to investigate the effects of different techniques causing noradrenergic and serotonergic depletions in the brain and spinal cord on brief shock-induced analgesia. Newborn pups were administered N-2-choloroethyl-N-ethyl-2-bromobenzylamine systemically (2 x 50 mg/kg, ip) and 6-hydroxydopamine administered either systemically (100 micrograms/g, sc) or directly (8 micrograms in 1 microliter, bilaterally) into the locus coeruleus region, or intrathecally (20 micrograms in 10 microliter) into the lumbar subarachnoidal space, caused notable and consistent attenuations of the analgesia caused by brief shock. These treatments reduced noradrenaline concentrations in the spinal cord drastically. A potentiation of brief shock-induced analgesia was caused by the administration of p-chlorophenyl-alanine, whereas administration of 5,7-dihydroxytryptamine, into the nucleus raphe magnus or intrathecally into the subarachnoidal space, produced attenuation of the analgesic effect. Biochemical analyses revealed marked 5-hydroxytryptamine depletions in the spinal cord. The present findings are discussed with regard to the role of spinal noradrenaline and 5-hydroxytryptamine involvement in brief shock-induced analgesia and in reactions to stressful events.

Blockade and reversal of 5-methoxy-N,N-dimethyltryptamine-induced analgesia following noradrenaline depletion.

The acute effects of the 5-hydroxytryptamine agonist, 5-Methoxy-N,N-dimethyltryptamine (5-MeO-DMT), upon pain sensitivity, using shock titration, tail-flick and hot-plate methods, in noradrenaline- and 5-hydroxytryptamine-depleted rats were examined. Noradrenaline depletion, following the systemic administration of N-2-chloroethyl-N-ethyl-2-bromobenzylamine hydrochloride (DSP4, 2 X 50 mg/kg, i.p.), caused a reversal of the analgesic effect of 5-MeO-DMT on shock-titration from hypo- to hypersensitivity, and a total blockade of the antinociceptive effect of 5-MeO-DMT upon pain responses in the hot-plate and tail-flick tests. Pretreatment with either p-chloroamphetamine (2 X 10 mg/kg) or p-chlorophenylalanine (200, 100, 100 mg/kg), that depletes central 5-hydroxytryptamine stores, failed to alter the analgesia caused by acute 5-MeO-DMT. Strong evidence is provided for the effect of central noradrenaline depletion upon the analgesic effect of the 5-HT agonist. These findings suggest an important tonic influence of the noradrenaline system upon the descending spinal 5-HT pathway in rats.

Analgesia induced by 5-hydroxytryptamine receptor agonists is blocked or reversed by noradrenaline-depletion in rats.

The antinociceptive effect of acute administration of 5-HT receptor agonists and agents releasing 5-HT from neuronal terminals was studied in rats by using the hot-plate, tail-flick and shock-titration tests. Noradrenaline depletion by the noradrenaline-neurotoxin N-2-chloroethyl-N-ethyl-2-bromo-benzylamine hydrochloride (DSP4, 2 X 50 mg/kg) blocked the analgesia induced by the 5-hydroxytryptamine (5-HT) receptor agonists 5-methoxy-N,N-dimethyltryptamine (5-MeODMT) and quipazine, as well as that induced by acute release of 5-HT by p-chloroamphetamine (PCA) and increased 5-HT synthesis by 5-hydroxytryptophan (5-HTP). Analgesia in the tail-flick test was partly blocked by both methergoline and mianserin, whereas the analgesic effects of 5-MeODMT in the hot-plate and shock-titration tests were unaffected by the 5-HT antagonists. In the shock-titration test it was found that the DSP4-pretreated animals were made hyperalgesic by acute 5-MeODMT, and this hyperalgesia was blocked by both mianserin and methergoline, implying that this effect was 5-HT receptor mediated. It is therefore concluded that a functional central noradrenergic system is required for eliciting 5-HT receptor mediated analgesia, and that these interactions, at least in part, are probably spinally located.

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.

Selective attention and place navigation in rats treated prenatally with methylazoxymethanol.

Prenatal treatment of rats on gestation day 15 with methylazoxymethanol (MAM) caused forebrain microencephaly. The behavioral analyses included measures of spontaneous motor activity and tests for cognitive deficits, and were performed when the rats had reached adult age. Female MAM-treated rats failed to demonstrate contextual control of latent inhibition, which confirms earlier findings with male rats. Male MAM-treated rats demonstrated a notable impairment of place navigation in a swim-maze, but showed as strong sensory preconditioning as the control animals. Biochemical analyses indicated considerable increases in catecholamine levels in the cerebral cortex, hippocampus and striatum. The cognitive deficits, characterised by the various conditioning (taste-aversion) and instrumental learning (swim-maze) tasks, suggested that the MAM rats are deficient in their capacity to attend selectively to the relevant stimulus in complex arrangements of the stimulus situation.

Noradrenergic-serotonergic interactions and nociception in the rat.

Spinal noradrenaline (NA) depletion in rats, via either systemic N-2-chloroethyl-N-ethyl-2-bromobenzylamine (DSP4) or intrathecal 6-hydroxydopamine (6-OHDA), reversed and/or abolished the analgesic effects of the 5-hydroxytryptamine (5-HT) agonists, 5-methoxy-N,N-dimethyltryptamine (5-MeODMT) and p-chloroamphetamine (PCA), in shock titration, hot-plate and tail-flick measures of pain sensitivity. Spinal NA depletion also abolished the analgesic effects of 5-HT itself, administered intrathecally, in all three nociception tests and potentiated the analgesic effects of intrathecal NA, a demonstration of receptor supersensitivity. Spinal 5-HT depletion, via intrathecal 5,7-dihydroxytryptamine (5,7-DHT), only attenuated 5-MeODMT-induced analgesia in the tail-flick test but potentiated the 5-MeODMT effect in the hot-plate test. Intrathecal 5,7-DHT treatment caused a drastic potentiation of NA-induced analgesia in the shock titration and tail-flick tests but not in the hot-plate test. Biochemical analyses confirmed the NA and 5-HT depletion. The spinal noradrenergic system appears to be an important tonic factor modulating the function of the descending 5-hydroxytryptaminergic pathway.

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.

Blockade of intrathecal 5-hydroxytryptamine-induced antinociception in rats by noradrenaline depletion.

The analgesic effect of 5-hydroxytryptamine (5-HT, 200 and 250 micrograms/kg) administered into the lumbar intrathecal space of rats was blocked in the tail-flick, hot-plate and shock titration tests of nociception when the animals had previously been treated with N-2-chloroethyl-N-ethyl-2-bromobenzylamine (DSP4), which depletes noradrenaline in the spinal cord. These results suggest an important modulatory role of noradrenaline upon spinal 5-HT mechanisms.

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.

(+)-8-OH-DPAT and 5-MeODMT induced analgesia is antagonised by noradrenaline depletion.

In experiments with both rats and mice the 5-HT agonists 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and 5-methoxy-N,N-dimethyl-tryptamine (5-MeODMT) were shown to produce reliable analgesic effects after acute administration (1 mg/kg SC) in the tail-flick, hot-plate and shock-titration tests of nociception. Prior treatment with the noradrenaline neurotoxin, N-2-chloroethyl-N-ethyl-2-bromobenzylamine (DSP4), systemically administered to both rats and mice abolished the analgesic effects of both the 5-HT agonist compounds in all the tests of nociception used. Intrathecal 6-hydroxydopamine (6-OHDA) treatment also abolished the analgesic effects of 8-OH-DPAT and 5-MeODMT; in the tail-flick test the analgesia induced by 8-OH-DPAT was reversed to an hyperalgesia. Biochemical analyses confirmed notable noradrenaline depletions in the spinal cord. It is concluded that an important interaction between presynaptic noradrenergic terminals and serotonergic receptor sites, possibly 5-HT1A, mediates spinal nociception processes.

Alpha 1-adrenoceptor function in the rat hippocampus as assessed by noradrenaline-stimulated inositol phospholipid breakdown after destruction of noradrenergic neurons by neonatal 6-hydroxydopamine treatment.

Neonatal 6-hydroxydopamine treatment of male Sprague-Dawley rats resulted in a large depletion of noradrenaline 84-87 days later, as demonstrated by an 82% reduction in the intra-synaptosomal deamination of [14C]noradrenaline within noradrenergic synaptosomes with only marginal effects upon the extrasynaptosomal deamination. The hippocampal stimulation of inositol phospholipid breakdown by noradrenaline was, however, unaffected by the neonatal 6-hydroxydopamine treatment.

Attenuation of the post-decapitation convulsions after repeated treatment of rats with desipramine, imipramine and maprotiline.

The effects of repeated treatment of rats with the antidepressant or potential antidepressant agents alaproclate, citalopram, clomipramine, desipramine, imipramine, maprotiline, mianserin and zimeldine on the convulsions released by decapitation were examined. The noradrenaline uptake inhibitors desipramine, imipramine and maprotiline increased significantly the latency of onset of the post-decapitation convulsions (PDC's) after repeated administration of 10 mumol/kg orally twice daily, or 66 mumol/kg orally once daily (desipramine), for 15 days. The duration of the PDC's was slightly prolonged by these agents. A single acute dose of desipramine (20 mg/kg) administered at various time intervals before decapitation (1 to 24 hours) had no effect on the PDC's nor did repeated treatment with the other compounds examined, alaproclate, citalopram, clomipramine, mianserin and zimeldine, have any effect upon the PDC latency. The results are interpreted as evidence for noradrenaline receptor subsensitivity following chronic treatment.

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.

Comparison of 40 milliliters of 0.25% intrapleural bupivacaine with epinephrine with 20 milliliters of 0.5% intrapleural bupivacaine with epinephrine after cholecystectomy.

To determine the influence of the volume of local anesthetic injected for intrapleural analgesia, 40 patients undergoing cholecystectomy were randomly allocated to two groups of 20 patients each. One group received 40 mL of 0.25% bupivacaine with epinephrine injected intrapleurally postoperatively. The other group received 20 mL of 0.5% bupivacaine with epinephrine. The onset time of analgesia was nearly the same in both groups and within 25 min all patients were nearly pain free. Our data demonstrate that 100 mg of bupivacaine with epinephrine elicits effective analgesia after cholecystectomy. There are only minor differences between 20 and 40 mL with regard to pain relief. The authors conclude that the volume of local anesthetic within the range of 20-40 mL in an adult has little influence on the extent or duration of intrapleural analgesia.

Distribution of bupivacaine after interpleural injection in rats.

The absorption and distribution of bupivacaine in several tissues was studied after interpleural injection of 14C-labelled bupivacaine through a small needle interpleurally on the right side in male rats. 14C-labelled bupivacaine 10 microCi/mmol was injected at a dose of 2.5 mg/kg. The animals, a total of 42, six at each testing time, were sacrificed at 5, 10, 20, 30, 60, 120 and 180 minutes after the interpleural injection. Bupivacaine concentration was then determined on the basis of the amount of radioactivity in tissues taken from 23 different locations. The highest concentration of bupivacaine, about 40 pmol/micrograms tissue, was found in the right lung in the middle and inferior lobes. In the inferior and dorsal aspects of the right thoracic wall, the concentrations were in the same range. In the more peripheral organs, such as the musculature in the legs, the concentration was as low as less than 5 pmol/micrograms. In the central nervous system, the concentration was less than 10 pmol/micrograms. Our conclusion is that interpleurally administered bupivacaine remains in the area of injection for a considerable time.

Evidence for crosstolerance to the analgesic effects between morphine and selective alpha 2-adrenoceptor agonists.

Rats were injected subcutaneously (s.c.) with morphine (5 mg/kg) until tolerance developed to its antinociceptive action, or with 0.9% saline which was used as vehicle for morphine. Subsequently, both groups of animals were given an intrathecal (i.th.) dose of either noradrenaline (2 micrograms), clonidine (12.5 micrograms) or guanfacine (12.5 micrograms), that had been found previously to be reliably antinociceptive. In the saline-treated animals, these doses of noradrenaline, clonidine or guanfacine induced clear antinociceptive effects, but not in the morphine-group. It is therefore concluded that cross-tolerance to the antinociceptive effects of systemic morphine and the alpha-adrenoceptor agonists was obtained. The cross-tolerance between morphine on one hand, and noradrenaline, clonidine and guanfacine on the other, implies that a substantial opiate-adrenoceptor interaction exists in antinociceptive processes.

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.

The inhibition of the cage-leaving response--a model for studies of the serotonergic neurotransmission in the rat.

It was observed that rats that had been given drugs that enhance serotonergic neurotransmission, e.g. the serotonin releasing compounds p-chloroamphetamine (PCA) and fenfluramine, the MAO-A inhibitors and serotonin releasing agents amiflamine and alpha-ethyltryptamine and the serotonin agonists 5-methoxy-N, N-dimethyltryptamine (5-MeODMT), 8-hydroxy-2-(di-n-propylamino) tetraline (8-OH-DPAT), m-chlorophenyl piperazine (m-CPP) and 5-methoxy-3 (1,2,3,6-tetrahydropyridin-4-yl)1H-indole (RU 24969), did not leave their home-cages when the grid-covers were removed in contrast to normal rats who almost immediately left the cages. The association between the serotonin neurotransmission and the inhibitory effect of PCA on the cage-leaving response was indicated by the findings that 1. Serotonin uptake inhibitors (alaproclate and citalopram) antagonized the effect of PCA. 2. High, neurotoxic doses of PCA antagonized the effect of PCA when tested one week after the former administration. The serotonin uptake inhibitor zimeldine counteracted the effect of neurotoxic PCA. 3. Depletion of brain serotonin with p-chlorophenylalanine counteracted the effect of acute PCA. 4. Repeated treatment of rats for 7 days with zimeldine, amiflamine, alpha-ethyltryptamine or clorgyline plus a low dose of PCA counteracted the effect of acute PCA probably due to a functional down-regulation at postsynaptic receptors. Clorgyline or a low dose of PCA by themselves had no effect. 5. Compounds interacting with dopamine or noradrenaline mechanisms, e.g. alpha-methyltyrosine, N-2-chloroethyl-N-ethyl-2-bromobenzylamine (DSP 4), pimozide, remoxipride and prazosin did not antagonize the effect of PCA nor did (+)-amphetamine inhibit the cage-leaving response. None of the serotonin receptor antagonists (cinanserin, ketanserin, metergoline, methysergide, metitepine, mianserin, pirenperone) blocked the inhibition of the cage-leaving response produced by PCA, indicating that the receptors involved may not be of the S1- and S2- types. Observation of the cage-leaving response may be a valuable technique in studies of drugs that enhance the serotonin neurotransmission in the rat brain.