Development of tolerance to the antinociceptive effect of metergoline

Metergoline given IP reduced the response to noxious stimulation in the mouse formalin test. Tolerance to this effect developed after a chronic treatment schedule consisting of ten daily injections of 5 mg/kg. Twenty four hours after the last injection a test dose of metergoline (2.5 mg/kg) reduced the licking time in the formalin test by 28% in the chronic metergoline group, compared to 68% reduction in the vehicle-treated animals. In addition, the antinociceptive effect of the 5-hydroxytryptamine releasing compound p-chloramphetamine (PCA) was reduced following chronic treatment with metergoline. The reduced effect of PCA may have been caused by down-regulation of 5-HT2 receptors. However, this finding is also compatible with the contention that metergoline may act as an agonist at postsynaptic serotonergic receptors.

Effects of the putative 5-HT1A receptor agonist 8-OH-2-(di-n-propylamino)tetralin on nociceptive sensitivity in mice

The ability of 8-OH-2-(di-n-propylamino)tetralin (8-OH-DPAT) to alter nociceptive sensitivity in mice was studied using the tail-flick, hot-plate and formalin tests. Subcutaneous (SC) administration of 8-OH-DPAT (0.63-1.0 mg/kg) dose-dependently increased the temperature at which hindpaw lick occurred in a hot-plate test using slowly rising temperature and increased the latencies to hindpaw lick, but reduced the latencies to jump in a conventional hot-plate test. Intracerebroventricular (ICV) injections (0.25-1.0 microgram) produced similar results in the conventional hot plate test. Following intrathecal (ITH) injections (0.25-1.0 microgram), however, the latencies to hindpaw lick were elevated without any change in jump latencies. In the formalin test a low systemic dose of 8-OH-DPAT (0.063 mg/kg) elicited hyperalgesia, while hypoalgesia was found after a high dose (1.0 mg/kg). ICV injection of 1.0 microgram produced hypoalgesia in the formalin test while the same dose injected ITH was without effect. 8-OH-DPAT did not alter tail-flick latencies, either by SC, ICV or ITH administration. Previous studies have shown that 8-OH-DPAT stimulates central serotonergic receptors, and shows selectivity for the 5-HT1A recognition site. The present findings indicate an involvement of 5-HT1A receptors in the processing of nociceptive information both at spinal and supraspinal sites. However, stimulation of 5-HT1A receptors does not seem to affect spinal, nociceptive reflexes.

A modified hot-plate test sensitive to mild analgesics

The present study compares a modified hot-plate test in which the temperature is slowly increased from non-noxious levels with a standard constant temperature hot-plate test. In both tests, hindpaw lick was found to be a more reliable criterion response than forepaw lick, and was employed throughout the experiments. In the constant temperature hot-plate test, 1-min exposure to the test apparatus the day before testing significantly reduced the response latencies of both rats and mice. No effect of pre-exposure was found in the increasing temperature test. In both tests and in both species, sessions of repeated testing were conducted with only insignificant alterations in responsiveness between trials. In both rats and mice, dose-related increases in response temperature were obtained in the increasing temperature hot-plate test after administration of morphine, paracetamol and acetylsalicylic acid, whereas only morphine had consistent effects in the constant temperature test. Thus, the increasing temperature hot-plate is a useful analgesimetric test in both rats and mice, superior to the conventional hot-plate test with regard to consistency of results and sensitivity to non-narcotic analgesics.

Orphenadrine citrate increases and prolongs the antinociceptive effects of paracetamol in mice

Orphenadrine, a muscle relaxant with antinociceptive effects, was shown to increase and prolong the antinociceptive effects of paracetamol in mice. Both in the increasing temperature hot plate test and in the formalin test, a combination of the two drugs showed a significantly improved effect compared to either of the drugs alone. The time course of the effects was tested in the increasing temperature hot plate test. The group treated with the drug combination showed a prolonged effect compared to both single drug treated groups, the effect lasting longer than 120 min. for the combination and about 80 min. for the single drugs. Orphenadrine and paracetamol increased antinociception even when orphenadrine was injected 90 min. after paracetamol, which by that time did not exert antinociceptive effects by itself. Thus the combination of orphenadrine and paracetamol enhances the antinociceptive effect of either drug in mice.

Evidence from behavioural and in vitro receptor binding studies that the enkephalinergic system does not mediate acute ethanol effects

The behavioural disturbances produced by acute exposure to ethanol have been related to changes in function of the opioid systems in the CNS. However, evidence in the literature is conflicting. The present report concerns the possible role of the enkephalinergic system in the mediation of acute ethanol effects. We used rats to study the ability of a selective opioid delta receptor antagonist (ICI 154129) to prevent the effect of ethanol on pain sensitivity, body temperature, sensorimotor performance and level of consciousness. Furthermore, in vitro receptor binding was measured to investigate whether or not ethanol, within a non-lethal concentration range, would change the binding parameters of the delta receptor ligand [3H][D-ala2, D-Leu5]enkephalin. ICI 154129 did not significantly influence the effects of ethanol in the behavioural experiments. Ethanol did not significantly change the binding parameters whether saturation or competition was measured in the receptor binding experiments. Thus, there was no evidence that the enkephalinergic system mediated the acute ethanol effects.

Changes in nociception after 6-hydroxydopamine lesions of descending catecholaminergic pathways in mice

Intrathecal (ITH) administration of 5 micrograms 6-hydroxydopamine (6-OHDA) in mice selectively lesioned descending catecholaminergic pathways. Uptake of 3H-noradrenaline (3H-NA) into synaptosomes from the lumbar spinal cord was reduced by 95%, without any change in the uptake of 14C-5-hydroxytryptamine (14C-5-HT). Synaptosomal uptake of 3H-NA and 14C-5-HT in the brain was not altered. The nociceptive sensitivity was evaluated using the tail-flick, hot plate and formalin tests 3 and 14 days after injection of 6-OHDA. At day 3 hyperalgesia was found in the hot-plate test, unchanged response latency in the tail-flick test and hypoalgesia in the formalin test. At day 14 there were no statistically significant differences from controls in any of the tests. The present findings support the contention that catecholaminergic pathways participate in the tonic regulation of nociception in the spinal cord. However, while supraspinally integrated responses to acute thermal pain, as measured with the hot-plate test, are inhibited by these pathways, responses to prolonged chemical pain are enhanced.

Learned and pharmacologically-induced tolerance to ethanol and cross-tolerance to morphine and clonidine

The development of tolerance to the inhibitory effect of ethanol on the tail-flick reflex was studied in the spinal rat. This preparation was used in order to avoid uncontrolled learning effects. Tolerance due to intoxicated practice (learned tolerance) and tolerance due to mere ethanol exposure (pharmacologically-induced tolerance) were studied in separate experiments. It was found that that learned tolerance to ethanol also caused tolerance to morphine and clonidine, whereas pharmacologically-induced tolerance did not have the same effect. The results challenge the concept of "behaviorally augmented tolerance" and suggest that learned and pharmacologically-induced tolerance involve different basal mechanisms in the CNS.

Lesions of the ascending serotonergic pathways and antinociceptive effects after systemic administration of p-chloroamphetamine in mice

The present study reports a method for lesioning of the ascending serotonergic system. The neurotoxic substance p-chloroamphetamine (PCA) was given IP on 2 consecutive days (40 mg/kg/day). After each injection, the animals were kept at 4 degrees C for 4 hr since a lower dose of PCA (25 mg/kg) induced severe hyperthermia. The mortality rate was 12%, considerably lower than previously reported in similar studies. Evaluated 9 days after the last injection of PCA, the uptake of 14C-5-HT into cortical and hippocampal crude synaptosomal preparations was reduced by 50 and 60%, respectively, while the uptake into spinal synaptosomes was unaffected. The uptake of 3H-NA was not significantly altered in any of the structures. Measurements of PCA performed 30 min to 4 hr after IP injections of 5 to 40 mg/kg demonstrated higher concentrations in the cortex than in the lumbar spinal cord. Administration of PCA (5 mg/kg) had an acute antinociceptive effect in the hot-plate and formalin tests, but not in the tail-flick test. Prior treatment with neurotoxic doses of PCA prevented the antinociception but had in itself no effect on the responsiveness in any of the tests. Thus systemic administration of PCA produces highly selective and functional lesions of the ascending serotonergic pathways in mice.

Acetylsalicylic acid, paracetamol and morphine inhibit behavioral responses to intrathecally administered substance P or capsaicin

Intrathecally administered substance P (SP) or capsaicin in mice elicited a pain-related behavioral response consisting of vigorous biting, licking and scratching of the caudal part of the body. Pretreatment of the animals with intraperitoneally injected acetylsalicylic acid (300 and 400 mg/kg), paracetamol (300 and 400 mg/kg) and morphine (2.5 and 5 mg/kg) reduced the responses in a dose-dependent manner. The analgesia is probably mediated by inhibition of a postsynaptic SP sensitive mechanism. Thus these results demonstrate central antinociceptive effects of acetylsalicylic acid and paracetamol.

Changes in nociception after lesions of descending serotonergic pathways induced with 5,6-dihydroxytryptamine. Different effects in the formalin and tail-flick tests

Intrathecal administration of 5,6-dihydroxytryptamine (5,6-DHT) in mice selectively lesioned descending serotonergic pathways. Nociception was evaluated 3 days after injection of 5,6-DHT using the tail-flick and formalin tests. In the tail-flick test shortened latencies were found in the lesioned animals. In contrast, the initial behavioural response (0-15 min) to formalin was reduced, while the late response (15-40 min) was not altered. Fourteen days after intrathecal administration of 5,6-DHT the changes in nociception, both in the tail-flick and in the formalin test, had returned to the control level. These findings support the contention that the raphe-spinal serotonergic system participates in the tonic regulation of nociception in the spinal cord. Apparently this system tonically inhibits spinal nociceptive reflexes, but tonically enhances the initial behavioural responses to noxious chemical stimulation, as measured with the formalin test.

Formalin test in mice, a useful technique for evaluating mild analgesics

A modification of the formalin test appropriate for testing of mice is described. Formalin 1 or 5% was injected into the dorsal surface of a hindpaw, and the time the animal spent licking the paw was recorded. On the basis of the response pattern, two distinct periods of intensive licking activity were identified; an early (0-5 min after injection) and a late response (20-30 min after injection). The following analgesics were investigated (dose range): acetylsalicylic acid (100-400 mg/kg), paracetamol (100-400 mg/kg) and morphine (0.6-10 mg/kg). Acetylsalicylic acid (200-400 mg/kg early response, 300-400 mg/kg late response), paracetamol (200-400 mg/kg early response, 300-400 mg/kg late response) and morphine (2.5-10 mg/kg) inhibited the responses in a dose-dependent manner. The results indicate that the test is useful for evaluating mild analgesics. It may have advantages over some of the tests that are commonly used for testing analgesics.

Antinociceptive effects of orphenadrine citrate in mice

The antinociceptive effect of orphenadrine citrate, a muscle relaxant, was investigated in mice. Four different pain tests were selected to involve different noxious stimuli. Clear antinociceptive effects were found in the formalin test. The increasing temperature hot-plate test showed a biphasic dose-response relationship with slight hyperalgesia after low doses and hypoalgesia after higher doses. No significant effects of orphenadrine (0-25 mg/kg) were found in the tail flick and constant temperature hot-plate tests. The data suggest that orphenadrine may reduce, enhance or leave unaffected different types of nociceptive transmission. Orphenadrine may also possess analgesic properties in conditions not involving muscle spasm.

Selective neurotoxic lesions of descending serotonergic and noradrenergic pathways in the rat

The ability of neurotoxic substances to induce selective lesions of the descending monoaminergic pathways in rats was investigated. Saline, 6-hydroxydopamine, 5,6-dihydroxytryptamine, or 5,7-dihydroxytryptamine were administered into the lumbar subarachnoid space through a chronically indwelling catheter. The lesions were evaluated 2-3 weeks later by in vitro uptake of [3H]noradrenaline and [14C]5-hydroxytryptamine into synaptosomal preparations from the frontal cortex, brainstem, cervical spinal cord, and lumbar spinal cord of each animal. There was no difference in uptake between saline-injected and noncatheterized controls and no significant changes in cortical uptake after any of the treatments (dose range of neurotoxins: 0.6-80 micrograms). In the lumbar spinal cord, 6-hydroxydopamine (5-80 micrograms) reduced the [3H]noradrenaline uptake by approximately 90% with no effects on [14C]5-hydroxytryptamine uptake, whereas 5,6-dihydroxytryptamine reduced the uptake of [14C]5-hydroxytryptamine by 90% (20-80 micrograms). [3H]Noradrenaline uptake was unaffected by lower doses of 5,6-dihydroxytryptamine but fell by 45-55% after 40-80 micrograms. 5,7-Dihydroxytryptamine (10-80 micrograms) reduced [3H]noradrenaline uptake by 90-95% and [14C]5-hydroxytryptamine uptake by approximately 80% (5-80 micrograms) in the lumbar cord. It is concluded that intrathecal administration of suitable doses of neurotoxins may produce extensive selective lesions of descending noradrenergic and serotonergic pathways.

The putative serotonin receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin antagonizes the antinociceptive effect of morphine

The effect of the selective 5-hydroxytryptamine (5-HT-1A) receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) on nociception and morphine analgesia was tested with the tail-flick method in mice. 8-OH-DPAT (0.06-1.0 mg/kg) had no apparent effect on the general behavior of the animals and did not change their reactivity to stimulation with noxious radiant heat. The compound did, however, dose-dependently attenuate the antinociception induced by administration of morphine hydrochloride (5.0 and 10.0 mg/kg). Thus, stimulation of a subpopulation of serotonin receptors may counteract the antinociceptive effect of morphine in the tail-flick test.

Learned tolerance to ethanol in a spinal reflex separated from supraspinal control

We have recently reported that ethanol-induced inhibition of the tail flick reflex in intact and spinal rats is diminished during an eight day period if the animals are tested daily under the influence of ethanol. Ethanol only, or testing before ethanol administration, is not followed by tolerance. In the present study we used the tail flick testing of spinal rats to investigate the effect on tolerance development of repetitively triggering the tail flick reflex during intoxication, and of just placing the intoxicated animals in the test apparatus. We also investigated if damage to the tail tissue, due to repetitive prolonged test exposure, would facilitate the reflex and thereby reduce the inhibitory effect of ethanol. The results indicated that triggering of the reflex in the presence of ethanol was necessary for the tolerance to develop. Facilitation of the tail flick reflex, due to damage of the tail tissue, was not revealed. Thus the tolerance observed seems to be caused by an adaption to ethanol learned by structures involved in the tail flick reflex.

Test-dependent antinociceptive effect of spinal serotonin release induced by intrathecal p-chloroamphetamine in mice

The effect of direct intrathecal injection of p-chloroamphetamine (PCA) into the lumbar subarachnoid space was investigated in mice. PCA (0.6 - 20 micrograms) induced a dose-related prolongation of response latencies in the tail-flick test, but failed to affect the hind-paw lick response in a hot-plate test employing slowly rising temperature. PCA (5 micrograms) given intracerebroventricularly did, however, significantly elevate the response temperature in the hot-plate test. The antinociceptive effect of PCA in the tail-flick test was prevented by spinalization, by pretreatment with the selective serotonergic re-uptake blocker zimelidine (20 mg X kg-1 i.p.) and by the serotonin synthesis inhibitor p-chlorophenylalanine (300 + 300 + 150 mg X kg-1 i.p. 72, 48 and 24 h before test). It is concluded that PCA given intrathecally releases serotonin from spinal terminals, which may under certain conditions induce antinociception.

Transport and distribution of homocarnosine after intracerebroventricular and intravenous injection in the rat

Rats were injected intracerebroventricularly (i.c.v.) or i.v. with [14C]homocarnosine (250 nmol). Distribution of the dipeptide in brain structures, transport from the brain to the blood, distribution in peripheral organs, and excretion in the urine were studied by measuring radioactivity in tissue, plasma, and urine samples by liquid scintillation counting 15-120 min after injection. After i.c.v. injection, [14C]homocarnosine was taken up into all parts of the brain investigated (highest uptake in structures close to the site of injection), it was transported to the blood, and radioactive substances were found in low concentration in muscle, spleen, and liver, in high concentration in the kidneys, and very high concentration in the urine. Investigations using high pressure liquid chromatography (HPLC) showed that no degradation took place in the brain, all radioactivity was found in the homocarnosine fraction. In the plasma 86% of the radioactivity was found in the GABA fraction presumed to be formed by cleavage of the peptide.

Learned tolerance to ethanol in the spinal cord

Learning has been claimed to be of major importance in the development of tolerance to ethanol. In the present study we investigated the influence of learning on tolerance to ethanol-induced inhibition of a spinal reflex (tail-flick response) in intact and spinal rats. On day 1 and 9, groups of rats were injected with either ethanol 2.5 g/kg IP or saline 30 min prior to tail-flick testing. On days 2-8 the groups were treated differently in order to reveal the importance of the drug alone, the test alone and the combination of the two on development of tolerance. On day 10, the rats rendered tolerant in the home room were transferred to a new test room to be tested. Both in intact and spinal rats development of tolerance was observed only if the animals were repetitively tested while intoxicated. Tolerance acquired in the home room was not attenuated by transfer to a new environment. Results in the spinal rats suggested that adaptive mechanisms leading to tolerance may also be located in the spinal cord. The tolerance observed may be regarded as learned from practice while intoxicated.

Immobilization-induced analgesia: possible involvement of a non-opioid circulating substance

Stress induced analgesia (SIA) and stress-induced changes in body temperature were studied in mice and rats. Immobilization was used as the stressor. Nociception was measured with the tail-flick method and body temperature was recorded in the colon. Within 5 min of immobilization a similar increase in tail-flick latencies was observed in the two species. Concomitantly, the body temperature increased in the rats and decreased in the mice. Transection of the spinal cord 7 days before the experiments tended to increase the effect of stress on the tail-flick latencies in both species. Pretreatment with naloxone HCL (2 mg/kg SC, 5 min before immobilization) did not influence SIA in either intact or spinal rats. Thus, analgesia induced by immobilization may be due to a non-opioid substance acting peripherally or reaching the spinal cord via the systemic circulation.

Metergoline elevates or reduces nociceptive thresholds in mice depending on test method and route of administration

Intrathecal injection of metergoline reduced the response latencies in the tail-flick and hot-plate tests, supporting the contention that descending 5-hydroxytryptamine (5-HT) pathways tonically inhibit pain sensitivity. Elevated latencies were, however, observed after both intraperitoneal (IP) and intracerebroventricular (ICV) injections in the hot-plate test, when hindpaw lick was used as the response criterion. These findings may indicate that supraspinal 5-HT pathways tonically increase pain responsiveness in certain test situations . Alternative hypotheses are that metergoline in supraspinal structures acts as an agonist at post-synaptic 5-HT receptors mediating antinociception, or as an antagonist at pre-synaptic 5-HT receptors. Recording of first reaction latencies on the hot-plate showed increased thresholds after IP, but not after ICV injections. This may indicate an action on 5-HT receptors in the brain not accessible after ICV injections, or that the effect is mediated by blockade of peripheral 5-HT receptors.

Serotonin receptor antagonists induce hyperalgesia without preventing morphine antinociception

5-Hydroxytryptamine (5-HT) receptor blockade by administration of mianserin (1 mg/kg) or metergoline (0.25 mg/kg) shortened the response latencies of rats in the hot-plate (hind-paw lick response) and tail-flick tests, but did not consistently attenuate the antinociceptive effect of morphine (1.25--5.0 mg/kg). Injection of the opiate receptor antagonist naloxone (1 mg/kg) did not change tail-flick response latencies and did not interfere with the antinociceptive action of the 5-HT receptor agonist 5-methoxy-N,N-dimethyltryptamine (5-MeODMT). The antinociceptive effect of morphine was reduced in chronically spinal rats, although significant increases in tail-flick latencies were observed after 2.5 and 5.0 mg/kg. Concomitant administration of 5-MeODMT failed to restore the effect of morphine in spinal rats. In the hot-plate test, morphine did not reliably prolong latencies to forepaw lick, indicating that this response is not a useful measure of pain sensitivity. The results suggest that different mechanisms underlie the analgesia induced by systemic administration of morphine and 5-HT mediated tonic inhibition of nociception.

Changes in nociception after intrathecal administration of 5,6-dihydroxytryptamine in mice

Intrathecal administration of 5,6-dihydroxytryptamine (5,6-DHT) (5 micrograms) to mice selectively lesioned descending serotonergic pathways, reducing spinal levels of 5-hydroxytryptamine (5-HT) by 80%, without significantly changing the levels of noradrenaline. Increased sensitivity to noxious stimulation, as measured by the tail-flick and hot-plate tests, was observed 2 days after injection of 5,6-DHT. The tail-flick latencies returned to normal on day 6, but were again reduced by administration of the 5-HT receptor blocker metergoline, suggesting that the normalization process involved compensatory mechanisms in the remaining 5-HT system. In the hot-plate test, the latencies both to shaking or kicking of a hindpaw (kick) and to hindpaw lick were recorded, but the time course for the changes of these two responses was found to be different. The latencies to hindpaw lick were normalized within 2 weeks, whereas the hindpaw kick latencies remained reduced throughout the 21 day observation period.

Attenuation of morphine-induced analgesia by p-chlorophenylalanine and p-chloroamphetamine: test-dependent effects and evidence for brainstem 5-hydroxytryptamine involvement

A number of studies on the role of 5-HT in morphine analgesia and regulation of nociception are reviewed. Highly divergent conclusions are found in the literature with regard to the importance of serotonergic structures. Several methodological differences, particularly with regard to lesion and depletion techniques and testing procedures, may account for the controversies in the literature. The experimental findings presented demonstrate attenuation of morphine-induced analgesia in the hot-plate and tail-flick tests, and increased responsiveness to noxious electrical shock following depletion of 5-HT in ascending and descending 5-HT pathways by PCPA (200 + 100 + 100 mg/kg on 3 consecutive days prior to testing) as well as following destruction of cerebral 5-HT terminals by PCA (2 X 10 mg/kg, 7 and 8 days before testing). This was also the case when the neurotoxic effect of PCA was largely restricted to the brainstem by pretreatment with the 5-HT reuptake inhibitor zimelidine (20 mg/kg prior to each PCA injection). Attenuation of morphine analgesia was not found in the flinch-jump test or in the hot-plate test when conducted immediately after flinch-jump testing. It is concluded that brainstem 5-HT connections may contribute to the analgetic effect of morphine, but only under certain test conditions.

Inhibitory effect of 5-methoxy-N,N-dimethyltryptamine on the synaptosomal uptake of 5-hydroxytryptamine

5-Methoxy-N,N-dimethyltryptamine (5-MeODMT) in concentrations of 0.5-500 microM produced a significant inhibition of [14C]5-hydroxytryptamine [14C]5-HT) uptake in striatal, hippocampal and hypothalamic crude synaptosomes from rat brain. Higher concentrations of 5-MeODMT (10 microM) also inhibited the uptake of [3H]dopamine [3H]DA) and induced the release of [14C]5-HT and [3H]DA from preloaded synaptosomes. It appears that the 5-HT agonist properties of 5-MeODMT may involve reuptake inhibition in addition to the previously documented direct receptor stimulation.

Similar behavioural effects of 5-hydroxytryptamine and substance P injected intrathecally in mice

Intrathecal injection of 5-hydroxytryptamine (5-HT) or of substance P in mice elicited a behavioural syndrome consisting of reciprocal hindlimb scratching and biting or licking, directed towards the caudal parts of the body. 5-Hydroxtryptamine elicited more scratching than did substance P, which in turn caused a greater number of biting or licking responses. The 5-HT-induced responses were mimicked by 5,6-dihydroxytryptamine and inhibited by the 5-HT receptor blocker metergoline. The present findings indicate that 5-HT, injected intrathecally, may have similar effects as substance P in stimulating sensory pathways in the spinal cord.

Time course of changes in nociception after 5,6-dihydroxytryptamine lesions of descending 5-HT pathways

Intrathecal injection of 5,6-dihydroxytryptamine (5,6-DHT) in rats produced selective lesions of the descending 5-HT pathways. Spinal 5-HT levels gradually fell to less than 10% of controls within 10 days of 5,6-DHT administration with no recovery evident within 4 weeks. The uptake of 14C-5-HT into crude spinal synaptosomes was similarly reduced. The uptake of 3H-NA into spinal synaptosomes was unaffected, as was the uptake of 14C-5-HT and 3H-NA into cortical synaptosomes. Following 5,6-DHT, tail-flick latencies were reduced by 20-30% during the first post-injection week, but returned to control levels during the second week. Intrathecal or systemic administration of the 5-HT receptor antagonist metergoline significantly reduced latencies of normal rats and of 5,6-DHT treated rats tested after the second week when the response was normalized. Metergoline did not, however, further reduce the latencies of lesioned rats during the first post-injection week. It is concluded that functional adaptation involving 5-HT neurotransmission compensated for the selective lesion of descending 5-HT pathways induced by 5,6-DHT.

Does ethanol stimulate brain opiate receptors? Studies on receptor binding and naloxone inhibition of ethanol-induced effects

The ability of ethanol to stimulate opiate receptors was investigated. Administration of ethanol 2-3.5 g/kg in rats induced analgesia as tested by the tail flick method, reduced body temperature, impaired sensorimotor performance and induced "sleep". Naloxone HCI (0.5-10 mg/kg) did not significantly attenuate these ethanol-induced behavioural changes. [3H]Etorphine binding to opiate receptors in membrane preparations from rat and mouse brains was unchanged by ethanol concentrations of 0.05-3% (v/v). It was concluded that the ethanol effects studied were not mediated by naloxone-sensitive opiate receptors.

Nociception is enhanced after low doses and reduced after high doses of the serotonin receptor agonist 5-methoxy-N,N-dimethyltryptamine

The effects on pain sensitivity of intracerebroventricular injections of 5-methoxy-N,N-dimethyltryptamine were tested by the tail-flick method. Following administration of 1.6, 3.1, 6.3, 12.5 and 25 micrograms (n = 8 for each dose), tail-flick latencies were reduced by 13-24%. Fifty and 100 micrograms caused a biphasic response (hyperalgesia followed by analgesia), whereas 400 micrograms increased mean latencies by 28-39%. The hyperalgesia observed after low doses was most likely due to reduced activity in descending serotonergic neurons following presynaptic stimulation. Higher doses caused analgesia, probably by stimulating spinal postsynaptic serotonergic receptors as well.

Patterns of peptides and protein-associated-peptide complexes in psychiatric disorders

Peptidic neurones may be considered as multisignal intergrators and transducers. When formation or release of peptide outstrips genetically determined breakdown capacity, overflow of peptides to the body fluids and urine may be expected. In this paper, pathological urinary chromatographic patterns of peptides are shown for genetic, functional and mixed disorders. Part symptoms of the disorders may be induced with the biologically isolated and purified peptides as well as with chemically synthesized peptides.

A sexually dimorphic rhythm in oestradiol-activated lordosis behaviour in the rat

Ovariectomized rats exposed to constant plasma levels of oestradiol showed a daily rhythm in lordosis behaviour, with high levels of lordosis occurring during the dark portion of the daily light: darkness cycle and low levels during the light period. Similarly treated male rats failed to show a rhythm in lordosis behaviour. However, neonatal castration permitted the expression of the lordosis rhythm in male rats; conversely, an injection of 1.25 mg testosterone propionate on day 4 of life abolished the rhythm in female rats. Pinealectomy, adrenalectomy or depletion of brain 5-hydroxytryptamine levels did not affect the periodicity in lordosis behaviour but lesions in the suprachiasmatic nuclei of the hypothalamus disrupted the rhythm. It is suggested that the daily rhythm in lordosis behaviour participates in the control of the termination of heat in the female rat and that the perinatal hormone milieu may exert permanent effects on periodic functions.

Effects of p-chloroamphetamine and 5,7-dihydroxytryptamine on the sexual behavior of gonadectomized male and female rats

Treatment with the 5-HT neurotoxins p-chloroamphetamine (PCA, 2 X 10 mg/kg) or 5,7-dihydroxytryptamine (5,7-DHT, 2 X 6 microgram intracerebrally) stimulated the display of all aspects of sexual behavior, including ejaculations, by castrated male rats in the absence of testosterone (T) treatment and increased the behavioral sensitivity to a low level of T stimulation. The reduction of the (3H) 5-HT uptake after PCA treatment was more pronounced in the cortex than in the hypothalamus. 5,7-DHT treatment reduced the (3H) 5-HT uptake in the septum, hippocampus, amygdala, hypothalamus and cortex but the behavioral effects produced by the 5,7-DHT treatment could not be correlated to the biochemical effects in any of these brain areas. Since the behavioral effect of PCA appears to be stronger than that of 5,7-DHT, the 5-HT neurotoxins may exert their effect on sexual behavior in forebrain structures rather than in the hypothalamus. PCA treatment had a very small effect on mounting behavior but 5,7-DHT treatment stimulated the display of mounts and intromission patterns by ovariectomized female rats given no hormone treatment. Neither PCA nor 5,7-DHT had any effect on lordosis behavior tested before and after treatment with estradiol benzoate alone or in combination with progesterone. The observations support the conclusion that 5-HT is involved in the control by T of sexual behavior in male rats, but argue against a role of 5-HT in the neural control of lordosis behavior.

5,7-Dihydroxytryptamine lesions of the ascending 5-hydroxytryptamine pathways: habituation, motor activity and agonistic behavior

Rats were injected stereotactically in mesencephalon with 5,7-dihydroxytryptamine (5,7-DHT) in the medial 5-hydroxytryptamine (5-HT) pathway (n = 8) and in the medial plus the lateral 5-HT pathways (n = 7) or injected with vehicle (n = 8), or sham-operated (n = 8). The 5,7-DHT lesions reduced the in vitro 3H-5-HT uptake in the hypothalamus and the cortex cerebri to 27-51% of control values, 3H-noradrenaline uptake was not significantly changed. 5,7-DHT lesions of the medial, and of the medial plus the lateral, 5-HT induced mouse killing behavior and increased number of boxing positions in the shock elicited fighting test. Both lesions also reduced the rate of habituation to touch, but only the lesion of the medial plus the lateral 5-HT pathway significantly reduced the rate of habituation to acoustic stimulation. Activity in the home cage was not significantly changed by the lesions. It was concluded that selective chemical lesions of the ascending 5-HT pathways result in prolonged habituation of the orienting response and increase in particular components of agonistic behavior. The increase in locomotor activity observed after electrolytic lesions of nucleus raphe medianus seems not to be due only to lesion of the 5-HT neurons ascending from this nucleus.

Activity, avoidance learning and regional 5-hydroxytryptamine following intra-brain stem 5,7-dihydroxytryptamine and electrolytic midbrain raphe lesions in the rat

Rats underwent one of the following treatments: (1) electrocoagulation of both the dorsal and median midbrain raphe nuclei; (2) 5,7-dihydroxytryptamine creatinine sulfate (5,7-DHT) injection (10 mug, as the salt, in 5 mul vehicle) into the vicinity of each midbrain raphe nucleus; (3) intra-brain stem vehicle (5 mul of 0.2% ascorbic acid in isotonic saline) injections; or, (4) a control operation. Open field activity and one-way avoidance conditioning were examined on postoperative days 16-23. Regional central 5-hydroxytryptamine (5-HT) and catecholamine (CA) concentrations were determined 25-27 days postoperatively. Regional 5-HT levels were greatly reduced following 5,7-DHT administration and electrolytic raphe lesions. The 5,7-DHT rats also showed a reduction in spinal 5-HT content. Central CA concentrations were not affected. Variation in the pattern of regional 5-HT changes after 5,7-DHT treatment was observed but appeared to be related to the adequacy of the dorsal raphe (B7) injection. Only the electrolytic raphe lesion animals, however, showed increased locomotor activity and retarded acquisition and forced-extinction of the one-way avoidance response. In contrast, no significant differences were observed in the open field and avoidance behavior of the 5,7-DHT, vehicle, and control groups. The hyperactivity and impaired one-way avoidance performance observed after electrolytic midbrain raphe lesions are not related simply to reductions in regional forebrain 5-HT and may well be due to damage of non-serotonergic neural systems. Clearly, the behavioral effects of central 5-HT depletion depend on the method employed. The role of 5-HT in regulating activity level and mediating avoidance behavior, furthermore, remains to be determined.

Behavioral effects of 5, 7-dihydroxytryptamine lesions of ascending 5-hydroxytryptamine pathways

In protriptyline (25 mg/kg) pretreated rats stereotactic 5,7-dihydroxytryptamine (5,7-DHT) lesions of the medial plus laternal 5-hydroxytryptamine (5-HE) bundles in the mesencephalon increased the 5-HT fluorescence in these bundles, and reduced the in vitro uptake of [3H] 5-HT in the hypothalamus to 16% of control values after 2 mug 5,7-DHT/4mul and 12% after 4 mug 5,7-DHT/4mul, and in the cortex cerebri to 35 and 34% of control values, respectively. Selective lesion of the medial 5-HT bundle reduced [3H] 5-HT uptake both in hypothalamus and in cortex cerebri to 45-48% of control values, while selective lesion of the lateral 5-HT bundles significantly reduced [3H] 5-HT uptake only in cortex (to 73-75%). No significant change was observed in [3H] noradreanaline uptake after any injection, or in [3H] 5-HT uptake after vehicle injections. Locomotor activity in an open field 3-10 days postoperatively was significantly reduced by lesions of the medial plus lateral 5-HT bundles. 5-Hdroxytryptophan (50 mg/kg) and a peripheral decarboxylase inhibitor (MK 486, 75 mg/kg) 17 days postoperatively induced a pronounced behavioral "5-HT syndrome" in these rats with medial plus lateral lesions but not in controls. Pain sensitivity, as measured by the hot plate test, was not changed by any lesion, even when tryptophan hydroxylase was partly inhibited with alpha-propyldopacetamide (100 mg/kg). Morphine analgesia and acquisition of a one-way avoidance response also were unchanged. Apomorphine (2 mg/kg)-induced locomotor activity and stereotyped behavior, as measured in an Animex activity meter, were not significantly different from control values in the 5,7-DHT groups. It was concluded that the medial 5-JT BUNDLE INNERVATES BOTH THE HYPOTHALAMUS AND THE CORTEX CEREBRI AND THE LATERAL 5-HT bundle mainly the cortex. These ascending 5-HT neurons are involved in maintaining open field ambulation. No wupport was obtained for the view that they are involved in pain mechanisms, in morphine-induced analgesia, in apomorphine-induced motor behavior, or in one-way avoidance learning.

Unchanged sensitivity to electric shock in L-tryptophan treated rats

Injections of L-tryptophan (50, 100 and 200 mg/kg) i.p. in rats resulted in dose dependent increases in brain concentrations of tryptophan, 5-hydroxytryptamine and 5-hydroxyindoleacetic acid. Sensitivity to electric shock as measured by flinch and jump thresholds was not changed by L-tryptophan 100 and 200 mg/kg. The results indicate that increased 5-HT turnover does not influence sensitivity to electric shock.

Response to electric shock in rats: effects of selective midbrain raphe lesions

The forebrain serotonin (5-HT) concentrations of rats with lesions in the median (M; n equal to 5), dorsal (D; n equal to 5), and both (DM; n equal to 6) midbrain raphe nuclei were, respectively, 22, 48, and 70% lower than in control animals (n equal to 10). The lesion and control groups, however, did not evidence differences in pain sensitivity as measured by the flinch-jump technique. On the other hand, of the animals tested, those with M (n equal to 3) and DM (n equal to 4) lesions required more trials than controls (n equal to 6) to acquire a one-way avoidance response. D lesion rats (n equal to 2) did not differ from controls in one-way avoidance learning, except in terms of prolonged escape latencies during the first three trials. The previously reported increased sensitivity to painful stimuli subsequent to medial forebrain bundle lesions or para-chlorophenylalanine administration, therefore, does not appear to be due exclusively to disruption of ascending 5-HT fibers originating in the dorsal and median raphe nuclei. The effects of midbrain raphe lesions of avoidance learning, furthermore, depend on lesion locus, and are not due to either hypo- or hyperalgesia.