Illness-induced hyperalgesia is mediated by spinal neuropeptides and excitatory amino acids

The spinal cord dorsal horn contains neural mechanisms which can greatly facilitate pain. We have recently shown that ‘illness’-inducing agents, such as intraperitoneally administered lipopolysaccharide (LPS; bacterial endotoxin), can produce prolonged hyperalgesia. This hyperalgesic state is mediated at the level of the spinal cord via activation of the NMDA-nitric oxide cascade. However, prolonged neuronal depolarization is required before such a cascade can occur. The present series of experiments were aimed at identifying spinal neurotransmitters which might be responsible for creating such a depolarized state. These studies show that LPS hyperalgesia is mediated at the level of the spinal cord by substance P, cholecystokinin and excitatory amino acids acting at non-NMDA sites. No apparent role for serotonin or kappa opiate receptors was found.     

Spinal cholinergic and monoamine receptors mediate the antinociceptive effect of morphine microinjected in the periaqueductal gray on the rat tail, but not the feet

The antinociceptive effects of morphine (5 μg) microinjected into the ventrolateral periaqueductal gray were determined using both the tail flick and the foot withdrawal responses to noxious radiant heating in lightly anesthetized rats. Intrathecal injection of appropriate antagonists was used to determine whether the antinociceptive effects of morphine were mediated byα₂-noradrenergic, serotonergic, opioid, or cholinergic muscarinic receptors. The increase in the foot withdrawal response latency produced by microinjection of morphine in the ventrolateral periaqueductal gray was reversed by intrathecal injection of the cholinergic muscarinic receptor antagonist atropine, but was not affected by the a2-adrenoceptor antagonist yohimbine, the serotonergic receptor antagonist methysergide, or the opioid receptor antagonist naloxone. In contrast, the increase in the tail flick response latency produced by morphine was reduced by either yohimbine, methysergide or atropine. These results indicate that microinjection of morphine in the ventrolateral periaqueductal gray inhibits nociceptive responses to noxious heating of the tail by activating descending neuronal systems that are different from those that inhibit the nociceptive responses to noxious heating of the feet. More specifically, serotonergic, muscarinic cholinergic andα₂-noradrenergic receptors appear to mediate the antinociception produced by morphine using the tail flick test. In contrast, muscarinic cholinergic, but not monoamine receptors appear to mediate the antinociceptive effects of morphine using the foot withdrawal response.     

Changes in central serotoninergic transmission affect clonidine analgesia in monkeys

Summary 1. The effects of changes in central serotoninergic transmission on clonidine analgesia were assessed in monkeys. The minimum electrical current required for producing jaw opening is referred to as the pain threshold. Pain was induced by electrical stimulation of tooth pulp afferents. 2. In the first series of studies, intracerebroventricular administration of clonidine (5–30 g) produced dose-dependent analgesia in monkeys. The clonidine-induced analgesia was abolished or attenuated by prior injection of the animals with p-chlorophenylalanine or 5,7-dihydroxytryptamine into the third cerebral ventricle. On the other hand, pretreatment of the animals by injecting 5-HT or its precursor 5-hydroxytryptophan into the cerebral ventricle potentiated the clonidine-induced analgesia in monkeys. 3. In the second series of experiments, administration of clonidine (1–10 g) into the diencephalic periventricular gray (of the anterior hypothalamic portion), the periaqueductal gray, or the dorsal raphe nuclei also produced dose-dependent analgesia in monkeys. The analgesia induced by clonidine injection into the diencephalic periventricular gray or the periaqueductal gray was effectively antagonized by pretreatment of the animals by injecting two 5-HT receptor antagonists (such as ketanserine and methysergide) into the diencephalic periventricular gray or the periaqueductal gray. The clonidine-induced analgesia in monkeys was not affected by pretreatment of the animals with injections of either ketanserine or methysergide into the dorsal raphe nuclei. 4. The results suggest that the functional activity of central 5-HT neurons correlate well with the analgesic sensitivity of clonidine microinjected centrally. In addition, the analgesia induced by clonidine microinjected into the diencephalic periventricular gray or the periaqueductal gray was mediated by the 5-HT receptors at the site of injection.This study was supported by grants from the National Science Council of the Republic of China and the Student Summer Fellowship of National Cheng Kung University Medical College (1986) Send offprint requests to Mao-Tsun Lin at the above address     

Hallucinogen-induced rotational behavior in rats

LSD, mescaline, and MDMT (5-methoxy-N,N-dimethyltryptamine) in normal rats induced dose-dependent rotation (circling behavior), which was consistent in direction from week to week (1 week separating hallucinogen administration). The direction of LSD-induced rotation for individual animals was the same as amphetamine-induced, but not apomorphine-induced, rotation. Of the three postsynaptic serotonin antagonists (methysergide, cyproheptadine, and 2-bromo-LSD) tested, only methysergide induced rotation; this rotation was consistent in direction from week to week, and was in the same direction as LSD-induced rotation. l-LSD induced weak rotation and was approximately six times less potent than d-LSD. p-Chlorophenylalanine pretreatment increased the sensitivity to LSD, whereas -methyl-p-tyrosine pretreatment blocked LSD-induced rotation. Simultaneous administration of LSD and amphetamine induced rotation significantly greater than amphetamine alone; a similar effect was observed with LSD plus scopolamine. However, apomorphine plus LSD induced rotation similar in magnitude to apomorphine alone. These results suggest that the mechanism by which hallucinogens induce rotation is consistent with an inhibitory action on the serotonin-containing midbrain raphe neurons. The inhibition of raphe neuronal firing could disinhibit nigrostriatal activity (possibly at the level of the substantia nigra). Methysergide-induced rotation could result from partial antagonism of postsynaptic serotonin receptors in the substantia nigra or striatum. The dopaminergic properties of LSD may attenuate rotation resulting from disinhibition of nigrostriatal activity by interacting with presynaptic nigrostriatal dopamine autoreceptors.     

Influence of naloxone and methysergide on the analgesic effect of clomipramine in rats

The effects of the tricyclic antidepressant clomipramine were studied in two analgesic tests in rats: (1) vocalization threshold response; and (2) scored behavioral response to electric shock to the tail. Clomipramine (20-50 mg/kg i.p.) produced analgesia, decreasing behavioral response scores and increasing vocalization threshold. Morphine also reduced the response scores in the second test. Naloxone (0.8 mg/kg i.p) or methysergide (20 mg/kg i.p.) (no effect when given alone) abolished the analgesic effect of clomipramine as evaluated by vocalization threshold response. Naloxone alone (0.6 or 2 mg/kg i.p.) increased the behavioral response at 20 and 30 V but did not modify the score at 40 V. Naloxone reduced the analgesic effect of clomipramine or morphine in the behavioral test. These results suggest that the analgesic effect of clomipramine could involve both serotonergic and endorphin central systems.     

Effect of the selective 5-HT3 receptor antagonists ICS 205-930 and MDL 72222 on 5-HTP-induced head shaking and behavioral symptoms induced by 5-methoxy-N,N,dimethyltryptamine in rats: comparison with some other 5-HT receptor antagonists

The effect of the selective 5-HT₃ receptor antagonists ICS 205-930 and MDL 72222 on head shaking behavior induced by l-5-HTP and behavioral symptoms induced with 5-methoxy-N,N,-dimethyltryptamine (5-MeODMT) in rats was evaluated. Both drugs dose-dependently reduced l-5-HTP-induced head shaking but were at least 600 times less potent than pirenperone and ketanserin and at least 50 times less potent than methysergide. ICS 205-930 and MDL 72222 were more than 1000 times less potent than pirenperone or methysergide and 100 times less potent than ketanserin in blocking 5-MeODMT-induced forepaw treading and tremor. Since it appears that head shakes induced by l-5-HTP are mediated by 5-HT₂ receptors, these data suggest that ICS 205-930 and MDL 72222 do not significantly interact with 5-HT₂ receptors in the brain. Furthermore, the data suggest that ICS 205-930 and MDL 72222 lack appreciable antagonistic activity at the 5-HT receptor(s) mediating those behavioral effects induced by 5-MeODMT.     

Non-reversal of scopolamine- or age-related EEG changes by ondansetron,methysergide or alaproclate

The present studies investigates the effects of a 5HT3-antagonist (ondansetron: 0.01, 0.1, 1, 10 µg), a 5HT2-antagonist (methysergide: 2, 10, 20 mg/kg) and a serotonin uptake inhibitor (alaproclate: 2, 10, 20 mg/kg) on the neocortical electrical activity of young scopolamine-treated and aged rats. The scopolamine (0.2 and 0.8 mg/kg)-induced increase in EEG spectral components was not reversed by ondansetron, methysergide or alaproclate. The scopolamine (0.8 mg/kg)-induced EEG amplitude increase reversing potency of a subthreshold dose of the muscarinic agonist pilocarpine (2 mg/kg) was not potentiated by ondansetron, methysergide or alaproclate. A higher dose of pilocarpine (10 mg/kg) reversed scopolamine-induced EEG slowing. Age-related increase in high voltage spindles (HVS) was not alleviated by either ondansetron, methysergide or alaproclate. The HVS activity stabilizing effect of pilocarpine (2 mg/kg) was not enhanced by ondansetron, methysergide or alaproclate. These results suggest that the serotonergic agents investigated could not alleviate cortical cholinergic activation deficit and once again implicate the role of cholinergic system in both the neocortical electrical activation and age-related cortical electrical arousal deficit.     

Serotonergic neurons in the brainstem modulate animal hypnosis

We previously found that the center of animal hypnosis production in the rabbit is located around the locus ceruleus and brachium conjunctivum (LC-BC) of the brainstem. The involvement of serotonergic neurons in this area of animal hypnosis was investigated by microinjection of serotonin into these regions. The duration of animal hypnosis (DAH) induced by inversion was diminished to about 65% of the controls by serotonin microinjection into the LC-BC and microinjection of methysergide prolonged the DAH to 3.2 times that of the controls. Flexor muscle contraction (CFM) of the upper extremities induced by electrical stimulation of the motor cortex was enhanced by serotonin. In normal rabbits, hard pressure on the ear base or the lumbar paravertebral area reduced CFM and this effect was partially antagonized by serotonin microinjected into the LC-BC. The results suggest that serotonergic neurons in the LC-BC modulate animal hypnosis.     

Monoamine uptake inhibitors attenuate ethanol intake in alcohol-preferring (P) rats

The P line of alcohol-preferring rats drink pharmacologically significant amounts of ethanol when given free choice between a 10 percent ethanol solution and water. Serotonin (5-HT) uptake inhibitors and desipramine, a norepinephrine (NE) uptake inhibitor, were found to significantly reduce their ethanol consumption for up to 24 hours after intraperitoneal injection. To determine if this effect of 5-HT uptake inhibitors could be altered by receptor antagonists, some of which are short acting, P rats were trained to drink ethanol by free choice during scheduled availability, with ethanol being presented one hour every four hours during the light cycle. The majority of the ethanol was consumed during the first hour of availability, and the ethanol intake was significantly reduced by the 5-HT uptake inhibitors, fluoxetine and fluvoxamine. Pretreatment with antagonists for 5-HT1, 5-HT2 and alpha- and beta-NE receptor systems failed to alter the fluvoxamine attenuation of ethanol intake. The mechanism by which 5-HT uptake inhibitors alter ethanol preference remains unclear.     

Peripheral conversion of L-5-Hydroxytryptophan to serotonin induces drinking in rats

Female rats administered serotonin (0.25 to 4.0 mg/kg, s.c.) showed a dose-dependent increase in water intake. The dipsogenic response was nearly maximal when 2 mg/kg was administered s.c. and plateaued by 2 hr after treatment. l-5Hydroxytryptophan (5-HTP), the precursor of serotonin, is also a potent dipsogen which induces drinking by way of the renin-angiotensin system. The possibility that the dipsogenic activity of 5-HTP is dependent on decarboxylation to serotonin was the objective of these studies. Either benserazide (30 mg/kg. s.c.), a central and peripheral decarboxylase inhibitor, or carbidopa (6.5 mg/kg, s.c.), a peripheral decarboxylase inhibitor, was administered 15 min prior to the dipsogen. Both decarboxylase inhibitors attenuated the dipsogenic response to 5-HTP (25 mg/kg, s.c.) but not to serotonin (2 mg/kg, s.c.). The peripheral serotonergic receptor antagonist, methysergide (3 mg/kg, i.p.), blocked the dipsogenic responses to both 5-HTP (25 mg/kg, s.c.) and serotonin (2 mg/kg, s.c.). There was no interaction between 5-HTP (18 mg/kg, s.c.) and serotonin (1 mg/kg, s.c.) when administered simultaneously with respect to their dipsogenic effects. Thus, the drinking response accompanying administration of 5-HTP occurs following peripheral conversion to serotonin which, in turn, activates peripheral serotonergic receptors. The mechanism(s) by which activation of peripheral serotonergic receptors increases water intake is not known, but appears to involve release of renin from the kidney.