Effect of 5,7-dihydroxytryptamine on pain sensitivity and analgesic activity of morphine

Electron microscopy of rats ultrathin sections from dorsal and central raphe nucleus and spinal cord after 5,7-dihydroxytryptamine intracisternal microinjection (200 micrograms) has revealed neurones and axonal terminals distruction, which associated with tail-flick hypoalgesia and blood pressure nociceptive reactions diminished. In this condition the morphine (2 mg/kg) analgesia and drug depressive effect on pain hemodynamic manifestations increase significantly.     

Long-term effects of 5,7-dihydroxytryptamine on brain monoamines

5,7-dihydroxytryptamine (75 and 150 μg) was injected intraventricularly to adult male rats; animals were killed at various times after the injection and brains were examined for changes in the concentration of tryptophan, serotonin, 5-hydroxyindole acetic acid, norepinephrine and dopamine. Brain 5-hydroxyindoleamines were markedly depleted at all time periods examined, even after the administration of a tryptophan load (50 mg/kg). A small but significant decline in brain norepinephrine but not dopamine was also noted after the administration of the dihydroxytryptamine.     

ODC-polyamine system is involved in morphine analgesia

alpha-Difluoromethylornithine (DFMO) directly infused into a brain-lateral ventricle (12.5, 25 and 50 micrograms/rat) dose- and time-dependently inhibited brain ODC activity. While having no influence per se on pain threshold, DFMO significantly inhibited the analgesic activity of morphine (15 mg/kg i.p.), this effect being obtained when brain ODC activity was reduced by at least 80%. On the other hand, DFMO had no influence on number and affinity of brain opiate binding sites. Morphine per se neither modified whole brain ODC activity nor significantly affected the ODC inhibitory effect of DFMO. In more discrete brain areas (midbrain, brainstem) morphine actually increased ODC activity. The present results indicate that brain ODC/polyamines system may play a role in the analgesic activity of opioids, probably at a post-receptorial level or through a non-opiate receptor-linked mechanism.     

Peroxidase-promoted oxidation and peroxidation of the serotonergic neurotoxin 5,7-dihydroxytryptamine

Spectral data provide the first evidence that lactoperoxidase, a model enzyme for most mammalian peroxidases, catalyzed the one-electron oxidation and/or peroxidation of 5,7-dihydroxytryptamine. This process correlates with the production of superoxide radicals as is evident from the observed inhibitory effect of superoxide dismutase on product formation. 5,7-Dihydroxytryptamine is a classical peroxidase-oxidase substrate acting as a one-electron donor for enzyme compounds I, II and III. The one-electron peroxidatic oxidation of this serotonergic neurotoxin, responsible for the selective degeneration of central (5-hydroxytryptamine) neurons, is a fast process requiring measurement on the ms time scale. Attention is drawn to the biochemical and toxicological implications, because this fast reaction results in formation of known cell damaging species: free radicals, superoxide radicals and quinoidal products probably involved in the toxic action of 5,7-dihydroxytryptamine.Abbreviations 5-HT 5-Hydroxytryptamine, Serotonin - 5,7-DHT 5,7-Dihydroxytryptamine - 5,6-DHT 5,6-Dihydroxytryptamine - 5-HT-4,7-Dione 5-Hydroxytryptamine-4,7-Dione - GPO Glutathione Peroxidase - MAO Monoamine Oxidase - LPO Lactoperoxidase, the Roman numerals I, II and III added to LPO indicate compounds I, III and III of the enzyme - TPO Thyroid Peroxidase - IPO Intestinal Peroxidase - UPO Uterine Peroxidase - EPO Eosinophil Peroxidase - SOD Superoxide Dismutase - DPPH 1,1-Diphenyl-2-Picrylhydrazil radical - _max absorption maxima - ESR Electron Spin Resonance     

The effect of 5,6-dihydroxytryptamine on post-decapitation convulsions

Previous data (1) have shown that L-DOPA increases the duration of the clonic phase of post-decapitation convulsions (PDC) in mice. It was suggested that this effect is produced by depleting 5-hydroxytryptamine (5-HT) in the inhibitory bulbospinal pathways and thus enhancing reflex activity in the spinal cord. If this were true then L-DOPA administration should not influence clonic PDC in animals whose 5-HT pathways were destroyed. We therefore tested the effects of L-DOPA on mice 3 weeks after pretreatment with the 5-HT neurotoxin, 5,6-dihydroxytryptamine (5, 6-DHT) (50 μg/kg, intracerebroventricularly). All mice were given the peripheral decarboxylase inhibitor, Ro 4-4602. 5,6-DHT halved the brain 5-HT levels and significantly increased the duration of clonic PDC. The administration of L-DOPA (320 mg/kg i.p.) to 5,6 DHT treated mice did not produce any further significant increases in duration. The administration of 5-hydroxytryptophan (5-HTP) (100 mg/kg, i.v.) to 5,6-DHT treated mice, however, increased 5-HT to above control levels and reduced convulsions to control levels. Administration of both 5-HTP and L-DOPA to 5,6-DHT treated mice resulted in 5-HT levels and convulsion times which were also not significantly different from the controls. These data give additional indication that intact 5-HT nerve terminals are necessary for L-DOPA to prolong the duration of clonic PDC.     

Molecular mechanism of biological action of the serotonergic neurotoxin 5,7-dihydroxytryptamine

5,7-Dihydroxytryptamine (5,7-DHT) is a selective serotonergic neurotoxin by virtue of its selective uptake into 5-hydroxytryptamine neurons and its ability to undergo autoxidation. The mechanism by which 5,7-DHT induces neurodegenerative effects remains enigmatic. The mechanism of autoxidation of 5,7-DHT, which has been recently discovered, is unique among the autoxidizable neurotoxins and involves incorporation of oxygen to produce the 4-hydroperoxy-5-keto derivative of 5,7-DHT and thence the (4,7) p-quinone of 4,5,7-trihydroxytryptamine (4,5,7-THTQ), a relatively unreactive quinone. In addition, no reduced oxygen species such as hydrogen peroxide, superoxide and hydroxyl radical are produced during autoxidation of 5,7-DHT. Yet, there is evidence to suggest that both the covalent modification of endogenous macromolecules by 5,7-DHT derived products and the toxic effects of reduced oxygen species are, at least in part, responsible for the neurodegenerative effects of 5,7-DHT. Here we propose that (1) the 4-hydroperoxy-5-keto derivative of 5,7-DHT may serve as a substrate for glutathione peroxidase to eventually produce reduced oxygen species and 4,5,7-THTQ, (2) 4,5,7-THTQ may undergo redox cycling thereby generating reduced oxygen species and lowering the reducing equivalents of the neuron, (3) rapid oxygen consumption by 5,7-DHT and the products derived from it may lead to hypoxia, and (4) the product of autoxidation of 5,7-dihydroxyindole-3-acetaldehyde, the monoamine oxidase metabolite of 5,7-DHT, may serve as an alkylating (crosslinking) agent of proteins.     

The 5,7-DHT-induced anticonflict effect is dependent on intact adrenocortical function.

We have recently reported that the anxiolytic-like effect observed in rats severely depleted of brain serotonin (5-HT) by means of 5,7-DHT is indirect and probably involves the GABA(A)/benzodiazepine chloride ionophore receptor complex (GABAA/BDZ-RC). One tentative explanation for this effect considered the involvement of corticosteroids. In the present series of experiments we have therefore investigated the effect of adrenalectomy (ADX) on the 5,7-DHT-induced anxiolytic-like effect displayed by rats in Vogel's conflict test. ADX totally abolished the anticonflict effect of the 5,7-DHT lesion. Replacement treatment with corticosterone, but not with dexamethasone, reinstated the anticonflict effect. These results indicate that an intact adrenocortical function, possibly via brain steroid type I receptors, is required for the expression of the 5,7-DHT-induced anxiolytic-like effect. It is postulated that ADX lowers the concentration of endogenous positive modulators at the GABAA/BDZ-RC to a level no longer sufficient to produce anxiolytic-like effects in 5,7-DHT-lesioned animals. The finding that 5,7-DHT-lesioned animals were more sensitive than sham-lesioned controls to the anticonflict effect of the barbiturate-like corticosteroid THDOC provides further support for the contention that an increased endogenous activity at the GABAA/BDZ-RCes is involved in the anxiolytic-like effect observed in rats with a severe depletion of brain 5-HT.     

The effect of intracerebroventricular infusion of morphine, methionine-enkephalin and D-Ala2-enkephalinamide on body temperature of rabbits

The effect of intracerebroventricular infusions of two synthetically obtained peptides: Met-enkephalin hydrochloride and D-Ala2-Met-enkephalinamide hydrochloride, and of morphine hydrochloride on rectal temperature was investigated in conscious rabbits. Morphine hydrochloride in a dose of 240 micrograms and D-Ala2-Met-enkephalinamide hydrochloride in doses of 240 and 3000 micrograms produced a hyperthermia which was accompanied by ear vasoconstriction and shivering. No such effect ensued after Met-enkephalin, possibly due to rapid enzymatic degradation of this compound. The concept of opioid involvement in the central thermoregulatory mechanism is discussed.     

Effect of intraventricular injection of 5,7-dihydroxytryptamine on regional tryptophan hydroxylase of rat brain

5,6-DIHYDROXYTRYPTAMINE has been shown to cause selective degeneration of serotonergic neurons in the central nervous sytem (BAUMGARTEN, LACHENMAYER and SCHLOSSBERGER, 1972b). This degeneration is accompanied by depletion of serotonin (BAUMGARTEN et al., 1971; 1972a) and loss of tryptophan hydroxylase activity (VICTOR, BAUMGARTEN and LOVENBERG, 1973) in certain regions of the brain. In the current experiments, the effect of 5,7-dihydroxytryptamine (another dihydroxylated tryptamine derivative) on tryptophan hydroxylase activity has been examined. Since tryptophan hydroxylase is the rate-limiting enzyme in serotonin biosynthesis and has a similar distribution to that of serotonin in the brain, it is used as a biochemical marker of serotonergic neurons, Recent experiments also indicate that 5,7-dihydroxytryptamine causes morphological damage to serotonergic neurons of the central nervous system (BAUMGARTEN and LACHENMAYER, 1972).     

The short term effects of 5,7-dihydroxytryptamine on peripheral serotonin stores in Rhodnius prolixus and their long-term recovery

The serotonin neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) appears to affect invertebrate systems differently from vertebrate ones. The basis for toxicity in vertebrates appears to involve the intraneuronal actions of monoamine oxidase (MAO) upon the toxin. In insects, MAO is not present in appreciable amounts. In this study, we demonstrate that in vitro 5.7-DHT competitively inhibits the uptake of [³H]serotonin by serotonergic neurohaemal areas. The apparent K_M increases from 4.9 × 10⁻⁷ to 1.7 × 10⁻⁶ M. This neurotoxin also causes a significant release of previously accumulated [³H]serotonin in nominally Ca²⁺-free saline. While 5,7-DHT does not affect the uptake of [³H]tryptophan, it reduces the subsequent synthesis of [³H]serotonin. In vivo, the tissues appear to have recovered 2 weeks after toxin treatment, as determined by immunohistochemistry. At 24 h, 1 week and 2 weeks after injection, the tissues are able to take up and release [³H]serotonin normally. 1 and 2 weeks after injection, insects ingest a normal-sized blood meal, a behaviour acutely disrupted by 5,7-DHT treatment. The results of this and other invertebrate studies suggest that 5,7-DHT does not destroy serotonergic neurons, as it does in vertebrates. 5,7-DHT may be a more useful tool to study the functions of serotonin in invertebrates as one may transiently affect serotonin stores.     

Effect of substance P on the 5,7-dihydroxytryptamine induced alteration of the postnatal development of central serotonin neurons

Systemic treatment with the serotonin neurotoxin 5,7-dihydroxytryptamine [5,7-HT]in the neonatal stage leads to a permanent alteration of the postnatal development of the serotonin neurons in rat brain with denervation of distant nerve terminal projections and hyperinnervation in regions close to the serotonin perikarya. Intracisternal administration of substance P was found to counteract both the denervation and the hyperinnervation, as evaluated by measuring endogenous serotonin levels and [3H]-serotonin uptake in vitro. Furthermore, substance P was found to potentiate the reduction of serotonin induced by tryptophan hydroxylase inhibition with alpha-propyldopacetamide, indicating that substance P can produce an increase in serotonin utilization and turnover. The results suggest that substance P has a degeneration preventing and/or regrowth stimulatory effect on damaged serotonin neurons during ontogeny.     

Chronic ACTH treatment: influence on 5-HT2 receptors and behavioral supersensitivity induced by 5,7-dihydroxytryptamine lesions

The capacity of the serotonin (5-HT) precursor 5-HIP to induce the ACTH-responsive myoclonic-convulsive disorder infantile spasms in patients with Down's syndrome has been cited as evidence for altered serotonergic neurotransmission in infantile spasms. Since there is no animal model of infantile spasms, the suitability of behavioral supersensitivity (myoclonus) evoked by 5-HTP in rats with 5,7-dihydroxytryptamine (DHT) lesions as a model was tested by determining the effect of chronic treatment with ACTH (40 IU/kg) on 5-HTP-evoked myoclonus. In rats treated with DHT as adults, ACTH administration did not alter the "serotonergic behaviors," such as myoclonus, induced by 30 mg/kg 5-hydroxytryptophan (5-HTP), but induced a small significant increase in Bmax of neocortical 5-HT2 sites of the DHT group, with no change in rats without lesions. In rats treated with DHT as neonates, there was also no significant difference in behaviors evoked by several doses of 5-HTP. These data suggest that ACTH minimally modifies the effects on 5-HT receptors of DHT lesions, but the intracisternal DHT model is not a suitable model for infantile spasms because chronic ACTH was not antimyoclonic.     

Acupuncture analgesia in rats and its changes under the effect of morphine and naloxone

It was established in chronic experiments on rats that electric acupuncture of the acupuncture point noticeably decreases pain reaction to electric stimulation of the tail. Morphine given in a subanalgetic dose (5 mg/kg) potentiated acupuncture analgesia, while 5 mg/kg of naloxone completely abolished it. Potential mechanisms of analgesia realization during electric acupuncture are discussed.     

Effects of the intraventricular administration of 5,7-dihydroxytryptamine on FSH, LH and prolactin secretion in neonatally estrogenized male rats

The role of the serotoninergic system in the control of LH, FSH and prolactin secretion was analyzed in control and neonatally estrogenized male rats. Animals injected s.c. with 500 micrograms of estradiol benzoate (EB) on day 1 of life, or their corresponding sham-treated controls, were divided on day 75 into the following groups: (1) orchidectomized; (2) injected intraventricularly with 5,7-dihydroxytryptamine (5,7-DHT); (3) orchidectomized and treated with 5,7-DHT, and (4) sham operated. 15 days later, the animals were decapitated and their FHS, LH and prolactin plasma values measured by specific RIA systems. After the treatment with 5,7-DHT, control animals showed a decline in basal prolactin levels but no modification in basal LH and FSH values. After castration, 5,7-DHT-treated animals showed a reduced LH increase and a more marked prolactin decrease. In neonatal estrogen-treated animals, the 5,7-DHT injection did not change FSH, LH or prolactin levels but did partially or completely abolish the post-castration rise in FSH and LH levels, respectively. These data seem to indicate that neonatal estrogenization induced a modification of the serotoninergic role in the control of LH, FSH and prolactin.     

Luteinizing hormone secretion following intracerebroventricular administration of morphine in the prepuberal gilt

Antagonism of endogenous opioids with naloxone stimulates luteinizing hormone (LH) release in mature but not prepuberal gilts. The present report demonstrates that the opiate agonist morphine (500 micrograms), administered intracerebroventricularly (ICV), reduced LH secretion in both ovariectomized mature and prepuberal gilts. We suggest that opioid receptors are functionally coupled to the GnRH secretory system in prepuberal gilts even though endogenous opioid peptide modulation of LH secretion was not demonstrable in our previous studies.     

Morphine-3-glucuronide--a potent antagonist of morphine analgesia

In this study, morphine-3-glucuronide (M3G), the major plasma and urinary metabolite of morphine, was shown to be a potent antagonist of morphine analgesia when administered to rats by the intra-cerebroventricular (i.c.v.) route. The antagonism of morphine analgesia was observed irrespective of whether i.c.v. M3G (2.5 or 3.0 micrograms) was administered 15 mins prior to or 15 mins after i.c.v. morphine (20 micrograms). When M3G (10mg) was administered intraperitoneally (i.p.) to rats 30-40 mins prior to morphine (1.5mg i.p.), the analgesic response was significantly reduced compared to administration of morphine (1.5mg i.p.) alone. It was further demonstrated that i.c.v. M3G (2.0 micrograms) antagonized the analgesic effects of subsequently administered i.c.v. morphine-6-glucuronide (0.25 micrograms).