Jodie C. McGuffin

Neurotensin: Antinocisponsive action in rodents

Neurotensin administered intracisternally in mice increased reaction time in the hot plate test (minimum effective dose = 25 ng) and decreased writhing induced by acetic acid (minimum effective dose = 0.25 ng). At the maximum effective dose of 250 ng, the antinocisponsive action of neurotensin lasted for about 1 h. In the hot plate test, neurotensin, [Gln4]-neurotensin and [D-Ala2]-methionine enkephalinamide were all equiactive at the three dose levels examined, 0.00015, 0.015 and 1.5 nmoles/mouse. At antinocisponsive doses, mice treated intracisternally with neurotensin also were hypothermic and had reduced locomotor activity. Neurotensin, 2500 ng, administered intravenously did not produce any of the effects described above. D,L-5-Hydroxytryptophan, oxotremorine and chlorpromazine given systemically caused hypothermia but did not change reaction time in the hot plate procedure. Neurotensin, even at doses as high as 2500 ng given intracisternally, did not disrupt coordinated motor control or change the current-response relationship for electroshock-induced seizures. The antinocisponsive action of neurotensin in mice (hot plate test) was not antagonized by naloxone, cyproheptadine, methergoline, atropine, benzotropine, mecamylamine, chlorpheniramine, pyribenzamine, cimetidine, HEAT, propranolol, phenoxybenzamine, haloperidol or indomethacin. Neurotensin administered intracisternally was also an effective antinocisponsive agent in rats, as assessed in the hot plate procedure, but not in the tail-flick test. Naloxone did not antagonize this action of neurotensin in the rat. It is suggested that the antinocisponsive effect of neurotensin results from an interaction of the peptide with neurotensin-specific receptor sites in the central nervous system.

Role of monoamines in the anorexigenic actions of fenfluramine, amphetamine and p-chloromethamphetamine

Abstract Rats pretreated with methergoline, cyproheptadine, cinanserin, methysergide or chlorimipramine were partially protected against the reduction in food intake elicited by a subsequent injection of 6 mg/kg of fenfluramine. With the exception of methergoline, none of these compounds attenuated the anorexigenic action of 3 or 1.5 mg/kg of fenfluramine. Prior treatment with α-methyl-p-tyrosine diminished the anorexic activity of 6 mg/kg of amphetamine, but this compound was ineffective against 3 or 1.5 mg/kg. The anorexigenic action of amphetamine, 6 or 3 mg/kg, was reduced by prior administration of haloperidol, whereas 1.5 mg/kg of amphetamine was not affected. All of the substances used for pretreatment exhibited selectivity, i.e. in no instance did an antagonist of fenfluramine also reduce the effect of amphetamine and vice versa. Animals pretreated with the indoleamine antagonists, chlorimipramine, α-methyl-p-tyrosine and haloperidol, were also tested with p-chloromethamphetamine. The results indicated that p-chloromethamphetamine cannot be classified as either primarily fenfluramine-like or amphetamine-like in its mode of action. It was concluded that 5-hydroxytryptamine is involved in the anorexigenic effect of high doses of fenfluramine, that catecholamines (principally dopamine) are important in the action of high (and perhaps intermediate) dose levels of amphetamine, and that the anorexia which follows low doses of fenfluramine and amphetamine occurs via mechanisms not involving 5-hydroxytryptamine, norepinephrine or dopamine.

Fenfluramine: long-term reduction in brain serotonin (5-hydroxytryptamine).

A single oral dose of 15 mg/kg of fenfluramine reduced the level of serotonin in rat brain to 48, 51 and 63% of control at 1, 15 and 30 days, respectively, after administration. 3 mg/kg p.o. of the drug caused a smaller but significant diminution in brain serotonin. At the 3 mg/kg dose level, the decreases in serotonin were, at least partially, cumulative following multiple injections spaced 24 hr apart. Brains removed 14 days after the 5th daily injection of 5 mg/kg p.o. of fenfluramine had only 60% of the concentration of serotonin found in brains from control animals. These findings demonstrate that fenfluramine has a long-lasting action on serotonin-containing neurons in brain.

Central serotonin-like activity of 6-chloro-2-[1-piperazinyl]-pyrazine (CPP; MK-212).

CPP, administered systemically, elicited four distinct responses characteristic of serotonin-receptor activation in the central nervous system. The crossed extensor reflex in the acutely spinalized rat was enhanced by treatment with CPP, 1-16 mg/kg. CPP, 1.11-10 mg/kg, elicited a dose-related increase in head twitches in mice. A complex motor syndrome in rats similar to that produced by pargyline plus tryptophan (or other treatments effecting increased activation of central serotonin receptors) was produced by 1.25-5 mg/kg of CPP. An increase in twitch frequency of the mylohyoideus muscle in the urethane anesthetized rat occured after CPP at 0.1 mg/kg or less. Complete abolition of all four effects of CPP was achieved by pretreatment with the centrally acting indoleamine antagonist methergoline. The peripherally acting serotonin antagonist xylamidine was ineffective or only weakly active, depending upon the test procedure, in preventing the serotonin-like actions of CPP. These findings indicate that CPP has a serotoninmimetic action in the central nervous system.

Anorexigenic and Ancillary Actions of MK-212 (6-Chloro-2- (1-Piperazinyl)-Pyrazine; CPP)

In rats allowed to eat for 2h/day and injected i.p. 30 min before feeding, MK-212, ED50=1.5 mg/kg, was two times more potent as an anorexigen than fenfluramine. However, the compounds were equiactive in the rat following p.o. administration 1.5 or 3 h before the test, while fenfluramine was more potent if the interval was extended to 6 h. In cats permitted to eat for 3 h/day, the ED50 dose (mg/kg p.o.) for MK-212 determined at 0.5, 1 and 3 h after feeding was, respectively, 15, 10 and 3 times less than that of fenfluramine. Emesis and diarrhea were frequently observed ancillary effects in cats treated with fenfluramine, whereas apparent sedation and salivation were commonly detected in animals after MK-212. In rats or cats pretreated with methergoline, the decrease in food consumption elicited by MK-212 was markedly inhibited, suggesting that the mechanism of action involves a serotoninlike effect. Compared with the marked stimulant action of amphetamine, MK-212 had only a minor and inconsistent effect on motor activity in rats and mice. Similar results were obtained with fenfluramine. MK-212 was not self-administered by rats, while the self-administration of amphetamine and morphine were demonstrated using the same experimental protocol.

A 5‐HYDROXYTRYPTAMINE‐LIKE MODE OF ANORECTIC ACTION FOR 6‐CHLORO‐2‐[1‐PIPERAZINYL]‐PYRAZINE (MK‐212)

1 The mechanism of the reduction in food consumption elicited by 6‐chloro‐2‐[1‐piperazinyl]‐pyrazine (MK‐212) administered systemically was investigated in the rat. (+)‐Fenfluramine and (+)‐amphetamine were included in some studies for comparative purposes. 2 Pretreatment with methergoline, a 5‐hydroxytryptamine (5‐HT) antagonist, reduced the magnitude of the anorectic effect of 1.5 and 3 mg/kg of MK‐212, while the anti‐5‐HT agents, cyproheptadine and cinanserin, were likewise effective against the 3 mg/kg dose. 3 Xylamidine, an antagonist of 5‐HT that penetrates poorly into the central nervous system, completely blocked the decrease in food intake caused by 5‐HT administered peripherally, while not antagonizing an equianorectic dose of MK‐212. 4 Reduction of brain 5‐HT by intraventricular injection of 5,6‐dihydroxytryptamine, intraperitoneal administration of p‐chloroamphetamine or placement of a lesion in the region of the median raphe nucleus diminished the anorectic response to 3 mg/kg of MK‐212. The anorectic effect of amphetamine was reduced by p‐chloroamphetamine or lesion in the raphe, but not by 5,6‐dihydroxytryptamine. The decrease in food consumption produced by 1.5 mg/kg of MK‐212 was antagonized by prior treatment with p‐chloroamphetamine, but not by 5,6‐dihydroxytryptamine. 5 Haloperidol, which blocks receptors for dopamine, antagonized the anorexigenic effect of amphetamine, but was ineffective in offsetting the action of MK‐212, 3 mg/kg. 6 Pretreatment with chlorimipramine to inhibit the 5‐hydroxytryptaminergic uptake mechanism did not affect the anorectic response to 3 mg/kg of MK‐212, whereas the response to fenfluramine was diminished. 7 The results indicate that the anorectic action of MK‐212 involves a 5‐HT‐like component which is more evident at the higher dose level of the compound. The anorexigenic property of MK‐212 may depend, at least partly, upon the integrity of 5‐HT‐containing neurones in the central nervous system.

Pharmacological differentiation of the central 5-hydroxytryptamine-like actions of MK-212 (6-chloro-2-[1-piperazinyl]-pyrazine), p-methoxyamphetamine and fenfluramine in an in vivo model system

Abstract Three compounds believed to posses 5-hydroxytryptamine-like (5-HT-like) activity in the central nervous system (CNS) were studied in the suprahyoideal muscle twitch-frequency model. MK-212 (0.1 mg/kg), p-methoxy-amphetamine (p-MA; 0.75 mg/kg) and fenfluramine (2.5 mg/kg) caused an increase in the frequency of twitches over the spontaneous rate of approximately 1/min. Xylamidine, an antagonist of 5-HT that penetrates poorly into the CNS, did not effect this action of MK-212, p-MA or fenfluramine, whereas the centrally acting 5-HT antagonist, methergoline, abolished the effect of all three compounds. Pretreatment with fluoxetine to inhibit the reuptake mechanism present at the level of the serotoninergic neuronal membrane caused a reduction in the abilities of p-MA and fenfluramine, but not MK-212, to increase twitch-frequency. Inhibition of 5-HT synthesis by treatment with p-chlorophenylalanine (p-CPA) resulted in a marked diminution in the action of p-MA and an initial decrease in the activity of fenfluramine followed, at later times after injecting fenfluramine, by a modest enhancement of activity. The ability of MK-212 to increase the frequency of muscle twitching was greatly potentiated by treatment with p-CPA. The findings are consistent with all three compounds having a 5-HT-like action in the CNS. However, p-MA and fenfluramine appear to use the uptake system to gain access into the serotoninergic neuron and then act indirectly via the release of 5-HT, whereas MK-212 acts directly on receptors for 5-HT in the CNS. Treatment with p-CPA apparently induced a state of supersensitivity to serotoninmimetic agonists, accounting for potentiation by p-CPA of the action of MK-212 and the complex interaction between p-CPA and fenfluramine.

Inhibition of the serotoninergic uptake system by MK-212 (6-chloro-2-[1-piperazinyl]-pyrazine)

Summary MK-212 inhibited the uptake of 3H-serotonin in cerebral cortical tissue with an EC50 of about 10 μM. At 10 μM, MK-212 did not affect the uptake of 3H-norepinephrine. Inhibition of serotonin and catecholamine uptake in vivo was assessed by determining the effect of pretreatment with MK-212 on the ability of p-chloroamphetamine and H 77/77 to decrease, respectively, the concentration of serotonin and catecholamines in rat brain. MK-212, 2.5–10 mg/kg i.p., produced a dose-related antagonism of the serotonin-lowering action of p-chloroamphetamine, while the H 77/77-induced reductions in norepinephrine and dopamine were not inhibited by prior administration of MK-212. Inhibition of the neuronal reuptake system for serotonin may contribute to the previously described serotoninmimetic action of MK-212 in the central nervous system.

Fenfluramine-induced enhancement of confinement motor activity: An indirect 5-hydroxytryptamine-like action?

Abstract Fenfluramine, administered to rats at anorectic dose levels, reduced open field activity and enhanced confinement motor activity, demonstrating the mixed depressant and stimulant characteristics of the compound. Studies designed to elucidate the mechanism of action for the confinement motor activity-enhancing effect of fenfluramine produced the following results: (1) confinement motor activity was also increased by giving MK-486 + 5-hydroxytryptophan, (2) methergoline and cinanserin, antagonists of 5-hydroxytryptamine, reduced the confinement motor activity-enhancing actions of fenfluramine and MK-486 + 5-hydroxytryptophan but not that of methylphenidate, (3) following p -chlorophenylalanine-induced reduction in brain monoamines (5-hydroxytryptamine: −73%, norepinephrine: −16%, dopamine: −9%), the action of fenfluramine was diminished, (4) 13 days after injecting 5,6-dihydroxytryptamine intraventricularly (5-hydroxytryptamine: −51%, norepinephrine: −9%, dopamine: −10%), the enhancement of confinenment motor activity elicited by fenfluramine or MK-486 + 5-hydroxytryptophan was augmented. It was concluded that that fenfluramine increases confinement motor activity via a 5-hydroxytryptamine-like action. That this action of fenfluramine was affected by both p -chlorophenylalanine and 5,6-dihydroxytryptamine indicates an interaction with central 5-hydroxytryptamine-containing neurones. Denervation supersentivity in 5,6-dihydroxytryptamine-treated animals may accout for the opposite nature of the effect produced by pretreatment with p -chlorophenylalanine and 5,6-dihydroxytryptamine.

Central catecholamine receptor blocking actions of BE-2254 ('heat'): comparison with chlorpromazine and haloperidol.

BE-2254, 2-[beta-(4-hydroxyphenyl)-ethylaminomethyl]-tetralone, (ED50 = 3.4 mg/kg i.p.) was about equal to chlorpromazine (ED50 = 4.4) as an antagonist of central noradrenergic receptor stimulation produced by clonidine (enhancement of the flexor reflex in spinalized rats). Haloperidol and phentolamine had essentially no effect at 9 mg/kg i.p...