5-HT1F receptor agonists inhibit neurogenic dural inflammation in guinea pigs.

The serotonin (5-HT) receptor subtype mediating inhibition of neurogenic dural inflammation in guinea pigs was investigated using a series of serotonin agonists with differing affinities for the 5-HT1B, 5-HT1D and 5-HT1F receptors. When agonist potencies for inhibiting neurogenic inflammation were compared with affinities for these receptor subtypes, a significant positive correlation was seen only with the 5-HT1F receptor. The potency of agonists in inhibiting adenylate cyclase in cells transfected with human 5-HT1F receptor was also highly correlated with their potency in the animal model of migraine. In situ hybridization demonstrated 5-HT1F receptor mRNA in guinea pig trigeminal ganglion neurons. These data suggest that the 5-HT1F receptor is a rational target for migraine therapeutics.

Characterization of LY344864 as a pharmacological tool to study 5-HT1F receptors: binding affinities, brain penetration and activity in the neurogenic dural inflammation model of migraine.

LY344864 is a selective receptor agonist with an affinity of 6 nM (Ki) at the recently cloned 5-HT1F receptor. It possesses little affinity for the 56 other serotonergic and non-serotonergic neuronal binding sites examined. When examined for its ability to inhibit forskolin-induced cyclic AMP accumulation in cells stably transfected with human 5-HT1F receptors, LY344864 was shown to be a full agonist producing an effect similar in magnitude to serotonin itself. After an intravenous dose of 1 mg/kg, rat plasma LY344864 levels declined with time whereas brain cortex levels remained relatively constant for the first 6 hours after injection. Oral and intravenous LY344864 administration potently inhibited dural protein extravasation caused by electrical stimulation of the trigeminal ganglion in rats. Taken together, these data demonstrate that LY344864 is a selective 5-HT1F receptor agonist that can be used to explore both the in vitro and in vivo functions of this receptor.

Pharmacological characterization of LY293284: A 5-HT1A receptor agonist with high potency and selectivity.

(-)-LY293284, (-)-4R-6-acetyl-4-(di-n-propylamino)1,3,4,5- tetrahydrobenz[c,d]indole, is a conformationally restricted tryptamine derivative with an acetyl group serving as a protophilic substitution for the hydroxyl in serotonin (5-HT). In ligand displacement studies, LY293284 had a Ki of 0.07 nM for the 5-HT1A receptor but no affinity for other monoaminergic receptors within 3 orders of magnitude. LY293284 was evaluated in in vivo models, which have been used as markers for presynaptic and postsynaptic 5-HT1A receptor activity. LY293284 decreased hypothalamic 5-hydroxyindoleacetic acid levels (ED50, 2.9 micrograms/kg s.c.) and dorsal raphe serotonergic neuron firing rate (ED50, 0.08 micrograms/kg s.c.), which are accepted indices of presynaptic activity. LY293284 also induced a reduction in body temperature in rats (ED50, 3.6 micrograms/kg s.c.), which was blocked by pretreatment with (+/-)-pindolol. Hypothermic responses of rats to 5-HT1A agonists have had both pre- and postsynaptic characteristics in previous studies. The ED50 values for 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT) in these tests were 15 to 45 times higher than those observed for LY293284. In models for postsynaptic activity, the ED50 for LY293284 for elevating serum corticosterone levels was 9.7 micrograms/kg s.c. and the minimum effective doses to induce lower lip retraction and flat posture were 3 micrograms/kg s.c. For comparison, the same indices obtained for 8-OH-DPAT were 222.4 and 100 micrograms/kg, respectively. The 5-HT syndrome responses induced by LY293284 were also attenuated by pretreatment with (+/-)-pindolol. LY293284 was 10 times more potent than 8-OH-DPAT in a drug discrimination test that used pigeons trained to identify 8-OH-DPAT. In sexual behavior tests with male rats, LY293284 induced a maximal reduction in ejaculatory latency at 0.01 micrograms/kg s.c., which was approximately 10 times higher potency than 8-OH-DPAT. In the pigeon conflict model for anxiolytic activity, LY293284 was 100 times more potent than 8-OH-DPAT in increasing punished responding. In the rat forced swim model for antidepressant-like activity, LY293284 was 30 and 35 times more potent than 8-OH-DPAT in decreasing immobility time and defecation rate. These studies have demonstrated that LY293284 is a highly selective and extremely potent 5-HT1A receptor agonist and represents a useful pharmacological tool for studying 5-HT1A receptor-mediated effects.

Preclinical studies on LY228729: a potent and selective serotonin1A agonist.

LY228729 is a conformationally restricted tryptamine derivative with a carboxamide serving as a protophilic group to mimic the hydroxyl in serotonin (5-HT). LY228729 has high affinity for the 5-HT1A receptor, weak affinity for the 5-HT1D receptor and no significant affinity for other monoaminergic receptors studied. LY228729 was less effective than 5-carboxamidotrytamine in suppressing K(+)-evoked release of 3H-5-HT from parietal-occipital cortical slices from guinea pigs, which is in agreement with its weak 5-HT1D receptor affinity. LY228729 reduced hypothalamic 5-hydroxyindole-3-acetic acid levels and increased serum corticosterone levels in rats. LY228729 reduced hypothalamic 5-hydroxytryptophan accumulation after decarboxylase inhibition. LY228729 increased flat posture and lower lip retraction scores in rats at doses between 0.1 and 1 mg/kg s.c. (p.o. doses were 10 times higher) and these effects were blocked by (+/-) pindolol. LY228729 induced a hypothermic response in rats, which was blocked by (+/-) pindolol. These in vivo responses are characteristics of compounds with 5-HT1A agonist activity. In the preclinical efficacy models, LY228729 suppressed motion sickness responses in cats; decreased ejaculatory latency and the increased copulatory efficiency and rate in rats and increased punished responding at lower doses than it lowered unpunished responding in rats. Collectively, these results indicate that LY228729 is potent 5-HT1A agonist with bioavailability properties sufficient for clinical evaluation and with efficacy in preclinical models of anxiety, sexual disorders and motion sickness. Since the 5-HT1A agonists that have been studied previously have antidepressant activity, this indication will also be evaluated.

6-substituted 1,3,4,5-tetrahydrobenz[cd]indol-4-amines: potent serotonin agonists.

A series of 6-substituted tricyclic ergoline partial structures has been synthesized and found to possess very strong serotonin agonist activity. A methoxy group at the 6-position greatly enhances activity, but at the expense of compound stability. Substituting the 6-position with protophyllic groups that are also electron-withdrawing in character enhances both activity and stability.

Development of drugs to modify the actions of melatonin.

One of the best documented effects of melatonin in mammals is its antigonadotrophic effect. Melatonin has been shown to inhibit LH release and ovulation in rats and we chose this model to investigate the effects of new melatonin analogs. The analogs studied include 6-chloromelatonin, 6-fluoromelatonin, and 2-methyl-6,7-dichloromelatonin. All derivatives were more potent than melatonin and appeared to act on the brain and not on the pituitary to inhibit LH release. An exceptionally long duration of activity was found for 2-methyl-6,7-dichloromelatonin. The greatly increased stability of this derivative may provide a valuable tool in search for the role(s) of melatonin in mammals.

Inhibition of luteinizing hormone release and ovulation by 6-chloro- and 6-fluoromelatonin.

The effects of 6-chloromelatonin and 6-fluoromelatonin on ovulation and LH and prolactin release in rats were determined. Both halogenated melatonin analogs were more potent ovulation blockers than melatonin. The halogenated melatonin analogs inhibited the ovulatory surge of luteinizing hormone (LH) but did not alter the proestrous prolactin surge, nor did they alter basal serum levels of LH or prolactin. The plasma half-life of 6-chloromelatonin in rats and rhesus monkeys was 27 min compared to 11 min for melatonin. The results indicate that 6-chloromelatonin is a melatonin agonist with greater metabolic stability than melatonin.

Synthesis and evaluation of the antiovulatory activity of a variety of melatonin analogues.

A series of melatonin analogues was synthesized and examined for ovulation-blocking activity. Deviation from the 5-methoxy group or substitution of the 1 position prevented activity. Activity was not particularly sensitive to minor variations in the N-acyl group nor was it significantly altered by methylation of position 2 or the alpha-methylene; however, a pronounced enhancement resulted from halogenation of the 6 position.