The pineal hormone melatonin elicits two effects on the suprachiasmatic nuclei (SCN): acute neuronal inhibition and phase-shifting. Melatonin evokes its biological effects through G protein-coupled receptors. Since the Mel1a melatonin receptor may transduce the major neurobiological actions of melatonin in mammals, we examined whether it mediates both melatonin effects on SCN function by using mice with targeted disruption of the Mel1a receptor. The Mel1a receptor accounts for all detectable, high affinity melatonin binding in mouse brain. Functionally, this receptor is necessary for the acute inhibitory action of melatonin on the SCN. Melatonin-induced phase shifts, however, are only modestly altered in the receptor-deficient mice; pertussis toxin still blocks melatonin-induced phase shifts in Mel1a receptor-deficient mice. The other melatonin receptor subtype, the Mel1b receptor, is expressed in mouse SCN, implicating it in the phase-shifting response. The results provide a molecular basis for two distinct, mechanistically separable effects of melatonin on SCN physiology.

No abstract

During ischemic stroke, neurons at risk are exposed to pathologically high levels of intracellular calcium (Ca++), initiating a fatal biochemical cascade. To protect these neurons, we have developed openers of large-conductance, Ca++-activated (maxi-K or BK) potassium channels, thereby augmenting an endogenous mechanism for regulating Ca++ entry and membrane potential. The novel fluoro-oxindoles BMS-204352 and racemic compound 1 are potent, effective and uniquely Ca++-sensitive openers of maxi-K channels. In rat models of permanent large-vessel stroke, BMS-204352 provided significant levels of cortical neuroprotection when administered two hours after the onset of occlusion, but had no effects on blood pressure or cerebral blood flow. This novel approach may restrict Ca++ entry in neurons at risk while having minimal side effects.

Large-conductance calcium-activated potassium channels (maxi-K channels) have an essential role in the control of excitability and secretion. Only one gene Slo is known to encode maxi-K channels, which are sensitive to both membrane potential and intracellular calcium. We have isolated a potassium channel gene called Slack that is abundantly expressed in the nervous system. Slack channels rectify outwardly with a unitary conductance of about 25-65 pS and are inhibited by intracellular calcium. However, when Slack is co-expressed with Slo, channels with pharmacological properties and single-channel conductances that do not match either Slack or Slo are formed. The Slack/Slo channels have intermediate conductances of about 60-180 pS and are activated by cytoplasmic calcium. Our findings indicate that some intermediate-conductance channels in the nervous system may result from an interaction between Slack and Slo channel subunits.

The hormone, melatonin, is produced rhythmically in the vertebrate pineal gland (13). Rhythmic melatonin production plays a critical role in the regulation of reproduction in seasonally breeding mammals (1, 24). In nonmammalian vertebrates, melatonin also plays a major role in the regulation of circadian rhythms (4, 46). While endogenous melatonin appears to play only a subtle role in the regulation of circadian rhythms in mammals, exogenous melatonin influences circadian rhythms in several rodent species and in humans (2,3,9,21,22,39,46,47). While the magnitude of this phase shifting effect is small for adult mammals, it is nevertheless critical in some situations; e.g., daily melatonin administration appears to be useful for entrainment of non-24-h circadian cycles to the 24-hour day for blind individuals (22).Two high-affinity receptors for melatonin have been identified in mammals (for reviews, see references 33 and 43). These receptors, the Mel 1a and Mel 1b receptor subtypes, are members of the G protein-coupled receptor superfamily (29,30). A melatonin receptor-related receptor (also called H9 and GPR50) has sequence homology to the Mel 1a and Mel 1b receptors but does not bind melatonin (7,16,32). In addition to these three members of the gene family in mammals, nonmammalian species have a third high-affinity melatonin receptor subtype, the Mel 1c receptor (31). The mammalian Mel 1a and Mel 1b receptors are also called the MT1 and MT2 receptors, respectively [10]. The inability of this MT nomenclature system to accommodate the nonmammalian Mel 1c receptor has resulted in persistence of dual nomenclature systems; here, we use the original nomenclature.The relative importance of the mammalian melatonin receptor subtypes in mediating circadian responses to the hormone is unclear. The circadian clocks of neonatal hamsters are very efficiently set by even a single injection of melatonin, and several physiological and in vitro responses of the suprachiasmatic nucleus (SCN) to melatonin have been observed in hamsters (25,36,39,47). Remarkably, however, the Mel 1b receptor gene of several hamster species, including those with robust circadian and reproductive responses to the hormone, does not encode a functional melatonin receptor (47; see also GenBank accession number AY145849). Pharmacological studies with mice, however, suggest that the mouse Mel 1b receptor mediates circadian responses (9,19). Other data, derived from mice with targeted disruption of the Mel 1a melatonin receptor, indicate that this subtype mediates an acute suppressive effect of melatonin on SCN neuronal firing, and they reveal apparent redundancy of receptor subtypes in mediating the phase shifting response to melatonin (23).To determine whether the mouse Mel 1b receptor contributes

A human homolog of the large-conductance calcium-activated potassium (BK) channel beta subunit (hSlobeta) was cloned, and its effects on a human BK channel (hSlo) phenotype are reported. Coexpression of hSlo and hSlobeta, in both oocytes and human embryonic kidney 293 cells, resulted in increased Ca2+ sensitivity, marked slowing of BK channel activation and relaxation, and significant reduction in slow inactivation. In addition, coexpression changed the pharmacology of the BK channel phenotype: hSlo-mediated currents in oocytes were more sensitive to the peptide toxin iberiotoxin than were hSlo + hSlobeta currents, and the potency of blockade by the alkaloid BK blocker tetrandrine was much greater on hSlo + hSlobeta- mediated currents compared with hSlo currents alone. No significant differences in the response to charybdotoxin or the BK channel opener NS1619 were observed. Modulation of BK channel activity by phosphorylation was also affected by the presence of the hSlobeta subunit. Application of cAMP-dependent protein kinase increased P(OPEN) of hSlo channels, but decreased P(OPEN)of most hSlo + hSlobeta channels. Taken together, these altered characteristics may explain some of the wide diversity of BK channel phenotypes observed in native tissues.

An important goal of biomedical research is to translate basic research findings into useful medical advances. In the field of neuropharmacology this requires understanding disease mechanisms as well as the effects of drugs and other compounds on neuronal function. Our hope is that this information will result in new or improved treatment for CNS disease. Despite great progress in our understanding of the structure and functions of the CNS, the discovery of new drugs and their clinical development for many CNS disorders has been problematic. As a result, CNS drug discovery and development programs have been subjected to significant cutbacks and eliminations over the last decade. While there has been recent resurgence of interest in CNS targets, these past changes in priority of the pharmaceutical and biotech industries reflect several well-documented realities. CNS drugs in general have higher failure rates than non-CNS drugs, both preclinically and clinically, and in some areas, such as the major neurodegenerative diseases, the clinical failure rate for disease-modifying treatments has been 100%. The development times for CNS drugs are significantly longer for those drugs that are approved, and post-development regulatory review is longer. In this introduction we review some of the reasons for failure, delineating both scientific and technical realities, some unique to the CNS, that have contributed to this. We will focus on major neurodegenerative disorders, which affect millions, attract most of the headlines, and yet have witnessed the fewest successes. We will suggest some changes that, when coupled with the approaches discussed in the rest of this special volume, may improve outcomes in future CNS-targeted drug discovery and development efforts.

Amyotrophic lateral sclerosis (ALS) is characterized by upper and lower motor neuron dysfunction and loss, rapidly progressive muscle weakness, wasting and death. Many factors, including mitochondrial dysfunction, may contribute to ALS pathogenesis. Riluzole, which has shown only modest benefits in a measure of survival time without demonstrated effects on muscle strength or function, is the only approved treatment for ALS. We tested the putative mitochondrial modulator dexpramipexole (KNS-760704; (6R)-4,5,6,7-tetrahydro-N6-propyl-2,6-benzothiazole-diamine) in subjects with ALS in a two-part, double-blind safety and tolerability study, with a preliminary assessment of its effects on functional decline and mortality. In part 1, the effects of dexpramipexole (50, 150 or 300 mg d(-1)) versus placebo were assessed over 12 weeks. In part 2, after a 4-week, single-blind placebo washout, continuing subjects were re-randomized to dexpramipexole at 50 mg d(-1) or 300 mg d(-1) as double-blind active treatment for 24 weeks. Dexpramipexole was safe and well tolerated. Trends showing a dose-dependent attenuation of the slope of decline of the ALS Functional Rating Scale-Revised (ALSFRS-R) in part 1 and a statistically significant (P = 0.046) difference between groups in a joint rank test of change from baseline in ALSFRS-R and mortality in part 2 strongly support further testing of dexpramipexole in ALS.