News and Publications

Molecular Neurobiology

J.G. Sutcliffe, L. de Lecea, D.D. Gerendasy, M.J. Carson, E.A. Thomas, P.B. Hedlund, C.E. Alvarez, P.E. Danielson, P.E. Foye, M. Calbet-Murtro, A. Dao

OLEAMIDE SITE OF ACTION

Oleamide, an amidated lipid isolated from the cerebrospinal fluid of sleep-derived cats, has diverse sleep-inducing and hypothermic effects when administered to rodents; however, the mechanisms of these actions remain unknown. We examined the effects of oleamide on responses mediated by receptors for serotonin (5-HT) in cultured mammalian cells. In cells endogenously expressing the 5HT_2A receptor, oleamide significantly potentiated 5-HT--induced hydrolysis of phosphoinositide. In cells expressing the 5HT₇ receptor, oleamide caused a concentration-dependent increase in the accumulation of cAMP but with lower efficacy than that observed with 5-HT. In the presence of 5-HT, oleamide had the opposite effect on cAMP, causing insurmountable antagonism of the concentration-effect curve to 5-HT, but had no effect on cAMP levels elicited by isoproterenol or forskolin. These results indicate that oleamide can modulate 5-HT--mediated signal transduction at different subtypes of mammalian 5-HT receptors in vitro and that oleamide acts at an apparent allosteric site on the 5-HT₇ receptor and elicits functional responses via activation of this site. This allostery is a unique mechanism of activation for 5-HT G protein--coupled receptors.

To identify neuronal populations activated in vivo by oleamide, we measured induction of c-fos in response to oleamide in mouse brain. Oleamide elicited dramatic increases in c-fos mRNA and protein in distinct regions of the brain, including cingulate and somatosensory cortical areas and numerous nuclei of the thalamus and hypothalamus. In the last 2 areas, most of the neurons that had induction of c-fos also expressed the 5-HT₇ receptor, a target for oleamide in the in vitro studies. These data suggest that oleamide acts at 5-HT₇ receptors to elicit some of its physiological effects.

SLEEP-INDUCING PEPTIDE

Cortistatin is a presumptive neuropeptide; 11 of its 14 amino acids are identical to amino acids in somatostatin. Cortistatin mRNA accumulates postnatally in sparse interneurons of the cerebral cortex and hippocampus that contain -aminobutyric acid. The neuropeptide binds to all 5 cloned somatostatin receptors when the receptors are expressed in transfected cells and depresses neuronal activity, but, unlike somatostatin, it reduces locomotor activity and induces slow-wave sleep. Cortistatin mRNA accumulates during sleep deprivation, and the neuropeptide acts by antagonizing the effects of acetylcholine on cortical excitability, thereby causing synchronization of brain slow waves.

PEPTIDES FOR FEEDING

Another mRNA of postnatal onset that is restricted to large neuronal cell bodies of the dorsal-lateral hypothalamus encodes preprohypocretin, the precursor of 2 peptides that share homology with each other and with members of the secretin family of peptides. The peptides are detected immunohistochemically in secretory vesicles at synapses of fibers that project to the posterior part of the hypothalamus and to diverse targets in other regions of the brain. The peptides are excitatory when applied to cultured hypothalamic neurons. Recent studies indicated that the hypocretin peptides are ligands for 2 orphan receptors. When bound to the receptors, the peptides stimulate feeding behavior. These peptides, also called the orexins, accumulate during fasting.

PEPTIDES AS PRESSURES FOR VOLUNTARY BUT NECESSARY BEHAVIORS

The studies on cortistatin and the hypocretins suggest a common mechanism of regulation for necessary, but voluntary, behaviors (sleep and feeding): the presumably transcription-based accumulation of peptide transmitters creates pressures for the voluntary activities. Both cortistatin and the hypocretins accumulate as the physiologic requirement for a particular behavior increases: for cortistatin, sleep; for the hypocretins, feeding. Both of these behaviors are necessary, but they are voluntary, because an animal has considerable flexibility as to when these needs must be satisfied.

Despite the restricted locations of the cell bodies that express these peptides, each peptide appears to be involved in more than a single system, and neither peptide is the only signal for the behavior to which it has been most convincingly linked. Cortistatin is involved in sleep, but it also appears to function in short-term memory. Other substances have been implicated in sleep regulation: oleamide, for example. Similarly, the hypocretins are involved in feeding but, given their projections, probably several other processes also, including blood pressure and arousal. In addition, several other neuropeptides have been implicated as promoters of food consumption: neuropeptide Y, galanin, and melanin-concentrating hormone.

Thus, to maintain flexibility in acceding to the multiplicity of demands imposed by internal physiology and the external natural and social environments, animals have evolved complex, overlapping neurohormonal signaling systems. The overlapping systems allow both attention to individual demands and integration of several demands, some of which may have conflicting solutions. Many additional signaling molecules most likely exist.

PUBLICATIONS

Carson, M.J., Reilly, C.R., Sutcliffe, J.G., Lo, D. Mature microglia resemble immature antigen-presenting cells. Glia 22:72, 1998.

de Lecea, L., Criado, J.R., Rivera, S., Wen, W., Henriksen, S.J., Taylor, S.S., Gall, C.M., Sutcliffe, J.G. Endogenous protein kinase A inhibitor (PKI/alphapub/) modulates different forms of synaptic plasticity. J. Neurosci. Res., in press.

de Lecea, L., Kilduff, T.S., Peyron, C., Gao, X.-B., Foye, P.E., Danielson, P.E., Fukuhara, C., Battenberg, E.L.F., Gautvik, V.T., Bartlett, F.S., Frankel, W.N., van den Pol, A.N., Bloom, F.E., Gautvik, K.M., Sutcliffe, J.G. The hypocretins: Hypothalamus-specific peptides with neuroexcitatory activity. Proc. Natl. Acad. Sci. U.S.A. 95:322, 1998.

Gerendasy, D.D., Sutcliffe, J.G. RC3/neurogranin, a postsynaptic calpacitin for setting the response threshold to calcium influxes. Mol. Neurobiol. 15:131, 1997.

Kilduff, T.S., de Lecea, L., Usui, H., Sutcliffe, J.G. Isolation and identification of specific transcripts by subtractive hybridization. In: Molecular Regulation of Arousal States. Lydic, R. (Ed.). CRC Press, Boca Raton, FL, 1997, p. 103.

Thomas, E.A., Carson, M.J., Neal, M.J., Sutcliffe, J.G. Unique allosteric regulation of 5-hydroxytryptamine receptor-mediated signal. Proc. Natl. Acad. Sci. U.S.A. 94:14115, 1997.

Thomas, E.A., Cravatt, B.F., Danielson, P.E., Gilula, N.B., Sutcliffe, J.G. Fatty acid amide hydrolase, the degradative enzyme for anandamide and oleamide, has selective distribution in neurons within the rat central nervous system. J. Neurosci. Res. 50:1047, 1997.

Thomas, E.A., Danielson, P.E., Sutcliffe, J.G. RGS9: A regulator of G-protein signalling with specific expression in rat and mouse striatum. J. Neurosci. Res. 52:118, 1998.