Diurnal variations of s-adenosyl-l-methionine and adenosine content in the rat pineal gland

Adenosine stimulates neuromedin U mRNA expression in the rat pars tuberalis

Neuromedin U (NMU) shows circadian expression in the rat pars tuberalis (PT), and is known to be suppressed by melatonin. Here we examined the involvement of adenosine in the regulation of Nmu expression. We found that the rat PT expressed adenosine receptor A2b and that an adenosine receptor agonist, NECA, stimulated Nmu expression in brain slice cultures. In vitro promoter assays revealed that NECA stimulated Nmu promoter activity via a cAMP response element (CRE) in the presence of adenosine receptor A2b. NECA also increased the levels of phosphorylated CRE-binding protein. These findings suggest that adenosine stimulates Nmu expression by activating the cAMP signaling pathway through adenosine receptor A2b in the rat PT. This is the first report to demonstrate that Nmu expression in the PT is regulated by adenosine, which acts as an intravital central metabolic signal, in addition to melatonin, which acts as an external photoperiodic environmental signal.

Just the two of us: melatonin and adenosine in rodent pituitary function

The function of the pituitary gland is tightly controlled by neuronal and hormonal afferents of the brain. In this review, the role of the neurohormone melatonin and the neuromodulator adenosine for rodent pituitary function will be elucidated. Adenosine is known as an important paracrine modulator for pituitary endocrine and folliculostellate cells, with availability regulated by local metabolic cellular activity. In general, adenosine inhibits the cyclic adenosine monophosphate (AMP) pathway in pituitary cells by binding to A1-, and A3-adenosinergic receptors, and activates it via A2-adenosinergic receptors. The neurohormone melatonin integrates time-of-day and time-of-year into pituitary function via binding to MT1-melatonin receptors. Melatonin impacts at the hypothalamic level neurons that synthesize releasing and release-inhibiting hormones, and at the pituitary level only cells of the hypophyseal pars tuberalis (PT). Thereby, the daily changes in the duration of the nocturnal melatonin surge are decoded and subsequently relayed to the pars distalis to adapt gonadotropin and prolactin release, respectively, to season. An exciting integration of time within the regulation of pituitary function was deciphered by analysing transmembrane signalling events in cells of the hypophyseal PT: a consecutive daily impact of initially the neurohormone melatonin and later the neuromodulator adenosine on rodent PT cells leads to a circadian rhythm in the transcription of cyclic-AMP-sensitive genes.

Adenosine stimulates cone photoreceptor myoid elongation via an adenosine A2-like receptor

In several parts of the nervous system, adenosine has been shown to function as an extracellular neuromodulator binding to surface receptors on target cells. This study examines the possible role of adenosine in mediating light and circadian regulation of retinomotor movements in teleost cone photoreceptors. Teleost cones elongate in the dark and contract in the light. In continuous darkness, the cones continue to elongate and contract at subjective dusk and dawn in response to circadian signals. We report here that exogenous adenosine triggers elongation (the dark/night movement) in isolated cone inner segment-cone outer segment preparations (CIS-COS) in vitro. Agonist/antagonist potency profiles indicate that adenosine's effect on cone movement is mediated by an A2-like adenosine receptor, which like other A2 receptors enhances adenylate cyclase activity. Although closest to that expected for A2 receptors, the antagonist potency profile for CIS-COS does not correspond exactly to any known A2 receptor subtype, suggesting that the cone receptor may be a novel A2 subtype. Our findings are consistent with previous reports that retinal adenosine levels are higher in the dark, and further suggest that adenosine could act as a neuromodulatory "dark signal" influencing photoreceptor metabolism and function in the fish retina.

Adenosine effects on the rat pineal gland in vitro: cyclic adenosine monophosphate levels, N-acetyltransferase, and thyroxine type II 5'-deiodinase activities and melatonin production

The presence of adenosine and its receptors in the pineal gland suggests that they could have a regulatory influence on pineal physiology. Rat pineal glands cultured in the presence of the adenosine analog 5'-N-ethylcarboxamido adenosine (NECA) promoted a significant increase in pineal adenosine 3'5'-monophosphate (cAMP) content, similar to that caused by the beta-adrenergic receptor agonist isoproterenol (ISO). A potentiation of the cAMP response occurred when the glands were simultaneously stimulated with both NECA and ISO. Whereas high cAMP levels induced by ISO were associated with an increased N-acetyltransferase (NAT) activity and rise in the melatonin production and release, the NECA-induced rise in cAMP concentration failed to promote an increase in the activity of either NAT or another pineal enzyme thyroxine 5'-deiodinase type II; likewise, pineal melatonin levels did not rise.

Pineal indoles: significance and measurement

Despite intensive investigation, particularly over the past fifteen years, many aspects of pineal function with respect to mammalian physiology remain obscure. Much of this work is reviewed and particular attention focussed on indole metabolism within the pineal gland. Emphasis is placed on the development of new analytical techniques with special reference to high performance liquid chromatography coupled with electrochemical detection. The growth in knowledge regarding pineal indole synthesis which can be attributed to the use of this technique is discussed. The possibility that pineal indoles other than melatonin may function as hormones or neuromodulators is considered. A functional role for 5-hydroxytryptophol as a neuromodulator, possibly associated with diffuse neuroendocrine function (amine precursor, uptake and decarboxylation, APUD) is suggested.