Receptor-mediated modulation of avian caecal muscle contraction by melatonin: role of tyrosine protein kinase

Mechanisms Regulating Spontaneous Contractions in the Bovine Epididymal Duct1

Muscular autorhythmicity provides propulsion of spermatozoa through the epididymal duct, thereby ensuring sperm maturation. In the present study, the mechanisms underlying the bovine epididymal spontaneous phasic contractions (SCs) were analyzed by using muscle-tension recording and patch-clamp techniques. SCs were recorded from the caput, the corpus, and the proximal cauda region and found to be predominantly myogenic in origin. Removal of the luminal fluid induced a burstlike contraction pattern, and removal of the epithelium, a complete loss of SCs. Application of nifedipine, but not heparin and cyclopiazonic acid, suppressed SCs, indicating that influx of Ca2+ through L-type Ca2+ channels, but not Ca2+ release from intracellular stores, was crucial for maintaining SCs. The prostaglandin-endoperoxide synthase 2 (PTGS2) inhibitor NS-398 caused a region-dependent decrease in SCs and tone. These effects were mimicked by the mitogen-activated protein kinase (MAPK) kinase inhibitor PD-98059. Similarly, the prostaglandin F(2alpha) (PGF(2alpha))-receptor antagonist AL-8810 reduced SC generation, whereas PGF(2alpha) induced SC-like activity in epithelium-denuded segments. Cell-isolation experiments revealed the existence of three morphologically different types of contractile cells, which also showed distinct biophysical properties: typical smooth muscle cells in the cauda, myofibroblast-like cells all along the duct, and atypical muscle cells (ATMs) with filament-like spurs in all regions with SCs. These data suggest that the bovine epididymal autorhythmicity is based on an epithelial PTGS2-dependent release of (an) excitatory prostaglandin(s) and a MAPK-dependent activation of L-type Ca2+ channels in the contractile cells. ATM cells may provide electrical coupling between myofibroblasts, which is essential for the generation of regular myogenic activity.

Heterogeneous expression of melatonin receptor MT1 mRNA in the rat intestine under control and fasting conditions

Melatonin is found in mammalian central nervous system and various peripheral tissues including gastrointestinal tract (GIT) where it participates in the regulation of intestinal motility, blood flow, immunomodulation, ion transport, cell proliferation and scavenging of free radicals. Some of these effects are achieved via melatonin binding to specific receptors, MT1 and MT2. As no thorough study on the expression of these receptors in the GIT has yet been done, the aim of this study was to determine the MT1 mRNA expression in the rat intestine under both control and fasting conditions. Our results suggest that MT1 mRNA is present in epithelial as well as subepithelial layer, with higher expression in the latter in all intestinal segments studied. The highest signal of the MT1 transcript along the rostro-caudal intestinal axis was found both in epithelial and subepithelial layers of the duodenum. Nevertheless, duodenal MT1 mRNA expression did not reach the level found in pituitary gland. In a 12:12-hr light:dark cycle a MT1 receptor expression in the subepithelial layer of rat distal colon did not manifest a significant diurnal rhythm. Short-term fasting increased the expression of MT1 transcript in the subepithelial layer of both the small and large intestine. During long-term fasting the increase persisted only in distal colon while a return to control levels was observed in small intestinal segments. In conclusion we demonstrated heterogeneous expression of MT1 receptor in the rat intestine and showed that its expression is up-regulated by nutritional deprivation.

17Beta-estradiol modulates hMT1 melatonin receptor function

Estrogen modulates expression and function of G-protein-coupled receptors. The goal of this study was to assess the effect of 17beta-estradiol (10 nM) exposure for 1 (E1) or 6 (E6) days on density and function of hMT1 and hMT2 melatonin receptors expressed in Chinese hamster ovary (CHO) cells (CHO-MT1/CHO-MT2 cells). This strain of CHO cells expressed both estrogen receptor alpha and beta mRNAs, as determined by RT-PCR amplification. 17beta-Estradiol treatment did not modify the affinity of either receptor; however, it significantly increased the density of 2-[125I]iodomelatonin-binding sites in CHO-MT2 cells. 17beta-Estradiol treatment (1-6 days) did not affect the potency of melatonin to inhibit forskolin stimulation of cAMP formation through activation of either MT1 or MT2 receptors; however, it significantly attenuated the maximal inhibition of forskolin-stimulated cAMP formation induced by melatonin (0.01-1 microM) in CHO-MT1 cells. Melatonin stimulation of [35S]GTPgammaS binding to CHO-MT1 cell membranes was also attenuated following estradiol treatment. The inverse agonist luzindole reduced basal [35S]GTPgammaS binding in estradiol-treated cells but not in control CHO-MT1 cells, suggesting that estradiol promotes constitutive activity of MT1 melatonin receptors. We suggest that 17beta-estradiol differentially affects MT1 and MT2 melatonin receptor functions, attenuates melatonin responses through activation of MT1 receptors, and increases the MT2 receptors density.

Effects of intrathecal injections of melatonin analogs on capsaicin-induced secondary mechanical allodynia and hyperalgesia in rats

Melatonin, its agonists/antagonists were administered intrathecally (i.t.) before/after intradermal injection of capsaicin. Capsaicin produced an increase in the paw withdrawal frequency (PWF) in the presumed area of secondary mechanical allodynia and hyperalgesia. Melatonin agonists in the absence of a capsaicin injection decreased the PWF significantly, whereas melatonin antagonists given intrathecally alone were ineffective in the absence of a capsaicin injection. Pre-treatment with a melatonin agonist i.t. caused a reduction in the PWF after capsaicin. In contrast, the PWF increased after capsaicin with pre-administration of a melatonin antagonist i.t. Combined pre-treatment with melatonin and a melatonin antagonist i.t. prevented the change in PWF induced by melatonin alone after capsaicin. Intrathecal post-treatment with a melatonin agonist reduced the enhanced PWF that followed an injection of capsaicin, but treatment with a combination of a melatonin agonist and its antagonist did not alter the responses. The PWF was unaffected when melatonin analogs were applied i.t. at the T6 level or were injected intramuscularly adjacent to the L4 vertebra. In spinal rats, the data showed comparable effects of melatonin analogs on capsaicin-induced secondary mechanical hyperalgesia. Animal motor function tested by 'activity box' showed that motion activity was not affected by i.t. melatonin or its antagonist. These results suggest that activation of the endogenous melatonin system in the spinal cord can reduce the generation, development and maintenance of central sensitization, with a resultant inhibition of capsaicin-induced secondary mechanical allodynia and hyperalgesia.