Cyclic GMP and the second messenger hypothesis

Sperm chemotaxis in marine invertebrates--molecules and mechanisms

Sperm are attracted by chemical substances which are released by the egg. This process is called chemotaxis. Several molecules that are involved in chemotactic signaling of sperm from marine invertebrates are described and a model of the signaling pathway is presented. We discuss the motor response during chemotaxis and propose a model of the navigation strategy of sperm.

Ovarian hormone dependence of alpha(1)-adrenoceptor activation of the nitric oxide-cGMP pathway: relevance for hormonal facilitation of lordosis behavior

The ovarian hormones estradiol (E(2)) and progesterone (P) facilitate rat lordosis behavior in part by regulating the expression of and signal transduction by adrenoceptors in the hypothalamus (HYP) and preoptic area (POA). The major adrenoceptor subtype mediating E(2) and P facilitation of lordosis is the alpha(1)-adrenoceptor. In the present studies, we tested the hypotheses that (1) alpha(1)-adrenoceptors in the HYP enhance lordosis responses by activating the nitric oxide (NO)-cGMP signaling pathway, and (2) coupling of alpha(1)-adrenoceptors to this signal transduction pathway is hormone-dependent. Basal levels of cGMP were significantly higher in HYP and POA slices from animals treated with E(2) and P when compared with slices from ovariectomized controls or females treated with only E(2) or P. When slices of HYP and POA from ovariectomized female rats were incubated with norepinephrine or the selective alpha(1)-adrenoceptor agonist phenylephrine, cGMP accumulation was observed only if slices had been derived from females treated with both E(2) and P before experimentation. Moreover, alpha(1)-adrenoceptor stimulation of cGMP synthesis was blocked by an inhibitor of NO synthase, confirming that these receptors act by NO-mediated stimulation of soluble guanylyl cyclase. Behavioral studies demonstrated further that the cell-permeable cGMP analog 8-bromoadenosine-cGMP reverses the inhibitory effects of the alpha(1)-adrenoceptor antagonist prazosin on lordosis behavior in E(2)- and P-treated female rats. Thus, the NO-cGMP pathway mediates the facilitatory effects of alpha(1)-adrenoceptors on lordosis behavior in female rats, and previous exposure of the HYP and POA to both E(2) and P are required to link alpha(1)-adrenoceptors to this pathway.

External stimuli and intracellular signalling in the modification of the nematode surface during transition to the mammalian host environment

Previous work has shown that the surface of infective larvae of parasitic nematodes will not bind the fluorescent lipid analogue 5-N-(octadecanoyl)aminofluorescein (AF18) until after exposure of the parasite to mammalian tissue-culture conditions. In this study, culture media which are permissive or non-permissive for the acquisition of lipophilicity for AF18 were altered in order to examine possible stimuli involved. This showed that external alkaline pH and high sodium ion concentration were highly stimulatory. The internal signalling pathways which may be involved in the surface alteration were then examined using agents which are known to affect intracellular signalling in mammalian cells. The results indicated that elevation of cGMP levels was stimulatory whereas inhibition of a putative Na+/H+ antiporter or calcium mobilization was inhibitory, and it is argued that high intracellular levels of cAMP may be inhibitory. Whilst the precise effects of the agents used on nematode cells remain to be established, these results provide a framework for the examination of the processes involved in the modification of the nematode surface which takes place immediately after the infection event.

Angiotensin II decreases cGMP levels in neuronal cultures from rat brain

In previous studies we have determined that both cultured neuronal and astrocyte glial cells prepared from the hypothalamus and brain stem of 1-day-old rats contain specific receptors for angiotensin II (ANG II). Astrocyte glial receptors are coupled to inositol phospholipid hydrolysis, but there is little indication of the intracellular messengers or signal transduction mechanisms coupled to the neuronal ANG II receptors. In the present study, we have determined that ANG II decreases cellular guanosine 3',5'-cyclic monophosphate (cGMP) levels in neuronal but not in astrocyte glial cultures. This effect is both time and concentration dependent and is inhibited by the ANG II-receptor antagonist [Sar1,Ile8]ANG II, showing the involvement of specific ANG II receptors. ANG II has no effects on particulate or soluble guanylate cyclase activities or on efflux of cGMP from neuronal cultures. However, the effects of ANG II on cellular cGMP content are abolished by pretreatment with the calcium channel blockers cadmium and nifedipine, and by the nonselective phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine. These results suggest that calcium entry and possibly activation of a phosphodiesterase enzyme are involved in this ANG II-induced effect. This represents the first demonstration of a receptor-mediated effect of ANG II on an intracellular messenger in neuronal cultures. The functional role of cGMP as an intracellular messenger coupled to ANG II receptors in cultured neurons remains to be determined.