Additional Evidence for the Cyclic GMP Signaling Pathway Resulting in the Photophobic Behavior of Stentor coeruleus¶

Abstract

We report that exo- and endogenous guanosine 3',5'-cyclic monophosphate (cGMP) specifically influenced the photophobic response. In behavioral experiments the slowly hydrolyzable and membrane-permeable analogs of cGMP (8-bromo-cGMP [Br-cGMP] and N6,2'-o-dibutyryl-cGMP) dramatically prolonged the time for ciliary stop response and decreased the duration of ciliary reversal in a dose-dependent manner. When analogs of adenosine 3',5'-cyclic monophosphate (cAMP) (8-bromo-cAMP or N6,2'-o-dibutyryl-cAMP) were used, no essential effects were detected on the kinetics of the photophobic response. Both nonspecific cyclic nucleotide phosphodiesterase (PDE) activity inhibitors (3-isobutyl-1-methylxanthine [IBMX] and 1,3-dimethylxanthine [theophylline]) and the highly specific cGMP-PDE activity inhibitor 1,4-dihydro-5-[2-propoxyphenyl]-7H-1,2,3-triazolo[4,5-d]pyrimidine-7-one (zaprinast) mimicked the effects of cGMP analogs. Treatment of cells with an inhibitor of guanylate cyclase activity (6-anilino-5,8-quinolinedione [LY 83583]) exerted an effect opposite to that of cGMP analogs and PDE activity inhibitors. The positive physiological effect of LY 83583 was significantly diminished in ciliates that were treated simultaneously with Br-cGMP. In an assay of cell cyclic nucleotide content, the exposure of dark-adapted Stentor to light evoked a transient decrease in the basal level of intracellular cGMP. Alterations in internal cGMP levels were more distinct when the intensity of applied illumination was increased. In the presence of IBMX or theophylline the basal content of cGMP was markedly enhanced, and the photoinduced changes in cGMP level were less pronounced. In this paper the possible whole molecular mechanism by which the ciliary orientation in Stentor is controlled by light is presented.

A Videomicroscopic Study of the Effect of l-cis-Diltiazem on the Photobehavior of Stentor coeruleus¶

“…2 and 3) in cells after application of l-cis-diltiazem. A similar modulation of photobehavior of S. coeruleus was observed on treatment of cells with LY 83583, a substance that strongly inhibits guanylate cyclase activity and effectively lowers the internal cGMP level (11). These findings indicate that the inactivation of CNG ion channels, and hence the modulation of photoresponses of S. coeruleus, can be achieved by a specific channel blocker, as found in the present experiments, or by a lowered level of internal cGMP, as observed earlier (11,26).…”

“…Light of increased intensity also induces a prolonged afterpotential membrane depolarization, which resembles, in turn, the persistence of ciliary reversal duration (T CRD ) (4). Such a reciprocal relationship between T CRL and T CRD values of the photophobic response also occurs when the signal turnover pathway is modified with some drugs (4,11). Thus, the decrease in T CRL and increase in T CRD parameters of the cell's photophobic response may result from an application of light of increased intensity or from an acceleration of the signal turnover pathway.…”

“…where T t and T c indicate the time values for T CRL or T CRD measured for individual cells after application of the tested substance and under control conditions, respectively. The changes in the latency of ciliary stop response (ÁT CRL ) and duration of cell backward swimming (ÁT CRD ) allow a precise evaluation of the efficiency of the cell phototransduction system as described elsewhere (11). In addition, cell photoresponsiveness (R L ), another behavioral parameter of cell population, was estimated.…”

“…In dark-adapted S. coeruleus, illumination elicits a depolarizing photoreceptor potential, which in turn triggers an action potential correlating closely with the ciliary reversal (4). Light also induces a transient decrease in the content of internal cyclic guanosine monophosphate (cGMP) in the cell (10,11) that may result in an alteration of cGMP-gated conductance (11). Such a possibility is quite likely because patch-clamp experiments have confirmed the presence of cGMP-gated channels in native membrane patches derived from blistering cells of S. coeruleus (12).…”

Cyclic GMP and Cilia Motility

“…High alcohol (5%) stimulated large amounts of cGMP formation within 30 s, which very rapidly returned to baseline levels by 2 min, as a membrane calmodulin-dependent GC stimulates cGMP levels [ 37 ]. A similar dark-adapted protozoan, Stentor coeruleus , demonstrates a light-aversion response whereby the reverse or backup motility in response to light is governed by a cGMP-stimulation of cilia [ 38 ].…”