Ultra-violet light-induced changes in membrane properties in secretory cells

cGMP-mediated facilitation in nerve terminals by enhancement of the spike afterhyperpolarization

cGMP has long been suspected to play a role in synaptic plasticity, but the inaccessibility of nerve terminals to electrical recording has impeded tests of this hypothesis. In posterior pituitary nerve terminals, nitric oxide enhanced Ca(2+)-activated K+ channel activity by activating guanylate cyclase and PKG. This enhancement occurred only at depolarized potentials, so the spike threshold remained unaltered but the afterhyperpolarization became larger. During spike trains, the enhanced afterhyperpolarization promoted Na+ channel recovery from inactivation, thus reducing action potential failures and allowing more Ca(2+) to enter. Activating guanylate cyclase, either with applied nitric oxide, or with physiological stimulation to activate nitric oxide synthase, increased action potential firing. Thus, the cGMP/nitric oxide cascade generates a short-term, use-dependent enhancement of release.

Modification of cyclic nucleotide-gated ion channels by ultraviolet light

We irradiated cyclic nucleotide-gated ion channels in situ with ultraviolet light to probe the role of aromatic residues in ion channel function. UV light reduced the current through excised membrane patches from Xenopus oocytes expressing the alpha subunit of bovine retinal cyclic nucleotide-gated channels irreversibly, a result consistent with permanent covalent modification of channel amino acids by UV light. The magnitude of the current reduction depended only on the total photon dose delivered to the patches, and not on the intensity of the exciting light, indicating that the functionally important photochemical modification(s) occurred from an excited state reached by a one-photon absorption process. The wavelength dependence of the channels' UV light sensitivity (the action spectrum) was quantitatively consistent with the absorption spectrum of tryptophan, with a small component at long wavelengths, possibly due to cystine absorption. This spectral analysis suggests that UV light reduced the currents at most wavelengths studied by modifying one or more "target" tryptophans in the channels. Comparison of the channels' action spectrum to the absorption spectrum of tryptophan in various solvents suggests that the UV light targets are in a water-like chemical environment. Experiments on mutant channels indicated that the UV light sensitivity of wild-type channels was not conferred exclusively by any one of the 10 tryptophan residues in a subunit. The similarity in the dose dependences of channel current reduction and tryptophan photolysis in solution suggests that photochemical modification of a small number of tryptophan targets in the channels is sufficient to decrease the currents.

Direct actions of nitric oxide on rat neurohypophysial K+ channels

1. Nitric oxide (NO) has been shown to modulate neuropeptide secretion from the posterior pituitary. Here we show that NO activates large-conductance Ca2+-activated K+ (BK) channels in posterior pituitary nerve terminals. 2. NO, generated either by the photolysis of caged-NO or with chemical donors, irreversibly enhanced the component of whole-terminal K+ current due to BK channels and increased the activity of BK channels in excised patches. NO also inhibited the transient A-current. The time courses of these effects on K+ current were very different; activation of BK channels developed slowly over several minutes whereas inhibition of A-current immediately followed NO uncaging. 3. Activation of BK channels by NO occurred in the presence of guanylyl cyclase inhibitors and after removal of ATP or GTP from the pipette solution, suggesting a cGMP-independent signalling pathway. 4. The sulfhydryl alkylating agent N-ethyl maleimide (NEM) increased BK channel activity. Pretreatment with NEM occluded NO activation. 5. NO activation of BK channels occurred independently of voltage and cytoplasmic Ca2+ concentration. In addition, NO removed the strict Ca2+ requirement for channel activation, rendering channels highly active even at nanomolar Ca2+ levels. 6. These results suggest that NO, or a reactive nitrogen byproduct, chemically modifies nerve terminal BK channels or a closely associated protein and thereby produces an increase in channel activity. Such activation is likely to inhibit impulse activity in posterior pituitary nerve terminals and this may explain the inhibitory action of NO on secretion.