The light-suppressible K+ conductance and evaluation of internal messenger candidates in the molluscan extraocular photoreceptor

Molecular and functional characterization of an I(h)-channel from lobster olfactory receptor neurons

We isolated a cDNA named PAIH encoding a member of the I(h)-channel family expressed in olfactory receptor neurons (ORNs) of the spiny lobster Panulirus argus. Functional expression of recombinant PAIH in HEK293 cells generated a slowly activating, noninactivating inward current under whole-cell voltage-clamp to hyperpolarizing voltage steps, the amplitude and activation rate of which increase with increasing hyperpolarization. The channel is weakly selective for K+. Intracellular cAMP or cGMP shifts activation of the current to less negative potentials in a concentration-dependent manner. Finally, the channel is blocked by the I(h)-channel blocker ZD7288. An I(h)-channel sharing the properties of the recombinant channel occurs in cultured lobster ORNs. PAIH immunoreactivity localizes the protein to the transduction compartment of the ORNs in situ, and selectively applying the blocker to the transduction compartment reduces spontaneous activity in the ORN. Collectively, these results implicate for the first time a functional role for an I(h)-channel in olfactory signal transduction.

Characterization of recombinant and native Ih-channels from Apis mellifera

Recently, a novel class of genes coding for Ih-channels has been identified in several vertebrates and invertebrates. We isolated a cDNA (AMIH) encoding a putative member of these ion channels from Apis mellifera heads by means of polymerase chain reaction and homology screening. High similarity (88% identical amino acids) to the putative Drosophila melanogaster Ih-channel suggests that the Apis cDNA codes for a hyperpolarization-activated and cyclic nucleotide-gated channel. Functional expression of recombinant AMIH in HEK293 cells gave unitary currents that were preferentially selective for potassium over sodium ions and were activated by hyperpolarizing voltage steps. Cyclic nucleotides shifted the voltage activation curve to more positive membrane potentials. The current kinetics, activation by hyperpolarizing voltage steps and modulatory influence of cyclic nucleotides properties closely resemble those of mammalian Ih-channels. RT-PCR analysis showed pronounced mRNA expression in the antennae, head and body of Apis mellifera. Investigation of hyperpolarization-activated currents in olfactory receptor neurons (ORNs) in a primary cell culture of Apis mellifera antennal cells revealed activation properties similar to the heterologously expressed Ih-channel. By in-situ hybridization and immunohistochemistry, expression of AMIH was seen in olfactory receptor neurons of the bee antennae. We conclude that AMIH is the ion channel responsible for the hyperpolarization-activated currents in olfactory receptor neurons of bee.

Roles of cyclic GMP and inositol trisphosphate in phototransduction of the molluscan extraocular photoreceptor

The internal messengers mediating the photocurrent of the molluscan extraocular photoreceptor, A-P-1, were examined. In the dark, pressure-injection of cGMP into the A-P-1, voltage-clamped at resting levels, produced a rapid outward current, associated with an increase in conductance. However, the cGMP-induced current and increase in conductance were suppressed by subsequent photostimulation, suggesting hydrolysis of cGMP by light. The steady-state I/V relation for the cGMP-induced current was non-linear. The I/V relation for the instantaneous cGMP-induced current, measured 50 ms after the beginning of a voltage step, was linear, and reversed at the membrane potential, -67 mV, which corresponded to the K+ equilibrium potential of A-P-1 in 10 mM K+ normal saline. These findings indicate that the internal cGMP induces a voltage- and time-dependent K+ current. Since the photocurrent results from the suppression of a voltage- and time-dependent K+ current similar to above, the photocurrent is considered to be equivalent to the suppression of the cGMP-induced current. Short pressure-injection of GDP-beta-S into A-P-1 reduced the subsequent photocurrent. The photocurrent was also suppressed after an external application of Pertussis toxin. On the other hand, the photocurrent was amplified by prior pressure-injection of inositol 1,4,5-trisphosphate (IP3). However, a short pressure-injection of neomycin into A-P-1 depressed the subsequent photocurrent. These results suggested that the cGMP-induced (dark) current is mediated by cGMP, and that hydrolysis of cGMP by light leads to the photocurrent, then being modified by another messenger, IP3, to be amplified.(ABSTRACT TRUNCATED AT 250 WORDS)