Effects of calcium-activated potassium channel modulators on afterhyperpolarizing potentials in identified motor and mechanosensory neurons of the medicinal leech.

Calcium-activated potassium (KCa) channels contribute to multiple neuronal properties including spike frequency and afterhyperpolarizing potentials (AHPs). KCa channels are classified as KCa1.1, KCa2, or KCa3.1 based on single-channel conductance and pharmacology. Ca2+-dependent AHPs in vertebrates are categorized as fast, medium, or slow. Fast and medium AHPs are generated by KCa1.1 and KCa2 channels, respectively. The KCa subtype responsible for slow AHPs is unclear. Prolonged, Ca2+-dependent AHPs have been described in several leech neurons. Unfortunately, apamin and other KCa blockers often prove ineffective in the leech. An alternative approach is to utilize KCa modulators, which alter channel sensitivity to Ca2+. Vertebrate KCa2 channels are targeted selectively by the positive modulator CyPPA and the negative modulator NS8593. Here we show that AHPs in identified motor and mechanosensory leech neurons are enhanced by CyPPA and suppressed by NS8593. Our results indicate that KCa2 channels underlie prolonged AHPs in these neurons and suggest that KCa2 modulators may serve as effective tools to explore the role of KCa channels in leech physiology.

9-Phenanthrol modulates postinhibitory rebound and afterhyperpolarizing potentials in an excitatory motor neuron of the medicinal leech.

Postinhibitory rebound (PIR) responses in leech dorsal excitatory motor neurons (cell DE-3) are eliminated by Ca2+ channel blockers but also exhibit a strong dependence on extracellular Na+. These features could be explained by a voltage-gated Ca2+ current acting in concert with a Ca2+-activated nonspecific current (ICAN). In vertebrates, ICAN is associated with TRPM4 channels which are blocked selectively by 9-phenanthrol. Here, we show that 9-phenanthrol selectively inhibits a late phase of PIR and simultaneously enhances afterhyperpolarizing potentials (AHPs). Bath application of NNC 55-0396 or Cd2+ combined with ion substitution experiments indicate that a low-voltage-activated Ca2+ current plays a key role in generating PIR and that Ca2+ influx through low- or high-voltage-activated Ca2+ channels can trigger AHPs via activation of a Ca2+-dependent K+ current. We also demonstrate modulation of rebound responses by other ICAN blockers such as gadolinium and flufenamic acid, as well as the calmodulin antagonist W-7. We discuss how these results provide additional insights into the specific types of ionic currents underlying rebound responses of motor neuron DE-3 in the medicinal leech.

Riluzole suppresses postinhibitory rebound in an excitatory motor neuron of the medicinal leech.

Postinhibitory rebound (PIR) is an intrinsic property often exhibited by neurons involved in generating rhythmic motor behaviors. Cell DE-3, a dorsal excitatory motor neuron in the medicinal leech exhibits PIR responses that persist for several seconds following the offset of hyperpolarizing stimuli and are suppressed in reduced Na(+) solutions or by Ca(2+) channel blockers. The long duration and Na(+) dependence of PIR suggest a possible role for persistent Na(+) current (I NaP). In vertebrate neurons, the neuroprotective agent riluzole can produce a selective block of I NaP. This study demonstrates that riluzole inhibits cell DE-3 PIR in a concentration- and Ca(2+)-dependent manner. In 1.8 mM Ca(2+) solution, 50-100 µM riluzole selectively blocked the late phase of PIR, an effect similar to that of the neuromodulator serotonin. However, 200 µM riluzole blocked both the early and late phases of PIR. Increasing extracellular Ca(2+) to 10 mM strengthened PIR, but high riluzole concentrations continued to suppress both phases of PIR. These results indicate that riluzole may suppress PIR via a nonspecific inhibition of Ca(2+) conductances and suggest that a Ca(2+)-activated nonspecific current (I(CAN)), rather than I NaP, may underlie the Na(+)-dependent component of PIR.

Mechanisms of postinhibitory rebound and its modulation by serotonin in excitatory swim motor neurons of the medicinal leech.

Postinhibitory rebound (PIR) is defined as membrane depolarization occurring at the offset of a hyperpolarizing stimulus and is one of several intrinsic properties that may promote rhythmic electrical activity. PIR can be produced by several mechanisms including hyperpolarization-activated cation current (I(h)) or de-inactivation of depolarization-activated inward currents. Excitatory swim motor neurons in the leech exhibit PIR in response to injected current pulses or inhibitory synaptic input. Serotonin, a potent modulator of leech swimming behavior, increases the peak amplitude of PIR and decreases its duration, effects consistent with supporting rhythmic activity. In this study, we performed current clamp experiments on dorsal excitatory cell 3 (DE-3) and ventral excitatory cell 4 (VE-4). We found a significant difference in the shape of PIR responses expressed by these two cell types in normal saline, with DE-3 exhibiting a larger prolonged component. Exposing motor neurons to serotonin eliminated this difference. Cs+ had no effect on PIR, suggesting that I(h) plays no role. PIR was suppressed completely when low Na+ solution was combined with Ca2+-channel blockers. Our data support the hypothesis that PIR in swim motor neurons is produced by a combination of low-threshold Na+ and Ca2+ currents that begin to activate near -60 mV.

Single-cell analysis reveals cell-specific patterns of expression of a family of putative voltage-gated sodium channel genes in the leech.

To understand the molecular basis of nervous system function in the leech, Hirudo medicinalis, we have isolated four novel cDNAs encoding putative voltage-gated sodium (Na) channel alpha subunits, and have analyzed the expression of these genes in individual neurons of known function. To begin, degenerate oligonucleotide primers were used in combination with pre-existing cDNA libraries and reverse transcriptase-coupled polymerase chain reactions (RT-PCR). The putative leech Na channel cDNAs (LeNas) exhibit a higher degree of sequence homology to Na channel genes in other species than to voltage-gated calcium or potassium channel genes, including those expressed in leech. All LeNa cDNAs contain sequences corresponding to regions of functional importance in Na channel alpha subunits, including the "S4 region" involved in activation, the "pore loops" responsible for ion selectivity, and the "inactivation loop" between the third and fourth domains, though the latter lacks the highly conserved "IFM" motif critical for mammalian Na channel inactivation. Sequences corresponding to important determinants of tetrodotoxin sensitivity are found in some, but not all, LeNa cDNAs, consistent with prior electrophysiological evidence of Na channel heterogeneity in the leech with respect to this toxin. Subsequently, two different sets of isoform-specific primers and methods of RT-PCR, including a sensitive, fluorescence-based "real time" RT-PCR, were used to analyze LeNa isoform expression in functionally distinct neurons. The results from both approaches were consistent, and not only demonstrated that individual neurons often express more than one LeNa isoform, but also revealed cell-specific patterns of Na channel isoform expression in the leech nervous system.