Characterization of voltage-gated K+ currents contributing to subthreshold membrane potential oscillations in hippocampal CA1 interneurons.

CA1 inhibitory interneurons at the stratum lacunosum-moleculare and radiatum junction (LM/RAD-INs) display subthreshold membrane potential oscillations (MPOs) involving voltage-dependent Na(+) and A-type K(+) currents. LM/RAD-INs also express other voltage-gated K(+) currents, although their properties and role in MPOs remain unclear. Here, we characterized these voltage-gated K(+) currents and investigated their role in MPOs. Using outside-out patch recordings from LM/RAD-IN somata, we distinguished four voltage-gated K(+) currents based on their pharmacology and activation/inactivation properties: a fast delayed rectifier current (I(Kfast)), a slow delayed rectifier current (I(Kslow)), a rapidly inactivating A-type current (I(A)), and a slowly inactivating current (I(D)). Their relative contribution to the total K(+) current was I(A) > I(Kfast) > I(Kslow) = I(D). The presence of I(D) and the relative contributions of K(+) currents in LM/RAD-INs are different from those of other CA1 interneurons, suggesting the presence of differential complement of K(+) currents in subgroups of interneurons. We next determined whether these K(+) currents were sufficient for MPO generation using a single-compartment model of LM/RAD-INs. The model captured the subthreshold voltage dependence of MPOs. Moreover, all K(+) currents were active at subthreshold potentials but I(D), I(A), and the persistent sodium current (I(NaP)) were most active near threshold. Using impedance analysis, we found that I(A) and I(NaP) contribute to MPO generation by modulating peak spectral frequency during MPOs and governing the voltage range over which MPOs occur. Our findings uncover a differential expression of a complement of K(+) channels that underlies intrinsic rhythmic activity in inhibitory interneurons.

Noradrenergic modulation of intrinsic and synaptic properties of lumbar motoneurons in the neonatal rat spinal cord.

Although it is known that noradrenaline (NA) powerfully controls spinal motor networks, few data are available regarding the noradrenergic (NAergic) modulation of intrinsic and synaptic properties of neurons in motor networks. Our work explores the cellular basis of NAergic modulation in the rat motor spinal cord. We first show that lumbar motoneurons express the three classes of adrenergic receptors at birth. Using patch-clamp recordings in the newborn rat spinal cord preparation, we characterized the effects of NA and of specific agonists of the three classes of adrenoreceptors on motoneuron membrane properties. NA increases the motoneuron excitability partly via the inhibition of a K(IR) like current. Methoxamine (alpha(1)), clonidine (alpha(2)) and isoproterenol (beta) differentially modulate the motoneuron membrane potential but also increase motoneuron excitability, these effects being respectively inhibited by the antagonists prazosin (alpha(1)), yohimbine (alpha(2)) and propranolol (beta). We show that the glutamatergic synaptic drive arising from the T13-L2 network is enhanced in motoneurons by NA, methoxamine and isoproterenol. On the other hand, NA, isoproterenol and clonidine inhibit both the frequency and amplitude of miniature glutamatergic EPSCs while methoxamine increases their frequency. The T13-L2 synaptic drive is thereby differentially modulated from the other glutamatergic synapses converging onto motoneurons and enhanced by presynaptic alpha(1) and beta receptor activation. Our data thus show that the NAergic system exerts a powerful and complex neuromodulation of lumbar motor networks in the neonatal rat spinal cord.

Kv4.3-mediated A-type K+ currents underlie rhythmic activity in hippocampal interneurons.

Hippocampal-dependent learning and memory processes are associated with theta frequency rhythmic activity. Interneuron and pyramidal cell network interactions underlie this activity, but contributions of interneuron voltage-gated membrane conductances remain unclear. We show that interneurons at the CA1 lacunosum-moleculare (LM) and radiatum (RAD) junction (LM/RAD) display voltage-dependent subthreshold membrane potential oscillations (MPOs) generated by voltage-gated tetrodotoxin-sensitive Na+ and 4-aminopyridine (4-AP)-sensitive K+ currents. They also exhibit prominent 4-AP-sensitive A-type K+ currents, with gating properties showing activation at subthreshold membrane potentials. We found that LM/RAD cells are part of specific interneuron subpopulations expressing the K+ channel subunit Kv4.3 and their transfection with Kv4.3 small interfering RNA selectively impaired A-type K+ currents and MPOs. Thus, our findings reveal a novel function of Kv4.3-mediated A-type K+ currents in the generation of intrinsic MPOs in specific subpopulations of interneurons that may participate in hippocampal theta-related rhythmic activity.

mGluR1/5 subtype-specific calcium signalling and induction of long-term potentiation in rat hippocampal oriens/alveus interneurones.

Hippocampal inhibitory interneurones demonstrate pathway- and synapse-specific rules of transmission and plasticity, which are key determinants of their role in controlling pyramidal cell excitability. Mechanisms underlying long-term changes at interneurone excitatory synapses, despite their importance, remain largely unknown. We use two-photon calcium imaging and whole-cell recordings to determine the Ca2+ signalling mechanisms linked specifically to group I metabotropic glutamate receptors (mGluR1alpha and mGluR5) and their role in hebbian long-term potentiation (LTP) in oriens/alveus (O/A) interneurones. We demonstrate that mGluR1alpha activation elicits dendritic Ca2+ signals resulting from Ca2+ influx via transient receptor potential (TRP) channels and Ca2+ release from intracellular stores. By contrast, mGluR5 activation produces dendritic Ca2+ transients mediated exclusively by intracellular Ca2+ release. Using Western blot analysis and immunocytochemistry, we show mGluR1alpha-specific extracellular signal-regulated kinase (ERK1/2) activation via Src in CA1 hippocampus and, in particular, in O/A interneurones. Moreover, we find that mGluR1alpha/TRP Ca2+ signals in interneurone dendrites are dependent on activation of the Src/ERK cascade. Finally, this mGluR1alpha-specific Ca2+ signalling controls LTP at interneurone synapses since blocking either TRP channels or Src/ERK and intracellular Ca2+ release prevents LTP induction. Thus, our findings uncover a novel molecular mechanism of interneurone-specific Ca2+ signalling, critical in regulating synaptic excitability in hippocampal networks.

Synapse-specific mGluR1-dependent long-term potentiation in interneurones regulates mouse hippocampal inhibition.

Hippocampal CA1 inhibitory interneurones control the excitability and synchronization of pyramidal cells, and participate in hippocampal synaptic plasticity. Pairing theta-burst stimulation (TBS) with postsynaptic depolarization, we induced long-term potentiation (LTP) of putative single-fibre excitatory postsynaptic currents (EPSCs) in stratum oriens/alveus (O/A) interneurones of mouse hippocampal slices. LTP induction was absent in metabotropic glutamate receptor 1 (mGluR1) knockout mice, was correlated with the postsynaptic presence of mGluR1a, and required a postsynaptic Ca2+ rise. Changes in paired-pulse facilitation and coefficient of variation indicated that LTP expression involved presynaptic mechanisms. LTP was synapse specific, occurring selectively at synapses modulated by presynaptic group II, but not group III, mGluRs. Furthermore, the TBS protocol applied in O/A induced a long-term increase of polysynaptic inhibitory responses in CA1 pyramidal cells, that was absent in mGluR1 knockout mice. These results uncover the mechanisms of a novel form of interneurone synaptic plasticity that can adaptively regulate inhibition of hippocampal pyramidal cells.

Gabapentin actions on Kir3 currents and N-type Ca2+ channels via GABAB receptors in hippocampal pyramidal cells.

Gabapentin is a clinically effective anticonvulsant with an unclear mechanism of action. It was described as a GABA(B(1a,2)) receptor subtype-selective agonist, activating postsynaptic K(+) currents and inhibiting postsynaptic Ca(2+) channels in CA1 pyramidal cells, but without presynaptic actions. These activities appeared controversial and we therefore sought to further clarify gabapentin actions in rat hippocampal slices by characterizing K(+) currents and Ca(2+) channels targeted by gabapentin using whole-cell recording and multiphoton Ca(2+) imaging. 1) We found that gabapentin and baclofen induced inwardly rectifying K(+) currents (K(Gbp) and K(Bac), respectively), sensitive to Ba(2+) and Cs(+). 2) A constitutively active K(IR) current, independent of GABA(B) receptor activation and sensitive to Ba(2+) and Cs(+) was also present. 3) K(Gbp), K(Bac), and K(IR) currents showed some differences in sensitivity to Ba(2+) and Cs(+), indicating the possible activation of distinct Kir3 currents, independent of K(IR), by gabapentin and baclofen. 4) Gabapentin inhibition of Ca(2+) channels was abolished by omega-conotoxin GVIA, but not by omega-agatoxin IVA and nimodipine, indicating a predominant action of gabapentin on N-type Ca(2+) channels. 5) Gabapentin actions were linked to activation of pertussis toxin-sensitive G-proteins since N-ethylmaleimide (NEM) blocked K(Gbp) activation and Ca(2+) channel inhibition by gabapentin. 6) Finally, gabapentin reduced epileptiform discharges in slices via GABA(B) receptor activation. The anticonvulsant actions of gabapentin in hippocampal cells may thus involve GABA(B) receptor coupling to G-proteins and modulation of Kir3 and N-type Ca(2+) channels. Moreover, gabapentin and baclofen activation of GABA(B) receptors may couple to distinct cellular targets.

Different actions of gabapentin and baclofen in hippocampus from weaver mice.

The pre- and postsynaptic effects of baclofen, a broad-spectrum gamma-aminobutyric acid (GABA)B receptor agonist, and gabapentin, a selective agonist at GABA(B) receptors composed of GABA(B)(1a,2) heterodimers, were examined in CA1 pyramidal cells using whole-cell patch-clamp recordings in hippocampal slices from different strains of mice. In slices from C57BL/6 mice, by means of GABA(B) receptors, gabapentin and baclofen activated outward K+ currents at resting membrane potential. In weaver mice with a Kir3.2 channel mutation, baclofen and gabapentin failed to activate postsynaptic K+ currents. However, in littermate controls of weaver mice, gabapentin failed to evoke K+ currents, whereas baclofen activated currents in the same cells. Thus, postsynaptic actions of gabapentin and baclofen on K+ currents are different in this mouse strain. Via presynaptic GABA(B) receptors, baclofen significantly reduced GABA(A) inhibitory postsynaptic currents (IPSCs) in slices from C57BL/6 mice, as well as weaver and control mice. In contrast, gabapentin did not affect IPSCs significantly in any group of mice. These results indicate that although baclofen and gabapentin are agonists at postsynaptic GABA(B) receptors positively coupled to K+ channels, their mechanism of action differs in certain strains of mice, including the weaver wild-type mice, suggesting a dissociation in their signaling mechanism and coupling to K+ channels.