HIPPOCAMPAL INTERNEURONS AND THEIR ROLE IN THE CONTROL OF NETWORK EXCITABILITY
, Head, Section on Cellular and Synaptic Physiology
J. Joshua Lawrence, PhD, Staff Scientist
Tue Banke, PhD, Visiting Fellow
Michael Daw, PhD, Visiting Fellow
Kenneth Pelkey, PhD, Visiting Fellow
Christine Torborg, PhD, Visiting Fellow
Ludovic Tricoire, PhD, Visiting Fellow
Brian Jefferies, BS, Biologist
Xiaoqing Yuan, MSc, Biologist
Christian Cea-Del Rio, BS, Predoctoral Fellow
Tsz-wan Michelle Ho, BS, Predoctoral Fellow
GABAergic inhibitory interneurons constitute a population of hippocampal cells whose high degree of anatomical and functional divergence makes them suitable candidates for controlling the activity of large populations of principal neurons. GABAergic inhibitory interneurons play a major role in the synchronization of neuronal activity; they are also involved in the generation of large-scale network oscillations. Thus, many interneurons function as a clock that dictates when principal cells fire during suprathreshold excitatory drive. Interneurons receive strong excitatory glutamatergic innervation via numerous anatomically distinct afferent projections, and recent evidence has demonstrated that the molecular composition of the AMPA-preferring class of glutamate receptors expressed at interneuron synapses is often distinct from that found at principal cell synapses. Furthermore, single inhibitory interneurons can synthesize distinct AMPA receptors with a defined subunit composition and then target the receptors to synaptic domains innervated by different afferent inputs. We use high-resolution whole-cell patch clamp recording techniques in brain slices of the rodent hippocampus to investigate (1) differential mechanisms of synaptic transmission onto hippocampal inhibitory interneurons and (2) the role of intrinsic voltage-gated channels in regulating interneuron excitability.
GABAergic input onto CA3 hippocampal interneurons remains shunting in nature throughout development
In the mammalian hippocampal formation, the inhibitory input provided by the many populations of local circuit–inhibitory interneurons controls the net flow of excitability. In principal cells, GABAA receptor–mediated inhibitory synaptic input undergoes a highly coordinated shift that begins early in life and progresses from a depolarizing to a more conventional, hyperpolarizing inhibition on maturation. The developmental regulation of two chloride cotransporters (NKCC1 and KCC2) controls this switch in inhibitory input polarity and results in a net shift from high to low intracellular Cl−. It remained unclear, however, whether inhibitory input onto inhibitory interneurons would demonstrate a similar developmental shift in intracellular Cl−. Using the gramicidin-perforated patch configuration, we recorded from CA3 hippocampal stratum lucidum interneurons and pyramidal cells to monitor inhibitory input across a broad developmental range. Evoked GABAA receptor–mediated synaptic input onto stratum lucidum interneurons was shunting in nature across the entire developmental age range that we tested while resting membrane potential and the inhibitory postsynaptic current (IPSC) reversal potential remained within a few millivolts between postnatal days 5 and 31. Furthermore, sensitivity to block of the two chloride cotransporters KCC2 and NKCC1 did not differ across the same age range, suggesting that the cotransporters’ relative expression is fixed across development. In contrast, pyramidal cell synaptic inhibition demonstrated the well-described switch from depolarizing to hyperpolarizing over the same age range. Thus, in contrast to principal cells, inhibitory synaptic input onto CA3 interneurons remains shunting throughout development.
Banke TG, McBain CJ. GABAergic input onto CA3 hippocampal interneurons remains shunting throughout development. J Neurosci 2006;26:11720-5.
State-dependent cAMP sensitivity of feedforward inhibition in the hippocampal mossy fiber pathway
Activity-dependent alterations in synaptic efficacy are thought to represent the cellular substrate underlying learning and memory formation; they have been implicated in neurodegenerative disorders such as epilepsy and chronic pain. Despite rapid progress in elucidating cellular mechanisms responsible for long-term plasticity at excitatory synapses between principal cells, researchers have gained relatively little insight into the plasticity of excitatory transmission onto inhibitory interneurons. Indeed, bidirectional plasticity of excitatory drive onto any identified interneuron population has not been observed. Moreover, excitatory transmission typically displays cell target–specific regulation, indicating that the rules governing plasticity at principal cell synapses cannot be applied to inputs onto interneurons. For instance, hippocampal mossy fiber (MF) inputs to CA3 stratum lucidum interneurons (SLINs) undergo long-term depression (LTD) following high-frequency stimulation (HFS); in contrast, long-term potentiation (LTP) occurs in MF-pyramid (PYR) synapses. Furthermore, activity-induced potentiation of MF-SLIN transmission has not been observed.
We recently provided evidence that mGluR7 activation and surface expression critically regulate bidirectional plasticity of feedforward inhibition in the hippocampal MF pathway. At naive MF inputs to SLINs, surface mGluR7 activation during HFS yields presynaptic LTD through a PKC-dependent mechanism. This activity-induced LTD is mimicked by pairing basal synaptic stimulation with mGluR7 activation via exogenously applied agonist. Surprisingly, induction of LTP by the receptor agonist L-AP4 does not simply occlude HFS-induced LTD but rather unmasks HFS’s ability to induce presynaptic LTP in MF-SLINs. We hypothesized that agonist-induced mGluR7 internalization converts MF-SLIN terminals to a state permissive for HFS-induced potentiation of release. We recently demonstrated that L-AP4–induced LTD unlocks the ability of MF-SLIN inputs to enhance release in response to increased cAMP levels. As previously reported, forskolin treatment did not significantly affect transmission, that is, excitatory postsynaptic potentials (EPSCs) at naive MF-SLIN synapses; in fact, forskolin treatment did not significantly affect transmission, which remained at 87 percent of control transmission. In contrast, following L-AP4–induced MF-SLIN LTD, forskolin de-depressed MF-SLIN EPSCs, more than a doubling the responses. In control experiments, forskolin similarly increased MF-CA3 pyramid EPSCs with or without prior L-AP4 treatment, consistent with ongoing cAMP sensitivity at this synapse. Finally, we found that conversion of depressing MF-SLIN inputs to potentiating inputs is also revealed with several rounds of HFS, suggesting that the polarity switch in plasticity is physiologically relevant. After two rounds of HFS MF-SLIN, EPSCs were roughly halved but, following subsequent HFS EPSCs, de-depressed to 99 percent of initial control values measured before any HFS conditioning stimulation. Based on these findings, we propose that MF-SLIN LTP proceeds by a cAMP-dependent cascade that can be engaged only following agonist-induced mGluR7 sequestration from MF-SLIN terminals, yielding state-dependent cAMP sensitivity of MF-SLIN transmission.
Pelkey KA, Lavezzari G, Racca C, Roche KW, McBain CJ. mGluR7 is a metaplastic switch controlling bi-directional plasticity of feedforward inhibition. Neuron 2005;46:89-102.
Pelkey KA, Topolnik L, Lacaille J-C, McBain CJ. Compartmentalized Ca2+ channel regulation at functionally divergent release sites of single mossy fibers underlies target-cell dependent plasticity. Neuron 2006;52:497-510.
Pelkey KA, Yuan X-Q, Lavezzari G, Roche KW, McBain CJ. mGluR7 undergoes rapid internalization in response to activation by the allosteric agonist AMN082. Neuropharmacology 2006;52:108-17.
Developmental expression of Ca2+-permeable AMPA receptors underlies depolarization-induced LTD at mossy fiber–CA3 pyramid synapses
As a result of changes in subunit composition, many central excitatory synapses undergo developmental alterations in the molecular and biophysical characteristics of postsynaptic ionotropic glutamate receptors. With respect to AMPA receptors (AMPARs), GluR2-containing, Ca2+-impermeable AMPARs (CI-AMPARs) prevail at synapses between mature principal neurons. However, accumulating evidence indicates that GuR2-lacking, Ca2+-permeable AMPARs (CP-AMPARs) contribute to these synapses early in development. We used a combination of two-photon Ca-imaging and electrophysiological recording techniques to investigate the potential role of CP-AMPARs at developing hippocampal mossy fiber–CA3 pyramidal cell (MF-PYR) synapses. We found that transmission at nascent MF-PYR synapses is mediated by a mixed population of CP- and CI-AMPARs, as evidenced by polyamine-dependent inwardly rectifying current-voltage (I-V) relations, and by partial philanthotoxin sensitivity of synaptic events. Of particular interest is the fact that CP-AMPAR expression at MF-PYR synapses is transient; that is, it is limited to the first three postnatal weeks and then is downregulated and permanently replaced by CI-AMPARs. It is the PDZ domain–containing protein PICK1 that regulates expression of CP-AMPARs. Strikingly, MF-PYR transmission via CP-AMPARs is selectively depressed during depolarization-induced LTD (DiLTD), a postsynaptic form of MF-PYR plasticity that exhibits a developmental profile overlapping with CP-AMPAR expression. The selective depression of CP-AMPARs during DiLTD manifests as both a loss of postsynaptic CP-AMPAR–mediated Ca2+ transients in PYR spines and reduced rectification of MF-PYR synaptic currents following DiLTD induction. Thus, the transient participation of CP-AMPARs at young MF-PYR synapses dictates the developmental window for observing DiLTD and confers a novel postsynaptic form of plasticity to a synapse well known as the prototypical model of presynaptic plasticity.
Ho TW, Pelkey KA, Topolnik L, Petralia RS, Takamiya K, Xia J, Huganir RL, Lacaille J-C, McBain CJ. Developmental expression of Ca2+-permeable AMPARs underlies depolarization-induced LTD at mossy fiber-CA3 pyramid synapses. J Neurosci 2007;27:11651-62.
Pelkey KA, McBain CJ. How to dismantle a detonator synapse. Neuron 2005;5:327-9.
PICK1 is required for the activity-dependent regulation of AMPA receptor GluR2 subunit composition during hippocampal long-term potentiation
The PDZ domain–containing protein PICK1 interacts with GluR2/3 AMPAR subunits and is important for the expression of hippocampal and cerebellar forms of LTD. However, PICK1 can also increase AMPAR function and regulate the GluR2 subunit composition of AMPARs. The regulation of GluR2 subunit composition has recently been described for hippocampal LTP; therefore, we tested whether PICK1 plays a novel role in LTP. We showed that overexpression of PICK1 in CA1 pyramidal neurons produces NMDA receptor–dependent potentiation of AMPAR-mediated synaptic transmission and regulation of GluR2 subunit composition, preventing LTP induction. Moreover, blockade of the PICK1-GluR2 interaction prevents LTP, which is absent in slices from PICK1 knockout mice. Thus, PICK1 is required for LTP and is involved in a novel activity-dependent mechanism that rapidly regulates the GluR2-content, and hence the calcium permeability, of AMPARs in principal neurons.
Plant K, Pelkey KA, Bortolotto ZA, Terashima A, McBain CJ, Collingridge GL, Isaac JTR. Transient incorporation of native GluR2-lacking AMPA receptors mediates an initial component of hippocampal CA1 LTP. Nat Neurosci 2006;9:602-4.
Somatodendritic Kv7/KCNQ/M channels are specialized to control interspike interval in hippocampal interneurons: a physiological and multicompartment modeling approach
The M-current (IM) is a subthreshold voltage-dependent K+ conductance that plays a key role in the control of cell excitability. In hippocampal principal cells, IM controls action potential (AP) accommodation and contributes to medium-duration afterhyperpolarization. Kv7 subunits, the molecular correlates of IM, have been detected in hippocampal interneurons, but the role of IM in control of interneuron excitability is unclear. Using immunocytochemical, electrophysiological, and computational approaches, we investigated the function and cellular localization of IM in hippocampal stratum oriens (SO) interneuron. We observed heterogeneous somatodendritic expression of both Kv7.2 and Kv7.3 subunits on SO interneurons, including oriens-lacunosum/moleculare (O-LM) interneurons. Upon deactivation from –30 to –50 mV, we observed a relaxation characteristic of IM that was attenuated by TEA, XE-991, and linopirdine, consistent with a Kv7.2-containing phenotype. Upon strong depolarization, the Kv7-mediated outward current activated within 5 ms and accounted for up to 20 percent of the total outward current. In loose-patch and whole-cell recordings, inhibition of IM increased AP frequency without influencing half-width or first-spike latency. In a multicompartment O-LM interneuron model that incorporated IM, somatodendritic placement of KCNQ channels best reproduced experimentally measured IM. The model also indicated that IM- and Kv3-mediated delayed rectifier K+ conductances are activated during single APs but play fundamentally different roles. While Kv3 channels mediate rapid repolarization of the AP, the slow deactivation of Kv7 channels is specialized to influence dendritic excitability for hundreds of milliseconds following an AP.
Lawrence JJ, Saraga F, Churchill J, Statland JM, Travis KE, Skinner FK, McBain CJ. Somatodendritic Kv7/KCNQ/M channels control interspike interval in hippocampal interneurons. J Neurosci 2006;26:12325-38.
Pre- and postsynaptic cholinergic neuromodulation of parvalbumin-positive (PV+) CA1 basket cells
Basket cells, interneurons that provide inhibition to the perisomatic regions of principal cells, play critical roles in sustaining network oscillations both in vitro and in vivo. In network oscillations evoked by bath application of carbachol, the firing of perisomatically projecting interneurons appears to rely heavily on recurrent excitation from pyramidal cells (PCs) (Mann et al., J Physiol 2005;562:55). However, cholinergically induced oscillations can be observed in isolated hippocampal interneuron networks (e.g., Reich et al., J Neurophysiol 2005;94:4290), suggesting a role for direct cholinergic neuromodulation of interneuron networks during network oscillations. Using transgenic mice in which green fluorescent protein (GFP) is expressed in a subset of vertically oriented interneurons largely restricted to CA1 and CA3 PC layers, we determined how cholinergic neuromodulation affects the intrinsic membrane and output properties of PV+ basket cells. In hippocampal slices from P15–P20 mice, we performed whole-cell recordings from GFP+ cells within the CA1 pyramidal cell layer in the presence of AMPA and NMDA antagonists. Consistent with a PV+ interneuron population, GFP+ cells possessed a fast-spiking phenotype and low input resistance. In response to a 1-second, +500 pA current step from −60 mV, bath application of 25 µM carbachol increased action potential frequency, induced a small afterdepolarization, and increased the holding current. In dual whole-cell recordings from anatomically confirmed CA1 basket cells, we observed that eliciting APs in the presynaptic cells induced fast, depressing IPSCs in the postsynaptic CA1 PCs. Consistent with a CB1 receptor–lacking basket cell population (Glickfeld and Scanziani, Nat Neurosci 2006;9:807), we found that eliciting a depolarization in the postsynaptic PC did not suppress PV+ GFP-PC transmission. However, bath application of 25 µM carbachol greatly attenuated PV+ GFP–PC transmission, consistent with a direct, endocannabinoid-independent presynaptic mechanism of cholinergic neuromodulation (Fukudome et al., Eur J Neurosci 2004;19:2682). Therefore, our results suggest that, during carbachol-induced network oscillations, PV+ basket cells not only receive and transmit enhanced excitatory input from PCs but also actively shape oscillatory activity through direct cholinergic neuromodulation of pre- and postsynaptic cholinergic specializations.
Cholinergic neuromodulation of neurochemically distinct hippocampal interneuron subtypes
Cholinergic receptor activation elicits distinct voltage response profiles across morphologically distinct hippocampal interneuron subtypes, but it is unclear whether the heterogeneity in response profiles reflects cell type–specific specializations in neurochemically distinct interneuron subtypes. To examine cholinergic phenotypes in interneuron subpopulations, we performed patch clamp recordings in acute slices from mice expressing GFP in either GAD67 (GIN mice) or GAD65 interneuron populations. In cell-attached recordings, GAD67-GFP cells fired spontaneously under control conditions and exhibited an increase in firing frequency in response to bath application of 5 µM carbachol. In contrast, 50 percent of the GAD65-GFP cells exhibited an increase in firing in the presence of 5 µM carbachol while 12 of 24 were silent under both control and 5 µM carbachol conditions. In whole-cell recordings from GAD67-GFP cells located within SO, bath application of 12.5 µM carbachol increased firing frequency induced by a 1-second, 90–100 pA current step from −60 mV, consistent with the O-LM phenotype (Lawrence et al., 2006b). In GAD65-GFP cells located in stratum pyramidale or radiatum, 12.5 µM carbachol also induced an enhancement of firing frequency. In GAD65-GFP cells, the increase in firing was accompanied by a reduction in slow afterhyperpolarization (AHP). Interestingly, in one-third of the GAD65-GFP cells examined, the slow AHP was converted to an afterdepolarization (ADP). However, the GAD65-GFP ADP differed from the O-LM ADP in several respects. First, GAD65-GFP cells contained a carbachol-resistant component of the AHP. Second, in contrast to the simple increase in holding current at −60 mV accompanying the O-LM ADP, the GAD65-GFP ADP was accompanied by a transient decrease in holding current at −60 mV, suggesting a biphasic response profile. Consistent with the several cholinergic response profiles observed, the GAD65-GFP population consisted of different interneuron types, including calbindin, calretinin, and CCK populations. In contrast, GAD67-GFP cells were restricted solely to somatostatin-positive interneuron types. In summary, the data provide further evidence that cholinergic neuromodulation proceeds in a cell type–specific manner.
Lawrence JJ, Grinspan ZM, Statland JM, McBain CJ. Muscarinic receptor activation tunes mouse stratum oriens interneurons to amplify spike reliability. J Physiol 2006a;571:555-62.
Lawrence JJ, Statland JM, Grinspan ZM, McBain CJ. Cell type-specific dependence of muscarinic signaling in mouse hippocampal stratum oriens interneurons. J Physiol 2006b;570:595-610.
COLLABORATOR
Graham Collingridge, PhD, University of Bristol, Bristol, UK
Richard Huganir, PhD, Howard Hughes Medical Institute, The Johns Hopkins University, Baltimore, MD
John Isaac, PhD, Developmental Plasticity Unit, NINDS, Bethesda, MD
Jean-Claude Lacaille, PhD, University of Montreal, Montreal, Canada
Ronald Petralia, PhD, Laboratory of Neurochemistry, NIDCD, Bethesda, MD
Fernanda Saraga, PhD, University Toronto, Toronto, Canada
Francis Skinner, PhD, University Toronto, Toronto, Canada
Gabor Szabo, PhD, Institute of Experimental Medicine of the Hungarian Academy of Science, Budapest, Hungary
For further information, contactmcbainc@mail.nih.gov.
