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hippocampal
interneurons and their role in controlling
excitability
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Chris J. McBain, PhD, Head, Section on Cellular and Synaptic Physiology J.
Joshua Lawrence, PhD, Staff Scientist Tue
Banke, PhD, Visiting Fellow Andre
Fisahn, PhD, Visiting Fellow Saobo
Lei, PhD, Visiting Fellow Kenneth
Pelkey, PhD, Visiting Fellow Brian Jefferies,
BS, Biologist Xiaoqing
Yuan, MSc, Biologist Zachary
Grinspan, BS, HHMI Scholara Jeff Statland, BS, HHMI Scholarb |
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GABA-ergic 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. GABA-ergic inhibitory interneurons
play a major role in the synchronization of neuronal activity and are
involved in the generation of large-scale network oscillations. Thus,
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; recent evidence has demonstrated that the molecular
composition of both AMPA-preferring classes of glutamate receptors expressed
at interneuron synapses are often distinct from those found at principal cell
synapses. Furthermore, single inhibitory interneurons can synthesize distinct
AMPA receptors with defined subunit composition and target the receptors to
synaptic domains innervated by different afferent inputs. Using
high-resolution whole-cell patch clamp recording techniques in brain slices
of hippocampus and auditory and barrel cortex, we investigate differential
mechanisms of synaptic transmission onto hippocampal inhibitory interneurons
and the role of intrinsic voltage-gated channels in regulating interneuron
excitability. Quantal
transmission at mossy fiber targets in the CA3 region of the rat hippocampus Lawrence, Grinspan Recent anatomical evidence that inhibitory
interneurons receive approximately 10 times as many synapses from mossy fibers
as do principal neurons has led to a re-examination of the extent to which
interneurons are involved in CA3 network excitability. Despite previous
descriptions of many of the anatomical and physiological properties of mossy
fiber–CA3 interneuron synapses, an investigation into the quantal
nature of transmission at these synapses had not yet been conducted. We
employed variance-mean (VM) analysis to compare the release probability,
quantal size (q), and number of
release sites (n) at mossy fiber
target neurons in CA3. At interneuron synapses in which we experimentally
imposed a high concentration of Ca2+, the variance-mean
relationship was approximated by a parabolic function. Estimates of n were 1–2, and the weighted
release probability in normal Ca2+ conditions ranged from 0.34 to
0.51. At pyramidal cell synapses, the variance-mean relationship approximated
a linear relationship, suggesting that the release probability was
significantly lower. The weighted quantal amplitude was similar at interneuron
synapses and pyramidal cell synapses, although the variability in quantal
amplitude was larger at interneuron synapses. Mossy fiber transmission at CA3
interneuron synapses can be explained by a lower number of release sites, a
broader range of release probabilities, and a larger range of quantal
amplitudes than at CA3 pyramidal synapses. Finally, unitary quantal synaptic
events onto interneurons elicited spike transmission, owing in part to the
more depolarized membrane potential than that of pyramidal cells. The results
suggest that (1) although mossy fiber synapses onto pyramidal cells are
associated with a larger number of release sites per synapse, higher
connectivity, and higher initial release probability; and (2) a larger
relative impact per quantum onto CA3 interneurons generates strong
feedforward inhibition at physiological firing frequencies of dentate granule
cells. Given the central role of CA3 interneurons in mossy fiber synaptic
transmission, these details of transmission should provide insight into CA3
network dynamics under both physiological and pathophysiological
circumstances. Lei S, McBain CJ. Two loci of expression for
long-term depression at hippocampal mossy fiber-interneuron synapses. J Neurosci 2004;24:2112-2121. Control
of bidirectional plasticity of mossy fiber-stratum lucidum feedforward
inhibition by mGluR7 Pelkey; in collaboration with Roche Activity-dependent alterations in synaptic
efficacy are thought to represent the cellular substrate underlying learning
and memory formation and 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, much remains unknown about the plasticity
of excitatory transmission onto inhibitory interneurons. Indeed,
bidirectional plasticity of excitatory drive onto any identified interneuron
population has yet to be 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 to MF-pyramidal
(PYR) synapses, where long-term potentiation (LTP) occurs. We have now shown
that the metabotropic glutamate receptor subtype 7 (mGluR7) is a metaplastic
switch at MF-SLIN synapses that governs the direction of plasticity. In naive
slices, mGluR7 activation during HFS generates presynaptic MF-SLIN LTD
through a PKC-dependent mechanism. Agonist-induced mGluR7 internalization
unmasks HFS-induced MF-SLIN LTP. Thus, selective mGluR7 targeting to MF
terminals contacting SLINs rather than PYRs provides cell
target–specific plasticity and bidirectional control of feedforward
inhibition. Depolarization-induced
long-term depression at hippocampal mossy fiber-CA3 pyramidal neuron synapses Lei, Pelkey; in collaboration with
Congar, Lacaille, Topolnik Hippocampal CA3 pyramidal neurons receive two
types of excitatory afferent innervation: MFs from granule cells of the
dentate gyrus and recurrent collateral fibers (CFs) from other CA3 pyramidal
neurons. At CF-CA3 pyramidal neuron synapses, membrane depolarization paired
with low (0.33 Hz) presynaptic stimulation generated a heterogeneous response
that ranged from LTP, LTD, to no alteration of synaptic strength. However,
the same induction paradigm applied at MF-CA3 pyramidal neuron synapses
consistently induced LTD. This novel form of LTD was independent of NMDARs,
mGluRs, cannabinoid receptors, opioid receptors, or coincident synaptic
activity but was dependent on postsynaptic Ca2+ elevation through
L-type Ca2+ channels and release from InsP3 receptor–sensitive intracellular stores. Ca2+
imaging of both proximal and distal CA3 pyramidal neuron dendrites
demonstrated that the depolarizing induction paradigm differentially elevated
intracellular Ca2+ levels. We observed L-type Ca2+
channel activation only at the most proximal locations where MFs make
synapses. Depolarization-induced LTD did not occlude the conventional 1
Hz–induced LTD or vice versa, suggesting that independent mechanisms
underlie each form of plasticity. The paired-pulse ratio and coefficient of
variation of synaptic transmission remained unchanged after LTD induction,
suggesting that the expression locus of LTD is postsynaptic. Moreover, peak-scaled
nonstationary variance analysis indicated that depolarization-induced LTD
correlated with a reduction in postsynaptic AMPA receptor numbers without a
change in AMPA receptor conductance. Our results suggest that this novel form
of LTD is selectively expressed at proximal dendritic locations closely
associated with L-type Ca2+ channels. Lei S, Pelkey K, Topolnik L, Congar P,
Lacaille J-C, McBain CJ. Depolarization-induced long-term depression at
hippocampal mossy fiber-CA3 pyramidal neuron synapses. J Neurosci 2003;23:9786-9795. Muscarinic
control of spike timing and reliability in hippocampal interneurons Lawrence, Grinspan, Statland Hippocampal theta oscillations are thought to be
the primary means by which neuronal assemblies link distinct aspects of
spatial experience. The activation of hippocampal interneurons by cholinergic
afferents of the septum may play an important role in the initiation of theta
oscillations. Recently, we showed that several interneuron subclasses
discharge at different times during the theta cycle, a pattern that is
related both to the nature of the interneurons’ reciprocal connectivity
with pyramidal cells and the intrinsic membrane properties of each interneuron
class. Although cholinergic neuromodulation has been shown to have potent
effects on the resting membrane potential of different interneuron types, the
impact of cholinergic neuromodulation on spike timing and reliability is
unknown. We focused on a physiologically and anatomically well-described
interneuron subtype, the stratum oriens/lacunosum-moleculare (O-LM)
interneurons, which are poised both to receive cholinergic input from the
medial septum and modulate the flow of information from the entorhinal cortex
to the hippocampus. We find that O-LM interneurons consistently and potently
induce an after-depolarization (ADP) in the presence of muscarine, causing a
sustained increase in action potential firing frequency upon depolarization.
The ADP was triggered in part by the downregulation of two potassium
conductances, an M-current and a calcium-dependent after-hyperpolarization
current. Furthermore, the ADP persisted not only in the presence of M1 and M4
antagonists but also in M2 knockout mice, strongly suggesting that the ADP
was controlled by muscarinic M3 receptors. To assess how muscarine influences the
oscillatory properties of O-LM interneurons, we introduced sinusoidal current
injections at a variety of frequencies. Muscarine caused an increase in
action potential reliability, a shift in action potential timing to earlier
in the cycle, and an increase in spike timing precision. Interestingly, the
enhanced spike reliability and increased spike timing precision were limited
to approximately theta frequencies, suggesting that cholinergic
neuromodulation tunes the intrinsic membrane properties of O-LM interneurons
to match the frequency of extrinsic synaptic input. Such postsynaptic
specializations would afford cholinergic afferents from the medial septum
fine control over the degree of engagement of O-LM interneurons in network
activity. M-current
modulation in hippocampal interneurons Lawrence, Statland; in collaboration with
Saraga, Skinner Potassium channels consisting of the KCNQ
subtype are thought to underlie the macroscopic M-current (IM).
M-currents have long been known to play a critical role in controlling action
potential generation and firing pattern. While this conductance has been well
described in principal cells, it is unclear whether interneurons also possess
IM. We employed patch clamp techniques from acute mouse slices to
determine whether IM was present in visually identified
hippocampal CA1 oriens/alveus (OA) interneurons. We already showed that OA interneurons
possess both transient and sustained outward currents, with the latter
current thought to be mediated predominantly by delayed rectifier (Kv3)
potassium channels. However, the selective IM antagonist
linopirdine (10 micoM) removed about 22 percent of the sustained current
present at +40mV. Test pulses between -60 and +40 mV revealed a
linopirdine-sensitive (LS) current component with a half activation of about
-6.1 mV and slope of about 12.1. At +40mV, the time to peak was about 60.6
milliseconds (ms). An analysis of the time course of deactivation revealed
that linopirdine removed an intermediately decaying component of the outward
current while leaving both faster- and slower-decaying components unchanged.
Such a kinetic profile suggests that M-currents contribute considerably to
the intermediate component of the after-hyperpolarization. The relaxation
curve for a –20mV step from a holding potential of 30mV could be fit
with a single exponential, yielding a time constant of 231 ms. Moreover, IM
was inhibited 58 percent by 0.5mM tetraethylammonium, a specific inhibitor of
KCNQ channels. Thus, the voltage dependence, kinetics, and pharmacology all
strongly suggest that KCNQ2 channels mediate the LS conductance in OA
interneurons. Finally, to test whether block of IM affected cell
excitability, we applied 10 microM linopirdine in current clamp mode. In
cells exhibiting a detectable IM, linopirdine caused an increase
in action potential firing of about 3.3 Hz given a 500ms 90pA current
injection. Using these quantitative characteristics, we are currently
modeling the M-current in multicompartmental models of anatomically
reconstructed OA interneurons. In summary, IM is a previously
unrecognized component of the sustained current in OA interneurons, revealing
a novel mechanism by which interneuron excitability can be regulated. Given
that some genetic epilepsies feature a mutation of KCNQ genes, modeling
M-current control of interneuron excitability could offer insight into the
epileptic brain and suggest possibilities for treatment. The
role of the kainate receptor subunits GluR5 and GluR6 in hippocampal gamma
oscillations Fisahn; in collaboration with Buhl,
Contractor, Heinemann, Traub Activation of kainate receptors (but not AMPA
receptors) induces gamma oscillations in the in vitro hippocampus. The oscillations depend on intact
inhibitory GABA-ergic neurotransmission but are independent of excitatory
transmission via NMDA receptors, metabotropic glutamate receptors, and
cholinergic receptors. We investigated the connection between gamma
oscillations and the kainate receptor subtypes GluR5 and GluR6. Using
extracellular field recordings, we demonstrated that gamma oscillations of
comparable power were induced by much lower concentrations of kainate in GluR5–/–
(100nM) CA3 hippocampus than in wild type (600nM). Increasing concentrations
of kainate (>150nM) degraded the gamma oscillation and precipitated
electrographic seizure activity in GluR5–/–
hippocampus, but not in wild type. In contrast, kainate failed to induce
gamma oscillations or seizures at concentrations up to 600nM in GluR6–/–
hippocampus. Given that gamma oscillations were readily evoked by other
oscillogenic drugs such as muscarine (20microM), the altered neuronal
circuitry in GluR6–/– mice did not explain the lack of
effect of kainate. We concluded that activation of the GluR6 kainate receptor
subtype is necessary for the induction of gamma oscillations. Using
whole-cell recordings, we demonstrated that kainate increased spontaneous
inhibitory postsynaptic current (sIPSC) amplitude by about 50 percent in
wild-type mice and GluR5–/– but was without effect on
sIPSCs in GluR6–/–. Furthermore, kainate activated an
inward current and depolarized pyramidal neurons in both wild-type and GluR5–/–
mice, but not in GluR6–/– mice. These effects of
kainate on the hippocampal network are consistent with the expression
patterns of GluR5 and GluR6, which are primarily localized to inhibitory
interneuron presynaptic terminals and to somata and dendrites of interneurons
and pyramidal cells, respectively. Consistent with a presynaptic expression
pattern of GluR5, the paired-pulse ratio of evoked IPSCs was increased,
suggesting that GluR5 may act tonically to depolarize presynaptic inhibitory
terminals and provide an elevated basal state of inhibition within the
hippocampal network. Similarly, the increased sensitivity to kainate-induced
oscillations in the GluR5–/– hippocampus likely
results from a reduction in the inhibitory control over the CA3 network,
leading to enhanced excitability of pyramidal neurons and reduced ability of
the interneuron network to synchronize itself at gamma frequencies. Fisahn A, Contractor A, The
kainate receptor subunit GluR6 mediates metabotropic regulation of the slow
and medium AHP currents Fisahn; in collaboration with Heinemann Kainate is a well-known excitotoxic that
increases excitability and generates seizure-like activity as well as gamma
oscillations in the hippocampus. Changes in excitability have consequences
for action potential firing characteristics of neurons, which in turn are important
for the overall behavior and output of neuronal networks such as synchronized
oscillations or seizure-like activity. Prolonged depolarization of neurons
above their firing threshold leads to initially sustained generation of
action potentials. In most principal neuron types in the hippocampus,
however, the generation of action potentials slows down and stops before the
depolarizing stimulus has abated. Such spike frequency accommodation is
brought about by a number of conductances, including a calcium-activated
potassium current with slow decay time that hyperpolarizes the cell membrane
(IsAHP). The slow after-hyperpolarization (AHP) current therefore
governs the length and frequency of bursts of action potentials. Likewise,
the length and frequency of single action potentials is influenced by
calcium-activated hyperpolarizing potassium currents, which in pyramidal
neurons are distinguished by their medium and fast decay times (ImAHP,
IfAHP). A recent study showed that IsAHP is decreased
by direct activation of kainate receptors (KAR) on CA1 pyramidal cells.
Moreover, this action of kainate was metabotropic, activating a PKC-based
signaling pathway. We were interested in investigating the role of different
KAR subunits in modulating IsAHP and ImAHP and their
influence on repetitive action potential generation. To investigate the role
of KAR subunits in the metabotropic regulation of the slow and medium AHP
currents (IsAHP, ImAHP), we used whole-cell patch clamp
recordings from CA3 pyramidal cells of wild-type and KAR knockout mice. The
kainate-induced decrease of IsAHP and ImAHP amplitude
was PKC-dependent and absent in GluR6–/–, but not in
GluR5–/– neurons. Our findings suggest that activation
of GluR6-containing Publication Related to Other Work Spruston N, McBain CJ. Anatomy and physiology
of hippocampal neurons. In: Morris R, Amaral D, Bliss TV, eds. The Hippocampus. 2005, in press. aLeft
in 2002. bLeft in 2003. COLLABORATORS Eberhard Buhl, PhD, Patrice Congar, PhD, Anis Contractor, PhD, Northwestern Stephen Heinemann, PhD, The Salk Institute, Jean-Claude Lacaille, PhD, Katherine Roche, PhD, Basic Neurosciences Program, NINDS, Fernanda
Saraga, PhD, University of Toronto,
Canada Frances Skinner, PhD, Lisa Topolnik, PhD, Roger Traub, MD, SUNY, For further information, contact mcbainc@mail.nih.gov |
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