J. Josh Lawrence

Differential Mechanisms of Transmission at Three Types of Mossy Fiber Synapse

The axons of the dentate gyrus granule cells, the so-called mossy fibers, innervate their inhibitory interneuron and pyramidal neuron targets via both anatomically and functionally specialized synapses. Mossy fiber synapses onto inhibitory interneurons were comprised of either calcium-permeable (CP) or calcium-impermeable (CI) AMPA receptors, whereas only calcium-impermeable AMPA receptors existed at CA3 principal neuron synapses. In response to brief trains of high-frequency stimuli (20 Hz), pyramidal neuron synapses invariably demonstrated short-term facilitation, whereas interneuron EPSCs demonstrated either short-term facilitation or depression. Facilitation at all CI AMPA synapses was voltage independent, whereas EPSCs at CP AMPA synapses showed greater facilitation at −20 than at −80 mV, consistent with a role for the postsynaptic unblock of polyamines. At pyramidal cell synapses, mossy fiber EPSCs possessed marked frequency-dependent facilitation (commencing at stimulation frequencies >0.1 Hz), whereas EPSCs at either type of interneuron synapse showed only moderate frequency-dependent facilitation or underwent depression. Presynaptic metabotropic glutamate receptors (mGluRs) decreased transmission at all three synapse types in a frequency-dependent manner. However, after block of presynaptic mGluRs, transmission at interneuron synapses still did not match the dynamic range of EPSCs at pyramidal neuron synapses. High-frequency stimulation of mossy fibers induced long-term potentiation (LTP), long-term depression (LTD), or no change at pyramidal neuron synapses, interneuron CP AMPA synapses, and CI AMPA synapses, respectively. Induction of LTP or LTD altered the short-term plasticity of transmission onto both pyramidal cells and interneuron CP AMPA synapses by a mechanism consistent with changes in release probability. These data reveal differential mechanisms of transmission at three classes of mossy fiber synapse made onto distinct targets.

Interneuron Diversity series: Containing the detonation – feedforward inhibition in the CA3 hippocampus

Feedforward inhibitory circuits are involved both in the suppression of excitability and timing of action potential generation in principal cells. In the CA3 hippocampus, a single mossy fiber from a dentate gyrus granule cell forms giant boutons with multiple release sites, which are capable of detonating CA3 principal cells. By contrast, mossy fiber terminals form a larger number of Lilliputian-sized synapses with few release sites onto local circuit interneurons, with distinct presynaptic and postsynaptic properties. This dichotomy between the two synapse types endows the circuit with exquisite control over pyramidal cell discharge. Under pathological conditions where feedforward inhibition is compromised, focal excitation is no longer contained, rendering the circuit susceptible to hyperexcitability.

Somatodendritic Kv7/KCNQ/M Channels Control Interspike Interval in Hippocampal Interneurons

The M-current (IM), comprised of Kv7 channels, is a voltage-activated 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 the medium-duration afterhyperpolarization, but the role of IM in control of interneuron excitability remains unclear. Here, we investigated IM in hippocampal stratum oriens (SO) interneurons, both from wild-type and transgenic mice in which green fluorescent protein (GFP) was expressed in somatostatin-containing interneurons. Somatodendritic expression of Kv7.2 or Kv7.3 subunits was colocalized in a subset of GFP+ SO interneurons, corresponding to oriens-lacunosum moleculare (O-LM) cells. Under voltage clamp (VC) conditions at −30 mV, the Kv7 channel antagonists linopirdine/XE-991 abolished the IM amplitude present during relaxation from −30 to −50 mV and reduced the holding current (Ihold). In addition, 0.5 mm tetraethylammonium reduced IM, suggesting that IM was composed of Kv7.2-containing channels. In contrast, the Kv7 channel opener retigabine increased IM amplitude and Ihold. When strongly depolarized in VC, the linopirdine-sensitive outward current activated rapidly and comprised up to 20% of the total current. In current-clamp recordings from GFP+ SO cells, linopirdine induced depolarization and increased AP frequency, whereas retigabine induced hyperpolarization and arrested firing. In multicompartment O-LM interneuron models that incorporated IM, somatodendritic placement of Kv7 channels best reproduced experimentally measured IM. The models suggest that Kv3- and Kv7-mediated channels both rapidly activate during single APs; however, Kv3 channels control rapid repolarization of the AP, whereas Kv7 channels primarily control the interspike interval.

Quantal transmission at mossy fibre targets in the CA3 region of the rat hippocampus

Recent anatomical evidence that inhibitory interneurones receive approximately 10 times more synapses from mossy fibres than do principal neurones ( Acsády et al. 1998 ) has led to the re‐examination of the extent to which interneurones are involved in CA3 network excitability. Although many of the anatomical and physiological properties of mossy fibre–CA3 interneurone synapses have been previously described ( Acsády et al. 1998 ; Tóth et al. 2000 ), an investigation into the quantal nature of transmission at this synapse has not yet been conducted. Here, we employed variance–mean (VM) analysis to compare the release probability, quantal size (q) and number of release sites (n) at mossy fibre target neurones in CA3. At six of seven interneurone synapses in which a high concentration of Ca2+ was experimentally imposed, the variance–mean relationship could be approximated by a parabola. 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 release probability was significantly lower. The weighted quantal amplitude was similar at interneurone synapses and pyramidal cell synapses, although the variability in quantal amplitude was larger at interneurone synapses. Mossy fibre transmission at CA3 interneurone synapses can be explained by a lower number of release sites, a broader range of release probabilities, and larger range of quantal amplitudes than at CA3 pyramidal synapses. Finally, quantal events on to interneurones elicited spike transmission, owing in part to the more depolarized membrane potential than pyramidal cells. These results suggest that although mossy fibre synapses on to pyramidal cells are associated with a larger number of release sites per synapse, the higher connectivity, higher initial release probability, and larger relative impact per quantum on to CA3 interneurones generate strong feedforward inhibition at physiological firing frequencies of dentate granule cells. Given the central role of CA3 interneurones in mossy fibre synaptic transmission, these details of mossy fibre synaptic transmission should provide insight into CA3 network dynamics under both physiological and pathophysiological circumstances.

Cholinergic control of GABA release : emerging parallels between neocortex and hippocampus

Release of acetylcholine (ACh) into the neocortex and hippocampus profoundly alters cellular excitability, network synchronization and behavioral state. Despite its diverse cellular and synaptic targets, the actions of ACh can be highly specific, altering the excitability of distinct inhibitory and excitatory cell types. This review presents evidence for the selectivity of cholinergic neuromodulation in GABAergic interneurons and identifies emerging parallels between the neocortex and hippocampus. In light of growing evidence that neuromodulatory specializations relate to neurochemical identity, I propose that differential engagement of neurochemically distinct interneuron subtypes is a unifying principle by which ACh orchestrates the flow of sensory information in the neocortex and hippocampus.

Cell type-specific dependence of muscarinic signalling in mouse hippocampal stratum oriens interneurones

Cholinergic signalling is critically involved in learning and memory processes in the hippocampus, but the postsynaptic impact of cholinergic modulation on morphologically defined subtypes of hippocampal interneurones remains unclear. We investigated the influence of muscarinic receptor (mAChR) activation on stratum oriens interneurones using whole‐cell patch clamp recordings from hippocampal slices in vitro. Upon somatic depolarization, mAChR activation consistently enhanced firing frequency and produced large, sustained afterdepolarizations (ADPs) of stratum oriens–lacunosum moleculare (O‐LM) interneurones. In contrast, stratum oriens cell types with axon arborization patterns different from O‐LM cells not only lacked large muscarinic ADPs but also appeared to exhibit distinct responses to mAChR activation. The ADP in O‐LM cells, mediated by M1/M3 receptors, was associated with inhibition of an M current, inhibition of a slow calcium‐activated potassium current, and activation of a calcium‐dependent non‐selective cationic current (ICAT). An examination of ionic conductances generated by firing revealed that calcium entry through ICAT controls the emergence of the mAChR‐mediated ADP. Our results indicate that cholinergic specializations are present within anatomically distinct subpopulations of hippocampal interneurones, suggesting that there may be organizing principles to cholinergic control of GABA release in the hippocampus.

Pharmacological Characterization of Heterodimeric NMDA Receptors Composed of NR 1a and 2B Subunits: Differences with Receptors Formed from NR 1a and 2A

Abstract: Pharmacological and molecular biological evidence indicates the existence of multiple types of NMDA receptors within the CNS. We have characterized pharmacological properties of receptors assembled from the combination of NR 1a and NR 2B subunits (NR 1a/2B) expressed in transfected cells using both 125I‐MK‐801 binding assays and electrophysiological measures. Binding of 125I‐MK‐801 to cells transfected with NR 1a/2B is saturable with a KD of 440 pM. The binding is potently inhibited by ketamine, dextromethorphan, phencyclidine, and MK‐801 and is stimulated by low concentrations of magnesium. These properties resemble those of native receptors and receptors produced by NR 1a/2A. However, 125I‐MK‐801 binding to membranes from cells transfected with NR 1a/2B is inhibited with high affinity by ifenprodil and is stimulated by spermidine, unlike receptors assembled from NR 1a/2A. NMDA‐induced currents measured in cells transfected with either NR 1a/2A or NR 1a/2B have pharmacological properties that correlate well with the binding studies. Currents in cells transfected with NR 1a/2B are potentiated by spermidine and blocked with high affinity by ifenprodil, whereas currents in cells transfected with NR 1a/2A are not enhanced by spermidine and are weakly inhibited by ifenprodil. These data suggest that pharmacological heterogeneity in native NMDA receptors may be explained by combinations of different subunits.

Muscarinic receptor activation tunes mouse stratum oriens interneurones to amplify spike reliability

Cholinergic activation of hippocampal targets can initiate and sustain network oscillations in vivo and in vitro, yet the impact of cholinergic modulation on the oscillatory properties of interneurones remains virtually unexplored. Using whole cell current clamp recordings in acute hippocampal slices, we investigated the influence of muscarinic receptor (mAChR) activation on the oscillatory properties of CA1 stratum oriens (SO) interneurones in vitro. In response to suprathreshold oscillatory input, mAChR activation increased spike reliability and precision, and extended the bandwidth that interneurone firing phase‐locked. These suprathreshold effects were largest at theta frequencies, indicating that mAChR activation tunes active conductances to enhance firing reliability and precision to theta frequency input. Muscarinic tuning of the intrinsic oscillatory properties of interneurones is a novel mechanism that may be crucial for the genesis of the theta rhythm.

Direct excitation of parvalbumin‐positive interneurons by M1 muscarinic acetylcholine receptors: roles in cellular excitability, inhibitory transmission and cognition

Parvalbumin‐containing (PV) neurons from mouse CA1 hippocampus (HC) and prefrontal cortex exhibit a fast spiking phenotype in vitro. Within CA1, HC PV cells are mainly comprised of basket and bistratified cell types. Direct activation of muscarinic acetylcholine receptors (mAChRs) enhances excitability more in CA1 HC than in prefrontal cortex PV cells. mAChR‐induced excitation of CA1 PV cells occurs through direct activation of M1 mAChRs. Transgenetic deletion of M1 mAChRs from PV cells diminishes M1 mAChR expression and cholinergic excitation of CA1 PV cells. mAChR‐induced excitation exclusively in PV cells enhances GABAergic transmission in CA1 pyramidal cells. In vivo activation of M1 mAChRs in PV cells is important in recognition and working memory but not spatial memory.

Cholinergic modulation amplifies the intrinsic oscillatory properties of CA1 hippocampal cholecystokinin‐positive interneurons

In the mammalian hippocampus, the neurotransmitter acetylcholine (ACh) promotes learning and memory storage. During sensory processing and learning, large ACh‐dependent electrical oscillatory events are observed, which involve the synchronization of both inhibitory and excitatory neural circuits. While the actions of ACh are known on excitatory hippocampal circuits, its actions on specific inhibitory circuits are poorly understood. We show that two types of cholecystokinin‐positive local circuit inhibitory interneuron, the so‐called ‘basket cells’ and ‘Schaffer collateral‐associated’ cells, which innervate separately the cell body and dendritic regions of principal cells, are modulated similarly by cholinergic receptor activation. In both cell types activation of their muscarinic receptors triggers a general increase of excitability and intrinsic oscillatory activity, and a more efficient engagement to slow network oscillations. Knowledge of how cholinergic neuromodulation acts on neurochemically identical but morphologically distinct inhibitory interneurons will allow us to understand the role played by this important neuromodulator during hippocampal‐dependent tasks in vivo.