Neurogliaform and Ivy Cells: A Major Family of nNOS Expressing GABAergic Neurons

Abstract

Neurogliaform and Ivy cells are members of an abundant family of neuronal nitric oxide synthase (nNOS) expressing GABAergic interneurons found in diverse brain regions. These cells have a defining dense local axonal plexus, and display unique synaptic properties including a biphasic postsynaptic response with both a slow GABA(A) component and a GABA(B) component following even a single action potential. The type of transmission displayed by these cells has been termed "volume transmission," distinct from both tonic and classical synaptic transmission. Electrical connections are also notable in that, unlike other GABAergic cell types, neurogliaform family cells will form gap junctions not only with other neurogliaform cells, but also with non-neurogliaform family GABAergic cells. In this review, we focus on neurogliaform and Ivy cells throughout the hippocampal formation, where recent studies highlight their role in feedforward inhibition, uncover their ability to display a phenomenon called persistent firing, and reveal their modulation by opioids. The unique properties of this family of cells, their abundance, rich connectivity, and modulation by clinically relevant drugs make them an attractive target for future studies in vivo during different behavioral and pharmacological conditions.

Keywords: GABAA,slow; GABAB; feedforward inhibition; opioid; persistent firing.

Figure 1

Characteristics of neurogliaform family cells. Neurogliaform and Ivy cells in the hippocampus: (A) dentate gyrus neurogliaform cell, (B) CA3 Ivy cell, (C) CA1 Ivy cell, (D) CA1 neurogliaform cell; these cells are characterized by their dense local axonal plexus (black: axon; blue: somata and dendrites). These cells also show a characteristic late-spiking firing pattern in response to depolarizing current steps [insets, (A,C,D); scale bars 20 mV, 200 ms]. ML, molecular layer; GCL, granule cell layer; so, stratum oriens; sp, stratum pyramidale; sl, stratum lucidum; sr, stratum radiatum; slm, stratum lacunosum-moleculare. Cells represented in (A,B) are from rat. Examples in (C,D) are from mouse. (E) Schematic diagram of the hippocampus, illustrating the approximate relative locations of cells in (A–D) (scale bar = 100 μm). Note that the Ivy cell reconstructed in (C) targets proximal dendrites, while the neurogliaform cell in (D) targets distal dendrites of CA1 pyramidal cells. (F) Recorded, biocytin-filled cells confirmed as neurogliaform cells in the dentate gyrus (three separate cells, numbered 1–3) express a variety of markers, including COUP TFII, nNOS, reelin, and NPY (scale bar = 20 μm) among others (see text). (G) Light microscopic image, showing the characteristic neurogliaform axonal arborization, with multiple axonal branches passing through a single plane of focus and frequent, small en passant boutons [same cell as (A), scale bar 20 μm]. (H) In addition to the classical late-spiking pattern characteristic of these cells, a high percentage of neurogliaform and Ivy cells display the recently described phenomenon referred to as persistent firing. After hundreds of action potentials induced by repeated depolarizing current steps (note that only the last depolarizing step is shown here), the cell continues to fire action potentials after the cessation of depolarizing input (bottom trace: schematic of current injection; top trace: current clamp recording, dashed line indicates −60 mV). Note the apparent low action potential threshold (arrow). Due to the long duration of the persistent firing state, the trace has segments omitted (indicated by hash marks) for illustration purposes. Reproduced and modified with permission from Armstrong et al. (2011) (A,F,G), Szabadics et al. (2010) (B), and Krook-Magnuson et al. (2011) (C,D,H).

Figure 2

Unique synaptic properties of neurogliaform cells. (A) Action potentials in a neurogliaform cell (top trace, example from neocortex) produce a slow IPSC (GABA_A,slow – bottom trace) in the postsynaptic cell. (B) In contrast, action potentials in fast-spiking basket cell (top trace) produce an IPSC with fast kinetics in the postsynaptic cell (bottom trace). (C) Neurogliaform cells (averaged examples from dentate gyrus) produce a biphasic postsynaptic response, consisting of a slow GABA_A and a GABA_B component, which can be distinguished by application of the GABA_B antagonist CGP55845 (red trace). This biphasic response can be seen following even a single presynaptic action potential (inset). (D) Neurogliaform cells (NGFC, black traces) can form both electrical and chemical synaptic connections with other GABAergic cell types. In this example, an electrical and unidirectional synaptic connection between a NGFC (black traces) and a non-NGFC (green traces) can be appreciated (example from dentate gyrus). Left traces: a hyperpolarizing current step (Pre, current clamp responses shown in the upper traces of each example), evokes an outward current in the electrically connected cell (Post, voltage clamp responses shown in the lower traces of each example). Right: When an action potential is evoked in the presynaptic non-NGFC, only a short inward current, due to electrical coupling, is observed in the connected NGFC, while both electrical and chemical synaptic responses can be appreciated in the postsynaptic non-NGFC in response to NGFC stimulation (upper traces, both pre and postsynaptic traces are averaged). Note the electrical responses in the non-NGFC riding on top of the slow GABA-mediated IPSC. Reproduced and modified with permission from Szabadics et al. (2007) (A,B) and Armstrong et al. (2011) (C,D).

Figure 3

Feedforward network functions of neurogliaform family cells in the hippocampus. The schematic outlines the connectivity of hippocampal neurogliaform and Ivy cells with principal cells. Entorhinal cortical input (green) directly excites NGFCs (red cells) both in the dentate gyrus and in the CA1. These cells provide feedforward inhibition to principal cells (blue) of the dentate gyrus or CA1, respectively. In the dentate, neurogliaform family cells targeting proximal dendrites (Ivy, brown) provide inhibition to both adult granule cells (GC) and newly born granule cells (nGC). Granule cells of the dentate, as well as granule cells of the CA3 provide mossy fiber input to CA3 Ivy cells, which provide both feedforward inhibition to pyramidal cells (Pyr) and feedback inhibition to CA3, but not dentate, granule cells. Schaffer collateral input to the CA1 contacts CA1 NGFCs and presumably, Ivy cells which both provide feedforward inhibition to CA1 pyramidal cells. CA1 pyramidal cells also provide feedback excitation to Ivy cells. Both NGFCs and Ivy cells in the CA1 express the μ-opioid receptor (μOR, yellow triangle). In this simplified schematic, excitatory input to excitatory cells has been omitted. (A) indicates the observed connectivity of NGFCs (red) in the dentate gyrus with other interneurons (light blue), consisting of five different connectivity motifs (1: bidirectional chemical synaptic; 2: unidirectional chemical synaptic; 3: electrical only; 4 electrical and bidirectional chemical synaptic; and 5: electrical and unidirectional chemical synaptic). (B) shows the response in a dentate gyrus NGFC to perforant path stimulation. (C) illustrates that individual mossy fiber boutons (MF, blue trace) form strong unitary connections (left box) with postsynaptic Ivy cells in the CA3 (brown traces) while unitary connections between mossy fiber boutons and fast-spiking basket cells (FSBC, orange traces) are smaller in amplitude. However (right box), while Ivy cells receive relatively fewer individual connections from mossy fibers, FSBCs receive more frequent mossy fiber bouton input. (D) demonstrates, in a paired recording, that Ivy cells (action potential, brown trace) provide inhibition to CA3 GCs (IPSCs, black traces). (E) shows the effect of the μ-opioid receptor agonist, DAMGO on an Ivy cell (brown trace) to CA1 pyramidal cell (lower traces) pair. The paired connection (control ACSF, black) is nearly abolished by application of DAMGO (light blue), and can be restored by addition of the μ-opioid receptor antagonist, CTAP (DAMGO + CTAP, purple). (F) indicates the relative timing of NGFC and Ivy cell firing during theta rhythms recorded from the pyramidal cell layer in vivo. NGFCs tend to fire right after the peak of theta, shortly following entorhinal input, while Ivy cells tend to fire right after the trough, shortly following CA3 input and just after the firing of CA1 cells. Reproduced and modified with permission from Armstrong et al. (2011) (A,B); Szabadics and Soltesz (2009) (C); Szabadics et al. (2010) (D), and Krook-Magnuson et al. (2011) (E).