Target-specific control of olfactory bulb periglomerular cells by GABAergic and cholinergic basal forebrain inputs

  1. Didier De Saint Jan

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    Evaluation Summary:

    This study reports on the synaptic impact of basal forebrain stimulation on a population of olfactory bulb interneurons in acute mouse brain slices. The author reveals that optogenetic stimulation of GABAergic basal forebrain afferents by and large inhibits the discharge of periglomerular cells, whereas cholinergic afferents evoke a prolonged, M1 receptor-mediated depolarization and increase in firing in a subpopulation of periglomerular cells. The current study would further our understanding of the olfactory neural circuit and how different co-released neurotransmitters shape postsynaptic neuronal responses.

    (This preprint has been reviewed by eLife. We include the public reviews from the reviewers here; the authors also receive private feedback with suggested changes to the manuscript. The reviewers remained anonymous to the authors.)

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Abstract

The olfactory bulb (OB), the first relay for odor processing in the brain, receives dense GABAergic and cholinergic long-range projections from basal forebrain (BF) nuclei that provide information about the internal state and behavioral context of the animal. However, the targets, impact, and dynamic of these afferents are still unclear. How BF synaptic inputs modulate activity in diverse subtypes of periglomerular (PG) interneurons using optogenetic stimulation and loose cell-attached or whole-cell patch-clamp recording in OB slices from adult mice were studied in this article. GABAergic BF inputs potently blocked PG cells firing except in a minority of calretinin-expressing cells in which GABA release elicited spiking. Parallel cholinergic projections excited a previously overlooked PG cell subtype via synaptic activation of M1 muscarinic receptors. Low-frequency stimulation of the cholinergic axons drove persistent firing in these PG cells, thereby increasing tonic inhibition in principal neurons. Taken together, these findings suggest that modality-specific BF inputs can orchestrate synaptic inhibition in OB glomeruli using multiple, potentially independent, inhibitory or excitatory target-specific pathways.

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  1. Version published to 10.7554/elife.71965 on eLife
  2. eLife

    Evaluation Summary:

    This study reports on the synaptic impact of basal forebrain stimulation on a population of olfactory bulb interneurons in acute mouse brain slices. The author reveals that optogenetic stimulation of GABAergic basal forebrain afferents by and large inhibits the discharge of periglomerular cells, whereas cholinergic afferents evoke a prolonged, M1 receptor-mediated depolarization and increase in firing in a subpopulation of periglomerular cells. The current study would further our understanding of the olfactory neural circuit and how different co-released neurotransmitters shape postsynaptic neuronal responses.

    (This preprint has been reviewed by eLife. We include the public reviews from the reviewers here; the authors also receive private feedback with suggested changes to the manuscript. The reviewers remained anonymous to the authors.)

  3. eLife

    Reviewer #1 (Public Review):

    In this manuscript, de Saint Jan examines the diversity of responses of periglomerular (PG) neurons in response to cell-type-specific stimulation of basal forebrain (BF) inputs. These experiments build upon previous work done by the lab examining the circuit organization of BF inputs to the olfactory bulb. For type II PG neurons that are responsive to BF stimulation, the author finds that responses can be grouped into three broad classes containing: excitation, inhibition, and the excitation-inhibition. The manuscript primarily focuses on the third cell type which has the unique characteristic of having low activity at baseline, is inhibited by BF stimulation, but then increases their firing rate following the first optical stimulation. Through a rigorous set of experiments, the author shows that this bidirectional response is due to the interaction of GABA and ACh release. He then goes on to show that the increase in firing in type 2.3 PG neurons is due to activation of M1 AChRs and closure of M-type potassium channels.

    The characterization of the range of responses of PG 2.1-2.3 neurons is interesting and novel. On the whole, this is a well-designed and systematic study that may be appropriate for the journal. However, since the data comes only from slice recordings it is unclear what the in vivo relevance, such as in odor discrimination, maybe. Addressing this point would greatly increase the impact of this work, but it may be beyond the scope of the current manuscript.

  4. eLife

    Reviewer #2 (Public Review):

    The author used AAVs to conditionally express ChR2 in subsets of HDB neurons, using Dlx5/6-Cre mice and ChAT-Cre mice to restrict expressions to GABAergic and cholinergic neurons, respectively. In Dlx5/6 mice, optogenetic stimulations of labelled fibers in the OB result in observable modulation of spiking patterns in some PG cells. These effects were heterogeneous: (1) brief excitation, (2) mixture of phasic inhibition and slower excitation, or (3) inhibition on its own. The author teased out the basis of this heterogeneity, by revealing that the three types of optogenetic effects correspond to excitatory GABAA-mediaed effect on "type 2.1" PG cells, GABAA-mediated and M1-mediated effects on "type 2.3" PG cells, and finally GABAA-mediated effect on "type 2.X" cells. Further, the cholinergic effect was separable from the GABAergic effect in ChAT-Cre mice.

    Both HDB and the PG cells comprise heterogenous groups of cells, so a mechanistic understanding of cholinergic modulation of the OB function necessarily requires a cell-type specific investigation of this type and precision. The current study therefore advances our understanding in this crucial aspect. The specific receptor contributions with pharmacology is convincing. It is useful to know that the run-down prevents long-term monitoring of M1-mediated effect.

    The definition of PG cell types as presented is somewhat ambiguous and not done consistently, especially with respect to the author's previous publication, but this is something that further analyses of the existing data will be able to address.

  5. eLife

    Reviewer #3 (Public Review):

    This manuscript dissects the synaptic impact of basal forebrain (BF) afferents onto periglomerular (PG) cells in the olfactory bulb (OB) of mice using optogenetic stimulation of BF axons and in vitro slice electrophysiological recordings of postsynaptic responses. It is known that the BF projects widely throughout the brain, including in the OB, where it influences cellular physiology and behavior. The BF consists of multiple populations of cholinergic, glutamatergic and GABAergic neurons, and it is unclear how each of these populations contributes to BF's various effects on behavior. Here, the author carries out a series of experiments to dissect the synaptic actions of GABAergic and cholinergic axons on the discharge of PG cells, which function as inhibitory interneurons to OB output neurons. In a series of carefully conducted and clearly described experiments, the author uncovers heterogeneity in how PG cells respond to optogenetic stimulation of BF afferents: some are inhibited by GABA, others are excited by GABA, and others still are excited by type 1 muscarinic acetylcholine receptors (M1 receptors). The study focuses most of the latter, which has not been described previously and reliably evokes a prolonged increase in the firing in approximately 12% of PG cells via closure of M-type potassium channels. Despite the small number of PG cells responding to BF cholinergic axon stimulation in this fashion, a significant increase in spontaneous inhibitory synaptic currents (IPSCs) can be detected in tufted cells, potentially underlying an effect of BF afferents on the gain of OB output.

    One of the strengths of this study is that the experiments are conducted carefully and are well described, making it easy for experimental results to be understood and interpreted. Each finding is supported by a sufficient number of observations, large enough to reveal the rich diversity of cell types and their subtypes within the OB and differences in their response to BF axon stimulation. An exciting discovery is that a small population of poorly studied PG cells displays a classical M1 receptor and M-channel-mediated increase in tonic firing to cholinergic afferent stimulation. Another interesting finding, though not entirely novel nor extensively investigated here is that GABAergic inputs can drive another subpopulation of PG cells to fire an action potential, possibly because their chloride reversal potential is elevated above spike threshold. Overall, the analyses are thorough and statistics sound, yielding a solid functional and anatomical foundation for future analyses investigating the impact of BF axons on OB function.

    A weakness of this study is that it is largely observational and amounts to a functional anatomical study of a specialized brain circuit in vitro, offering little insights as to when or how the synapses under investigation contribute to brain function in vivo. For example, the authors show that optogenetic stimulation of BF cholinergic axons increases the firing of a small fraction of PG cells for several seconds, which in turn increases the frequency of spontaneous IPSCs in principal neurons. It is unclear when these BF axons are active in vivo: if tonically active at >0.5 Hz, the effect revealed here may be occluded at baseline. It is also unclear how the increase in IPSC frequency in tufted cells influences their discharge or that or mitral cells to alter the encoding of odorants. The same is true for GABAergic inputs to the OB, which are only peripherally studied here, and mainly exert an inhibitory effect on PG cells. The conclusions of this study therefore remain limited to the existence at least 2 populations of afferents from the BF - one cholinergic and another GABAergic - that synaptically influence largely non-overlapping subpopulations of PG cells to mediate phasic inhibition through GABA release or phasic excitation through acetylcholine release. Overall, the manuscript offers few additional insights into how these specialized BF signaling pathways arise (i.e. pre or postsynaptic specialization), or how differential modulation of subgroups in interneurons affects OB output and the sense of smell in the mammalian brain.

  6. Version published to 10.1101/2021.07.06.451255v1 on bioRxiv