Excitatory cholecystokinin neurons of the midbrain integrate diverse temporal responses and drive auditory thalamic subdomains

The Mesencephalic Locomotor Region: Multiple Cell Types, Multiple Behavioral Roles, and Multiple Implications for Disease

The mesencephalic locomotor region (MLR) controls locomotion in vertebrates. In humans with Parkinson disease, locomotor deficits are increasingly associated with decreased activity in the MLR. This brainstem region, commonly considered to include the cuneiform and pedunculopontine nuclei, has been explored as a target for deep brain stimulation to improve locomotor function, but the results are variable, from modest to promising. However, the MLR is a heterogeneous structure, and identification of the best cell type to target is only beginning. Here, I review the studies that uncovered the role of genetically defined MLR cell types, and I highlight the cells whose activation improves locomotor function in animal models of Parkinson disease. The promising cell types to activate comprise some glutamatergic neurons in the cuneiform and caudal pedunculopontine nuclei, as well as some cholinergic neurons of the pedunculopontine nucleus. Activation of MLR GABAergic neurons should be avoided, since they stop locomotion or evoke bouts flanked with numerous stops. MLR is also considered a potential target in spinal cord injury, supranuclear palsy, primary progressive freezing of gait, or stroke. Better targeting of the MLR cell types should be achieved through optimized deep brain stimulation protocols, pharmacotherapy, or the development of optogenetics for human use.

Lineage-tracing reveals an expanded population of NPY neurons in the inferior colliculus

Growing evidence suggests that neuropeptide signaling shapes auditory computations. We previously showed that neuropeptide Y (NPY) is expressed in the inferior colliculus (IC) by a population of GABAergic stellate neurons and that NPY regulates the strength of local excitatory circuits in the IC. NPY neurons were initially characterized using the NPY-hrGFP reporter mouse, in which hrGFP expression indicates NPY expression at the time of assay, i.e., an expression-tracking approach. However, studies in other brain regions have shown that NPY expression can vary based on a range of factors, suggesting that the NPY-hrGFP mouse might miss NPY neurons not expressing NPY proximal to the experiment date. Here, we hypothesized that neurons with the ability to express NPY represent a larger population of IC GABAergic neurons than previously reported. To test this hypothesis, we used a lineage-tracing approach to irreversibly tag neurons that expressed NPY at any point prior to the experiment date. We then compared the physiological and anatomical features of neurons labeled with this lineage-tracing approach to our prior data set, revealing a larger population of NPY neurons than previously found. In addition, we used optogenetics to test the local connectivity of NPY neurons and found that NPY neurons routinely provide inhibitory synaptic input to other neurons in the ipsilateral IC. Together, our data expand the definition of NPY neurons in the IC, suggest that NPY expression might be dynamically regulated in the IC, and provide functional evidence that NPY neurons form local inhibitory circuits in the IC.

Parvalbumin and Somatostatin: Biomarkers for Two Parallel Tectothalamic Pathways in the Auditory Midbrain

The inferior colliculus (IC) represents a crucial relay station in the auditory pathway, located in the midbrain's tectum and primarily projecting to the thalamus. Despite the identification of distinct cell classes based on various biomarkers in the IC, their specific contributions to the organization of auditory tectothalamic pathways have remained poorly understood. In this study, we demonstrate that IC neurons expressing parvalbumin (ICPV+) or somatostatin (ICSOM+) represent two minimally overlapping cell classes throughout the three IC subdivisions in mice of both sexes. Strikingly, regardless of their location within the IC, these neurons predominantly project to the primary and secondary auditory thalamic nuclei, respectively. Cell class-specific input tracing suggested that ICPV+ neurons primarily receive auditory inputs, whereas ICSOM+ neurons receive significantly more inputs from the periaqueductal gray and the superior colliculus (SC), which are sensorimotor regions critically involved in innate behaviors. Furthermore, ICPV+ neurons exhibit significant heterogeneity in both intrinsic electrophysiological properties and presynaptic terminal size compared with ICSOM+ neurons. Notably, approximately one-quarter of ICPV+ neurons are inhibitory neurons, whereas all ICSOM+ neurons are excitatory neurons. Collectively, our findings suggest that parvalbumin and somatostatin expression in the IC can serve as biomarkers for two functionally distinct, parallel tectothalamic pathways. This discovery suggests an alternative way to define tectothalamic pathways and highlights the potential usefulness of Cre mice in understanding the multifaceted roles of the IC at the circuit level.

Identifying neuron types and circuit mechanisms in the auditory midbrain

The inferior colliculus (IC) is a critical computational hub in the central auditory pathway. From its position in the midbrain, the IC receives nearly all the ascending output from the lower auditory brainstem and provides the main source of auditory information to the thalamocortical system. In addition to being a crossroads for auditory circuits, the IC is rich with local circuits and contains more than five times as many neurons as the nuclei of the lower auditory brainstem combined. These results hint at the enormous computational power of the IC, and indeed, systems-level studies have identified numerous important transformations in sound coding that occur in the IC. However, despite decades of effort, the cellular mechanisms underlying IC computations and how these computations change following hearing loss have remained largely impenetrable. In this review, we argue that this challenge persists due to the surprisingly difficult problem of identifying the neuron types and circuit motifs that comprise the IC. After summarizing the extensive evidence pointing to a diversity of neuron types in the IC, we highlight the successes of recent efforts to parse this complexity using molecular markers to define neuron types. We conclude by arguing that the discovery of molecularly identifiable neuron types ushers in a new era for IC research marked by molecularly targeted recordings and manipulations. We propose that the ability to reproducibly investigate IC circuits at the neuronal level will lead to rapid advances in understanding the fundamental mechanisms driving IC computations and how these mechanisms shift following hearing loss.

Neuropeptide Y Signaling Regulates Recurrent Excitation in the Auditory Midbrain

Neuropeptides play key roles in shaping the organization and function of neuronal circuits. In the inferior colliculus (IC), which is in the auditory midbrain, Neuropeptide Y (NPY) is expressed by a class of GABAergic neurons that project locally and outside the IC. Most neurons in the IC have local axon collaterals; however, the organization and function of local circuits in the IC remain unknown. We previously found that excitatory neurons in the IC can express the NPY Y1 receptor (Y1R+) and application of the Y1R agonist, [Leu31, Pro34]-NPY (LP-NPY), decreases the excitability of Y1R+ neurons. As NPY signaling regulates recurrent excitation in other brain regions, we hypothesized that Y1R+ neurons form interconnected local circuits in the IC and that NPY decreases the strength of recurrent excitation in these circuits. To test this hypothesis, we used optogenetics to activate Y1R+ neurons in mice of both sexes while recording from other neurons in the ipsilateral IC. We found that nearly 80% of glutamatergic IC neurons express the Y1 receptor, providing extensive opportunities for NPY signaling to regulate local circuits. Additionally, Y1R+ neuron synapses exhibited modest short-term synaptic plasticity, suggesting that local excitatory circuits maintain their influence over computations during sustained stimuli. We further found that application of LP-NPY decreased recurrent excitation in the IC, suggesting that NPY signaling strongly regulates local circuit function in the auditory midbrain. Our findings show that Y1R+ excitatory neurons form interconnected local circuits in the IC, and their influence over local circuits is regulated by NPY signaling.SIGNIFICANCE STATEMENT Local networks play fundamental roles in shaping neuronal computations in the brain. The IC, localized in the auditory midbrain, plays an essential role in sound processing, but the organization of local circuits in the IC is largely unknown. Here, we show that IC neurons that express the Neuropeptide Y1 receptor (Y1R+ neurons) make up most of the excitatory neurons in the IC and form interconnected local circuits. Additionally, we found that NPY, which is a powerful neuromodulator known to shape neuronal activity in other brain regions, decreases the extensive recurrent excitation mediated by Y1R+ neurons in local IC circuits. Thus, our results suggest that local NPY signaling is a key regulator of auditory computations in the IC.

Organization of orbitofrontal-auditory pathways in the Mongolian gerbil

Sound perception is highly malleable, rapidly adjusting to the acoustic environment and behavioral demands. This flexibility is the result of ongoing changes in auditory cortical activity driven by fluctuations in attention, arousal, or prior expectations. Recent work suggests that the orbitofrontal cortex (OFC) may mediate some of these rapid changes, but the anatomical connections between the OFC and the auditory system are not well characterized. Here, we used virally mediated fluorescent tracers to map the projection from OFC to the auditory midbrain, thalamus, and cortex in a classic animal model for auditory research, the Mongolian gerbil (Meriones unguiculatus). We observed no connectivity between the OFC and the auditory midbrain, and an extremely sparse connection between the dorsolateral OFC and higher order auditory thalamic regions. In contrast, we observed a robust connection between the ventral and medial subdivisions of the OFC and the auditory cortex, with a clear bias for secondary auditory cortical regions. OFC axon terminals were found in all auditory cortical lamina but were significantly more concentrated in the infragranular layers. Tissue-clearing and lightsheet microscopy further revealed that auditory cortical-projecting OFC neurons send extensive axon collaterals throughout the brain, targeting both sensory and non-sensory regions involved in learning, decision-making, and memory. These findings provide a more detailed map of orbitofrontal-auditory connections and shed light on the possible role of the OFC in supporting auditory cognition.

Neuropeptide Y signaling regulates recurrent excitation in the auditory midbrain

Neuropeptides play key roles in shaping the organization and function of neuronal circuits. In the inferior colliculus (IC), which is located in the auditory midbrain, Neuropeptide Y (NPY) is expressed by a large class of GABAergic neurons that project locally as well as outside the IC. The IC integrates information from numerous auditory nuclei making the IC an important hub for sound processing. Most neurons in the IC have local axon collaterals, however the organization and function of local circuits in the IC remains largely unknown. We previously found that neurons in the IC can express the NPY Y1 receptor (Y 1 R + ) and application of the Y 1 R agonist, [Leu 31 , Pro 34 ]-NPY (LP-NPY), decreases the excitability of Y 1 R + neurons. To investigate how Y 1 R + neurons and NPY signaling contribute to local IC networks, we used optogenetics to activate Y 1 R + neurons while recording from other neurons in the ipsilateral IC. Here, we show that 78.4% of glutamatergic neurons in the IC express the Y1 receptor, providing extensive opportunities for NPY signaling to regulate excitation in local IC circuits. Additionally, Y 1 R + neuron synapses exhibit modest short-term synaptic plasticity, suggesting that local excitatory circuits maintain their influence over computations during sustained stimuli. We further found that application of LP-NPY decreases recurrent excitation in the IC, suggesting that NPY signaling strongly regulates local circuit function in the auditory midbrain. Together, our data show that excitatory neurons are highly interconnected in the local IC and their influence over local circuits is tightly regulated by NPY signaling.

Metabolic cues impact non-oscillatory intergeniculate leaflet and ventral lateral geniculate nucleus: standard versus high-fat diet comparative study

The intergeniculate leaflet and ventral lateral geniculate nucleus (IGL/VLG) are subcortical structures involved in entrainment of the brain's circadian system to photic and non-photic (e.g. metabolic and arousal) cues. Both receive information about environmental light from photoreceptors, exhibit infra-slow oscillations (ISO) in vivo, and connect to the master circadian clock. Although current evidence demonstrates that the IGL/VLG communicate metabolic information and are crucial for entrainment of circadian rhythms to time-restricted feeding, their sensitivity to food intake-related peptides has not been investigated yet. We examined the effect of metabolically relevant peptides on the spontaneous activity of IGL/VLG neurons. Using ex vivo and in vivo electrophysiological recordings as well as in situ hybridisation, we tested potential sensitivity of the IGL/VLG to anorexigenic and orexigenic peptides, such as cholecystokinin, glucagon-like peptide 1, oxyntomodulin, peptide YY, orexin A and ghrelin. We explored neuronal responses to these drugs during day and night, and in standard vs. high-fat diet conditions. We found that IGL/VLG neurons responded to all the substances tested, except peptide YY. Moreover, more neurons responded to anorexigenic drugs at night, while a high-fat diet affected the IGL/VLG sensitivity to orexigenic peptides. Interestingly, ISO neurons responded to light and orexin A, but did not respond to the other food intake-related peptides. In contrast, non-ISO cells were activated by metabolic peptides, with only some being responsive to light. Our results show for the first time that peptides involved in the body's energy homeostasis stimulate the thalamus and suggest functional separation of the IGL/VLG cells. KEY POINTS: The intergeniculate leaflet and ventral lateral geniculate nucleus (IGL/VLG) of the rodent thalamus process various signals and participate in circadian entrainment. In both structures, cells exhibiting infra-slow oscillatory activity as well as non-rhythmically firing neurons being observed. Here, we reveal that only one of these two groups of cells responds to anorexigenic (cholecystokinin, glucagon-like peptide 1 and oxyntomodulin) and orexigenic (ghrelin and orexin A) peptides. Neuronal responses vary depending on the time of day (day vs. night) and on the diet (standard vs. high-fat diet). Additionally, we visualised receptors to the tested peptides in the IGL/VLG using in situ hybridisation. Our results suggest that two electrophysiologically different subpopulations of IGL/VLG neurons are involved in two separate functions: one related to the body's energy homeostasis and one associated with the subcortical visual system.

Long-Duration Sound-Induced Facilitation Changes Population Activity in the Inferior Colliculus

The inferior colliculus (IC) is at the midpoint of the auditory system and integrates virtually all information ascending from the auditory brainstem, organizes it, and transmits the results to the auditory forebrain. Its abundant, excitatory local connections are crucial for this task. This study describes a long duration sound (LDS)-induced potentiation in the IC that changes both subsequent tone-evoked responses and spontaneous activity. Afterdischarges, changes of spontaneous spiking following an LDS, were seen previously in single neurons. Here, we used multi-channel probes to record activity before and after a single, tetanic sound and describe the changes in a population of IC neurons. Following a 60 s narrowband-noise stimulation, a subset of recording channels (∼16%) showed afterdischarges. A facilitated response spike rate to tone pips following an LDS was also observed in ∼16% of channels. Both channels with an afterdischarge and channels with facilitated tone responses had higher firing rates in response to LDS, and the magnitude of the afterdischarges increased with increased responses to the LDS. This is the first study examining the effect of LDS stimulation on tone-evoked responses. This observed facilitation in vivo has similarities to post-tetanic potentiation in vitro as both manner of induction (strong stimulation for several seconds) as well as time-course of the facilitation (second to minute range) are comparable. Channels with and without facilitation appear to be intermixed and distributed widely in the central nucleus of IC, and this suggests a heretofore unknown property of some IC neurons or their circuits. Consequently, this sound-evoked facilitation may enhance the sound-evoked output of these neurons, while, simultaneously, most other IC neurons have reduced or unchanged output in response to the same stimulus.

Synaptic mechanisms of top-down control in the non-lemniscal inferior colliculus

Corticofugal projections to evolutionarily ancient, subcortical structures are ubiquitous across mammalian sensory systems. These 'descending' pathways enable the neocortex to control ascending sensory representations in a predictive or feedback manner, but the underlying cellular mechanisms are poorly understood. Here, we combine optogenetic approaches with in vivo and in vitro patch-clamp electrophysiology to study the projection from mouse auditory cortex to the inferior colliculus (IC), a major descending auditory pathway that controls IC neuron feature selectivity, plasticity, and auditory perceptual learning. Although individual auditory cortico-collicular synapses were generally weak, IC neurons often integrated inputs from multiple corticofugal axons that generated reliable, tonic depolarizations even during prolonged presynaptic activity. Latency measurements in vivo showed that descending signals reach the IC within 30 ms of sound onset, which in IC neurons corresponded to the peak of synaptic depolarizations evoked by short sounds. Activating ascending and descending pathways at latencies expected in vivo caused a NMDA receptor-dependent, supralinear excitatory postsynaptic potential summation, indicating that descending signals can nonlinearly amplify IC neurons' moment-to-moment acoustic responses. Our results shed light upon the synaptic bases of descending sensory control and imply that heterosynaptic cooperativity contributes to the auditory cortico-collicular pathway's role in plasticity and perceptual learning.

Juxtacellular Labeling of Stellate, Disk and Basket Neurons in the Central Nucleus of the Guinea Pig Inferior Colliculus

We reconstructed the intrinsic axons of 32 neurons in the guinea pig inferior colliculus (IC) following juxtacellular labeling. Biocytin was injected into cells in vivo, after first analyzing physiological response properties. Based on axonal morphology there were two classes of neuron: (1) laminar cells (14/32, 44%) with an intrinsic axon and flattened dendrites confined to a single fibrodendritic lamina and (2) translaminar cells (18/32, 56%) with axons that terminated in two or more laminae in the central nucleus (ICc) or the surrounding cortex. There was also one small, low-frequency cell with bushy-like dendrites that was very sensitive to interaural timing differences. The translaminar cells were subdivided into three groups of cells with: (a) stellate dendrites that crossed at least two laminae (8/32, 25%); (b) flattened dendrites confined to one lamina and that had mainly en passant axonal swellings (7/32, 22%) and (c) short, flattened dendrites and axons with distinctive clusters of large terminal boutons in the ICc (3/32, 9%). These terminal clusters were similar to those of cortical basket cells. The 14 laminar cells all had sustained responses apart from one offset response. Almost half the non-basket type translaminar cells (7/15) had onset responses while the others had sustained responses. The basket cells were the only ones to have short-latency (7–9 ms), chopper responses and this distinctive temporal response should allow them to be studied in more detail in future. This is the first description of basket cells in the auditory brainstem, but more work is required to confirm their neurotransmitter and precise post-synaptic targets.

α3β4 ∗ Nicotinic Acetylcholine Receptors Strongly Modulate the Excitability of VIP Neurons in the Mouse Inferior Colliculus

The inferior colliculus (IC), the midbrain hub of the central auditory system, receives extensive cholinergic input from the pontomesencephalic tegmentum. Activation of nicotinic acetylcholine receptors (nAChRs) in the IC can alter acoustic processing and enhance auditory task performance. However, how nAChRs affect the excitability of specific classes of IC neurons remains unknown. Recently, we identified vasoactive intestinal peptide (VIP) neurons as a distinct class of glutamatergic principal neurons in the IC. Here, in experiments using male and female mice, we show that cholinergic terminals are routinely located adjacent to the somas and dendrites of VIP neurons. Using whole-cell electrophysiology in brain slices, we found that acetylcholine drives surprisingly strong and long-lasting excitation and inward currents in VIP neurons. This excitation was unaffected by the muscarinic receptor antagonist atropine. Application of nAChR antagonists revealed that acetylcholine excites VIP neurons mainly via activation of α₃β₄^∗ nAChRs, a nAChR subtype that is rare in the brain. Furthermore, we show that acetylcholine excites VIP neurons directly and does not require intermediate activation of presynaptic inputs that might express nAChRs. Lastly, we found that low frequency trains of acetylcholine puffs elicited temporal summation in VIP neurons, suggesting that in vivo-like patterns of cholinergic input can reshape activity for prolonged periods. These results reveal the first cellular mechanisms of nAChR regulation in the IC, identify a functional role for α₃β₄^∗ nAChRs in the auditory system, and suggest that cholinergic input can potently influence auditory processing by increasing excitability in VIP neurons and their postsynaptic targets.