Ipsilateral Inferior Colliculus Research Articles

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 mouse, in which humanized renilla Green Fluorescent Protein (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 several factors, suggesting that the NPY-hrGFP mouse might miss NPY neurons not expressing NPY on 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.

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.

Neurotransmitter phenotype and axonal projection patterns of VIP-expressing neurons in the inferior colliculus

Neurons in the inferior colliculus (IC), the midbrain hub of the central auditory pathway, send ascending and descending projections to other auditory brain regions, as well as projections to other sensory and non-sensory brain regions. However, the axonal projection patterns of individual classes of IC neurons remain largely unknown. Vasoactive intestinal polypeptide (VIP) is a neuropeptide expressed by subsets of neurons in many brain regions. We recently identified a class of IC stellate neurons that we called VIP neurons because they are labeled by tdTomato (tdT) expression in VIP-IRES-Cre x Ai14 mice. Here, using fluorescence in situ hybridization, we found that tdT+ neurons in VIP-IRES-Cre x Ai14 mice express Vglut2, a marker of glutamatergic neurons, and VIP, suggesting that VIP neurons use both glutamatergic and VIPergic signaling to influence their postsynaptic targets. Next, using viral transfections with a Cre-dependent eGFP construct, we labeled the axonal projections of VIP neurons. As a group, VIP neurons project intrinsically, within the ipsilateral and contralateral IC, and extrinsically to all the major targets of the IC. Within the auditory system, VIP neurons sent axons and formed axonal boutons in higher centers, including the medial geniculate nucleus and the nucleus of the brachium of the IC. Less dense projections terminated in lower centers, including the nuclei of the lateral lemniscus, superior olivary complex, and dorsal cochlear nucleus. VIP neurons also project to several non-auditory brain regions, including the superior colliculus, periaqueductal gray, and cuneiform nucleus. The diversity of VIP projections compared to the homogeneity of VIP neuron intrinsic properties suggests that VIP neurons play a conserved role at the microcircuit level, likely involving neuromodulation through glutamatergic and VIPergic signaling, but support diverse functions at the systems level through their participation in different projection pathways.

Downregulated miRNAs associated with auditory deafferentation and compensatory neural plastic changes following single-sided deafness in the inferior colliculi of rats

Background and aimsDeafferentation and compensatory neural plastic changes in the inferior colliculus (IC) have been suggested following single-sided deafness (SSD). We explored related miRNA changes in the IC of SSD rats using miRNA microarray analyses. MethodsEight-week-old rats were divided into control and SSD rats (n = 8 for each group). SSD rats underwent right-side cochlear ablation surgery, with the IC harvested two weeks post-surgery. miRNA microarray analysis was performed using GeneChip miRNA 4.0, microarray (Affymetrix Inc.). miRNAs whose expression levels differed between SSD and control rats with a fold-change ≥ 1.5 and P < 0.05 were examined using quantitative reverse transcription-polymerase chain reaction (qRT-PCR). Target genes of differentially expressed miRNAs were predicted using TargetScan software. The pathways related to predicted target genes were analyzed. mRNA levels of predicted target genes were estimated using qRT-PCR. ResultsThe expression of miR-15b-5p, miR-202-5p, and miR-212-3p was lower in the contralateral (left) IC of SSD rats than that of control rats. In SSD rats, miRNA expression levels in the contralateral IC were 0.45-, 0.25-, and 0.50-fold lower for miR-15b-5p, miR-202-5p, and miR-212-3p, respectively (P < 0.05). The expression of predicted target genes (Spred1, Rasa1, Lsm11, and Srsf1) was higher in the contralateral IC of SSD rats than in control rats. The targets were predicted to be related with cleavage of growing transcripts in the termination region, mitogen-activated protein kinase family signaling cascades, RAF/AMP kinase cascade, regulation of RAS by GTPase activating proteins (GAPs), and RNA polymerase II transcription termination. For ipsilateral ICs, miR-425-3p, miR-199a-5p, and miR-134-3p showed lower expressions in SSD rats than in control rats, which were 0.55-, 0.61-, and 0.69-fold lower, respectively (P < 0.05). The expression of predicted target genes (Atp2b2, Grin2b, Foxp1, Ztbt20, Zfp91, and Strn) was higher in the ipsilateral IC of SSD rats; the regulation of synaptic plasticity, cAMP signaling pathway, metal ion binding, and calcium ion transport can be associated with these target genes. ConclusionAdult rats with unilateral auditory deprivation showed miRNA changes in the IC. The contralateral IC showed decreased miRNA expression predicted to be related to MAPK and RAS signaling, whereas the ipsilateral IC revealed decreased miRNA expression predicted to be associated with synaptic plasticity and calcium ion transport.

Tonotopic distribution and inferior colliculus projection pattern of inhibitory and excitatory cell types in the lateral superior olive of Mongolian gerbils.

Sound localization critically relies on brainstem neurons that compare information from the two ears. The conventional role of the lateral superior olive (LSO) is extraction of intensity differences; however, it is increasingly clear that relative timing, especially of transients, is also an important function. Cellular diversity within the LSO that is not well understood may underlie its multiple roles. There are glycinergic inhibitory and glutamatergic excitatory principal neurons in the LSO, however, there is some disagreement regarding their relative distribution and projection pattern. Here we employ in situ hybridization to definitively identify transmitter types combined with retrograde labeling of projections to the inferior colliculus (IC) to address these questions. Excitatory LSO neurons were more numerous (76%) than inhibitory ones. A smaller proportion of inhibitory neurons were IC-projecting (45% vs. 64% for excitatory) suggesting that inhibitory LSO neurons may have more projections to other regions such the lateral lemniscus or more distributed IC projections. Inhibitory LSO neurons almost exclusively projected ipsilaterally making up a sizeable proportion (41%) of the transmitter type-labeled ipsilateral IC projection from LSO and exhibited a moderate low frequency bias (10% difference H-L). Two thirds of excitatory neurons projected contralaterally and had a slight high frequency bias (4%). One third of excitatory LSO neurons projected ipsilaterally to the IC and these cells were strongly biased toward the low frequency limb of the LSO (37%). This projection appears to be species specific in animals with good low frequency hearing suggesting that it may be a specialization for such ability.

Effect of Unilateral Acoustic Trauma on Neuronal Firing Activity in the Inferior Colliculus of Mice.

Neural hyperactivity induced by sound exposure often correlates with the development of hyperacusis and/or tinnitus. In laboratory animals, hyperactivity is typically induced by unilateral sound exposure to preserve one ear for further testing of hearing performance. Most ascending fibers in the auditory system cross into the superior olivary complex and then ascend contralaterally. Therefore, unilateral exposure should be expected to mostly affect the contralateral side above the auditory brain stem. On the other hand, it is well known that a significant number of neurons have crossing fibers at every level of the auditory pathway, which may spread the effect of unilateral exposure onto the ipsilateral side. Here we demonstrate that unilateral sound exposure causes development of hyperactivity in both the contra and ipsilateral inferior colliculus in mice. We found that both the spontaneous firing rate and bursting activity were increased significantly compared to unexposed mice. The neurons with characteristic frequencies at or above the center frequency of exposure showed the greatest increase. Surprisingly, this increase was more pronounced in the ipsilateral inferior colliculus. This study highlights the importance of considering both ipsi- and contralateral effects in future studies utilizing unilateral sound exposure.

Changes in the Gene Expression Profiles of the Inferior Colliculus Following Unilateral Cochlear Ablation in Adult Rats.

This study aimed to explore gene expression changes in the inferior colliculus (IC) after single-sided deafness (SSD). Forty 8-week-old female Sprague-Dawley rats were used. Twenty rats underwent right-side cochlear ablation, and IC tissues were harvested after 2weeks (SSD 2-week group). Twenty rats underwent a sham operation and were sacrificed after 2weeks (control group). Both sides of the IC were analyzed using a gene expression array. Pathway analyses were performed on genes that were differentially expressed compared with their levels in the control group. The expression levels of genes involved in the candidate pathways were confirmed using reverse transcription polymerase chain reaction (RT-PCR). Among the genes with ≥ 1.5-fold changes in expression levels and P < 0.05, there were 7 and 9 genes with increased and decreased expression, respectively, in the ipsilateral IC and 10 and 12 genes with increased and decreased expression, respectively, in the contralateral IC. The pathway analysis did not identify significantly related pathway. In the bilateral analysis, a total of 14 genes were ≥ 1.3-fold downregulated in both the ipsilateral and contralateral IC in the SSD 2-week group compared with their expression in the control group. Pathway analyses of these 14 genes included 7 genes, namely, amine compound solute carrier (Slc)5a7; Slc18a3; Slc6a5; synaptic vesicle glycoprotein 2C (Sv2c); S100 calcium binding protein A10 (S100a10); a gene with sequence similarity to family 111, member A (Fam111a); and peripherin (Prph), that were related to the acetylcholine neurotransmitter release cycle, SLC transporters, and the neurotransmitter release cycle pathways. RT-PCR showed reduced expression of Slc5a7, Sv2c, and Prph in the contralateral IC and Slc18a3 and Slc6a5 in the ipsilateral IC of the SSD 2-week group compared with that in the control group.

Dynamic Changes in Synaptic Plasticity Genes in Ipsilateral and Contralateral Inferior Colliculus Following Unilateral Noise-induced Hearing Loss

Unilateral noise-induced hearing loss reduces the input to the central auditory pathway disrupting the excitatory and inhibitory inputs to the inferior colliculus (IC), an important binaural processing center. Little is known about the compensatory synaptic changes that occur in the IC as a consequence of unilateral noise-induced hearing loss. To address this issue, Sprague–Dawley rats underwent unilateral noise exposure resulting in severe unilateral hearing loss. IC tissues from the contralateral and ipsilateral IC were evaluated for acute (2-d) and chronic (28-d) changes in the expression of 84 synaptic plasticity genes on a PCR array. Arc and Egr1 genes were further visualized by in situ hybridization to validate the PCR results. None of the genes were upregulated, but many were downregulated post-exposure. At 2-d post-exposure, more than 75% of the genes were significantly downregulated in the contralateral IC, while only two were downregulated in the ipsilateral IC. Many of the downregulated genes were related to long-term depression, long-term potentiation, cell adhesion, immediate early genes, neural receptors and postsynaptic density. At 28-d post-exposure, the gene expression pattern was reversed with more than 85% of genes in the ipsilateral IC now downregulated. Most genes previously downregulated in the contralateral IC 2-d post-exposure had recovered; less than 15% remained downregulated. These time-dependent, asymmetric changes in synaptic plasticity gene expression could shed new light on the perceptual deficits associated with unilateral hearing loss and the dynamic structural and functional changes that occur in the IC days and months following unilateral noise-induced hearing loss.

Bilateral projections to the thalamus from individual neurons in the inferior colliculus.

The medial geniculate body (MG) receives a large input from the ipsilateral inferior colliculus (IC) and a smaller but substantial input from the contralateral IC. Both crossed and uncrossed inputs comprise a large percentage of glutamatergic cells and a smaller percentage of GABAergic cells. We used double labeling with fluorescent retrograde tracers to identify individual IC cells that project bilaterally to the MGs in adult guinea pigs. We also used immunohistochemistry for glutamic acid decarboxylase to distinguish GABAergic from glutamatergic cells that project bilaterally to the MG. We found cells in the IC that contained both retrograde tracers, indicating that they project bilaterally. Across cases, the bilaterally projecting cells constituted up to 37% of the cells that project to the ipsilateral MG and up to 73% of the cells that project to the contralateral MG. GABAergic cells averaged 20% of the bilaterally-projecting population. We conclude that a population of IC cells sends branching axonal projections to innervate the MG bilaterally. Most of the neurons in this population are glutamatergic, with a minority that are GABAergic. A mixed projection, with glutamatergic cells outnumbering GABAergic cells, originates from each of the major IC subdivisions (central nucleus, dorsal cortex, and lateral cortex). The bilaterally projecting cells are likely to serve functions different from the larger unilateral projections, perhaps synchronizing activity on the two sides of the auditory brain.

Inhibitory Projections in the Mouse Auditory Tectothalamic System.

The medial geniculate body (MGB) is the target of excitatory and inhibitory inputs from several neural sources. Among these, the inferior colliculus (IC) is an important nucleus in the midbrain that acts as a nexus for auditory projections, ascending and descending, throughout the rest of the central auditory system and provides both excitatory and inhibitory inputs to the MGB. In our study, we assessed the relative contribution from presumed excitatory and inhibitory IC neurons to the MGB in mice. Using retrograde tract tracing with cholera toxin beta subunit (CTβ)-Alexa Fluor 594 injected into the MGB of transgenic, vesicular GABA transporter (VGAT)-Venus mice, we quantitatively analyzed the projections from both the ipsilateral and contralateral IC to the MGB. Our results demonstrate inhibitory projections from both ICs to the MGB that likely play a significant role in shaping auditory processing. These results complement prior studies in other species, which suggest that the inhibitory tectothalamic pathway is important in the regulation of neuronal activity in the auditory forebrain.

Effects of unilateral acoustic trauma on tinnitus-related spontaneous activity in the inferior colliculus.

This study describes the long-term effects of sound-induced cochlear trauma on spontaneous discharge rates in the central nucleus of the inferior colliculus (ICC). As in previous studies, single-unit recordings in Sprague-Dawley rats revealed pervasive increases in spontaneous discharge rates. Based on differences in their sources of input, it was hypothesized that physiologically defined neural populations of the auditory midbrain would reveal the brainstem sources that dictate ICC hyperactivity. Abnormal spontaneous activity was restricted to target neurons of the ventral cochlear nucleus. Nearly identical patterns of hyperactivity were observed in the contralateral and ipsilateral ICC. The elevation in spontaneous activity extended to frequencies well below and above the region of maximum threshold shift. This lack of frequency organization suggests that ICC hyperactivity may be influenced by regions of the brainstem that are not tonotopically organized. Sound-induced hyperactivity is often observed in animals with behavioral signs of tinnitus. Prior to electrophysiological recording, rats were screened for tinnitus by measuring gap pre-pulse inhibition of the acoustic startle reflex (GPIASR). Rats with positive phenotypes did not exhibit unique patterns of ICC hyperactivity. This ambiguity raises concerns regarding animal behavioral models of tinnitus. If our screening procedures were valid, ICC hyperactivity is observed in animals without behavioral indications of the disorder. Alternatively, if the perception of tinnitus is strictly linked to ongoing ICC hyperactivity, our current behavioral approach failed to provide a reliable assessment of tinnitus state.

Behavioral consequences of unilateral inferior colliculus lesions in the rat

This study was carried out to determine the behavioral sensitivity to sound of rats with unilateral lesions of inferior colliculus (IC) located ipsilateral or contralateral to the projection pathway from one ear. Absolute thresholds for the detection of a broad-band noise burst were compared for rats with a profound conductive hearing loss in one ear and a lesion placed either ipsilateral or contralateral to the normally functioning ear. The rats were trained to make withdrawal responses to avoid a shock when they detected the presence of a noise burst. Sound pressure level was systematically lowered to obtain psychophysical curves from which absolute thresholds could be determined. Complete lesions of the contralateral IC resulted in substantial elevations in absolute threshold relative to normal whereas equivalent lesions of the ipsilateral IC produced relatively little elevation. In neither case did unilateral destruction of the IC produce a total inability to respond to sound. Contralateral IC lesions that included the dorsal nucleus of the lateral lemniscus (DNLL) produced a significantly greater elevation in behavioral thresholds than complete lesions limited to the IC. The results indicate a predominance of the contralateral over the ipsilateral pathway to IC for maintaining normal thresholds. They also indicate that other pathways that bypass the IC are likely involved in detecting the presence of a sound.

Bilateral Collicular Interaction: Modulation of Auditory Signal Processing in Amplitude Domain

In the ascending auditory pathway, the inferior colliculus (IC) receives and integrates excitatory and inhibitory inputs from many lower auditory nuclei, intrinsic projections within the IC, contralateral IC through the commissure of the IC and from the auditory cortex. All these connections make the IC a major center for subcortical temporal and spectral integration of auditory information. In this study, we examine bilateral collicular interaction in modulating amplitude-domain signal processing using electrophysiological recording, acoustic and focal electrical stimulation. Focal electrical stimulation of one (ipsilateral) IC produces widespread inhibition (61.6%) and focused facilitation (9.1%) of responses of neurons in the other (contralateral) IC, while 29.3% of the neurons were not affected. Bilateral collicular interaction produces a decrease in the response magnitude and an increase in the response latency of inhibited IC neurons but produces opposite effects on the response of facilitated IC neurons. These two groups of neurons are not separately located and are tonotopically organized within the IC. The modulation effect is most effective at low sound level and is dependent upon the interval between the acoustic and electric stimuli. The focal electrical stimulation of the ipsilateral IC compresses or expands the rate-level functions of contralateral IC neurons. The focal electrical stimulation also produces a shift in the minimum threshold and dynamic range of contralateral IC neurons for as long as 150 minutes. The degree of bilateral collicular interaction is dependent upon the difference in the best frequency between the electrically stimulated IC neurons and modulated IC neurons. These data suggest that bilateral collicular interaction mainly changes the ratio between excitation and inhibition during signal processing so as to sharpen the amplitude sensitivity of IC neurons. Bilateral interaction may be also involved in acoustic-experience-dependent plasticity in the IC. Three possible neural pathways underlying the bilateral collicular interaction are discussed.

Cortical Auditory Deafferentation Induces Long-Term Plasticity in the Inferior Colliculus of Adult Rats: Microarray and qPCR Analysis

The cortico-collicular pathway is a bilateral excitatory projection from the cortex to the inferior colliculus (IC). It is asymmetric and predominantly ipsilateral. Using microarrays and RT-qPCR we analyzed changes in gene expression in the IC after unilateral lesions of the auditory cortex, comparing the ICs ipsi- and contralateral to the lesioned side. At 15 days after surgery there were mainly changes in gene expression in the IC ipsilateral to the lesion. Regulation primarily involved inflammatory cascade genes, suggesting a direct effect of degeneration rather than a neuronal plastic reorganization. Ninety days after the cortical lesion the ipsilateral IC showed a significant up-regulation of genes involved in apoptosis and axonal regeneration combined with a down-regulation of genes involved in neurotransmission, synaptic growth, and gap junction assembly. In contrast, the contralateral IC at 90 days post-lesion showed an up-regulation in genes primarily related to neurotransmission, cell proliferation, and synaptic growth. There was also a down-regulation in autophagy and neuroprotection genes. These findings suggest that the reorganization in the IC after descending pathway deafferentation is a long-term process involving extensive changes in gene expression regulation. Regulated genes are involved in many different neuronal functions, and the number and gene rearrangement profile seems to depend on the density of loss of the auditory cortical inputs.

Cholinergic cells in the tegmentum send branching projections to the inferior colliculus and the medial geniculate body

The pontomesencephalic tegmentum (PMT) provides cholinergic input to the inferior colliculus (IC) and the medial geniculate body (MG). PMT cells are often characterized as projecting to more than one target. The purpose of this study was to determine whether individual PMT cholinergic cells, (1) innervate the auditory pathways bilaterally via collateral projections to left and right auditory thalamus; or, (2) innervate multiple levels of the auditory pathways via collateral projections to the auditory thalamus and inferior colliculus. We used multiple retrograde tracers to identify individual PMT cells that project to more than one target. We combined the retrograde tracer studies with immunohistochemistry for choline acetyltransferase to determine whether the projecting cells were cholinergic. We found that individual PMT cells send branching axonal projections to two or more auditory targets in the midbrain and thalamus. The collateral projection pattern that we observed most frequently was to the ipsilateral IC and ipsilateral MG. Cells projecting to both MGs were somewhat less common, followed by cells projecting to the contralateral IC and ipsilateral MG. Both cholinergic and non-cholinergic cells contribute to each of these projection patterns. Less often, we found cells that project to one IC and both MGs; there was no evidence for non-cholinergic cells in this projection pattern. It is likely that collateral projections from PMT cells could have coordinated effects bilaterally and at multiple levels of the ascending auditory pathways.

Connections of the Superior Paraolivary Nucleus of the Rat: II. Reciprocal Connections with the Tectal Longitudinal Column

The superior paraolivary nucleus (SPON), a prominent GABAergic center of the mammalian auditory brainstem, projects to the ipsilateral inferior colliculus (IC) and sends axons through the commissure of the IC (CoIC). Herein we demonstrate that the SPON is reciprocally connected with the recently discovered tectal longitudinal column (TLC). The TLC is a long and narrow structure that spans nearly the entire midbrain tectum longitudinally, immediately above the periaqueductal gray matter (PAG) and very close to the midline. Unilateral injections of biotinylated dextran into the SPON of the rat label abundant terminal fibers in the TLC of both sides, with an ipsilateral predominance. The SPON provides a dense innervation of the entire rostrocaudal extent of the ipsilateral TLC, and a relatively sparser innervation of the caudal and rostral portions of the contralateral TLC. SPON fibers reach the TLC by two routes: as collaterals of axons of the CoIC, and as axons that circumvent the ipsilateral IC before traveling in the deep layers of the superior colliculus (SC). The density of these projections identifies SPON as a significant source of input to the TLC. Other targets of the SPON discovered in this study include the deep layers of the SC and the PAG. The same experiments reveal numerous labeled cell bodies in the TLC, interspersed among the labeled SPON fibers. This observation suggests that the SPON is a significant target of TLC projections. The discovery of novel reciprocal connections between the SPON and the TLC opens unexpected avenues for investigation of sound processing in mammalian brainstem circuits.

Projections from auditory cortex to midbrain cholinergic neurons that project to the inferior colliculus

We have shown that auditory cortex projects to cholinergic cells in the pedunculopontine tegmental nucleus (PPT) and laterodorsal tegmental nucleus (LDT). PPT and LDT are the sources of cholinergic projections to the inferior colliculus, but it is not known if the cortical inputs contact the cholinergic cells that project to the inferior colliculus. We injected FluoroRuby into auditory cortex in pigmented guinea pigs to label cortical projections to PPT and LDT. In the same animals, we injected Fast Blue into the left or right inferior colliculus to label PPT and LDT cells that project to the inferior colliculus. We processed the brain to identify cholinergic cells with an antibody to choline acetyltransferase, which was visualized with a green fluorescent marker distinguishable from both FluoroRuby and Fast Blue. We then examined the PPT and LDT to determine whether boutons of FluoroRuby-labeled cortical axons were in close contact with cells that were double-labeled with the retrograde tracer and the immunolabel. Apparent contacts were observed ipsilateral and, less often, contralateral to the injected cortex. On both sides, the contacts were more numerous in PPT than in LDT. The results indicate that auditory cortex projects directly to brainstem cholinergic cells that innervate the ipsilateral or contralateral inferior colliculus. This suggests that cortical projections could elicit cholinergic effects on both sides of the auditory midbrain.

Connections of the superior paraolivary nucleus of the rat: projections to the inferior colliculus

GABAergic neurotransmission contributes to shaping the response properties of inferior colliculus (IC) neurons. In rodents, the superior paraolivary nucleus (SPON) is a prominent and well-defined cell group of the superior olivary complex that sends significant but often neglected GABAergic projections to the IC. To investigate the trajectory, distribution and morphology of these projections, we injected the neuroanatomical tracer biotinylated dextran amine into the SPON of albino rats. Our results demonstrate that: (1) the SPON innervates densely all three subdivisions of the ipsilateral IC: central nucleus (CNIC), dorsal cortex (DCIC) and external cortex (ECIC). The SPON also sends a sparse projection to the contralateral DCIC via the commissure of the IC. (2) SPON axons are relatively thick (diameter >1.2 μm), ascend to the midbrain tectum in the medial aspect of the lateral lemniscus, and, for the most part, do not innervate the nuclei of the lateral lemniscus. (3) SPON fibers ramify profusely within the IC and bear abundant en passant and terminal boutons. (4) The axons of neurons in discrete regions of the SPON form two laminar terminal plexuses in the ipsilateral IC: a medial plexus that spans the CNIC and DCIC parallel to the known fibrodendritic laminae of the CNIC, and a lateral plexus located in the ECIC and oriented more or less parallel to the surface of the IC. (5) The projection from SPON to the ipsilateral IC is topographic: medial SPON neurons innervate the ventromedial region of the CNIC and DCIC and the ventrolateral region of the ECIC, whereas more laterally situated SPON neurons innervate more dorsolateral regions of the CNIC and DCIC and more dorsomedial regions of the ECIC. Thus, SPON fibers follow a pattern of distribution within the IC similar to that previously reported for intracollicular and corticocollicular projections.

Sources of cholinergic input to the inferior colliculus

We combined retrograde tracing with immunohistochemistry for choline acetyltransferase to identify the source of cholinergic input to the inferior colliculus (IC) in guinea pigs. Injection of a retrograde tracer into one IC labeled cells in many brainstem nuclei. Retrogradely-labeled cells that were also immunoreactive for choline acetyltransferase were identified in two nuclei in the midbrain tegmentum: the pedunculopontine tegmental nucleus (PPT) and the laterodorsal tegmental nucleus (LDT). More PPT and LDT cells project ipsilaterally than contralaterally to the IC and, on both sides, there are more projecting cells in the PPT than in the LDT. Double-labeled cells were not found in any other brainstem nucleus. A common feature of cholinergic cells in PPT and LDT is collateral projections to multiple targets. We placed different retrograde tracers into each IC to identify cells in PPT and LDT that project to both ICs. In both PPT and LDT, a substantial proportion (up to 57%) of the immunoreactive cells that contained tracer from the contralateral IC also contained tracer from the ipsilateral IC. We conclude that acetylcholine in the IC originates from the midbrain tegmental cholinergic nuclei: PPT and LDT. These nuclei are known to participate in arousal, the sleep/wake cycle and prepulse inhibition of acoustic startle. It is likely that the cholinergic input to the IC is directly associated with these functions.

Projections to the inferior colliculus from layer VI cells of auditory cortex

A large injection of a retrograde tracer into the inferior colliculus of guinea pigs labeled two bands of cells in the ipsilateral auditory cortex: a dense band of cells in layer V and a second band of cells in layer VI. On the contralateral side, labeled cells were restricted to layer V. The ipsilateral layer VI cells were distributed throughout temporal cortex, suggesting projections from multiple auditory areas. The layer VI cells included pyramidal cells as well as several varieties of non-pyramidal cells. Small tracer injections restricted to the dorsal cortex or external cortex of the inferior colliculus consistently labeled cells in layer VI. Injections restricted to the central nucleus of the inferior colliculus labeled layer VI cells only rarely. Overall, 10% of the cells in temporal cortex that project to the ipsilateral inferior colliculus were located in layer VI, suggesting that layer VI cells make a significant contribution to the corticocollicular pathway.