ABSTRACT: The inferior colliculus (IC) and the locus coeruleus (LC) are two midbrain nuclei that integrate multimodal information and play a major role in novelty detection to elicit an orienting response. Despite the reciprocal connections between these two structures, the projection pattern and target areas of the LC within the subdivisions of the rat IC are still unknown. Here, we used tract-tracing approaches combined with immunohistochemistry, densitometry, and confocal microscopy (CM) analysis to describe a projection from the LC to the IC. Biotinylated dextran amine (BDA) injections into the LC showed that the LC-IC projection is mainly ipsilateral (90%) and reaches, to a major extent, the dorsal and lateral part of the IC and the intercollicular commissure. Additionally, some LC fibers extend into the central nucleus of the IC. The neurochemical nature of this projection is noradrenergic, given that tyrosine hydroxylase (TH) and dopamine beta hydroxylase (DBH) colocalize with the BDA-labeled fibers from the LC. To determine the total field of the LC innervations in the IC, we destroyed the LC neurons and fibers using a highly selective neurotoxin, DSP-4, and then studied the distribution and density of TH- and DBH-immunolabeled axons in the IC. In the DSP-4 treated animals, the number of axonal fibers immunolabeled for TH and DBH were deeply decreased throughout the entire rostrocaudal extent of the IC and its subdivisions compared to controls. Our densitometry results showed that the IC receives up to 97% of its noradrenergic innervations from the LC neurons and only 3% from non-coeruleus neurons. Our results also indicate that TH immunoreactivity in the IC was less impaired than the immunoreactivity for DBH after DSP-4 administration. This is consistent with the existence of an important dopaminergic projection from the substantia nigra to the IC. In conclusion, our study demonstrates and quantifies the noradrenergic projection from the LC to the IC and its subdivisions. The re-examination of the TH and DBH immunoreactivity after DSP-4 treatment provides insights into the source, extent, and topographic distribution of the LC efferent network in the IC, and hence, contributes to our understanding of the role of the noradrenaline (NA) system in auditory processing.
Frontiers in Neural Circuits 01/2012; 6:41. · 5.10 Impact Factor
ABSTRACT: Cochlear root neurons (CRNs) provide short-latency acoustic inputs to the caudal pontine reticular nucleus that elicit an
acoustic startle reflex (ASR). Auditory prepulse inhibition (PPI) of the acoustic startle response is the reduction in ASR
magnitude that is observed when a strong acoustic startling stimulus (pulse) is shortly preceded by a weak sound (prepulse).
It has been suggested that a short descending auditory pathway conveys auditory prepulses to the level of the CRNs to mediate
the inhibition of ASR. However, no electrophysiological data is available to confirm such inhibition. We here investigated
the effects of auditory prepulses on the neuronal activity of CRNs using extracellular recordings in vivo from single CRNs.
Our results show that CRN responses are strongly inhibited by auditory prepulses. As occurs in the behavioral paradigm, the
inhibition of the CRN responses depended on parameters of the auditory prepulse, such as intensity and interstimulus interval,
showing their strongest inhibition at high intensity level and short interstimulus intervals. Furthermore, we tested the auditory
PPI on the activity of different neuron types in the ventral cochlear nucleus. Of all neuron types tested, only CRNs exhibited
a strong attenuation of activity. Our results corroborate our previous hypothesis that CRNs might be involved in the neural
circuit of the inhibition of the ASR and suggest that several neuronal pathways participate in that circuit.
KeywordsSound processing-Cochlear root neurons-Extracellular recordings-Ventral nucleus of the trapezoid body
12/2010: pages 79-90; , ISBN: ISBSN 978-1-4419-5685-9
ABSTRACT: Geometry of the dendritic tree and synaptic organization of afferent inputs are essential factors in determining how synaptic input is integrated by neurons. This information remains elusive for one of the first brainstem neurons involved in processing of the primary auditory signal from the ear, the bushy cells (BCs) of the ventral cochlear nucleus (VCN). Here, we labeled the BC dendritic trees with retrograde tracing techniques to analyze their geometry and synaptic organization after immunofluorescence for excitatory and inhibitory synaptic markers, electron microscopy, morphometry, double tract-tracing methods, and 3D reconstructions. Our study revealed that BC dendrites provide space for a large number of compartmentalized excitatory and inhibitory synaptic interactions. The dendritic inputs on BCs are of cochlear and noncochlear origin, and their proportion and distribution are dependent on the branching pattern and orientation of the dendritic tree in the VCN. Three-dimensional reconstructions showed that BC dendrites branch and cluster with those of other BCs in the core of the VCN. Within the cluster, incoming synaptic inputs establish divergent multiple-contact synapses (dyads and triads) between BCs. Furthermore, neuron-neuron connections including puncta adherentia, sarcoplasmic junctions, and gap junctions are common between BCs, which suggests that these neurons are electrically coupled. Overall, our study demonstrates the existence of a BC network in the rat VCN. This network may establish the neuroanatomical basis for acoustic information processing by individual BCs as well as for enhanced synchronization of the output signal of the VCN.
The Journal of Comparative Neurology 07/2009; 516(4):241-63. · 3.81 Impact Factor
ABSTRACT: Brain stem pathways are essential for the modulation of the acoustic startle reflex by sounds; nevertheless, the neural circuits that convey fast auditory information to the primary acoustic startle circuit are still unclear. In the rat, cochlear root neurons (CRNs) comprise the first component of the primary acoustic startle circuit and are critical in the initiation and full expression of the acoustic startle reflex. To determine whether CRNs receive auditory descending inputs, we developed tract-tracing studies combined with immunohistochemistry, electron microscopy, morphometry, and confocal microscopy. Either FluoroGold or biotinylated dextran amine (BDA) injections in CRNs showed retrogradely labeled neurons in the ventral nucleus of the trapezoid body (VNTB). We verified the projection to CRNs by injecting BDA into the VNTB. Our results showed that neurons from VNTB project bilaterally and directly to CRNs, giving off numerous endings onto cell bodies and preferentially dendrites of CRNs. Electron microscopy analysis of labeled VNTB terminals demonstrated that they made multiple symmetric synapses and contained small round vesicles. Colocalization of the vesicular acetylcholine transporter and fluorescein dextran after injection in the VNTB indicated that these terminals use acetylcholine as neurotransmitter. We also revealed that the inferior colliculus, an important nucleus mediating the auditory prepulse inhibition, projects to VNTB neurons that innervate CRNs. Our data show a novel and short descending auditory pathway from the VNTB to the first nucleus of the primary acoustic startle circuit that might play an important role in the auditory prepulse inhibition of the startle reflex elicited by sounds.
The Journal of Comparative Neurology 02/2008; 506(3):452-68. · 3.81 Impact Factor