Topography of projections from the auditory cortex to the inferior colliculus in the rat

Projection to the inferior colliculus from the basal nucleus of the amygdala

This report describes a projection from the amygdala, a forebrain center mediating emotional expression, to the inferior colliculus (IC), the midbrain integration center of the ascending auditory system. In the IC of mustached bats (Pteronotus parnellii) and pallid bats (Antrozous pallidus), we placed deposits of retrograde tracers at physiologically defined sites and then searched for retrogradely labeled somata in the forebrain. Labeling was most sensitive in experiments using cholera toxin B-subunit as tracer. We consistently observed retrograde labeling in a single amygdalar subdivision, the magnocellular subdivision of the basal nucleus (Bmg). The Bmg is distinctive across mammals, containing the largest cells in the amygdala and the most intense acetylcholinesterase staining. Labeled amygdalar cells occurred ipsilateral and contralateral to IC deposits, but ipsilateral labeling was greater, averaging 72%. Amygdalar labeling was observed after tracer deposits throughout the IC, including its central nucleus (ICC). In comparison, labeling in the auditory cortex (layer V) was heavily ipsilateral (averaging 92%). Cortical labeling depended on the location of IC deposits: dorsomedial deposits resulted in the most labeled cells, whereas ventrolateral deposits labeled few or no cortical cells. Cortical labeling occurred after several deposits in the ICC. Across experiments, the average number of labeled cells in the amygdala was similar to that in the auditory cortex, indicating that the amygdalocollicular projection is significant. The results demonstrate a direct, widespread projection from the basal amygdala to the IC. They also suggest the presence of a rapid thalamoamygdalocollicular feedback circuit that may impose emotional content onto processing of sensory stimuli at a relatively low level of an ascending sensory pathway.

Temporal processing speed in the inferior colliculus of young and aged rats

A common problem among the elderly is a difficulty in discriminating speech. One factor that may contribute to this is a deterioration in the ability to process dynamic aspects of speech such as formant transitions. Recently, Mendelson and Ricketts [Mendelson, J.R., Ricketts, C., Hear. Res. 158 (2001) 84-94] showed that cells recorded from the auditory cortex of aged animals exhibited a decrease in temporal processing speed compared to young animals. In the present study, we examined whether this age-related effect was exclusive to the auditory cortex or whether it was apparent subcortically. To this end, single units were recorded from the inferior colliculus (IC) of young and aged rats in response to frequency modulated (FM) sweeps. Results showed that there was no age-related difference in speed or direction selectivity of FM sweep responses in the IC. The present results suggest that the effect of aging on temporal processing speed occurs in the cortex, but not subcortically.

Effects of paired-pulse and repetitive stimulation on neurons in the rat medial geniculate body

Many behaviorally relevant sounds, including language, are composed of brief, rapid, repetitive acoustic features. Recent studies suggest that abnormalities in producing and understanding spoken language are correlated with abnormal neural responsiveness to such auditory stimuli at higher auditory levels [Tallal et al., Science 271 (1996) 81-84; Wright et al., Nature 387 (1997) 176-178; Nagarajan et al., Proc. Natl. Acad. Sci. USA 96 (1999) 6483-6488] and with abnormal anatomical features in the auditory thalamus [Galaburda et al., Proc. Natl. Acad. Sci. USA 91 (1994) 8010-8013]. To begin to understand potential mechanisms for normal and abnormal transfer of sensory information to the cortex, we recorded the intracellular responses of medial geniculate body thalamocortical neurons in a rat brain slice preparation. Inferior colliculus or corticothalamic axons were excited by pairs or trains of electrical stimuli. Neurons receiving only excitatory collicular input had tufted dendritic morphology and displayed strong paired-pulse depression of their large, short-latency excitatory postsynaptic potentials. In contrast, geniculate neurons receiving excitatory and inhibitory collicular inputs could have stellate or tufted morphology and displayed much weaker depression or even paired-pulse facilitation of their smaller, longer-latency excitatory postsynaptic potentials. Depression was not blocked by ionotropic glutamate, GABA(A) or GABA(B) receptor antagonists. Facilitation was unaffected by GABA(A) receptor antagonists but was diminished by N-methyl-D-aspartate (NMDA) receptor blockade. Similar stimulation of the corticothalamic input always elicited paired-pulse facilitation. The NMDA-independent facilitation of the second cortical excitatory postsynaptic potential lasted longer and was more pronounced than that seen for the excitatory collicular inputs. Paired-pulse stimulation of isolated collicular inhibitory postsynaptic potentials generated little change in the second GABA(A) potential amplitude measured from the resting potential, but the GABA(B) amplitude was sensitive to the interstimulus interval. Train stimuli applied to collicular or cortical inputs generated intra-train responses that were often predicted by their paired-pulse behavior. Long-lasting responses following train stimulation of the collicular inputs were uncommon. In contrast, corticothalamic inputs often generated long-lasting depolarizing responses that were dependent on activation of a metabotropic glutamate receptor. Our results demonstrate that during repetitive afferent firing there are input-specific mechanisms controlling synaptic strength and membrane potential over short and long time scales. Furthermore, they suggest that there may be two classes of excitatory collicular input to medial geniculate neurons and a single class of small-terminal corticothalamic inputs, each of which has distinct features.

Modulatory effect of cortical activation on the lemniscal auditory thalamus of the Guinea pig

In the present study, we investigated the point-to-point modulatory effects from the auditory cortex to the thalamus in the guinea pig. Corticofugal modulation on thalamic neurons was studied by electrical activation of the auditory cortex. The modulation effect was sampled along the frontal or sagittal planes of the auditory thalamus, focusing on the ventral division (MGv) of the medial geniculate body (MGB). Electrical activation was targeted at the anterior and dorsocaudal auditory fields, to which the MGv projects and from which it assumptively receives reciprocal projections. Of the 101 MGv neurons examined by activation of the auditory cortex through passing pulse trains of 100-200 microA current into one after another of the three implanted electrodes (101 neurons x 3 stimulation sites = 303 cases), 208 cases showed a facilitatory effect, 85 showed no effect, and only 10 cases (7 neurons) showed an inhibitory effect. Among the cases of facilitation, 63 cases showed a facilitatory effect >100%, and 145 cases showed a facilitatory effect from 20-100%. The corticofugal modulatory effect on the MGv of the guinea pig showed a widespread, strong facilitatory effect and very little inhibitory effect. The MGv neurons showed the greatest facilitations to stimulation by the cortical sites, with the closest correspondence in BF. Six of seven neurons showed an elevation of the rate-frequency functions when the auditory cortex was activated. The comparative results of the corticofugal modulatory effects on the MGv of the guinea pig and the cat, together with anatomical findings, hint that the strong facilitatory effect is generated through the strong corticothalamic direct connection and that the weak inhibitory effect might be mainly generated via the interneurons of the MGv. The temporal firing pattern of neuronal response to auditory stimulus was also modulated by cortical stimulation. The mean first-spike latency increased significantly from 15.7 +/- 5.3 ms with only noise-burst stimulus to 18.3 +/- 4.9 ms (n = 5, P < 0.01, paired t-test), while the auditory cortex was activated with a train of 10 pulses. Taking these results together with those of previous experiments conducted on the cat, we speculate that the relatively weaker inhibitory effect compared with that in the cat could be due to the smaller number of interneurons in the guinea pig MGB. The corticofugal modulation of the firing pattern of the thalamic neurons might enable single neurons to encode more auditory information using not only the firing rate but also the firing pattern.

Interaction of excitation and inhibition in inferior collicular neurons of the big brown bat, Eptesicus fuscus

Neurons in the inferior colliculus (IC) of the midbrain receive convergent excitatory and inhibitory inputs from both lower and higher auditory nuclei. Interaction of these two opposing inputs shapes different response properties of IC neurons. In this study, we examined this interaction of excitation and inhibition in IC neurons using a probe (excitatory pulse) and a masker (inhibitory pulse) under different stimulation conditions. Inhibition of probe-elicited responses by a masker, i.e. masking, occurred when the masker was presented at certain inter-pulse intervals (the temporal window) in relation to the probe. At the best inter-pulse interval, masking was maximal such that a neuron had the minimal number of impulses, the longest response latency, and the smallest excitatory frequency tuning curve. The temporal window for masking expanded with increasing masker duration. The inhibition decreased with increasing probe intensity but increased with increasing masker intensity. Increasing masker intensity also produced progressive shrinkage in excitatory frequency tuning curves. Similarly, increasing probe intensity produced progressive shrinkage of inhibitory frequency tuning curves. Possible mechanisms underlying the time and intensity dependence of inhibition are discussed.

Corticofugal modulation of midbrain sound processing in the house mouse

An understanding of the neural mechanisms responsible for auditory information processing is incomplete without a careful examination of substantial descending pathways. This study focuses on the functional role of corticofugal projections. Our work with the house mouse reveals that the focal electrical stimulation of the primary auditory cortex leads to profound changes in auditory response properties in the central nucleus of the inferior colliculus of the midbrain. Cortical stimulation does not impact on the collicular best frequencies when the best frequencies of stimulated cortical neurons and recorded collicular neurons are similar. Rather, collicular best frequencies are shifted toward the stimulated cortical best frequencies when cortical and collicular frequencies are different. Such a shift is unrelated to the differences in minimum thresholds between cortical and collicular neurons. In addition to frequency-specific shifts in collicular best frequencies, cortical stimulation elevates collicular minimum thresholds and reduces both dynamic ranges and response magnitudes if cortical and collicular best frequencies are different. If cortical and collicular best frequencies are similar but minimum thresholds are different, collicular minimum thresholds are shifted toward the stimulated cortical thresholds; dynamic ranges and response magnitudes may either increase or decrease in this scenario. Our results suggest that the corticofugal adjustment has a centre-surround organization with regard to both cortical best frequencies and cortical minimum thresholds. The midbrain processing of sound components in the centre of cortical feedback is largely enhanced while processing in the surround is suppressed.

Descending projections to the inferior colliculus from the posterior thalamus and the auditory cortex in rat, cat, and monkey

Projections from the posterior thalamus and medial geniculate body were labeled retrogradely with wheat germ agglutinin conjugated to horseradish peroxidase injected into the rat, cat, and squirrel monkey inferior colliculus. Neurons were found ipsilaterally in the (1) medial division of the medial geniculate body, (2) central gray, (3) posterior limitans nucleus, and the (4) reticular part of the substantia nigra. Bilateral projections involved the (5) peripeduncular/suprapeduncular nucleus, (6) subparafascicular and posterior intralaminar nuclei, (7) nucleus of the brachium of the inferior colliculus, (8) lateral tegmental/lateral mesencephalic areas, and (9) deep layers of the superior colliculus. The medial geniculate projection was concentrated in the caudal one-third of the thalamus; in contrast, the labeling in the subparafascicular nucleus, substantia nigra, and central gray continued much further rostrally. Robust anterograde labeling corresponded to known patterns of tectothalamic projection. Biotinylated dextran amine deposits in the rat inferior colliculus revealed that (1) many thalamotectal cells were elongated multipolar neurons with long, sparsely branched dendrites, resembling neurons in the posterior intralaminar system, and that other labeled cells were more typical of thalamic relay neurons; (2) some cells have reciprocal projections. Similar results were seen in the cat and squirrel monkey. The widespread origins of descending thalamic influences on the inferior colliculus may represent a phylogenetically ancient feedback system onto the acoustic tectum, one that predates the corticocollicular system and modulates nonauditory centers and brainstem autonomic nuclei. Besides their role in normal hearing such pathways may influence behaviors ranging from the startle reflex to the genesis of sound-induced seizures.

Brief and short-term corticofugal modulation of acoustic signal processing in the bat midbrain

This article reviews our recent studies of brief and short-term corticofugal modulation of signal processing in the central nucleus of the inferior colliculus (ICc) by electrical stimulation in the primary auditory cortex (AC). When cortical electrical stimulation was synchronized with an acoustic stimulus, auditory responses of ICc neurons were either inhibited or facilitated and the modulative effect typically vanished within 5-10 s after the stimulation. When cortical electrical stimulation synchronized with an acoustic stimulus was repetitively delivered for 30 min, corticofugal modulation of collicular responses typically persisted up to 40 min after the stimulation. In the frequency domain, cortical electrical stimulation decreased the excitatory frequency tuning curves (FTCs) and asymmetrically increased the lateral inhibitory FTCs of corticofugally inhibited ICc neurons but produced the opposite effect on corticofugally facilitated ICc neurons. Cortical electrical stimulation facilitated auditory responses of neurons in the external nucleus of the inferior colliculus (ICx) while electrical stimulation in the ICx decreased auditory responses of ICc neurons. Auditory responses of simultaneously recorded ICx and ICc neurons varied in opposite ways during cortical electrical stimulation or drug application to recorded ICx neurons. In the amplitude domain, cortical electrical stimulation compressed rate-amplitude functions so as to increase the slope of rate-amplitude functions of ICc neurons. This modulative effect decreased with increasing stimulus amplitude. The possible biological relevance of these findings is discussed.

Serotonin in the inferior colliculus

It has been recognized for some time that serotonin fibers originating in raphe nuclei are present in the inferior colliculi of all mammalian species studied. More recently, serotonin has been found to modulate the responses of single inferior colliculus neurons to many types of auditory stimuli, ranging from simple tone bursts to complex species-specific vocalizations. The effects of serotonin are often quite strong, and for some neurons are also highly specific. A dramatic illustration of this is that serotonin can change the selectivity of some neurons for sounds, including species-specific vocalizations. These results are discussed in light of several theories on the function of serotonin in the IC, and of outstanding issues that remain to be addressed.

The effect of two-tone stimulation on responses of two simultaneously recorded neurons in the inferior colliculus of the big brown bat, Eptesicus fuscus

This study examined auditory responses of two simultaneously recorded neurons in the central nucleus of bat inferior colliculus (IC) under two-tone stimulation conditions. We specifically examined how a sound within the excitatory frequency tuning curve (FTC) of one IC neuron might affect responses of the other IC neuron in amplitude and frequency domains. Under this specific two-tone stimulation condition, responses of 82% neurons were suppressed and their excitatory FTCs sharpened. Responses of the other 18% neurons were facilitated and their excitatory FTCs broadened. Two-tone suppression was greater at low than at high stimulus amplitudes. Two-tone suppression also decreased with increasing recording depth and best frequency (BF) difference between each pair of neurons. The suppressive or facilitatory FTC of a neuron plotted under two-tone stimulation conditions was always within the excitatory FTC of the other neuron. Two-tone suppression or two-tone facilitation was weak near the BF but became increasingly strong with frequencies away from the BF. Biological significance of these findings is discussed.

The cellular origin of corticofugal projections to the superior olivary complex in the rat

Corticofugal pathways originating in auditory cortex innervate most subcortical auditory nuclei in the ascending pathway [Auditory Neurosci. 1 (1995) 287-308; J. Comp. Neurol. 371 (1996) 15-40]. Our goal is to determine if these projections arise from the same neurons or if different neurons project to each of the separate structures. We also seek to identify the layers and fields of auditory cortex from which these neurons originate. In the present study, we answer these questions with respect to the projections to the superior olivary complex (SOC). Fluorescent retrograde tracers, Fast Blue (FB) or Diamidino Yellow (DiY), were injected into the SOC and the pattern of labeled cells was determined in temporal neocortex. We also injected FB into the granule cell domain (GCD) of the cochlear nucleus. Cortical projections to the GCD derive exclusively from layer V pyramidal cells in primary auditory cortex [Brain Res. 706 (1996) 97-102]. Thus the pattern of labeling produced by injections in the GCD provided a reference for interpreting the labeling after SOC injections. Layer V pyramidal cells project to the SOC, and these neurons were distributed bilaterally in primary and secondary areas of auditory cortex. The projections to the SOC from primary auditory cortex are predominantly uncrossed, whereas those from secondary auditory cortex are nearly equal for the two hemispheres. In animals that received injections of FB in the GCD and DiY in the SOC, cells labeled by each injection had a different laminar distribution and very few cells were double labeled. These data suggest that the cortical pathways ending in the cochlear nucleus and SOC are largely independent. We discuss the implications of these findings with respect to the multifunctional nature of the SOC in brainstem auditory processing.

Effect of auditory cortex lesions on NADPH-diaphorase staining in the inferior colliculus of rat

Projections from the auditory cortex (AC) in the rat terminate in the dorsal cortex (DC) and in the external cortex (EC) of the inferior colliculus (IC), areas which exhibit a moderate number of nicotinamide-adenine dinucleotide phosphate-diaphorase (NADPH-d) positive neurons. NADPH-d co-localizes with nitric oxide synthase, which is responsible for the production of the transcellular messenger, nitric oxide. Changes in NADPH-d staining in the IC were found after unilateral lesions of the AC. Lesions resulted in a reduction in NADPH-d staining in neurons and neuropil within the ipsilateral DC and EC with the maximum reduction occurring 3-4 days after lesion. The reduction in NADPH-d staining in the contralateral IC was less pronounced. Lesions affecting auditory areas Te 1 and Te 3 produced the largest decrease in NADPH-d staining in neurons and neuropil. This finding may be related to the abolition of the influence of glutamatergic corticocollicular and commissural pathways.

Role of mammalian auditory cortex in the perception of elementary sound properties

Studies in several mammalian species have demonstrated that bilateral ablations of the auditory cortex have little effect on simple sound intensity and frequency-based behaviors. In the rat, for example, early experiments have shown that auditory ablations result in virtually no effect on the rat's ability to either detect tones or discriminate frequencies. Such lesion experiments, however, typically examine an animal's performance some time after recovery from ablation surgery. As such, they demonstrate that the cortex is not essential for simple auditory behaviors in the long run. Our study further explores the role of cortex in basic auditory perception by examining whether the cortex is normally involved in these behaviors. In these experiments we reversibly inactivated the rat primary auditory cortex (AI) using the GABA agonist muscimol, while the animals performed a simple auditory task. At the same time we monitored the rat's auditory activity by recording auditory evoked potentials (AEP) from the cortical surface. In contrast to lesion studies, the rapid time course of these experimental conditions preclude reorganization of the auditory system that might otherwise compensate for the loss of cortical processing. Soon after bilateral muscimol application to their AI region, our rats exhibited an acute and profound inability to detect tones. After a few hours this state was followed by a gradual recovery of normal hearing, first of tone detection and, much later, of the ability to discriminate frequencies. Surface muscimol application, at the same time, drastically altered the normal rat AEP. Some of the normal AEP components vanished nearly instantaneously to unveil an underlying waveform, whose size was related to the severity of accompanying behavioral deficits. These results strongly suggest that the cortex is directly involved in basic acoustic processing. Along with observations from accompanying multiunit experiments that related the AEP to AI neuronal activity, our results suggest that a critical amount of activity in the auditory cortex is necessary for normal hearing. It is likely that the involvement of the cortex in simple auditory perceptions has hitherto not been clearly understood because of underlying recovery processes that, in the long-term, safeguard fundamental auditory abilities after cortical injury.

Evidence of peripheral auditory activity modulation by the auditory cortex in humans

At the auditory periphery, the medial olivocochlear system is assumed to be involved in complex sound processing and may be influenced by feedback from higher auditory nuclei. Indeed, the descending auditory pathway includes fibers coming from the auditory cortex that are anatomically well positioned to influence the superior olivary complex, and thus the medial efferent system. The aim of the present study was to verify the hypothesis of an implied influence of the auditory cortex on the peripheral auditory system. In three rare cases of patients presenting with intractable temporal lobe epilepsy, Heschl's gyrus (i.e. the temporal superior gyrus) was surgically removed in the right hemisphere in two patients and in the left hemisphere in a third patient, in order to minimize epilepsy attacks, as preoperative stereoencephalography had shown the epileptic focus or tumor to be situated in those locations. In all three cases, several weeks after the operation the medial olivocochlear system was clearly less functional on both sides, but especially on the side contralateral to the resection. In healthy controls, no such pattern was obtained. In four other epileptic patients, who were operated unilaterally at the anterior temporal pole, amygdala and hippocampus with the temporal gyrus partially spared, efferent suppression grew stronger in the ear ipsilateral to surgery. These results revealed that, in humans, the primary and secondary auditory cortex play a role in modulating auditory periphery activity through direct or indirect efferent fibers. In accordance with previous findings, this descending influence may improve the auditory afferent message by adapting the hearing function according to cortical analysis of the ascending input.

Comparison of the fine structure of cortical and collicular terminals in the rat medial geniculate body

Neurons throughout the rat medial geniculate body, including the dorsal and ventral divisions, display a variety of responses to auditory stimuli. To investigate possible structural determinants of this variability, measurements of axon terminal profile area and postsynaptic dendrite diameter were made on inferior colliculus and corticothalamic terminal profiles in the medial geniculate body identified by anterograde tracer labeling following injections into the inferior colliculus or cortex. Over 90% of the synapses observed were axodendritic, with few axosomatic synapses. Small (<0.5 microm(2)) and large (>1.0 microm(2)) collicular profiles were found throughout the medial geniculate, but were smaller on average in the dorsal division (0.49+/-0.49 microm(2)) than in the ventral division (0.70+/-0.64 microm(2)). Almost all corticothalamic profiles were small and ended on small-caliber dendrites (0.57+/-0.25 microm diameter) throughout the medial geniculate. A few very large (>2.0 microm(2)) corticothalamic profiles were found in the dorsal division and in the marginal zone of the medial geniculate. GABA immunostaining demonstrated the presence of GABAergic profiles arising from cells in the inferior colliculus. These profiles were compared with GABAergic profiles not labeled with anterograde tracer, which were presumed to be unlabeled inferior colliculus profiles or thalamic reticular nucleus profiles. The distributions of dendritic diameters postsynaptic to collicular, cortical and unlabeled GABAergic profiles were compared with dendritic diameters of intracellularly labeled medial geniculate neurons from rat brain slices. Our results demonstrate a corticothalamic projection to medial geniculate body that is similar to other sensory corticothalamic projections. However, the heterogeneous distributions of excitatory inferior collicular terminal sizes and postsynaptic dendritic diameters, along with the presence of a GABAergic inferior collicular projection to dendrites in the medial geniculate body, suggest a colliculogeniculate projection that is more complex than the ascending projections to other sensory thalamic nuclei. These findings may be useful in understanding some of the differences in the response characteristics of medial geniculate neurons in vivo.

Brief and short-term corticofugal modulation of subcortical auditory responses in the big brown bat, Eptesicus fuscus

Recent studies show that the auditory corticofugal system modulates and improves ongoing signal processing and reorganizes frequency map according to auditory experience in the central nucleus of bat inferior colliculus. However, whether all corticofugally affected collicular neurons are involved in both types of modulation has not been determined. In this study, we demonstrate that one group (51%) of collicular neurons participates only in corticofugal modulation of ongoing signal processing, while a second group (49%) of collicular neurons participates in both modulation of ongoing signal processing and in reorganization of the auditory system.

Corticofugal inhibition compresses all types of rate-intensity functions of inferior collicular neurons in the big brown bat

Recent studies have shown that the auditory corticofugal system modulates and improves signal processing in the frequency, time and spatial domains. In this study, we examine corticofugal modulation of rate-intensity functions of inferior collicular (IC) neurons of the big brown bat, Eptesicus fuscus, by electrical stimulation in the primary auditory cortex (AC). Cortical electrical stimulation compressed all types of rate-intensity functions so as to increase the slope but decrease the dynamic range of IC neurons. Cortical electrical stimulation also shifts the responsive intensity of IC neurons to higher levels. These data indicate that corticofugal modulation also improves subcortical signal processing in intensity domain. The implication of these findings to bat echolocation is discussed.

Functional organization of auditory cortex in the Mongolian gerbil (Meriones unguiculatus). IV. Connections with anatomically characterized subcortical structures

The subcortical connections of the four tonotopically organized fields of the auditory cortex of the Mongolian gerbil, namely the primary (AI), the anterior (AAF), the dorsoposterior (DP) and the ventroposterior field (VP), were studied predominantly by anterograde transport of biocytin injected into these fields. In order to allow the localization of connections with respect to subdivisions of subcortical auditory structures, their cyto-, fibre- and chemoarchitecture was characterized using staining methods for cell bodies, myelin and the calcium-binding protein parvalbumin. Each injected auditory cortical field has substantial and reciprocal connections with each of the three subdivision of the medial geniculate body (MGB), namely the ventral (MGv), dorsal (MGd) and medial division (MGm). However, the relative strengths of these connections vary: AI is predominantly connected with MGv, AAF with MGm and MGv, and DP and VP with MGd and MGv. The connections of at least AI and MGv are topographic: injections into caudal low-frequency AI label laterorostral portions of MGv, whereas injections into rostral high-frequency AI label mediocaudal portions of MGv. All investigated auditory fields send axons to the suprageniculate, posterior limitans, laterodorsal and lateral posterior thalamic nuclei, with strongest projections from DP and VP, as well as to the reticular and subgeniculate thalamic nuclei. AI, AAF, DP and VP project to all three subdivisions of the inferior colliculus, namely the dorsal cortex, external cortex and central nucleus ipsilaterally and to the dorsal and external cortex contralaterally. They also project to the deep and intermediate layers of the ipsilateral superior colliculus, with strongest projections from DP and VP to the lateral and basolateral amygdaloid nuclei, the caudate putamen, globus pallidus and the pontine nuclei. In addition, AAF and particularly DP and VP project to paralemniscal regions around the dorsal nucleus of the lateral lemniscus (DNLL), to the DNLL itself and to the rostroventral aspect of the superior olivary complex. Moreover, DP and VP send axons to the dorsal lateral geniculate nucleus. The differences with respect to the existence and/or relative strengths of subcortical connections of the examined auditory cortical fields suggest a somewhat different function of each of these fields in auditory processing.

Evidence for direct cortical innervation of medial olivocochlear neurones in rats

We have investigated the morphological relationship between auditory cortex efferents and medial olivocochlear neurones. Using combined retrograde and anterograde tracing we describe close contacts between medial olivocochlear neurones and corticofugal terminals in the ventral nucleus of the trapezoid body. The data indicate a possible direct action of the auditory cortex on the activity of the medial olivocochlear neurones and thus possibly the sensitivity of the cochlea.

Synaptic response patterns of neurons in the cortex of rat inferior colliculus

The present study examined synaptic potentials of neurons in inferior colliculus (IC) cortex slice and the roles of GABA and glutamate receptors in generating these potentials. Multipolar (82%) and elongated (18%) cells were observed with intracellular biocytin staining. Electrical stimulation of the IC commissure (CoIC) elicited only inhibitory postsynaptic potentials (IPSPs) (10% of cells), only excitatory postsynaptic potentials (EPSPs) (51%), or both (38%). IPSPs were elicited at lower thresholds and shorter latencies than EPSPs (mean: 1.6+/-1.2 ms) and IPSPs were observed in all neurons following membrane depolarization. Short-latency EPSPs were blocked by non-NMDA receptor antagonists, and longer-latency EPSPs were blocked by NMDA antagonists. CoIC stimulation evoked short-latency IPSPs (mean: 0.55+/-0.33 ms) in 48% of neurons, and the IPSPs persisted despite glutamate receptor blockade, which implies monosynaptic inhibitory input. A GABA(A) antagonist blocked IPSPs and paired pulse inhibition of EPSPs, suggesting GABA(A) receptor mediation. A GABA(B) antagonist reduced paired pulse inhibition of IPSPs, suggesting GABA(B) receptor modulation. Thus, GABA-mediated inhibition plays a critical role in shaping synaptic responses of IC cortex neurons. Normal GABAergic function in IC has been shown to be important in acoustic coding, and reduced efficacy of GABA function in IC neurons is critical in IC pathophysiology in presbycusis, tinnitus and audiogenic seizures.

corticofugal regulation of excitatory and inhibitory frequency tuning curves of bat inferior collicular neurons

Corticofugal regulation of excitatory and inhibitory frequency tuning curves (FTCs) of neurons in the central nucleus of bat inferior colliculus (ICc) was studied by electrical stimulation of the primary auditory cortex (AC stimulation) under free field stimulation conditions using a two-tone inhibition paradigm. AC stimulation narrowed the excitatory FTCs and asymmetrically expanded the lateral inhibitory FTCs of corticofugally inhibited ICc neurons. The opposite results were observed for excitatory and inhibitory FTCs of corticofugally facilitated ICc neurons. These data support previous reports that corticofugal systems work together with widespread lateral inhibition to regulate subcortical frequency processing.

Corticofugal amplification of facilitative auditory responses of subcortical combination-sensitive neurons in the mustached bat

Recent studies on the bat's auditory system indicate that the corticofugal system mediates a highly focused positive feedback to physiologically "matched" subcortical neurons, and widespread lateral inhibition to physiologically "unmatched" subcortical neurons, to adjust and improve information processing. These findings have solved the controversy in physiological data, accumulated since 1962, of corticofugal effects on subcortical auditory neurons: inhibitory, excitatory, or both (an inhibitory effect is much more frequent than an excitatory effect). In the mustached bat, Pteronotus parnellii parnellii, the inferior colliculus, medial geniculate body, and auditory cortex each have "FM-FM" neurons, which are "combination-sensitive" and are tuned to specific time delays (echo delays) of echo FM components from the FM components of an emitted biosonar pulse. FM-FM neurons are more complex in response properties than cortical neurons which primarily respond to single tones. In the present study, we found that inactivation of the entire FM-FM area in the cortex, including neurons both physiologically matched and unmatched with subcortical FM-FM neurons, on the average reduced the facilitative responses to paired FM sounds by 82% for thalamic FM-FM neurons and by 66% for collicular FM-FM neurons. The corticofugal influence on the facilitative responses of subcortical combination-sensitive neurons is much larger than that on the excitatory responses of subcortical neurons primarily responding to single tones. Therefore we propose the hypothesis that, in general, the processing of complex sounds by combination-sensitive neurons more heavily depends on the corticofugal system than that by single-tone sensitive neurons.

Sound stimulation increases calcium-binding protein immunoreactivity in the inferior colliculus in mice

The numerical density of calbindin D-28k and parvalbumin immunopositive neurons in the inferior colliculus (IC) in mice was increased after sound stimulation. An increased number of calbindin positive neurons was found in the deep layers of the external cortex (EC) and particularly in the dorsal cortex (DC) and commissural nucleus (NCO). An increase of parvalbumin positive neurons was found in the EC, central nucleus (ICC) and DC, but not in the NCO. The increased immunoreactivity related to sound exposure suggests the appearance of neurons which express these proteins after sound stimulation. The up-regulation of calcium-binding proteins in these neurons may be due to their protective role against overstimulation, their response to a higher auditory metabolic activity, or increasing effect of excitatory inputs after noise-induced hearing loss.

Area- and lamina-specific organization of a neuronal subpopulation defined by expression of latexin in the rat cerebral cortex

The aim of the present study was to investigate the density, laminar distribution, size, morphology, and neurotransmitter phenotype of rat cortical neurons expressing latexin, an inhibitor of carboxypeptidase A. Immunohistochemical analyses established that latexin-immunoreactive neurons are restricted essentially to the infragranular layers of lateral cortical areas in the rat. The overall density, laminar or sublaminar localization, and cell size distribution of latexin-positive neurons differed substantially across cytoarchitectonic areas within lateral cortex. Numerous latexin-positive neurons had the morphology of modified pyramidal cells especially of layer VI. The vast majority of latexin-positive neurons were glutamate-immunoreactive in the six lateral neocortical areas examined, while neurons immunoreactive for both latexin and GABA were virtually absent. Thus the majority of latexin-positive neurons are likely to be excitatory projection neurons. The area- and lamina-specific distribution of the latexin-expressing subpopulation of glutamate-immunoreactive neurons is a distinctive feature that may contribute to the functional specialization of the lateral cortical areas.

Ethanol and neurotransmitter interactions--from molecular to integrative effects

There is extensive evidence that ethanol interacts with a variety of neurotransmitters. Considerable research indicates that the major actions of ethanol involve enhancement of the effects of gamma-aminobutyric acid (GABA) at GABAA receptors and blockade of the NMDA subtype of excitatory amino acid (EAA) receptor. Ethanol increases GABAA receptor-mediated inhibition, but this does not occur in all brain regions, all cell types in the same region, nor at all GABAA receptor sites on the same neuron, nor across species in the same brain region. The molecular basis for the selectivity of the action of ethanol on GaBAA receptors has been proposed to involve a combination of benzodiazepine subtype, beta 2 subunit, and a splice variant of the gamma 2 subunit, but substantial controversy on this issue currently remains. Chronic ethanol administration results in tolerance, dependence, and an ethanol withdrawal (ETX) syndrome, which are mediated, in part, by desensitization and/or down-regulation of GABAA receptors. This decrease in ethanol action may involve changes in subunit expression in selected brain areas, but these data are complex and somewhat contradictory at present. The sensitivity of NMDA receptors to ethanol block is proposed to involve the NMDAR2B subunit in certain brain regions, but this subunit does not appear to be the sole determinant of this interaction. Tolerance to ethanol results in enhanced EAA neurotransmission and NMDA receptor upregulation, which appears to involve selective increases in NMDAR2B subunit levels and other molecular changes in specific brain loci. During ETX a variety of symptoms are seen, including susceptibility to seizures. In rodents these seizures are readily triggered by sound (audiogenic seizures). The neuronal network required for these seizures is contained primarily in certain brain stem structures. Specific nuclei appear to play a hierarchical role in generating each stereotypical behavioral phases of the convulsion. Thus, the inferior colliculus acts to initiate these seizures, and a decrease in effectiveness of GABA-mediated inhibition in these neurons is a major initiation mechanism. The deep layers of superior colliculus are implicated in generation of the wild running behavior. The pontine reticular formation, substantia nigra and periaqueductal gray are implicated in generation of the tonic-clonic seizure behavior. The mechanisms involved in the recruitment of neurons within each network nucleus into the seizure circuit have been proposed to require activation of a critical mass of neurons. Achievement of critical mass may involve excess EAA-mediated synaptic neurotransmission due, in part, to upregulation as well as other phenomena, including volume (non-synaptic diffusion) neurotransmission. Effects of ETX on receptors observed in vitro may undergo amplification in vivo to allow the excess EAA action to be magnified sufficiently to produce synchronization of neuronal firing, allowing participation of the nucleus in seizure generation. GABA-mediated inhibition, which normally acts to limit excitation, is diminished in effectiveness during ETX, and further intensifies this excitation.

Corticofugal modulation of the midbrain frequency map in the bat auditory system

The auditory system, like the visual and somatosensory systems, contains topographic maps in its central neural pathways. These maps can be modified by sensory deprivation, injury and experience in both young and adult animals. Such plasticity has been explained by changes in the divergent and convergent projections of the ascending sensory system. Another possibility, however, is that plasticity may be mediated by descending corticofugal connections. We have investigated the role of descending connections from the cortex to the inferior colliculus of the big brown bat. Electrical stimulation of the auditory cortex causes a downward shift in the preferred frequencies of collicular neurons toward that of the stimulated cortical neurons. This results in a change in the frequency map within the colliculus. Moreover, similar changes can be induced by repeated bursts of sound at moderate intensities. Thus, one role of the mammalian corticofugal system may be to modify subcortical sensory maps in response to sensory experience.

Corticofugal amplification of subcortical responses to single tone stimuli in the mustached bat

Since 1962, physiological data of corticofugal effects on subcortical auditory neurons have been controversial: inhibitory, excitatory, or both. An inhibitory effect has been much more frequently observed than an excitatory effect. Recent studies performed with an improved experimental design indicate that corticofugal system mediates a highly focused positive feedback to physiologically "matched" subcortical neurons, and widespread lateral inhibition to "unmatched" subcortical neurons, in order to adjust and improve information processing. These results lead to a question: what happens to subcortical auditory responses when the corticofugal system, including matched and unmatched cortical neurons, is functionally eliminated? We temporarily inactivated both matched and unmatched neurons in the primary auditory cortex of the mustached bat with muscimol (an agonist of inhibitory synaptic transmitter) and measured the effect of cortical inactivation on subcortical auditory responses. Cortical inactivation reduced auditory responses in the medial geniculate body and the inferior colliculus. This reduction was larger (60 vs. 34%) and faster (11 vs. 31 min) for thalamic neurons than for collicular neurons. Our data indicate that the corticofugal system amplifies collicular auditory responses by 1.5 times and thalamic responses by 2.5 times on average. The data are consistant with a scheme in which positive feedback from the auditory cortex is modulated by inhibition that may mostly take place in the cortex.

Bilateral ablation of the auditory cortex in the rat alters conditioned emotional suppression to a sound as appraised through a latent inhibition study

Latent inhibition consists of a retardation of conditioning seen when the to be conditioned stimulus is presented a number of times with no other consequence. This phenomenon likely reflects processes of selective attention whereby irrelevant stimuli come to be ignored. Using physiological models for auditory attention, some investigators have suggested that selective attention acts as a filtering mechanism capable of inhibiting or gating unattended stimuli relative to attended ones in the auditory cortex. In the present work, an on-baseline conditioned suppression response procedure was used to study the effects of stimulus preexposure in rats submitted to bilateral auditory cortex ablation. Our results indicate that both auditory cortex lesioned and control animals exhibit latent inhibition to a sound. However, learning after preexposure to that sound was particularly slow in animals with bilateral auditory cortex lesion, i.e. in these animals, the latent inhibition effect appeared to be enhanced. Conditioning from one day to the next also varied slightly. Thus, the auditory cortex appears to modulate learning when the conditioned stimulus is a sound.

Age-related changes in calbindin D-28k and calretinin immunoreactivity in the inferior colliculus of CBA/CaJ and C57Bl/6 mice

This study examines calbindin D-28k and calretinin immunoreactivity in the inferior colliculus (IC) of young and old mice of two strains. The CBA/CaJ mouse maintains good hearing until very late in life, whereas the C57Bl/6 strain exhibits severe sensorineural hearing loss at an early age. Young and old mice of both strains were selected with matching auditory brainstem response audiograms and gap detection thresholds. Brain sections were reacted with anti-calbindin D-28k (CB) and anti-calretinin (CR). Staining patterns were characterized and cell counts performed. CB immunoreactivity was high only in the nucleus of the commissure (NCO); counts revealed a 22.3% decrease in the number of CB+ cells in old CBA mice and a 25.1% decrease in old C57 mice. Calretinin immunoreactivity was high in the pericentral regions of the IC, but the central nucleus was devoid of CR+ cells. The dorsal cortex, lateral nucleus, and NCO showed increases of 42.3, 49.0, and 61%, respectively, in the number of CR+ cells, but only in the old CBA mice. No significant change was observed in the old C57 mice. Whereas decreases in CB immunoreactivity are common with age, this study is the first to report an age-related increase in CR immunoreactivity in the auditory system. The increase in CR+ cells is a possible compensatory adaptation to the decrease in CB+ cells. That the number of CR+ cells remains constant with age in C57 mice suggests this compensation may depend upon stimulus-driven activity, but this requires further study.

Corticofugal modulation of frequency processing in bat auditory system

Auditory signals are transmitted from the inner ear through the brainstem to the higher auditory regions of the brain. Neurons throughout the auditory system are tuned to stimulus frequency, and in many auditory regions are arranged in topographical maps with respect to their preferred frequency. These properties are assumed to arise from the interactions of convergent and divergent projections ascending from lower to higher auditory areas; such a view, however, ignores the possible role of descending projections from cortical to subcortical regions. In the bat auditory system, such corticofugal connections modulate neuronal activity to improve the processing of echo-delay information, a specialized feature. Here we show that corticofugal projections are also involved in the most common type of auditory processing, frequency tuning. When cortical neurons tuned to a specific frequency are inactivated, the auditory responses of subcortical neurons tuned to the same frequency are reduced. Moreover, the responses of other subcortical neurons tuned to different frequencies are increased, and their preferred frequencies are shifted towards that of the inactivated cortical neurons. Thus the corticofugal system mediates a positive feedback which, in combination with widespread lateral inhibition, sharpens and adjusts the tuning of neurons at earlier stages in the auditory processing pathway.

Cortical and medullary somatosensory projections to the cochlear nuclear complex in the hedgehog tenrec

Various tracer substances were injected into the spinal cord, the dorsal column nuclei, the trigeminal nuclear complex and the somatosensory cortex in Madagascan hedgehog tenrecs. With the exception of the cases injected exclusively into the spinal cord all injections gave rise to sparse, but distinct anterograde projections to the cochlear nuclear complex, particularly the granular cell domain within and outside of the dorsal cochlear nucleus. Among these cochlear afferents the projection from the primary somatosensory cortex is the most remarkable because the hedgehog tenrec has one of the lowest encephalisation indices among mammals and a similar cortico-cochlear connection has not been demonstrated so far in other species.

Responses to exponential frequency modulations in the rat inferior colliculus

We examined responses to pure tones and exponentially frequency-modulated (FM) stimuli in the inferior colliculus of ketamine anesthetized rats. All units responded to both pure-tone and FM stimulation: units responding selectively to FM stimuli were not found. The comparison between responses to many different FM sweeps revealed that activity was elicited when the instantaneous frequency of a FM sweep entered the unit's pure-tone tuning curve. Units were tuned to the rate of frequency modulation. Most modulation rate transfer functions had bandpass characteristics. Best modulation rates covered a range from 4.8 to 1904 octaves/s with more than 90% between 10 and 400 octaves/s. In contrast to previous studies, modulation direction was not coded in unit responses and only few units demonstrated a weak change in response strength when sweep direction was altered. This is at least partly attributable to the FM stimulus design which, in the present study, was adapted to the logarithmic frequency representation in the rat auditory system and carefully matched to the units' pure-tone responses area. In spite of the close relationship between pure tone and FM response behavior, modulation rate tuning cannot be completely explained on the basis of the units' pure-tone responses.

Corticofugal modulation of time-domain processing of biosonar information in bats

The Jamaican mustached bat has delay-tuned neurons in the inferior colliculus, medial geniculate body, and auditory cortex. The responses of these neurons to an echo are facilitated by a biosonar pulse emitted by the bat when the echo returns with a particular delay from a target located at a particular distance. Electrical stimulation of cortical delay-tuned neurons increases the delay-tuned responses of collicular neurons tuned to the same echo delay as the cortical neurons and decreases those of collicular neurons tuned to different echo delays. Cortical neurons improve information processing in the inferior colliculus by way of the corticocollicular projection.

Sources of GABAergic input to the inferior colliculus of the rat

We have studied the GABAergic projections to the inferior colliculus (IC) of the rat by combining the retrograde transport of horseradish peroxidase (HRP) and immunohistochemistry for gamma-amino butyric acid (GABA). Medium-sized (0.06-0.14 microliter) HRP injections were made in the ventral part of the central nucleus (CNIC), in the dorsal part of the CNIC, in the dorsal cortex (DCIC), and in the external cortex (ECIC) of the IC. Single HRP-labeled and double (HRP-GABA)-labeled neurons were systematically counted in all brainstem auditory nuclei. Our results revealed that the IC receives GABAergic afferent connections from ipsi- and contralateral brainstem auditory nuclei. Most of the contralateral GABAergic input originates in the IC and the dorsal nucleus of the lateral lemniscus (DNLL). The dorsal region of the IC (DCIC and dorsal part of the CNIC) receives connections mostly from its homonimous contralateral region, and the ventral region from the contralateral DNLL. The commissural GABAergic projections originate in a morphologically heterogeneous neuronal population that includes small to medium-sized round and fusiform neurons as well as large and giant neurons. Quantitatively, the ipsilateral ventral nucleus of the lateral lemniscus is the most important source of GABAergic input to the CNIC. In the superior olivary complex, a smaller number of neurons, which lie mainly in the periolivary nuclei, display double labeling. In the contralateral cochlear nuclei, only a few of the retrogradely labeled neurons were GABA immunoreactive. These findings give us more information about the role of GABA in the auditory system, indicating that inhibitory inputs from different ipsi- and contralateral, mono- and binaural auditory brainstem centers converge in the IC.

Projections from auditory cortex to the cochlear nucleus in rats: synapses on granule cell dendrites

Previous work has demonstrated that layer V pyramidal cells of primary auditory cortex project directly to the cochlear nucleus. The postsynaptic targets of these centrifugal projections, however, are not known. For the present study, biotinylated dextran amine, an anterograde tracer, was injected into the auditory cortex of rats, and labeled terminals were examined with light and electron microscopy. Labeled corticobulbar axons and terminals in the cochlear nucleus are found almost exclusively in the granule cell domain, and the terminals appear as boutons (1-2 microns in diameter) or as small mossy fiber endings (2-5 microns in diameter). These cortical endings contain round synaptic vesicles and form asymmetric synapses on hairy dendritic profiles, from which thin (0.1 micron in diameter), nonsynaptic "hairs" protrude deep into the labeled endings. These postsynaptic dendrites, which are typical of granule cells, surround and receive synapses from large, unlabeled mossy fiber endings containing round synaptic vesicles and are also postsynaptic to unlabeled axon terminals containing pleomorphic synaptic vesicles. No labeled fibers were observed synapsing on profiles that did not fit the characteristics of granule cell dendrites. We describe a circuit in the auditory system by which ascending information in the cochlear nucleus can be modified directly by descending cortical influences.

Distribution of descending projections from primary auditory neocortex to inferior colliculus mimics the topography of intracollicular projections

To ascertain whether the auditory neocortex also innervates the central nucleus of the inferior colliculus (CNIC) and not only its dorsal (DCIC) and external (ECIC) cortices, the anterograde tracers Phaseolus vulgaris-leucoagglutinin (PHA-L) and biotinylated dextran (BD) were injected into the primary auditory neocortex of albino rats (Te1), and labeled corticocollicular fibers were studied via light and electron microscopy. Axons from discrete regions of Te1 form two rostrocaudally oriented laminar plexuses of terminal fibers in the ipsilateral inferior colliculus (IC) and one in the contralateral IC. The first ipsilateral plexus, located in the medial half of the IC, has a dorsomedial to ventrolateral orientation, parallel to the isofrequency planes of the IC; is continuous through the CNIC and DCIC; and extends into the rostral ECIC. The second plexus is located in the deep layers of the lateral ECIC. These two plexuses meet caudally and ventrally, at the border between the CNIC and the lateral ECIC. The plexus in the contralateral IC is less dense and shorter than the two ipsilateral plexuses and is symmetric to the medial plexus. The thickness of the three plexuses is correlated with the size of the injection site, and their mediolateral and dorsoventral positions change as the injection site in Te1 is displaced rostrocaudally, with more caudal injections resulting in more dorsolateral medial plexuses and more dorsomedial lateral plexuses. Furthermore, the ventromedial border of the IC receives nontopographic, convergent projections from wide regions of rostral portions of Te1. The distribution of these corticocollicular plexuses mimics the topography of previously described intracollicular fibers. Electron microscopy shows that, in all three subdivisions of the ipsilateral IC, corticocollicular fibers form small boutons with features generally associated with excitatory transmission; i.e., they contain round synaptic vesicles and form asymmetric synapses with thin dendritic shafts and spines. These results demonstrate that the auditory corticocollicular projections innervate more extensive regions of the IC than were previously observed. Although peripheral regions receive the densest projection, the entire IC appears to be the target of corticofugal input.

Sound-evoked electrocortical desynchronization is inhibited by N omega-nitro-L-arginine methyl ester microinfused into the inferior colliculi in rats

In previous experiments we have shown that systemic or intracerebroventricular administration of N omega-nitro-L-arginine methyl ester (L-NAME), an inhibitor of nitric oxide (NO) synthase, is able to significantly reduce sound-evoked electrocortical (ECoG) desynchronization in rats. The present experiments were aimed at identifying the site(s) of the brain through which these effects are mediated. L-NAME (200 and 300 nmol), oxyhaemoglobin (200 and 300 nmol), a NO-trapping agent, and methylene blue (100 and 150 nmol), an inhibitor of guanylate cyclase and NO synthase, given bilaterally into the inferior colliculi, but not in other relay stations of the acoustic pathway, prevented the reduction in ECoG amplitude induced by sound stimulation in rats. Significant reduction of sound-evoked ECoG desynchronization has also been observed in rats receiving injection of CGP37849 (125 and 500 pmol) and LY274614 (125 pmol), two competitive N-methyl-D-aspartate receptor antagonists into the inferior colliculi. The present results show that the inferior colliculus represents the main site where sound-evoked ECoG desynchronization is prevented by L-NAME and provide further support for the hypothesis that NO may play a role at this level in the control of the measured response.

Multiple projections from the ventral nucleus of the trapezoid body in the rat

An analysis of the central projections of the ventral nucleus of the trapezoid body (VNTB) in the rat, a region of the superior olivary complex known for its neuronal heterogeneity, was made using two anterograde axonal tracers, [3H]leucine and biotinylated dextran amine (BDA). A mixture of these tracers was injected iontophoretically into the VNTB and the results analyzed by first assessing magnitudes of autoradiographic signal in nuclei receiving projections and then identifying the axons and terminals responsible for this signal in parallel sets of sections processed for BDA. Our analysis showed that in addition to its projections to each cochlea via the olivocochlear bundle, the VNTB has 3 major central sites of axonal terminations: (1) the cochlear nucleus, particularly the molecular layer of the contralateral dorsal cochlear nucleus, (2) the contralateral lateral superior olive, and (3) the ipsilateral inferior colliculus. Other sites receiving projections from the VNTB included the VNTB itself and the nuclei of the lateral lemniscus. Significantly, the relative magnitudes of labeling within the nuclei receiving inputs from the VNTB varied consistently as a function of the dorsoventral location of the injection site, confirming previous work showing that there is a partial segregation within this nucleus of neurons according to their projections. Our data also revealed an orderly topographic pattern of projections to the cochlear nuclei, lateral superior olive and the inferior colliculus which is consistent with the known tonotopic organization both of the VNTB and these projection targets. Methodologically, the co-injection of two tracers was advantageous in that patterns of silver grains in autoradiographs could be used to confirm whether axons and terminals labeled with BDA had originated from labeled somata at the injection site or were the result of uptake of BDA by fibers of passage.

Pyramidal cells in primary auditory cortex project to cochlear nucleus in rat

Recent work has demonstrated that the auditory cortex in rat sends direct projections to the auditory nuclei of the brainstem, including the cochlear nucleus and superior olive. To determine the cortical origin of the projections to cochlear nucleus, Fast Blue, a retrograde fluorescent tracer, was injected into the cochlear nucleus. Labeled cells in the forebrain were then studied with light microscopy and mapped. The projection was found to originate from large pyramidal neurons in layer V of primary auditory cortex. The projection was predominantly ipsilateral, and no labeled neurons were found in other cortical areas. These data imply that primary auditory cortex exerts influence over ascending auditory information at the earliest stages of the central auditory system.

Distribution of calcium-binding protein calbindin-D28k in the auditory system of adult and developing rats

Calbindin-D28k (CaBP) is a calcium-binding protein, which appears to be involved in the buffering of free intracellular calcium and may thereby contribute to calcium homeostasis. This study attempted to determine the distribution pattern of CaBP immunoreactivity in the central auditory system of adult rats and during development, when calcium ions play key roles in several aspects of nerve cell function. It was found that most steps during CaBP development occur postnatally in the central auditory system. With the exception of the lateral superior olive, the ventral and the intermediate nuclei of the lateral lemniscus, and the auditory cortex, which already express CaBP prenatally, CaBP immunoreactivity is not present before postnatal day 2 (P2). Development proceeds until about P24, when the pattern characteristic of adult animals can be seen. There was no detectable sequence in CaBP development from lower to higher stations in the auditory pathway, i.e., the different nuclei appear to express CaBP independently of each other, indicating that intrinsic, rather than peripheral, maturation processes may predominantly influence CaBP expression. Neurons in four brainstem nuclei (the lateral superior olive, the ventral and intermediate nuclei of the lateral lemniscus, and the central nucleus of the inferior colliculus) express CaBP only transiently. In these nuclei, CaBP immunoreactivity peaks between P6 and P18, which coincides with the period of synapse stabilization. Therefore, CaBP may play a specific role during neuronal development, by buffering the concentration of intracellular free Ca2+, which may be necessary for modification of synaptic efficiency.

Dorsal nucleus of the lateral lemniscus in the rat: concentric organization and tonotopic projection to the inferior colliculus

A basic principle of organization in auditory centers is the topographic-tonotopic order. Whether this applies to the dorsal nucleus of the lateral lemniscus (DNLL), however, is still debated. To clarify this problem, we have utilized the neuroanatomical tracers horseradish peroxidase (HRP) and biotinylated dextran (BD) injected into different regions of the central nucleus of the inferior colliculus (CNIC) in the rat. After large injections of HRP that included most of the CNIC, retrogradely labelled neurons were found all across the ipsi- and contralateral DNLL, showing that all parts of this nucleus innervate the CNIC bilaterally. More neurons were seen consistently on the side contralateral to the injection site. Labelled fibers, however, were abundant ipsilaterally, but scarce in the contralateral DNLL. Single, small injections of HRP or BD into the CNIC resulted in labelling in restricted areas of the ipsi- and contralateral DNLL. In coronal sections, the neurons and fibers labelled in the ipsilateral DNLL formed a well-defined, ring-shaped structure made of dendrites and axons oriented parallel to each other, which we termed "annular band." The observation of serial sections revealed that the annular band seen in any individual section represents a slice through a more or less complete three-dimensional, hollow, ovoid structure oriented rostrocaudally. The position and diameter of the annular band changed as the injection site was shifted along the tonotopic axis of the CNIC. Single injections placed in the ventromedial, high-frequency region of the CNIC produced a large annular band along the periphery of the DNLL. After injections placed in progressively more dorsolateral, lower-frequency regions of the CNIC, the annular band became smaller in diameter and occupied a successively more central position in the DNLL. Double injections along the tonotopic axis of the CNIC resulted in two roughly concentric annular bands. The labelled neurons and fibers in the contralateral DNLL systematically occupied a position symmetric to the annular band seen ipsilaterally. These findings indicate that the rat DNLL is primarily composed of neurons with flattened dendritic arbors and flattened fields of terminal fibers. These two elements intermingle, forming concentric layers around the geometric center of the nucleus. The axons of neurons within corresponding layers on the two sides converge onto the CNIC of both sides in a strict topographic fashion: the peripheral layers project to the ventromedial, high-frequency region of the CNIC, and the central layers project to the dorsolateral, low-frequency region. These results suggest that the concentric arrangement of the DNLL is the substrate of its tonotopic organization.

Development of frequency-selective domains in inferior colliculus of normal and neonatally noise-exposed rats

Topographic patterns of pure-tone responses in inferior colliculus (IC) of Wistar rats were mapped using immunohistochemical staining for the nuclear protein Fos, the translation product of the c-fos proto-oncogene. Patterns were compared in ICs of immature and mature rats and in mature rats which experienced auditory deprivation beginning on day 14, an age near the developmental onset of hearing. Neonatal hearing losses, caused here by exposure to potentially deafening noise, are known to result in audiogenic seizure susceptibility in neonatal rats. These seizures can be triggered only by high-frequency stimuli and are believed to be initiated in IC. Thus, it seemed possible that susceptibility might depend on derangements of topographic frequency representation due to neonatal auditory deprivation. The band-like frequency-response domains, characteristic of adult IC, were found to be poorly differentiated in ICs of immature rats. On day 12, only lower-frequency stimuli induced discrete bands of Fos immunoreactivity while responses to higher frequencies remained exceptionally diffuse within ventral portions of IC. Only after day 24 did responses to the highest frequencies also appear mature. Furthermore, most significantly, adult rats which were transiently deafened on day 14, retained the more voluminous response patterns which were characteristic of immature IC. Because frequency selectivity in cochlea also develops by a low-to-high frequency sequence, results are consistent with a hypothesis that topographic organization arises in IC by an activity-dependent process. Whereas neonatal noise exposure also conferred audiogenic seizure susceptibility, it appears the arrest of tonotopic organization of IC is the probable basis of this reflex epilepsy.

Organization of auditory callosal connections in hypothyroid adult rats

Callosal connections were studied with tracers (horseradish peroxidase (HRP) and wheat germ agglutinin-horseradish peroxidase (WGA-HRP)) in normal rats and rats deprived of thyroid hormones with methimazole (Sigma) since embryonic day 14 and thyroidectomized at postnatal day 6. In hypothyroid rats, the auditory areas, in particular the primary auditory area, showed cytoarchitectonic changes including blurred lamination and decrease in the size of layer V pyramidal neurons. In control rats, callosally-projecting neurons were found between layers II and VI with a peak in layer III and upper layer IV. In hypothyroid rats, labelled neurons were found between layers IV and VI with two peaks corresponding to layer IV and upper layer V, and in upper layer VI. Quantitative analysis of radial distribution of callosally-projecting neurons confirmed their shift to infragranular layers in hypothyroid rats. Three-dimensional reconstructions showed a more continuous tangential distribution of callosally-projecting neurons in hypothyroid rats which may be due to the maintenance of a juvenile 'exuberant' pattern of projections. These changes in cortical connectivity may be relevant for understanding epilepsy and mental retardation associated with early hypothyroidism in humans and to clarify basic mechanisms of cortical development.

Topography of descending projections from the inferior colliculus to auditory brainstem nuclei in the rat

We examined the organization of descending projections from the inferior colliculus (IC) to auditory brainstem nuclei and to pontine and reticular nuclei in the rat by employing the anterograde axonal tracer Phaseolus vulgaris-leucoagglutinin (PHA-L). Small PHA-L injections into cytologically defined subnuclei of the IC revealed that each subnucleus has a unique pattern of efferent projections. The central nucleus of the IC projects in a topographic order to the dorsal nucleus of the lateral lemniscus (DLL), the rostral periolivary nucleus (RPO), the ventral nucleus of the trapezoid body (VNTB), and the dorsal cochlear nucleus (DCN). It is assumed that this topography represents a cochleotopic arrangement. The external cortex of the IC projects to the nucleus sagulum (Sag), the RPO, the VNTB, and the DCN. Minor projections were found to pontine and reticular nuclei. Efferent fibers from the dorsal cortex of the IC terminate mainly in the Sag, while other nuclei of the auditory and extra-auditory brainstem receive only minor projections. The intercollicular zone sends a moderate number of fibers to the DLL and very few, if any, to the remaining auditory brainstem nuclei. In contrast, fairly strong projections from the intercollicular zone to the reticular formation were found. The present data demonstrate that the four subnuclei of the IC have a differential pattern of descending projections to nuclei in the pontine and medullary brainstem. These parallel colliculofugal pathways, assumed to belong to functionally separate circuits, may modulate auditory processing at different levels of the auditory neuraxis.

Tectal and related target areas of spinal and dorsal column nuclear projections in hedgehog tenrecs

The terminal distributions of spinal and dorsal column nuclear projections to tectum, pretectum, and central gray of hedgehog tenrecs (Echinops telfairi and Setifer setosus) were investigated using anterograde axonal flow and various tracer substances. In the inferior colliculus, the densest and most extensive mesencephalic projections were found within the pericentral regions. One target area, referred to as the external portion of the inferior colliculus, was represented as a semicircle of grain patches lateral and caudal to the central nucleus. This region received somesthetic afferents from the dorsal column nuclei and from spinal segments at various levels. In contrast, after high cervical injections, the pericentral portion dorsomedial to the rostral half of the central nucleus was labeled almost exclusively. This area of labeling was distinct from the labeling in the central gray and might be best compared with the intercollicular zone in other species. The superior colliculus received projections predominantly from the high cervical cord; minor projections also arose from lumbar spinal segments and the dorsal column nuclei. The terminal field covered roughly the caudal half of the colliculus and involved the stratum griseum intermediale in a patch-like fashion. Some labeling was also found in the stratum griseum profundum and in the stratum griseum superficiale. Other than in the colliculi, weak pretectal projections were observed following dorsal column nuclear injections, while the nucleus of Darkschewitsch was labeled best following lumbosacral injections. All mesencephalic target areas were labeled consistently on the contralateral side, while their ipsilateral side was involved to a varying degree: The relatively most prominent ipsilateral labeling was seen in the central gray, being roughly similar on both sides; scarcely any labeling was noted in the ipsilateral superior colliculus. Tectal injections of retrograde tracer, in addition, revealed a considerable number of labeled neurons in a relatively cell-poor region immediately ventral to the high cervical dorsal horn. This region might correspond to the lateral cervical nucleus, an aggregation of neurons that so far has only been demonstrated in higher mammals.

Intrinsic and commissural connections of the rat inferior colliculus

This study analyzes the distribution of the intrinsic and commissural fiber plexuses originating in the central nucleus of the inferior colliculus in the rat. The anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHA-L) was injected iontophoretically at different places along the tonotopic axis of the central nucleus and visualized immunohistochemically. In coronal sections the terminal fields of axons originating at each injection site are seen to create four well-defined bands across the rostrocaudal extent of the inferior colliculus, two in the ipsilateral and two in the contralateral side. The "ipsilateral main band" extends dorsomedially and ventrolaterally from the injection site, in register with the known isofrequency contours of the central nucleus, spanning this nucleus and extending into the dorsal cortex of the inferior colliculus. The "ipsilateral external band" is located in the external cortex, where it is oriented dorsoventrally, slightly oblique to the pial surface. In caudal sections, the ventral portion of these two bands appear to join. The two bands in the contralateral inferior colliculus occupy a symmetric position to those of the ipsilateral side, forming a mirror-like image. The position of the four bands changes as the position of the injection site is varied along the frequency gradient axis of the central nucleus. After ventromedial (high frequency area) injections, the main band is ventral and medial, and the external band ventral and lateral. After more dorsolateral (lower frequency) injections, the main band is more dorsal and lateral, whereas the external band is more dorsal but more medial. Thus, the change in the position of the external band is separate and opposite to that of the main band. We suggest that the main bands represent isofrequency contours. Since the projection from the central nucleus to the external cortex of the inferior colliculus also appears to be tonotopic, we also propose a tonotopic organization for the external cortex. The main bands overlap the terminal field of the lemniscal fibers in the central nucleus; thus, it is concluded that the intracollicular fibers contribute to the formation of the known fibrodendritic laminae of the central nucleus. A possible role in preservation of frequency information and integration of other different acoustic parameters is proposed for the main bands. The external bands could participate in polysensory integration, and the commissural connections could be involved in hitherto unknown stages of binaural processing of sound. Based on our results, several modifications are proposed for delineating the subdivisions of the inferior colliculus.