Rat Inferior Colliculus Research Articles

Differences in FM response correlate with morphology of neurons in the rat inferior colliculus.

The response characteristics to linear frequency sweeps were studied in two groups of FM (frequency modulation) sensitive neurons in the rat inferior colliculus. 'FM specialized' cells responded to frequency sweeps but not to pure tones. 'Mixed' cells responded to both frequency sweeps and pure tones. FM specialized cells preferred faster and broader sweeps of higher intensity than did mixed cells and were more directionally selective. In addition, FM specialized cells were more sharply tuned to FM velocity and FM range and had longer response latencies. Physiologically identified FM cells stained intracellularly with horseradish peroxidase revealed differences in morphology correlating with the differences in their responses to tones. FM specialized cells had larger dendritic fields, more dendritic branching and more dendritic spines than did mixed cells. The findings are taken as evidence that the two groups of inferior colliculus neurons are both functionally and morphologically distinct.

Opposite effects of low and high doses of morphine on neural substrates of aversion in the inferior colliculus

The inferior colliculus (IC) is a well known relay station for auditory pathways in the brainstem. Much evidence has implicated this structure as part of a brain system mediating aversive states, and changes in its functioning as responsible for the occurrence of audiogenic seizures. Rats were implanted with chemitrodes, devices which allow electrical stimulation and microinjections of drugs in the same brain sites. Stepwise increases in the intensity of electrical stimulation of the IC of rats placed inside a circular arena allowed the determination of thresholds for freezing and escape behavior. Morphine (10-40nmol) caused dose-dependent increases in both aversive thresholds. A systemic injection of naloxone reversed the effects of morphine on the centrally induced aversive responses. These data suggest that neural substrates controlling defensive behavior in this structure are under opioid inhibitory control. Higher doses of morphine (80nmol) induced a non-naloxone reversible fearful hyper activity. It is suggested that opioid mechanisms exert an inhibitory control on the neural substrates of aversion in the IC and that high doses of morphine microinjected into IC cause pro-aversive actions probably through non-opioid mechanisms.

Binaural responses in rat inferior colliculus following kainic acid lesions of the superior olive: Interaural intensity difference functions

The binaural responses of cells in the rat's inferior colliculus were determined following either unilateral or bilateral lesions of the superior olivary complex (SOC). The lesions were made by local injection of kainic acid through a glass micropipette lowered stereotaxically into the auditory brain stem After a recovery period of at least two weeks, neural responses to pure tone pulses were recorded from the inferior colliculus with tungsten microelectrodes inserted into the central nucleus. Attention was focused on neurons that exhibited binaural suppression, ie., were excited by contralateral and inhibited by bilateral stimulation. Binaural suppression responses were still present following complete unilateral destruction of the superior olive either ipsilateral or contralateral to the recording site. Binaural responses were also present following bilateral lesions of the SOC. Futhermore, the shape and slope of interaural intensity difference functions were not statistically different for normal animals and animals with either unilateral or bilateral SOC lesions. No differences were found among lesion groups or between recording sites ipsilateral or contralateral to the lesions. These data suggest that the superior olive is not essential for binaural interaction and that supraolivary structures play a significant role in shaping binaural responses in the inferior colliculus.

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.

Wild running elicited by microinjections of bicuculline or morphine into the inferior colliculus of rats: Lack of effect of periaqueductal gray lesions

Bicuculline methiodide, a GABA A receptor antagonist, or a high dose of morphine was injected at the same site within the inferior colliculus (IC) of rats. Both drugs elicited the same behavioral activity (wild running). However, the time course and magnitude of the effects of the two drugs differed. Since the behavioral activation elicited was reminiscent of what was found with microinjections of bicuculline methiodide or morphine into the periaqueductal gray (PAG), we lesioned the PAG in another group of rats. It was found that extensive lesions of the PAG including those extending to the medial part of the superior colliculus did not significantly reduce the wild running.

Effects of superior olivary comples lesions on binaural responses in rat inferior colliculus

Unilateral or bilateral lesions of the superior olivary complex (SOC) were made by local injection of kainic acid through a micropipette lowered stereotaxically into the rat's auditory brainstem. The lesions had the effect of destroying cell bodies in the superior olive without disrupting fibers of passage. After a recovery period of approximately one month, physiological recordings were made with tungsten microelectrodes from the central nucleus of the inferior colliculus of animals with SOC lesions. For animals with unilateral lesions recordings were made either ipsilateral or contralateral to the site of damage. Results were compared with data from normal control cases. Monaural and binaural tone bursts were presented through a sealed sound delivery system and binaural interaction patterns were determined. Both binaural summation and suppression responses were still present following unilateral SOC lesions. Interaural intensity difference threslholds were within the normal range and no differences were found between animals with lesions placed ipsilateral or contralateral to the recording site. Binaural summation and suppression responses were also present following bilateral lesions. Bilateral lesions had the effect of shifting the average interaural intensity difference thresholds in favor of the contralateral ear. But, in general, binaural interactions were remarkably little affected by bilateral destruction of the SOC.

Differential sound-evoked metabolic activity in fetal tectum grafted into damaged adult rat inferior colliculus

Long-Evans rats with unilateral lesions of the dorsal inferior colliculus (IC) received transplants of fetal tectal tissue to determine functional efficacy. Acoustic stimulation increases metabolic activity in both graft and host tissues relative to spontaneous activity in these regions. During periods of quiescence, graft tissue shows basal metabolic activity similar to that found in host IC. Coupled with previous anatomical findings, the results suggest that tectal grafts not only possess a neural structure resembling normal adult IC, but also contain cellular constituents which are responsive to sound. Given the apparent system-appropriate function of implanted fetal tectum, the graft tissue may be able to have a restorative effect within the damaged central auditory pathway.

Sound frequency and binaural response properties of single neurons in rat inferior colliculus

Sound frequency and binaural response properties were determined for single neurons in the rat's inferior colliculus. Nerve cell responses in the central nucleus of the inferior colliculus were narrowly tuned and had clearly defined characteristic frequencies (CF). The central nucleus was tonotopically organized with low frequencies represented dorsolaterally and high frequencies ventromedially from 0.87 to 45 kHz. Sharpness of tuning, as indicated by Q 10, covered a wide range of values for neurons with the same CF, but the maximum Q 10 at each frequency increased monotonically with CF. Maximum Q 10s were larger than previously reported for auditory cortex at the same CF. Binaural responses were classified as either suppression, summation or mixed. Most of the units encountered exhibited binaural suppression but there were substantial numbers of both summation and mixed responses. Each major binaural response type was distributed broadly across sound frequencies within the rat's hearing range. Binaural suppression responses were most numerous at high frequencies and summation responses at low frequencies. The binaural response types, their relative proportions and their distribution by CF were similar for neurons in the central nucleus of inferior colliculus and primary auditory cortex of the albino rat.

Structural features of neurons in whole grafts of the rat inferior colliculus

The inferior colliculus (IC) is a midbrain structure that receives ascending auditory input from brainstem nuclei via the lateral lemniscus, sends efferent fibers to the medial geniculate body of thalamus and receives descending projections from auditory cortex. In the rat, the IC consists of dorsal and external cortices surrounding the central nucleus of IC (CNIC) which is populated by discoid and stellate neurons; the CNIC has a laminar appearance arising from organization of lemniscal fibers and processes of discoid cells. The IC of adult rats was chosen for implantation of whole grafts of E16–17 caudal tectum into unilateral lesion sites. Dendritic and somal architecture of graft neurons was examined 1 to 4.5 months following implantation using rapid Golgi, HRP and Nissl methods. The CNIC of rat is dominated by principal neurons with relatively flattened dendritic fields. In grafts of caudal tectum the most common neuron class observed possesses flattened dendritic arbors which often parallel one another. These neurons also resemble CNIC neurons of host tissue adjacent to the graft border. Spine formations appear on both proximal and distal dendrites of this neural type in both normal and implanted tissues. In addition, comparable somal features of graft neurons include ovoid or fusiform shapes with regular nuclear membranes as found in the normal colliculus. In Golgi stained material fewer stellate class neurons appear as in the normal CNIC, although stellate cell classes are more abundant in the pericentral areas of normal tissue. Both neuron populations are retrogradely labelled in graft and normal IC after HRP injection into the medial geniculate body. These features suggest that the graft core typically consists of prototypic CNIC cells. Other features of neuron and glial cell density vary in graft material which also shows a complex network of vasculature. These results demonstrate that whole grafts of caudal tectum placed into the inferior colliculus can form organized neural architecture similar to the normal CNIC. The somal, dendritic and spine features of these neurons form a potential substrate for connectional and functional properties which establish this preparation as suitable for further investigation as a model for development and recovery of function in the central auditory system.

Basic determinants for FM responses in the inferior colliculus of rats

The response of 835 click-sensitive neurons in the inferior colliculus (IC) to ramp frequency modulated (FM) tones was studied in the anaesthetized rat. More than 70% of the cells were sensitive to the FM sound, and over 25% were "FM specialized". Systematic variations of the stimulus parameters showed that sweep velocity, sweep range, and intensity of the FM signal were the 3 basic determinants for the unit response. For an "FM specialized" cell, the response pattern to each of the parameters was either monotonic or bell-shaped. The population statistics of response patterns to the FM parameters, including the tuning factors, were generated. A stimulus domain was proposed to represent the "receptive space" of the FM cells.

Laminar arrangement of C-fos-immunoreactive neurons in rat inferior colliculus following acoustic stimuli

Laminar arrangement of C-fos-immunoreactive neurons in rat inferior colliculus following acoustic stimuli

Auditory projections to the inferior colliculus of the rat are present by birth

Despite extensive literature on the anatomical organization of the adult mammalian auditory system, the ontogeny of internuclear connections is still obscure. We studied the postnatal development of afferent brainstem connections to the inferior colliculus in rat pups using WGA-HRP and DiI as tracers. At birth, connections to the inferior colliculus from the cochlear nuclei and nuclei of the superior olivary complexes and the lateral lemnisci are present. During successive development, there are no obvious changes in the quantity of labeled neurons or the basic labeling pattern. These results provide evidence that all input connections from auditory brainstem nuclei to the rat's inferior colliculus are established prenatally.

Functional reorganization of neural auditory maps by differential learning

A new perspective on neural mechanisms of learning is emerging from studies showing that some sensory maps in mature brains can be reorganized in response to conditioned stimuli. Autoradiography with [14C] 2-deoxy-2-fluoro-D-glucose was used to examine changes in the functional tonotopic organization of the rat inferior colliculus during differential Pavlovian conditioning. Metabolic comparisons of adjacent tonotopic maps were made within the same inferior colliculus. They showed that tonotopic maps were enhanced during differential learning of tones associated with an arousing stimulus. The findings support a concept of sensory learning that postulates a functional reorganization of sensory maps in adult brains.

Fetal tectum grafted as a cell suspension into the adult rat inferior colliculus

The known structural and functional features of the inferior colliculus provided an advantageous substrate for examining the outcome of neural transplantation. In the present study, tissue from the midbrain tectum of Sprague-Dawley rat fetuses was removed at 15, 16 and 19 days of gestation (E15, E16 and E19), incubated as a dissociated cell suspension for 12 to 15 h in a medium containing True Blue for prelabeling, and injected into the central nucleus of the inferior colliculus (CNIC) of 28 normal male adult conspecifics. Viable tectal grafts were found in 57.5% of host animals with 65.2% located in CNIC. Grafts of E16 tissue had a survival rate of 84.2%, which was twice that of E15 or E19 donors, and there was no difference in graft survival rate in hosts sacrificed up to 6 months after the implantation procedure. Neurons prelabeled with True Blue and counterstained with thionin were identified as uniquely green stained cells in brightfield illumination and brilliant opaque cells in darkfield microscopy. Grafts were organized in tightly packed clusters of cells including large rounded polygonal and smaller cells similar in size to normal CNIC large and small multipolar neurons. Cell morphology of the larger ovoid and fusiform grafted neurons most closely resembled that of typical adult CNIC cell populations; other implanted cell types appeared immature even when host post-implant survival was extended. At longer post-implant intervals graft and host cells further intermingled, and both cellular and fibrous bridges were observed at the interface between host and graft tissue. The present work demonstrates the viability and development of homotypic cells grafted into a central auditory structure utilizing a simple method of prelabeling donor tissue with a fluorescent dye.

Efferent connections from the auditory cortex to the inferior colliculus in rats : Junsei Horikawa, Yutaka Hosokawa, Tomoo Homma, Susumu Ito, and Keiichi Murata, Department of Neurophysiology, Medical Research Institute, Tokyo Medical and Dental University, 2-3-10, Kandasurugadai, Chiyodaku, Tokyo 101, Japan.

Efferent connections from the auditory cortex to the inferior colliculus in rats : Junsei Horikawa, Yutaka Hosokawa, Tomoo Homma, Susumu Ito, and Keiichi Murata, Department of Neurophysiology, Medical Research Institute, Tokyo Medical and Dental University, 2-3-10, Kandasurugadai, Chiyodaku, Tokyo 101, Japan.

Efferent connections from the auditory cortex to the inferior colliculus in rats

Efferent connections from the auditory cortex to the inferior colliculus in rats

Postnatal development of vascularity in the inferior colliculus of the young rat

The inferior colliculus in the rat midbrain is an auditory relay center whose functional maturation occurs postnatally. We examined by morphometry the vascularity and the nuclear profile density of the inferior colliculus in normal young rats at different ages (before and after the onset of auditory input). We also compared this region with a frontal region of the cerebral cortex in 24-day-old rats. The inferior colliculus from aldehyde-perfused Sprague-Dawley rats aged 5, 9, 14, and 24 days was analyzed by light microscopy of semithin plastic sections. The central region (mostly the central nucleus) was sampled at 5 levels representing its entire rostrocaudal extent. Patent-blood-vessel profiles were counted and classified according to their size and profile orientation. Counts of nuclear profiles in the same sections were also made. In the inferior colliculus of rats between 5 and 24 days of age, the small (less than 10-microns diameter) cross-sectioned vessel profiles increased over 6-fold in number per unit area. Correspondingly the vascular volume density, estimated by differential point counting, increased between these ages. However, there was a decrease in the number of neuronal and glial nuclear profiles per unit area, probably because of growth in the volume of the neuronal perikarya and processes, along with cell emigration reported to occur at early postnatal ages. This study has shown that an increase in vascularity in the central region of the rat inferior colliculus continues for up to 2 weeks after the onset of hearing.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino acid profiles in Long-Evans rat superior colliculus, visual cortex, and inferior colliculus.

An ultrasensitive triple-column ion-exchange/fluorometric method was utilized to measure the levels of over 30 amino acids and related primary amino compounds in Long-Evans rat superior colliculus (SC), visual cortex (VC) and inferior colliculus (IC). Comparison of levels of amino compounds revealed distinctly different profiles for each region. Major constituents were the neurotransmitters and related compounds glutamate, glutamine, GABA, taurine, aspartate and glycine. Glutathione levels were also relatively high in all three regions. SC exhibited a significantly higher level of GABA and beta-alanine compared to both VC and IC. VC had significantly higher levels of glutamate and taurine. VC exhibited the lowest level of glycine and IC the highest. A time-course experiment using SC documented that levels of eleven of thirty-four compounds, including GABA, were subject to significant postmortem alteration in vitro. SC GABA stability experiments indicated that significant in vitro increases of free GABA levels between 1 and 4 min postmortem were associated with equimolar decreases of conjugated GABA levels.

Nigrotectal projection to the inferior colliculus: Horseradish peroxidase transport and tyrosine hydroxylase immunohistochemical studies in rats, cats, and bats

The present study investigated descending projections from the substantia nigra to the auditory tectum. Small (0.02-0.05 microliters) injections of a 30-60% aqueous solution of horseradish peroxidase (HRP) were made unilaterally into the inferior colliculus in rats, cats, and bats (Eptesicus fuscus). Tissue blocks including the substantia nigra, superior colliculus, and inferior colliculus were removed, sectioned, and processed for visualization of HRP. Results show that the substantia nigra, pars lateralis, projects to the inferior colliculus ipsilaterally. In addition, retrogradely labeled cells are found dorsal to the pars lateralis, in a column within the lateral tegmental area of the midbrain. Analysis of injection sites suggests that the principal target of this nigral projection is the dorsal and rostral pericentral region of the inferior colliculus. Immunohistochemical studies with an antibody to tyrosine hydroxylase demonstrate catecholaminergic neurons within the pars lateralis and lateral tegmentum that are similar in location and morphology to one class of HRP retrogradely labeled cells within these structures. These immunohistochemical studies also demonstrate a plexus of fine, varicose tyrosine hydroxylase-positive axons in the rostral pericentral region of the colliculus. The presence of this nigrotectal projection to the inferior colliculus is discussed in relation to its possible role in the control of acousticomotor behavior.

Effects of unilateral microinjections of gabaergic drugs into the inferior colliculus on auditory evoked potentials and on audiogenic seizure susceptibility

Alteration of GABAergic neurotransmission within the inferior colliculus (IC) appears to be involved in the generation of the audiogenic seizure (AGS) susceptibility. In the present study, we provide evidence indicating that this susceptibility may result from IC neuronal hyperexcitability to sound induced by a decreased GABAergic inhibition. In a first experiment, a unilateral microinjection of bicuculline, a GABAa antagonist, into the IC of normal rats increased the amplitude of the collicular auditory evoked potential, while the microinjection of THIP, a GABAa agonist, decreased this response. In a second experiment, a unilateral microinjection of bicuculline into the IC induced AGS susceptibility in normal rats.