C-fos immunocytochemical evidence for acoustic pathway mapping in rats

Prenatal valproic acid exposure disrupts tonotopic c-Fos expression in the rat brainstem

Autism spectrum disorder (ASD) is a group of neurodevelopmental conditions characterized by difficulties in communication and social interactions, restricted, repetitive behaviors and sensory abnormalities. Notably, the vast majority of individuals with ASD experience some degree of auditory dysfunction and we have recently reported consistent hypoplasia and dysmorphology in auditory brainstem centers in individuals with ASD. Prenatal exposure to the antiepileptic drug valproic acid (VPA) is associated with an increased risk of ASD. In rodents, prenatal exposure to VPA is employed as an animal model of ASD and is associated with a number of anatomical, physiological and behavioral deficits, including hypoplasia and dysmorphology of auditory brainstem centers. Based on these observations, we hypothesized that such dysmorphology in VPA-exposed animals would translate into abnormal neuronal activity in brainstem circuits and irregular tonotopic maps. Herein, we have subjected control and VPA-exposed animals to 4- or 16-kHz tones and examined neuronal activation with immunohistochemistry for c-Fos. After these exposures, we identified significantly more c-Fos-positive neurons in the auditory brainstem of VPA-exposed animals. Additionally, we observed a larger dispersion of c-Fos-positive neurons and shifted tonotopic bands in VPA-exposed rats. We interpret these findings to suggest hyper-responsiveness to sounds and disrupted mapping of sound frequencies after prenatal VPA exposure. Based on these findings, we suggest that such abnormal patterns of activation may play a role in auditory processing deficits in ASD.

Prenatal valproic acid exposure disrupts tonotopic c-Fos expression in the rat brainstem

Autism spectrum disorder (ASD) is a neurodevelopmental condition characterized by difficulties with communication and social interactions, restricted, repetitive behaviors and sensory abnormalities. Additionally, the vast majority of subjects with ASD suffer some degree of auditory dysfunction and we have previously identified significant hypoplasia and dysmorphology in auditory brainstem centers in individuals with ASD. Prenatal exposure to the antiepileptic drug valproic acid (VPA) is associated with an increased risk of ASD. In rodents, prenatal exposure to VPA is utilized as an animal model of ASD and is associated with a number of anatomical, physiological and behavioral deficits, including hypoplasia and dysmorphology in the auditory brainstem. Based on these observations, we hypothesized that such dysmorphology in VPA-exposed animals would translate into abnormal activity in brainstem circuits and irregular tonotopic maps. Herein, we have subjected control and VPA-exposed animals to 4 or 16 kHz tones and examined neuronal activation with immunohistochemistry for c-Fos. After these sound exposures, we found significantly more c-Fos-positive neurons in the auditory brainstem of VPA-exposed animals. Further, we found a larger dispersion of c-Fos-positive neurons and shifted tonotopic bands in VPA-exposed rats. We interpret these findings to suggest hyper-responsiveness to sounds and disrupted mapping of sound frequencies after prenatal VPA exposure. Based on these findings, we suggest that such abnormal patterns of activation may play a role in auditory processing deficits in ASD.

Transient Down-Regulation of Sound-Induced c-Fos Protein Expression in the Inferior Colliculus after Ablation of the Auditory Cortex

We tested whether lesions of the excitatory glutamatergic projection from the auditory cortex (AC) to the inferior colliculus (IC) induce plastic changes in neurons of this nucleus. Changes in neuronal activation in the IC deprived unilaterally of the cortico-collicular projection were assessed by quantitative c-Fos immunocytochemistry. Densitometry and stereology measures of sound-induced c-Fos immunoreactivity in the IC showed diminished labeling at 1, 15, 90, and 180 days after lesions to the AC suggesting protein down-regulation, at least up to 15 days post-lesion. Between 15 and 90 days after the lesion, c-Fos labeling recovers, approaching control values at 180 days. Thus, glutamatergic excitation from the cortex maintains sound-induced activity in neurons of the IC. Subdivisions of this nucleus receiving a higher density of cortical innervation such as the dorsal cortex showed greater changes in c-Fos immunoreactivity, suggesting that the anatomical strength of the projection correlates with effect strength. Therefore, after damage of the corticofugal projection, neurons of the IC down-regulate and further recover sound-induced c-Fos protein expression. This may be part of cellular mechanisms aimed at balancing or adapting neuronal responses to altered synaptic inputs.

Acoustic stress activates tuberoinfundibular peptide of 39 residues neurons in the rat brain

Strong acoustic stimulation (105 dB SPL white noise) elicited c-fos expression in neurons in several acoustic system nuclei and in stress-sensitive hypothalamic nuclei and limbic areas in rats. In the present study, using this type of loud noise for 30 min, Fos-like immunoreactivity (Fos-ir) was investigated in neurons that synthesize tuberoinfundibular peptide of 39 residues (TIP39) in the rat brain: in the subparafascicular area of the thalamus, the posterior intralaminar complex of the thalamus and the medial paralemniscal nucleus in the lateral part of the pons. By double labeling, Fos-ir was shown in nearly 80% of TIP39-positive cells in the medial paralemniscal nucleus, 43% in the posterior intralaminar complex and 18.5% in the subparafascicular area 30 min after the end of a 30-min loud noise period. In control rats, only few neurons, including 0-4% of TIP39-positive neurons showed Fos-ir. While the majority of the Fos-ir neurons were TIP39-positive in the subparafascicular area and medial paralemniscal nucleus, a fairly high number of TIP39-immunonegative, chemically uncharacterized neurons expressed c-fos in the subparafascicular area and the posterior intralaminar complex of the thalamus. These observations clearly show that some TIP39 neurons in the so-called "acoustic thalamus" and the majority of TIP39 neurons in the medial paralemniscal nucleus are sensitive to loud noise and they may participate in the central organization of responses to acoustic stress. Furthermore, the present data suggest that non-TIP39-expressing neurons may play a prevalent role in the activity of the "acoustic thalamus".

Immediate early gene expression invoked by electrical intracochlear stimulation in some but not all types of neurons in the rat auditory brainstem

Specific patterns of sensory activity may induce plastic remodeling of neurons and the communication network they form in the adult mammalian brain. Among the indicators for the initiation of neuronal remodeling is the expression of immediate early genes (IEGs). The IEGs c-fos and egr-1 encode transcription factors. Following spectrally and temporally precisely defined unilateral electrical intracochlear stimulation (EIS) that corresponded in strength to physiological acoustic stimuli and lasted for 2 h under anesthesia, we characterized those neuronal cell types in ventral (VCN) and dorsal cochlear nucleus (DCN), lateral superior olive (LSO) and central nucleus of the inferior colliculus (CIC) of the rat brain that expressed IEGs. We found that EIS affected only specific types of neurons. Whereas sub-populations of glutamatergic and glycinergic cells responded in all four regions, GABAergic neurons failed to do so except in DCN. Combining immunocytochemistry with axonal tracing, neurons participating in major ascending pathways, commissural cells of VCN and certain types of neurons of the descending auditory system were seen to respond to EIS with IEG expression. By contrast, principal LSO cells projecting to the contralateral CIC as well as collicular efferents of the DCN did not. In total, less than 50% of the identified neurons turned up expression of the IEGs studied. The pattern of IEG expression caused by unilateral EIS led us to suggest that dominant sensory activity may quickly initiate a facilitation of central pathways serving the active ear at the expense of those serving the unstimulated ear.

Tinnitus behavior and hearing function correlate with the reciprocal expression patterns of BDNF and Arg3.1/arc in auditory neurons following acoustic trauma

The molecular changes following sensory trauma and the subsequent response of the CNS are poorly understood. We focused on finding a molecular tool for monitoring the features of excitability which occur following acoustic trauma to the auditory system. Of particular interest are genes that alter their expression pattern during activity-induced changes in synaptic efficacy and plasticity. The expression of brain-derived neurotrophic factor (BDNF), the activity-dependent cytoskeletal protein (Arg3.1/arc), and the immediate early gene c-Fos were monitored in the peripheral and central auditory system hours and days following a traumatic acoustic stimulus that induced not only hearing loss but also phantom auditory perception (tinnitus), as shown in rodent animal behavior models. A reciprocal responsiveness of activity-dependent genes became evident between the periphery and the primary auditory cortex (AI): as c-Fos and BDNF exon IV expression was increased in spiral ganglion neurons, Arg3.1/arc and (later on) BDNF exon IV expression was reduced in AI. In line with studies indicating increased spontaneous spike activity at the level of the inferior colliculus (IC), an increase in BDNF and GABA-positive neurons was seen in the IC. The data clearly indicate the usefulness of Arg3.1/arc and BDNF for monitoring trauma-induced activity changes and the associated putative plasticity responses in the auditory system.

Granule cells in the cochlear nucleus sensitive to sound activation detected by Fos protein expression

Granule cells are the smallest neuronal type in the cochlear nucleus (CN). Due to their small size, it is extremely difficult to record their sound-evoked activity with microelectrodes. Compared with large, non-granule cells, much less is known about their response properties to sound stimulation. Here, we use Fos, the nuclear regulatory protein, as a neuronal activity marker to determine the responsiveness of granule cells to sound in comparison to the larger neurons. The present study determined the threshold sensitivity and activation pattern of neurons in the three subdivisions of the CN with free-field sound stimulation in monaural, awake rats. Immunocytochemical localization of Fos was used as our metric for "sound activation." Neuronal types upregulating Fos expression in response to sound stimulation were further identified with Nissl counterstaining. Our results show that most CN cell types can upregulate Fos expression when sound activated and the number of Fos-expressing neurons is directly related to sound intensity. The threshold for Fos activation in granule cells is lower than that for non-granule cells. The number of Fos activated granule cells saturates at high sound intensity, while the number of Fos activated non-granule cells is a monotonic function. By comparing the patterns of sound-induced Fos expression in different CN cell types, it may be possible to predict features of sound-evoked activity in granule cells.

Reconnecting neuronal networks in the auditory brainstem following unilateral deafening

When we disturbed the auditory input of the adult rat by cochleotomy or noise trauma on one side, several substantial anatomical, cellular, and molecular changes took place in the auditory brainstem. We found that: (1) cochleotomy or severe noise trauma both lead to a considerable increase of immunoreactivity of the growth-associated protein GAP-43 in the ventral cochlear nucleus (VCN) of the affected side; (2) the expression of GAP-43 in VCN is restricted to presynaptic endings and short fiber segments; (3) axon collaterals of the cholinergic medial olivocochlear (MOC) neurons are the path along which GAP-43 reaches VCN; (4) partial cochlear lesions induce the emergence of GAP-43 positive presynaptic endings only in regions tonotopically corresponding to the extent of the lesion; (5) judging from the presence of immature fibers and growth cones in VCN on the deafened side, at least part of the GAP-43 positive presynaptic endings appear to be newly formed neuronal contacts following axonal sprouting while others may be modified pre-existing contacts; and (6) GAP-43 positive synapses are formed only on specific postsynaptic profiles, i.e., glutamatergic, glycinergic and calretinin containing cell bodies, but not GABAergic cell bodies. We conclude that unilateral deafening, be it partial or total, induces complex patterns of reconnecting neurons in the adult auditory brainstem, and we evaluate the possibility that the deafness-induced chain of events is optimized to remedy the loss of a bilaterally balanced activity in the auditory brainstem.

Electrical stimulation of the cochlear nerve in rats: analysis of c-Fos expression in auditory brainstem nuclei

We investigated functional activation of central auditory brainstem nuclei in response to direct electrical stimulation of the cochlear nerve using c-Fos immunoreactivity as a marker for functional mapping. The cochlear nerve was stimulated in the cerebellopontine angle of Lewis rats applying biphasic electrical pulses (120-250 muA, 5 Hz) for 30 min. In a control group, bilateral cochlectomy was performed in order to assess the basal expression of c-Fos in the auditory brainstem nuclei. The completeness of cochlear ablations and the response of auditory brainstem nuclei to electrical stimulation were electrophysiologically verified. C-Fos immunohistochemistry was performed using the free floating method. In anaesthetized animals with unilateral electrical stimulation of the cochlear nerve, increased expression of c-Fos was detected in the ipsilateral ventral cochlear nucleus (VCN), in the dorsal cochlear nucleus bilaterally (DCN), in the ipsilateral lateral superior olive (LSO) and in the contralateral inferior colliculus (IC). A bilateral slight increase of c-Fos expression in all subdivisions of the lateral lemniscus (LL) did not reach statistical significance. Contralateral inhibition of the nuclei of the trapezoid body (TB) was observed. Our data show that unilateral electrical stimulation of the cochlear nerve leads to increased expression of c-Fos in most auditory brainstem nuclei, similar to monaural auditory stimulation. They also confirm previous studies suggesting inhibitory connections between the cochlear nuclei. C-Fos immunoreactivity mapping is an efficient tool to detect functional changes following direct electrical stimulation of the cochlear nerve on the cellular level. This could be particularly helpful in studies of differential activation of the central auditory system by experimental cochlear and brainstem implants.

c-Fos immunohistochemical mapping of the audiogenic seizure network and tonotopic neuronal hyperexcitability in the inferior colliculus of the Frings mouse

The Frings mouse is a model of audiogenic seizure (AGS) susceptibility. The genetic locus responsible for the AGS phenotype in the Frings mouse has been named monogenic audiogenic seizure-susceptible (MASS1). MASS1 is unique in that it is one of only two identified seizure loci that are not associated with an ion channel mutation. Furthermore, Frings mice display a robust AGS phenotype demonstrating very high and prolonged susceptibility to sound-induced tonic extension seizures. The purpose of this investigation was to use c-Fos immunohistochemistry to map the brain structures involved in the Frings AGS and to examine neuronal hyperexcitability in the inferior colliculus, the brain structure that is recognized as the site of AGS initiation. AGS mapping revealed that intense seizure-induced neuronal activation was mostly limited to structures involved in a brainstem seizure network, including the external and dorsal nuclei of the inferior colliculus, as observed in other AGS rodents. Acoustically induced c-Fos expression in the central nucleus of the inferior colliculus to sub-AGS threshold tone stimulations displayed a greater level of neuronal activation in AGS-susceptible Frings, DBA/2J and noise-primed C57BL/6J mice compared to AGS-resistant C57BL/6J and CF1 mice. The AGS-susceptible mice also displayed c-Fos immunoreactivity that was more focused within the tonotopic response domain of the inferior colliculus compared to AGS-resistant mice. Furthermore, Frings mice displayed significantly greater tonotopic hyper-responsiveness compared to other AGS-susceptible mice.

Episodic blockade of cranial nerve VIII provokes asymmetric changes in lobule X of the rat

Although debilitating syndromes like Ménière's disease are in part characterized by recurrent or episodic vestibular disturbance the study of episodic vestibular disruption has only recently been possible with the introduction of a new model utilizing tetrodotoxin (TTX). In the present study, serial unilateral transtympanic administration of TTX produced behavioral symptoms indicative of transient vestibular disruption and novel patterns of Fos activity in the brainstem and cerebellum. Following two or three serial injections of TTX and a final survival time of 2 h, Fos immunocytochemistry revealed a distinct pattern of labeling in the brainstem that differed temporally from that observed following a single unilateral TTX injection. Specifically there was protracted expression of Fos in the beta subdivision of the inferior olive (IO) on the side ipsilateral to TTX treatment. In the cerebellum, the hallmark of episodic vestibular blockade was an asymmetric pattern of Fos labeling that involved all three layers of the cortex. In particular, there was prominent Fos labeling of Purkinje cells in the contra-TTX half of lobule X. In view of the fact that Fos labeling is not found in Purkinje cells following a single transient event or following peripheral vestibular ablation, it is suggested that Fos expression in Purkinje cells is a unique feature of episodic vestibular disruption and may represent a novel plastic response by a select population of Purkinje cells to episodic functional deafferentation.

Mild carbon monoxide exposure impairs the developing auditory system of the rat

The object of this study was to determine if chronic exposure to mild concentrations of CO in air caused changes in the integrity of the inferior colliculus during the most active period of synaptogenesis/auditory development. We examined all subregions of the inferior colliculus (IC) of rats by immunocytochemical approaches after pups were exposed chronically to CO concentrations of, 0, 12.5, 25, and 50 ppm in air starting at Day 8 through 20-22 days of age. Mother-reared pups were compared to the gastrostomy-reared pups with or without CO exposure for basal neural activity, using c-Fos immunoreactivity as a marker. Half the rats were examined at 27 days of age, 5 days after the end of CO exposure, and the other half were examined 50 days later at 75-77 days of age. In the central nucleus of the IC, the number of cells expressing a basal level of c-Fos was decreased significantly in the CO-exposed animals when compared to controls; however, there was little or no difference in the number of cells expressing c-Fos in the other subregions of the IC. We conclude that the central nucleus of the inferior colliculus is affected selectively by mild CO exposure (0.0012% in air) and that this reduction in neuronal activity persists into adulthood.

C-Fos immunoreactivity mapping of the auditory system after electrical stimulation of the cochlear nerve in rats

The aim of this study was to establish the use of c-Fos immunoreactivity as a marker for functional mapping in the auditory system in response to direct electrical stimulation of the cochlear nerve in the cerebellopontine angle. In rats the cochlear nerve was electrically stimulated with a biphasic current (120-250 microA, 5 Hz) for 30 min using a bipolar concentric Tungsten electrode. Bilateral cochlectomy was performed in a control group in order to investigate basal expression of c-Fos in the auditory brainstem nuclei. The response of auditory brainstem nuclei to electrical stimulation and the completeness of cochlear ablations were electrophysiologically verified. After the experiments, the animals were prepared for cryotomy and c-Fos immunohistochemistry. The results were morphologically analyzed and statistically compared among groups. In anesthetized animals with unilateral electrical stimulation of the cochlear nerve increased expression of c-Fos was detected in the ipsilateral ventral (VCN) and bilateral dorsal cochlear nucleus (DCN), whereas the VCN of the contralateral side revealed only few immunoreactive cells. In animals with bilateral cochlear ablation the number of c-Fos reactive cell nuclei representing basal expression was generally low in the VCN and DCN of both sides. Our data show that electrical stimulation of the cochlear nerve leads to increased expression of c-Fos in the cochlear nucleus. It also confirms bilateral connections between the cochlear nuclei. These experimental results suggest that c-Fos immunoreactivity mapping provides a powerful tool for functional investigations on the cellular level after direct electrical stimulation of the cochlear nerve. Future functional studies analyzing the effect of electrical stimulation of the central auditory system as performed by auditory brainstem implants could be investigated in detail by mapping c-Fos expression on cellular level.

Acute audiogenic stress-induced activation of CRH neurons in the hypothalamic paraventricular nucleus and catecholaminergic neurons in the medulla oblongata

Strong c-fos expression was induced in neuronal cells of several brain nuclei and the auditory cortex by a short duration auditory stimulation (white noise) in rats. By double immunostaining, Fos-immunoreactive cell nuclei appeared in corticotropin-releasing hormone (CRH)-containing neurons in the hypothalamic paraventricular nucleus, but not in CRH neurons elsewhere in the brain including the central nucleus of the amygdala. Among brain catecholaminergic neurons, only cells in the medulla oblongata (in the A1/C1and A2/C2 cell groups) established double immunostaining for Fos and tyrosine hydroxylase. Sound stimulus in rats with unilateral tympanotomy and plugging the airways resulted in side differences of Fos immunoreactivity in neurons of the auditory pathways and the auditory cortex, but the effect was bilateral in hypothalamic and amygdaloid nuclei. The present data provide evidence for the participation of CRH-synthesizing neurons in hypothalamus and medullary catecholaminergic neurons in the central organization of responses to audiogenic stress stimuli.

Differential changes in Fos-immunoreactivity at the auditory brainstem after chronic injections of salicylate in rats

In human, salicylate-induced tinnitus sometimes occurs a few days after its administration, but the chronic effects of salicylate in animal models are not fully known. In this study, we revealed the distribution of active cells in the rat auditory brainstem by staining an activity marker Fos-protein after multiple daily injections of salicylate. Experimental animals were first given five daily doses of sodium salicylate (250 mg/kg, i.p.). On day 6 they were placed inside a sound room for 8 h before sacrifice. Immunohistochemistry showed a significant increase in the number of Fos-positive cells at the inferior colliculus (IC), particularly its central division. At the cochlear nucleus (CN), only a few Fos-stains were found at the dorsal nucleus while no Fos-stain appeared at the ventral nucleus. The scarcity of Fos-stains at the CN reflected more a lack of external sound inputs than an adaptation in Fos-expression. Since Fos-stains in CN could still be induced on day 6 following brief tonal stimulation. Results are consistent with the hypothesis that salicylate-induced tinnitus is a phantom sound perception related to overactivity of cells at the IC.

Altered tone-induced Fos expression in the mouse inferior colliculus after early exposure to intense noise

Mice become highly susceptible to audiogenic seizures (AGS) after being exposed to intense, high-frequency noise during a critical period of early life (priming). To determine the critical site for AGS priming in the auditory brainstem, animals in the experimental group were primed at 21 days, and the tone-induced Fos immunoreactivity was examined 1, 7, and 14 days after priming as an index of excitability of neurons. Enhanced Fos immunoreactivity was observed in the inferior colliculus (IC) of the primed mice 7 and 14 days after priming as compared to that of non-primed mice and attenuated Fos expression was observed 1 day after priming. No significant elevation of Fos expression was observed in the cochlear nucleus and the deep layer of the superior colliculus of either type of mice. These results strongly suggest that the IC is the target site of AGS priming.

Acoustic trauma induces reemergence of the growth- and plasticity-associated protein GAP-43 in the rat auditory brainstem

We explored the consequences of unilateral acoustic trauma to intracochlear and central nervous system structures in rats. An acoustic trauma, induced by applying click stimuli of 130 dB (sound pressure level; SPL) for 30 minutes, resulted in an instant and permanent threshold shift of 95.92 +/- 1.08 dB (SEM) in the affected ear. We observed, as a consequence, a structural deterioration of the organ of Corti. Deprivation-dependent changes of neurons of the auditory brainstem were determined using antibodies against neurofilament and the growth-associated protein GAP-43 and compared with those following cochleotomy, studied earlier. By 231 days posttrauma, spiral ganglion cell bodies and their processes were almost entirely lost from all cochlear regions with destroyed organ of Corti. In the lateral superior olive (LSO) ipsilateral to the trauma, cell bodies of lateral olivocochlear neurons turned transiently GAP-43 positive within the first 1.5 years posttrauma. The time course of emergence and disappearance of this population of neurons was similar to that found after cochleotomy. Additionally, after noise trauma, principal cells in contralateral LSO and in medial superior olive (MSO) on both sides of the brainstem developed an expression of GAP-43 that began 3 and 16 days posttrauma, respectively, and lasted for at least 1 year. Such cells were rarely observed after cochleotomy. An unequivocal rise in GAP-43 immunoreactivity was also found in the neuropil of the inferior colliculus and the ventral cochlear nucleus, both preferentially on the acoustically damaged side. We conclude that the degree and specific cause of sudden unilateral deafness entail specific patterns of plasticity responses in the auditory brainstem, possibly to prevent the neural network dedicated to locate sounds in the environment from delivering erroneous signals centralward.

Transcription factor modulation and expression in the rat auditory brainstem following electrical intracochlear stimulation

Neuronal activity in sensory organs elicited by adequate or electrical stimulation not only invokes fast electrical responses but may also trigger complex molecular changes inside central neurons. Following electrical intracochlear stimulation with a cochlear implant under urethane anesthesia, we observed changes in the phosphorylation state of the cAMP response element binding protein (CREB) and the expression of the immediate-early genes c-fos and egr-1, molecules known to act as transcription factors, in a tonotopically precise pattern in central auditory neurons. These neurons resided in the posteroventral and anteroventral cochlear nucleus, the dorsal cochlear nucleus, the lateral superior olive, the medial nucleus of the trapezoid body, the dorsal and ventral nucleus of the lateral lemniscus, and the central nucleus of the inferior colliculus. Moreover, effects of electrical stimulation were identified in the medial vestibular nucleus and the lateral parabrachial nucleus. Regionally, CREB was dephosphorylated wherever immediate-early gene expression went up. These massive stimulation-dependent modulations of transcription factors in the ascending auditory system are indicative of ongoing changes that modify the chemistry and structure of the affected cells and, consequently, their response characteristics to subsequent stimulation of the inner ear.

Age dependent changes in the medial nucleus of the trapezoid body in gerbils

The results of a quantitative light microscopic analysis of serial glycine immunoreacted sections through the medial nucleus of the trapezoid body (MNTB) of young and old gerbils are presented. Spongiform lesions were prominent in the MNTB of gerbils that were 3 years and older, but were virtually absent in animals below 1 year of age. In old animals the prevalence and density of spongiform lesions were most pronounced in the caudal MNTB and decreased towards the rostral MNTB. Total MNTB volume and rostro-caudal extent were independent of age and the cross-sectional area of MNTB varied in an identical fashion along the MNTB in young and old gerbils. Mean MNTB soma size (cross-sectional area) varied with the age of the animal. In young gerbils soma size increased between 1 and 6 months of age reaching a maximum near 160 microm(2). In old gerbils mean soma size was significantly reduced to 130 microm(2). At all three rostro-caudal positions analyzed along MNTB, soma size varied systematically being largest in the ventro-lateral and smallest in the dorso-medial part of MNTB. The reduction of soma size in old animals appeared uniform across MNTB.

Inner ear lesion alters acoustically induced c-Fos expression in the rat auditory rhomboencephalic brainstem

The pattern of c-Fos expression was mapped in the adult rat's brain following unilateral cochlear lesions. In normal and cochlear lesioned rats, c-Fos expression was induced with sound stimuli. Acoustic stimulation consisted of pulses of four tones. An additional control group consisted of non-stimulated rats. In the cochlear nuclei (CN), c-Fos activation was scarce in isolated rats and increased strongly following sound stimulation. Following unilateral cochlear lesion, acoustically driven expression was decreased in all CN in both the lesioned and the untreated sides. The ventromedial periolivary nucleus and the rostral periolivary nucleus showed c-Fos activation in isolated conditions and were strongly activated following sound stimulation. The rest of the superior olivary complex showed no c-Fos activation in isolated rats and a weak activation following sound stimulation. Following unilateral cochlear lesions, acoustically driven expression was decreased in some, but not all superior olivary nuclei in both the lesioned and the untreated sides. In the lateral lemniscus complex, c-Fos activation was scarce in isolated rats and increased strongly after stimulation. Following unilateral cochlear lesion, acoustically driven expression decreased bilaterally in all nuclei. We have found that unilateral inner ear lesions lead to bilateral impairment of the capability of acoustic pathway neurons, to being c-Fos-activated following sound stimulation.

Activity-dependent plasticity in the adult auditory brainstem

Over the past few years we have studied the plasticity of the adult auditory brainstem in the rat following unilateral changes to the pattern of sensory activation, either by intracochlear electrical stimulation or by deafening. We discovered that modifications to afferent activity induced changes in the molecular composition and cellular morphology throughout the auditory brainstem, including its major centers: the cochlear nucleus complex, the superior olivary complex, and the inferior colliculus. The time window studied ranged from 2 h to over 1 year following induction of changes to afferent activity. The molecular markers employed include the NMDA receptor subunit type 1, the cAMP response element binding protein (CREB), the immediate early gene products c-Fos, c-Jun and Egr-1, the growth and plasticity-associated protein GAP-43 and its mRNA, the calcium binding protein calbindin, the cell adhesion molecule integrin-alpha(1), the microtubule-associated protein MAP-1b, and the neurofilament light chain (NF-L). As a consequence of the specific electrical stimulation of the auditory afferents or the loss of hearing, a cascade of events is triggered that apparently modifies the integrative action and computational abilities of the central auditory system. An attempt is made to relate the diverse phenomena observed to a common molecular signaling network that is suspected to bridge sensory experience to changes in the structure and function of the brain. Eventually, a thorough understanding of these events will be essential for the specific diagnosis of patients, optimal timing for implantation, and suitable parameters for running of a cochlear implant or an auditory brainstem implant in humans. In this report an overview of the results obtained in the past years in our lab is presented, flanked by an introduction into the history of plasticity research and a model proposed for intracellular signal cascades related to activity-dependent plasticity.

Fos imaging reveals differential neuronal activation of areas of rat temporal cortex by novel and familiar sounds

To provide information about the possible regions involved in auditory recognition memory, this study employed an imaging technique that has proved valuable in the study of visual recognition memory. The technique was used to image populations of neurons that are differentially activated by novel and familiar auditory stimuli, thereby paralleling previous studies of visual familiarity discrimination. Differences evoked by novel and familiar sounds in the activation of neurons were measured in different parts of the rat auditory pathway by immunohistochemistry for the protein product (Fos) of the immediate early gene c-fos. Significantly higher counts of stained neuronal nuclei (266 +/- 21/mm2) were evoked by novel than by familiar sounds (192 +/- 17/mm2) in the auditory association cortex (area Te3; AudA). No such significant differences were found for the inferior colliculus, primary auditory cortex, postrhinal cortex, perirhinal cortex (PRH), entorhinal cortex, amygdala or hippocampus. These findings are discussed in relation to the results of lesion studies and what is known of areas involved in familiarity discrimination for visual stimuli. Differential activation is produced by novel and familiar individual stimuli in sensory association cortex for both auditory and visual stimuli, whereas the PRH is differentially activated by visual but not auditory stimuli. It is suggested that this latter difference is related to the nature of the particular auditory and visual stimuli used.

Chorda tympani nerve stimulation evokes Fos expression in regionally limited neuron populations within the gustatory nucleus of the solitary tract

The distribution of neurons in the rostral nucleus of the solitary tract (rNST) that respond to gustatory input from the anterior tongue was visualized by Fos protein immunohistochemistry following electrical stimulation of the chorda tympani (CT) nerve in rats. Maps of Fos-immunoreactive (Fos-ir) neurons were compared with the distribution of CT afferent terminal fields labeled by transganglionic transport of rhodamine-dextran in a separate group of animals. The primary concentration of Fos-ir neurons localized in register with the major terminal fields of CT afferent fibers, in the central third of the rostral 1.0 mm of the NST ipsilateral to the stimulated nerve. A similar correspondence in location and degree of labeling of Fos-ir neurons and afferent terminals was observed in the ipsilateral dorsal spinal trigeminal complex (Sp5) pars caudalis, near the obex, and the Sp5 pars oralis near the rostral pole of the rNST. Thus, the magnitude of Fos upregulation in brainstem targets of the CT nerve having chemosensory or nociceptive function, was proportional to the relative density of the CT afferent input. This correspondence, and the absence of labeling in neurons known to be one additional synapse away from the afferent input within gustatory or oral reflex pathways, suggests that the cell map obtained represents mainly neurons that are directly activated via primary afferent synapses from CT fibers. The availability of a method to histochemically identify a population of putative second-order taste neurons will facilitate analysis of the cellular/molecular properties of these neurons and of synaptic circuitry in the rNST.

Modulation of P-CREB and expression of c-fos in cochlear nucleus and superior olive following electrical intracochlear stimulation

Investigating activity-dependent plasticity in the auditory brain stem of the adult rat, we observed that electrical intracochlear stimulation led to a tonotopically localized modulation of the phosphorylation of the cAMP response element binding protein (CREB) and an equally localized expression of the immediate early gene product c-Fos in cochlear nucleus and superior olive. As P-CREB is thought to act as transcription factor on one promoter site of the c-fos gene, we compared immunolabeling for P-CREB and c-fos in adjacent brain sections. Following 2h sustained stimulation in previously deafened animals, labelling for P-CREB declined in regions where c-Fos labelling increased. This suggests that the level or state of P-CREB (e.g. whether it is phosphorylated or not) are affected by intracochlear stimulation in a process that appears to be linked to the stimulation-dependent expression of c-Fos in auditory brain stem nuclei.

Plasticity of the superior olivary complex

The superior olivary complex (SOC) is part of the auditory brainstem of the vertebrate brain. Residing ventrally in the rhombencephalon, it receives sensory signals from both cochleae through multisynaptic pathways. Neurons of the SOC are also a target of bilateral descending projections. Ascending and descending efferents of the SOC affect the processing of auditory signals on both sides of the brainstem and in both organs of Corti. The pattern of connectivity indicates that the SOC fulfills functions of binaural signal integration serving sound localization. But whereas many of these connectional features are shared with the inferior colliculus (with the important exception of a projection to the inner ear), cellular and molecular investigations have shown that cells residing in SOC are unique in several respects. Unlike those of other auditory brainstem nuclei, they specifically express molecules known to be involved in development, plasticity, and learning (e.g., GAP-43 mRNA, specific subunits of integrin). Moreover, neurons of the SOC in adult mammals respond to various kinds of hearing impairment with the expression of plasticity-related substances (e.g., GAP-43, c-Jun, c-Fos, cytoskeletal elements), indicative of a restructuring of auditory connectivity. These observations suggest that the SOC is pivotal in the developmental and adaptive tuning of binaural processing in young and adult vertebrates.

c-Fos expression in the auditory pathways related to the significance of acoustic signals in rats performing a sensory-motor task

Neuronal activity was established in the auditory pathways in relation to behavioural response and cognitive information processing during a sensory-motor acoustic learning. Rats were trained in three consecutive phases. The first phase was an association between an auditory stimulus and a food reward; the second phase a simple discrimination between two sounds of different frequency components, and the third phase a more complex discrimination involving both spectral and spatial sound dimensions. Auditory stimuli were bursts of complex sounds lasting 500 ms. Neuronal activity related to the behaviourally relevant stimuli was established in 20 "learning" rats undergoing this protocol, which were progressively sacrificed at the beginning, middle and end of each phase. For comparison, activity was also established in four "control" rats exposed to the same stimuli delivered pseudo-randomly, thus carrying no behavioural meaning. Neuronal activity was assessed immunocytochemically using the functional marker Fos. To establish a baseline, two rats were unexposed to controlled acoustic stimulation ("unstimulated" rats). In the superior olivary complex (SOC), inferior colliculus (IC) and medial geniculate body (MGB), the number of Fos-like immunopositive cells was comparable in "learning" and "control" animals, but higher than in the "unstimulated" rats. In the auditory cortex (AC), most prominently in the secondary area Te2, the number of Fos-like positive cells differed between "learning" and "control" rats, suggesting that the auditory cortical areas may be involved in the encoding of the behavioural significance of the acoustic stimuli.

Glossopharyngeal nerve transection eliminates quinine-stimulated fos-like immunoreactivity in the nucleus of the solitary tract: implications for a functional topography of gustatory nerve input in rats

The relationship between specific gustatory nerve activity and central patterns of taste-evoked neuronal activation is poorly understood. To address this issue within the first central synaptic relay in the gustatory system, we examined the distribution of neurons in the nucleus of the solitary tract (NST) activated by the intraoral infusion of quinine using Fos immunohistochemistry in rats with bilateral transection of the chorda tympani (CTX), bilateral transection of the glossopharyngeal nerve (GLX), or combined neurotomy (DBLX). Compared with nonstimulated and water-stimulated controls, quinine evoked significantly more Fos-like-immunoreactive (FLI) neurons across the rostrocaudal extent of the gustatory NST (gNST), especially within its dorsomedial portion (subfield 5). Although the somatosensory aspects of fluid stimulation contributed to the observed increase in FLI neurons, the elevated number and spatial distribution of FLI neurons in response to quinine were remarkably distinguishable from those in response to water. GLX and DBLX produced a dramatic attenuation of quinine-evoked FLI neurons and a shift in their spatial distribution such that their number and pattern were indiscernable from those observed in water-stimulated controls. Although CTX had no effect on the number of quinine-evoked FLI neurons within subfield 5 at intermediate levels of the gNST, it produced intermediate effects elsewhere; yet, the spatial distribution of the quinine-evoked FLI neurons was not altered by CTX. These findings suggest that the GL provides input to all FLI neurons responsive to quinine, however, some degree of convergence with CT input apparently occurs in this subpopulation of neurons. Although the role of these FLI neurons in taste-guided behavioral responses to quinine remains speculative, their possible function in oromotor reflex control is considered.

Cochlear ablation alters acoustically induced c-fos mRNA expression in the adult rat auditory brainstem

Expression of c-fos mRNA was studied in the adult rat brain following cochlear ablations by using in situ hybridization. In normal animals, expression was produced by acoustic stimulation and was found to be tonotopically distributed in many auditory nuclei. Following unilateral cochlear ablation, acoustically driven expression was eliminated or decreased in areas normally activated by the ablated ear, e.g., the ipsilateral dorsal and ventral cochlear nuclei, dorsal periolivary nuclei, and lateral nucleus of the trapezoid body and the contralateral medial and ventral nuclei of the trapezoid body, lateral lemniscal nuclei, and inferior colliculus. These deficits did not recover, even after long survivals up to 6 months. Results also indicated that neurons in the dorsal cochlear nucleus could be activated by contralateral stimulation in the absence of ipsilateral cochlear input and that the influence of the contralateral ear was tonotopically organized. Results also indicated that c-fos expression rose rapidly and persisted for up to 6 months in neurons in the rostral part of the contralateral medial nucleus of the trapezoid body following a cochlear ablation, even in the absence of acoustic stimulation. This response may reflect a release of constitutive excitatory inputs normally suppressed by missing afferent input or changes in homeostatic gene expression related to sensory deprivation. Instances of transient, surgery-dependent increases in c-fos mRNA expression in the absence of acoustic stimulation were observed in the superficial dorsal cochlear nucleus and the cochlear nerve root on the ablated side.

Inducible and constitutive transcription factors in the mammalian nervous system: control of gene expression by Jun, Fos and Krox, and CREB/ATF proteins

This article reviews findings up to the end of 1997 about the inducible transcription factors (ITFs) c-Jun, JunB, JunD, c-Fos, FosB, Fra-1, Fra-2, Krox-20 (Egr-2) and Krox-24 (NGFI-A, Egr-1, Zif268); and the constitutive transcription factors (CTFs) CREB, CREM, ATF-2 and SRF as they pertain to gene expression in the mammalian nervous system. In the first part we consider basic facts about the expression and activity of these transcription factors: the organization of the encoding genes and their promoters, the second messenger cascades converging on their regulatory promoter sites, the control of their transcription, the binding to dimeric partners and to specific DNA sequences, their trans-activation potential, and their posttranslational modifications. In the second part we describe the expression and possible roles of these transcription factors in neural tissue: in the quiescent brain, during pre- and postnatal development, following sensory stimulation, nerve transection (axotomy), neurodegeneration and apoptosis, hypoxia-ischemia, generalized and limbic seizures, long-term potentiation and learning, drug dependence and withdrawal, and following stimulation by neurotransmitters, hormones and neurotrophins. We also describe their expression and possible roles in glial cells. Finally, we discuss the relevance of their expression for nervous system functioning under normal and patho-physiological conditions.

Attenuation of Fos expression to airpuff startle stimuli following tympanic membrane rupture

The airpuff startle stimulus consists of two modalities, tactile and acoustic. Tympanic membrane rupture (TMR) effectively deafens a rat, thus preventing it from perceiving the acoustic component of the airpuff and permitting study of the tactile component in isolation. Previous studies have shown that the tactile modality is sufficient to drive the cardiovascular response to the airpuff, but cannot elicit the full behavioral startle response. In the present study Fos protein was used as a marker of neuronal activation to identify brain regions activated by the airpuff in both intact and TMR rats. Results show an attenuation of Fos expression following TMR in the dorsal and ventral cochlear nuclei, ventral nucleus of the lateral lemniscus and medial geniculate nucleus. In contrast, Fos expression following TMR was unchanged in the locus coeruleus, the laterodorsal tegmental nucleus, the supramammilary nucleus, and the ventromedial hypothalamic nucleus. Analysis of behavioral data confirmed that the startle response to the airpuff was diminished following TMR. These data are the first of which we know to employ an immediate early gene approach to discriminate between brain regions activated by the tactile and acoustic startle stimulus modalities. The results are discussed in terms of the classical acoustic startle circuit, and the central autonomic pathways activated by the tactile component of the airpuff.

Latent inhibition in conditioned emotional response: c-fos immunolabelling evidence for brain areas involved in the rat

Latent inhibition refers to the fact that the formation of a conditioned association between a conditioned and an unconditioned stimulus is delayed by prior exposure to the conditioned stimulus. Latent inhibition is often investigated in the context of the conditioned emotional response, in which a tone serves as the conditioned and a footshock as the unconditioned stimulus. Such a paradigm was used for the present experiments in which some rats had been pre-exposed to the tone. Two hours after a subsequent exposure to the tone, c-fos immunocytochemistry was used to map activated brain areas. The density of immunoreactive neurones was measured in brain areas involved in audition, fear, stress and memory. For the basic conditioning group, pre-exposure to the tone decreased the density of labelled cells in the auditory system, areas involved in fear and stress and a number of limbic areas, namely the amygdala, the Ammon's horn of the hippocampus and the entorhinal cortex. In contrast, the density increased in three limbic areas: the dentate gyrus, the subiculum and the nucleus accumbens. Taken together, these data suggest that latent inhibition corresponds to alterations of sensory processing which renders difficult to state about the alteration of the transfers of the sensory information to structures involved in the control of emotional responses. As some brain areas show a specific increase of activity in cases of latent inhibition, further studies will investigate how the latter brain areas contribute to the other cell density alterations reported in this study and to the latent inhibition phenomenon itself.