Pre- and postnatal development of efferent connections of the cochlear nucleus in the rat

Although the connections of the auditory brainstem nuclei are well described in adult mammals, almost nothing is known concerning how and when these connections develop. The purpose of the present study was to describe the development of the efferent projections of the cochlear nucleus (CN), the first central relay station in the ascending auditory pathway of mammals. We used two tracers in rats aged between embryonic day 15 (E15) and postnatal day 14 (P14; birth in the rat is at E22 = P0). The carbocyanine dye DiI was applied into the CN in aldehyde-fixed tissue. The second tracer, biocytin, was applied into the ventral acoustic stria in an in vitro slice preparation. The ontogeny of the efferent projections from the CN could be divided into three periods. The first period (E15-E17) is characterized by axonal outgrowth. Axons traverse nuclei in the superior olivary complex and the lateral lemniscus and finally grow up into the inferior colliculus, but axon collaterals do not form during this period. The second period (E18-P5) is marked by pronounced collateral branching of CN fibers in auditory brainstem nuclei. Collateralisation in the contralateral inferior colliculus starts shortly before that in the ipsilateral superior olivary complex. The remaining auditory nuclei become successively innervated, as indicated by collaterals found in them. During the third period (P5-P14) terminal structures mature further, as shown by the morphological changes of the calyces of Held in the medial nucleus of the trapezoid body. In conclusion, our results show that the efferent connections from the cochlear nucleus form over a period of almost two weeks and are laid down without forming aberrant internuclear connections. On a nuclear level, an adult-like projection pattern is already achieved one week prior to the onset of physiological hearing.

Divergent projections of physiologically characterized rat ventral cochlear nucleus neurons as shown by intra-axonal injection of horseradish peroxidase

An attempt was made to correlate electrophysiological and morphological characteristics of rat ventral cochlear nucleus neurons. Their axonal course and their soma morphology were investigated using the intra-axonal horseradish peroxidase method. Prior to labeling, neurons were characterized by recording their response patterns to acoustic stimulation with pure tones. Three types of cells were found: Category I (37 neurons) exhibited "primarylike" responses and a spontaneous firing rate below 10 spikes/s. Category II (21 neurons) showed "on" responses and little spontaneous activity. Category III (9 neurons) had "primarylike" responses like neurons in category I. However, the spontaneous activity rate of these neurons was significantly higher (mean: 95 spikes/s). Among the response categories, the morphological characteristics differed in some prominent aspects. Within each category, however, the morphological properties were rather similar. All neurons in category I were globular/bushy cells located in the area of the entrance of the cochlear nerve. The axon of each cell coursed along the ventral acoustic stria and consistently innervated the lateral superior olive ipsilaterally, and the nucleus of the trapezoid body and the nucleus of the lateral lemniscus contralaterally. Some neurons also projected to periolivary nuclei ipsilaterally and contralaterally. Neurons in category II were located in the posteroventral cochlear nucleus and were presumably multipolar/stellate cells. Their axons coursed via the intermediate acoustic stria and innervated mainly contralateral periolivary regions as well as the contralateral nucleus of the lateral lemniscus. Ipsilaterally, the lateral superior olive and the superior periolivary nucleus were innervated by some of the category II neurons. Somata types of neurons in category III could not be identified morphologically, but somata were located in caudal parts of the posteroventral cochlear nucleus that correspond to the octopus cell area. Their axons coursed via the intermediate acoustic stria and innervated periolivary regions and the contralateral nucleus of the lateral lemniscus. Thus, their axonal distribution differed only slightly from neurons in category II. These data confirm and extend previous findings regarding the efferent connections of ventral cochlear neurons. They emphasize the complexity of the axonal projection patterns of single cochlear nucleus cells. Since two types of response patterns and three types of axonal projection patterns have been observed, there remains an ambiguous relation between response pattern and axonal projection site.(ABSTRACT TRUNCATED AT 250 WORDS)

Distribution of the calcium-binding proteins parvalbumin and calretinin in the auditory brainstem of adult and developing rats

Parvalbumin (PV), calretinin (CR), and calbindin (CB) are calcium-binding proteins which are presumably involved in the regulation of the intracellular calcium concentration. Within the rat auditory system, CB is transiently expressed in several nuclei during the period of synapse refinement, indicating a specific function of CB during development, yet little is known in this regard about PV and CR. In order to gather more information about calcium-binding proteins during development, we analyzed the spatiotemporal distribution of PV and CR in the rat auditory brainstem using immunocytochemistry. In the adult, PV was heavily present in somata and neuropil of all nuclei and in fibers of all tracts. CR was found in somata of the cochlear nucleus and peripheral aspects of the inferior colliculus as well as in fibers extending into the superior olivary complex and the nuclei of the lateral lemniscus. The developmental expression of PV was characterized by a relatively late appearance in somata (at postnatal day 8), followed by a rapid increase to adult levels. In contrast, CR immunoreactivity was already strong two days before birth, yet the number and intensity of labeled neurons subsequently decreased and CR disappeared almost completely in the superior olivary complex, nuclei of the lateral lemniscus, and central aspects of the inferior colliculus. These data, together with those on CB, show that CR, CB, and PV are sequentially expressed during auditory brainstem development. They also suggest that the presence of the three proteins can be correlated with definite developmental stages.

Shift from depolarizing to hyperpolarizing glycine action in rat auditory neurones is due to age-dependent Cl- regulation

1. The inhibitory neurotransmitter glycine can elicit depolarizing responses in immature neurones. We investigated the changes in glycine responses and their ionic mechanism in developing neurones of the rat lateral superior olive (LSO), an auditory brainstem nucleus involved in sound localization. 2. Whole-cell and gramicidin perforated-patch recordings were performed from visually identified LSO neurones in brain slices and glycine was pressure applied for 3-100 ms to the soma. Glycine-evoked currents were reversibly blocked by strychnine. They were mostly monophasic, but biphasic responses occurred in approximately 30 % of P8-11 neurones in perforated-patch recordings. 3. In whole-cell recordings from P2-11 neurones, the reversal potential of glycine-evoked currents (EGly) was determined by the transmembranous Cl- gradient and corresponded closely to the Nernst potential for Cl-, regardless of age. This indicates that Cl- is the principle ion permeating glycine receptors, but is also consistent with a low relative (10-20 %) permeability for HCO3-. The Cl- gradient also determined the polarity and amplitude of glycine-evoked membrane potential changes. 4. Leaving the native intracellular [Cl-] undisturbed with gramicidin perforated-patch recordings, we found a highly significant, age-dependent change of EGly from -46.8 +/- 1.8 mV (P1-4, n = 28) to -67.6 +/- 3.3 mV (P5-8, n = 10) to -82.2 +/- 4.1 mV (P9-11, n = 18). The majority of P1-4 neurones were depolarized by glycine ( approximately 80 %) and spikes were evoked in approximately 30 %. In contrast, P9-11 neurones were hyperpolarized. 5. In perforated-patch recordings, EGly was influenced by the voltage protocol and the glycine application interval; it could be shifted in the positive and negative direction. For a given application interval, these shifts were always larger in P1-4 than in P8-11 neurones, pointing to less effective Cl- regulation mechanisms in younger neurones. 6. Furosemide (frusemide), a blocker of cation-Cl- cotransporters, reversibly shifted EGly in the negative direction in P2-4 neurones, yet in the positive direction in P8-10 neurones, suggesting the blockade of net inward and net outward Cl- transporters, respectively. 7. Taken together, age-dependent changes in active Cl- regulation are likely to cause the developmental shift from depolarizing to hyperpolarizing glycine responses. A high intracellular [Cl-] is generated in neonatal LSO neurones which decreases during maturation.

Principal cells of the rat medial nucleus of the trapezoid body: an intracellular in vivo study of their physiology and morphology

The medial nucleus of the trapezoid body (MNTB) is one of several principal nuclei in the superior olivary complex (SOC) of mammals. It is classically thought to function as a relay station between the contralateral ventral cochlear nucleus and the lateral superior olive (LSO), playing a role among those brainstem nuclei that are involved in binaural hearing. In order to characterise the physiology and morphology at the cellular level of the major neuronal component of the MNTB, the principal cells, we have analysed these neurons in rats in vivo using intracellular recordings and horseradish peroxidase-labelling. Our data demonstrate that MNTB principal cells, when being stimulated acoustically via the contralateral ear, show a phasic-tonic response with an onset latency of 3.5 ms and a suppression of their spontaneous activity following stimulus offset. These neurons have an axonal morphology whose complexity has not yet been described. All cells (n = 10) projected exclusively ipsilaterally and had terminal axonal arbors in a variety of auditory brainstem nuclei. At least two and maximally seven auditory targets were innervated by an individual cell. Each cell projected into the LSO and the superior paraolivary nucleus (SPN). Additional projections that were intrinsic to the SOC were often observed in the lateral nucleus of the trapezoid body and in periolivary regions, with only one cell projecting into the medial superior olive. Most, if not all, MNTB principal cells also had projections that were extrinsic to the SOC, as their axons ascended into the lateral lemniscus. In two neurons the ascending axon formed terminal arbors in the ventral nucleus of the lateral lemniscus, and the dorsal nucleus of the lateral lemniscus could be identified as a target of one neuron. The location of the cell bodies of the MNTB principal cells correlated with the neurons' best frequencies, thereby demonstrating a tonotopic organisation of the MNTB, with high frequencies being represented medially and low frequencies laterally. The axonal projections into the LSO and the SPN were also tonotopically organised and the alignment of the tonotopically organised and the alignment of the tonotopic axes was similar to that in the MNTB. Our results confirm previous data from other species and suggest that MNTB principal cells have a great amount of physiological and morphological similarities across mammalian species. Furthermore, the complexity of the axonal projections indicates that these neurons play a role in auditory information processing which goes far beyond their previously described classical role.

Changing patterns of synaptic input to subplate and cortical plate during development of visual cortex

1. The development of excitatory activation in the visual cortex was studied in fetal and neonatal cats. During fetal and neonatal life, the immature cerebral cortex (the cortical plate) is sandwiched between two synaptic zones: the marginal zone above, and an area just below the cortical plate, the subplate. The subplate is transient and disappears by approximately 2 mo postnatal. Here we have investigated whether the subplate and the cortical plate receive functional synaptic inputs in the fetus, and when the adultlike pattern of excitatory synaptic input to the cortical plate appears during development. 2. Extracellular field potential recording to electrical stimulation of the optic radiation was performed in slices of cerebral cortex maintained in vitro. Laminar profiles of field potentials were converted by the current-source density (CSD) method to identify the spatial and temporal distribution of neuronal excitation within the subplate and the cortical plate. 3. Between embryonic day 47 (E47) and postnatal day 28 (P28; birth, E65), age-related changes occur in the pattern of synaptic activation of neurons in the cortical plate and the subplate. Early in development, at E47, E57, and P0, short-latency (probably monosynaptic) excitation is most obvious in the subplate, and longer latency (presumably polysynaptic) excitation can be seen in the cortical plate. Synaptic excitation in the subplate is no longer apparent at P21 and P28, a time when cell migration is finally complete and the cortical layers have formed. By contrast, excitation in the cortical plate is prominent in postnatal animals, and the temporal and spatial pattern has changed. 4. The adultlike sequence of synaptic activation in the different cortical layers can be seen by P28. It differs from earlier ages in several respects. First, short-latency (probably monosynaptic) excitation can be detected in cortical layer 4. Second, multisynaptic, long-lasting activation is present in layers 2/3 and 5. 5. Our results show that the subplate zone, known from anatomic studies to be a synaptic neurophil during development, receives functional excitatory inputs from axons that course in the developing white matter. Because the only mature neurons present in this zone are the subplate neurons, we conclude that subplate neurons are the principal, if not the exclusive, recipients of this input. The results suggest further that the excitation in the subplate in turn is relayed to neurons of the cortical plate via axon collaterals of subplate neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Development of auditory brainstem circuitry. Activity-dependent and activity-independent processes

Despite its complexity, the neural circuitry in the auditory brainstem of vertebrates displays a fascinating amount of order. How is this order established in such a precise manner during ontogeny? In this review, we will summarize evidence for both activity-independent and activity-dependent processes involved in the generation of the auditory brainstem circuitry of birds and mammals. An example of activity-independent processes is the emergence of crude topography, which, most probably, is determined by molecular markers whose expression is genetically controlled. On the other hand, neuronal activity supports cell survival, affects dendritic and axonal growth, and influences fine tuning of maps. It appears that various types of neuronal activity, namely spontaneous versus acoustically evoked, bilateral versus unilateral, uncoordinated versus patterned, play a role during different aspects of development and cooperate with the activity-independent processes to ensure the proper formation of neuronal circuitry.

Giant neurons in the caudal pontine reticular formation receive short latency acoustic input: an intracellular recording and HRP-study in the rat

The reticular formation is composed of heterogeneous cell populations with multiple functions. Among these multiple functions is the processing of sensory information in the context of behavior. The purpose of the present study was to identify and characterize neurons in the reticular formation of the rat that receive auditory input. In order to do so, we combined intracellular electrophysiology in vivo with intracellular injection of horseradish peroxidase, enabling us to correlate electrophysiology unequivocally with anatomy at the single cell level. We found that many neurons in the caudal pontine reticular nucleus (PnC), which we analyzed intracellularly, responded to acoustic stimuli and were excited at short latency (mean EPSP latency: 2.6 ms; mean spike latency: 5.2 ms). This short latency suggests a direct input from the cochlear nucleus, the first central nucleus of the auditory pathway. The morphology revealed that the acoustically driven PnC neurons have very large somata (mean diameter: 44.0 microns). They can therefore be referred to as "giant PnC neurons." Complex dendritic arbors extended from these neurons into the reticular formation and thus formed a large membrane surface for the integration of multimodal inputs. Most of the giant PnC neurons sent their axons caudally into the medial longitudinal fasciculus and can therefore be regarded as reticulospinal neurons. The results demonstrate that the giant reticulospinal PnC neurons are in a position to transmit acoustic information very quickly to spinal cord neurons and to receive converging input from other parts of the brain. They are thus good candidates for participation in the mediation and modulation of acoustically elicited behaviors, such as the short latency acoustic startle response.

Development and influence of inhibition in the lateral superior olivary nucleus

While studies of neuronal development and plasticity have focused on excitatory pathways, the inhibitory projection from the MNTB to the LSO provides a favorable model for studies of synaptic inhibition. This review covers recent studies from our laboratories indicating that inhibitory connections are quite dynamic during development. These findings suggest that there are two phases inhibitory transmission. During an initial depolarizing phase is growth and branching of pre- and postsynaptic elements in the LSO. During a second hyperpolarizing phase there is refinement of inhibitory afferent arborizations and the LSO dendrites that they innervate.

Development of adult-type inhibitory glycine receptors in the central auditory system of rats

Inhibitory synaptic activity is crucial for many aspects of acoustic information processing and mainly mediated by glycine and gamma-aminobutyric acid, the two principal inhibitory neurotransmitters in the auditory system. Glycine exerts its inhibitory action via binding to postsynaptic receptors existing in various isoforms. Here we have investigated the spatiotemporal distribution of adult-type, strychnine-sensitive glycine receptors (GlyRs) in the rat auditory system by using a specific antibody against the ligand-binding alpha1 GlyR subunit. In adults, alpha1 GlyRs were found at all relay stations of the auditory pathway except for the medial geniculate body and the auditory cortex. In most brainstem nuclei, labeling was characterized by dense clusters of heavily immunoreactive puncta outlining the somata and proximal dendrites, indicative of a powerful glycinergic inhibition. No alpha1 immunoreactivity was seen in the auditory system of fetal rats, consistent with results obtained by others in the spinal cord. At birth, labeling was weak and restricted to defined nuclei of the cochlear nuclear complex and the superior olivary complex. By postnatal day 8, labeling was seen in all brainstem nuclei. At the first appearance of immunoreactivity, alpha1 GlyRs were diffusely distributed on the neuronal surface, yet they became clustered with age, finally densely incrusting the somata and proximal dendrites between the 3rd and 4th postnatal week, when the mature pattern of immunoreactivity was established. We never observed an overexpression of alpha1 GlyRs or a transient appearance in areas that are devoid of the receptor in adults. The late formation of glycinergic synapses harboring the adult-type GlyRs in the auditory system, at a time when internuclear connections have already formed, indicates that alpha1 GlyRs do not participate in early synaptogenesis.

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.

Transient appearance of calbindin-D28k-positive neurons in the superior olivary complex of developing rats

Calbindin-D28k (CaBP) is a calcium-binding protein that is prominent in various parts of the mammalian auditory system. In order to shed some light on the possible role of CaBP during ontogeny, when calcium ions play key roles in several processes, the location of CaBP was examined immunocytochemically in the auditory system of the developing rat. This study focuses on the principal nuclei of the superior olivary complex, which show distinct CaBP labeling in the adult. Consistent with previous reports in the rat and other mammals, CaBP immunoreactivity in adults was intense in somata of the medial nucleus of the trapezoid body (MNTB) and in the neuropil (presumably in axons) of the lateral superior olive (LSO), the superior paraolivary nucleus (SPN), and the medial superior olive (MSO). In fetal and neonatal animals, however, the labeling pattern was strikingly different. Around birth, MNTB neurons are immunonegative for CaBP, whereas somata and processes in the LSO, probably neuronal, are heavily labeled at that age. This labeling pattern persists throughout the first week of postnatal life and begins to change at P8, when MNTB neurons become immunopositive for CaBP. During the next 10 days labeling intensity in MNTB neurons increases considerably, and the increase is paralleled by an increase in labeling intensity of the neuropil in the LSO, SPN, and MSO, indicating that the labeled processes in these nuclei may be axons originating from MNTB neurons. Immunoreactivity in LSO cells begins to decline around P8, decays rapidly between P10 and P18, and reaches its adult level around P28, when the CaBP labeling pattern in the whole superior olivary complex is indistinguishable from that in the adult. The present results show that the development of CaBP immunoreactivity in the rat superior olivary complex is characterized by two reciprocally related processes: as immunoreactivity within MNTB somata and fibers in the SPN, and LSO, and the MSO increases between P8 and about P21, the immunoreactivity in LSO neurons declines. CaBP immunoreactivity in LSO neurons is only transiently present, suggesting a critical period in development during which the control of Ca2+ homeostasis in LSO neurons may be of particular importance.

Development of electrical membrane properties and discharge characteristics of superior olivary complex neurons in fetal and postnatal rats

Although hearing onset occurs relatively late during ontogeny of rats [around postnatal day (P) 12], anatomical brainstem connections are formed much earlier and are present before birth, indicating that a substantial amount of maturation occurs without acoustic input. Electrical activity is thought to influence neuronal development, but the physiological properties of auditory brainstem neurons during perinatal maturation are barely known. The present study focuses on the development of electrophysiological membrane properties of neurons in the rat's superior olivary complex (SOC), the first binaural station in the mammalian auditory brainstem. In in vitro slice preparations, intracellular recordings were obtained from 115 SOC cells from embryonic day (E) 18 to P17, and cells were morphologically identified by intracellular injection of biocytin or neurobiotin. By E18, i.e. 4 days before birth, SOC neurons were capable of generating Na(+)-dependent action potentials. Several passive and active membrane properties, including the resting potential, spike threshold and spike amplitude, did not change with development. In contrast, input resistance, time constant and spike duration decreased significantly, and maximal spike frequency increased significantly during the age period sampled. Our results show that rat SOC neurons display mature as well as immature electrical membrane properties during the same developmental period when anatomical connections are refined and when the soma-dendritic morphology develops. We conclude, therefore, that their membrane properties represent adequate physiological adaptations to the immature auditory brainstem microcircuits and that they form a basis upon which the development of these microcircuits is shaped.

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.

Immunohistochemical localization of the somatostatin sst2(b) receptor splice variant in the rat central nervous system

Somatostatin is a neuromodulator in the mammalian CNS. To date, genes for at least five different somatotrophin release inhibiting factor receptors, termed sst1-sst5, have been cloned. The rat sst2 receptor exists in two splice variants, sst(alpha)a) and sst2(b), which differ in their carboxy-termini. When heterologously expressed in Chinese hamster ovary-K1 cells, these splice variants show little difference in their operational characteristics. Recently, the distribution of the sst2(a) receptor was documented, yet at present no data are available about the distribution of the sst2(b) receptor in the CNS. Here, we present the characterization of a novel polyclonal anti-peptide antibody that is selective for the sst2(b) receptor splice variant. The antibody was raised against the unique intracellular carboxy-terminal portion of the receptor protein. Using this affinity-purified antibody in western blotting experiments, the sst2(b) receptor expressed in Chinese hamster ovary-K1 cells was shown to be a glycoprotein with a molecular weight centred at about 85,000. The antibody showed no cross-reactivity to any of the recombinant human sst1-5 receptors, the rat sst2(a) receptor or wild-type Chinese hamster ovary-K1 cells. Employing immunohistochemistry, we investigated the distribution of the sst2(b) receptor in the brain and spinal cord of adult rats. A distinct distribution was found throughout the rostrocaudal axis of the CNS. Somatodendritic as well as axonal staining was observed. Somatodendritic labelling was particularly obvious in the olfactory bulb, cerebral cortex, hippocampal formation, mesencephalic trigeminal nucleus and cerebellum, as well as in cranial and spinal motor areas. The results show that the distribution of the sst2(b) receptor partially overlaps with that of the sst2(b) receptor, although there were differences in a number of brain areas. The location of the sst2(b) receptor implies that it may mediate a modulatory role of somatostatin inhibitory releasing factor on sensory as well as motor functions.

C-fos immunocytochemical evidence for acoustic pathway mapping in rats

Principles of the functional organization of brain structures are manifested in the response characteristics of the composing neuronal elements. In order to visualize and map some of the fundamental organization principles in the mammalian auditory system, namely tonotopy, ipsilateral versus contralateral inputs, and excitatory and inhibitory interactions, the present study employed the c-fos immunocytochemistry technique for imaging changes in acoustically evoked synaptic activity. Young rats were either monaurally or binaurally stimulated with pure tone pulses, and the distribution of immunoreactive neurons was analyzed in auditory brainstem nuclei. In a quantitative approach, i.e. by counting the number of labeled neurons in a given nucleus, monaural nuclei could be distinguished from binaural nuclei. Moreover, by relating areas with increased or decreased labeling to the stimulus conditions, further information was obtained on the nature of the inputs, i.e. whether ipsilateral and contralateral stimuli elicit predominantly excitatory and/or inhibitory responses. Taken together, the present data demonstrate the suitability of the c-fos immunocytochemistry technique to identify tone-evoked excitation (and inhibition) in auditory brainstem neurons and to visualize fundamental principles of organization.

Development of a topographically organized auditory network in slice culture is calcium dependent

Inhibitory and excitatory connections of remarkably precise topographic order are characteristic features of the mammalian auditory system, particularly within the superior olivary complex (SOC). Little is known about the requirements for the correct development of these specific connections. Previous in vivo experiments have demonstrated a high expression of calcium-binding proteins in this system during development, pointing to the need for precise calcium regulation. Here, we have employed an organotypic slice culture from the above neuronal network and analyzed the requirements for the maintenance and development of this system in vitro. When slices from neonatal rats were incubated in standard culture medium for up to 7 days, we found no organotypic features. Only if 25 mM KCl was added to the culture medium, the cytoarchitecture of the nuclei, the neuronal morphology, and the specificity and topography of internuclear connections were indistinguishable from that in vivo. The addition of calcium channel blockers (MgCl2 and nifedipine) to the high-KCl medium reduced organotypicity drastically, indicating that a depolarization-induced increase of intracellular calcium is indispensable. Furthermore, the temporal course of the expression of the calcium-binding protein parvalbumin in culture under high KCl mimics that in vivo, demonstrating developmental processes during incubation. The need for calcium influx into neurons of this auditory network in vitro (which is not seen in other slice culture systems) strengthens the hypothesis that an optimal calcium concentration is exceptionally important in auditory neurons. The effect of KCl in the slice cultures may substitute for input activity regulating intracellular calcium in auditory neurons in vivo.

Morphology of motoneurons in different subdivisions of the rat facial nucleus stained intracellularly with horseradish peroxidase

Horseradish peroxidase was injected into single facial motoneurons of the rat. Neurons were identified by antidromic stimulation of either the buccal or the marginal mandibular or the posterior auricular nerve branches. Motoneuronal cell bodies supplying the buccal branch were located in the lateral subdivision of the facial nucleus, those supplying the marginal mandibular branch were in the intermediate subdivision, and those supplying the posterior auricular branch were in the medial subdivision. Eleven motoneurons were reconstructed with a computer-assisted technique. Their soma diameters averaged 20 microns; the average number of primary dendrites was 7.9 and the combined lengths of the dendritic trees averaged 17,650 microns. There was no distinction between the three motoneuron groups in terms of these and other quantitative data. However, on the basis of reconstructed dendritic tree orientation (i.e., dendritic distribution), major differences were observed between motoneurons of the three groups. Dendrites from all groups extended beyond the boundaries of the facial nucleus into the reticular formation. The border between the intermediate and the lateral subdivision was crossed by some dendrites but the overlap was small. In contrast, no dendrite of a motoneuron in the medial subdivision entered the intermediate subdivision and vice versa. The dendritic extent was totally restricted by the borders between these two subdivisions. Outside the Nissl-defined nuclear border, however, dendrites from cells in adjacent subdivisions overlapped. It is concluded that the medial subdivision of the facial nucleus can be distinguished from the intermediate and lateral subdivisions not only by its sharp Nissl-defined border but also by the discrete organization of its dendritic field.

Topographic organization of facial motoneurons to individual pinna muscles in rat (Rattus rattus) and bat (Rousettus aegyptiacus)

The location and number of motoneurons to individual pinna muscles were determined by retrograde transport of horseradish peroxidase in rat and flying fox. The degree of ear mobility differs considerably between these species in that rats perform simpler ear movements while flying foxes move their pinnae in a sophisticated way. Five pinna muscles were investigated in each species. Motoneurons lay within the medial subdivision of the facial motor nucleus extending over its entire rostrocaudal length. They were topographically organized; however, a somatotopic order could not be observed. With one exception homologous pinna muscles were represented in corresponding areas in both species, supporting the idea of a common representation of ear muscles in mammals. In rat, motoneuron pools overlapped considerably, whereas in flying fox overlap was minute. A total of 1,110 and 1,646 motoneurons were labeled in rat and flying fox, respectively. We conclude that the higher number of pinna motoneurons in the latter species in addition to the more clear-cut topography provide the structural substrates that underlie differences in the quality of ear movements as seen in bats vis-a-vis other mammals.

Sensory neurons and motoneurons of the jaw-closing reflex pathway in rats: a combined morphological and physiological study using the intracellular horseradish peroxidase technique

Motoneurons and muscle spindle afferents of the rat masseter muscle were physiologically and morphologically characterized. Their soma-dendritic morphology and axonal course were investigated using the intracellular horseradish peroxidase method. Following electrical stimulation of the masseter nerve, individual motoneurons were identified by antidromic all-or-none action potentials and individual sensory neurons by orthodromic action potentials. Using threshold separation an excitatory input from muscle spindles to a masseter motoneuron was demonstrated. The short latency difference of 0.34 ms between the mean orthodromic response in the sensory neurons and the beginning of the synaptic potential in the masseter motoneuron suggests a monosynaptic connection between the spindle afferents and the motoneurons. Following intrasomatic horseradish peroxidase injection large multipolar cell bodies of masseter motoneurons were found within the motor nucleus. Their positions corresponded to the topographic organization of the motor trigeminal nucleus as described in retrograde tracing studies. Dendrites of masseter motoneurons were complex and could be found far beyond the nuclear borders. Distal dendrites extended to the mesencephalic trigeminal nucleus, the supratrigeminal nucleus, the lateral lemniscus and the reticular formation. Within the reticular formation dendrites were seen in the intertrigeminal nucleus and the peritrigeminal zone. Unipolar cell bodies of muscle spindle afferents were found in the mesencephalic trigeminal nucleus after intra-axonal injection of horseradish peroxidase. For all reconstructed sensory neurons a similar axonal course was found. Axonal terminals were found ipsilateral in the motor trigeminal nucleus, indicating a direct connection between sensory neurons and motoneurons. Further collaterals were found ipsilateral in the supratrigeminal nucleus and caudal to the motor trigeminal nucleus in the parvocellular reticular nucleus alpha. Since the latter termination areas are important for bilateral control of jaw-movements, the muscle spindle afferents are likely to participate not only in a monosynaptic motor reflex, but also in more complex neuronal circuits involved in jaw-movements.

Physiology and pharmacology of native glycine receptors in developing rat auditory brainstem neurons

Glycinergic neurotransmission is mediated via inhibitory glycine receptors (GlyRs) which are heterogeneous during development. Electrophysiological studies performed on recombinant GlyRs have identified different pharmacological properties and attributed them to differences in their subunit composition. Here, we report on age-related changes in the response properties of native GlyRs in the mammalian brain. Whole-cell patch-clamp recordings were obtained from neurons of the medial nucleus of the trapezoid body (MNTB), a major relay station in the mammalian auditory brainstem. Experiments were performed in acute medullary slices of rats between postnatal day (P) 1 and P15, a period during which synapse maturation occurs. Glycine-induced currents were present throughout the period under investigation and displayed age-related modifications in their amplitude, kinetic characteristics, and sensitivity to drugs. Current amplitudes and GlyR desensitization behavior increased with age. The alpha 1 subunit-specific GlyR antagonist cyanotriphenylborate (CTB) was barely effective in reducing glycine-induced currents during the first few postnatal days, yet a significant increase of the inhibitory effect occurred after the first postnatal week. This finding indicates that alpha 1 subunit-containing GlyRs become expressed only postnatally in the MNTB. Picrotoxin, which most effectively blocks recombinant alpha 2-homooligomers, reduced glycine-induced currents in neonatal MNTB neurons, suggesting that alpha 2-homooligomers may form native GlyR isoforms. Our results show that the physiology and pharmacology of GlyRs in the auditory brainstem underlie age-related changes which are most probably produced through a replacement of "neonatal" alpha 2 subunits with "adult" alpha 1 subunits.

Glycine-activated currents are changed by coincident membrane depolarization in developing rat auditory brainstem neurones

1. During early ontogeny, glycine receptors (GlyRs) exert depolarizing responses which may be of developmental relevance. We have used the gramicidin-perforated patch technique to elucidate the mechanism of glycine-activated currents in developing neurones of the rat lateral superior olive (LSO). 2. When the holding potential was set to -60 mV, perforated-patch recordings revealed glycine-induced inward currents in 59 %, outward currents in 5 % and biphasic currents in 34 % of the LSO neurones tested (n = 44). The biphasic currents were characterized by a transient outward phase which was followed by an inward phase. 3. Ion substitution experiments showed that both Cl- and HCO3- contributed to the glycine- induced biphasic current responses. 4. In the biphasic responses, the reversal potential of the glycine-induced current (Egly) depended on the response phase. A strong shift of Egly from a mean of -72 mV during the outward phase of the glycine response to a mean of -51 mV during the inward phase was observed, suggesting a shift of an ion gradient. 5. When the membrane potential was depolarized, 'tail' currents were induced in the presence of glycine. An increased duration or amplitude of the evoked depolarizations resulted in a proportional enlargement of these tail currents, indicating that they were produced by a shift of an ion gradient. Since changes of the HCO3- gradient are negligible, because of the carbonic anhydrase activity, we suggest that these tail currents were caused by a shift of the Cl- gradient. 6. We conclude that Cl- accumulates intracellularly during the activation of GlyRs and, consequently, Egly moves towards more positive values. 7. Coincident depolarizing stimuli enhanced intracellular Cl- accumulation and the shift of Egly, thereby switching hyperpolarizing to depolarizing action. This change could assist in an activity-dependent strengthening and refinement of glycinergic synapses during the maturation of inhibitory connectivity.

Development of chondroitin sulfate proteoglycans in the central auditory system of rats correlates with acquisition of mature properties

Chondroitin sulfate proteoglycans (CSPGs) are macromolecules which regulate the structural organization of the extracellular matrix and can mediate cell migration and axonal growth. Here the spatiotemporal distribution of CSPGs in the central auditory system of rats was investigated using a polyclonal antiserum. In adult brains, CSPGs surrounded many neuronal cell bodies and proximal dendrites at all stations of the auditory pathway except the medial geniculate body. During development, CSPG expression became visible at postnatal day (P) 4 in the superior olivary complex, at P8 in the midbrain, and at P18 in the cortex. Immunoreactivity increased strongly until P12 in the brainstem and until P24 in the cortex. The adult-like pattern in the pontine nuclei, the midbrain, and the cortex was seen at P12, P29, and P35, respectively. The relatively late expression of strong immunoreactivity indicates that the CSPGs are involved in the maturation of axonal connections, but not in early processes such as cell migration or neurite outgrowth.

Enrichment of integral membrane proteins from small amounts of brain tissue

Subcellular fractionation represents an essential technique for functional proteome analysis. Recently, we provided a subcellular fractionation protocol for minute amounts of tissue that yielded a nuclear fraction, a membrane and organelle fraction, and a cytosolic fraction. In the current study, we attempted to improve the protocol for the isolation of integral membrane proteins, as these are particularly important for brain function. In the membrane and organelle fraction, we increased the yield of membranes and organelles by about 50% by introducing a single re-extraction step. We then tested two protocols towards their capacity to enrich membrane proteins present in the membrane and organelle fraction. One protocol is based on sequential solubilization using subsequent increases of chaotropic conditions such as urea, thereby partitioning hydrophobic proteins from hydrophilic ones. The alternative protocol applies high-salt and high-pH washes to remove non-membrane proteins. The enrichment of membrane proteins by these procedures, as compared to the original membrane and organelle fraction, was evaluated by 16-BAC-SDS-PAGE followed by mass spectrometry of randomly selected spots. In the original membrane and organelle fraction, 7 of 50 (14%) identified proteins represented integral membrane proteins, and 15 (30%) were peripheral membrane proteins. In the urea-soluble fraction, 4 of 33 (12%) identified proteins represented integral membrane proteins, and 10 (30%) were peripheral membrane proteins. In the high-salt/high-pH resistant sediment, 12 of 45 (27%) identified proteins were integral membrane proteins and 13 (29%) represented peripheral membrane proteins. During the analysis, several proteins involved in neuroexocytosis were detected, including syntaxin, NSF, and Rab3-interaction protein 2. Taken together, differential centrifugation in combination with high-salt and high-pH washes resulted in the highest enrichment of integral membrane proteins and, therefore, represents an adequate technique for region-specific profiling of membrane proteins in the brain.