Postnatal maturation of the cochlear neclei in the cat: a neurophysiological study

Functional Development of Principal Neurons in the Anteroventral Cochlear Nucleus Extends Beyond Hearing Onset

Sound information is transduced into graded receptor potential by cochlear hair cells and encoded as discrete action potentials of auditory nerve fibers. In the cochlear nucleus, auditory nerve fibers convey this information through morphologically distinct synaptic terminals onto bushy cells (BCs) and stellate cells (SCs) for processing of different sound features. With expanding use of transgenic mouse models, it is increasingly important to understand the in vivo functional development of these neurons in mice. We characterized the maturation of spontaneous and acoustically evoked activity in BCs and SCs by acquiring single-unit juxtacellular recordings between hearing onset (P12) and young adulthood (P30) of anesthetized CBA/J mice. In both cell types, hearing sensitivity and characteristic frequency (CF) range are mostly adult-like by P14, consistent with rapid maturation of the auditory periphery. In BCs, however, some physiological features like maximal firing rate, dynamic range, temporal response properties, recovery from post-stimulus depression, first spike latency (FSL) and encoding of sinusoid amplitude modulation undergo further maturation up to P18. In SCs, the development of excitatory responses is even more prolonged, indicated by a gradual increase in spontaneous and maximum firing rates up to P30. In the same cell type, broadly tuned acoustically evoked inhibition is immediately effective at hearing onset, covering the low- and high-frequency flanks of the excitatory response area. Together, these data suggest that maturation of auditory processing in the parallel ascending BC and SC streams engages distinct mechanisms at the first central synapses that may differently depend on the early auditory experience.

Spontaneous activity in the developing auditory system

Spontaneous electrical activity is a common feature of sensory systems during early development. This sensory-independent neuronal activity has been implicated in promoting their survival and maturation, as well as growth and refinement of their projections to yield circuits that can rapidly extract information about the external world. Periodic bursts of action potentials occur in auditory neurons of mammals before hearing onset. This activity is induced by inner hair cells (IHCs) within the developing cochlea, which establish functional connections with spiral ganglion neurons (SGNs) several weeks before they are capable of detecting external sounds. During this pre-hearing period, IHCs fire periodic bursts of Ca(2+) action potentials that excite SGNs, triggering brief but intense periods of activity that pass through auditory centers of the brain. Although spontaneous activity requires input from IHCs, there is ongoing debate about whether IHCs are intrinsically active and their firing periodically interrupted by external inhibitory input (IHC-inhibition model), or are intrinsically silent and their firing periodically promoted by an external excitatory stimulus (IHC-excitation model). There is accumulating evidence that inner supporting cells in Kölliker's organ spontaneously release ATP during this time, which can induce bursts of Ca(2+) spikes in IHCs that recapitulate many features of auditory neuron activity observed in vivo. Nevertheless, the role of supporting cells in this process remains to be established in vivo. A greater understanding of the molecular mechanisms responsible for generating IHC activity in the developing cochlea will help reveal how these events contribute to the maturation of nascent auditory circuits.

Electrophysiological properties of octopus neurons of the cat cochlear nucleus: an in vitro study

Electrophysiological studies from mice in vitro have suggested that octopus cells of the mammalian ventral cochlear nucleus (VCN) are anatomically and biophysically specialized for detecting the coincident firing of a population of auditory nerve fibers. Recordings from cats in vivo have shown that octopus cells fire rapidly and with exceptional temporal precision as they convey the timing of that coincidence to higher auditory centers. The current study addresses the question whether the biophysical properties of octopus cells that have until now been examined only in mice, are shared by octopus cells in cats. Whole-cell patch-clamp recordings confirm that octopus cells in brain slices from kittens share the anatomical and biophysical features of octopus cells in mice. As in mice, octopus cells in kittens have large cell bodies and thick dendrites that extend in one direction. Voltage changes produced by depolarizing and hyperpolarizing current injection were small and rapid. Input resistances and membrane time constants in octopus cells of 16-day-old kittens were 15.8 +/- 1.5 MOmega (n = 16) and 1.28 +/- 0.3 ms (n = 16), respectively. Octopus cells fired only a single action potential at the onset of a depolarizing current pulse; suprathreshold stimuli were greater than 1.8 nA. A tetrodotoxin (TTX)-sensitive sodium conductance (gNa) was responsible for the generation of the action potentials. Octopus cells displayed outward rectification that lasted for the duration of the depolarizing pulses. Hyperpolarizations produced by the injection of current exhibited a depolarizing sag of the membrane potential toward the resting value. A 4-aminopyridine (4-AP) and alpha-dendrotoxin (alpha-DTX)-sensitive, low-voltage-activated potassium conductance (gKL) and a ZD7288-sensitive, mixed-cation conductance (gh) were partially activated at rest, giving the octopus cells low input resistances and, as a consequence, brief time constants. In 7-day-old kittens, action potentials were taller and broader, input resistances higher, and both inward and outward rectification was weaker than in 16-day-old kittens. Also as in mice, stellate cells of the VCN fired trains of action potentials with constant interspike intervals when they were depolarized (n = 10) and bushy cells of the VCN fired only a single action potential at the onset of depolarizations (n = 6). In conclusion, the similarity of octopus cells in mice and kittens suggests that the anatomical and biophysical specializations that allow octopus cells to detect and convey synchronous firing among auditory nerve fibers are common to all mammals.

Effect of prenatal auditory enrichment on developmental expression of synaptophysin and syntaxin 1 in chick brainstem auditory nuclei

Neural activity plays an important role in shaping the developing brain. We have determined the consequence of increased auditory stimulation on the developmental profile of synaptic proteins, synaptophysin and syntaxin 1, in the chick brainstem auditory nuclei, nucleus magnocellularis and nucleus laminaris, by immunohistochemistry and western blotting techniques. The chick embryos were provided with patterned sounds of species-specific calls or musical notes of a sitar, a stringed instrument, in a graded manner from embryonic day 10 (E10) through hatching, for 15 min every hour. During normal synaptogenesis of nucleus magnocellularis and nucleus laminaris, synaptophysin immunoreactivity increased significantly from E8 to E20, in parallel with synapse formation, and reduced at hatching. The embryos receiving species-specific sound stimuli exhibited a similar pattern with higher levels of immunoreactivity, though the difference between the study groups was not statistically significant. The music stimulated embryos showed an earlier peak at E16, followed by a gradual decline until hatching. In all three groups studied, syntaxin immunoreactivity showed a surge at E12, followed by a decline at E16 and subsequent stabilization. The stimulated groups continually expressed higher amounts of syntaxin immunoreactivity. The results suggest that prenatal sound stimulation enhances the normal pattern of synaptic protein expression in these auditory nuclei.

Topography of spiral ganglion projections to cochlear nucleus during postnatal development in cats

A fundamenntal organizational principle of the central auditory system is that virtually all areas are tonotopically organized. However, we know very little about the timing or mechanisms that are responsible for the development of this organization. When cats are born, their auditory nervous systems are extremely immature, and their hearing thresholds are very high. Until postnatal days 7-10 (P7-10), cats have behavioral and physiological thresholds which are near or above the pain threshold for adults and also have poor frequency selectivity. Physiological thresholds for auditory nerve fibers and cochlear nucleus neurons are typically above 100-120 dB SPL (sound pressure level re 20 microPa). Three weeks later (at approximately P31), the sensitivity and frequency discrimination (tuning) of these neurons approximate adult values. This study examines the development of the tonotopic projections from the spiral ganglion to the cochlear nucleus during the period in cat development in which the auditory system undergoes the transition from being essentially nonfunctional to having adult-like function. With the animals heavily anesthetized, the cochleas were surgically exposed in kittens ranging in age from P6 to P45. Focal injections of Neurobiotin (NB) were made into Rosenthal's canal, labeling a small cluster of cells in the spiral ganglion of each cochlea. The projections of these labeled cells were visualized as frequency-specific bands of labeled axons and terminals in all major subdivisions of the cochlear nucleus. The thickness of these bands (i.e., the dimension of the bands orthogonal to the isofrequency representation and across the frequency gradient) were measured and compared to similar projections in adults. As in adult cats, the thickness of the bands varied only slightly with the location of the injection site (frequency representation) over a range of 1-7 mm from the cochlear base (45-13 kHz). Moreover, band thickness did not vary significantly with age. These data indicate that the tonotopic organization of spiral ganglion projections to the cochlear nucleus is as precise in kittens as young as P6 as it is in adults.

Encoding of amplitude-modulated tones by neurons of the inferior colliculus of the kitten

Responses of single neurons of the central nucleus of the inferior colliculus (ICC) of kittens 4-43 days of age were studied using sinusoidally amplitude-modulated (AM) tones delivered monaurally or binaurally via sealed and calibrated earphones. The carrier frequency of the AM signal was set to the CF of the neuron. CFs ranged from 2-26 kHz. During the about first 2 weeks of postnatal life, ICC neurons responded to sound with periodic bursts of activity. In response to AM tones, discharges of ICC neurons at all ages studied were phase-locked to the envelope of the modulation waveform over a wide range of stimulus level and modulation depth. A linear relationship, independent of SPL, was found between the average phase of discharge on the modulation cycle and modulation frequency. The slope of the line represents a time delay, which was highly correlated with the first-spike latency to tone onset, and hence with the age of the animal. The mean effective phase of the discharge remained relatively constant with age. There was little systematic change in average phase of discharge with changing stimulus level or modulation depth, although the number of spikes evoked and the temporal pattern of the spikes within a modulation cycle could vary. The sensitivity function relating spike synchrony or spike count to modulation frequency was typically band-pass in nature. The most effective modulation frequency (MEMF) was, on average, 15 Hz, far below that reported for adult cat ICC cells. When AM tones were delivered binaurally, the discharge was a periodic function of the interaural phase difference of the stimulus envelopes. The results indicate that prior to the time the cochlea is able to respond to most environmental sounds, monaural and binaural circuits involving the ICC faithfully transmit information pertaining to amplitude-modulated signals in the rate and timing of their discharges. During the next several weeks, when neural thresholds fall to adult levels, ICC circuits are activated by amplitude modulated sounds at levels encountered in the normal acoustic environment even though they are restricted to modulation frequencies below those encoded by the adult.

Postnatal development of central auditory frequency maps

In the early postnatal period of many mammals and in the perihatching period of chicks the auditory ranges are restricted to the species-specific low- and mid-frequency ranges. During subsequent development, the high frequency hearing expands (depending on the species) by 1-4 octaves. Adult-like audition is established between the 4th and the 7th week. It is still discussed controversially, how the extension of the auditory ranges relates to the maturation of orderly frequency representation in the cochleae of the respective species. The present review summarizes investigations of the development of tonotopy in nuclei of the central auditory system, and discusses how the centrally acquired data might contribute to the understanding of the maturation of cochlear stimulus transduction and to the development of frequency maps.

Ontogenesis of auditory fovea representation in the inferior colliculus of the Sri Lankan rufous horseshoe bat, Rhinolophus rouxi

This report describes the ontogenesis of tonotopy in the inferior colliculus (IC) of the rufous horseshoe bat (Rhinolophus rouxi). Horseshoe bats are deaf at birth, but consistent tonotopy with a low-to-high frequency gradient from dorsolateral to ventromedial develops from the 2nd up to the 5th week. The representation of the auditory fovea is established in ventro-medio-caudal parts of the IC during the 3rd postnatal week (Fig. 3). Then, a narrow frequency band 5 kHz in width, comprising 16% of the bat's auditory range, captures 50-60 vol% of the IC (Fig. 3c). However, foveal tuning is 10-12 kHz (1/3 octave) lower than in adults; foveal tuning in females (65-68 kHz) is 2-3 kHz higher than in males (62-65 Khz). Thereafter, foveal tuning increases by 1-1.5 kHz per day up to the 5th postnatal week, when the adult hearing range is established (Figs. 4, 5). The increase of sensitivity and of tuning sharpness of single units also follows a low-to-high frequency gradient (Fig. 6). Throughout this development the foveal tuning matches the second harmonic of the echolocation pulses vocalised by these young bats. The results confirm the hypothesis of developmental shifts in the frequency-place code for the foveal high frequency representation in the IC.

Rhythmic discharge properties of caudal cochlear nucleus neurons during postnatal development in cats

Action potentials recorded extracellularly from neurons within the caudal cochlear nuclei of developing cats exhibited distinctive temporal characteristics (i.e., rhythmic responses) in response to long-duration acoustic stimuli including both tone and noise bursts. Unlike the homogeneous response characteristics of auditory nerve fibers, cochlear nucleus neurons exhibited many variations in rhythmic discharge patterns. The majority of neurons within the caudal CN of kittens younger than 10 days of age responded rhythmically to long-duration acoustic stimuli, however, the percentage of neurons responding rhythmically steadily decreased thereafter, and by the end of the second postnatal week most tonically-responding neurons maintained sustained steady-state discharge rates throughout stimulation. Discharges of neurons recorded during the transitional ages (around 13 days) were rhythmic at low sensation levels and exhibited adultlike sustained patterns at higher levels. Using constant sensation level stimuli (re individual neuron thresholds), burst frequencies remained essentially constant during the period of development in which rhythmic responses were observed. Intervals separating discharge bursts decreased as stimulus intensities increased for all neurons studied during the relevant period, but were not related in an orderly way to stimulus frequency. The effects of intensity on response periodicity were not mimicked by altering the amount of neurotransmitter present at the postsynaptic cell through microiontophoresis of excitatory amino acids and their antagonists onto the surface of neurons within the caudal CN. In addition, some immature neurons which responded phasically to acoustic stimuli responded rhythmically during the simultaneous presentation of acoustic stimuli and neuroexcitatory agents (i.e., glutamate). These results suggest that the source of the rhythmicity is not intrinsic to neurons in the caudal CN. Based on these and other observations we conclude that the most probable source of response periodicity observed early in development is the domination of inner hair cell output by efferent projections of the olivocochlear bundle, the temporal discharge patterns of which are also periodic.

Sensitivity of auditory cortical neurons of kittens to monaural and binaural high frequency sound

The experiments reported here describe the abilities of young auditory cortical neurons to encode information about tone bursts having frequencies above 2.5 kHz. The studies were carried out in anesthetized kittens ranging from 8 to 44 days of age. At all ages studied, stimulation of the contralateral ear was most effective in evoking spikes. Typically the response was confined to stimulus onset. Thresholds were comparatively high and response latencies were comparatively long in the youngest kittens studied. The time course of threshold development was very similar to that of the auditory nerve and cochlear nuclei indicating that most, if not all, age related thresholds and threshold changes at the cortical level are accounted for by mechanisms operating at the level of the cochlea and auditory nerve. Response latency shortened progressively over the first month of postnatal life and while the absolute change in response latency differed considerably from that of cells in the cochlear nuclei the proportional changes were very similar. These data indicate that the comparatively long response latency and latency changes recorded at the cortex are imposed by underdeveloped central auditory processes. Response areas of kitten cortical neurons resembled those of the adult. At all ages studied, binaural interactions were robust and similar in kind to those recorded in adult cats. We conclude that cortical neurons of kittens preserve the results of interactions occurring at lower brainstem levels and that the development of the circuits of which these neurons are a part develop as a functional unit.

Maturation of synapses and electrical properties of cells in the cochlear nuclei

Auditory nerve fibers carry impulses from the cochlea to the cochlear nuclei. There the temporal firing patterns of auditory nerve fibers are preserved by some cells and altered by others. The two factors which govern how firing patterns are shaped are (1) the intrinsic electrical properties of cells that determine the size and time course of voltage changes caused by synaptic currents and (2) the synaptic circuitry between cells. The electrical properties of cells were measured by recording the responses to current injected intracellularly into brain slice preparations. The synaptic responses to electrical shocks of the auditory nerve were used to determine the functional properties of synaptic connections. The three distinct types of electrical properties of cells that can be distinguished electrophysiologically in similar preparations of mature tissue, bushy and stellate cells in the ventral cochlear nucleus [(1984) J. Neurosci. 4, 1577-1588] and cells in the dorsal cochlear nucleus [Hirsch and Oertel (1987) (submitted); Oertel et al. (1987) In: Functions of the Auditory System, Editor: S. Hassler. J. Wiley and Sons (in press)] can be differentiated at least as early as 7 days after birth. Young cells, however, have higher input resistances and lower input capacitances than mature cells, and they cannot sustain high firing rates. Bushy and stellate cells in the ventral cochlear nucleus respond to electrical stimulation of the auditory nerve with both excitatory and inhibitory postsynaptic potentials as early as 4 days after birth. The synaptic potentials occur with longer and more variable latencies than in mature cells and synapses fatigue more easily, however. Cells of the dorsal cochlear nucleus also receive both excitatory and inhibitory synaptic inputs 4 days after birth, upon stimulation of the auditory nerve. No systematic changes were detected in these synaptic responses as a function of age but this may have been because the variability in the shape and timing of synaptic responses was large even in mature tissue.

Binaural interactions in cortical area AI of cats reared with unilateral atresia of the external ear canal

Binaural interactions were recorded in auditory cortical (AI) neurons of anesthetized adult cats that had unilateral atresias created shortly after birth by surgically ligating and cutting one external ear canal. At the time of the recording experiment, the atresia and associated debris were removed and tones were delivered to both tympanic membranes via a sealed and calibrated acoustic system. The majority of neurons recorded were in the cortex ipsilateral to the previously occluded ear. Thresholds for monaural stimulation of either the operated or unoperated ear were within normal range although thresholds to stimulation of the previously operated ear tended to occupy the upper end of the normal distribution. Monotonic and nonmonotonic spike count-vs-intensity functions derived from responses to monaural stimulation of the atretic ear were indistinguishable in their shape from those recorded in normal cats. All binaural classes were represented in our sample in proportions similar to those reported in cats with two normal ears. The forms of the functions relating spike count to interaural intensity differences and interaural phase differences were essentially the same as those seen in normal animals. The main binaural deficit observed under these conditions was a shift in the interaural intensity difference to which an AI neuron was most sensitive. The intensity needed for a stimulus to the atretic ear to participate in the binaural response was as much as 50 dB higher than that at the opposite normal ear in contrast to the nearly equal SPLs required for binaural interactions in cats with two ears intact. It was suggested that elevated thresholds at the previously operated ear could account for much of the shift observed.

Ontogeny of neural discharge patterns in the ventral cochlear nucleus of the mongolian gerbil

Discharge patterns were recorded extracellularly from single neurons in the ventral cochlear nucleus (VCN) of Mongolian gerbils ranging in age from 10 days after birth (DAB) to adult, a period which includes the onset of responsiveness to acoustic stimulation. At 10 DAB none of the neurons encountered within the VCN responded to acoustic stimulation. At 12 DAB approximately 15% of the neurons isolated in VCN were responsive. This coincided with the earliest cochlear microphonic potentials and preceded the appearance of the cochlear compound action potential (AP) by two days. At 14 DAB, or older, the great majority of neurons isolated in VCN responded to acoustic stimulation. Most parameters of VCN neural function exhibited significant changes between 12 and 18 DAB: neural thresholds improved approximately 100 dB; mean spontaneous discharge rate increased; the high-frequency range of characteristic frequency (CF) values increased from 10.0 to 24.0 kHz; the upper limit for phase locking increased from 0.8 kHz to 3.0 kHz; dynamic range increased from 16 dB to 44 dB, and the proportion of units with well-defined initial onset peaks in their post-stimulus-time (PST) response patterns increased from 40% to 100% of units. Most of the neural parameters examined achieved adult characteristics by 18 DAB. Frequency tuning (Q10dB) matured earlier for high-CF units. The most sharply tuned neurons with high CFs (greater than 4 kHz) at 12 DAB had Q10dB values equal to those for adults. None of the neurons with low CFs (less than 4 kHz) had Q10dB values greater than 1.2 at this age. Classical 'on' PST response patterns were not seen at 12 and 14 DAB. A unique PST response type, characterized by very long latency phasic discharge, was observed only at 12 DAB. None of the VCN neurons recorded from 12 DAB subjects displayed rhythmic 'bursting' or 'pulsing' PST response patterns, as has been reported at the earliest stages of functional development in the VCN of the cat. Most units were capable of sustained discharge, even with long stimulus durations. Units with 'primary-like' PST response patterns at 12 exhibited greater variability in first spike latency and less pronounced initial rates of firing than was characteristic in adults, resulting in poorly defined onset peaks. In contrast, units with chopper PST response patterns showed well-defined onset peaks.

Behavioural studies on auditory development in mammals in relation to higher nervous system functioning

The development of hearing measured behaviourally is compared in a quantitative way with studies on the physiological development in the auditory pathway of cats, house mice and humans. The similarity of time constants and of the beginning and end of the developments suggests that behavioural threshold sensitivity measured by unconditioned and conditioned reflexes is determined at or below the midbrain level.

Functional properties of auditory-nerve fibers during postnatal development in the kitten

The discharges of the auditory-nerve fibers were studied in kittens between 2-40 days of age. Up to the 10th postnatal day, fibers could be divided into two main categories: fibers with spontaneous activity (SA) that respond to sound and fibers without SA but with evoked responses. A third, smaller, category, fibers having neither SA nor evoked activity, was also present. The development of SA comprises two phases. The first, lasting from birth up to the third postnatal week, shows a relatively fast increase and the second, lasting up to adulthood, a slower increase. Typical tone burst responses can be recorded at the end of the first postnatal week. Thereafter reactivity steadily increases especially after the 10th postnatal day. In young animals, rate level function is characterized by a steep segment with a low dynamic range followed by a decrease in activity that lasts until the end of the second week. At this point adult-like functions may be observed, although maximal firing still increases for some weeks. Tuning curves and threshold sensitivity tend to develop inversely at corresponding frequencies. Fibers with low characteristic frequencies reach adult threshold before that of high frequency fibers and high frequency fibers reach adult tuning before low frequency fibers. A comparison of auditory-nerve fiber activity in kittens show that maturation of most functional characteristics lasts several weeks after birth and in some cases continues after the first postnatal month.

The development of auditory function in the cochlea of the mongolian gerbil

Cochlear microphonic (CM) potentials were recorded throughout the development of auditory function in the Mongolian gerbil. CM responses were first recorded at 12 days after birth (DAB), with thresholds exceeding 103 dB SPL. CM thresholds subsequently improved rapidly in a parallel fashion across the responsive frequency range, achieving adult levels by 18 DAB. There was no evidence from CM thresholds of a preferential rate of maturation for either the high or low frequency ranges. CM responses to suptrathreshold stimulation were also studied throughout development. At all responsive ages, CM input-output functions increased logarithmically to a saturating maximum value and then declined. Maximum CM responses increased continuously throughout development in a parallel fashion across all frequencies. The dynamic range of the CM input-output functions (the intensity interval from CM threshold to maximum CM value) also developed in a parallel manner across frequencies, but reached mature assymptotic values by 16 DAB. The results suggest that throughout the adult CM frequency range cochlear hair cell function develops simultaneously.

Morphology of primary axosomatic endings in the anteroventral cochlear nucleus of the cat: a study of the endbulbs of Held

The central axons of Type I spiral ganglion neurons travel in the auditory nerve and terminate in the cochlear nucleus. The ascending branches of these axons innervate the anteroventral cochlear nucleus and give rise to large axosomatic endings, called the endbulbs of Held, and smaller boutons. This paper reports a study of the endbulbs of Held, stained by horseradish peroxidase and variants of the Golgi method in kittens 2, 5, 10, 20, and 45 days postnatal and adult cats. Endbulbs tend to fall into two extreme groups with some endbulbs having an intermediate appearance; consequently, we have defined three descriptive stages of endbulbs that are conceived of as representing a developmental sequence. One group of endbulbs is found mostly in kittens younger than 10 days postnatal and is similar to the classic description of endbulbs by Ramón y Cajal ('09). The other extreme group of endbulbs is found mostly in adult cats. In these cases, the parent axonal trunk divides into several thick, gnarled branches that in turn branch again, sometimes repeatedly. These branches display irregular varicosities and form a cup-shaped arborization into which the postsynaptic cell body nestles. A chronology of postnatal endbulb development has been inferred from the relative proportions of the different endbulb stages at various ages. Maturation transforms the endbulb of Held from a large, spoon-shaped swelling having many filipodia into an elaborate tree with broad trunks and many smaller branches. Some implications of the proposed development sequence are discussed.

A study of alcohol effects on the brain during gestation and lactation

Pregnant Wistar rats were maintained on a diet of food pellets and a 10% (v/v) solution of ethanol in water during pregnancy and up to 21 days postpartum. Control groups were given food pellets and water ad libitum. The consumption of food and liquid was monitored throughout this period, so that daily food- and ethanol-derived caloric intake, daily liquid intake, and dose of alcohol ingested could be determined. The body weights and whole brain weights of ethanol-exposed offspring did not differ significantly from controls at any time from birth up to 21 days postpartum. However, cerebellar weights were significantly reduced in 21-day-old ethanol-exposed offspring. Areal analysis of the cerebellar vermis showed a 9% reduction in 21-day-old ethanol-exposed offspring compared with controls. Although there were no differences in total vermal area between control and treated rats at 12 days, ethanol-exposed offspring had a significantly more extensive external granular layer. In midgestation, pregnant females registered peak alcohol levels of 118 mg/100 ml blood 1 hr prior to the end of the dark cycle, while during lactation peak levels of 135 mg/100 ml blood were obtained. Neonate alcohol levels peaked slightly later and reached much lower values (44 mg/100 ml blood) during the lactation period. Ingested alcohol intake and blood alcohol levels fall within the range associated in man with the fetal alcohol syndrome, and it is suggested that the experimental regime described is a useful method for investigating the effects of alcohol on the developing nervous system.

Development of the dorsal and ventral cochlear nuclei in rat and effects of acoustic deprivation

Patterns of normal development of the rat dorsal and ventral cochlear nuclei (DCN, VCN) and the changes induced in the adult by postnatal acoustic deprivation were examined using volumetric determinations. VCN and DCN showed striking increases in size during the interval from postnatal days 10 to 16 which includes meatus opening. Sound deprivation from postnatal day 10 resulted in the most pronounced reduction in both DCN and VCN. The findings suggest a sensitive period during which structural development in the cochlear nuclei is influenced by acoustic stimuli.

Neurophysiological correlates of auditory maturation

At the time of its birth, the auditory system of the cat is not completely developed. Anatomic maturity of the peripheral auditory system, ie, the cochlea, auditory nerve and cochlear nucleus, is attained at about two or three weeks of age. Physiological response properties of neural elements of the peripheral auditory system change radically during the first few weeks of life, with most measures of responsiveness to acoustic stimuli reaching adult status by the end of the third of fourth postnatal week. The physiological maturation of neural responses correlates well with the anatomic maturation of the auditory structures. At least one physiological response property, namely the ability of cochlear nucleus neurons to time their discharges in response to low-frequency tones, is not fully achieved until the sixth postnatal week or later. Although considerably less is known about the development of the central auditory system, it appears that it is in part dependent upon the maturation of more peripheral elements. Auditory evoked responses, for example, follow roughly the same developmental time course as do the majority of response properties of peripheral neurons. This implies that the central system is ready to function at birth, but that it must await the maturation of more peripheral elements before it can function properly. In contrast to that of the cat and other mammals, the auditory system of the human is relatively well advanced at birth. Certain aspects of brain development, such as dendritic aborization and axonal myelination, undergo considerable change postnatally. How these factors influence hearing is not known.

Developmental changes in neurophysiological taste responses from the medulla in sheep

To determine whether functional characteristics of the taste system change during development, electrophysiological taste responses were recorded from neurons in the solitary complex (nucleus and tractus solitarius) in the medulla of fetal, newborn and adult sheep. Taste stimuli included NH4Cl, KCl, NaCl, LiCl, citric acid, and HCl, applied to the anterior tongue. Fetal neurons at all ages (84-137 days of gestation) responded to stimulation of the tongue with NH4Cl and KCl, but responses to NaCl and LiCl were only obtained in older fetuses (after 114 days of gestation), lambs and adults. Responses to citric aicd were obtained at all ages; however, HCl responses were only infrequently obtained in young fetuses. Other developmental changes included a progressive decrease in latency of the responses to NH4Cl, KCl, citric acid and HCl, and an increase in the duration of the neural response discharge as a function of gestational age. Since taste buds do not acquire the structural characteristics of the adult until the last third of gestation (approximately 100-147 days), these functional changes in taste response characteristics take place concurrently with structural development. Mammalian fetuses swallow amniotic fluid in utero, and, therefore, the fetal taste system is stimulated during structural and functional development. Thus, there is an opportunity for fetal gustatory experience to influence the developing taste system.

Brain stem auditory evoked response development in the kitten

The development of brain stem auditory evoked responses (BAERs), recorded from a surface electrode as short-latency, volume-conducted potentials, was studied in a series of kittens over a postnatal period ranging from birth to 60 days. Repeated, longitudinal observations on particular kittens were supplemented with observations on additional kittens during the first and second postnatal week to determine age of onset of the BAERs. The position of the animal and sound source within the recording chamber were held constant across recording sessions, as was click intensity except during recordings in which intensity effects were specifically studied. Click rates of 1, 10, 50 and 100/sec were routinely presented. Reference electrodes at the tongue, pinna and neck showed volume-conducted responses to the click stimuli and resulted in considerable distortion of the activity recorded by the vertex electrode; the forepaw, in contrast, showed no activity and a vertex-forepaw electrode configuration provided good resolution of the BAERs across development. A number of new observations were made. BAERs were first observed at 4 days of age, approximately the same age at which depth evoked potentials are first recorded in brain stem auditory nuclei. Initially the BAERs were diffuse, high threshold and fatigued rapidly, characteristics shared with depth evoked potentials in the early postnatal period. Over the first two weeks, the potentials showed marked decrease in threshold, increased resistance to fast click rates, and better definition of wave forms. All BAER components showed exponential decreases in latency. Because all of the brain stem evoked potentials could be recorded concurrently and longitudinally in the same subject a number of developmental comparisons were possible among the BAER components. Wave 1, related to the acoustic nerve in the adult cat, showed a developmental time course and adult latency similar to that reported for N1. Wave 2, related to the cochlear nucleus in the adult, showed a marked bimodality over the first month; wave 2a was a large amplitude clearly separated wave which gradually fused as an inconspicuous leading shoulder on wave 2b. Wave 2b developed with a time course and adult latency similar to that reported for the ventral cochlear nucleus. Wave 3, related to the region of the superior olivary complex in the adult, showed a clear but transient bimodality during the third week of development. Wave 5, related to the inferior colliculus in the adult, appeared later than waves 1-4 and showed a significantly slower rate of development than waves 1-4. These data indicate that differential developmental changes occur within the brain stem auditory pathway and that the BAERs provide a dynamic probe of concurrent maturational interactions.

The development of some peripheral and central auditory responses in the neonatal cat

The development of cochlear nerve action potential thresholds at different frequencies (AP audiograms) and inferior colliculus (IC) single unit thresholds and tuning was examined in barbiturate-anaesthetized kittens. AP thresholds decreased over the whole frequency spectrum during the first 5 weeks of life. Thresholds to high-frequency stimulation remained higher in 7-week-old animals than in adults. These results are in contrast to previous reports which have suggested that the AP response and gross cochlear anatomy are mature by the end of the second week. These differences may be due to the fact that the AP audiogram technique provides a measure of the activity of discrete regions along the cochlear partition. IC units in animals younger than 3.5 weeks had significantly elevated thresholds and broader tuning than those of the adult cat. Comparison of AP audiogram and IC unit thresholds in the adult revealed that these indices show similar frequency-dependent sensitivity. The slower maturation revealed by the AP audiogram may be due to the greater number and/or synchrony of cochlear nerve discharges needed to produce the gross AP. If this were the case, perception of suprathreshold sounds might not develop as quickly as thresholds for sound detection.

Neurological development of kittens

The neurological development of the kitten was studied from birth to 120 days of age. Three motor features present at birth disappeared within the 1st 45 days of life. The labyrinthine head and body righting reactions were present at birth; the latter matured only by 25 days of age when the air righting reaction (mature by 35 days) started to appear. Limb placing reactions developed progressively with proprioceptive components being present at birth whereas tactile components evolved slowly from early contact lifting to forepaws contact placing (60 days) and narrow plank walking (75 days). Standing and walking were well developed by 45 days. Eye opening occurred at 9.5 days and several eyeblink reactions (including blink to light) were present at birth. Adequate binocular coordination was seen by 47 days. Vision progressed parallel to the clearing of the ocular media which were fairly transparent by 32 days. Visually guided paw placing and the visuopalpebral blink reflex matured later (by 37 and 59 days, respectively). The external auditory canal and pinna were fully developed by 12 and 31 days, respectively; spontaneous and tactile pinna movements were present at birth; orienting to animal and nonanimal sounds was well developed by 6.5 and 18 days, respectively; spatial sound localization was developed by 16 days and differential responding to animal sounds matured by 24.5 days. Somatic responses and olfaction were present at birth but matured further thereafter. Playful interactions between kittens started by 10--15 days and well developed play behavior was seen by 36--40 days. In brief, all neurological functions mature progressively during the 1st 3 postnatal months.

Different patterns of cochlear innervation during the development of the kitten

The postnatal development of neuro-epithelial junctions inside the kitten cochlea has been investigated by electron microscopy, and correlated with previous electrophysiological results. Two main stages of development are described. During the first postnatal days, outer hair cells look very immature with only a few afferent endings adjoining them. The inner hair cells, on the other hand, are surrounded by numerous endings with mature afferent and efferent synapses. Thus, when the efferent olivo-cochlear system begins to function during the first postnatal day s, it is able to modify only inner hair cell responses. The second postnatal week is characterized by maturation of the large efferent endings below the outer hair cells. At the same time, direct efferent connections become sparce at the level of inner hair cells. The maturation of hearing, at the receptor level, seems to proceed in two steps, one related to inner hair cells and corresponding to a gross and primitive hearing, the other related to outer hair cells and corresponding to more precise and discriminative hearing abilities.

Development of the octopus cell area in the cat ventral cochlear nucleus

The octopus cell area (OCA) of the posteroventral cochlear nucleus was studied electron microscopically in kittens. The adult OCA, a region of morphologically homogeneous neurons receiving heterotypic synapses from the cochlea, was used to define the mature state. The OCA reaches cytological maturity at three weeks postnatally, after progression through four stages, defined on the basis of octopus cell cytology (including relative numbers of somatic and dendritic filopodia and spines) and the frequency, ultrastructure and location of previously defined synaptic terminals. Octopus cell size was also studied in rapid Golgi impregnations. The OCA from birth through three postnatal days (stage 1) showed small neurons, few identifiable synaptic types, small, mostly unmyelinated axons, mitotic cells and undifferentiated glia. Between the fourth and seventh postnatal days (stage 2) distinct type 1 and type 2 endings appeared and dendrites thickened, expanded peripherally and developed mature spines. During stage 3 (8-19 days) loss of filopodia, increased somatic spicules, larger somas and clearer differentiation of type 1 and type 2 synapses occurred. After three postnatal weeks (stage 4) the OCA contained morphologically mature octopus cell somas, all three synaptic types ending upon somas and thick basal dendrites, and fascicles of myelinated fibers. Although cytologically mature, the OCA at this stage (about 20-35 days) is substantially smaller than the adult OCA. This smaller size will facilitate further study of OCA synaptic organization.

The structural maturation of the stato-acoustic nerve in the cat

The maturation of the stato-acoustic nerve in the cat was studied by light and electron microscopy from the fetal stage to the adult. Measurement of the outer diameter of the fibers and the study of the myelination process revealed that myelination begins earlier for the vestibular nerve than for the cochlear nerve: by the fifty-third day of gestation 64% of the vestibular fibres have already passed the promyelin stage whereas for the cochlear nerve this promyelin stage begins for the majority of fibers on the fifty-seventh gestation day. Afterward, maturation proceeds more rapidly for the cochlear nerve. In the case of both nerves, maturation is still incomplete at two months of age. Concerning the relationship between the thickness of the myelin sheath and the axoplasmic diameter, there is already a good correlation by the fifty-seventh day of gestation in the vestibular nerve, whereas it appears several days after birth in the cochlear nerve.