Genetic absence epilepsy: Effective connectivity from piriform cortex to mediodorsal thalamus

OBJECTIVE

The objective of the study was to quantify effective connectivity from the piriform cortex to mediodorsal thalamus, in Genetic Absence Epilepsy Rats from Strasbourg (GAERS).

METHODS

Local field potentials (LFPs) were recorded using microelectrode arrays implanted in the mediodorsal thalamus and piriform cortex, in three urethane anesthetized GAERS and three control rats. Screw electrodes were placed in the primary motor cortex to identify epileptiform discharges. We used transfer entropy to measure effective connectivity from piriform cortex to mediodorsal thalamus prior to and during generalized epileptiform discharges.

RESULTS

We observed increased theta band effective connectivity from piriform cortex to mediodorsal thalamus, prior to and during epileptiform discharges in GAERS compared with controls. Increased effective connectivity was also observed in beta and gamma bands from the piriform cortex to mediodorsal thalamus, but only during epileptiform discharges.

CONCLUSIONS

This preliminary study suggests that increased effective theta connectivity from the piriform cortex to the mediodorsal thalamus may be a feature of the 'epileptic network' associated with genetic absence epilepsy. Our findings indicate an underlying predisposition of this direct pathway to propagate epileptiform discharges in genetic absence epilepsy.

Multiunit cluster firing patterns of piriform cortex and mediodorsal thalamus in absence epilepsy

OBJECTIVE

The objective of the study were to investigate patterns of multiunit cluster firing in the piriform cortex (PC) and mediodorsal thalamus (MDT) in a rat model of genetic generalized epilepsy (GGE) with absence seizures and to assess whether these regions contribute to the initiation or spread of generalized epileptiform discharges.

METHODS

Multiunit clusters and their corresponding local field potentials (LFPs) were recorded from microelectrode arrays implanted in the PC and MDT in urethane anesthetized Genetic Absence Epilepsy Rats from Strasbourg (GAERS) and nonepileptic control (NEC) rats. Peristimulus time histograms (PSTHs) and cross-correlograms were used to observe transient changes in both the rate of firing and synchrony over time. The phase locking of multiunit clusters to LFP signals (spike-LFP phase locking) was calculated for frequency bands associated with olfactory communication between the two brain regions.

RESULTS

There were significant increases in both rate of firing and synchronous activity at the onset of generalized epileptiform discharges in both PC and MDT. Prior to and following these increases in synchronous activity, there were periods of suppression. Significant increases in spike-LFP phase locking were observed within the PC prior to the onset of epileptiform discharges across all spectral bands. There were also significant increases in spike-LFP phase locking within the theta band of the MDT prior to onset. Between the two brain regions, there was a significant decrease in spike-LFP phase locking -0.5 s prior to onset in the theta band which coincided with a significant elevation in spike-LFP phase locking in the gamma band.

CONCLUSIONS

Both the PC and MDT are engaged in the absence epilepsy network. Early spike-LFP phase locking between these two brain regions suggests potential involvement in the initiation of seizure activity.

Predicting neural recording performance of implantable electrodes

Recordings of neural activity can be used to aid communication, control prosthetic devices or alleviate disease symptoms. Chronic recordings require a high signal-to-noise ratio that is stable for years. Current cortical devices generally fail within months to years after implantation. Development of novel devices to increase lifetime requires valid testing protocols and a knowledge of the critical parameters controlling electrophysiological performance. Here we present electrochemical and electrophysiological protocols for assessing implantable electrodes. Biological noise from neural recording has significant impact on signal-to-noise ratio. A recently developed surgical approach was utilised to reduce biological noise. This allowed correlation of electrochemical and electrophysiological behaviour. The impedance versus frequency of modified electrodes was non-linear. It was found that impedance at low frequencies was a stronger predictor of electrophysiological performance than the typically reported impedance at 1 kHz. Low frequency impedance is a function of electrode area, and a strong correlation of electrode area with electrophysiological response was also seen. Use of these standardised testing protocols will allow future devices to be compared before transfer to preclinical and clinical trials.

The Interplay Between Multisensory Associative Learning and IQ in Children

This study assessed the developmental profile of unisensory and multisensory processes, and their contribution to children's intellectual abilities (8- and 11-year olds, N = 38, compared to adults, N = 19) using a simple audiovisual detection task and three incidental associative learning tasks with different sensory signals: visual-verbal with pseudowords, novel audiovisual, and visual-visual. The level of immaturity throughout childhood was dependent on both, the sensory signal type and the task. Associative learning was significantly enhanced with verbal sounds, compared to novel audiovisual and unisensory visual learning. Visual-verbal learning was also the best predictor of children's general intellectual abilities. The results demonstrate a separate developmental trajectory for visual and verbal multisensory processes and independent contributions to the development of cognitive abilities throughout childhood.

Electrical stimulation of the piriform cortex for the treatment of epilepsy: A review of the supporting evidence

In this review, we consider how the piriform cortex is engaged in both focal and generalized epilepsy networks and postulate the various neural pathways that can be effectively neuromodulated by stimulation at this site. This highlights the common involvement of the piriform cortex in epilepsy. We address both current and future preclinical studies of deep brain stimulation (DBS) of the piriform cortex, with attention to the critical features of these trials that will enable them to be of greatest utility in informing clinical translation. Although recent DBS trials have utilized thalamic targets, electrical stimulation of the piriform cortex may also be a useful intervention for people with epilepsy. However, more work is required to develop a solid foundation for this approach before considering human trials.

Correlation of Impedance and Effective Electrode Area of Iridium Oxide Neural Electrodes

Iridium oxide is routinely used for bionic applications owing to its high charge injection capacity. The electrode impedance at 1 kHz is typically reported to predict neural recording performance. In this article, the impedance of activated iridium oxide films (AIROFs) has been examined. The impedance of unactivated iridium electrodes was half that of platinum electrodes of similar geometry, indicating some iridium oxide was present on the electrode surface. A two time constant equivalent circuit was used to model the impedance of activated iridium. The impedance at low and intermediate frequencies decreased with increasing number of activation pulses and total activation charge. The impedance at 12 Hz correlated with the steady-state diffusion electroactive area. The impedance at 12 Hz also correlated with the charge density of the electrode. The high charge density and low impedance of AIROFs may provide improved neural stimulation and recording properties compared with typically used platinum electrodes.

Utilising reinforcement learning to develop strategies for driving auditory neural implants

OBJECTIVE

In this paper we propose a novel application of reinforcement learning to the area of auditory neural stimulation. We aim to develop a simulation environment which is based off real neurological responses to auditory and electrical stimulation in the cochlear nucleus (CN) and inferior colliculus (IC) of an animal model. Using this simulator we implement closed loop reinforcement learning algorithms to determine which methods are most effective at learning effective acoustic neural stimulation strategies.

APPROACH

By recording a comprehensive set of acoustic frequency presentations and neural responses from a set of animals we created a large database of neural responses to acoustic stimulation. Extensive electrical stimulation in the CN and the recording of neural responses in the IC provides a mapping of how the auditory system responds to electrical stimuli. The combined dataset is used as the foundation for the simulator, which is used to implement and test learning algorithms.

MAIN RESULTS

Reinforcement learning, utilising a modified n-Armed Bandit solution, is implemented to demonstrate the model's function. We show the ability to effectively learn stimulation patterns which mimic the cochlea's ability to covert acoustic frequencies to neural activity. Time taken to learn effective replication using neural stimulation takes less than 20 min under continuous testing.

SIGNIFICANCE

These results show the utility of reinforcement learning in the field of neural stimulation. These results can be coupled with existing sound processing technologies to develop new auditory prosthetics that are adaptable to the recipients current auditory pathway. The same process can theoretically be abstracted to other sensory and motor systems to develop similar electrical replication of neural signals.

Broadband onset inhibition can suppress spectral splatter in the auditory brainstem

In vivo intracellular responses to auditory stimuli revealed that, in a particular population of cells of the ventral nucleus of the lateral lemniscus (VNLL) of rats, fast inhibition occurred before the first action potential. These experimental data were used to constrain a leaky integrate-and-fire (LIF) model of the neurons in this circuit. The post-synaptic potentials of the VNLL cell population were characterized using a method of triggered averaging. Analysis suggested that these inhibited VNLL cells produce action potentials in response to a particular magnitude of the rate of change of their membrane potential. The LIF model was modified to incorporate the VNLL cells’ distinctive action potential production mechanism. The model was used to explore the response of the population of VNLL cells to simple speech-like sounds. These sounds consisted of a simple tone modulated by a saw tooth with exponential decays, similar to glottal pulses that are the repeated impulses seen in vocalizations. It was found that the harmonic component of the sound was enhanced in the VNLL cell population when compared to a population of auditory nerve fibers. This was because the broadband onset noise, also termed spectral splatter, was suppressed by the fast onset inhibition. This mechanism has the potential to greatly improve the clarity of the representation of the harmonic content of certain kinds of natural sounds.

Correlation of the impedance and effective electrode area of doped PEDOT modified electrodes for brain–machine interfaces

An analytical solution to impedance allows correlation of an effective electrode area with the impedance and phase angle at low frequencies.

In vivo electrophysiological recordings in amygdala subnuclei reveal selective and distinct responses to a behaviorally identified predator odor

Chemosensory cues signaling predators reliably stimulate innate defensive responses in rodents. Despite the well-documented role of the amygdala in predator odor-induced fear, evidence for the relative contribution of the specific nuclei that comprise this structurally heterogeneous structure is conflicting. In an effort to clarify this we examined neural activity, via electrophysiological recordings, in amygdala subnuclei to controlled and repeated presentations of a predator odor: cat urine. Defensive behaviors, characterized by avoidance, decreased exploration, and increased risk assessment, were observed in adult male hooded Wistar rats (n = 11) exposed to a cloth impregnated with cat urine. Electrophysiological recordings of the amygdala (777 multiunit clusters) were subsequently obtained in freely breathing anesthetized rats exposed to cat urine, distilled water, and eugenol via an air-dilution olfactometer. Recorded units selectively responded to cat urine, and frequencies of responses were distributed differently across amygdala nuclei; medial amygdala (MeA) demonstrated the greatest frequency of responses to cat urine (51.7%), followed by the basolateral and basomedial nuclei (18.8%) and finally the central amygdala (3.0%). Temporally, information transduction occurred primarily from the cortical amygdala and MeA (ventral divisions) to other amygdala nuclei. Interestingly, MeA subnuclei exhibited distinct firing patterns to predator urine, potentially revealing aspects of the underlying neurocircuitry of predator odor processing and defensiveness. These findings highlight the critical involvement of the MeA in processing olfactory cues signaling predator threat and converge with previous studies to indicate that amygdala regulation of predator odor-induced fear is restricted to a particular set of subnuclei that primarily include the MeA, particularly the ventral divisions.

Calorie restriction and corticosterone elevation during lactation can each modulate adult male fear and anxiety-like behaviour

Early life events, such as calorie restriction (CR) and elevated glucocorticoids, can calibrate the lifelong behavioural and physiological profile of an individual. Stress reactivity in adulthood is particularly sensitive to early life events; however, the consequence to fear and anxiety-like behaviour is less clear. Consequently, the current study sought to examine the effects of post-natal CR and glucocorticoid elevation, long considered powerful programming stimuli, on the subsequent fear and anxiety behaviour of the adult offspring. Rat dams received either corticosterone (200 μg/ml) supplementation in drinking water (CORT) or a 25% CR from post-natal day (PND) 1 to 11. Responses to the elevated plus maze (EPM), open field and a predator odour (TMT; 2,5-dihydro-2,4,5-trimethylthiazoline) were characterised in the adult male offspring. Both treatment conditions resulted in enhanced fear responses to TMT, characterised by heightened risk assessment and increased avoidance of TMT. CORT nursed offspring further demonstrated an anxiogenic profile in the open field. Basal hypothalamic-pituitary-adrenal function was unchanged in CORT adult offspring, whilst corticosterone concentration was elevated by post-natal CR. CR and CORT treated dams both exhibited greater anxiety-like behaviour in the EPM. A modest and temporary enhancement of maternal care was observed in CR and CORT treated dams, with CR dams further exhibiting rapid pup retrieval latencies. The results indicate enhanced emotionality in the adult male progeny of dams exposed to CR and corticosterone supplementation during the post-natal period. The modest enhancement of maternal care observed by both treatments is unlikely to have influenced the behavioural profile of the offspring.

Calorie restriction dose-dependently abates lipopolysaccharide-induced fever, sickness behavior, and circulating interleukin-6 while increasing corticosterone

In mice a 50% calorie restriction (CR) for 28days attenuates sickness behavior after lipopolysaccharide (LPS) and these mice demonstrate a central anti-inflammatory bias. This study examined the dose-dependent effect of CR on sickness behavior (fever, anorexia, cachexia) and peripheral immune markers post-LPS. Male Sprague-Dawley rats fed ad libitum or CR by 50% for 14, 21, or 28days were injected on day 15, 22, or 29 with 50μg/kg of LPS or saline (1mL/500g). Changes in body temperature (Tb), locomotor activity, body weight, and food intake were determined. A separate cohort of rats was fed ad libitum or CR by 50% for 28days and serum levels of corticosterone (CORT), interleukin 6 (IL-6), and IL-10 were determined at 0, 2, and 4h post-LPS. The rats CR for 28days demonstrated the largest attenuation of sickness behavior: no fever, limited reduction in locomotor activity, no anorexia, and reduced cachexia following LPS. Rats CR for 14 and 21days demonstrated a partial attenuation of sickness behavior. Rats CR for 14days demonstrated a larger increase in Tb, larger reduction in locomotor activity, and larger weight loss compared to rats CR for 21days. Serum CORT was increased at 2h post-LPS in ad libitum and CR groups; however it was two times larger in the CR animals. Levels of IL-6 were significantly attenuated at 2h post-LPS in the CR animals. IL-10 levels were similar post-LPS. CR results in an enhanced anti-inflammatory response in the form of increased CORT and diminished pro-inflammatory signals.

A method for systematic electrochemical and electrophysiological evaluation of neural recording electrodes

New materials and designs for neural implants are typically tested separately, with a demonstration of performance but without reference to other implant characteristics. This precludes a rational selection of a particular implant as optimal for a particular application and the development of new materials based on the most critical performance parameters. This article develops a protocol for in vitro and in vivo testing of neural recording electrodes. Recommended parameters for electrochemical and electrophysiological testing are documented with the key steps and potential issues discussed. This method eliminates or reduces the impact of many systematic errors present in simpler in vivo testing paradigms, especially variations in electrode/neuron distance and between animal models. The result is a strong correlation between the critical in vitro and in vivo responses, such as impedance and signal-to-noise ratio. This protocol can easily be adapted to test other electrode materials and designs. The in vitro techniques can be expanded to any other nondestructive method to determine further important performance indicators. The principles used for the surgical approach in the auditory pathway can also be modified to other neural regions or tissue.

An in vivo investigation of inferior colliculus single neuron responses to cochlear nucleus pulse train stimulation

The auditory brain stem implant (ABI) is being used clinically to restore hearing to patients unable to benefit from a cochlear implant (CI). Speech perception outcomes for ABI users are typically poor compared with most CI users. The ABI is implanted either on the surface of or penetrating through the cochlear nucleus in the auditory brain stem and uses stimulation strategies developed for auditory nerve stimulation with a CI. Although the stimulus rate may affect speech perception outcomes with current stimulation strategies, no studies have systematically investigated the effect of stimulus rate electrophysiologically or clinically. We therefore investigated rate response properties and temporal response properties of single inferior colliculus (IC) neurons from penetrating ABI stimulation using stimulus rates ranging from 100 to 1,600 pulses/s in the rat. We found that the stimulus rate affected the proportion of response types, thresholds, and dynamic ranges of IC activation. The stimulus rate was also found to affect the temporal properties of IC responses, with higher rates providing more temporally similar responses to acoustic stimulation. Suppression of neural firing and inhibition in IC neurons was also found, with response properties varying with the stimulus rate. This study demonstrated that changes in the ABI stimulus rate results in significant differences in IC neuron response properties. Due to electrophysiological differences, the stimulus rate may also change perceptual properties. We suggest that clinical evaluation of the ABI stimulus rate should be performed.

Behavioral determination of stimulus pair discrimination of auditory acoustic and electrical stimuli using a classical conditioning and heart-rate approach

Acute animal preparations have been used in research prospectively investigating electrode designs and stimulation techniques for integration into neural auditory prostheses, such as auditory brainstem implants and auditory midbrain implants. While acute experiments can give initial insight to the effectiveness of the implant, testing the chronically implanted and awake animals provides the advantage of examining the psychophysical properties of the sensations induced using implanted devices. Several techniques such as reward-based operant conditioning, conditioned avoidance, or classical fear conditioning have been used to provide behavioral confirmation of detection of a relevant stimulus attribute. Selection of a technique involves balancing aspects including time efficiency (often poor in reward-based approaches), the ability to test a plurality of stimulus attributes simultaneously (limited in conditioned avoidance), and measure reliability of repeated stimuli (a potential constraint when physiological measures are employed). Here, a classical fear conditioning behavioral method is presented which may be used to simultaneously test both detection of a stimulus, and discrimination between two stimuli. Heart-rate is used as a measure of fear response, which reduces or eliminates the requirement for time-consuming video coding for freeze behaviour or other such measures (although such measures could be included to provide convergent evidence). Animals were conditioned using these techniques in three 2-hour conditioning sessions, each providing 48 stimulus trials. Subsequent 48-trial testing sessions were then used to test for detection of each stimulus in presented pairs, and test discrimination between the member stimuli of each pair. This behavioral method is presented in the context of its utilisation in auditory prosthetic research. The implantation of electrocardiogram telemetry devices is shown. Subsequent implantation of brain electrodes into the Cochlear Nucleus, guided by the monitoring of neural responses to acoustic stimuli, and the fixation of the electrode into place for chronic use is likewise shown.

Midbrain responses to micro-stimulation of the cochlea using high density thin-film arrays

A broader activation of auditory nerve fibres than normal using a cochlear implant contributes to poor frequency discrimination. As cochlear implants also deliver a restricted dynamic range, this hinders the ability to segregate sound sources. Better frequency coding and control over amplitude may be achieved by limiting current spread during electrical stimulation of the cochlea and positioning electrodes closer to the modiolus. Thin-film high density microelectrode arrays and conventional platinum ring electrode arrays were used to stimulate the cochlea of urethane-anaesthetized rats and responses compared. Neurophysiological recordings were taken at 197 multi-unit clusters in the central nucleus of the inferior colliculus (CIC), a site that receives direct monaural innervation from the cochlear nucleus. CIC responses to both the platinum ring and high density electrodes were recorded and differences in activity to changes in stimulation intensity, thresholds and frequency coding of neural activation were examined. The high density electrode array elicited less CIC activity at nonspecific frequency regions than the platinum ring electrode array. The high density electrode array produced significantly lower thresholds and larger dynamic ranges than the platinum ring electrode array when positioned close to the modiolus. These results suggest that a higher density of stimulation sites on electrodes that effectively 'aim' current, combined with placement closer to the modiolus would permit finer control over charge delivery. This may equate to improved frequency specific perception and control over amplitude when using future cochlear implant devices.

The relationship between multisensory integration and IQ in children

It is well accepted that multisensory integration has a facilitative effect on perceptual and motor processes, evolutionarily enhancing the chance of survival of many species, including humans. Yet, there is limited understanding of the relationship between multisensory processes, environmental noise, and children's cognitive abilities. Thus, this study investigated the relationship between multisensory integration, auditory background noise, and the general intellectual abilities of school-age children (N = 88, mean age = 9 years, 7 months) using a simple audiovisual detection paradigm. We provide evidence that children with enhanced multisensory integration in quiet and noisy conditions are likely to score above average on the Full-Scale IQ of the Wechsler Intelligence Scale for Children-Fourth Edition (WISC-IV). Conversely, approximately 45% of tested children, with relatively low verbal and nonverbal intellectual abilities, showed reduced multisensory integration in either quiet or noise. Interestingly, approximately 20% of children showed improved multisensory integration abilities in the presence of auditory background noise. The findings of the present study suggest that stable and consistent multisensory integration in quiet and noisy environments is associated with the development of optimal general intellectual abilities. Further theoretical implications are discussed.

Calorie restriction attenuates LPS-induced sickness behavior and shifts hypothalamic signaling pathways to an anti-inflammatory bias

Calorie restriction (CR) has been demonstrated to alter cytokine levels; however, its potential to modify sickness behavior (fever, anorexia, cachexia) has not. The effect of CR on sickness behavior was examined in male C57BL/6J mice fed ad libitum or restricted 25% (CR25%) or restricted 50% (CR50%) in food intake for 28 days and injected with 50 μg/kg of LPS on day 29. Changes in body temperature, locomotor activity, body weight, and food intake were determined. A separate cohort of mice were fed ad libitum or CR50% for 28 days, and hypothalamic mRNA expression of inhibitory factor κB-α (IκB-α), cyclooxygenase-2 (COX-2), prostaglandin E(2) (PGE(2)), suppressor of cytokine signaling 3 (SOCS3), IL-10, neuropeptide Y (NPY), leptin, proopiomelanocortin (POMC), and corticotrophin-releasing hormone (CRH) were determined at 0, 2, and 4 h post-LPS. CR50% mice did not develop fevers, whereas the CR25% mice displayed a fever shorter in duration but with the same peak as the controls. Both CR25% and CR50% mice showed no sign of anorexia and reduced cachexia after LPS administration. Hypothalamic mRNA expression of NPY and CRH were both increased by severalfold in CR50% animals preinjection compared with controls. The CR50% mice did not demonstrate the expected rise in hypothalamic mRNA expression of COX-2, microsomal prostaglandin E synthase-1, POMC, or CRH 2 h post-LPS, and leptin expression was decreased at this time point. Increases in SOCS3, IL-10, and IκB-α expression in CR50% animals were enhanced compared with ad libitum-fed controls at 4 h post-LPS. CR results in a suppression of sickness behavior in a dose-dependent manner, which may be due to CR attenuating proinflammatory pathways and enhancing anti-inflammatory pathways.

Inferior colliculus responses to dual-site intralamina stimulation in the ventral cochlear nucleus

A major limitation of the present auditory brainstem implant (ABI) is its inability to access the tonotopic organization of the ventral cochlear nucleus (VCN). A previous study by our group indicated that stimulation of single sites within a given VCN frequency region did not always elicit frequency-specific responses within the central nucleus of the inferior colliculus (CIC) and in some cases did not elicit a response at all. For this study, we hypothesized that sequential stimulation (with a short interpulse delay of 320 μsec) of two VCN sites in similar frequency regions would enhance responsiveness in CIC neurons. Multiunit neural recordings in response to pure tones were obtained at 58 VCN and 164 CIC sites in anesthetized rats. Among the 58 VCN sites, 39 pairs of sites with similar characteristic frequencies were chosen for electrical stimulation. Each member of a VCN pair was electrically stimulated individually, followed by sequential stimulation of the pair, while recording CIC responses. On average, CIC sites were found to respond to dual-site VCN stimulation with significantly lower thresholds, wider dynamic ranges, a greater extent of activation with increasing current levels, and a higher degree of frequency specificity compared with single-site stimulation. Although these effects were positive for the most part, in some cases dual-site stimulation resulted in increased CIC thresholds and decreased dynamic ranges, extent of activation, and frequency specificity. The results suggest that multisite stimulation within VCN isofrequency laminae using penetrating electrodes could significantly improve ABI stimulation strategies and implant performance.

An in vivo investigation of first spike latencies in the inferior colliculus in response to multichannel penetrating auditory brainstem implant stimulation

The cochlear nucleus (CN) is the first auditory processing site within the brain and the target location of the auditory brainstem implant (ABI), which provides speech perception to patients who cannot benefit from a cochlear implant (CI). Although there is variance between ABI recipient speech performance outcomes, performance is typically low compared to CI recipients. Temporal aspects of neural firing such as first spike latency (FSL) are thought to code for many speech features; however, no studies have investigated FSL from CN stimulation. Consequently, ABIs currently do not incorporate CN-specific temporal information. We therefore systematically investigated inferior colliculus (IC) neuron's FSL response to frequency-specific electrical stimulation of the CN in rats. The range of FSLs from electrical stimulation of many neurons indicates that both monosynaptic and polysynaptic pathways were activated, suggesting initial activation of multiple CN neuron types. Electrical FSLs for a single neuron did not change irrespective of the CN frequency region stimulated, indicating highly segregated projections from the CN to the IC. These results present the first evidence of temporal responses to frequency-specific CN electrical stimulation. Understanding the auditory system's temporal response to electrical stimulation will help in future ABI designs and stimulation strategies.

Enhanced spatial memory and hippocampal long-term potentiation in p75 neurotrophin receptor knockout mice

Previous reports have described increases in the size and number of cholinergic neurons in the basal forebrain in p75 neurotrophin receptor (p75(NTR)) knockout mice. In an earlier study, we also found improved spatial memory in these mice, raising the possibility that p75(NTR) regulates hippocampal function by its effects on the cholinergic basal forebrain. We therefore investigated hippocampal long-term potentiation in p75(NTR) knockout mice that shared the same genetic background as control 129/Sv mice. We also investigated heterozygous mice, carrying just one functional p75(NTR) allele. The p75(NTR) knockout mice had enhanced long-term potentiation in the Schafer collateral fiber synapses of the hippocampus. Heterozygous mice had an intermediate level, greater than controls but less than knockout mice. Hippocampal choline acetyltransferase activity was also markedly elevated in p75(NTR) knockout mice, with a smaller increase in heterozygous mice. In the Barnes maze, p75(NTR) knockout mice displayed markedly superior learning to controls, and this was evident over the three age brackets tested. At each age, the performance of heterozygous mice was intermediate to the other groups. In the open field test, p75(NTR) knockout mice exhibited greater stress-related behavioral responses, including freezing, than did control animals. There were no differences between the three groups in a test of olfactory function. The dose-dependent effects of p75(NTR) gene copy number on hippocampal plasticity and spatial memory indicate that p75(NTR) has profound effects on hippocampal function. Bearing in mind that p75(NTR) is very sparsely expressed in the adult hippocampus and has a potent effect on hippocampal choline acetyltransferase activity, the effects of p75(NTR) on hippocampal function are likely to be mediated indirectly, by its actions on basal forebrain cholinergic neurons.

Neural synchrony in ventral cochlear nucleus neuron populations is not mediated by intrinsic processes but is stimulus induced: implications for auditory brainstem implants

The aim of this investigation was to elucidate if neural synchrony forms part of the spike time-based theory for coding of sound information in the ventral cochlear nucleus (VCN) of the auditory brainstem. Previous research attempts to quantify the degree of neural synchrony at higher levels of the central auditory system have indicated that synchronized firing of neurons during presentation of an acoustic stimulus could play an important role in coding complex sound features. However, it is unknown whether this synchrony could in fact arise from the VCN as it is the first station in the central auditory pathway. Cross-correlation analysis was conducted on 499 pairs of multiunit clusters recorded in the urethane-anesthetized rat VCN in response to pure tones and combinations of two tones to determine the presence of neural synchrony. The shift predictor correlogram was used as a measure for determining the synchrony owing to the effects of the stimulus. Without subtraction of the shift predictor, over 65% of the pairs of multiunit clusters exhibited significant correlation in neural firing when the frequencies of the tones presented matched their characteristic frequencies (CFs). In addition, this stimulus-evoked neural synchrony was dependent on the physical distance between electrode sites, and the CF difference between multiunit clusters as the number of correlated pairs dropped significantly for electrode sites greater than 800 microm apart and for multiunit cluster pairs with a CF difference greater than 0.5 octaves. However, subtraction of the shift predictor correlograms from the raw correlograms resulted in no remaining correlation between all VCN pairs. These results suggest that while neural synchrony may be a feature of sound coding in the VCN, it is stimulus induced and not due to intrinsic neural interactions within the nucleus. These data provide important implications for stimulation strategies for the auditory brainstem implant, which is used to provide functional hearing to the profoundly deaf through electrical stimulation of the VCN.

Testosterone, social and sexual behavior of perinatally and lifelong calorie restricted offspring

Calorie restriction (CR) during sensitive perinatal periods has consistently been demonstrated to alter the development of a variety of physiological systems, which consequently affect behavior. This study compared the social behavior and sexual behavior of the adult male offspring of mothers administered a 25% CR at one of four times in the perinatal period: a brief period preconception, during gestation, during lactation, or a lifelong restriction (beginning at conception and continuing throughout life). Levels of serum testosterone were also determined in these animals. Social interaction increased in the gestation and lifelong CR groups. The lifelong group also exhibited more dominant type behaviors. CR during preconception and lactation resulted in offspring that displayed an enhanced and more efficient copulatory pattern compared to all other conditions. This was demonstrated by a reduced frequency of intromissions, shorter latency to ejaculation, and a greater frequency of ejaculations by the preconception and lactation group compared to some, if not all of the other CR groups and controls. Serum testosterone was significantly higher in the preconception group compared to controls. These findings indicate that CR during specific periods of development can differentially alter the social behavioral phenotype and hormone levels in adulthood.

Maturation of auditory brainstem projections and calyces in the congenitally deaf (dn/dn) mouse

The deaf dn/dn mouse is a valuable model of human congenital deafness. In this study we used the lipophylic dye DiA to trace auditory nerve and cochlear nucleus projections in the dn/dn mouse. In both normal and deaf mice, the ipsilateral projections from the anteroventral cochlear nucleus (AVCN) to the lateral superior olive (LSO), and the contralateral projections from the AVCN to the medial nucleus of the trapezoid body (MNTB) were intact. With age, there was a noted increase in the fenestration of the endbulb and calyx of Held, and this morphological maturation was also observed in the deaf mice, although there was a significant difference in total endbulb volume at P20 between normal and deaf mice. However, total calyceal volume was not significantly different between normal and deaf mice. There was electrophysiological evidence of in vivo spontaneous ventral cochlear nucleus activity in normal and deaf animals, indicating that this activity may be responsible for the appropriate connectivity in the deaf mice. Our results indicate that congenital deafness caused by the dn/dn mutation does not result in aberrant projections between the AVCN and the ipsilateral MNTB and contralateral LSO but can cause abnormalities in endbulb size.

Effects of adult-onset calorie restriction on anxiety-like behavior in rats

Calorie restriction (CR) has consistently been shown to increase lifespan and ameliorate disease outcomes. Its effects on behavior are less clear, although anxiolytic-like effects have been observed. Rats were subjected to 1 of 4 dietary regimens: control, CR25%, CR50% and, an acute episode of CR and tested in 3 tests of anxiety: the open field test, the elevated plus maze, and the modified open field test. In the open field test, the CR25% and CR50% groups made more central zone entries than the control and Acute groups, which was primarily due to differences in the initial 5 min of the test. Moreover, both CR groups engaged in greater exploration of the central zone than the control group in the initial 5 min of the test. The Acute group also exhibited significantly longer latencies to leave the central zone at test onset than the control and CR50% group. In the elevated plus maze, the Acute group also displayed longer latencies to open arm entry as compared to the control and CR50% group and showed a lower ratio of open to total arm entries compared to all other groups. There were no effects of CR on any variable of the modified open field test. Possible neurochemical mechanisms underlying the anxiolytic-like effect of CR are discussed.

Inferior colliculus responses to multichannel microstimulation of the ventral cochlear nucleus: implications for auditory brain stem implants

Multichannel techniques were used to assess the frequency specificity of activation in the central nucleus of the inferior colliculus (CIC) produced by electrical stimulation of localized regions within the ventral cochlear nucleus (VCN). Data were recorded in response to pure tones from 141 and 193 multiunit clusters in the rat VCN and the CIC, respectively. Of 141 VCN sites, 126 were individually stimulated while recording responses in the CIC. A variety of CIC response types were seen with an increase in both electrical and acoustic stimulation levels. The majority of sites exhibited monotonic rate-level types acoustically, whereas spike rate saturation was achieved predominantly with electrical stimulation. In 20.6% of the 364 characteristic frequency aligned VCN-CIC pairs, the CIC sites did not respond to stimulation. In 26% of the 193 CIC sites, a high correlation was observed between acoustic tuning and electrical tuning obtained through VCN stimulation. A high degree of frequency specificity was found in 58% of the 118 lowest threshold VCN-CIC pairs. This was dependent on electrode placement within the VCN because a higher degree of frequency specificity was achieved with stimulation of medial, central, and posterolateral VCN regions than more anterolateral regions. Broadness of acoustic tuning in the CIC played a role in frequency-specific activation. Narrowly tuned CIC sites showed the lowest degree of frequency specificity on stimulation of the anterolateral VCN regions. These data provide significant implications for auditory brain stem implant electrode placement, current localization, power requirements, and facilitation of information transfer to higher brain centers.

The commissural pathway and cochlear nucleus bushy neurons: An in vivo intracellular investigation

A direct commissural connection formed between cochlear nuclei allows information from the contralateral ear to rapidly influence the processing of the ascending auditory signal. Among the neuronal groups proposed to both receive, and contribute to, commissural input is the bushy cell population in the ventral cochlear nucleus (VCN). In this in vivo electrophysiological study we examine the intracellular recordings of bushy neurons during electrical stimulation of the contralateral cochlear nucleus (CN) for evidence of both their contribution to, and input from commissural projections. Activation of the commissural pathway revealed short-latency fast hyperpolarisation in 19.5% of the 41 bushy neurons examined. The hyperpolarising potentials were small in amplitude, displayed a highly variable time course between neurons, and in some cases were eliminated with injection of depolarising current. There was no indication of antidromic activity, or short-latency excitatory potentials. These results suggest that i) bushy neurons do not contribute projections to the commissural connection, and ii) a small portion of bushy neurons are hyperpolarised following commissural stimulation.

Intracellular responses and morphology of rat ventral complex of the lateral lemniscus neurons in vivo

The function of the ventral and intermediate nuclei of the lateral lemniscus (VNLL and INLL), collectively termed ventral complex of the lateral lemniscus (VCLL), is unclear. Several studies have suggested that it plays a role in coding the temporal aspects of sound. In our study, a sample (n = 161) of intracellular responses to dichotically presented noise or tone bursts was collected from the VCLL of urethane-anesthetized rats in vivo. Intracellular recordings revealed six distinct response types to tones, distinguished by their synaptic and membrane characteristics as well as firing pattern. Three of these response types were correlated with distinct cellular morphologies revealed by intracellular injection of neurobiotin. 3D reconstructions of recorded neurons within the VCLL showed the spatial distribution of various response properties, including response type, laterality, characteristic frequency (CF), and binaural influences. Cells that responded to monaural (55%) or binaural (45%) stimulation were distributed throughout the VCLL. Almost all VCLL units were responsive to contralateral stimulation (97%). Most neurons were excited by contralateral stimulation (83%), many exclusively (43%), and some in conjunction with ipsilateral inhibition (28%) or excitation (12%). The INLL contained mostly binaural neurons (65%), typically with ipsilateral inhibition and contralateral excitation. These results indicate that the VCLL is not a monaural structure and there is a dorsal-ventral segregation of binaural and monaural cells. 3D reconstructions of intracellular CFs did not reveal the presence of any tonotopic arrangement within the VCLL. Presumably, the proposed timing role of this structure does not require a systematic representation of tonal frequency.

Effect of Age and Cognition on Childhood Speech in Noise Perception Abilities

This research on children's speech in noise and cognitive abilities aimed to determine the age-related trends in speech in noise perception abilities and the relationship between speech in noise perception and cognitive abilities. Monosyllabic distinguishable (consonant-vowel-consonant) words was the most recognisable word category, followed by monosyllabic confusable words (consonant-vowel-consonant), disyllabic non-words (/aCa/) and monosyllabic syllables (/Ca/), demonstrating that phoneme distinctiveness and a reduction in word confusability contribute to their recognition. Older children outperformed younger children on all speech in noise tasks, indicating that there are age-related trends in speech in noise abilities. Children with higher cognitive abilities did not outperform children with lower cognitive abilities on speech in noise tasks, indicating that the ability to hear speech in noise may be an intrinsic feature of the auditory system that matures with age.

Neural timing, inhibition and the nature of stellate cell interaction in the ventral cochlear nucleus

The ventral cochlear nucleus (VCN) stellate cell population comprises two clusters: narrowly-tuned, excitatory T stellate neurons, and D stellate neurons, a broadly-tuned population of inhibitory cells. These neurons respond to best frequency (BF) tone bursts in a chopper or onset manner, respectively. Through extensive local and commissural projections the D stellate population provides a source of fast inhibitory input to both intrinsic and contralateral T stellate neurons. Whilst the nature of interactions between intrinsic stellate populations is difficult to examine, our previous intracellular investigations of the commissural pathway have provided a means by which to study this relationship in the in vivo preparation. It is the aim of this paper to both review and extend our understanding of the link between stellate populations and their involvement in the commissural pathway by presenting an overview of the results attained in our recently expanded study. The sample of 17 intracellular and 34 extracellular onset chopper (O(C)) and late/ideal (On(L)/On(I)) neurons revealed antidromic activity in 31.4% of neurons following contralateral stimulation, providing physiological evidence that On(L)/On(I) neurons also contribute projections to the commissural connection. Alternatively, 64.7% of the 34 intracellularly-recorded chopper neurons displayed fast, monosynaptic inhibitory potentials. This commissural input was found to influence the timing of neural activity in chopper neurons, providing insight into the relationship that exists between T and D stellate neurons.

Topographic organization in the auditory brainstem of juvenile mice is disrupted in congenital deafness

There is an orderly topographic arrangement of neurones within auditory brainstem nuclei based on sound frequency. Previous immunolabelling studies in the medial nucleus of the trapezoid body (MNTB) have suggested that there may be gradients of voltage-gated currents underlying this tonotopic arrangement. Here, our electrophysiological and immunolabelling results demonstrate that underlying the tonotopic organization of the MNTB is a combination of medio-lateral gradients of low-and high-threshold potassium currents and hyperpolarization-activated cation currents. Our results also show that the intrinsic membrane properties of MNTB neurones produce a topographic gradient of time delays, which may be relevant to sound localization, following previous demonstrations of the importance of the timing of inhibitory input from the MNTB to the medial superior olive (MSO). Most importantly, we demonstrate that, in the MNTB of congenitally deaf mice, which exhibit no spontaneous auditory nerve activity, the normal tonotopic gradients of neuronal properties are absent. Our results suggest an underlying mechanism for the observed topographic gradient of neuronal firing properties in the MNTB, show that an intrinsic neuronal mechanism is responsible for generating a topographic gradient of time-delays, and provide direct evidence that these gradients rely on spontaneous auditory nerve activity during development.

Balanced inhibition and excitation underlies spike firing regularity in ventral cochlear nucleus chopper neurons

Ventral cochlear nucleus stellate cells respond to characteristic frequency (CF) tones with sustained (C(S)), transient (C(T)) or onset chopping (O(C)) activity. The mechanisms underlying these different response patterns are not fully understood, and the present study used in vivo intracellular recordings (n = 42) in urethane-anaesthetized rats to examine the possible influence of inhibition on action potential regularity. Hyperpolarization following the offset of a CF tone burst was used as a measure of on-CF inhibition. A cluster analysis based on several membrane potential features, including on-CF inhibition, discriminated three groups in addition to the C(S) response type - two types of C(T) responses and the O(C) type. The different patterns of firing regularity exhibited by C(S/T) neurons reflected different thresholds or degrees of overlap between these cells' narrowly tuned excitatory and inhibitory inputs. C(T) cells with closely matched inhibitory and excitatory response areas showed substantial on-CF inhibition and the greatest decline in firing regularity during a CF tone, whereas those with a mismatch between their response areas showed lateral inhibition and a less marked decline in firing regularity. The presence of inhibition in C(S) neurons did not alter their firing regularity, possibly because of the lower threshold for excitation compared with inhibition. The latency, duration and frequency extent of sustained hyperpolarization in C(S/T) cells is inconsistent with the response properties of O(C) neurons, suggesting that another source(s) of inhibition influences firing regularity, and presumably response magnitude, in these neurons.

Powerful, onset inhibition in the ventral nucleus of the lateral lemniscus

The function of the ventral nucleus of the lateral lemniscus (VNLL), a secondary processing site within the auditory brain stem, is unclear. It is known to be a major source of inhibition to the inferior colliculus (IC). It is also thought to play a role in coding the temporal aspects of sound, such as onsets and the periodic components of complex stimuli. In vivo intracellular recordings from VNLL neurons (n = 56) in urethane anesthetized rats revealed the presence of large-amplitude, short-duration, onset inhibition in a subset of neurons (14.3%). This inhibition occurred before the first action potential (AP) elicited by noise or tone bursts, was broadly tuned to tonal frequency and was shown to delay the first AP. Our data suggest it is a result of an intrinsic circuit activated by the octopus cell pathway originating in the contralateral cochlear nucleus; this pathway is known to convey exquisitely timed and broadly tuned onset information. This powerful inhibition within the VNLL appears to control the timing of this structure's inhibitory output to higher centers, which has important auditory processing outcomes. The circuit also provides a pathway for fast, broadly tuned, onset inhibition to the IC.