Vestibular compensation after ganglionectomy: ultrastructural study of the tangential vestibular nucleus and behavioral study of the hatchling chick

Understanding the Pathophysiology of Congenital Vestibular Disorders: Current Challenges and Future Directions

In congenital vestibular disorders (CVDs), children develop an abnormal inner ear before birth and face postnatal challenges to maintain posture, balance, walking, eye-hand coordination, eye tracking, or reading. Only limited information on inner ear pathology is acquired from clinical imaging of the temporal bone or studying histological slides of the temporal bone. A more comprehensive and precise assessment and determination of the underlying mechanisms necessitate analyses of the disorders at the cellular level, which can be achieved using animal models. Two main criteria for a suitable animal model are first, a pathology that mirrors the human disorder, and second, a reproducible experimental outcome leading to statistical power. With over 40 genes that affect inner ear development, the phenotypic abnormalities resulting from congenital vestibular disorders (CVDs) are highly variable. Nonetheless, there is a large subset of CVDs that form a common phenotype of a sac-like inner ear with the semicircular canals missing or dysplastic, and discrete abnormalities in the vestibular sensory organs. We have focused the review on this subset, but to advance research on CVDs we have added other CVDs not forming a sac-like inner ear. We have included examples of animal models used to study these CVDs. Presently, little is known about the central pathology resulting from CVDs at the cellular level in the central vestibular neural network, except for preliminary studies on a chick model that show significant loss of second-order, vestibular reflex projection neurons.

A New Model for Congenital Vestibular Disorders

Many developmental disorders of the inner ear are manifested clinically as delayed motor development and challenges in maintaining posture and balance, indicating involvement of central vestibular circuits. How the vestibular circuitry is rewired in pediatric cases is poorly understood due to lack of a suitable animal model. Based on this, our lab designed and validated a chick embryo model to study vestibular development in congenital vestibular disorders. The developing inner ear or "otocyst" on the right side of 2-day-old chick embryos (E2) was surgically rotated 180° in the anterior-posterior axis, forming the "anterior-posterior axis rotated otocyst chick" or ARO chick. The ARO chick has a reproducible pathology of a sac with truncated or missing semicircular canals. A sac is the most common inner ear defect found in children with congenital vestibular disorders. In E13 ARO chicks, the sac contained all three cristae and maculae utriculi and sacculi, but the superior crista and macula utriculi were shortened in anterior-posterior extent. Also, the number of principal cells of the tangential vestibular nucleus, a major avian vestibular nucleus, was decreased 66 % on the rotated side. After hatching, no difference was detected between ARO and normal chicks in their righting reflex times. However, unlike normal chicks, ARO hatchlings had a constant, right head tilt, and after performing the righting reflex, ARO chicks stumbled and walked with a widened base. Identifying the structure and function of abnormally developed brain regions in ARO chicks may assist in improving treatments for patients with congenital vestibular disorder.

Presynaptic GABA(B) receptors decrease neurotransmitter release in vestibular nuclei neurons during vestibular compensation

Unilateral damage to the peripheral vestibular receptors precipitates a debilitating syndrome of oculomotor and balance deficits at rest, which extensively normalize during the first week after the lesion due to vestibular compensation. In vivo studies suggest that GABA(B) receptor activation facilitates recovery. However, the presynaptic or postsynaptic sites of action of GABA(B) receptors in vestibular nuclei neurons after lesions have not been determined. Accordingly, here presynaptic and postsynaptic GABA(B) receptor activity in principal cells of the tangential nucleus, a major avian vestibular nucleus, was investigated using patch-clamp recordings correlated with immunolabeling and confocal imaging of the GABA(B) receptor subunit-2 (GABA(B)R2) in controls and operated chickens shortly after unilateral vestibular ganglionectomy (UVG). Baclofen, a GABA(B) agonist, generated no postsynaptic currents in principal cells in controls, which correlated with weak GABA(B)R2 immunolabeling on principal cell surfaces. However, baclofen decreased miniature excitatory postsynaptic current (mEPSC) and GABAergic miniature inhibitory postsynaptic current (mIPSC) events in principal cells in controls, compensating and uncompensated chickens three days after UVG, indicating the presence of functional GABA(B) receptors on presynaptic terminals. Baclofen decreased GABAergic mIPSC frequency to the greatest extent in principal cells on the intact side of compensating chickens, with concurrent increases in GABA(B)R2 pixel brightness and percentage overlap in synaptotagmin 2-labeled terminals. In uncompensated chickens, baclofen decreased mEPSC frequency to the greatest extent in principal cells on the intact side, with concurrent increases in GABA(B)R2 pixel brightness and percentage overlap in synaptotagmin 1-labeled terminals. Altogether, these results revealed changes in presynaptic GABA(B) receptor function and expression which differed in compensating and uncompensated chickens shortly after UVG. This work supports an important role for GABA(B) autoreceptor-mediated inhibition in vestibular nuclei neurons on the intact side during early stages of vestibular compensation, and a role for GABA(B) heteroreceptor-mediated inhibition of glutamatergic terminals on the intact side in the failure to recover function.

Basic Concepts in Understanding Recovery of Function in Vestibular Reflex Networks during Vestibular Compensation

Unilateral peripheral vestibular lesions produce a syndrome of oculomotor and postural deficits with the symptoms at rest, the static symptoms, partially or completely normalizing shortly after the lesion due to a process known as vestibular compensation. The symptoms are thought to result from changes in the activity of vestibular sensorimotor reflexes. Since the vestibular nuclei must be intact for recovery to occur, many investigations have focused on studying these neurons after lesions. At present, the neuronal plasticity underlying early recovery from the static symptoms is not fully understood. Here we propose that knowledge of the reflex identity and input–output connections of the recorded neurons is essential to link the responses to animal behavior. We further propose that the cellular mechanisms underlying vestibular compensation can be sorted out by characterizing the synaptic responses and time course for change in morphologically defined subsets of vestibular reflex projection neurons. Accordingly, this review focuses on the perspective gained by performing electrophysiological and immunolabeling studies on a specific subset of morphologically defined, glutamatergic vestibular reflex projection neurons, the principal cells of the chick tangential nucleus. Reference is made to pertinent findings from other studies on vestibular nuclei neurons, but no comprehensive review of the literature is intended since broad reviews already exist. From recording excitatory and inhibitory spontaneous synaptic activity in principal cells, we find that the rebalancing of excitatory synaptic drive bilaterally is essential for vestibular compensation to proceed. This work is important for it defines for the first time the excitatory and inhibitory nature of the changing synaptic inputs and the time course for changes in a morphologically defined subset of vestibular reflex projection neurons during early stages of vestibular compensation.

Plasticity of spontaneous excitatory and inhibitory synaptic activity in morphologically defined vestibular nuclei neurons during early vestibular compensation

After unilateral peripheral vestibular lesions, the brain plasticity underlying early recovery from the static symptoms is not fully understood. Principal cells of the chick tangential nucleus offer a subset of morphologically defined vestibular nuclei neurons to study functional changes after vestibular lesions. Chickens show posture and balance deficits immediately after unilateral vestibular ganglionectomy (UVG), but by 3 days most subjects begin to recover, although some remain uncompensated. With the use of whole cell voltage-clamp, spontaneous excitatory and inhibitory postsynaptic currents (sEPSCs and sIPSCs) and miniature excitatory and inhibitory postsynaptic currents (mEPSCs and mIPSCs) were recorded from principal cells in brain slices 1 and 3 days after UVG. One day after UVG, sEPSC frequency increased on the lesion side and remained elevated at 3 days in uncompensated chickens only. Also by 3 days, sIPSC frequency increased on the lesion side in all operated chickens due to major increases in GABAergic events. Significant change also occurred in decay time of the events. To determine whether fluctuations in frequency and kinetics influenced overall excitatory or inhibitory synaptic drive, synaptic charge transfer was calculated. Principal cells showed significant increase in excitatory synaptic charge transfer only on the lesion side of uncompensated chickens. Thus compensation continues when synaptic charge transfer is in balance bilaterally. Furthermore, excessive excitatory drive in principal cells on the lesion side may prevent vestibular compensation. Altogether, this work is important for it defines the time course and excitatory and inhibitory nature of changing spontaneous synaptic inputs to a morphologically defined subset of vestibular nuclei neurons during critical early stages of recovery after UVG.

GABA and glycine immunolabeling in the chicken tangential nucleus

In the vestibular nuclei, GABAergic and glycinergic neurons play important roles in signal processing for normal function, during development, and after peripheral vestibular lesions. The chicken tangential nucleus is a major avian vestibular nucleus, whose principal cells are projection neurons with axons transmitting signals to the oculomotor nuclei and/or cervical spinal cord. Antibodies against GABA, glycine and glutamate were applied to study immunolabeling in the tangential nucleus of 5-7 days old chicken using fluorescence detection and confocal imaging. All the principal cells and primary vestibular fibers were negative for GABA and glycine, but positive for glutamate. GABA is the predominant inhibitory neurotransmitter in the tangential nucleus, labeling most of the longitudinal fibers in transverse tissue sections and more than 50% of all synaptic terminals. A large fraction of GABAergic terminals were derived from the longitudinal fibers, with fewer horizontal GABAergic fibers detected. GABA synapses terminated mainly on dendrites in the tangential nucleus. In contrast, glycine labeling represented about one-third of all synaptic terminals, and originated from horizontally-coursing fibers. A distinct pool of glycine-positive terminals was found consistently around the principal cell bodies. While no GABA or glycine-positive neuron cell bodies were found in the tangential nucleus, several pools of immunopositive neurons were present in the neighboring vestibular nuclei, mainly in the descending vestibular and superior vestibular nuclei. GABA and glycine double-labeling experiments revealed little colocalization of these two neurotransmitters in synaptic terminals or fibers in the tangential nucleus. Our data support the concept of GABA and glycine playing critical roles as inhibitory neurotransmitters in the tangential nucleus. The two inhibitory neurotransmitters have distinct and separate origins and display contrasting subcellular termination patterns, which underscore their discrete roles in vestibular signal processing.

Adaptation of chicken vestibular nucleus neurons to unilateral vestibular ganglionectomy

Vestibular compensation refers to the behavioral recovery after a unilateral peripheral vestibular lesion. In chickens, posture and balance deficits are present immediately following unilateral vestibular ganglionectomy (UVG). After three days, most operated chickens begin to recover, but severe deficits persist in others. The tangential nucleus is a major avian vestibular nucleus whose principal cells are vestibular reflex projection neurons. From patch-clamp recordings on brain slices, the percentage of spontaneous spike firing principal cells, spike discharge rate, ionic conductances, and spontaneous excitatory postsynaptic currents (sEPSCs) were investigated one and three days after UVG. Already by one day after UVG, sEPSC frequency increased significantly on the lesion side, although no differences were detected in the percentage of spontaneous spike firing cells or discharge rate. In compensated chickens three days after UVG, the percentage of spontaneous spike firing cells increased on the lesion side and the discharge rate increased bilaterally. In uncompensated chickens three days after UVG, principal cells on the lesion side showed increased discharge rate and increased sEPSC frequency, whereas principal cells on the intact side were silent. Typically, silent principal cells exhibited smaller persistent sodium conductances and higher activation thresholds for the fast sodium channel than spiking cells. In addition, silent principal cells on the intact side of uncompensated chickens had larger dendrotoxin-sensitive potassium conductance, with a higher ratio of Kv1.1 surface/cytoplasmic expression. Increased sEPSC frequency in principal cells on the lesion side of uncompensated chickens was accompanied by decreased Kv1.2 immunolabeling of presynaptic terminals on principal cell bodies. Thus, both intrinsic ionic conductances and excitatory synaptic inputs play crucial roles at early stages after lesions. Unlike the principal cells in compensated chickens which showed similar percentages of spontaneous spike firing cells, discharge rates, and sEPSC frequencies bilaterally, principal cells in uncompensated chickens displayed gross asymmetry in these properties bilaterally.

Modelling the anxiety-depression continuum in chicks

The clinical syndromes of anxiety and depression are now thought to exist along a temporal continuum and this construct has been modelled in a preclinical setting in chicks separated from conspecifics. This research sought to further the validity of the chick anxiety-depression continuum model. Dose-response studies using two classes of anxiolytics (chlordiazepoxide: 2.5, 5.0, 10.0, 15.0 mg/kg, and clonidine: 0.1, 0.15, 0.2, 0.25 mg/kg) and three classes of antidepressants (imipramine: 1.0, 3.0, 10.0, 15.0 mg/kg, maprotoline: 2.5, 5.0, 10.0, 20.0 mg/kg and fluoxetine: 1.0, 5.0, 10.0, 20.0 mg/kg) showed an ability to detect anxiolytic activity of chlordiazepoxide, clonidine, imipramine and maprotoline in the anxiety-like phase of the model and to detect antidepressant effects of imipramine, maprotoline and fluoxetine in the depression-like phase of the model. In addition, blood plasma interleukin-6, a biomarker of stress, was found to be elevated in response to social-separation stress. Collectively, these findings further characterize the model as a simulation of the anxiety-depression continuum and begin to establish the paradigm as a high-utility adjuvant to rodent screening assays for putative anxiolytic and antidepressant compounds.

Dye coupling in developing vestibular nuclei

The chick tangential nucleus is a major vestibular nucleus whose principal cells participate in the vestibular reflexes. During development, most mature vestibular nucleus neurons must acquire repetitive firing of action potentials on depolarization and spontaneous spike activity to process signals effectively. In the chicken, these properties emerge gradually in the embryo, starting the week before (E13, E16) and continuing through the first week after hatching (H7). Since gap junction-mediated cell coupling may influence the emergence of neuronal excitability, we investigated whether neuron-neuron and neuron-glia coupling are present in this morphologically distinctive vestibular nucleus during the period for establishing signal processing. In brain slices, principal cells were injected with biocytin in the whole-cell configuration and visualized via confocal imaging at E13, E16, and H7. The incidence of dye coupling between the injected principal cell and neurons was 42% at E13, 75% at E16, and 7% at H7, whereas the incidence of dye coupling with glia was 100% at both embryonic ages but decreased to 27% by H7. For each injected principal cell at E13, one coupled neuron and 35 coupled glia were detected, whereas three coupled neurons and 12 coupled glia were observed at E16, and few if any coupled neurons and glia were detected at H7. These results suggest that neuron-neuron and neuron-glia coupling are developmentally regulated and present before, but not after, the onset of mature signal processing by these neurons. Thus, transient neuron-neuron and neuron-glia coupling may both play roles in establishing excitability in vestibular nucleus neurons during development.

[Dynamic vestibular compensation in vestibular peripheral diseases]

Vestibular compensation, or neuronal plasticity in the central vestibular system, is quite an important process in patients with acute unilateral peripheral vestibular disease, allowing them to lead a comfortable daily life when medical treatments fail to cure the peripheral vestibular function. Is the residual unilateral vestibular input from damaged vestibular endo-organs a positive or negative factor for the development of dynamic vestibular compensation in the central nervous system? To elucidate the true mechanism of vestibular compensation, we examined the ENG findings and dizziness handicap inventory questionnaire in patients with vestibular neuronitis (VN), sudden deafness with vertigo (SDV), Meniere's disease (MD) and acoustic tumor (AT) during remission of the vertigo attacks. We obtained neuro-otological findings from caloric tests and head shaking after nystagmus using ENG and information on motion-evoked dizziness in daily life using the questionnaire. There were no significant differences in the sex, age or canal paresis % (CP%) among the four groups. The results of the present study showed that dynamic vestibular compensation processes developed progressively in the order of patients with SDV, VN, MD and AT (Kruskal-Wallis : p < 0.05). This finding suggests that processes of dynamic vestibular compensation could be accelerated in patients with fixed vestibular lesions caused by SDV and VN more than in those with fluctuating vestibular functions caused by MD and AT. In patients with fixed vestibular lesions caused by SDV and VN, patients with lower CP% showed dynamic vestibular compensation (i.e. disappearance of head shaking after nystagmus (chi-square: p < 0.05) and motion-evoked dizziness (Mann-Whitney: p < 0.0005)) more rapidly than those with higher CP%. In patients with fluctuating vestibular functions caused by MD and AT, patients with lower CP% did not always develop dynamic vestibular compensation more smoothly than those with higher CP%.

Otolith fibers and terminals in chick vestibular nuclei

The distribution of gravity-sensing, otolith afferent fibers and terminals was studied in the vestibular nuclei of 4-5-day hatchling chicks by using single and double labeling of fibers and terminals with biocytin conjugated to Alexa Fluor and confocal imaging. The vestibular nuclei are represented in a series of five transverse sections of the brainstem immunolabeled with MAP2. Saccular fibers entered the medulla posterior to and at the level of the posterior tangential vestibular nucleus and coursed through ventral parts, producing ascending and descending branches. Small saccular terminals contacted a few dendrites in the tangential nucleus. In contrast, small saccular terminals contacted many dendrites and a few neuron cell bodies in the ventrolateral vestibular nucleus, vestibulocerebellar nucleus, and descending vestibular nuclei. Utricular fibers coursed through ventral parts of the central tangential nucleus before bifurcating into ascending and descending branches. In the tangential nucleus, utricular fibers formed a few large axosomatic terminals (spoon terminals) and a few small terminals on dendrites. In addition, small utricular terminals contacted numerous dendrites and a few neuron cell bodies in the ventrolateral, vestibulocerebellar, and descending vestibular nuclei. Thus, there was negligible overlap in the distribution of the otolith nerves, although each otolith afferent shared common regions with the canal afferents, previously shown, suggesting that some second-order vestibular neurons process convergent inputs from otolith and canal afferents. Taken together with previous results, the present findings identify discrete regions of the chick vestibular nuclei where second-order vestibular neurons likely process directly convergent otolith and canal inputs.

Fos-enkephalin signaling in the rat medial vestibular nucleus facilitates vestibular compensation

In the present study, we first observed up-regulation in preproenkephalin (PPE)-like immunoreactivity (-LIR), a precursor of Met- and Leu-enkephalin, in the rat ipsilateral medial vestibular nucleus (ipsi-MVN) after unilateral labyrinthectomy (UL). By means of double-staining immunohistochemistry with PPE and Fos, a putative regulator of PPE gene expression, we revealed that some of these PPE-LIR neurons were also Fos immunopositive. The time course of decay of these double-stained neurons was quite parallel to that of UL-induced behavioral deficits. This suggests that these double-labeled neurons could have something to do with development of vestibular compensation. We next examined correlation between Fos and PPE expression in the ipsi-MVN by means of a 15-min pre-UL application of antisense oligonucleotide probes against c-fos mRNA into the ipsi-MVN. Gel shift assay and Western blotting revealed that elimination of Fos expression significantly reduced both AP-1 DNA binding activity and PPE expression in the ipsi-MVN after UL. C-fos antisense study also revealed that depression of Fos-PPE signaling in the ipsi-MVN caused significantly more severe behavioral deficits during vestibular compensation. Furthermore, studies with PPE antisense and naloxone, an opioid receptor antagonist, demonstrated that specific depression of enkephalinergic effects in the ipsi-MVN significantly delayed vestibular compensation. All these findings suggest that, immediately after UL, Fos induced in some of the ipsi-MVN neurons could regulate consequent PPE expression via the AP-1 activation and facilitate the restoration of balance between bilateral MVN activities via the opioid receptor activation, resulting in progress of vestibular compensation.

Maturation of firing pattern in chick vestibular nucleus neurons

The principal cells of the chick tangential nucleus are vestibular nucleus neurons participating in the vestibuloocular and vestibulocollic reflexes. In birds and mammals, spontaneous and stimulus-evoked firing of action potentials is essential for vestibular nucleus neurons to generate mature vestibular reflex activity. The emergence of spike-firing pattern and the underlying ion channels were studied in morphologically-identified principal cells using whole-cell patch-clamp recordings from brain slices of late-term embryos (embryonic day 16) and hatchling chickens (hatching day 1 and hatching day 5). Spontaneous spike activity emerged around the perinatal period, since at embryonic day 16 none of the principal cells generated spontaneous action potentials. However, at hatching day 1, 50% of the cells fired spontaneously (range, 3 to 32 spikes/s), which depended on synaptic transmission in most cells. By hatching day 5, 80% of the principal cells could fire action potentials spontaneously (range, 5 to 80 spikes/s), and this activity was independent of synaptic transmission and showed faster kinetics than at hatching day 1. Repetitive firing in response to depolarizing pulses appeared in the principal cells starting around embryonic day 16, when <20% of the neurons fired repetitively. However, almost 90% of the principal cells exhibited repetitive firing on depolarization at hatching day 1, and 100% by hatching day 5. From embryonic day 16 to hatching day 5, the gain for evoked spike firing increased almost 10-fold. At hatching day 5, a persistent sodium channel was essential for the generation of spontaneous spike activity, while a small conductance, calcium-dependent potassium current modulated both the spontaneous and evoked spike firing activity. Altogether, these in vitro studies showed that during the perinatal period, the principal cells switched from displaying no spontaneous spike activity at resting membrane potential and generating one spike on depolarization to the tonic firing of spontaneous and evoked action potentials.

Vestibular ganglionectomy and otolith nerve identification in the hatchling chicken

Unilateral peripheral vestibular lesions are characterized by rapid recovery from the static symptoms, called vestibular compensation, a process likely involving brain plasticity. The hatchling chick offers a promising model for studies of this process. Ganglionectomy is performed, since it provides a reproducible lesion. Here, we describe a surgical approach for vestibular ganglionectomy and the identification of the otolith nerves, using drawings and digital images of the surgical field to assist in visualizing and accessing this small, complex, and highly vascular region of the inner ear. A retroauricular approach was used in 4-8-day-old hatchling chicks. Broad access and easy identification of the otolith nerves were achieved by cauterizing the caudal auricular artery and vein in the exoccipital bone and excising the surrounding exoccipital and squamosal bones. The vestibular ganglion was accessed by removing the bony medial wall of the vestibule. Dura mater covering the ganglion was opened, the primary vestibular fibers were cut at the lateral brain surface, and the anterior and posterior parts of the vestibular ganglion were extirpated. At 24 h after surgery, the survival rate was 87% and complete ganglionectomy was achieved in 85% of operated animals.

Spontaneous synaptic activity in chick vestibular nucleus neurons during the perinatal period

The principal cells of the chick tangential nucleus are second-order vestibular neurons involved in the vestibuloocular and vestibulocollic reflexes. The spontaneous synaptic activity of morphologically identified principal cells was characterized in brain slices from 1-day-old hatchlings (H1) using whole-cell voltage-clamp recordings and Cs-gluconate pipet solution. The frequency was 1.45 Hz for spontaneous excitatory postsynaptic currents (sEPSCs) and 1.47 Hz for spontaneous inhibitory postsynaptic currents (sIPSCs). Using specific neurotransmitter receptor antagonists, all of the sEPSCs were identified as AMPA receptor-mediated events, whereas 56% of the sIPSCs were glycine and 44% were GABA(A) receptor-mediated events. On exposure to TTX, the frequency of EPSCs decreased by 68%, while the frequency of IPSCs decreased by 33%, indicating greater EPSC dependency on presynaptic action potentials. These data on spontaneous synaptic activity at H1 were compared with those obtained in previous studies of 16-day old embryos (E16). After birth, the spontaneous synaptic activity exhibited increased EPSC frequency, increased ratio for excitatory to inhibitory events, increased percentage of TTX-dependent EPSCs, and faster kinetics. In addition, the ratio for glycine/GABA receptor-mediated events increased significantly. Altogether, these data indicate that at hatching spontaneous synaptic activity of vestibular nucleus neurons in brain slices of the chick tangential nucleus undergoes appreciable changes, with increased frequency of EPSCs and glycinergic activity playing more important roles compared with the late-term chick embryo when GABAergic activity prevailed. The definition of this developmental pattern of synaptic activity in vestibular nucleus neurons should contribute to understanding how vestibular reflex activity is established in the hatchling chick.

Characterization of the chick carrageenan response

The present study characterized carrageenan inflammatory nociception in the 7-day-old domestic chick. The time course effects of foot withdrawal latency to a thermal stimulus and edema were examined over a 6-h period following an intraplantar carrageenan (0.0-1.0%) injection. Carrageenan-induced hyperalgesia and edema had a similar course of action, enduring for approximately 6 h, with a peak effect at approximately 2 h post carrageenan injection. Carrageenan inflammation was produced in a robust concentration dependent manner. Carrageenan hyperalgesia was induced at all concentrations tested and no carrageenan concentration effects were discerned. In a subsequent series of experiments we challenged the carrageenan inflammation model with systemic administration of the opioid agonist morphine, the nonsteroidal anti-inflammatory drug naproxen or the steroidal antiinflammatory drug dexamethasone. Morphine produced a dose dependent attenuation of carrageenan hyperalgesia but had no effect upon carrageenan inflammation. Naproxen produced a moderate attenuation of carrageenan inflammation and hyperalgesia. Dexamethasone dramatically attenuated both carrageenan hyperalgesia and inflammation. Collectively, these experiments characterize the chick carrageenan response and demonstrate the potential of the chick carrageenan inflammation model as a less expensive adjunct model of inflammatory nociception.

Developmental change in expression and subcellular localization of two shaker-related potassium channel proteins (Kv1.1 and Kv1.2) in the chick tangential vestibular nucleus

The chick tangential nucleus is a major avian vestibular nucleus whose principal cells participate in two vestibular reflexes. Intracellular recordings have shown that the principal cells acquire their mature firing pattern gradually during development. At embryonic day 16 (E16), most principal cells fire a single spike, whereas shortly after hatching (H) the vast majority fire repetitively on depolarization. The transition in firing pattern was likely due in part to a downregulation of a low-threshold, sustained, dendrotoxin-sensitive (DTX) potassium current, I(DS). Since the DTX-sensitive potassium channel subunits Kv1.1 and Kv1.2 generate sustained currents, in the present study we applied fluorescence immunocytochemistry and confocal microscopy to characterize their developmental expression at E16, H1, and H9. At E16, both Kv1.1 and Kv1.2 staining were confined to the principal cell bodies. Immunolabeling decreased significantly for both proteins at H1, and more so by H9. Double-labeling with a monoclonal antibody against microtubule-associated protein 2 (MAP2) in hatchlings showed that some Kv1.1 remained as clusters within the cell body, at the base of the dendrites, and in the axon initial segment. In hatchlings, Kv1.2 staining decreased in the cell bodies and simultaneously appeared in the neuropil, colocalized with biocytin-labeled primary vestibular fibers and vestibular "spoon" terminals. Also, double-labeling with synaptotagmin showed that Kv1.2 colocalized with many nonvestibular terminals surrounding the principal cell bodies. These results identified developmental decreases in the staining of these two potassium channel protein subunits and changes in their subcellular localization corresponding to the downregulation of I(DS) defined electrophysiologically around hatching. Accordingly, both of these protein subunits could be involved in regulating excitability of the principal cells.

Differential expression of connexin 43 in the chick tangential vestibular nucleus

The chick tangential nucleus is a major vestibular nucleus whose principal cells receive convergent inputs from primary vestibular and nonvestibular fibers and participate in the vestibular reflexes. During development, the principal cells gradually acquire the mature firing pattern in part by losing a specific potassium current around hatching (H). Here we focus on characterizing the expression of connexin 43 (Cx43), a gap junction protein found mainly between astrocytes in the mature brain. The astrocytic syncytium plays an important role in maintaining extracellular potassium ion balance in the brain. Accordingly, it is important to characterize the potential of this syncytium to communicate during the critical developmental age of hatching. Using fluorescence immunocytochemistry, we investigated whether Cx43 staining was concentrated in specific cellular compartments at H1 by applying well-known markers for astrocytes (glial fibrillary acidic protein; GFAP), oligodendrocytes (antimyelin), neurons (microtubule-associated protein 2), and synaptic terminals (synaptotagmin). GFAP-positive astrocytes and GFAP-negative nonneuronal cells around the principal cell bodies were labeled with Cx43, suggesting that Cx43 was expressed exclusively by nonneuronal cells near the neuronal elements. Next, the developmental pattern of expression of Cx43 was studied at embryonic day 16 (E16), H1, and H9. At E16, Cx43 was present weakly as random small clusters in the tangential nucleus, whereas, at H1, overall staining became localized, with increases in size, brightness, and number of immunostained clusters. Finally, at H9, Cx43 staining decreased, but cluster size and location remained unchanged. These results suggest that Cx43 is developmentally regulated with a peak at birth and is associated primarily with astrocytes and nonneuronal cells near the principal cell bodies.