Assembling, Connecting, and Maintaining the Cochlear Nucleus

Activation of metabotropic glutamate receptors regulates ribosomes of cochlear nucleus neurons

The brain stem auditory system of the chick is an advantageous model for examining changes that occur as a result of deafness. Elimination of acoustic input through cochlear ablation results in the eventual death of approximately 30% of neurons in the chick cochlear nucleus, nucleus magnocellularis (NM). One early change following deafness is an alteration in NM ribosomes, evidenced both by a decrease in protein synthesis and reduction in antigenicity for Y10B, a monoclonal antibody that recognizes a ribosomal epitope. Previous studies have shown that mGluR activation is necessary to maintain Y10B antigenicity and NM viability. What is still unclear, however, is whether or not mGluR activation is sufficient to prevent deafness-induced changes in these neurons, or if other activity-dependent factors are also necessary. The current study investigated the ability of mGluR activation to regulate cochlear nucleus ribosomes in the absence of auditory nerve input. In vitro methods were employed to periodically pressure eject glutamate or mGluR agonists over neurons on one side of a slice preparation leaving the opposite side of the same slice untreated. Immunohistochemistry was then performed using Y10B in order to assess ribosomal changes. Application of glutamate and both group I and II selective mGluR agonists effectively rescued ribosomal antigenicity on the treated side of the slice in comparison to ribosomes on the untreated side. These findings suggest that administration of mGluR agonists is sufficient to reduce the early interruption of normal ribosomal integrity that is typically seen following loss of auditory nerve activity.

Neuronal differentiation of the early embryonic auditory hindbrain of the chicken in primary culture

Neurons in the auditory hindbrain pathway of the chicken are physiologically and morphologically highly specialized. It remains unclear to what extent independent differentiation vs. activity-dependent mechanisms determines the development of this system. To address this question we established a primary culture system of the early auditory hindbrain neurons. Primary cultures of neurons from nucleus magnocellularis and nucleus laminaris were prepared from embryonic day 6.5 chicken. These cells developed in culture under serum-free conditions for up to 15 days. Immunocytochemical staining and whole-cell patch recordings were used to characterize the development of the neurons. A stable expression of the calcium-binding protein calretinin, which serves as a characteristic marker of the auditory pathway, was found at all stages. A voltage-gated potassium channel (Kv3.1b) with a specific function in the auditory system was also expressed after about 1 week in culture. Electrophysiological recordings showed a general maturation of the neuronal phenotype as reflected by an increase in the mean resting membrane potential, a decrease in the mean input resistance as well as a maturation of action potential parameters. Four groups of neurons that generate action potentials could be distinguished. One of these showed the phasic firing pattern of auditory brainstem neurons known from slice preparations. In older cultures we demonstrated functional synaptogenesis in vitro by recording postsynaptic activity elicited by extracellular stimulation and styryl dye loading of vesicles. Thus, isolated neurons from the auditory region of the avian brainstem differentiate to specific neuronal subtypes and autonomously develop synaptic connections in vitro.

Expression of GABA(B) receptor in the avian auditory brainstem: ontogeny, afferent deprivation, and ultrastructure

Nucleus magnocellularis (NM), nucleus angularis (NA), and nucleus laminaris (NL), second- and third-order auditory neurons in the avian brainstem, receive GABAergic input primarily from the superior olivary nucleus (SON). Previous studies have demonstrated that both GABA(A) and GABA(B) receptors (GABA(B)Rs) influence physiological properties of NM neurons. We characterized the distribution of GABA(B)R expression in these nuclei during development and after deafferentation of the excitatory auditory nerve (nVIII) inputs. We used a polyclonal antibody raised against rat GABA(B)Rs in the auditory brainstem during developmental periods that are thought to precede and include synaptogenesis of GABAergic inputs. As early as embryonic day (E)14, dense labeling is observed in NA, NM, NL, and SON. At earlier ages immunoreactivity is present in somas as diffuse staining with few puncta. By E21, when the structure and function of the auditory nuclei are known to be mature, GABA(B) immunoreactivity is characterized by dense punctate labeling in NM, NL, and a subset of NA neurons, but label is sparse in the SON. Removal of the cochlea and nVIII neurons in posthatch chicks resulted in only a small decrease in immunoreactivity after survival times of 14 or 28 days, suggesting that a major proportion of GABA(B)Rs may be expressed postsynaptically or on GABAergic terminals. We confirmed this interpretation with immunogold TEM, where expression at postsynaptic membrane sites is clearly observed. The characterization of GABA(B)R distribution enriches our understanding of the full complement of inhibitory influences on central auditory processing in this well-studied neuronal circuit.

Development of the human fetal cochlear nerve: a morphometric study

Ontogenesis of the human peripheral auditory pathway is relatively less explored. While the distal part of the auditory perception apparatus (i.e. the cochlea) received attention, studies on the neural element carrying information to the brainstem (i.e. the cochlear nerve) are scarce. In the present study, axonal differentiation, maturation and myelination of the distal end of the human cochlear nerve (CN) were assessed using light and electron microscopy. Seven human fetuses of 12, 15, 18, 20, 22, 28 and 38 weeks' gestation (WG) were analyzed. Light microscopy revealed nerve fascicles as early as 12 WG, initially arranged loosely but later compacted by 18 WG. Myelinated fibers were clearly detected at 28 WG. Ultrastructurally, at 12 WG developing Schwann cells were present between the thin unmyelinated axons. At 15 WG, the fascicular arrangement was distinct with blood vessels in the perineurium. The maximum number of axons was found at 20 WG, which subsequently reduced to reach the adult level at 22 WG. The myelinated axons in the CN were first observed on the left side at 20 WG, following which the number and proportion of myelinated axons increased until term, incorporating both small and large axons. The right CN lagged behind in maturation. Axon size also increased with age. Thus, the maturation of the human CN commences during the mid-gestation period and produces exuberant axons that are eventually pruned at a time when axons start to myelinate. During this developmental period the human CN maintains maturational asymmetry, the functional consequences of which remain to be elucidated.