Postnatal developmental changes in the medial nucleus of the trapezoid body in a mouse model of auditory pathology

Cochlear ablation in neonatal rats disrupts inhibitory transmission in the medial nucleus of the trapezoid body

Inhibitory circuits in the auditory brainstem undergo multiple postnatal changes that are both activity-dependent and activity-independent. We tested to see if the shift from GABA- to glycinergic transmission, which occurs in the rat medial nucleus of the trapezoid body (MNTB) around the onset of hearing, depends on sound-evoked neuronal activity. We prevented the activity by bilateral cochlear ablations in early postnatal rats and studied ionotropic GABA and glycine receptors in MNTB neurons after hearing onset. The removal of the cochlea decreased responses of GABA_A and glycine receptors to exogenous agonists as well as the amplitudes of inhibitory postsynaptic currents. The reduction was accompanied by a decrease in the number of glycine receptor- or vesicular GABA transporter-immunopositive puncta. Furthermore, the ablations markedly affected the switch in presynaptic GABA_A to glycine receptors. The increase in the expression of postsynaptic glycine receptors and the shift in inhibitory transmitters were not prevented. The results suggest that inhibitory transmission in the MNTB is subject to multiple developmental signals and support the idea that auditory experience plays a role in the maturation of the brainstem glycinergic circuits.

The Expression of Hypoxia-Induced Gene 1 (Higd1a) in the Central Nervous System of Male and Female Rats Differs According to Age

HIGD1A (hypoxia-induced gene domain protein-1a), a mitochondrial inner membrane protein present in various cell types, has been mainly associated with anti-apoptotic processes in response to stressors. Our previous findings have shown that Higd1a mRNA is widely expressed across the central nervous system (CNS), exhibiting an increasing expression in the spinal cord from postnatal day 1 (P1) to 15 (P15) and changes in the distribution pattern from P1 to P90. During the first weeks of postnatal life, the great plasticity of the CNS is accompanied by cell death/survival decisions. So we first describe HIGD1A expression throughout the brain during early postnatal life in female and male pups. Secondly, based on the fact that in some areas this process is influenced by the sex of individuals, we explore HIGD1A expression in the sexual dimorphic nucleus (SDN) of the medial preoptic area, a region that is several folds larger in male than in female rats, partly due to sex differences in the process of apoptosis during this period. Immunohistochemical analysis revealed that HIGD1A is widely but unevenly expressed throughout the brain. Quantitative Western blot analysis of the parietal cortex, diencephalon, and spinal cord from both sexes at P1, P5, P8, and P15 showed that the expression of this protein is predominantly high and changes with age but not sex. Similarly, in the sexual dimorphic nucleus, the expression of HIGD1A varied according to age, but we were not able to detect significant differences in its expression according to sex. Altogether, these results suggest that HIGD1A protein is expressed in several areas of the central nervous system following a pattern that quantitatively changes with age but does not seem to change according to sex.

Development of glycinergic innervation to the murine LSO and SPN in the presence and absence of the MNTB

Neurons in the superior olivary complex (SOC) integrate excitatory and inhibitory inputs to localize sounds in space. The majority of these inhibitory inputs have been thought to arise within the SOC from the medial nucleus of the trapezoid body (MNTB). However, recent work demonstrates that glycinergic innervation of the SOC persists in Egr2; En1^CKO mice that lack MNTB neurons, suggesting that there are other sources of this innervation (Jalabi et al., 2013). To study the development of MNTB- and non-MNTB-derived glycinergic SOC innervation, we compared immunostaining patterns of glycine transporter 2 (GlyT2) at several postnatal ages in control and Egr2; En1^CKO mice. GlyT2 immunostaining was present at birth (P0) in controls and reached adult levels by P7 in the superior paraolivary nucleus (SPN) and by P12 in the lateral superior olive (LSO). In Egr2; En1^CKO mice, glycinergic innervation of the LSO developed at a similar rate but was delayed by one week in the SPN. Conversely, consistent reductions in the number of GlyT2⁺ boutons located on LSO somata were seen at all ages in Egr2; En1^CKO mice, while these numbers reached control levels in the SPN by adulthood. Dendritic localization of GlyT2+ boutons was unaltered in both the LSO and SPN of adult Egr2; En1^CKO mice. On the postsynaptic side, adult Egr2; En1^CKO mice had reduced glycine receptor α1 (GlyRα1) expression in the LSO but normal levels in the SPN. GlyRα2 was not expressed by LSO or SPN neurons in either genotype. These findings contribute important information for understanding the development of MNTB- and non-MNTB-derived glycinergic pathways to the mouse SOC.

Postinhibitory rebound neurons and networks are disrupted in retrovirus-induced spongiform neurodegeneration

Certain retroviruses induce progressive spongiform motor neuron disease with features resembling prion diseases and amyotrophic lateral sclerosis. With the neurovirulent murine leukemia virus (MLV) FrCasE, Env protein expression within glia leads to postsynaptic vacuolation, cellular effacement, and neuronal loss in the absence of neuroinflammation. To understand the physiological changes associated with MLV-induced spongiosis, and its neuronal specificity, we employed patch-clamp recordings and voltage-sensitive dye imaging in brain slices of the mouse inferior colliculus (IC), a midbrain nucleus that undergoes extensive spongiosis. IC neurons characterized by postinhibitory rebound firing (PIR) were selectively affected in FrCasE-infected mice. Coincident with Env expression in microglia and in glia characterized by NG2 proteoglycan expression (NG2 cells), rebound neurons (RNs) lost PIR, became hyperexcitable, and were reduced in number. PIR loss and hyperexcitability were reversed by raising internal calcium buffer concentrations in RNs. PIR-initiated rhythmic circuits were disrupted, and spontaneous synchronized bursting and prolonged depolarizations were widespread. Other IC neuron cell types and circuits within the same degenerative environment were unaffected. Antagonists of NMDA and/or AMPA receptors reduced burst firing in the IC but did not affect prolonged depolarizations. Antagonists of L-type calcium channels abolished both bursts and slow depolarizations. IC infection by the nonneurovirulent isogenic virus Friend 57E (Fr57E), whose Env protein is structurally similar to FrCasE, showed no RN hyperactivity or cell loss; however, PIR latency increased. These findings suggest that spongiform neurodegeneration arises from the unique excitability of RNs, their local regulation by glia, and the disruption of this relationship by glial expression of abnormal protein.