Maturation of glutamatergic transmission in the vestibulo-olivary pathway impacts on the registration of head rotational signals in the brainstem of rats

Neonatal GABAergic transmission primes vestibular gating of output for adult spatial navigation

GABAergic interneurons are poised with the capacity to shape circuit output via inhibitory gating. How early in the development of medial vestibular nucleus (MVN) are GABAergic neurons recruited for feedforward shaping of outputs to higher centers for spatial navigation? The role of early GABAergic transmission in assembling vestibular circuits for spatial navigation was explored by neonatal perturbation. Immunohistochemistry and confocal imaging were utilized to reveal the expression of parvalbumin (PV)-expressing MVN neurons and their perineuronal nets. Whole-cell patch-clamp recording, coupled with optogenetics, was conducted in vitro to examine the synaptic function of MVN circuitry. Chemogenetic targeting strategy was also employed in vivo to manipulate neuronal activity during navigational tests. We found in rats a neonatal critical period before postnatal day (P) 8 in which competitive antagonization of GABAergic transmission in the MVN retarded maturation of inhibitory neurotransmission, as evidenced by deranged developmental trajectory for excitation/inhibition ratio and an extended period of critical period-like plasticity in GABAergic transmission. Despite increased number of PV-expressing GABAergic interneurons in the MVN, optogenetic-coupled patch-clamp recording indicated null-recruitment of these neurons in tuning outputs along the ascending vestibular pathway. Such perturbation not only offset output dynamics of ascending MVN output neurons, but was further accompanied by impaired vestibular-dependent navigation in adulthood. The same perturbations were however non-consequential when applied after P8. Results highlight neonatal GABAergic transmission as key to establishing feedforward output dynamics to higher brain centers for spatial cognition and navigation.

cFOS as a biomarker of activity maturation in the hippocampal formation

We explored the potential for cFOS expression as a marker of functional development of "resting-state" waking activity in the extended network of the hippocampus and entorhinal cortex. We examined sleeping and awake mice at (P)ostnatal days 5, 9, 13, and 17 as well as in adulthood. We find that cFOS expression is state-dependent even at 5 days old, with reliable staining occurring only in the awake mice. Even during waking, cFOS expression was rare and weak at P5. The septal nuclei, entorhinal cortex layer (L)2, and anterodorsal thalamus were exceptional in that they had robust cFOS expression at P5 that was similar to or greater than in adulthood. Significant P5 expression was also observed in the dentate gyrus, entorhinal cortex L6, postsubiculum L4-6, ventral subiculum, supramammillary nucleus, and posterior hypothalamic nucleus. The expression in these regions grew stronger with age, and the expression in new regions was added progressively at P9 and P13 by which point the overall expression pattern in many regions was qualitatively similar to the adult. Six regions-CA1, dorsal subiculum, postsubiculum L2-3, reuniens nucleus, and perirhinal and postrhinal cortices-were very late developing, mostly achieving adult levels only after P17. Our findings support a number of developmental principles. First, early spontaneous activity patterns induced by muscle twitches during sleep do not induce robust cFOS expression in the extended hippocampal network. Second, the development of cFOS expression follows the progressive activation along the trisynaptic circuit, rather than birth date or cellular maturation. Third, we reveal components of the egocentric head-direction and theta-rhythm circuits as the earliest cFOS active circuits in the forebrain. Our results suggest that cFOS staining may provide a reliable and sensitive biomarker for hippocampal formation activity development, particularly in regard to the attainment of a normal waking state and synchronizing rhythms such as theta and gamma.

Timely insertion of AMPA receptor in developing vestibular circuits is required for manifestation of righting reflexes and effective navigation

Vestibular information processed first by the brainstem vestibular nucleus (VN), and further by cerebellum and thalamus, underlies diverse brain function. These include the righting reflexes and spatial cognitive behaviour. While the cerebellar and thalamic circuits that decode vestibular information are known, the importance of VN neurons and the temporal requirements for their maturation that allow developmental consolidation of the aforementioned circuits remains unclear. We show that timely unsilencing of glutamatergic circuits in the VN by NMDA receptor-mediated insertion of AMPAR receptor type 1 (GluA1) subunits is critical for maturation of VN and successful consolidation of higher circuits that process vestibular information. Delayed unsilencing of NMDA receptor-only synapses of neonatal VN neurons permanently decreased their functional connectivity with inferior olive circuits. This was accompanied by delayed pruning of the inferior olive inputs to Purkinje cells and permanent reduction in their plasticity. These derangements led to deficits in associated vestibular righting reflexes and motor co-ordination during voluntary movement. Vestibular-dependent recruitment of thalamic neurons was similarly reduced, resulting in permanently decreased efficiency of spatial navigation. The findings thus show that well-choreographed maturation of the nascent vestibular circuitry is prerequisite for functional integration of vestibular signals into ascending pathways for diverse vestibular-related behaviours.

The Role of Carotid Sinus Nerve Input in the Hypoxic-Hypercapnic Ventilatory Response in Juvenile Rats

In juvenile rats, the carotid body (CB) is the primary sensor of oxygen (O₂) and a secondary sensor of carbon dioxide (CO₂) in the blood. The CB communicates to the respiratory pattern generator via the carotid sinus nerve, which terminates within the commissural nucleus tractus solitarius (cNTS). While this is not the only peripheral chemosensory pathway in juvenile rodents, we hypothesize that it has a unique role in determining the interaction between O₂ and CO₂, and consequently, the response to hypoxic-hypercapnic gas challenges. The objectives of this study were to determine (1) the ventilatory responses to a poikilocapnic hypoxic (HX) gas challenge, a hypercapnic (HC) gas challenge or a hypoxic-hypercapnic (HH) gas challenge in juvenile rats; and (2) the roles of CSN chemoafferents in the interactions between HX and HC signaling in these rats. Studies were performed on conscious, freely moving juvenile (P25) male Sprague Dawley rats that underwent sham-surgery (SHAM) or bilateral transection of the carotid sinus nerves (CSNX) 4 days previously. Rats were placed in whole-body plethysmographs to record ventilatory parameters (frequency of breathing, tidal volume and minute ventilation). After acclimatization, they were exposed to HX (10% O₂, 90% N₂), HC (5% CO₂, 21% O₂, 74% N₂) or HH (5% CO₂, 10% O₂, 85% N₂) gas challenges for 5 min, followed by 15 min of room-air. The major findings were: (1) the HX, HC and HH challenges elicited robust ventilatory responses in SHAM rats; (2) ventilatory responses elicited by HX alone and HC alone were generally additive in SHAM rats; (3) the ventilatory responses to HX, HC and HH were markedly attenuated in CSNX rats compared to SHAM rats; and (4) ventilatory responses elicited by HX alone and HC alone were not additive in CSNX rats. Although the rats responded to HX after CSNX, CB chemoafferent input was necessary for the response to HH challenge. Thus, secondary peripheral chemoreceptors do not compensate for the loss of chemoreceptor input from the CB in juvenile rats.

5-HT1A receptor-mediated attenuation of synaptic transmission in rat medial vestibular nucleus impacts on vestibular-related motor function

KEY POINTS

Chemogenetic activation of medial vestibular nucleus-projecting 5-HT neurons resulted in deficits in vestibular-mediated tasks, including negative geotaxis, balance beam and rota-rod tests. The 5-HT_1A receptor mediates the vestibular-related behavioural effects of 5-HT in the vestibular nucleus. 5-HT_1A receptor activation attenuated evoked excitatory postsynaptic currents and evoked inhibitory postsynaptic currents via a presynaptic mechanism in the vestibular nucleus.

ABSTRACT

While the anxiolytic effects of serotonergic neuromodulation are well studied, its role in sensorimotor coordination and postural control is unclear. In this study, we show that an increase of serotonin (5-hydroxytryptamine, 5-HT) at the medial vestibular nucleus (MVN), a brainstem centre for vestibulospinal coordination, by either direct cannula administration or chemogenetic stimulation of MVN-projecting serotonergic neurons, adversely affected performance of rats in vestibular-mediated tasks, including negative geotaxis, balance beam and rota-rod tests. Application of the 5-HT₁ and 5-HT₇ receptor co-agonist 8-hydroxy-2-(di-n-propylamino) tetralin recapitulated the effect of 5-HT, while co-administration of the specific 5-HT_1A receptor antagonist WAY 100135 effectively abolished all 5-HT-induced behavioural deficits. This indicated that 5-HT_1A receptors mediated the effects of 5-HT in the rat MVN. Using whole-cell patch-clamp recording, we demonstrated that 5-HT_1A receptor activation attenuated both evoked excitatory and evoked inhibitory postsynaptic currents through a presynaptic mechanism in the rat MVN. The results thus highlight the 5-HT_1A receptor as the gain controller of vestibular-related brainstem circuits for posture and balance.

Low plasma concentrations of N-methyl-d-aspartate receptor subunits as a possible biomarker for psychosis

BACKGROUND

N-methyl-d-aspartate receptor (NMDAR) has been largely implicated in the neurobiology of schizophrenia and other psychosis. Aiming to evaluate their potential as peripheral biomarkers for psychosis, we quantified the plasma concentrations of NR1 and NR2 NMDAR subunits of first-episode psychosis patients in their first contact with mental health services due to psychotic symptoms, compared with siblings and matched community-based controls.

METHODS

The quantifications of NR1 and NR2 plasma concentrations were performed by ELISA. Data were analysed by nonparametric tests and Receiver Operating Curve (ROC) analysis.

RESULTS

We included 166 first-episode psychosis patients (mean age = 30.3 ± 12.2 years; 64% men), with the diagnosis of schizophrenia spectrum (n = 84), bipolar disorder (n = 51) and psychotic depression (n = 31), 76 siblings (mean age = 31.5 ± 11.0 years; 30.3% men) and 166 healthy community-based controls (mean age = 31.4 ± 12.0 years; 63.9% men). NMDAR subunits were significantly lower in patients compared with siblings and controls (p < 0.001), except by NR1 plasma concentrations of bipolar patients compared with siblings and controls. NR1 plasma concentrations lower than 17.65 pg/ml (AUC = 0.621) showed sensitivity of 42.8%, specificity of 84.3%, positive predictive value (PPV) of 73.2% and negative predictive value (NPV) of 59.6%. Individuals with NR2 plasma concentrations lower than 2.92 ng/ml (AUC = 0.801) presented a 10.61-fold increased risk of psychosis, with a sensibility of 71.9%, specificity of 80.6%, PPV of 79.0% and NPV of 73.9%.

CONCLUSIONS

This is the first study reporting the measurement and the reduction of NR1 and NR2 NMDAR subunits plasma concentrations in psychiatric disorders. In particular, the NR2 subunit may be a possible plasma biomarker for psychosis.

Regulatory roles of perineuronal nets and semaphorin 3A in the postnatal maturation of the central vestibular circuitry for graviceptive reflex

Perineuronal nets (PN) restrict neuronal plasticity in the adult brain. We hypothesize that activity-dependent consolidation of PN is required for functional maturation of behavioral circuits. Using the postnatal maturation of brainstem vestibular nucleus (VN) circuits as a model system, we report a neonatal period in which consolidation of central vestibular circuitry for graviception is accompanied by activity-dependent consolidation of chondroitin sulfate (CS)-rich PN around GABAergic neurons in the VN. Postnatal onset of negative geotaxis was used as an indicator for functional maturation of vestibular circuits. Rats display negative geotaxis from postnatal day (P) 9, coinciding with the condensation of CS-rich PN around GABAergic interneurons in the VN. Delaying PN formation, by removal of primordial CS moieties on VN with chondroitinase ABC (ChABC) treatment at P6, postponed emergence of negative geotaxis to P13. Similar postponement was observed following inhibition of GABAergic transmission with bicuculline, in line with the reported role of PN in increasing excitability of parvalbumin neurons. We further reasoned that PN-CS restricts bioavailability of plasticity-inducing factors such as semaphorin 3A (Sema3A) to bring about circuit maturation. Treatment of VN explants with ChABC to liberate PN-bound Sema3A resulted in dendritic growth and arborization, implicating structural plasticity that delays synapse formation. Evidence is thus provided for the role of PN-CS-Sema3A in regulating structural and circuit plasticity at VN interneurons with impacts on the development of graviceptive postural control.

Activation of 5-HT7 receptors reverses NMDA-R-dependent LTD by activating PKA in medial vestibular neurons

The medial vestibular nucleus (MVN) is a major output station for neurons that project to the vestibulo-spinal pathway. MVN neurons show capacity for long-term depression (LTD) during the juvenile period. We investigated LTD of MVN neurons using whole-cell patch-clamp recordings. High frequency stimulation (HFS) robustly induced LTD in 90% of type B neurons in the MVN, while only 10% of type A neurons were responsive, indicating that type B neurons are the major contributors to LTD in the MVN. The neuromodulator serotonin (5-HT) is known to modulate LTD in neural circuits of the cerebral cortex and the hippocampus. We therefore aim to determine the action of 5-HT on the LTD of type B MVN neurons and elucidate the relevant 5-HT receptor subtypes responsible for its action. Using specific agonists and antagonists of 5-HT receptors, we found that selective activation of 5-HT₇ receptor in type B neurons in the MVN of juvenile (P13-16) rats completely abolished NMDA-receptor-mediated LTD in a protein kinase A (PKA)-dependent manner. Our finding that 5-HT restricts plasticity of type B MVN neurons via 5-HT₇ receptors offers a mechanism whereby vestibular tuning contributes to the maturation of the vestibulo-spinal circuit and highlights the role of 5-HT in postural control.