Neonatal deafferentation does not alter membrane properties of trigeminal nucleus principalis neurons

Synaptic Origin of Early Sensory-evoked Oscillations in the Immature Thalamus

During the critical period of postnatal development, brain maturation is extremely sensitive to external stimuli. Newborn rodents already have functional somatosensory pathways and the thalamus, but the cortex is still forming. Immature thalamic synapses may produce large postsynaptic potentials in immature neurons, while non-synaptic membrane currents remain relatively weak and slow. The thalamocortical system generates spontaneous and evoked early gamma and spindle-burst oscillations in newborn rodents. How relatively strong synapses and weak intrinsic currents interact with each other and how they contribute to early thalamic activities remains largely unknown. Here, we performed local field potential (LFP), juxtacellular, and patch-clamp recordings in the somatosensory thalamus of urethane-anesthetized rat pups at postnatal days 6-7 with one whisker stimulation. We removed the overlying cortex and hippocampus to reach the thalamus with electrodes. Deflection of only one (the principal) whisker induced spikes in a particular thalamic cell. Whisker deflection evoked a group of large-amplitude excitatory events, likely originating from lemniscal synapses and multiple inhibitory postsynaptic events in thalamocortical cells. Large-amplitude excitatory events produced a group of spike bursts and could evoke a depolarization block. Juxtacellular recordings confirmed the partial inactivation of spikes. Inhibitory events prevented inactivation of action potentials and gamma-modulated neuronal firing. We conclude that the interplay of strong excitatory and inhibitory synapses and relatively weak intrinsic currents produces sensory-evoked early gamma oscillations in thalamocortical cells. We also propose that sensory-evoked large-amplitude excitatory events contribute to evoked spindle-bursts.

Brainstem local microglia induce whisker map plasticity in the thalamus after peripheral nerve injury

Whisker deafferentation in mice disrupts topographic connectivity from the brainstem to the thalamic ventral posteromedial nucleus (VPM), which represents whisker map, by recruiting "ectopic" axons carrying non-whisker information in VPM. However, mechanisms inducing this plasticity remain largely unknown. Here, we show the role of region-specific microglia in the brainstem principal trigeminal nucleus (Pr5), a whisker sensory-recipient region, in VPM whisker map plasticity. Systemic or local manipulation of microglial activity reveals that microglia in Pr5, but not in VPM, are necessary and sufficient for recruiting ectopic axons in VPM. Deafferentation causes membrane hyperexcitability of Pr5 neurons dependent on microglia. Inactivation of Pr5 neurons abolishes this somatotopic reorganization in VPM. Additionally, microglial depletion prevents deafferentation-induced ectopic mechanical hypersensitivity. Our results indicate that local microglia in the brainstem induce peripheral nerve injury-induced plasticity of map organization in the thalamus and suggest that microglia are potential therapeutic targets for peripheral nerve injury-induced mechanical hypersensitivity.

Patchwork-Type Spontaneous Activity in Neonatal Barrel Cortex Layer 4 Transmitted via Thalamocortical Projections

Establishment of precise neuronal connectivity in the neocortex relies on activity-dependent circuit reorganization during postnatal development; however, the nature of cortical activity during this period remains largely unknown. Using two-photon calcium imaging of the barrel cortex in vivo during the first postnatal week, we reveal that layer 4 (L4) neurons within the same barrel fire synchronously in the absence of peripheral stimulation, creating a "patchwork" pattern of spontaneous activity corresponding to the barrel map. By generating transgenic mice expressing GCaMP6s in thalamocortical axons, we show that thalamocortical axons also demonstrate the spontaneous patchwork activity pattern. Patchwork activity is diminished by peripheral anesthesia but is mostly independent of self-generated whisker movements. The patchwork activity pattern largely disappeared during postnatal week 2, as even L4 neurons within the same barrel tended to fire asynchronously. This spontaneous L4 activity pattern has features suitable for thalamocortical (TC) circuit refinement in the neonatal barrel cortex.

Sensory Activity-Dependent and Sensory Activity-Independent Properties of the Developing Rodent Trigeminal Principal Nucleus

The whisker-sensory trigeminal central pathway of rodents is an established model for studies of activity-dependent neural plasticity. The first relay station of the pathway is the trigeminal principal nucleus (PrV), the ventral part of which receives sensory inputs mainly from the infraorbital branch of the maxillary trigeminal nerve (ION). Whisker-sensory afferents play an important role in the development of the morphological and physiological properties of PrV neurons. In neonates, deafferentation by ION transection leads to the disruption of whisker-related neural patterns (barrelettes) and cell death within a specific time window (critical period), as revealed by morphological studies. Whisker-sensory inputs control synaptic elimination, postsynaptic AMPA receptor trafficking, astrocyte-mediated synaptogenesis, and receptive-field characteristics of PrV cells, without a postnatal critical period. Sensory activity-dependent synaptic plasticity requires the activation of NMDA receptors and involves the participation of glia. However, the basic physiological properties of PrV neurons, such as cell type-specific ion channels, presynaptic terminal function, postsynaptic NMDA receptor subunit composition, and formation of the inhibitory circuitry, are independent of sensory inputs. Therefore, the first relay station of the whisker sensation is largely mature-like and functional at birth. Delineation of activity-dependent and activity-independent features of the postnatal PrV is important for understanding the development and functional characteristics of downstream trigeminal stations in the thalamus and neocortex. This mini review focuses on such features of the developing rodent PrV.

Neonatal sensory nerve injury-induced synaptic plasticity in the trigeminal principal sensory nucleus

Sensory deprivation studies in neonatal mammals, such as monocular eye closure, whisker trimming, and chemical blockade of the olfactory epithelium have revealed the importance of sensory inputs in brain wiring during distinct critical periods. But very few studies have paid attention to the effects of neonatal peripheral sensory nerve damage on synaptic wiring of the central nervous system (CNS) circuits. Peripheral somatosensory nerves differ from other special sensory afferents in that they are more prone to crush or severance because of their locations in the body. Unlike the visual and auditory afferents, these nerves show regenerative capabilities after damage. Uniquely, damage to a somatosensory peripheral nerve does not only block activity incoming from the sensory receptors but also mediates injury-induced neuro- and glial chemical signals to the brain through the uninjured central axons of the primary sensory neurons. These chemical signals can have both far more and longer lasting effects than sensory blockade alone. Here we review studies which focus on the consequences of neonatal peripheral sensory nerve damage in the principal sensory nucleus of the brainstem trigeminal complex.

Development and critical period plasticity of the barrel cortex

In primary sensory neocortical areas of mammals, the distribution of sensory receptors is mapped with topographic precision and amplification in proportion to the peripheral receptor density. The visual, somatosensory and auditory cortical maps are established during a critical period in development. Throughout this window in time, the developing cortical maps are vulnerable to deleterious effects of sense organ damage or sensory deprivation. The rodent barrel cortex offers an invaluable model system with which to investigate the mechanisms underlying the formation of topographic maps and their plasticity during development. Five rows of mystacial vibrissa (whisker) follicles on the snout and an array of sinus hairs are represented by layer IV neural modules ('barrels') and thalamocortical axon terminals in the primary somatosensory cortex. Perinatal damage to the whiskers or the sensory nerve innervating them irreversibly alters the structural organization of the barrels. Earlier studies emphasized the role of the sensory periphery in dictating whisker-specific brain maps and patterns. Recent advances in molecular genetics and analyses of genetically altered mice allow new insights into neural pattern formation in the neocortex and the mechanisms underlying critical period plasticity. Here, we review the development and patterning of the barrel cortex and the critical period plasticity.

Astrocytes promote peripheral nerve injury-induced reactive synaptogenesis in the neonatal CNS

Neonatal damage to the trigeminal nerve leads to "reactive synaptogenesis" in the brain stem sensory trigeminal nuclei. In vitro models of brain injury-induced synaptogenesis have implicated an important role for astrocytes. In this study we tested the role of astrocyte function in reactive synaptogenesis in the trigeminal principal nucleus (PrV) of neonatal rats following unilateral transection of the infraorbital (IO) branch of the trigeminal nerve. We used electrophysiological multiple input index analysis (MII) to estimate the number of central trigeminal afferent fibers that converge onto single barrelette neurons. In the developing PrV, about 30% of afferent connections are eliminated within 2 postnatal weeks. After neonatal IO nerve damage, multiple trigeminal inputs (2.7 times that of the normal inputs) converge on single barrelette cells within 3-5 days; they remain stable up to the second postnatal week. Astrocyte proliferation and upregulation of astrocyte-specific proteins (GFAP and ALDH1L1) accompany reactive synaptogenesis in the IO nerve projection zone of the PrV. Pharmacological blockade of astrocyte function, purinergic receptors, and thrombospondins significantly reduced or eliminated reactive synaptogenesis without changing the MII in the intact PrV. GFAP immunohistochemistry further supported these electrophysiological results. We conclude that immature astrocytes, purinergic receptors, and thrombospondins play an important role in reactive synaptogenesis in the peripherally deafferented neonatal PrV.

Peripheral nerve damage does not alter release properties of developing central trigeminal afferents

The infraorbital branch of the trigeminal nerve (ION) is essential in whisker-specific neural patterning ("barrelettes") in the principal nucleus of the trigeminal nerve (PrV). The barrelettes are formed by the ION terminal arbors, somata, and dendrites of the PrV cells; they are abolished after neonatal damage to the ION. Physiological studies show that disruption of the barrelettes is accompanied by conversion of functional synapses into silent synapses in the PrV. In this study, we used whole cell recordings with a paired-pulse stimulation protocol and MK-801 blocking rate to estimate the presynaptic release probability (Pr) of ION central trigeminal afferent terminals in the PrV. We investigated Pr during postnatal development, following neonatal ION damage, and determined whether conversion of functional synapses into silent synapses after peripheral denervation results from changes in Pr. The paired-pulse ratio (PPR) was quite variable ranging from 40% (paired-pulse depression) to 175% (paired-pulse facilitation). The results from paired-pulse protocol were confirmed by MK-801 blocking rate experiments. The nonuniform PPRs did not show target cell specificity and developmental regulation. The distribution of PPRs fit nicely to Gaussian function with a peak at ∼ 100%. In addition, neonatal ION transections did not alter the distribution pattern of PPR in their central terminals, suggesting that the conversion from functional synapses into silent synapses in the peripherally denervated PrV is not caused by changes in the Pr.

N-methyl-D-aspartate receptor subunit composition in the rat trigeminal principal nucleus remains constant during postnatal development and following neonatal denervation

N-methyl-D-aspartate receptors (NMDARs) play a major role in various forms of developmental and adult synaptic plasticity (Lopez de Armentia M, Sah P (2003) J Neurosci 23:6876-6883). Activity-dependent shifts in NR2 subunits of the NMDARs have been proposed to be the molecular basis of critical period plasticity. Several supporting examples have been reported; however it is not clear whether the relationship between NMDAR subunit changes and neural plasticity are correlative or causal, nor whether such a relationship is universal across all sensory pathways with developmental plasticity. In the present study, we used voltage-clamp recording techniques to investigate whether subunit composition of NMDARs changes during development and after neonatal denervation in the principal sensory nucleus (PrV) of the trigeminal nerve. Relative AMPA receptor contribution to synaptic transmission increased linearly by the second postnatal week in the normal PrV. Denervation by peripheral nerve damage did not alter this process. We took the weighted decay time constant (τw) of NMDAR-mediated EPSCs as an index for NMDAR subunit composition. The τw measurement and Western blot analysis revealed that NMDARs contained both NR2A and NR2B subunits. The NR2A/NR2B ratio did not change during postnatal development or after neonatal denervation. Thus, critical period plasticity-related pattern formation in the PrV does not depend on changes in subunit composition of NMDARs. The mechanism underlying developmental synaptic plasticity in the PrV differs from those in higher trigeminal centers and other brain structures.

Conversion of functional synapses into silent synapses in the trigeminal brainstem after neonatal peripheral nerve transection

One of the major consequences of neonatal infraorbital nerve damage is irreversible morphological reorganization in the principal sensory nucleus (PrV) of the trigeminal nerve in the brainstem. We used the voltage-clamp technique to study synaptic transmission in the normal and the denervated PrV of neonatal rats in an in vitro brainstem preparation. Most of the synapses in the PrV are already functional at birth. Three days after peripheral deafferentation, functional synapses become silent, lacking AMPA receptor-mediated currents. Without sensory inputs from the whiskers, silent synapses persist through the second postnatal week, indicating that the maintenance of AMPA receptor function depends on sensory inputs. High-frequency (50 Hz) electrical stimulation of the afferent pathway, which mimics sensory input, restores synaptic function, whereas low-frequency (1 Hz) stimulation has no effect. Application of glycine, which promotes AMPA receptor exocytosis, also restores synaptic function. Therefore, normal synaptic function in the developing PrV requires incoming activity via sensory afferents and/or enhanced AMPA receptor exocytosis. Sensory deprivation most likely results in AMPA receptor endocytosis and/or lateral diffusion to the extrasynaptic membrane.

Emergence of intrinsic bursting in trigeminal sensory neurons parallels the acquisition of mastication in weanling rats

There is increasing evidence that a subpopulation of neurons in the dorsal principal sensory trigeminal nucleus are not simple sensory relays to the thalamus but may form the core of the central pattern generating circuits responsible for mastication. In this paper, we used whole cell patch recordings in brain stem slices of young rats to show that these neurons have intrinsic bursting abilities that persist in absence of extracellular Ca(2+). Application of different K(+) channel blockers affected duration and firing rate of bursts, but left bursting ability intact. Bursting was voltage dependent and was abolished by low concentrations of Na(+) channel blockers. The proportion of bursting neurons increased dramatically in the second postnatal week, in parallel with profound changes in several electrophysiological properties. This is the period in which masticatory movements appear and mature. Bursting was associated with the development of an afterdepolarization that depend on maturation of a persistent sodium conductance (I(NaP)). An interesting finding was that the occurrence of bursting and the magnitude of I(NaP) were both modulated by the extracellular concentration of Ca(2+). Lowering extracellular [Ca(2+)] increased both I(NaP) and probability of bursting. We suggest that these mechanisms underlie burst generation in mastication and that similar processes may be found in other motor pattern generators.

NMDA receptor-dependent regulation of axonal and dendritic branching

In the rodent trigeminal principal nucleus (PrV), trigeminal afferent terminals and postsynaptic cells form discrete modules ("barrelettes") that replicate the patterned array of whiskers and sinus hairs on the snout. Barrelette neurons of the PrV relay whisker-specific patterns to the contralateral thalamus and, subsequently, to the primary somatosensory barrel cortex. Genetic impairment of NMDA receptor (NMDAR) function blocks development of barrelettes in the PrV. Underlying cellular and functional defects are not known. Here, we examined morphological differentiation of whisker afferents, dendritic differentiation of barrelette cells, and their electrophysiological properties in mice with genetic perturbations of the essential subunit NR1 of NMDARs. We show that in NR1 gene knock-down (KD) and knock-out mice, whisker afferents begin their embryonic development normally but, over time, fail to segregate into patches, and instead they develop exuberant terminal arbors spanning most of the PrV. Postnatal NR1KD barrelette cells, with significantly reduced NMDA currents, retain their membrane and synaptic properties but develop longer dendrites with no orientation preference. These results indicate that NMDARs regulate growth of presynaptic terminal arbors and postsynaptic dendritic branching, thereby leading to consolidation of synapses and patterning of presynaptic and postsynaptic elements.

Activity-dependent regulation of the potassium channel subunits Kv1.1 and Kv3.1

Afferent activity, especially in young animals, can have profound influences on postsynaptic neuronal structure, function and metabolic processes. Most studies evaluating activity regulation of cellular components have examined the expression of ubiquitous cellular proteins as opposed to molecules that are specialized in the neurons of interest. Here we consider the regulation of two proteins (voltage-gated potassium channel subunits Kv1.1 and Kv3.1) that auditory brainstem neurons in birds and mammals express at uniquely high levels. Unilateral removal of the avian cochlea leads to rapid and dramatic reduction in the expression of both proteins in the nucleus magnocellularis (NM; a division of the avian cochlear nucleus) neurons as detected by immunocytochemistry. Uniform downregulation of Kv1.1 was reliable by 3 hours after cochlea removal, was sustained through 96 hours, and returned to control levels in the surviving neurons by 2 weeks. The activity-dependent changes in Kv3.1 appear to be bimodal and are more transient, being observed at 3 hours after cochlea removal and recovering to control levels within 24 hours. We also explored the functional properties of Kv1.1 in NM neurons deprived of auditory input for 24 hours by whole-cell recordings. Low-threshold potassium currents in deprived NM neurons were not significantly different from control neurons in their amplitude or sensitivity to dendrotoxin-I, a selective K+ channel antagonist. We conclude that the highly specialized abundant expression of Kv1.1 and 3.1 channel subunits is not permanently regulated by synaptic activity and that changes in overall protein levels do not predict membrane pools.

Development of intrinsic properties and excitability of layer 2/3 pyramidal neurons during a critical period for sensory maps in rat barrel cortex

The development of layer 2/3 sensory maps in rat barrel cortex (BC) is experience dependent with a critical period around postnatal days (PND) 10-14. The role of intrinsic response properties of neurons in this plasticity has not been investigated. Here we characterize the development of BC layer 2/3 intrinsic responses to identify possible sites of plasticity. Whole cell recordings were performed on pyramidal cells in acute BC slices from control and deprived rats, over ages spanning the critical period (PND 12, 14, and 17). Vibrissa trimming began at PND 9. Spiking behavior changed from phasic (more spike frequency adaptation) to regular (less adaptation) with age, such that the number of action potentials per stimulus increased. Changes in spiking properties were related to the strength of a slow Ca(2+)-dependent afterhyperpolarization. Maturation of the spiking properties of layer 2/3 pyramidal neurons coincided with the close of the critical period and was delayed by deprivation. Other measures of excitability, including I-f curves and passive membrane properties, were affected by development but unaffected by whisker deprivation.

L-type calcium channel-mediated plateau potentials in barrelette cells during structural plasticity

Development and maintenance of whisker-specific patterns along the rodent trigeminal pathway depends on an intact sensory periphery during the sensitive/critical period in development. Barrelette cells of the brain stem trigeminal nuclei are the first set of neurons to develop whisker-specific patterns. Those in the principal sensory nucleus (PrV) relay these patterns to the ventrobasal thalamus, and consequently, to the somatosensory cortex. Thus PrV barrelette cells are among the first group of central neurons susceptible to the effects of peripheral damage. Previously we showed that membrane properties of barrelette cells are distinct as early as postnatal day 1 (PND 1) and remain unchanged following peripheral denervation in newborn rat pups (Lo and Erzurumlu 2001). In the present study, we investigated the changes in synaptic transmission. In barrelette cells of normal PND 1 rats, weak stimulation of the trigeminal tract (TrV) that was subthreshold for inducing Na(+) spikes evoked an excitatory postsynaptic potential-inhibitory postsynaptic potential (EPSP-IPSP) sequence that was similar to the responses seen in older rats (Lo et al. 1999). Infraorbital nerve transection at birth did not alter excitatory and inhibitory synaptic connections of the barrelette cells. These observations suggested that local neuronal circuits are already established in PrV at birth and remain intact after deafferentation. Strong stimulation of the TrV induced a sustained depolarization (plateau potential) in denervated but not in normal barrelette neurons. The plateau potential was distinct from the EPSP-IPSP sequence by 1) a sustained (>80 ms) depolarization above -40 mV; 2) a slow decline slope (<0.1 mV/ms); 3) partially or totally inactivated Na(+) spikes on the plateau; and 4) a termination by a steep decay (>1 mV/ms) to a hyperpolarizing membrane level. The plateau potential was mediated by L-type Ca(2+) channels and triggered by a N-methyl-D-aspartate (NMDA) receptor-mediated EPSP. gamma-aminobutyric acid-A (GABA(A)) receptor-mediated IPSP dynamically regulated the latency and duration of the plateau potential. These results indicate that after neonatal peripheral damage, central trigeminal inputs cause a large and long-lasting Ca(2+) influx through L-type Ca(2+) channels in barrelette neurons. Increased Ca(2+) entry may play a key role in injury-induced structural remodeling, and/or transsynaptic cell death.

Synaptic plasticity in the trigeminal principal nucleus during the period of barrelette formation and consolidation

We examined whether the postsynaptic responses of cells in the principal sensory nucleus of the trigeminal nerve (PrV) are subject to long-term changes in synaptic strength, and if such changes were correlated the whisker-specific patterning during and just after the critical period for pattern formation. We used an in vitro brainstem preparation in which the trigeminal ganglion (TG) and PrV remained attached. By electrically activating TG afferents, we evoked large-amplitude extracellular field potentials. These responses were postsynaptic in origin and blocked by the glutamate antagonist, DNQX. At P1, a time when barrelettes are consolidating, high frequency stimulation of their afferents led to an immediate (<1 min) and long-lasting (> or =90 min) reduction (35%) in the amplitude of the evoked response. At P3-7, when the pattern of barrelettes have stabilized, the same form of tetanus led to an immediate and long-lasting increase (40%) in the amplitude of the response. Both forms of synaptic plasticity were mediated by the activation of L-type Ca(2+) channels. Application of the L-type channel blocker, nitrendipine, led to a complete blockade of any the tetanus induced changes. These associative processes may regulate the patterning and maintenance of whisker-specific patterns in the brainstem trigeminal nuclei.