Synaptically activated calcium responses in dendrites of hippocampal oriens-alveus interneurons

Synaptic Plasticity in Cortical Inhibitory Neurons: What Mechanisms May Help to Balance Synaptic Weight Changes?

Inhibitory neurons play a fundamental role in the normal operation of neuronal networks. Diverse types of inhibitory neurons serve vital functions in cortical networks, such as balancing excitation and taming excessive activity, organizing neuronal activity in spatial and temporal patterns, and shaping response selectivity. Serving these, and a multitude of other functions effectively requires fine-tuning of inhibition, mediated by synaptic plasticity. Plasticity of inhibitory systems can be mediated by changes at inhibitory synapses and/or by changes at excitatory synapses at inhibitory neurons. In this review, we consider that latter locus: plasticity at excitatory synapses to inhibitory neurons. Despite the fact that plasticity of excitatory synaptic transmission to interneurons has been studied in much less detail than in pyramids and other excitatory cells, an abundance of forms and mechanisms of plasticity have been observed in interneurons. Specific requirements and rules for induction, while exhibiting a broad diversity, could correlate with distinct sources of excitatory inputs and distinct types of inhibitory neurons. One common requirement for the induction of plasticity is the rise of intracellular calcium, which could be mediated by a variety of ligand-gated, voltage-dependent, and intrinsic mechanisms. The majority of the investigated forms of plasticity can be classified as Hebbian-type associative plasticity. Hebbian-type learning rules mediate adaptive changes of synaptic transmission. However, these rules also introduce intrinsic positive feedback on synaptic weight changes, making plastic synapses and learning networks prone to runaway dynamics. Because real inhibitory neurons do not express runaway dynamics, additional plasticity mechanisms that counteract imbalances introduced by Hebbian-type rules must exist. We argue that weight-dependent heterosynaptic plasticity has a number of characteristics that make it an ideal candidate mechanism to achieve homeostatic regulation of synaptic weight changes at excitatory synapses to inhibitory neurons.

Both decreased Akt expression and mTOR phosphorylation are related to decreased neuronal differentiation in the hippocampal alveus of aged mice

BACKGROUND

Aging is an inevitable process which results in many changes. These changes are closely related to the hippocampus which is in charge of long-term learning and episodic memory.

AIM

This study was to investigate age-related changes of the cell proliferation, neuroblast differentiation and Akt/mTOR signaling in the hippocampal alveus of aged mice.

METHODS

In the present study, we compared the differences of neurogenesis in the hippocampal alveus between adult (postnatal month 6) and aged (postnatal month 24) mice using immunohistochemistry and western blot analysis.

RESULTS

The cell proliferation, neuroblast differentiation, and the increased astrocyte activation in the hippocampal alveus of mice were decreased in an age-dependent manner. In addition, during normal aging, the protein level of AKT, mTOR and the phosphorylation of mTOR were all decreased. However, the protein level of AKT was increased.

DISCUSSION

These results indicate the neurogenesis in the immature neurons in the hippocampal alveus of aged mice was closely related to the normal aging process. In addition, during normal aging, the increased AKT phosphorylation and decreased mTOR phosphorylation in the hippocampus may play a role in aging development.

CONCLUSION

The result indicates that increased activation of astrocyte, increased phosphorylation of AKT and decreased phosphorylation of mTOR may be involved in the decreased cell proliferation and neuroblast differentiation in the alveus of hippocampus of aged mice.

Estimating intracellular Ca2+ concentrations and buffering in a dendritic inhibitory hippocampal interneuron

Calcium is known to regulate several phenomena like neuronal excitability and plasticity. Interestingly, the spatiotemporal profile of dendritic calcium depends on several processes, specific to each neuronal type. In this study, we investigated Ca(2+) buffering and action potential (AP)-evoked Ca(2+) signaling in the dendrites of anatomically identified oriens lacunosum-moleculare (O-LM) cells, a major type of dendrite-targeting interneurons in the hippocampal CA1 region, using a combination of whole-cell patch-clamp recording and fast Ca(2+) imaging in acute rat brain slices. Cells were loaded with fluorescent Ca(2+) indicators fura-2 or Oregon Green BAPTA-1 (OGB-1) via patch-clamping electrode, and the effect of fura-2 on AP-evoked dendritic Ca(2+) transients was determined by ratiometric Ca(2+) imaging. To estimate intracellular Ca(2+) concentrations ([Ca(2+)](i)) and endogenous Ca(2+)-binding ratio (kappa(s)) in the proximal dendrite, fluorescence signals were converted into [Ca(2+)](i) using the ratioing method and were analyzed on the basis of the "single compartment model." Resting [Ca(2+)](i) was 22+/-5 nM and the build-up of [Ca(2+)](i) during a single AP was up to 656+/-226 nM. Analysis of Ca(2+) transients revealed that O-LM cells have a relatively low endogenous Ca(2+)-binding ratio (kappa(s)): the kappa(s) was 20+/-8 estimated during fura-2 loading and 27 estimated under steady-state fura-2 concentrations, respectively. To further examine the spatial profile of dendritic Ca(2+) transients, we measured somatic AP-evoked Ca(2+) transients beyond proximal dendrites using OGB-1. Dendritic Ca(2+) transients evoked by single APs or AP trains are not limited to regions close to the soma. The amplitude and decay of [Ca(2+)](i) associated with backpropagating APs are relatively independent of the distance from the soma. In sum, O-LM cells exhibit low endogenous Ca(2+)-binding ratios and relatively distance-independent Ca(2+) dynamics in the dendrites. These special features of Ca(2+) signaling in O-LM cells may have important functional implications for both normal and pathological conditions.

Differential regulation of metabotropic glutamate receptor- and AMPA receptor-mediated dendritic Ca2+ signals by presynaptic and postsynaptic activity in hippocampal interneurons

Calcium plays a crucial role as a ubiquitous second messenger and has a key influence in many forms of synaptic plasticity in neurons. The spatiotemporal properties of dendritic Ca2+ signals in hippocampal interneurons are relatively unexplored. Here we use two-photon calcium imaging and whole-cell recordings to study properties of dendritic Ca2+ signals mediated by different glutamate receptors and their regulation by synaptic activity in oriens/alveus (O/A) interneurons of rat hippocampus. We demonstrate that O/A interneurons express Ca2+-permeable AMPA receptors (CP-AMPARs) providing fast Ca2+ signals. O/A cells can also coexpress CP-AMPARs, Ca2+-impermeable AMPARs (CI-AMPARs), and group I/II metabotropic glutamate receptors (mGluRs) (including mGluR1a), in the same cell. CI-AMPARs are often associated with mGluRs, resulting in longer-lasting Ca2+ signals than CP-AMPAR-mediated responses. Finally, CP-AMPAR- and mGluR-mediated Ca2+ signals demonstrate distinct voltage dependence and are differentially regulated by presynaptic and postsynaptic activity: weak synaptic stimulation produces Ca2+ signals mediated by CP-AMPARs, whereas stronger stimulation, or weak stimulation coupled with postsynaptic depolarization, recruits Ca2+ signals mediated by mGluRs. Our results suggest that differential activation of specific glutamate receptor-mediated Ca2+ signals within spatially restricted dendritic microdomains may serve distinct signaling functions and endow oriens/alveus interneurons with multiple forms of Ca2+-mediated synaptic plasticity. Specific activation of mGluR-mediated Ca2+ signals by coincident presynaptic and postsynaptic activity fulfills the conditions for Hebbian pairing and likely underlies their important role in long-term potentiation induction at O/A interneuron synapses.

Space matters: local and global dendritic Ca2+ compartmentalization in cortical interneurons

Dendrites of pyramidal neurons are complex, electrically active structures that can produce local and global Ca(2+) compartments. Recent studies indicate that dendrites of cortical GABAergic interneurons are also highly specialized, and that different subtypes vary in their morphology, in their intrinsic and synaptic conductances and in the Ca(2+) signals they generate. Because interneurons play a major role in oscillations, understanding their dendrites could offer key insights into rhythmogenesis. Different interneuron subtypes have different synaptic integration properties and generate differentially timed inhibition at distinct sites of the pyramidal neuraxis. In addition, interneuron dendrites generate diverse Ca(2+) signals that reflect this circuit function and probably also implement subclass-specific plasticity and homeostasis.

Astrocyte glutamate transporters regulate metabotropic glutamate receptor-mediated excitation of hippocampal interneurons

Clearance of extracellular glutamate is essential for limiting the activity of metabotropic glutamate receptors (mGluRs) at excitatory synapses; however, the relative contribution of transporters found in neuronal and glial membranes to this uptake is poorly understood. Hippocampal interneurons located at the oriens-alveus border express mGluR1alpha, a metabotropic glutamate receptor that regulates excitability and synaptic plasticity. To determine which glutamate transporters are essential for removing glutamate at these excitatory synapses, we recorded mGluR1-mediated EPSCs from oriens-lacunosum moleculare (O-LM) interneurons in acute hippocampal slices. Stimulation in stratum oriens reliably elicited a slow mGluR1-mediated current in O-LM interneurons if they were briefly depolarized to allow Ca2+ entry before stimulation. Selective inhibition of GLT-1 [for glutamate transporter; EAAT2 (for excitatory amino acid transporter)] with dihydrokainate increased the amplitude of these responses approximately threefold, indicating that these transporters compete with mGluRs for synaptically released glutamate. However, inhibition of all glutamate transporters with TBOA (DL-threo-b-benzyloxyaspartic acid) increased mGluR1 EPSCs >15-fold, indicating that additional transporters also shape activation of these receptors. To identify these transporters, we examined mGluR1 EPSCs in mice lacking GLAST (for glutamate-aspartate transporter; EAAT1) or EAAC1 (for excitatory amino acid carrier; EAAT3). A comparison of responses recorded from wild-type and transporter knock-out mice revealed that the astroglial glutamate transporters GLT-1 and GLAST, but not the neuronal transporter EAAC1, restrict activation of mGluRs in O-LM interneurons. Transporter-dependent potentiation of mGluR1 EPSCs led to a dramatic increase in interneuron firing and enhanced inhibition of CA1 pyramidal neurons, suggesting that acute or prolonged disruption of transporter activity could lead to changes in network activity as a result of enhanced interneuron excitability.

Group I metabotropic glutamate receptor actions in oriens/alveus interneurons of rat hippocampal CA1 region

Group I metabotropic glutamate receptors (mGluRs) are important for hippocampal interneuron function. We used whole-cell recording and confocal imaging to characterize group I mGluR actions in CA1 oriens/alveus interneurons in slices. In tetrodotoxin and ionotropic glutamate receptor antagonists, the group I mGluR specific agonist DHPG increased intradendritic Ca(2+) levels and depolarized interneurons, whereas the group II mGluR specific agonist DCG-IV and the group III mGluR specific agonist L-AP4 did not. DHPG-induced depolarizing and Ca(2+) responses were antagonized by the group I mGluR antagonist 4CPG, but only Ca(2+) responses were significantly inhibited by the mGluR1 antagonist CPCCOEt. DHPG-induced depolarizing responses were not blocked by the inositol-1,4,5-trisphosphate (IP(3)) receptor inhibitor heparin, the protein kinase C (PKC) antagonists GF-109203X, or the inhibitor of phospholipase C (PLC) U73122. Thus, these responses to DHPG may not be transduced by the PLC-->IP(3)/diacylglycerol (DAG) pathway classically linked to group I mGluRs. DHPG-induced depolarizations were not blocked by intracellular GDP beta S or bath-application of N-ethylmaleimide (NEM), suggesting the involvement of a G protein-independent pathway. Our findings indicate that group I mGluRs induce a depolarization of oriens/alveus interneurons via a G protein-independent mechanism different from their classic signalling pathway. Since depolarizations are associated with intracellular Ca(2+) rises, these actions may be important for their synaptic plasticity and vulnerability to excitotoxicity.

Global dendritic calcium spikes in mouse layer 5 low threshold spiking interneurones: implications for control of pyramidal cell bursting

Interneuronal networks in neocortex underlie feedforward and feedback inhibition and control the temporal organization of pyramidal cell activity. We previously found that lower layer neocortical interneurones can reach action potential threshold in response to the stimulation of a single presynaptic cell. To better understand this phenomenon and the circuit roles of lower layer neocortical interneurones, we combined two-photon calcium imaging with whole cell recordings and anatomical reconstructions of low threshold spiking (LTS) interneurones from mouse neocortex. In both visual and somatosensory cortex, LTS interneurones are somatostatin-positive, concentrated in layer 5 and possess dense axonal innervation to layer 1. Due to the LTS properties, these neurones operate in burst and tonic modes. In burst mode, dendritic T-type calcium channels boosted small synaptic inputs and triggered low threshold calcium spikes, while in tonic mode, sodium-based APs evoked smaller calcium influxes. In both modes, the entire dendritic tree of LTS interneurones behaved as a 'global' single spiking unit. This, together with the fact that synaptic inputs to layer 5 LTS cells are facilitating, and that their axons target the dendritic region of the pyramidal neurones where bursts are generated, make these neurones ideally suited to detect and control burst generation of individual lower layer pyramidal neurones.

Synapse-specific mGluR1-dependent long-term potentiation in interneurones regulates mouse hippocampal inhibition

Hippocampal CA1 inhibitory interneurones control the excitability and synchronization of pyramidal cells, and participate in hippocampal synaptic plasticity. Pairing theta-burst stimulation (TBS) with postsynaptic depolarization, we induced long-term potentiation (LTP) of putative single-fibre excitatory postsynaptic currents (EPSCs) in stratum oriens/alveus (O/A) interneurones of mouse hippocampal slices. LTP induction was absent in metabotropic glutamate receptor 1 (mGluR1) knockout mice, was correlated with the postsynaptic presence of mGluR1a, and required a postsynaptic Ca2+ rise. Changes in paired-pulse facilitation and coefficient of variation indicated that LTP expression involved presynaptic mechanisms. LTP was synapse specific, occurring selectively at synapses modulated by presynaptic group II, but not group III, mGluRs. Furthermore, the TBS protocol applied in O/A induced a long-term increase of polysynaptic inhibitory responses in CA1 pyramidal cells, that was absent in mGluR1 knockout mice. These results uncover the mechanisms of a novel form of interneurone synaptic plasticity that can adaptively regulate inhibition of hippocampal pyramidal cells.