Prior short-term synaptic disinhibition facilitates long-term potentiation and suppresses long-term depression at CA1 hippocampal synapses

Synaptic configuration and reconfiguration in the neocortex are spatiotemporally selective

Brain computation relies on the neural networks. Neurons extend the neurites such as dendrites and axons, and the contacts of these neurites that form chemical synapses are the biological basis of signal transmissions in the central nervous system. Individual neuronal outputs can influence the other neurons within the range of the axonal spread, while the activities of single neurons can be affected by the afferents in their somatodendritic fields. The morphological profile, therefore, binds the functional role each neuron can play. In addition, synaptic connectivity among neurons displays preference based on the characteristics of presynaptic and postsynaptic neurons. Here, the author reviews the "spatial" and "temporal" connection selectivity in the neocortex. The histological description of the neocortical circuitry depends primarily on the classification of cell types, and the development of gene engineering techniques allows the cell type-specific visualization of dendrites and axons as well as somata. Using genetic labeling of particular cell populations combined with immunohistochemistry and imaging at a subcellular spatial resolution, we revealed the "spatial selectivity" of cortical wirings in which synapses are non-uniformly distributed on the subcellular somatodendritic domains in a presynaptic cell type-specific manner. In addition, cortical synaptic dynamics in learning exhibit presynaptic cell type-dependent "temporal selectivity": corticocortical synapses appear only transiently during the learning phase, while learning-induced new thalamocortical synapses persist, indicating that distinct circuits may supervise learning-specific ephemeral synapse and memory-specific immortal synapse formation. The selectivity of spatial configuration and temporal reconfiguration in the neural circuitry may govern diverse functions in the neocortex.

GABAergic inhibition gates excitatory LTP in perirhinal cortex

The perirhinal cortex (PRh) is a key region downstream of auditory cortex (ACx) that processes familiarity linked mnemonic signaling. In gerbils, ACx-driven EPSPs recorded in PRh neurons are largely shunted by GABAergic inhibition (Kotak et al., 2015, Frontiers in Neural Circuits, 9). To determine whether inhibitory shunting prevents the induction of excitatory long-term potentiation (e-LTP), we stimulated ACx-recipient PRh in a brain slice preparation using theta burst stimulation (TBS). Under control conditions, without GABA blockers, the majority of PRh neurons exhibited long-term depression. A very low concentration of bicuculline increased EPSP amplitude, but under this condition TBS did not significantly increase e-LTP induction. Since PRh synaptic inhibition included a GABA_B receptor-mediated component, we added a GABA_B receptor antagonist. When both GABA_A and GABA_B receptors were blocked, TBS reliably induced e-LTP in a majority of PRh neurons. We conclude that GABAergic transmission is a vital mechanism regulating e-LTP induction in the PRh, and may be associated with auditory learning.

Essential roles of AMPA receptor GluA1 phosphorylation and presynaptic HCN channels in fast-acting antidepressant responses of ketamine

Although the molecular mechanism is not clear, the clinically tested drug ketamine has rapid antidepressant action that does not require the multiple weeks of treatment needed for other antidepressant drugs to have an effect. We showed that ketamine potentiated Schaffer collateral-CA1 cell excitatory synaptic transmission in hippocampal slice preparations from rodents and enhanced the phosphorylation of the GluA1 subunit on Ser⁸⁴⁵ of the AMPA-type glutamate receptor in the hippocampal area CA1. These effects persisted when γ-aminobutyric acid (GABA) receptors were pharmacologically blocked. Ketamine reduced behavioral despair in wild-type mice but had no effect in GluA1 S845A knock-in mutant mice. Presynaptic (CA3 pyramidal cell), but not postsynaptic (CA1 pyramidal cell), deletion of N-methyl-d-aspartate (NMDA)-type glutamate receptors eliminated the ketamine-induced enhancement of excitatory synaptic transmission in hippocampal slices and the antidepressant actions of ketamine in mice. The synaptic and behavioral actions of ketamine were completely occluded by inhibition or deletion of the hyperpolarization-activated cyclic nucleotide-gated channel 1 (HCN1). Our results implicate presynaptic NMDA receptor inhibition followed by reduced activity of presynaptic HCN1 channels, which would result in an increase in glutamate release and postsynaptic glutamate receptor activity, as a mechanism of ketamine action. These data provide a mechanism for changes in synaptic activity that could explain the fast-acting antidepressant effects of this drug.

Releasing the Cortical Brake by Non-Invasive Electromagnetic Stimulation? rTMS Induces LTD of GABAergic Neurotransmission

Repetitive Transcranial Magnetic Stimulation (rTMS) is a non-invasive brain stimulation technique which modulates cortical excitability beyond the stimulation period. However, despite its clinical use rTMS-based therapies which prevent or reduce disabilities in a functionally significant and sustained manner are scarce. It remains unclear how rTMS-mediated changes in cortical excitability, which are not task- or input-specific, exert beneficial effects in some healthy subjects and patients. While experimental evidence exists that repetitive magnetic stimulation (rMS) is linked to the induction of long-term potentiation (LTP) of excitatory neurotransmission, less attention has been dedicated to rTMS-induced structural, functional and molecular adaptations at inhibitory synapses. In this review article we provide a concise overview on basic neuroscience research, which reveals an important role of local disinhibitory networks in promoting associative learning and memory. These studies suggest that a reduction in inhibitory neurotransmission facilitates the expression of associative plasticity in cortical networks under physiological conditions. Hence, it is interesting to speculate that rTMS may act by decreasing GABAergic neurotransmission onto cortical principal neurons. Indeed, evidence has been provided that rTMS is capable of modulating inhibitory networks. Consistent with this suggestion recent basic science work discloses that a 10 Hz rTMS protocol reduces GABAergic synaptic strength on principal neurons. These findings support a model in which rTMS-induced long-term depression (LTD) of GABAergic synaptic strength mediates changes in excitation/inhibition-balance of cortical networks, which may in turn facilitate (or restore) the ability of stimulated networks to express input- and task-specific associative synaptic plasticity.

Ginkgolic Acid Protects against Aβ-Induced Synaptic Dysfunction in the Hippocampus

Ginkgo leaf is the most used form of supplement for cognitive ailments. The standardized extract formulation EGb 761 is a dietary supplement with proven benefit in several neurological and psychiatric conditions including memory decline in Alzheimer's disease, schizophrenia and dementia. Ginkgolic acid (GA) is a component of this extract which shows pleiotropic effects including antitumoral and anti-HIV action; however, its effect on memory is still unknown. Here, we carried out an electrophysiological analysis to investigate the effects of GA on long term potentiation and synaptic transmission at CA1 hippocampal synapses. We also evaluated the potential rescuing effect of GA on the synaptic dysfunction following in vitro application of Aβ. Data obtained indicate that GA exerts neuroprotective effects against Aβ-induced impairment of neurotransmitter release and synaptic plasticity.

Differential regulation of GluA1 expression by ketamine and memantine

Evidence from preclinical and clinical studies shows that ketamine, a noncompetitive NMDA receptor antagonist, exerts rapid and sustained antidepressant responses. However, ketamine's psychotomimetic side effects and abuse liability limit the clinical use of the compound. Interestingly, memantine, another NMDA receptor channel blocker, processes no defined antidepressant property but is much safer and clinical tolerated. Understanding why ketamine but not memantine exhibits rapid antidepressant responses is important to elucidate the cellular signaling underlying the fast antidepressant actions of ketamine and to design a new safer generation of fast-acting antidepressants. Here we show that ketamine but memantine caused a rapid and sustained antidepressant-like responses in forced swim test (FST). Both drugs enhanced GluA1 S845 phosphorylation and potentiated Schaffer collateral-CA1 synaptic transmission. However, ketamine but not memantine elevated the expression of GluA1. Incubating acutely prepared hippocampal slices with ketamine but not memantine enhanced mTOR phosphorylation in a time course parallel to the time course of GluA1 elevation. Our results suggest that distinct properties in regulation of mTOR phosphorylation and synaptic protein expression may underlie the differential effectiveness of ketamine and memantine in their antidepressant responses.

Cortical Synaptic Inhibition Declines during Auditory Learning

Auditory learning is associated with an enhanced representation of acoustic cues in primary auditory cortex, and modulation of inhibitory strength is causally involved in learning. If this inhibitory plasticity is associated with task learning and improvement, its expression should emerge and persist until task proficiency is achieved. We tested this idea by measuring changes to cortical inhibitory synaptic transmission as adult gerbils progressed through the process of associative learning and perceptual improvement. Using either of two procedures, aversive or appetitive conditioning, animals were trained to detect amplitude-modulated noise and then tested daily. Following each training session, a thalamocortical brain slice was generated, and inhibitory synaptic properties were recorded from layer 2/3 pyramidal neurons. Initial associative learning was accompanied by a profound reduction in the amplitude of spontaneous IPSCs (sIPSCs). However, sIPSC amplitude returned to control levels when animals reached asymptotic behavioral performance. In contrast, paired-pulse ratios decreased in trained animals as well as in control animals that experienced unpaired conditioned and unconditioned stimuli. This latter observation suggests that inhibitory release properties are modified during behavioral conditioning, even when an association between the sound and reinforcement cannot occur. These results suggest that associative learning is accompanied by a reduction of postsynaptic inhibitory strength that persists for several days during learning and perceptual improvement.

GABAergic Inhibition Regulated Pain Sensitization through STEP61 Signaling in Spinal Dorsal Horn of Mice

Background:

The reduction of γ-aminobutyric acid (GABA) type A receptor–mediated inhibition has long been implicated in spinal sensitization of nociceptive responses. However, it is largely unknown which signaling cascades in spinal dorsal horn neurons are initiated by the reduced inhibition to trigger pain hypersensitivity.

Methods:

GABAergic inhibition was manipulated by intrathecal application of GABA type A receptor antagonist bicuculline in intact mice or by GABA type A receptor agonist muscimol in complete Freund’s adjuvant–injected mice. Immunoblotting, coimmunoprecipitation, immunohistochemistry, and behavioral tests were used to explore the signaling pathways downstream of the altered GABAergic tone.

Results:

The study data revealed that the 61-kD isoform of striatal-enriched protein phosphatase (STEP61) was a key molecule that relayed the signals from GABAergic neurotransmission. The authors found that STEP61 was highly expressed in dorsal horn neurons. Under physiological conditions, STEP61 tonically interacted with and negatively controlled the activities of extracellular signal–regulated kinase and Src-family protein tyrosine kinases member Fyn, two critical kinases involved in spinal sensitization. Once GABAergic inhibition was impaired, STEP61 interaction with its substrates was substantially disturbed, allowing for activation of extracellular signal–regulated kinase and Fyn (n = 4 to 6). The hyperactivities of extracellular signal–regulated kinase and Fyn, along with STEP61 dysregulation, caused the tyrosine phosphorylation and synaptic accumulation of GluN2B subunit-containing N-methyl-d-aspartate subtype of glutamate receptors (n = 6), leading to GluN2B receptor-dependent pain hypersensitivity. Overexpression of wild-type STEP61 to resume its enzymatic activity significantly blocked the mechanical allodynia evoked by bicuculline and more importantly, alleviated chronic inflammatory pain (n = 6 in each group).

Conclusion:

These data identified STEP61 as a key intermediary for GABAergic inhibition to regulate pain sensitization.

Two sides to long-term potentiation: a view towards reconciliation

Almost since the discovery of long-term potentiation (LTP) in the hippocampus, its locus of expression has been debated. Throughout the years, convincing evidence has accumulated to suggest that LTP can be supported either presynaptically, by an increase in transmitter release, or postsynaptically, by an increase in α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor number. However, whereas postsynaptic enhancement appears to be consistently obtained across studies following LTP induction, presynaptic enhancement is not as reliably observed. Such discrepancies, along with the failure to convincingly identify a retrograde messenger required for presynaptic change, have led to the general view that LTP is mainly supported postsynaptically, and certainly, research within the field for the past decade has been heavily focused on the postsynaptic locus. Here, we argue that LTP can be expressed at either synaptic locus, but that pre- and postsynaptic forms of LTP are dissociable phenomena mediated by distinct mechanistic processes, which are sensitive to different patterns of neuronal activity. This view of LTP helps to reconcile discrepancies across the literature and may put to rest a decades-long debate.

Adrenergic receptor-mediated disinhibition of mitral cells triggers long-term enhancement of synchronized oscillations in the olfactory bulb

Norepinephrine (NE) is widely implicated in various forms of associative olfactory learning in rodents, including early learning preference in neonates. Here we used patch-clamp recordings in rat olfactory bulb slices to assess cellular actions of NE, examining both acute, short-term effects of NE as well as the relationship between these acute effects and long-term cellular changes that could underlie learning. Our focus for long-term effects was on synchronized gamma frequency (30-70 Hz) oscillations, shown in prior studies to be enhanced for up to an hour after brief exposure of a bulb slice to NE and neuronal stimulation. In terms of acute effects, we found that a dominant action of NE was to reduce inhibitory GABAergic transmission from granule cells (GCs) to output mitral cells (MCs). This disinhibition was also induced by clonidine, an agonist specific for alpha(2) adrenergic receptors (ARs). Acute NE-induced disinhibition of MCs appeared to be linked to long-term enhancement of gamma oscillations, based, first, on the fact that clonidine, but not agonists specific for other AR subtypes, mimicked NE's long-term actions. In addition, the alpha(2) AR-specific antagonist yohimbine blocked the long-term enhancement of the oscillations due to NE. Last, brief exposure of the slice to the GABA(A) receptor antagonist gabazine, to block inhibitory synapses directly, also induced the long-term changes. Acute disinhibition is a plausible permissive effect of NE leading to olfactory learning, because, when combined with exposure to a specific odor, it should lead to neuron-specific increases in intracellular calcium of the type generally associated with long-term synaptic modifications.

Protein kinase Calpha mediates a novel form of plasticity in the accessory olfactory bulb

Modification of synapses in the accessory olfactory bulb (AOB) is believed to underlie pheromonal memory that enables mate recognition in mice. The memory, which is acquired with single-trial learning, forms only with coincident noradrenergic and glutamatergic inputs to the AOB. The mechanisms by which glutamate and norepinephrine (NE) alter the AOB synapses are not well understood. Here we present results that not only reconcile the earlier, seemingly contradictory, observations on the role of glutamate and NE in changing the AOB synapses, but also reveal novel mechanisms of plasticity. Our studies suggest that initially, glutamate acting at Group II metabotropic receptors and NE acting at alpha(2)-adrenergic receptors inhibit N-type and R-type Ca(2+) channels in mitral cells via a G-protein. The N-type and R-type Ca(2+) channel inhibition is reversed by activation of alpha(1)-adrenergic receptors and protein kinase Calpha (PKCalpha). Based on these results, we propose a hypothetical model for a new kind of synaptic plasticity in the AOB that accounts for the previous behavioral data on pheromonal memory. According to this model, initial inhibition of the Ca(2+) channels suppresses the GABAergic inhibitory feedback to mitral cells, causing disinhibition and Ca(2+) influx. NE also activates phospholipase C (PLC) through alpha(1)-adrenergic receptors generating inositol 1,4,5-trisphosphate and diacylglycerol (DAG). Calcium and DAG together activate PKCalpha which switches the disinhibition to increased inhibition of mitral cells. Thus, PKCalpha is likely to be a coincidence detector integrating glutamate and NE input in the AOB and bridging the short-term signaling to long-term structural changes resulting in enhanced inhibition of mitral cells that is thought to underlie memory formation.

The metabotropic glutamate receptor mGluR3 is critically required for hippocampal long-term depression and modulates long-term potentiation in the dentate gyrus of freely moving rats

Group II metabotropic glutamate receptors (mGluRs) play an important role in the regulation of hippocampal synaptic plasticity in vivo: long-term potentiation (LTP) is inhibited and long-term depression (LTD) is enhanced by activation of these receptors. The contribution, in vivo, of the individual group II mGluR subtypes has not been characterized. We analysed the involvement of the subtype mGluR3 in LTD and LTP. Rats were implanted with electrodes to enable chronic measurement of evoked potentials from medial perforant path-dentate gyrus synapses. Neither the selective mGluR3 agonist, N-acetylaspartylglutamate (NAAG), nor the antagonist beta-NAAG, given intracerebrally, affected basal synaptic transmission. beta-NAAG significantly inhibited LTD expression. NAAG exhibited transient inhibitory effects on the intermediate phase of LTD. Whereas NAAG altered paired-pulse responses, beta-NAAG had no effect, suggesting that antagonism of mGluR3 prevents LTD via a postsynaptic mechanism, whereas agonist activation of mGluR3 modulates LTD at a presynaptic locus. NAAG impaired the expression of LTP, whereas beta-NAAG had no effect. NAAG effects on LTP were blocked by EGLU, a selective group II mGluR antagonist. Our data suggest an essential role for mGluR3 in LTD, and a modulatory role for mGluR3 in LTP, with effects being mediated by distinct pre- and post-synaptic loci.

Age-related effects of Ginkgo biloba extract on synaptic plasticity and excitability

EGb 761 is a standardized extract from the Ginkgo biloba leaf and is purported to improve age-related memory impairment. The acute and chronic effect of EGb 761 on synaptic transmission and plasticity in hippocampal slices from young adult (8-12 weeks) and aged (18-24 months) C57Bl/6 mice was tested because hippocampal plasticity is believed to be a key component of memory. Acutely applied EGb 761 significantly increased neuronal excitability in slices from aged mice by reducing the population spike threshold and increased the early phase of long-term potentiation, though there was no effect in slices from young adults. In chronically treated mice fed for 30 days with an EGb 761-supplemented diet, EGb 761 significantly increased the population spike threshold and long-term potentiation in slices from aged animals, but had no effect on slices from young adults. The rapid effects of EGb 761 on plasticity indicate a direct interaction with the glutamatergic system and raise interesting implications with respect to a mechanism explaining its effect on cognitive enhancement in human subjects experiencing dementia.

Bidirectional shift in the cornu ammonis 3 pyramidal dendritic organization following brief stress

The negative impact of chronic stress at the structure of apical dendrite branches of cornu ammonis 3 (CA3) pyramidal neurons is well established. However, there is no information available on the CA3 dendritic organization related to short-lasting stress, which suffices to produce long-term habituation or sensitization of anxiety behaviors and neuroendocrine responses. Here, we tested the effects evoked by brief stress on the arrangements of CA3 pyramidal neuron dendrites, and the activity-dependent properties of the commissural-associational (C/A) excitatory postsynaptic potentials (EPSPs). Adult male rats were socially defeated followed by 3 weeks without further treatment or as comparison exposed to a regimen of a social defeat every second day for the same time period. We assessed CA3 pyramidal neurons with somatic whole-cell recording and neurobiotin application in acute hippocampal slices. The results from morphometric analysis of post hoc reconstructions demonstrated that CA3 dendrites from repeatedly stressed rats were reduced in surface area and length selectively at the apical cone (70% of control, approximately 280 microm from the soma). Brief stress, however, produced a similar decrease in apical dendritic length (77% of control, approximately 400 microm from the soma), accompanied by an increased length (167% of control) and branch complexity at the basal cone. The structural changes of the dendrites significantly influenced signal propagation by shortening the onset latency of EPSPs and increasing input resistance (r=0.45, P<0.01), of which the first was significantly changed in repeatedly stressed animals. Both brief and repeated stress long-lastingly impaired long-term potentiation of C/A synapses to a similar degree (P<0.05). These data indicate that the geometric plasticity of CA3 dendrites is dissociated from repetition of aversive experiences. A double social conflict suffices to drive a dynamic reorganization, by site-selective elimination and de novo growth of dendrite branches over the course of weeks after the actual experience.

Effects of corticotropin-releasing factor on plasticity of optically recorded neuronal activity in the substantia gelatinosa of rat spinal cord slices

We examined the effects of corticotropin-releasing factor (CRF) on plasticity of optically recorded neuronal activity in the substantia gelatinosa (lamina II) of 12-18-day-old rat spinal cord slices stained with a voltage-sensitive dye. Single-pulse test stimulation to the dorsal root that activated A and C fibres evoked prolonged (>100 ms) light-absorption change in the lamina II. This response represents the gross membrane potential change of all elements along the slice depth. After conditioning high-frequency stimulation of A-fibre-activating strength, test stimulus elicited less neuronal activity [-27+/-1% (7), (average+/-SE (n)), P<0.01 (*) at 45-60 min after conditioning]. When CRF (1 microM, 10 min) was applied during conditioning, the neuronal activity was facilitated rather than suppressed [+20+/-3% (5), P<0.05]. CRF alone exhibited insignificant effect [-5+/-1% (4), P=0.2]. In the presence of the inhibitory amino acid antagonists bicuculline (1 microM) and strychnine (0.3 microM) in the perfusate, in contrast, the conditioning facilitated it [+27+/-1% (12)*], and CRF treatment during conditioning inhibited the facilitation dose-dependently [0.1 microM: +18+/-2% (5)*, 1 microM: +13+/-1% (7)*]. Although interneuronal actions might contribute, these results suggest that CRF may have dual effects on excitatory synaptic transmission within the lamina II depending upon cellular conditions: a conversion from the induction of long-term depression to long-term potentiation (LTP), and inhibition of LTP induction. Since the LTP is thought to be responsible at least in part for the persistent pain, CRF could regulate the induction.

Requirement of a critical period of GABAergic receptor blockade for induction of a cAMP-mediated long-term depression at CA3-CA1 synapses

Previous reports show that bath application of the adenosine 3' : 5'-cyclic monophosphate (cAMP) analog, Sp-cAMPS, induces a protein kinase A (PKA)-dependent and protein synthesis-dependent long-term potentiation (LTP) at hippocampal CA3-CA1 synapses. Recently, we reported a novel form of long-term depression (LTD) induced by concurrent application of Sp-cAMPS and picrotoxin, the gamma-aminobutyric acid type A (GABA(A)) receptor antagonist. In the present study, we further investigated the mechanisms underlying such cAMP-mediated LTD. Synaptically connected CA3 and CA1 cells of hippocampal slice cultures were impaled by sharp electrodes. Excitatory postsynaptic potentials recorded from a CA1 pyramidal cell were evoked by single action potentials in a CA3 cell. Picrotoxin was applied to slices at various time points after Sp-cAMPS was perfused. We found that Sp-cAMPS-induced potentiation could be converted to depression when picrotoxin was applied within 30 min after perfusion of Sp-cAMPS. Picrotoxin applied 1 h after perfusion of Sp-cAMPS had no effect on Sp-cAMPS-induced synaptic potentiation. Once LTP was induced by Sp-cAMPS and expressed for 1 h, the subsequent application of Sp-cAMPS and picrotoxin produced no new changes in synaptic strength. Also, once LTD was induced and expressed for 1 h, subsequent Sp-cAMPS produced no new changes in synaptic strength. These findings suggest that a synapse is committed irreversibly to cAMP-mediated LTP or LTD during a critical period and that later signals cannot interconvert these two fates.

Transient removal of extracellular Mg(2+) elicits persistent suppression of LTP at hippocampal CA1 synapses via PKC activation

Previous work has shown that seizure-like activity can disrupt the induction of long-term potentiation (LTP). However, how seizure-like event disrupts the LTP induction remains unknown. To understand the cellular and molecular mechanisms underlying this process better, a set of studies was implemented in area CA1 of rat hippocampal slices using extracellular recording methods. We showed here that prior transient seizure-like activity generated by perfused slices with Mg(2+)-free artificial cerebrospinal fluid (ACSF) exhibited a persistent suppression of LTP induction. This effect lasted between 2 and 3 h after normal ACSF replacement and was specifically inhibited by N-methyl-D-aspartate (NMDA) receptor antagonist D-2-amino-5-phosphovaleric acid (D-APV) and L-type voltage-operated Ca(2+) channel (VOCC) blocker nimodipine, but not by non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). In addition, this suppressive effect was specifically blocked by the selective protein kinase C (PKC) inhibitor NPC-15437. However, neither Ca(2+)/calmodulin-dependent protein kinase II inhibitor KN-62 nor cAMP-dependent protein kinase inhibitor Rp-adenosine 3', 5'-cyclic monophosphothioate (Rp-cAMPS) affected this suppressive effect. This persistent suppression of LTP was not secondary to the long-lasting changes in NMDA receptor activation, because the isolated NMDA receptor-mediated responses did not show a long-term enhancement in response to a 30-min Mg(2+)-free ACSF application. Additionally, in prior Mg(2+)-free ACSF-treated slices, the entire frequency-response curve of LTP and long-term depression (LTD) is shifted systematically to favor LTD. These results suggest that the increase of Ca(2+) influx through NMDA channels and L-type VOCCs in turn triggering a PKC-dependent signaling cascade is a possible cellular basis underlying this seizure-like activity-induced inhibition of LTP.