Activity-dependent regulation of inhibitory synaptic transmission in hippocampal neurons

“Curiously, the age-dependency of quantal responses has been demonstrated in dissociated cultures (e.g. Burrone et al., 2002; Han and Stevens, 2009; Hartman et al., 2006; Wierenga et al., 2006), albeit not necessarily in the same direction.”

“, 2006), it would be of interest to determine whether excitatory GABA actions during early development also affect synapse maturation and circuit formation in vivo, in a manner similar to that found for newborn granule cells in adult hippocampus (Ge et al., 2006; Hartman et al., 2006).”

“…culture reports that reduction of spiking in single GABAergic neurons does not seem to decrease synaptic output, although spiking activity was reduced at neonatal stage when GABA was still depolarizing, and different classes of GABAergic synapses were not distinguished (Hartman et al., 2006).”

“…that the loss of inhibitory structures may reflect a general reduction of inhibition after a decrease in cortical activity, which could be a result of either decreased activity levels in the inhibitory neurons themselves or in the activity levels of the network (as in Hartman et al., 2006).”

“Reported instances of activity-dependent regulation of GAD65 expression in other CNS neurons (Hartman et al., 2006) could have their origin in a BDNF-regulated pathway of synaptic GAD65 trafficking.”

“Second, recordings were made from cells voltage clamped at −70 mV in the presence of 500 nM TTX, 10 μM CNQX and 50 μM AP5, as previously describe”

“Our results corroborate and extend previous findings in rat hippocampal neurons (Hartman et al., 2006) that the quantal size of inhibitory transmission can be regulated by changes in vesicle filling, in addition to regulation by postsynaptic receptor composition (Kilman et al., 2002; Saliba et al.,…”

“However, these experiments involved activity blockade at later stages of synapse formation and maturation (TTX treatment from 5 div onward; Hartman et al., 2006; Kilman et al., 2002) suggesting that activity may have different effects on synapse formation at different developmental stages.”

“Indeed, several lines of evidence suggest that GABAergic efficacy is upregulated in hyperactive networks [25–29] and downregulated under conditions of reduced activity [5, 30, 31].”

“ifying postsynaptic GABAAR levels (Rutherford et al., 1997; Kilman et al., 2002; Hartman et al., 2006).”

“In vitro studies from cortical and hippocampal cells reported that activity-deprivation invariably resulted in decreases in mIPSC amplitude, either without [19,20] or with [17,38] concurrent decreases in event frequency.”

“As homeostatic plasticity is known to involve regulation of inhibitory transmission (Hartman et al., 2006; Kilman et al., 2002; Maffei et al., 2004), we next examined the protein components of GABAergic synaptic transmission that exhibit altered protein synthesis (Figure S5).”

“Apart from the total number of presynaptic terminal sites available, another factor mediating signaling strength and efficacy is the available pool of vesicles and their content of neurotransmitter (Hartman et al., 2006).”

“Responses from single cells were recorded with a standard patch clamping setup (HEKA epc10 double patch clamp amplifier, operating with HEKA Pulse software) (Hartman et al 2006).”

“This sets the interpretation of our data apart from any developmental influence; this is of relevance because responses to inactivity may vary widely depending on the maturity of the neural network (Burrone et al., 2002; Hartman et al., 2006; Karmarkar and Buonomano, 2006; Kirov et al., 2004).”

“Synaptic Scaling in Inhibitory Neurons While synaptic scaling has previously been shown to occur in inhibitory neurons in reduced preparations (Hartman et al., 2006), here we demonstrate that synaptic scaling occurs following in vivo deprivation in what are likely NPY-positive inhibitory neurons measured via an increa…”

“Although GABA in the cytosol can serve nonsignaling metabolic functions under some conditions, cytosolic GABA concentration may also be a critical determinant of vesicular filling in GABAergic interneurons (Hartman et al., 2006; Lau and Murthy, 2012).”

“…circuits, and a number of studies in cultures of dissociated neurons have provided evidence that chronic changes (i.e., days) in neuronal activity can lead to homeostatic scaling of GABAergic synaptic strength (Kilman et al , 2002; Hartman et al , 2006; Swanwick et al , 2006; Saliba et al , 2007).”

“neonatal cultured hippocampal neurons [65].”

“Homeostatic synaptic plasticity, for example, generally results in opposing alterations in glutamatergic and GABAergic transmission (Turrigiano et al., 1998; Kilman et al., 2002; Hartman et al., 2006; Frias and Wierenga, 2013).”

“Although activity-dependent excitatory SS is mediated through CaMKIV signaling, it remains unknown whether this pathway coordinately regulates homeostatic inhibitory synaptic scaling (Kilman et al., 2002; Vale and Sanes, 2002; Hartman et al., 2006) and intrinsic excitability (Turrigiano et al.”

“…that during chronic activity blockade, homeostatic regulation of inhibitory synapses required change in the activity level of multiple neurons within the network (Hartman et al., 2006), suggesting that homeostatic regulation of inhibitory synapses following activity blockade is noncell-autonomous.”

“In previous studies, the frequency of GABA-mediated mIPSCs in hippocampal pyramidal neurons correlated with the density of GABAergic synapses (Hartman et al., 2006; Swanwick et al., 2006).”

“This similarity to control neurons did not remain though, synaptic strength after network activity blockade arises from presynaptic changes (Hartman et al., 2006).”

“These dynamics may be reflective of the perpetual remodeling of synapses that takes place in response to signals arising from molecular interactions and synaptic activity (Liu, 2004; Hartman et al., 2006; Huang et al., 2007; Siddiqui and Craig, 2011).”

“10A), we observed a 30.4% reduction in the number of GABAergic synapses, compared with nontreated cultures, consistent with previously reported results (Hartman et al., 2006).”

“A classical paradigm to explore this process experimentally is elevating or suppressing the activity of cultured neurons, which triggers the potentiation or depression of spontaneous inhibitory synaptic currents into the perturbed neurons [66,67].”

“An increase of inhibition is most probably one of the first reactions in response to elevated neuronal activity, where the homeostatic inhibitory regulation seems to be influenced by the elevated network activity rather than cellular activity (Hartman et al. 2006).”

“Other studies have provided similar results using pharmacological and genetic manipulation of activity in developing hippocampal cultures (Marty et al., 2000; Hartman et al., 2006).”

“Reductions in mIPSC activity have been implicated as a potential mechanism of epileptogenesis (Hirsch et al., 1999; Shao et al., 2005). mIPSCs are known to play an important role in synaptic maintenance and function (Hartman et al., 2006).”

“In addition, compensatory changes in the vesicular inhibitory amino acid transporter, VIAAT, have been observed suggesting there are coordinated presynaptic contributions to homeostatic changes in mIPSC amplitude [18, 20, 21].”

“Puff application of GABA (100 μM in ACSF), glutamate (100 μM in ACSF), or ACSF was performed via a patch pipette positioned in the stratum radiatum close to the neuron of interest.”

“1 did not alter GABAergic inputs (Hartman et al., 2006).”

“Here, we observe a dramatic, developmentally dependent transient increase in inhibitory synaptic input onto excitatory principal neurons in the FXS amygdala that is reminiscent of homeostatic changes observed when global network activity is increased in normal networks (Gonzalez-Islas and Wenner, 2006; Hartman et al.,…”

“…genetic manipulation of glutamate receptors in developing neurons indicate that glutamate receptor activities regulate GABAergic synapse development (Aamodt et al., 2000; Gaı̈arsa, 2004; Hartman et al., 2006; Henneberger et al., 2005; Lu et al., 2013; Marty et al., 2000; Rosato-Siri et al., 2002).”

“The sequence 5 -CTACTGAGAACTAAGAGAG-3 (shc) was used as nontargeting control (Hartman et al., 2006).”

“Glutamate-induced acidification of vesicles also enables the ΔpH-dependent uptake of monoamines, acetylcholine (ACh) or GABA by H+ antiport in neurons that coexpress co-express VGLUT3 and VMAT2, VAChTor VGAT/ VIAAT (see [223] for review).”

“What mechanisms regulate the ratio of inhibitory to excitatory synapses on dendrites? It is likely that apart from glutamatergic transmission [60,61], GABAergic transmission itself plays a role in regulating the balance of inhibitory and excitatory inputs in developing CNS circuits [62,63].”

“In support of this hypothesis, alterations in vesicular GABA content have been implicated in changes in mIPSC amplitude (Lau and Murthy, 2012), although whether the intensity of GAD65 immunostaining is a relevant proxy for vesicular GABA content remains to be determined (Tian et al., 1999; Hartman et al., 2006).”

“Median mIPSC amplitudes, however, were not differentially affected by TPMPA (Fig.”

“6F, right), suggesting an amplification of inhibition in these networks that could help to gate network activity by increasing the functional drive to inhibitory subcircuits (Hartman et al., 2006).”

“Indeed, inactivity has been demonstrated to induce reduction in the amount of GABA released by individual vesicles (De Gois et al., 2005; Hartman et al., 2006) that should reduce the ambient [GABA]o. Alternatively, prolong synaptic disuse might trigger upregulation of GAT-1GABA transporter.”

“…may account for these results (on the assumption that these are not saturated by the release of GABA from a single vesicle; Barberis et al., 2004; Hartman et al., 2006), this was not the case since a similar number of receptor channels was revealed with peak-scaled non-stationary fluctuation…”

“However, a major locus of expression is also a dramatic reduction in the number of functional inhibitory synapses (33), at a time in vitro when there are no changes in excitatory synapse number (97); conversely, inhibitory synapse number does not change in older cultures (25) (at a time when excitatory synapse number…”

“The UPS is emerging as a powerful modulator of synaptic function, acting at both postsynaptic and presynaptic sites (Hegde, 2010).”

“PHluorin has a neutral pKa [8,10] and is widely used for studying synapse function [11,12,13,14,15,16,17], exocytosis and fusion pore behavior [18,19,20,21,22], endocytosis [23,24], receptor trafficking [25,26], and endosome function [27].”

“A reduced cortical activity leads normally to an impaired GABAergic innervation [107, 108] and to a decreased GABAergic neurotransmission [109–111].”

“Such a phenomenon has been shown to occur in GABAergic synapses from cultured neurons upon prolonged reduction of neuronal activity (Hartman et al., 2006).”

“Standard deviation values for the power analysis were estimated based on previously published values from dissociated hippocampal neuron electrophysiological recordings (Henry et al, 2012 and Hartman et al, 2006).”

“In the present study, the downregulation of GAD67 was coincidently observed with the impairment of GABA synaptic inhibition in the dorsal horn neurons of the modeled rats, which was consistent with previous studies showing that cytosolic concentration of the GABA transmitter is a critical factor in its vesicular loadi…”

“Under conditions of chronic activity blockade in neuronal and organotypic cultures from hippocampus, the amount of inhibition and density of GABAergic terminals were reported to decrease (Marty et al., 2000; Hartman et al., 2006).”

“Neuronal activity plays an important role in regulating many properties of neurons, including neuronal migration and synaptic plasticity, either cell autonomously (Burrone et al., 2002; Tong et al., 2009) or non-cell autonomously (Hartman et al., 2006; Wright and Ribera, 2010).”

“Prolonged blockade or potentiation of neuronal activity triggers homeostatic plasticity, which restores the neuron’s target firing rate [44, 109, 153, 160].”

“Journal compilation C© 2007 The Physiological Society (> 48 h) of activity deprivation (e.g. Murthy et al. 2001; Burrone et al. 2002; Nakayama et al. 2005; Stellwagen & Malenka, 2006; Hartman et al. 2006; Swanwick et al. 2006).”

“In vivo, NL2 would then recruit further postsynaptic density proteins (Poulopoulos et al., 2009), and perhaps this, together with the increased synaptic activity, would further strengthen and stabilize the connection (Hartman et al., 2006; Varoqueaux et al., 2006; Chubykin et al., 2007).”

“First, the excitation ⁄ inhibition balance is constantly adapted to changes in network activity (Esclapez & Houser, 1999; Hartman et al., 2006; Hartmann et al., 2008), a process called homeostatic plasticity (Turrigiano & Nelson, 2000; Erickson et al., 2006).”

“Such weakening of inhibitory connections may be a homeostatic regulation in response to a general reduction of activity level (Hartman et al., 2006), and is also reminiscent of results in adult cortex that MD for a few days increases loss of inhibitory synapses (Chen et al., 2012; van Versendaal et…”

“Second, the very modest changes in synaptic gephyrin contents induced by prolonged activity suppression (in agreement with [58]) would seem to be at odds with other studies suggesting that activity-deprivation results in decreases in mIPSC amplitude and reductions in GABAAR labeling (e.”

“Some in vitro studies have suggested that the homeostatic regulation of mIPSC amplitude is due to regulation of vesicular GABA content (Hartman et al., 2006; Hartmann et al., 2008).”