Opposite Changes in Glutamatergic and GABAergic Transmission Underlie the Diffuse Hyperexcitability of Synapsin I–Deficient Cortical Networks

Abstract

Synapsins (Syns) are synaptic vesicle (SV) phosphoproteins that play a role in synaptic transmission and plasticity. Mutation of the SYN1 gene results in an epileptic phenotype in mouse and man, implicating SynI in the control of network excitability. We used microelectrode array and patch-clamp recordings to study network activity in primary cortical neurons from wild-type (WT) or SynI knockout (KO) mice. SYN1 deletion was associated with increased spontaneous and evoked activities, with more frequent and sustained bursts of action potentials and a high degree of synchronization. Blockade of GABA(A) (gamma-aminobutyric acid(A)) receptors with bicuculline attenuated, but did not completely abolish, the differences between WT and SynI KO networks in both spontaneous and evoked activities. Patch-clamp recordings on cortical autaptic neurons revealed a reduced amplitude of evoked inhibitory postsynaptic currents (PSCs) and a concomitantly increased amplitude of evoked excitatory PSCs in SynI KO neurons, in the absence of changes in miniature PSCs. Cumulative amplitude analysis revealed that these effects were attributable to opposite changes in the size of the readily releasable pool of SVs. The results indicate distinct roles of SynI in GABAergic and glutamatergic neurons and provide an explanation for the high susceptibility of SynI KO mice to epileptic seizures.

Subconvulsant doses of pentylenetetrazol uncover the epileptic phenotype of cultured synapsin-deficient Helix serotonergic neurons in the absence of excitatory and inhibitory inputs

“…Their spontaneous activity was highly synchronized with respect to control neurons (Chiappalone et al, 2009). In our case, under control conditions, Helix neurons did not fire APs at rest, but they exhibited strong responses when exposed to PTZ.…”

“…It is generally accepted that Syn deficiency reduces GABAergic transmission and not affects or increases glutamatergic transmission, thus leading to a positive imbalance towards hyperexcitability (Terada et al, 1999;Gitler et al, 2004;Baldelli et al, 2007;Chiappalone et al, 2009;Ketzef et al, 2011;Farisello et al, 2012;Lignani et al, 2013;Feliciano et al, 2013;Medrihan et al, 2013;Medrihan et al, 2014 Klee, 1976 andAltrup, 2004). Finally, a key advantage of this monosynaptic cell model is that the presynaptic and postsynaptic compartments can be selectively targeted by injections and specific networks can be constructed by directly plating individual neurons on MEA electrodes.…”

Knock-down of synapsin alters cell excitability and action potential waveform by potentiating BK and voltage-gated Ca2+ currents in Helix serotonergic neurons

“…showing an imbalance between excitatory and inhibitory neurotransmission, reducing the GABAergic component and leaving unchanged or increased the glutamatergic component (Terada et al, 1999;Gitler et al, 2004;Baldelli et al, 2007;Chiappalone et al, 2009;Ketzef et al 2011;Farisello et al, 2012;Medrihan et al, 2013;Feliciano et al, 2013;Lignani et al, 2013;Medrihan et al, 2014), thus leading to an increased network excitability.…”

“…Several studies provided evidence that Syns play a fundamental role in the development of neurites (Ferreira et al 1994;, Kao et al, 2002, and in the formation, maintenance and rearrangements of synaptic contacts (Ferreira et al, 1998;Chin et al, 1995). It is also accepted that Syns modulate synaptic vesicle pools, modifying the readily releasable pool (RRP), the recycling pool, and the resting pool sizes (Humeau et al, 2001;Feng et al, 2002;Gitler et al, 2004;Gaffield and Betz, 2007;Chiappalone et al, 2009;Messa et al, 2010;Kile, 2010;Valtorta et al, 2011;Farisello et al, 2012;Orenbuch et al, 2012;Verstegen et al, 2014;Brenes et al, 2015). Epileptic disorders have been related with deficiencies in Syn-regulated processes such as neurite outgrowth, synapse formation or neurotransmitter release.…”

“…Epileptic disorders have been related with deficiencies in Syn-regulated processes such as neurite outgrowth, synapse formation or neurotransmitter release. In fact, the development of epilepsy in Syn mutants has been associated with i) alterations of neurite outgrowth and synaptogenesis leading to abnormal circuitry during maturation (Cesca et al, 2010), and ii) modification of the synaptic vesicle pool mobility and release with consequent differential changes of the rate of neurotransmitter release in excitatory and inhibitory synapses, thus causing an excitation/inhibition imbalance in network excitability (Gitler et al, 2004;Baldelli et al, 2007;Chiappalone et al, 2009;Noebels et al, 2010;Pitkänen and Lukasiuk, 2011;Farisello et al, 2012;Lignani et al, 2013).…”