Homeostatic regulation of axonal Kv1.1 channels accounts for both synaptic and intrinsic modifications in the hippocampal CA3 circuit

Homeostatic plasticity of intrinsic excitability goes hand in hand with homeostatic plasticity of synaptic transmission. However, the mechanisms linking the two forms of homeostatic regulation have not been identified so far. Using electrophysiological, imaging, and immunohistochemical techniques, we show here that blockade of excitatory synaptic receptors for 2 to 3 d induces an up-regulation of both synaptic transmission at CA3–CA3 connections and intrinsic excitability of CA3 pyramidal neurons. Intrinsic plasticity was found to be mediated by a reduction of Kv1.1 channel density at the axon initial segment. In activity-deprived circuits, CA3–CA3 synapses were found to express a high release probability, an insensitivity to dendrotoxin, and a lack of depolarization-induced presynaptic facilitation, indicating a reduction in presynaptic Kv1.1 function. Further support for the down-regulation of axonal Kv1.1 channels in activity-deprived neurons was the broadening of action potentials measured in the axon. We conclude that regulation of the axonal Kv1.1 channel constitutes a major mechanism linking intrinsic excitability and synaptic strength that accounts for the functional synergy existing between homeostatic regulation of intrinsic excitability and synaptic transmission.

Chronic modulation of activity regimes in neuronal circuits induces homeostatic plasticity. This implicates a regulation of both intrinsic excitability (homeostatic plasticity of intrinsic excitability) and synaptic transmission (homeostatic plasticity of synaptic transmission) to maintain network activity within physiological bounds (1). In most cases, these two forms of homeostatic plasticity act synergistically but involve different molecular actors. Homeostatic intrinsic plasticity is associated with the regulation of voltage-gated ion channels (2–9), while homeostatic synaptic plasticity involves the regulation of postsynaptic receptors to neurotransmitters (10–17) or the regulation of the readily releasable pool of synaptic vesicles (18–20). However, the function of voltage-gated ion channels is not limited to the control of intrinsic excitability. Several studies point to the role of axonal voltage-gated channels in shaping presynaptic action potential (AP) waveform and subsequently controlling neurotransmitter release and synaptic transmission (21–35). Moreover, some studies describe homeostatic plasticity of the AP waveform via voltage-gated channel regulation (36–38), while other studies report an absence of this phenomenon (39).Kv1.1 channels are responsible for the fast-activating, slow-inactivating D-type current (I_D) in CA3 neurons. This current has been shown to create a delay in the onset of the first AP and to determine intrinsic excitability in various neuronal types, including CA1 and CA3 pyramidal neurons of the hippocampus (3, 40), L5 pyramidal neurons of the cortex (26, 34), and L2/3 fast-spiking interneurons of the somatosensory cortex (41, 42). Furthermore, Kv1.1 channels have been shown to control axonal AP width and subsequently presynaptic calcium entry and neurotransmitter release. In fact, pharmacological blockade of Kv1.1 channels broadens presynaptic APs and increases synaptic transmission at neocortical and hippocampal glutamatergic synapses and at cerebellar GABAergic synapses (21, 22, 26, 30, 32, 41, 43, 44). Moreover, Kv1.1 channels have been shown to be responsible for the phenomenon of depolarization-induced analog digital facilitation of synaptic transmission (d-ADF). In fact, at CA3–CA3 and L5–L5 synapses, a somatic subthreshold depolarization of the presynaptic cell leads to inactivation of axonal Kv1.1 channels, inducing the broadening of the presynaptic AP, an increase in spike-evoked calcium entry, and a facilitation of presynaptic glutamate release (26, 31, 33, 34, 45, 46). Therefore, Kv1.1 channels control both intrinsic excitability and glutamate release in CA3 pyramidal neurons.Kv1.1 channels have been shown to be involved in homeostatic regulation of neuronal excitability. Chronic activity enhancement by kainate application leads to an increase in I_D current and a decrease in excitability in dentate gyrus granule cells (47). Conversely, chronic sensory deprivation leads to Kv1.1 channel down-regulation and enhancement of excitability in the avian cochlear nucleus (48). In this study, we examined whether the increase in synaptic transmission could also be due to Kv1.1 channel down-regulation, which would possibly explain the observed synergy between homeostatic plasticity of excitability and synaptic transmission.We show here that chronic activity deprivation induced with an antagonist of ionotropic glutamate receptors (kynurenate) in hippocampal organotypic cultures provokes both an increase in CA3 pyramidal cells excitability and an enhancement of synaptic transmission at monosynaptically connected CA3 neurons. Deprived cultures display a decrease in Kv1.1 channel staining in the axon initial segment. Bath application of dendrotoxin-K (DTX-K), a selective blocker of Kv1.1 channels, leads to a larger excitability increase in control cultures than in deprived cultures. Focal puffing of DTX-K on the axon increases excitability in control but not in deprived cultures, showing that homeostatic plasticity of excitability in deprived cultures is partly due to the down-regulation of axonal Kv1.1 channels. In addition, we found that axonal Kv1.1 down-regulation in deprived cultures is responsible for a spike broadening in CA3 neurons, leading to elevated release probability at CA3–CA3 synapses. Consistent with these observations, d-ADF, a Kv1.1-dependent form of synaptic facilitation, is present in control cultures but not in deprived cultures. Altogether, these results show that chronic activity blockade of the hippocampal CA3 circuit induces the down-regulation of axonal Kv1.1 channels leading to a homeostatic increase in both excitability and presynaptic release probability.