Selective refinement of glutamate and GABA synapses on dorsal raphe 5-HT neurons during postnatal life

Serotonin (5-hydroxytryptamine, 5-HT) neurons are implicated in the etiology and therapeutics of anxiety and depression. Critical periods of vulnerability during brain development enable maladaptive mechanisms to produce detrimental consequences on adult mood and emotional responses. 5-HT plays a crucial role in these mechanisms; however, little is known about how synaptic inputs and modulatory systems that shape the activity of early 5-HT networks mature during postnatal development. We investigated in mice the postnatal trajectory of glutamate and GABA synaptic inputs to dorsal raphe nucleus (DRN) 5-HT neurons, the main source of forebrain 5-HT. High-resolution quantitative analyses with array tomography and ex vivo electrophysiology indicate that cortical glutamate and subcortical GABA synapses undergo a profound refinement process after the third postnatal week, whereas subcortical glutamate inputs do not. This refinement of DRN inputs is not accompanied by changes in 5-HT1A receptor-mediated inhibition over 5-HT neurons. Our study reveals a precise developmental pattern of synaptic refinement of DRN excitatory and inhibitory afferents, when 5-HT-related inhibitory mechanisms are in place. These findings contribute to the understanding of neurodevelopmental vulnerability to psychiatric disorders. This article has an associated 'The people behind the papers' interview.

Genetic Deletion of KLHL1 Leads to Hyperexcitability in Hypothalamic POMC Neurons and Lack of Electrical Responses to Leptin

Kelch-like 1 (KLHL1) is a neuronal actin-binding protein that modulates voltage-gated calcium channels. The KLHL1 knockout (KO) model displays altered calcium channel expression in various brain regions. We analyzed the electrical behavior of hypothalamic POMC (proopiomelanocortin) neurons and their response to leptin. Leptin's effects on POMC neurons include enhanced gene expression, activation of the ERK1/2 pathway and increased electrical excitability. The latter is initiated by activation of the Jak2-PI3K-PLC pathway, which activates TRPC1/5 (Transient Receptor Potential Cation) channels that in turn recruit T-type channel activity resulting in increased excitability. Here we report over-expression of Ca_V3.1 T-type channels in the hypothalamus of KLHL1 KO mice increased T-type current density and enhanced POMC neuron basal excitability, rendering them electrically unresponsive to leptin. Electrical sensitivity to leptin was restored by partial blockade of T-type channels. The overexpression of hypothalamic T-type channels in POMC neurons may partially contribute to the obese and abnormal feeding phenotypes observed in KLHL1 KO mice.

TRPC1/5-Ca V 3 Complex Mediates Leptin-Induced Excitability in Hypothalamic Neurons

Leptin regulates hypothalamic POMC⁺ (pro-opiomelanocortin) neurons by inducing TRPC (Transient Receptor Potential Cation) channel-mediate membrane depolarization. The role of TRPC channels in POMC neuron excitability is clearly established; however, it remains unknown whether their activity alone is sufficient to trigger excitability. Here we show that the right-shift voltage induced by the leptin-induced TRPC channel-mediated depolarization of the resting membrane potential brings T-type channels into the active window current range, resulting in an increase of the steady state T-type calcium current from 40 to 70% resulting in increased intrinsic excitability of POMC neurons. We assessed the role and timing of T-type channels on excitability and leptin-induced depolarization in vitro in cultured mouse POMC neurons. The involvement of TRPC channels in the leptin-induced excitability of POMC neurons was corroborated by using the TRPC channel inhibitor 2APB, which precluded the effect of leptin. We demonstrate T-type currents are indispensable for both processes, as treatment with NNC-55-0396 prevented the membrane depolarization and rheobase changes induced by leptin. Furthermore, co-immunoprecipitation experiments suggest that TRPC1/5 channels and Ca_V3.1 and Ca_V3.2 channels co-exist in complex. The functional relevance of this complex was corroborated using intracellular Ca²⁺ chelators; intracellular BAPTA (but not EGTA) application was sufficient to preclude POMC neuron excitability. However, leptin-induced depolarization still occurred in the presence of either BAPTA or EGTA suggesting that the calcium entry necessary to self-activate the TRPC1/5 complex is not blocked by the presence of BAPTA in hypothalamic neurons. Our study establishes T-type channels as integral part of the signaling cascade induced by leptin, modulating POMC neuron excitability. Leptin activation of TRPC channels existing in a macromolecular complex with T-type channels recruits the latter by locally induced membrane depolarization, further depolarizing POMC neurons, triggering action potentials and excitability.

Simultaneous administration of cocaine and caffeine dysregulates HCN and T-type channels

RATIONALE

The abuse of psychostimulants has adverse consequences on the physiology of the central nervous system. In Argentina, and other South American countries, coca paste or "PACO" (cocaine and caffeine are its major components) is massively consumed with deleterious clinical consequences for the health and well-being of the general population. A scant number of studies have addressed the consequences of stimulant combination of cocaine and caffeine on the physiology of the somatosensory thalamocortical (ThCo) system.

OBJECTIVES

Our aim was to study ion conductances that have important implications regulating sleep-wake states 24-h after an acute or chronic binge-like administration of a cocaine and caffeine mixture following previously analyzed pasta base samples ("PACO"-like binge") using mice.

METHODS

We randomly injected (i.p.) male C57BL/6JFcen mice with a binge-like psychostimulants regimen during either 1 day (acute) or 1 day on/1 day off during 13 days for a total of 7 binges (chronic). Single-cell patch-clamp recordings of VB neurons were performed in thalamocortical slices 24 h after the last psychostimulant injection. We also recorded EEG/EMG from mice 24 h after being systemically treated with chronic administration of cocaine + caffeine versus saline, vehicle.

RESULTS

Our results showed notorious changes in the intrinsic properties of the VB nucleus neurons that persist after 24-h of either acute or chronic binge administrations of combined cocaine and caffeine ("PACO"-like binge). Functional dysregulation of HCN (hyperpolarization-activated cyclic nucleotide-gated) and T-type VGC (voltage-gated calcium) channels was described 24-h after acute/chronic "PACO"-like administrations. Furthermore, intracellular basal [Ca²⁺] disturbances resulted a key factor that modulated the availability and the activation of T-type channels, altering T-type "window currents." As a result, all these changes ultimately shaped the low-threshold spikes (LTS)-associated Ca²⁺ transients, regulated the membrane excitability, and altered sleep-wake transitions.

CONCLUSION

Our results suggest that deleterious consequences of stimulants cocaine and caffeine combination on the thalamocortical physiology as a whole might be related to potential neurotoxic effects of soaring intracellular [Ca²⁺].

Leptin alters somatosensory thalamic networks by decreasing gaba release from reticular thalamic nucleus and action potential frequency at ventrobasal neurons

Leptin is an adipose-derived hormone that controls appetite and energy expenditure. Leptin receptors are expressed on extra-hypothalamic ventrobasal (VB) and reticular thalamic (RTN) nuclei from embryonic stages. Here, we studied the effects of pressure-puff, local application of leptin on both synaptic transmission and action potential properties of thalamic neurons in thalamocortical slices. We used whole-cell patch-clamp recordings of thalamocortical VB neurons from wild-type (WT) and leptin-deficient obese (ob/ob) mice. We observed differences in VB neurons action potentials and synaptic currents kinetics when comparing WT vs. ob/ob. Leptin reduced GABA release onto VB neurons throughout the activation of a JAK2-dependent pathway, without affecting excitatory glutamate transmission. We observed a rapid and reversible reduction by leptin of the number of action potentials of VB neurons via the activation of large conductance Ca²⁺-dependent potassium channels. These leptin effects were observed in thalamocortical slices from up to 5-week-old WT but not in leptin-deficient obese mice. Results described here suggest the existence of a leptin-mediated trophic modulation of thalamocortical excitability during postnatal development. These findings could contribute to a better understanding of leptin within the thalamocortical system and sleep deficits in obesity.

Calcium current homeostasis and synaptic deficits in hippocampal neurons from Kelch-like 1 knockout mice

Kelch-like 1 (KLHL1) is a neuronal actin-binding protein that modulates voltage-gated Ca_V2.1 (P/Q-type) and Ca_V3.2 (α_1H T-type) calcium channels; KLHL1 knockdown experiments (KD) cause down-regulation of both channel types and altered synaptic properties in cultured rat hippocampal neurons (Perissinotti et al., 2014). Here, we studied the effect of ablation of KLHL1 on calcium channel function and synaptic properties in cultured hippocampal neurons from KLHL1 knockout (KO) mice. Western blot data showed the P/Q-type channel α_1A subunit was less abundant in KO hippocampus compared to wildtype (WT); and P/Q-type calcium currents were smaller in KO neurons than WT during early days in vitro, although this decrease was compensated for at late stages by increases in L-type calcium current. In contrast, T-type currents did not change in culture. However, biophysical properties and western blot analysis revealed a differential contribution of T-type channel isoforms in the KO, with Ca_V3.2 α_1H subunit being down-regulated and Ca_V3.1 α_1G up-regulated. Synapsin I levels were also reduced in the KO hippocampus and cultured neurons displayed a concomitant reduction in synapsin I puncta and decreased miniature excitatory postsynaptic current (mEPSC) frequency. In summary, genetic ablation of the calcium channel modulator resulted in compensatory mechanisms to maintain calcium current homeostasis in hippocampal KO neurons; however, synaptic alterations resulted in a reduction of excitatory synapse number, causing an imbalance of the excitatory-inhibitory synaptic input ratio favoring inhibition.

Down-regulation of endogenous KLHL1 decreases voltage-gated calcium current density

The actin-binding protein Kelch-like 1 (KLHL1) can modulate voltage-gated calcium channels in vitro. KLHL1 interacts with actin and with the pore-forming subunits of Cav2.1 and CaV3.2 calcium channels, resulting in up-regulation of P/Q and T-type current density. Here we tested whether endogenous KLHL1 modulates voltage gated calcium currents in cultured hippocampal neurons by down-regulating the expression of KLHL1 via adenoviral delivery of shRNA targeted against KLHL1 (shKLHL1). Control adenoviruses did not affect any of the neuronal properties measured, yet down-regulation of KLHL1 resulted in HVA current densities ~68% smaller and LVA current densities 44% smaller than uninfected controls, with a concomitant reduction in α(1A) and α(1H) protein levels. Biophysical analysis and western blot experiments suggest Ca(V)3.1 and 3.3 currents are also present in shKLHL1-infected neurons. Synapsin I levels, miniature postsynaptic current frequency, and excitatory and inhibitory synapse number were reduced in KLHL1 knockdown. This study corroborates the physiological role of KLHL1 as a calcium channel modulator and demonstrates a novel, presynaptic role.