Increased GABAergic transmission in neuropeptide Y-expressing neurons in the dopamine-depleted murine striatum

As the main input nucleus of the basal ganglia, the striatum plays a central role in planning, control, and execution of movement and motor skill learning. More than 90% of striatal neurons, so-called medium spiny neurons (MSN), are GABAergic projection neurons, innervating primarily the substantia nigra pars reticulata or the globus pallidus internus. The remaining neurons are GABAergic and cholinergic interneurons, synchronizing and controlling striatal output by reciprocal connections with MSN. Besides prominent local cholinergic influence, striatal function is globally regulated by dopamine (DA) from the nigrostriatal pathway. Little is known about whether DA depletion, as occurs in Parkinson’s disease, affects the activity of striatal interneurons. Here we focused on neuropeptide Y (NPY)-expressing interneurons, which are among the major subgroups of GABAergic interneurons in the striatum. We investigated the effects of striatal DA depletion on GABAergic transmission in NPY interneurons by electrophysiologically recording GABAergic spontaneous (s) and miniature (m) inhibitory postsynaptic currents (IPSCs) in identified NPY interneurons in slices from 6-hydroxydopamine (6-OHDA)- and vehicle-injected transgenic NPY-humanized Renilla green fluorescent protein (hrGFP) mice with the whole cell patch-clamp technique. We report a significant increase in sIPSC and mIPSC frequency as well as the occurrence of giant synaptic and burst sIPSCs in the 6-OHDA group, suggesting changes in GABAergic circuit activity and synaptic transmission. IPSC kinetics remained unchanged, pointing to mainly presynaptic changes in GABAergic transmission. These results show that chronic DA depletion following 6-OHDA injection causes activity-dependent and -independent increase of synaptic GABAergic inhibition onto striatal NPY interneurons, confirming their involvement in the functional impairments of the DA-depleted striatum. NEW & NOTEWORTHY Neuropeptide Y (NPY) interneurons regulate the function of striatal projection neurons and are upregulated upon dopamine depletion in the striatum. Here we investigated how dopamine depletion affects NPY circuits and show electrophysiologically that it leads to the occurrence of giant synaptic and burst GABAergic spontaneous inhibitory postsynaptic currents (IPSCs) and to an activity-independent increase in GABAergic miniature IPSC frequency in NPY neurons. We suggest that degeneration of dopaminergic terminals in the striatum causes functional changes in striatal GABAergic function.

The CXCL12/CXCR4 signaling is required for DPP4 regulation of spontaneous inhibitory postsynaptic currents (sIPSCs)

Background: Previous data suggested that dipeptidyl peptidase-IV (DPP4) involved in the occurrence of febrile seizure (FS), but its potential mechanism remains to be determined. Here, we investigated whether DPP4 regulated gamma-aminobutyric acid (GABA) mediated spontaneous inhibitory postsynaptic currents (sIPSCs) via the downstream C-X-C Motif Chemokine Ligand 12 (CXCL12)/ C-X-C chemokine receptor type 4 (CXCR4) signaling in cultured hippocampal neurons submitted to hyperthermia(39.5-40°C). Methods: Whole cell patch- clamp method was used to test sIPSC in vitro after DPP4 inhibition or CXCL12 administration. The level of CXCL12 and CXCR4 was tested using western blot analysis. The effect of CXCR4 antagonist AMD3100 (5 mg/ml, i.c.v) on seizures were tested using electroencephalogram (EEG) in a FS model. Results: We found that pharmacological DPP4 inhibitor sitagliptin (Sita,100μM) treatment or siRNA-mediated DPP4 knockdown enhanced the mean amplitude and frequency of sIPSCs in vitro. DPP4 knockdown with siRNA increased protein level of CXCL12 and CXCR4. Furthermore, CXCL12 (10 nM) treatment enhanced inhibitory transmission by increasing the mean frequency and amplitude of sIPSCs in vitro. AMD3100 administration decreased seizure severity by increasing hippocampal GABA content in vivo. Conclusions: Our data suggest that CXCL12/CXCR4 signaling is required for DPP4 regulation of sIPSCs, supporting that DPP4 played a key role in the pathogenesis of FS.

Serotonergic control of GABAergic inhibition in the lateral amygdala

Much evidence implicates the serotonergic regulation of the amygdala in anxiety. Thus the present study was undertaken to characterize the influence of serotonin (5-HT) on principal neurons (PNs) of the rat lateral amygdala (LA), using whole cell recordings in vitro. Because inhibition is a major determinant of PN activity, we focused on the control of GABAergic transmission by 5-HT. IPSCs were elicited by local electrical stimulation of LA in the presence of glutamate receptor antagonists. We found that 5-HT reduces GABAA inhibitory postsynaptic currents (IPSCs) via presynaptic 5-HT1B receptors. While the presynaptic inhibition of GABA release also attenuated GABAB currents, this effect was less pronounced than for GABAA currents because 5-HT also induced a competing postsynaptic enhancement of GABAB currents. That is, GABAB currents elicited by pressure application of GABA or baclofen were enhanced by 5-HT. In addition, we obtained evidence suggesting that 5-HT differentially regulates distinct subsets of GABAergic synapses. Indeed, GABAA IPSCs were comprised of two components: a relatively 5-HT-insensitive IPSC that had a fast time course and a 5-HT-sensitive component that had a slower time course. Because the relative contribution of these two components varied depending on whether neurons were recorded at proximity versus at a distance from the stimulating electrodes, we speculate that distinct subtypes of local-circuit cells contribute the two contingents of GABAergic synapses. Overall, our results indicate that 5-HT is a potent regulator of synaptic inhibition in LA. NEW & NOTEWORTHY We report that 5-HT, acting via presynaptic 5-HT1B receptors, attenuates GABAA IPSCs by reducing GABA release in the lateral amygdala (LA). In parallel, 5-HT enhances GABAB currents postsynaptically, such that GABAB inhibitory postsynaptic currents (IPSCs) are relatively preserved from the presynaptic inhibition of GABA release. We also found that the time course of 5-HT-sensitive and -insensitive GABAA IPSCs differ. Together, these results indicate that 5-HT is a potent regulator of synaptic inhibition in LA.

Structural and functional characterization of dendritic arbors and GABAergic synaptic inputs on interneurons and principal cells in the rat basolateral amygdala

The basolateral amygdala (BLA) is a complex brain region associated with processing emotional states, such as fear, anxiety, and stress. Some aspects of these emotional states are driven by the network activity of synaptic connections, derived from both local circuitry and projections to the BLA from other regions. Although the synaptic physiology and general morphological characteristics are known for many individual cell types within the BLA, the combination of morphological, electrophysiological, and distribution of neurochemical GABAergic synapses in a three-dimensional neuronal arbor has not been reported for single neurons from this region. The aim of this study was to assess differences in morphological characteristics of BLA principal cells and interneurons, quantify the distribution of GABAergic neurochemical synapses within the entire neuronal arbor of each cell type, and determine whether GABAergic synaptic density correlates with electrophysiological recordings of inhibitory postsynaptic currents. We show that BLA principal neurons form complex dendritic arborizations, with proximal dendrites having fewer spines but higher densities of neurochemical GABAergic synapses compared with distal dendrites. Furthermore, we found that BLA interneurons exhibited reduced dendritic arbor lengths and spine densities but had significantly higher densities of putative GABAergic synapses compared with principal cells, which was correlated with an increased frequency of spontaneous inhibitory postsynaptic currents. The quantification of GABAergic connectivity, in combination with morphological and electrophysiological measurements of the BLA cell types, is the first step toward a greater understanding of how fear and stress lead to changes in morphology, local connectivity, and/or synaptic reorganization of the BLA.