Large Inhibitory Postsynaptic Currents Research Articles

Rationale for an adjunctive therapy with fenofibrate in pharmacoresistant nocturnal frontal lobe epilepsy

Nocturnal frontal lobe epilepsy (NFLE) is an idiopathic partial epilepsy with a family history in about 25% of cases, with autosomal dominant inheritance (autosomal dominant NFLE [ADNFLE]). Traditional antiepileptic drugs are effective in about 55% of patients, whereas the rest remains refractory. One of the key pathogenetic mechanisms is a gain of function of neuronal nicotinic acetylcholine receptors (nAChRs) containing the mutated α4 or β2 subunits. Fenofibrate, a common lipid-regulating drug, is an agonist at peroxisome proliferator-activated receptor alpha (PPARα) that is a ligand-activated transcription factor, which negatively modulates the function of β2-containing nAChR. To test clinical efficacy of adjunctive therapy with fenofibrate in pharmacoresistant ADNFLE\NFLE patients, we first demonstrated the effectiveness of fenofibrate in a mutated mouse model displaying both disease genotype and phenotype. We first tested the efficacy of fenofibrate in transgenic mice carrying the mutation in the α4-nAChR subunit (Chrna4S252F) homologous to that found in humans. Subsequently, an add-on protocol was implemented in a clinical setting and fenofibrate was administered to pharmacoresistant NFLE patients. Here, we show that a chronic fenofibrate diet markedly reduced the frequency of large inhibitory postsynaptic currents (IPSCs) recorded from cortical pyramidal neurons in Chrna4S252F mice, and prevented nicotine-induced increase of IPSC frequency. Moreover, fenofibrate abolished differences between genotypes in the frequency of sleep-related movements observed under basal conditions. Patients affected by NFLE, nonresponders to traditional therapy, by means of adjunctive therapy with fenofibrate displayed a reduction of seizure frequency. Furthermore, digital video-polysomnographic recordings acquired in NFLE subjects after 6months of adjunctive fenofibrate substantiated the significant effects on control of motor-behavioral seizures. Our preclinical and clinical studies suggest PPARα as a novel disease-modifying target for antiepileptic drugs due to its ability to regulate dysfunctional nAChRs.

Intraspinal Sensory Neurons Provide Powerful Inhibition to Motor Circuits Ensuring Postural Control during Locomotion

In the vertebrate spinal cord, cerebrospinal fluid-contacting neurons (CSF-cNs) are GABAergic neurons whose functions are only beginning to unfold. Recent evidence indicates that CSF-cNs detect localspinal bending and relay this mechanosensory feedback information to motor circuits, yet many CSF-cN targets remain unknown. Using optogenetics, patterned illumination, and invivo electrophysiology, we show here that CSF-cNs provide somatic inhibition to fast motor neurons and excitatory sensory interneurons involved in the escape circuit. Ventral CSF-cNs respond to longitudinal spinal contractions and induce large inhibitory postsynaptic currents (IPSCs) sufficient tosilence spiking of their targets. Upon repetitive stimulation, these IPSCs promptly depress, enabling the mechanosensory response to the first bend to be the most effective. When CSF-cNs are silenced, postural control is compromised, resulting in rollovers during escapes. Altogether, our data demonstrate how GABAergic sensory neurons provide powerful inhibitory feedback to the escape circuit to maintain balance during active locomotion.

Nitric oxide signaling modulates synaptic inhibition in the superior paraolivary nucleus (SPN) via cGMP-dependent suppression of KCC2.

Glycinergic inhibition plays a central role in the auditory brainstem circuitries involved in sound localization and in the encoding of temporal action potential firing patterns. Modulation of this inhibition has the potential to fine-tune information processing in these networks. Here we show that nitric oxide (NO) signaling in the auditory brainstem (where activity-dependent generation of NO is documented) modulates the strength of inhibition by changing the chloride equilibrium potential. Recent evidence demonstrates that large inhibitory postsynaptic currents (IPSCs) in neurons of the superior paraolivary nucleus (SPN) are enhanced by a very low intracellular chloride concentration, generated by the neuronal potassium chloride co-transporter (KCC2) expressed in the postsynaptic neurons. Our data show that modulation by NO caused a 15 mV depolarizing shift of the IPSC reversal potential, reducing the strength of inhibition in SPN neurons, without changing the threshold for action potential firing. Regulating inhibitory strength, through cGMP-dependent changes in the efficacy of KCC2 in the target neuron provides a postsynaptic mechanism for rapidly controlling the inhibitory drive, without altering the timing or pattern of the afferent spike train. Therefore, this NO-mediated suppression of KCC2 can modulate inhibition in one target nucleus (SPN), without influencing inhibitory strength of other target nuclei (MSO, LSO) even though they are each receiving collaterals from the same afferent nucleus (a projection from the medial nucleus of the trapezoid body, MNTB).

Giant, TTX-insensitive, inhibitory postsynaptic currents in cultured rat spinal cord and medullary neurons.

1. In whole cell patch-clamp studies on cultured rat embryonic spinal cord and medullary neurons bathed in tetrodotoxin, DL-2-amino-5-phosphonovaleric acid, and 6-cyano-7-nitroquinoxaline-2,3-dione, large and long-lasting spontaneous inhibitory postsynaptic currents were occasionally recorded. The amplitudes of these events were 1 order of magnitude larger than those of spontaneous miniature inhibitory postsynaptic currents. Because these large currents had reduced amplitudes in calcium-free saline and in solutions containing glycinergic or GABAergic antagonists, we conclude that they were probably produced by large and prolonged release of glycine and/or 4-amino-n-butyric acid (GABA), which subsequently bind to their postsynaptic receptors. 2. The frequency of spontaneous miniature postsynaptic currents increased dramatically during the long, slow decay phase of these large postsynaptic currents. Considering the requirement for extracellular calcium for the occurrence of these large responses, we hypothesize that this increased frequency reflected an increased intracellular calcium concentration in the presynaptic terminal. 3. Similar evidence for large inhibitory postsynaptic currents and prolonged transmitter release was observed in cell-attached patches, which also exhibited the smaller, spontaneous miniature inhibitory postsynaptic currents, suggesting that these large events are properties of single synaptic terminals. 4. A comparison of the properties of these large inhibitory postsynaptic currents recorded in whole cell mode or cell-attached patches showed no statistically significant differences. The overall mean values, then, are 13.9 +/- 1.6 (SE) ms and 4.5 +/- 0.5 s for the 10-90% rise time and duration, respectively. Furthermore, these large events had amplitudes that were 11-fold larger than the mean amplitude of the miniatures (i.e., mean amplitude ratio of 10.8 +/- 0.5). 5. Periodic large increases in the frequency of spontaneous miniature inhibitory postsynaptic currents occurred in both cell-attached patches and in the whole cell mode, and these increases were only sometimes associated with the large inhibitory postsynaptic currents. The rhythmicity in both recording configurations had similar temporal characteristics, with average interburst intervals of 5 and 12-14 s. Presumably these bursts of spontaneous miniature postsynaptic currents reflected periodic oscillations in the Ca2+ concentration in presynaptic terminals. 6. Both the probability and the frequency of occurrence of large inhibitory postsynaptic currents doubled during the 7-day period of time in culture when experiments were performed, suggesting that these large currents may play a role during development.

Analysis of miniature spontaneous inhibitory postsynaptic currents (sIPSCs) from current noise in crayfish opener muscle

Deviating from the normal situation, some crayfish muscle fibres showed spontaneous inhibitory activity: discharge of large inhibitory postsynaptic currents, IPSCs, alternating with long lasting bursts of current noise. Analysis of the bursts of current noise revealed that they are composed of spontaneous miniature unit currents, sIPSCs. In the burst periods the sIPSCs occurred with an average rate of 3.5--10 k Hz and had an amplitude of about alpha = 90 pA at a driving force delta E = 10 mV. The peak conductance gamma alpha = alpha/delta E of the sIPSCs was gamma alpha = 9.2 nS +/- 0.5 (S.D., n = 5) for membrane potentials between E = --60 mV and E = --80 mV. gamma alpha seemed to decrease when the membrane was hyperpolarized. The time constants of decay, tau of the sIPSCs were identical with tau of the IPSCs. Further, tau and its potential dependence agreed with the mean lifetimes of inhibitory postsynaptic channels operated by gamma-aminobutyric acid (GABA) [Dudel et al. 1977, 1980]. Synchronized opening of about 750 inhibitory synaptic channels generates a sIPSC. Analysis of this anomalous bursting inhibitory activity thus yields the size of the inhibitory quantum of transmission, which could not be obtained from IPSCs.