M‐type channels selectively control bursting in rat dopaminergic neurons

Midbrain dopaminergic neurons in the substantia nigra, pars compacta and ventral tegmental area are critically important in many physiological functions. These neurons exhibit firing patterns that include tonic slow pacemaking, irregular firing and bursting, and the amount of dopamine that is present in the synaptic cleft is much increased during bursting. The mechanisms responsible for the switch between these spiking patterns remain unclear. Using both in‐vivo recordings combined with microiontophoretic or intraperitoneal drug applications and in‐vitro experiments, we have found that M‐type channels, which are present in midbrain dopaminergic cells, modulate the firing during bursting without affecting the background low‐frequency pacemaker firing. Thus, a selective blocker of these channels, 10,10‐bis(4‐pyridinylmethyl)‐9(10H)‐anthracenone dihydrochloride, specifically potentiated burst firing. Computer modeling of the dopamine neuron confirmed the possibility of a differential influence of M‐type channels on excitability during various firing patterns. Therefore, these channels may provide a novel target for the treatment of dopamine‐related diseases, including Parkinson’s disease and drug addiction. Moreover, our results demonstrate that the influence of M‐type channels on the excitability of these slow pacemaker neurons is conditional upon their firing pattern.     

Functional neuroanatomy of the human pre‐Bötzinger complex with particular reference to sudden unexplained perinatal and infant death

The authors are the first to identify in man the pre‐Bötzinger complex, a structure of the brainstem critical for respiratory rhythmogenesis, previously investigated only in rats. The evaluation of the neurokinin 1 receptors and somatostatin immunoreactivity in a total of 63 brains from 25 fetuses, nine newborns and 29 infants, allowed to delineate the anatomic structure and the boundaries of this human neural center in a restricted area of the ventrolateral medulla at the obex level, ventral to the semicompact ambiguus nucleus. The neurons of the pre‐Bötzinger complex were roundish in fetuses before 30 gestational weeks and lengthened after birth, embedded in a dendritic system belonging to the reticular formation. Besides, structural and/or functional alterations of the pre‐Bötzinger complex were present in a high percentage of sudden deaths (47%), prevalent in late fetal deaths. In particular, different developmental defects (hypoplasia with a decreased neuronal number and/or dendritic hypodevelopment of the reticular formation, abnormal neuronal morphology, immunonegativity of neurotransmitters, and agenesis) were found. The authors suggest that the pre‐Bötzinger complex contains a variety of neurons not only involved in respiratory rhythm generation, but more extensively, essential to the control of all vital functions. Sudden death and in particular sudden unexpected fetal death could therefore be ascribed to a selective process when developmental alterations of the pre‐Bötzinger complex arise.     

Two sides of the same coin: Sodium homeostasis and signaling in astrocytes under physiological and pathophysiological conditions

The intracellular sodium concentration of astrocytes is classically viewed as being kept under tight homeostatic control and at a relatively stable level under physiological conditions. Indeed, the steep inwardly directed electrochemical gradient for sodium, generated by the Na⁺/K⁺‐ATPase, contributes to maintain the electrochemical gradient of K⁺ and the highly K⁺‐based negative membrane potential, and is a central element in energizing membrane transport. As such it is tightly coupled to the homeostasis of extra‐ and intracellular potassium, calcium or pH and to the reuptake of transmitters such as glutamate. Recent studies, however, have demonstrated that this picture is far too simplistic. It is now firmly established that transmitters, most notably glutamate, and excitatory neuronal activity evoke long‐lasting sodium transients in astrocytes, the properties of which are distinctly different from those of activity‐related glial calcium signals. From these studies, it emerges that sodium homeostasis and signaling are two sides of the same coin: sodium‐dependent transporters, primarily known for their role in ion regulation and homeostasis, also generate relevant ion signals during neuronal activity. The functional consequences of activity‐related sodium transients are manifold and are just coming into view, enabling surprising and important new insights into astrocyte function and neuron‐glia interaction in the brain. The present review will highlight current knowledge about the mechanisms that contribute to sodium homeostasis in astrocytes, present recent data on the spatial and temporal properties of activity‐related glial sodium signals and discuss their functional consequences with a special emphasis on pathophysiological conditions. GLIA 2013;61:1191–1205     

Dopamine D1 receptor‐mediated upregulation of BKCa currents modifies Müller cell gliosis in a rat chronic ocular hypertension model

Müller cell gliosis is a common response in many retinal pathological conditions. We previously demonstrated that downregulation of Kir channels contributes to Müller cell gliosis in a rat chronic ocular hypertension (COH) model. Here, the possible involvement of outward K⁺ currents in Müller cell gliosis was investigated. Outward K⁺ current densities in Müller cells isolated from COH rats, as compared with those in normal rats, showed a significant increase, which was mainly contributed by large‐conductance Ca²⁺‐activated K⁺ (BK_Ca) channels. The involvement of BK_Ca channels in Müller cell gliosis is suggested by the fact that glial fibrillary acidic protein (GFAP) levels were augmented in COH retinas when these channels were suppressed by intravitreal injections of iberiotoxin. In COH retinas an increase in dopamine (DA) D1 receptor (D1R) expression in Müller cells was revealed by both immunohistochemistry and Western blotting. Moreover, protein levels of tyrosine hydroxylase were also increased, and consistent to this, retinal DA contents were elevated. SKF81297, a selective D1R agonist, enhanced BK_Ca currents of normal Müller cells through intracellular cAMP‐PKA signaling pathway. Furthermore, GFAP levels were increased by the D1R antagonist SCH23390 injected intravitreally through eliminating the BK_Ca current upregulation in COH retinas, but partially reduced by SKF81297. All these results strongly suggest that the DA‐D1R system may be activated to a stronger extent in COH rat retinas, thus increasing BK_Ca currents of Müller cells. The upregulation of BK_Ca channels may antagonize the Kir channel inhibition‐induced depolarization of Müller cells, thereby attenuating the gliosis of these cells.     

Differential contributions of Ca2+‐activated K+ channels and Na+/K+‐ATPases to the generation of the slow afterhyperpolarization in CA1 pyramidal cells

In many types of CNS neurons, repetitive spiking produces a slow afterhyperpolarization (sAHP), providing sustained, intrinsically generated negative feedback to neuronal excitation. Changes in the sAHP have been implicated in learning behaviors, in cognitive decline in aging, and in epileptogenesis. Despite its importance in brain function, the mechanisms generating the sAHP are still controversial. Here we have addressed the roles of M‐type K⁺ current (I_M), Ca²⁺‐gated K⁺ currents (I_Ca(K)'s) and Na⁺/K⁺‐ATPases (NKAs) current to sAHP generation in adult rat CA1 pyramidal cells maintained at near‐physiological temperature (35 °C). No evidence for I_M contribution to the sAHP was found in these neurons. Both I_Ca(K)'s and NKA current contributed to sAHP generation, the latter being the predominant generator of the sAHP, particularly when evoked with short trains of spikes. Of the different NKA isoenzymes, α₁‐NKA played the key role, endowing the sAHP a steep voltage‐dependence. Thus normal and pathological changes in α₁‐NKA expression or function may affect cognitive processes by modulating the inhibitory efficacy of the sAHP.     

Heterotrimeric guanosine triphosphate‐binding protein‐coupled modulatory actions of motilin on K+ channels and postsynaptic γ‐aminobutyric acid receptors in mouse medial vestibular nuclear neurons

Some central nervous system neurons express receptors of gastrointestinal hormones, but their pharmacological actions are not well known. Previous anatomical and unit recording studies suggest that a group of cerebellar Purkinje cells express motilin receptors, and motilin depresses the spike discharges of vestibular nuclear neurons that receive direct cerebellar inhibition in rats or rabbits. Here, by the slice‐patch recording method, we examined the pharmacological actions of motilin on the mouse medial vestibular nuclear neurons (MVNs), which play an important role in the control of ocular reflexes. A small number of MVNs, as well as cerebellar floccular Purkinje cells, were labeled with an anti‐motilin receptor antibody. Bath application of motilin (0.1 μm ) decreased the discharge frequency of spontaneous action potentials in a group of MVNs in a dose‐dependent manner (K_d, 0.03 μm ). The motilin action on spontaneous action potentials was blocked by apamin (100 nm ), a blocker of small‐conductance Ca²⁺‐activated K⁺ channels. Furthermore, motilin enhanced the amplitudes of inhibitory postsynaptic currents (IPSCs) and miniature IPSCs, but did not affect the frequencies of miniature IPSCs. Intracellular application of pertussis toxin (PTx) (0.5 μg/μL) or guanosine triphosphate‐γ‐S (1 mm ) depressed the motilin actions on both action potentials and IPSCs. Only 30% of MVNs examined on slices obtained from wild‐type mice, but none of the GABAergic MVNs that were studied on slices obtained from vesicular γ‐aminobutyric acid transporter‐Venus transgenic mice, showed such a motilin response on action potentials and IPSCs. These findings suggest that motilin could modulate small‐conductance Ca²⁺‐activated K⁺ channels and postsynaptic γ‐aminobutyric acid receptors through heterotrimeric guanosine triphosphate‐binding protein‐coupled receptor in a group of glutamatergic MVNs.     

Impaired learning and memory in rats induced by a high‐fat diet: Involvement with the imbalance of nesfatin‐1 abundance and copine 6 expression

Non‐alcoholic fatty liver disease (NAFLD) is the most common cause of chronic liver disease, resulting not only in liver dysfunction, glucose and lipid metabolism disorder, but also in neuropsychiatric damage. In the present study, a NAFLD rat model was established via feeding of a high‐fat diet, and behaviour was observed via the open field test (OFT), the sucrose preference test (SPT), the elevated plus maze (EPM), the forced swimming test (FST) and the Morris water maze (MWM). The plasma concentrations of alanine aminotransferase (ALT), glucose, free fatty acid (FFA), total cholesterol (TC), triglyceride (TG), high‐density lipoprotein cholesterol (HDL‐C) and low‐density lipoprotein cholesterol (LDL‐C) were detected using chemiluminescence technique. The plasma levels of nesfatin‐1, leptin and insulin were measured via enzyme‐linked immunosorbent assay, and the protein expressions of p‐glycogen synthase kinase‐3β (GSK‐3β), GSK‐3β, p‐β‐catenin, β‐catenin, cyclinD and copine 6 in the hippocampus and prefrontal cortex (PFC) were detected using western blotting. After 4 consecutive weeks of feeding with a high‐fat diet, the rats showed obesity; increased plasma concentrations of ALT, glucose, FFA, TC, TG, HDL‐C and LDL‐C; decreased plasma levels of leptin and insulin; and inflammation and mild hepatocyte steatosis in the liver. Although there was no significant difference between groups with regard to performance in the OFT, EPM or FST, the NAFLD rats showed a decreased sucrose preference index in the SPT and impaired learning and memory in the MWM task. Moreover, the present study provides the first evidence of an increased plasma nesfatin‐1 concentration in NAFLD rats, which was significantly correlated with plasma lipid concentrations and behavioural performance. Furthermore, copine 6 and p‐β‐catenin protein expression decreased and p‐GSK‐3β increased in the hippocampus and PFC of NAFLD rats. These results suggest that consuming of a high‐fat diet for 4 consecutive weeks could successfully induce a NAFLD rat model. More importantly, these results provide the first evidence that impaired learning and memory in NAFLD rats was, at least partly, associated with increased plasma nesfatin‐1 concentration and decreased copine 6 expression in the hippocampus and PFC.