Different regional neuroinhibitory effects of adenosine on stimulus-induced patterns of bioelectric activity of rat hippocampal and neocortical tissues

Drug resistance in cortical and hippocampal slices from resected tissue of epilepsy patients: no significant impact of p-glycoprotein and multidrug resistance-associated proteins

Drug resistant patients undergoing epilepsy surgery have a good chance to become sensitive to anticonvulsant medication, suggesting that the resected brain tissue is responsible for drug resistance. Here, we address the question whether P-glycoprotein (Pgp) and multidrug resistance-associated proteins (MRPs) expressed in the resected tissue contribute to drug resistance in vitro. Effects of anti-epileptic drugs [carbamazepine (CBZ), sodium valproate, phenytoin] and two unspecific inhibitors of Pgp and MRPs [verapamil (VPM) and probenecid (PBN)] on seizure-like events (SLEs) induced in slices from 35 hippocampal and 35 temporal cortex specimens of altogether 51 patients (161 slices) were studied. Although in slice preparations the blood brain barrier is not functional, we found that SLEs predominantly persisted in the presence of anticonvulsant drugs (90%) and also in the presence of VPM and PBN (86%). Following subsequent co-administration of anti-epileptic drugs and drug transport inhibitors, SLEs continued in 63% of 143 slices. Drug sensitivity in slices was recognized either as transition to recurrent epileptiform transients (30%) or as suppression (7%), particularly by perfusion with CBZ in PBN containing solutions (43, 9%). Summarizing responses to co-administration from more than one slice per patient revealed that suppression of seizure-like activity in all slices was only observed in 7% of patients. Patients whose tissue was completely or partially sensitive (65%) presented with higher seizure frequencies than those with resistant tissue (35%). However, corresponding subgroups of patients do not differ with respect to expression rates of drug transporters. Our results imply that parenchymal MRPs and Pgp are not responsible for drug resistance in resected tissue.

Modulation of synaptic transmission by adenosine in layer 2/3 of the rat visual cortex in vitro

Adenosine is a wide-spread endogenous neuromodulator. In the central nervous system it activates A1 and A2A receptors (A1Rs and A2ARs) which have differential distributions, different affinities to adenosine, are coupled to different G-proteins, and have opposite effects on synaptic transmission. Although effects of adenosine are studied in detail in several brain areas, such as the hippocampus and striatum, the heterogeneity of the effects of A1R and A2AR activation and their differential distribution preclude generalization over brain areas and cell types. Here we study adenosine's effects on excitatory synaptic transmission to layer 2/3 pyramidal neurons in slices of the rat visual cortex. We measured effects of bath application of adenosine receptor ligands on evoked excitatory postsynaptic potentials (EPSPs), miniature excitatory postsynaptic potentials (mEPSPs), and membrane properties. Adenosine reduced the amplitude of evoked EPSPs and excitatory postsynaptic currents (EPSCs), and reduced frequency of mEPSPs in a concentration-dependent and reversible manner. Concurrent with EPSP/C amplitude reduction was an increase in the paired-pulse ratio. These effects were blocked by application of the selective A1R antagonist DPCPX (8-cyclopentyl-1,3-dipropylxanthine), suggesting that activation of presynaptic A1Rs suppresses excitatory transmission by reducing release probability. Adenosine (20μM) hyperpolarized the cell membrane from -65.3±1.5 to -67.7±1.8mV, and reduced input resistance from 396.5±44.4 to 314.0±36.3MOhm (∼20%). These effects were also abolished by DPCPX, suggesting postsynaptic A1Rs. Application of the selective A2AR antagonist SCH-58261 (2-(2-furanyl)-7-(2-phenylethyl)-7H-pyrazolo[4,3-e][1,2,4]triazolo[1,5-c]pyrimidin-5-a-mine) on the background of high adenosine concentrations revealed an additional decrease in EPSP amplitude. Moreover, application of the A2AR agonist CGS-21680 (4-[2-[[6-amino-9-(N-ethyl-β-d-ribofuranuronamidosyl)-9H-purin-2-yl]amino]ethyl]benzenepropanoic acid hydrochloride) led to an A1R-dependent increase in mEPSP frequency. Dependence of the A2AR effects on the A1R availability suggests interaction between these receptors, whereby A2ARs exert their facilitatory effect on synaptic transmission by inhibiting the A1R-mediated suppression. Our results demonstrate functional pre and postsynaptic A1Rs and presynaptic A2ARs in layer 2/3 of the visual cortex, and suggest interaction between presynaptic A2ARs and A1Rs.

A₁ adenosine receptor modulation of chemically and electrically evoked lumbar locomotor network activity in isolated newborn rat spinal cords

It is not well-studied how the ubiquitous neuromodulator adenosine (ADO) affects mammalian locomotor network activities. We analyzed this here with focus on roles of 8-cyclopentyl-1,3-dipropylxanthine (DPCPX)-sensitive A(1)-type ADO receptors. For this, we recorded field potentials from ventral lumbar nerve roots and electrically stimulated dorsal roots in isolated newborn rat spinal cords. At ≥ 25μM, bath-applied ADO slowed synchronous bursting upon blockade of anion-channel-mediated synaptic inhibition by bicuculline (20 μM) plus strychnine (1 μM) and this depression was countered by DPCPX (1 μM) as tested at 100 μM ADO. ADO abolished this disinhibited rhythm at ≥ 500 μM. Contrary, the single electrical pulse-evoked dorsal root reflex, which was enhanced in bicuculline/strychnine-containing solution, persisted at all ADO doses (5 μM-2 mM). In control solution, ≥ 500 μM ADO depressed this reflex and pulse train-evoked bouts of alternating fictive locomotion; this inhibition was reversed by 1 μM DPCPX. ADO (5 μM-2 mM) did not depress, but stabilize alternating fictive locomotion evoked by serotonin (10 μM) plus N-methyl-d-aspartate (4-5 μM). Addition of DPCPX (1μM) to control solution did not change either the dorsal root reflex or rhythmic activities indicating lack of endogenous A(1) receptor activity. Our findings show A(1) receptor involvement in ADO depression of the dorsal root reflex, electrically evoked fictive locomotion and spontaneous disinhibited lumbar motor bursting. Contrary, chemically evoked fictive locomotion and the enhanced dorsal root reflex in disinhibited lumbar locomotor networks are resistant to ADO. Because ADO effects in standard solution occurred at doses that are notably higher than those occurring in vivo, we hypothesize that newborn rat locomotor networks are rather insensitive to this neuromodulator.

Network hyperexcitability in hippocampal slices from Mecp2 mutant mice revealed by voltage-sensitive dye imaging

Dysfunctions of neuronal and network excitability have emerged as common features in disorders associated with intellectual disabilities, autism, and seizure activity, all common clinical manifestations of Rett syndrome (RTT), a neurodevelopmental disorder caused by loss-of-function mutations in the transcriptional regulator methyl-CpG-binding protein 2 (MeCP2). Here, we evaluated the consequences of Mecp2 mutation on hippocampal network excitability, as well as synapse structure and function using a combination of imaging and electrophysiological approaches in acute slices. Imaging the amplitude and spatiotemporal spread of neuronal depolarizations with voltage-sensitive dyes (VSD) revealed that the CA1 and CA3 regions of hippocampal slices from symptomatic male Mecp2 mutant mice are highly hyperexcitable. However, only the density of docked synaptic vesicles and the rate of release from the readily releasable pool are impaired in Mecp2 mutant mice, while synapse density and morphology are unaffected. The differences in network excitability were not observed in surgically isolated CA1 minislices, and blockade of GABAergic inhibition enhanced VSD signals to the same extent in Mecp2 mutant and wild-type mice, suggesting that network excitability originates in area CA3. Indeed, extracellular multiunit recordings revealed a higher level of spontaneous firing of CA3 pyramidal neurons in slices from symptomatic Mecp2 mutant mice. The neuromodulator adenosine reduced the amplitude and spatiotemporal spread of VSD signals evoked in CA1 of Mecp2 mutant slices to wild-type levels, suggesting its potential use as an anticonvulsant in RTT individuals. The present results suggest that hyperactive CA3 pyramidal neurons contribute to hippocampal dysfunction and possibly to limbic seizures observed in Mecp2 mutant mice and RTT individuals.