Thomas J. Feuerstein

Gabapentin increases the hyperpolarization-activated cation current Ih in rat CA1 pyramidal cells.

Summary:  Purpose: Gabapentin (GBP) is a commonly used drug in the treatment of partial seizures, but its mode of action is still unclear. The genesis of seizures in temporal lobe epilepsy is thought to be crucially influenced by intrinsic membrane properties. Because the Ih substantially contributes to the intrinsic membrane properties of neurons, the effects of GBP on the Ih were investigated in CA1 pyramidal cells of rat hippocampus.

Expression of NMDA receptor subunit mRNAs in neurochemically identified projection and interneurons in the human striatum.

N‐methyl‐D‐aspartate (NMDA) receptors are composed of subunits from two families: NR1 and NR2. We used a dual‐label in situ hybridization technique to assess the levels of NR1 and NR2A‐D messenger ribonucleic acid (mRNA) expressed in projection neurons and interneurons of the human striatum. The neuronal populations were identified with digoxigenin‐tagged complementary RNA probes for preproenkephalin (ENK) and substance P (SP) targeted to striatal projection neurons, and somatostatin (SOM), glutamic acid decarboxylase 67 kD (GAD67), and choline acetyltransferase (ChAT) targeted to striatal interneurons. Intense NR1 signals were found over all striatal neurons. NR2A signals were high over GAD67‐positive neurons and intermediate over SP‐positive neurons. ENK‐positive neurons displayed low NR2A signals, whereas ChAT‐ and SOM‐positive neurons were unlabeled. NR2B signals were intense over all neuronal populations in striatum. Signals for NR2C and NR2D were weak. Only ChAT‐positive neurons displayed moderate signals, whereas all other interneurons and projection neurons were unlabeled. Moderate amounts of NR2D signal were detected over SOM‐ and ChAT‐positive neurons; GAD67‐ and SP‐positive striatal neurons displayed low and ENK‐positive neurons displayed no NR2D hybridization signal. These data suggest that all human striatal neurons have NMDA receptors, but different populations have different subunit compositions that may affect function as well as selective vulnerability. J. Comp. Neurol. 419:407–421, 2000.

Serotonin (5-HT) enhances hippocampal noradrenaline (NA) release: Evidence for facilitatory 5-HT receptors within the CNS

SummarySlices of rabbit hippocampus were preincubated with 3H-noradrenaline (3H-NA), then superfused continuously in the presence of the noradrenaline (NA) uptake inhibitor (+)oxaprotilin and twice stimulated electrically. The stimulation induced tritium overflow was increased by the 5-HT receptor agonists, 5-HT, 2-methyl-5-HT and 5-carboxamidotryptamine in a concentration dependent manner; a tyramine-like displacement of NA by the 5-HT agonists was prevented by (+)oxaprotilin. The 5-HT M-receptor antagonists, MDL 72222 and ICS 205-930, inhibited the facilitatory effects of 5-HT agonists as well as the enhanced tritium overflow due to the selective 5-HT uptake inhibitor, 6-nitroquipazine: in each case, concentrations much higher than those required to block M-receptors of the periphery were necessary. At high concentrations MDL 72222, in contrast to ICS 205-930, seems to have α-adrenoceptor antagonistic activity. The 5-HT2 receptor antagonist, ketanserin, had no effect on 5-HT-induced facilitation of transmitter release; metitepin facilitated stimulationevoked transmitter release per se both in the absence and presence of phentolamine.From our results we conclude that, as on peripheral nerve endings, also on central noradrenergic terminals, facilitatory 5-HT receptors are present that modulate NA release. The enhanced tritium overflow following 6-nitroquipazine may be due to an increased release of endogenous 5-HT, a suggestion which supports the hypothesis of a physiological innervation of these facilitatory 5-HT receptors on NA terminals.

Islet-activating protein (pertussis toxin) diminishes ?2-adrenoceptor mediated effects on noradrenaline release

SummaryThe effect of islet-activating protein (IAP) on α2-adrenoceptor mediated modulation of noradrenaline release in the rabbit hippocampus was studied. Slices of the hippocampus were incubated for 6 h with IAP, subsequently loaded with3H-noradrenaline and superfused continuously. IAP-pretreatment significantly enhanced the electrically evoked transmitter release and diminished the facilitatory effect of the α2-adrenoceptor antagonist yohimbine. In addition, the inhibitory effect of the α2-adrenoceptor agonist clonidine was reduced. These results provide circumstantial evidence that an inhibitory guanine-nucleotide-binding protein, most probably N1 of a presynaptically located adenylate cyclase, is involved in the α2-autoreceptor mediated modulation of noradrenaline release.

The Endocannabinoid Anandamide Is a Substrate for the Human Polymorphic Cytochrome P450 2D6

Members of the cytochrome P450 (P450) family of drug-metabolizing enzymes are present in the human brain, and they may have important roles in the oxidation of endogenous substrates. The polymorphic CYP2D6 is one of the major brain P450 isoforms and has been implicated in neurodegeneration, psychosis, schizophrenia, and personality traits. The objective of this study was to determine whether the endocannabinoid arachidonoylethanolamide (anandamide) is a substrate for CYP2D6. Anandamide is the endogenous ligand to the cannabinoid receptor CB1, which is also activated by the main psychoactive component in marijuana. Signaling via the CB1 receptor alters sensory and motor function, cognition, and emotion. Recombinant CYP2D6 converted anandamide to 20-hydroxyeicosatetraenoic acid ethanolamide and 5,6-, 8,9-, 11,12-, and 14,15-epoxyeicosatrienoic acid ethanolamides (EET-EAs) with low micromolar Km values. CYP2D6 further metabolized the epoxides of anandamide to form novel dioxygenated derivatives. Human brain microsomal and mitochondrial preparations metabolized anandamide to form hydroxylated and epoxygenated products, respectively. An inhibitory antibody against CYP2D6 significantly decreased the mitochondrial formation of the EET-EAs. To our knowledge, anandamide and its epoxides are the first eicosanoid-like molecules to be identified as CYP2D6 substrates. Our study suggests that anandamide may be a physiological substrate for brain mitochondrial CYP2D6, implicating this polymorphic enzyme as a potential component of the endocannabinoid system in the brain. This study also offers support to the hypothesis that neuropsychiatric phenotype differences among individuals with genetic variations in CYP2D6 could be ascribable to interactions of this enzyme with endogenous substrates.

Myoclonus in Epilepsy Patients with Anticonvulsive Add-On Therapy with Pregabalin

Summary:  Purpose: To report on the occurrence of myoclonus in patients receiving pregabalin (PGB) for the treatment of focal epilepsy.

Effects of riluzole on electrically evoked neurotransmitter release.

The main purpose of the present study was to investigate the effects of the neuroprotective agent riluzole on the electrically evoked release of [3H]‐glutamate ([3H]‐Glu) in mouse neocortical slices. The reported selectivity of riluzole for excitatory amino acids was tested in release experiments with further neurotransmitters. Also distinct species, mouse, rat and man were compared. [3H]‐Glu was formed endogenously during incubation of slices with [3H]‐glutamine ([3H]‐Gln). Released [3H]‐Glu and tissue [3H]‐Glu was separated by anion exchange chromatography. Electrically evoked [3H]‐Glu release was strongly diminished by tetrodotoxin (TTX) and Ca2+‐withdrawal. Riluzole (100 μM) depressed the release of [3H]‐Glu up to 77% (IC50=19.5 μM). Riluzole was also able to inhibit strongly the electrically evoked release of [3H]‐acetylcholine ([3H]‐ACh) (at 100 μM by 92%, IC50=3.3 μM, and [3H]‐dopamine ([3H]‐DA) (at 32 μM by 72%, IC50=6.8 μM). However, the release of [3H]‐serotonin ([3H]‐5‐HT) was less diminished (at 100 μM by 53%, IC50=39.8 μM). Riluzole up to 100 μM did not affect [3H]‐noradrenaline ([3H]‐NA) release. Between species, i.e. in mouse, rat and human neocortex, no significant differences between the effects of riluzole could be observed. The NMDA‐receptor blocker MK‐801 (1 μM) and the AMPA/Kainate‐receptor blocker NBQX (1 μM) did neither affect the electrically evoked [3H]‐ACh release nor its inhibition by riluzole, indicating that effects of riluzole on transmitter release were neither due to modulation of ionotropic Glu receptors, nor due to indirect inhibition of Glu release through these receptors. Taken together, riluzole inhibits the release of distinct neurotransmitters differently, but is not selective for the excitatory amino acid Glu.

Regulated expression of HCN channels and cAMP levels shape the properties of the h current in developing rat hippocampus

The hyperpolarization‐activated current (Ih) contributes to intrinsic properties and network responses of neurons. Its biophysical properties depend on the expression profiles of the underlying hyperpolarization‐activated, cyclic nucleotide‐gated (HCN) channels and the presence of cyclic AMP (cAMP) that potently and differentially modulates Ih conducted by HCN1, HCN2 and/or HCN4. Here, we studied the properties of Ih in hippocampal CA1 pyramidal cells, the developmental evolution of the HCN‐subunit isoforms that contribute to this current, and their interplay with age‐dependent free cAMP concentrations, using electrophysiological, molecular and biochemical methods. Ih amplitude increased progressively during the first four postnatal weeks, consistent with the observed overall increased expression of HCN channels. Activation kinetics of the current accelerated during this period, consonant with the quantitative reduction of mRNA and protein expression of the slow‐kinetics HCN4 isoform and increased levels of HCN1. The sensitivity of Ih to cAMP, and the contribution of the slow component to the overall Ih, decreased with age. These are likely a result of the developmentally regulated transition of the complement of HCN channel isoforms from cAMP sensitive to relatively cAMP insensitive. Thus, although hippocampal cAMP concentrations increased over twofold during the developmental period studied, the coordinated changes in expression of three HCN channel isoforms resulted in reduced effects of this signalling molecule on neuronal h currents.

The serotonin (5-HT) autoreceptor in the hippocampus of the rabbit: Role of 5-HT biophase concentration

Slices of hippocampus from the rabbit were preincubated with [3H]5-HT), then superfused continuously and twice stimulated electrically. The stimulation-evoked overflow of tritium was inhibited by the 5-HT autoreceptor ligands 5-carboxamido-tryptamine (5-COHT), 5-HT, 5-methoxy-N,N-dimethyl-tryptamine (5-MeOMT), (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), methysergide and (+/-)-cyanopindolol in a concentration-dependent manner. These effects were competitively inhibited by the 5-HT autoreceptor antagonists, metitepin and metergoline. (+/-)-Cyanopindolol also reduced the evoked release of 5-HT from slices of cortex from the rat. The inhibitor of the uptake of 5-HT, 6-nitroquipazine diminished the autoreceptor-mediated depression of release of 5-HT. In cortex tissue from the rat, 6-nitroquipazine reversed the decreased release of 5-HT, due to (+/-)-cyanopindolol, to a facilitation. The disinhibition of the release of 5-HT by autoreceptor antagonists was further enhanced by 6-nitroquipazine. Non-linear regression analysis of concentration-response curves for 5-COHT yielded the following pKd of endogenous 5-HT at the autoreceptor: 7.753 +/- 0.116. This value corresponds to the pKd of 5-HT at the 5-HT1B binding site. The 5-HT biophase concentration at the autoreceptor of 10(-8.220 +/- 0.132)M was markedly enhanced by 6-nitroquipazine (10(-6)M) to 10(-7.476 +/- 0.132)M. It is concluded that the 5-HT autoreceptor belongs to the 5-HT1B subtype of receptor; the corresponding 5-HT biophase concentration can be estimated quantitatively; 8-OH-DPAT decreased the evoked release of 5-HT through both 5-HT autoreceptors and alpha 2-heteroreceptors and (+/-)-cyanopindolol acts as partial agonist at the 5-HT autoreceptor.

Presynaptic receptors for dopamine, histamine, and serotonin.

Presynaptic receptors for dopamine, histamine and serotonin that are located on dopaminergic, histaminergic and sertonergic axon terminals, respectively, function as autoreceptors. Presynaptic receptors also occur as heteroreceptors on other axon terminals. Auto- and heteroreceptors mainly affect Ca(2+) -dependent exocytosis from the receptor-bearing nerve ending. Some additionally subserve other presynaptic functions.Presynaptic dopamine, histamine and serotonin receptors are involved in various (patho)physiological conditions. Examples are the following:Dopamine autoreceptors play a role in Parkinsons disease, schizophrenia and drug addiction. Dopamine heteroreceptors affecting the release of acetylcholine and of amino acid neurotransmitters in the basal ganglia are also relevant for Parkinsons disease. Peripheral dopamine heteroreceptors on postganglionic sympathetic terminals influence heart rate and vascular resistance through modulation of noradrenaline release. Blockade of histamine autoreceptors increases histamine synthesis and release and may support higher CNS functions such as arousal, cognition and learning. Peripheral histamine heteroreceptors on C fiber and on postganglionic sympathetic fiber terminals diminish neuropeptide and noradrenaline release, respectively. Both inhibititory effects are beneficial in myocardial ischemia. The inhibition of neuropeptide release also explains the antimigraine effects of some agonists of presynaptic histamine receptors. Upregulation of presynaptic serotonin autoreceptors is probably involved in the pathogenesis of major depression. Correspondingly, antidepressant treatments can be linked with a reduced density of 5-HT autoreceptors. 5-HT Heteroreceptor activation diminishes acetylcholine and GABA release and may therefore increase anxiety. In the periphery, presynaptic 5-HT heteroreceptor agonists shorten migraine attacks by inhibition of the release of neuropeptides from trigeminal afferents, apart from their constrictive action on meningeal vessels.