Henry Sershen

Effect of nicotine on extracellular levels of neurotransmitters assessed by microdialysis in various brain regions : role of glutamic acid

We studied the effect of local administration of nicotine on the release of monoamines in striatum, substantia nigra, cerebellum, hippocampus, cortex (frontal, cingulate), and pontine nucleus and on the release of glutamic acid in striatum of rats in vivo, using microdialysis for nicotine administration and for measuring extracellular amine and glutamic acid levels. Following nicotine administration the extracellular concentration of dopamine, increased in all regions except cerebellum; serotonin increased in cingulate and frontal cortex; and norepinephrine increased in substantia nigra, cingulate cortex, and pontine nucleus. Cotinine, the major nicotine metabolite, had no effect at similar concentrations. The cholinergic antagonists mecamylamine and atropine, the dopaminergic antagonists haloperidol and sulpiride, and the excitatory amino acid antagonist kynurenic acid all inhibited the nicotine-induced increase of extracellular dopamine in the striatum. The fact that kynurenic acid almost completely prevented the effects of nicotine, and nicotine at this concentration produced a 6-fold increase of glutamic acid release, suggests that the effect of nicotine is mainly mediated via glutamic acid release.

Inhibition of System A-mediated glycine transport in cortical synaptosomes by therapeutic concentrations of clozapine: implications for mechanisms of action

Clozapine is an atypical antipsychotic with particular efficacy in schizophrenia, possibly related to potentiation of brain N-methyl-D-aspartate receptor (NMDAR) -mediated neurotransmission. NMDARs are regulated in vivo by glycine, which is regulated in turn by glycine transporters. The present study investigates transport processes regulating glycine uptake into rat brain synaptosomes, along with effects of clozapine on synaptosomal glycine transport. Amino-acid uptake of amino acids was assessed in rat brain P2 synaptosomal preparations using a radiotransport assay. Synaptosomal glycine transport was inhibited by a series of amino acids and by the selective System A antagonist MeAIB (2-methyl-aminoisobutyric acid). Clozapine inhibited transport of both glycine and MeAIB, but not other amino acids, at concentrations associated with preferential clinical response (0.5–1 μg/ml). By contrast, other antipsychotics studied were ineffective. The novel glycine transport inhibitor N[3-(4′-fluorophenyl)-3-(4′-phenylphenoxy)propyl]sarcosine (NFPS) produced biphasic inhibition of [3H]glycine transport, with IC50 values of approximately 25 nM and 25 μM, respectively. NFPS inhibition of [3H]MeAIB was monophasic with a single IC50 value of 31 μM. Clozapine significantly inhibited [3H]glycine binding even in the presence of 100 nM NFPS. In conclusion, this study suggests first that System A transporters, or a subset thereof, may play a critical role in regulation of synaptic glycine levels and by extension of NMDA receptor regulation, and second that System A antagonism may contribute to the differential clinical efficacy of clozapine compared with other typical or atypical antipsychotics.

Nicotine-induced changes in neurotransmitter levels in brain areas associated with cognitive function.

Nicotine, one of the most widespread drugs of abuse, has long been shown to impact areas of the brain involved in addiction and reward. Recent research, however, has begun to explore the positive effects that nicotine may have on learning and memory. The mechanisms by which nicotine interacts with areas of cognitive function are relatively unknown. Therefore, this paper is part of an ongoing study to evaluate regional effects of nicotine enhancement of cognitive function. Nicotine-induced changes in the levels of three neurotransmitters, dopamine (DA), serotonin (5-HT), norepinepherine (NE), their metabolites, homovanillic acid (HVA), dihydroxyphenylacetic acid (DOPAC), 5-hydroxyindoleacetic acid (5-HIAA), and their precursor, l-DOPA, were evaluated in the ventral and dorsal hippocampus (VH and DH), prefrontal and medial temporal cortex (PFC and MTC), and the ventral tegmental area (VTA) using in vivo microdialysis in awake, freely moving, male Sprague-Dawley rats. The animals were treated with acute nicotine (0.5 mg/kg, s.c.) halfway through the 300-min experimental period. The reuptake blockers, desipramine (100 μM) and fluoxetine (30 μM), were given to increase the levels of NE and 5-HT so that they could be detected. Overall, a nicotine-induced DA increase was found in some areas, and this increase was potentiated by desipramine and fluoxetine. The two DA metabolites, HVA and DOPAC, increased in all the areas throughout the experiments, both with and without the inhibitors, indicating a rapid metabolism of the released DA. The increase in these metabolites was greater than the increase in DA. 5-HT was increased in the DH, MTC, and VTA in the presence of fluoxetine; its metabolite, 5-HIAA, was increased in the presence and absence of fluoxetine. Except in the VTA, NE levels increased to a similar extent with desipramine and fluoxetine. Overall, nicotine appeared to increase the release and turnover of these three neurotransmitters, which was indicated by significant increases in their metabolites. Furthermore, DA, and especially HVA and DOPAC, increased for the 150 min following nicotine administration; 5-HT and NE changes were shorter in duration. As gas chromatography experiments showed that nicotine levels in the brain decreased by 75% after 150 min, this may indicate that DA is more susceptible to lower levels of nicotine than 5-HT or NE. In conclusion, acute nicotine administration caused alterations in the levels of DA, 5-HT, and NE, and in the metabolism of DA and 5-HT, in brain areas that are involved in cognitive processes.

Reversal of Phencyclidine-Induced Hyperactivity by Glycine and the Glycine Uptake Inhibitor Glycyldodecylamide

Phencyclidine (PCP) induces a psychotic state that closely resembles schizophrenia. In preclinical studies, PCP has been shown to induce its unique behavioral effects by blocking excitatory neurotransmission mediated at the N-methyl-D-asparate (NMDA) receptors, suggesting that agents which potentiate NMDA receptor-mediated neurotransmission might have clinically beneficial effects. The present study demonstrates that the NMDA co-agonist glycine inhibits rodent hyperactivity induced by PCP, but not amphetamine. Glycyldodecylamide, a compound that blocks neuronal glycine uptake and which may therefore increase intrasynaptic glycine levels, inhibits PCP-induced hyperactivity more potently than glycine. These results complement recent clinical studies with glycine and suggest that glycine-uptake inhibitors, as well as glycine, may be beneficial in the treatment of PCP-induced psychosis and schizophrenia.

Changes in cerebral neurotransmitters and metabolites induced by acute donepezil and memantine administrations: a microdialysis study.

Cholinesterase inhibitors including donepezil, rivastigmine, and galantamine and the N-methyl-D-aspartate (NMDA) antagonist, memantine are the medications currently approved for the treatment of Alzheimers disease (AD). In addition to their beneficial effects on cognitive and functional domains typically disrupted in AD, these agents have also been shown to slow down the emergence of behavioral and psychotic symptoms associated with this disease. However, the underlying mechanisms for these therapeutic effects remain poorly understood and could involve effects of these medications on non-cholinergic or non-glutamatergic neurotransmitter systems respectively. These considerations prompted us to initiate a series of investigations to examine the acute and chronic effects of donepezil (Aricept (+/-)-2,3-dihydro-5,6-dimethoxy-2-[[1-(phenylmethyl)-4-piperidinyl]methyl]-1H-inden-1-1 hydrochloride and memantine (1-amino-3,5-dimethyladamantane hydrochloride C12H21N.HCl)). The present study focuses on the acute effects of donepezil and memantine on brain extracellular levels of acetylcholine, dopamine, serotonin, norepinephrine and their metabolites. We assayed changes in the ventral and dorsal hippocampus and the prefrontal and medial temporal cortex by microdialysis. Memantine resulted in significant increases in extracellular dopamine (DA), norepinephrine (NE), and their metabolites, in the cortical regions, and in a reduction of DA in the hippocampus. Donepezil produced an increase in extracellular DA in the cortex and in the dorsal hippocampus. Norepinephrine increased in the cortex; with donepezil it increased in the dorsal hippocampus and the medial temporal cortex, and decreased in the ventral hippocampus. Interestingly both compounds decreased extracellular serotonin (5HT) levels. The metabolites of the neurotransmitters were increased in most areas. We also found an increase in extracellular acetylcholine (ACh) by memantine in the nucleus accumbens and the ventral tegmental area. Our results suggest both region and drug specific neurotransmitter effects of these agents as well as some similarities. We conclude that drugs influencing cognitive mechanisms induce changes in a number of neurotransmitters with the changes being both region and drug specific. Release and metabolism are altered and extracellular neurotransmitter levels can be increased or decreased by the drugs. Other studies are in progress to determine the pharmacological effects associated with chronic treatment with these compounds, which may be more pertinent to the clinical situation in which patients take these medications for months or years.

Continuous Phencyclidine Treatment Induces Schizophrenia-Like Hyperreactivity of Striatal Dopamine Release

Functional dopaminergic hyperactivity is a key feature of schizophrenia. Recent in vivo imaging studies have demonstrated greater striatal dopamine release in response to amphetamine challenge in schizophrenia subjects than in normal controls. N-methyl-D-aspartate (NMDA) receptors are known to play a prominent role in regulation of striatal dopamine release. In humans, NMDA antagonists induce a psychotic state that closely resembles schizophrenia. The present study investigates the degree to which chronic continuous administration of the NMDA antagonist phencyclidine (PCP) induces schizophrenia-like hyperreactivity of striatal dopamine release to amphetamine in rodents. Rats were treated with 10 or 15 mg/kg/d PCP for two weeks by osmotic minipump, and striatal dopamine release to amphetamine challenge (1 mg/kg) was monitored by microdialysis. PCP-treated rats showed significant enhancement in amphetamine-induced dopamine release, along with significantly enhanced locomotor activity. These findings support the concept that NMDA receptor dysfunction may contribute to dopaminergic dysfunction in schizophrenia.

Specific Calpain Inhibition by Calpastatin Prevents Tauopathy and Neurodegeneration and Restores Normal Lifespan in Tau P301L Mice

Tau pathogenicity in Alzheimers disease and other tauopathies is thought to involve the generation of hyperphosphorylated, truncated, and oligomeric tau species with enhanced neurotoxicity, although the generative mechanisms and the implications for disease therapy are not well understood. Here, we report a striking rescue from mutant tau toxicity in the JNPL3 mouse model of tauopathy. We show that pathological activation of calpains gives rise to a range of potentially toxic forms of tau, directly, and by activating cdk5. Calpain overactivation in brains of these mice is accelerated as a result of the marked depletion of the endogenous calpain inhibitor, calpastatin. When levels of this inhibitor are restored in neurons of JNPL3 mice by overexpressing calpastatin, tauopathy is prevented, including calpain-mediated breakdown of cytoskeletal proteins, cdk5 activation, tau hyperphosphorylation, formation of potentially neurotoxic tau fragments by either calpain or caspase-3, and tau oligomerization. Calpastatin overexpression also prevents loss of motor axons, delays disease onset, and extends survival of JNPL3 mice by 3 months to within the range of normal lifespan. Our findings support the therapeutic promise of highly specific calpain inhibition in the treatment of tauopathies and other neurodegenerative states.

Food Reward-Induced Neurotransmitter Changes in Cognitive Brain Regions

Recent evidence indicates that mechanisms involved in reward and mechanisms involved in learning interact, in that reward includes learning processes and learning includes reward processes. In spite of such interactions, reward and learning represent distinct functions. In the present study, as part of an examination of the differences in learning and reward mechanisms, it was assumed that food principally affects reward mechanisms. After a brief period of fasting, we assayed the release of three neurotransmitters and their associated metabolites in eight brain areas associated with learning and memory as a response to feeding. Using microdialysis for the assay, we found changes in the hippocampus, cortex, amygdala, and the thalamic nucleus, (considered cognitive areas), in addition to those in the nucleus accumbens and ventral tegmental area (considered reward areas). Extracellular dopamine levels increased in the nucleus accumbens, ventral tegmental area, amygdala, and thalamic nucleus, while they decreased in the hippocampus and prefrontal cortex. Dopamine metabolites increased in all areas tested (except the dorsal hippocampus); changes in norepinephrine varied with decreases in the accumbens, dorsal hippocampus, amygdala, and thalamic nucleus, and increases in the prefrontal cortex; serotonin levels decreased in all the areas tested; although its metabolite 5HIAA increased in two regions (the medial temporal cortex, and thalamic nucleus). Our assays indicate that in reward activities such as feeding, in addition to areas usually associated with reward such as the mesolimbic dopamine system, other areas associated with cognition also participate. Results also indicate that several transmitter systems play a part, with several neurotransmitters and several receptors involved in the response to food in a number of brain structures, and the changes in transmitter levels may be affected by metabolism and transport in addition to changes in release in a regionally heterogeneous manner. Food reward represents a complex pattern of changes in the brain that involve cognitive processes. Although food reward elements overlap with other reward systems sharing some neurotransmitter compounds, it significantly differs indicating a specific reward to process for food consumption. Like in other rewards, both learning and cognitive areas play a significant part in food reward.

Nicotine: Alcohol Reward Interactions

It is well established that the continued intake of drugs of abuse is reinforcing—that is repeated consumption increases preference. This has been shown in some studies to extend to other drugs of abuse; use of one increases preference for another. In particular, the present review deals with the interaction of nicotine and alcohol as it has been shown that smoking is a risk factor for alcoholism and alcohol use is a risk factor to become a smoker. The review discusses changes in the brain caused by chronic nicotine and chronic alcohol intake to approach the possible mechanisms by which one drug increases the preference for another. Chronic nicotine administration was shown to affect nicotine receptors in the brain, affecting not only receptor levels and distribution, but also receptor subunit composition, thus affecting affinity to nicotine. Other receptor systems are also affected among others catecholamine, glutamate, GABA levels and opiate and cannabinoid receptors. In addition to receptor systems and transmitters, there are endocrine, metabolic and neuropeptide changes as well induced by nicotine. Similarly chronic alcohol intake results in changes in the brain, in multiple receptors, transmitters and peptides as discussed in this overview and also illustrated in the tables. The changes are sex and age-dependent—some changes in males are different from those in females and in general adolescents are more sensitive to drug effects than adults. Although nicotine and alcohol interact—not all the changes induced by the combined intake of both are additive—some are opposing. These opposing effects include those on locomotion, acetylcholine metabolism, nicotine binding, opiate peptides, glutamate transporters and endocannabinoid content among others. The two compounds lower the negative withdrawal symptoms of each other which may contribute to the increase in preference, but the mechanism by which preference increases—most likely consists of multiple components that are not clear at the present time. As the details of induced changes of nicotine and alcohol differ, it is likely that the mechanisms of increasing nicotine preference may not be identical to that of increasing alcohol preference. Stimulation of preference of yet other drugs may again be different –representing one aspect of drug specificity of reward mechanisms.

A novel alanine-insensitive D-serine transporter in rat brain synaptosomal membranes.

D-Serine is an endogenous modulator of brain N-methyl-D-aspartate receptors. This study investigates serine transport in brain synaptosomal fractions. Saturable, temperature-dependent uptake of both D- and L-[3H]serine was observed. Alanine was only partially effective in blocking transport, arguing against involvement of system ASC. Inhibitors of the systems A, L and GLY were also ineffective. Saturation studies suggested a submillimolar K(m) for both D- and L-serine. These data suggest the presence of a novel serine transport system in rodent synaptosomes.