Acetylcholinesterase inhibitors block acetylcholine-evoked release of dopamine in rat striatum, in vivo

Relationships and Interactions between Ionotropic Glutamate Receptors and Nicotinic Receptors in the CNS

Although ionotropic glutamate receptors and nicotinic receptors for acetylcholine (ACh) have usually been studied separately, they are often co-localized and functionally inter-dependent. The objective of this review is to survey the evidence for interactions between the two receptor families and the mechanisms underlying them. These include the mutual regulation of subunit expression, which change the NMDA:AMPA response balance, and the existence of multi-functional receptor complexes which make it difficult to distinguish between individual receptor sites, especially in vivo. This is followed by analysis of the functional relationships between the receptors from work on transmitter release, cellular electrophysiology and aspects of behavior where these can contribute to understanding receptor interactions. It is clear that nicotinic receptors (nAChRs) on axonal terminals directly regulate the release of glutamate and other neurotransmitters, α7-nAChRs generally promoting release. Hence, α7-nAChR responses will be prevented not only by a nicotinic antagonist, but also by compounds blocking the indirectly activated glutamate receptors. This accounts for the apparent anticholinergic activity of some glutamate antagonists, including the endogenous antagonist kynurenic acid. The activation of presynaptic nAChRs is by the ambient levels of ACh released from pre-terminal synapses, varicosities and glial cells, acting as a 'volume neurotransmitter' on synaptic and extrasynaptic sites. In addition, ACh and glutamate are released as CNS co-transmitters, including 'cholinergic' synapses onto spinal Renshaw cells. It is concluded that ACh should be viewed primarily as a modulator of glutamatergic neurotransmission by regulating the release of glutamate presynaptically, and the location, subunit composition, subtype balance and sensitivity of glutamate receptors, and not primarily as a classical fast neurotransmitter. These conclusions and caveats should aid clarification of the sites of action of glutamate and nicotinic receptor ligands in the search for new centrally-acting drugs.

Oxidative and nitrative stress in neurodegeneration

Aerobes require oxygen for metabolism and normal free radical formation. As a result, maintaining the redox homeostasis is essential for brain cell survival due to their high metabolic energy requirement to sustain electrochemical gradients, neurotransmitter release, and membrane lipid stability. Further, brain antioxidant levels are limited compared to other organs and less able to compensate for reactive oxygen and nitrogen species (ROS/RNS) generation which contribute oxidative/nitrative stress (OS/NS). Antioxidant treatments such as vitamin E, minocycline, and resveratrol mediate neuroprotection by prolonging the incidence of or reversing OS and NS conditions. Redox imbalance occurs when the antioxidant capacity is overwhelmed, consequently leading to activation of alternate pathways that remain quiescent under normal conditions. If OS/NS fails to lead to adaptation, tissue damage and injury ensue, resulting in cell death and/or disease. The progression of OS/NS-mediated neurodegeneration along with contributions from microglial activation, dopamine metabolism, and diabetes comprise a detailed interconnected pathway. This review proposes a significant role for OS/NS and more specifically, lipid peroxidation (LPO) and other lipid modifications, by triggering microglial activation to elicit a neuroinflammatory state potentiated by diabetes or abnormal dopamine metabolism. Subsequently, sustained stress in the neuroinflammatory state overwhelms cellular defenses and prompts neurotoxicity resulting in the onset or amplification of brain damage.

Galantamine attenuates the heroin seeking behaviors induced by cues after prolonged withdrawal in rats

BACKGROUND AND OBJECTIVE

Evidence shows that acetylcholinergic transmission in the ventral tegmental area (VTA) or nucleus accumbens (NAc) plays an important role in heroin-seeking induced by cues. Cholinergic modulation of VTA neurons arises from the lateral dorsal tegmental nucleus (LDT). The present studies investigated the effect of systemic or intra- LDT administration of galantamine, an inhibitor of acetylcholinesterase, on heroin-seeking induced by cues.

METHODS

Rats were trained to self-administer heroin for 12 days, underwent extinction training for 12 days followed by two weeks in their home cages. Then the conditioned cues were introduced for the reinstatement of heroin-seeking.

RESULTS

The reinstatement of heroin-seeking induced by cues was attenuated by the administration of galantamine (0, 0.3, 1 or 3mg/kg, i.p.) in a dose-dependent manner. In contrast, galantamine only at the dose of 3mg/kg could inhibit the reinstatement of sucrose-seeking. Galantamine at those doses failed to alter the locomotor activity in heroin-withdrawn rats. The inhibition of drug-seeking by galantamine was reversed by pretreatment with scopolamine (0.5mg/kg) but not with mecamylamine (3mg/kg) or scopolamine methobromide (1mg/kg). Moreover, the microinjection of galantamine into the LDT blocked cue-induced heroin-seeking, while the microinjection of scopolamine into the LDT reversed the inhibitory effect of galantamine on drug-seeking behavior.

CONCLUSION

The results suggest that cholinergic transmission in the LDT may play a critical role in heroin-seeking behavior induced by cues and that galantamine may have the beneficial effect of blocking heroin-seeking behavior, which is mediated through its actions on the muscarinic receptors.

CHF2819: pharmacological profile of a novel acetylcholinesterase inhibitor

CHF2819 is a novel orally active acetylcholinesterase inhibitor (AChEI) developed for the treatment of Alzheimer's disease (AD). CHF2819 is a selective inhibitor of AChE, it is 115 times more potent against this enzyme than against butyrylcholinesterase (BuChE). Moreover, CHF2819 is more selective for inhibition of central (brain) AChE than peripheral (heart) AChE. In vivo CHF2819, 0.5, 1.5, and 4.5 mg/kg p.o., significantly and in dose-dependent manner increased acetylcholine (ACh) levels in hippocampus of young adult rats. Moreover, aging animals, with lower basal ACh levels than young adult rats, also exhibit a marked increase in hippocampal levels of this neurotransmitter after administration of CHF2819. At 1.5 mg/kg p.o. CHF2819 attenuated scopolamine-induced amnesia in a passive avoidance task. Furthermore, it decreased dopamine (DA) levels and increased extracellular levels of 5-hydroxytryptamine (5-HT) in the hippocampus, without modifying norepinephrine (NE) levels. By oral administration to young adult rats CHF2819 did not affect extracellular hippocampal levels of glutamate (Glu), aspartate (Asp), gamma-aminobutyric acid (GABA), taurine (Tau), arginine (Arg) or citrulline (Cit). Functional observational battery (FOB) screening demonstrated that CHF2819 (1.5 and 4.5 mg/kg p.o.) does not affect activity, excitability, autonomic, neuromuscular, and sensorimotor domains, as well as physiological endpoints (body weight and temperature). CHF2819 induced, however, involuntary motor movements (ranging from mild tremors to myoclonic jerks) in a dose-dependent manner. The neurochemical and behavioral profiles of CHF2819 suggest that this orally active novel AChEI could be of clinical interest for the treatment of Alzheimer-type dementia associated with multiple neurotransmitter abnormalities in the brain. In particular, CHF2819 might be a useful therapeutic drug for AD patients with cognitive impairment accompanied by depression.

The effect of serotonin(1A) receptor agonism on antipsychotic drug-induced dopamine release in rat striatum and nucleus accumbens

Serotonin (5-HT)(1A) receptor agonism may be of interest in regard to both the antipsychotic action and extrapyramidal symptoms (EPS) of antipsychotic drugs (APD) based, in part, on the effect of 5-HT(1A) receptor stimulation on the release of dopamine (DA) in the nucleus accumbens (NAC) and striatum (STR), respectively. We investigated the effect of R(+)-8-hydroxy-2-(di-n-propylamino)-tetralin (R(+)-8-OH-DPAT) and n-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-n-(2-pyridinyl)cyclohe xanecarboxamide trihydrochloride (WAY100635), a selective 5-HT(1A) receptor agonist and antagonist, respectively, on basal and APD-induced DA release. In both STR and NAC, R(+)-8-OH-DPAT (0.2 mg/kg) decreased basal DA release; R(+)-8-OH-DPAT (0.05 mg/kg) inhibited DA release produced by the 5-HT(2A)/D(2) receptor antagonists clozapine (20 mg/kg), low dose risperidone (0.01 and 0. 03 mg/kg) and amperozide (10 mg/kg), but not that produced by high dose risperidone (0.1 and 1.0 mg/kg) or haloperidol (0.01-1.0 mg/kg), potent D(2) receptor antagonists. This R(+)-8-OH-DPAT-induced inhibition of the effects of clozapine, risperidone and amperozide was antagonized by WAY100635 (0.05 mg/kg). WAY100635 (0.1-0.5 mg/kg) alone increased DA release in the STR but not NAC. The selective 5-HT(2A) receptor antagonist M100907 (1 mg/kg) did not alter the effect of R(+)-8-OH-DPAT or WAY100635 alone on basal DA release in either region. These results suggest that 5-HT(1A) receptor stimulation inhibits basal and some APD-induced DA release in the STR and NAC, and that this effect is unlikely to be mediated by an interaction with 5-HT(2A) receptors. The significance of these results for EPS and antipsychotic action is discussed.

Dopaminergic pharmacology and antioxidant properties of pukateine, a natural product lead for the design of agents increasing dopamine neurotransmission

The dopaminergic and antioxidant properties of pukateine [(R)-11-hydroxy-1,2-methylenedioxyaporphine, PUK], a natural aporphine derivative, were analyzed in the rat central nervous system. At dopamine (DA) D1 ([3H]-SCH 23390) and D2 ([3H]-raclopride) binding sites, PUK showed IC50 values in the submicromolar range (0.4 and 0.6 microM, respectively). When the uptake of tritiated dopamine was assayed by using a synaptosomal preparation, PUK showed an IC50 = 46 microM. In 6-hydroxydopamine unilaterally denervated rats, PUK (8 mg/kg but not 4 mg/kg) elicited a significant contralateral circling, a behavior classically associated with a dopaminergic agonist action. When perfused through a microdialysis probe inserted into the striatum, PUK (340 microM) induced a significant increase in dopamine levels. In vitro experiments with a crude rat brain mitochondrial suspension showed that PUK did not affect monoamine oxidase activities, at concentrations as high as 100 microM. PUK potently (IC50 = 15 microM) and dose-dependently inhibited the basal lipid peroxidation of a rat brain membrane preparation. As a whole, PUK showed a unique profile of action, comprising an increase in extracellular DA, an agonist-like interaction with DA receptors, and antioxidant activity. Thus, PUK may be taken as a lead compound for the development of novel therapeutic strategies for Parkinson disease.

Effects of alpha-erabutoxin, alpha-bungarotoxin, alpha-cobratoxin and fasciculin on the nicotine-evoked release of dopamine in the rat striatum in vivo

Snake neurotoxins (NTX) have proven to be valuable tools for the characterisation of muscular nicotinic acetylcholine receptor structure and function. It is very likely that they could also be utilised to identify subtypes of neuronal nicotinic receptors controlling specific functions within the central nervous system. In this study we examined the effects of long alpha NTX (alpha-bungarotoxin, alpha-Bgt, and alpha-cobratoxin, alpha-Cbt) and short alpha NTX (alpha-erabutoxin a, alpha-Ebt) as well as the anticholinesterase toxin fasciculin-2 (FAS), on the nicotine-evoked release of dopamine (DA) in the striatum, using the in vivo push-pull technique. The short toxins alpha-Ebt and FAS blocked the extracellular increase of DA evoked by nicotine at 4.2 microM concentrations and alpha-Ebt was more potent, as reflected by the blockade at the lower dose of 0.42 microM. In contrast, the long toxins showed a different profile of action. Alpha-Cbt did not show any blockade of the nicotine-evoked release of DA at the doses studied while alpha-Bgt did block it only at the higher dose (4.2 microM) These results indicate that short neurotoxins show a stronger interaction with striatal nicotinic receptors subtypes controlling DA release when compared to the long ones. This interaction of short neurotoxin polypeptides and presynaptic receptors may permit the further elucidation of the particular nicotinic receptor populations responsible for the modulation of striatal DA release.