Manickavasagom Alkondon

Choline is a Selective Agonist of α7 Nicotinic Acetylcholine Receptors in the Rat Brain Neurons

In the present study, we demonstrate that choline, a precursor of acetylcholine (ACh) and a product of acetylcholine hydrolysis by acetylcholinesterase (AChE), acts as an efficient and relatively selective agonist of α7–containing nicotinic acetylcholine receptors (nAChR) in neurons cultured from the rat hippocampus, olfactory bulb and thalamus as well as in PC12 cells. Choline was able to activate postsynaptic and presynaptic α7 nAChRs, with the latter action resulting in the release of other neurotransmitters. Although choline was approximately one order of magnitude less potent than ACh (EC50 of 1.6 mM for choline and 0.13 mM for ACh), it acted as a full agonist at α7 nAChRs. In contrast, choline did not activate α4β2 agonist‐bearing nAChRs on hippocampal neurons, and acted as a partial agonist at α3β4‐containing nAChRs on PC12 cells. The ethyl alcohol moiety of choline is required for the selective action on α7 nAChR. Exposure of cultured hippocampal neurons for 10 min to choline (10–100 μM) resulted in desensitization of the native α7 nAChRs. Moreover, chronic exposure (10 days) of the cultured hippocampal neurons to a desensitizing concentration of choline (∼30 μM) decreased their responsiveness to ACh. The selective action of choline on native α7 nAChRs suggests that this naturally occurring compound may act in vivo as an endogenous ligand for these receptors. Putative physiological actions of choline include retrograde messenger activity during the development of the mammalian central nervous system and during periods of elevated synaptic activity that leads to long‐term potentiation.

NMDAR inhibition-independent antidepressant actions of ketamine metabolites

Major depressive disorder affects around 16 per cent of the world population at some point in their lives. Despite the availability of numerous monoaminergic-based antidepressants, most patients require several weeks, if not months, to respond to these treatments, and many patients never attain sustained remission of their symptoms. The non-competitive, glutamatergic NMDAR (N-methyl-d-aspartate receptor) antagonist (R,S)-ketamine exerts rapid and sustained antidepressant effects after a single dose in patients with depression, but its use is associated with undesirable side effects. Here we show that the metabolism of (R,S)-ketamine to (2S,6S;2R,6R)-hydroxynorketamine (HNK) is essential for its antidepressant effects, and that the (2R,6R)-HNK enantiomer exerts behavioural, electroencephalographic, electrophysiological and cellular antidepressant-related actions in mice. These antidepressant actions are independent of NMDAR inhibition but involve early and sustained activation of AMPARs (α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors). We also establish that (2R,6R)-HNK lacks ketamine-related side effects. Our data implicate a novel mechanism underlying the antidepressant properties of (R,S)-ketamine and have relevance for the development of next-generation, rapid-acting antidepressants.

α-Bungarotoxin- and methyllycaconitine-sensitive nicotinic receptors mediate fast synaptic transmission in interneurons of rat hippocampal slices

Abstract This study demonstrates for the first time that α7 nicotinic receptors (nAChRs) mediate fast synaptic transmission in conventional hippocampal slices. In the presence of antagonists of muscarinic, AMPA, NMDA, GABAA, ATP, and 5-HT3 receptors, spontaneous and evoked postsynaptic currents (PSCs) recorded from CA1 interneurons were blocked by the α7 nAChR antagonists methyllycaconitine and α-bungarotoxin and by a desensitizing concentration of the α7 nAChR agonist choline. Spontaneous nicotinic PSCs were also accompanied by Na+ transients, indicating that α7 nAChR-mediated transmission serves as an excitatory signal to the CA1 interneurons in the hippocampus.

Nicotinic Receptor Activation in Human Cerebral Cortical Interneurons: a Mechanism for Inhibition and Disinhibition of Neuronal Networks

Cholinergic control of the activity of human cerebral cortical circuits has long been thought to be accounted for by the interaction of acetylcholine (ACh) with muscarinic receptors. Here we report the discovery of functional nicotinic receptors (nAChRs) in interneurons of the human cerebral cortex and discuss the physiological and clinical implications of these findings. The whole-cell mode of the patch-clamp technique was used to record responses triggered by U-tube application of the nonselective agonist ACh and of the α7-nAChR-selective agonist choline to interneurons visualized by means of infrared-assisted videomicroscopy in slices of the human cerebral cortex. Choline induced rapidly desensitizing whole-cell currents that, being sensitive to blockade by methyllycaconitine (MLA; 50 nm), were most likely subserved by an α7-like nAChR. In contrast, ACh evoked slowly decaying whole-cell currents that, being sensitive to blockade by dihydro-β-erythroidine (DHβE; 10 μm), were most likely subserved by an α4β2-like nAChR. Application of ACh (but not choline) to the slices also triggered GABAergic postsynaptic currents (PSCs). Evidence is provided that ACh-evoked PSCs are the result of activation of α4β2-like nAChRs present in preterminal axon segments and/or in presynaptic terminals of interneurons. Thus, nAChRs can relay inhibitory and/or disinhibitory signals to pyramidal neurons and thereby modulate the activity of neuronal circuits in the human cerebral cortex. These mechanisms, which appear to be retained across species, can account for the involvement of nAChRs in cognitive functions and in certain neuropathological conditions.

Selective blockade of NMDA‐activated channel currents may be implicated in learning deficits caused by lead

The effect of Pb2+ on glutamate receptor activity in rat hippocampal neurons was investigated with a view of explaining the cognitive and learning deficits produced by this heavy metal. Pb2+ (2.5–50 μ;M) selectively inhibited N‐methyl‐D‐aspartate (NMDA)‐induced whole‐cell and single‐channel currents in a concentration‐dependent but voltage‐independent manner, without significantly altering currents induced by either quisqualate or kainate. The frequency of NMDA‐induced channel activation was decreased by Pb2+. Neither glycine (10–100 μ;M), nor Ca2+ (10 mM) reversed the effect of Pb2+. Pb2+ also inhibited the [3H]MK‐801 binding to rat hippocampal membranes in vitro. The elucidation of the actions of Pb2+ on the NMDA receptor ion channel complex provides important insights into the clinical and toxic effects of this cation.

The nicotinic acetylcholine receptor subtypes and their function in the hippocampus and cerebral cortex

Nicotinic acetylcholine receptors (nAChRs) are widely distributed in the central nervous system and have been implicated in multiple behavioral paradigms and pathological conditions. Nicotinic therapeutic interventions require an extensive characterization of native nAChRs including mapping of their distribution and function in different brain regions. Here, we describe the roles played by different nAChRs in affecting neuronal activity in the hippocampus and cerebral cortex. At least three distinct functional nAChR subtypes (alpha 7, alpha 4 beta 2, alpha 3 beta 4) can be detected in the hippocampal region, and in many instances a single neuron type is found to be influenced by all three nAChRs. Further, it became clear that GABAergic and glutamatergic inputs to the hippocampal interneurons are modulated via different subtypes of nAChRs. In the cerebral cortex, GABAergic inhibition to the layer V pyramidal neurons is enhanced predominantly via activation of alpha 4 beta 2 nAChR and to a minor extent via activation of alpha 7 nAChR. Such diversity offers pathways by which nicotinic drugs affect brain function.

Nicotinic Receptor Function in the Mammalian Central Nervous Systema

The diversity of neuronal nicotinic receptors (nAChRs) in addition to their possible involvement in such pathological conditions as Alzheimers disease have directed our research towards the characterization of these receptors in various mammalian brain areas. Our studies have relied on electrophysiological, biochemical, and immunofluorescent techniques applied to cultured and acutely dissociated hippocampal neurons, and have been aimed at identifying the various subtypes of nAChRs expressed in the mammalian central nervous system (CNS), at defining the mechanisms by which CNS nAChR activity is modulated, and at determining the ion permeability of CNS nAChR channels. Our findings can be summarized as follows: (1) hippocampal neurons express at least three subtypes of CNS nAChRs--an alpha 7-subunit-bearing nAChR that subserves fast-inactivating, alpha-BGT-sensitive currents, which are referred to as type IA, and alpha 4 beta 2 nAChR that subserves slowly inactivating, dihydro-beta-erythroidine-sensitive currents, which are referred to as type II, and an alpha 3 beta 4 nAChR that subserves slowly inactivating, mecamylamine-sensitive currents, which are referred to as type III; (2) nicotinic agonists can activate a single type of nicotinic current in olfactory bulb neurons, that is, type IA currents; (3) alpha 7-subunit-bearing nAChR channels in the hippocampus have a brief lifetime, a high conductance, and a high Ca2+ permeability; (4) the peak amplitude of type IA currents tends to rundown with time, and this rundown can be prevented by the presence of ATP-regenerating compounds (particularly phosphocreatine) in the internal solution; (5) rectification of type IA currents is dependent on the presence of Mg2+ in the internal solution; and (6) there is an ACh-insensitive site on neuronal and nonneuronal nAChRs through which the receptor channel can be activated. These findings lay the groundwork for a better understanding of the physiological role of these receptors in synaptic transmission in the CNS.

Nicotine at concentrations found in cigarette smokers activates and desensitizes nicotinic acetylcholine receptors in CA1 interneurons of rat hippocampus.

Behavioral effects of cigarette smoking are attributed to the interactions of nicotine with brain nicotinic acetylcholine receptors (nAChRs). However, the mechanisms by which nAChR function in developing and mature brain is affected by a smokers level of nicotine (50-500 nM) remain unclear. Thus, the objective of this study was to determine the concentration- and time-dependent effects of nicotine on alpha7 and alpha4beta2 nAChRs, the two major brain subtypes, natively expressed in CA1 interneurons of rat hippocampal slices. Only at concentrations > or =5 microM did nicotine (applied for 6-60 s) elicit action potentials or measurable whole-cell currents (EC(50)=158 microM) in stratum radiatum interneurons that express alpha7 nAChRs. Continuous exposure for 10-15 min of the neurons to nicotine (0.5-2.5 microM) inhibited alpha7 nAChR-mediated currents (IC(50)=640 nM) evoked by choline (10 mM). Nicotine (> or =0.125 microM) applied to the neurons for 1-5 min induced slowly desensitizing whole-cell currents (EC(50)=3.2 microM) in stratum lacunosum moleculare interneurons; this effect was mediated by alpha4beta2 nAChRs. Also via activation of alpha4beta2 nAChRs, nicotine (0.125-0.5 microM) increased the frequency and amplitude of GABAergic postsynaptic currents (PSCs) in stratum radiatum interneurons. However, exposure of the neurons for 10-15 min to nicotine (0.25-0.5 microM) resulted in desensitization of alpha4beta2 nAChRs. It is suggested that nanomolar concentrations of nicotine after acute intake suppress inhibitory inputs to pyramidal cells through a disinhibitory mechanism involving activation of alpha4beta2 nAChRs and desensitization of alpha7 nAChRs, and after chronic intake leads to up-regulation of both receptor subtypes via desensitization. These findings have direct implications to the actions of nicotine in cigarette smokers.

A possible involvement of cyclic AMP in the expression of desensitization of the nicotinic acetylcholine receptor: A study with forskolin and its analogs

Forskolin, an activator of adenylate cyclase, and its analogs were studied on the nicotinic acetylcholine receptor‐ion channel complex (AChR) of rat and frog skeletal muscles. At nanomolar concentrations, forskolin caused desensitization of the AChR located at the junctional region of innervated and the extrajunctional region of chronically denervated rat soleus muscles. The desensitization of the AChR occurred without alteration of the conducting state (channel lifetime, conductance or bursting) as shown by single channel currents. Accordingly, forskolin decreased the peak amplitude of the repetitive evoked endplate currents in frog sartorius muscles. These findings taken together with the good correlation found between the effects of forskolin and its analogs on the desensitization of the nicotinic AChR and their ability to activate adenylate cyclase suggested a possible involvement of phosphorylation of AChR via cyclic AMP on the desensitization process.

α7 Nicotinic acetylcholine receptors and modulation of gabaergic synaptic transmission in the hippocampus

The present report provides new findings regarding modulation of gamma-aminobutyric acid (GABA) transmission by alpha7 nicotinic receptor activity in CA1 interneurons of rat hippocampal slices. Recordings were obtained from tight-seal cell-attached patches of the CA1 interneurons, and agonists were delivered to the neurons via a modified U-tube. Application for 6 s of the alpha7 nicotinic receptor-selective agonist choline (> or =1 mM) to all CA1 interneurons tested triggered action potentials that were detected as fast current transients. The activity triggered by choline terminated well before the end of the agonist pulse, was blocked by the alpha7 nicotinic receptor antagonist methyllycaconitine (50 nM) and was concentration dependent; the higher the concentration of choline the higher the frequency of events and the shorter the delay for detection of the first event. In 40% of the neurons tested, choline-triggered action potentials decreased in amplitude progressively until no more events could be detected despite the presence of the agonist. Primarily, this finding could be explained by Na(+)-channel inactivation associated with membrane depolarization induced by alpha7 nicotinic receptor activation. In 60% of the neurons, the amplitude of choline-induced action potentials was sustained at the intial level, but again the activity did not last as long as the agonist pulse, in this case apparently because of agonist-induced receptor desensitization. These results altogether demonstrate that agonists interacting with alpha7 nicotinic receptors, including the natural transmitter acetylcholine and its metabolite choline, influence GABAergic transmission, not only by activating these receptors, but also by controlling the rate of Na(+)-channel inactivation and/or by inducing receptor desensitization.