fg-9041 and methyllycaconitine

In the hippocampus, activation of nicotinic receptors that include α4 and β2 subunits (α4β2*) facilitates memory formation. α4β2* receptors may also play a role in nicotine withdrawal, and their loss may contribute to cognitive decline in aging and Alzheimer's disease (AD). However, little is known about their cellular function in the hippocampus. Therefore, using optogenetics, whole cell patch clamping and voltage-sensitive dye (VSD) imaging, we measured nicotinic excitatory postsynaptic potentials (EPSPs) in hippocampal CA1. In a subpopulation of inhibitory interneurons, release of ACh resulted in slow depolarizations (rise time constant 33.2 ± 6.5 ms, decay time constant 138.6 ± 27.2 ms) mediated by the activation of α4β2* nicotinic receptors. These interneurons had somata and dendrites located in the stratum oriens (SO) and stratum lacunosum-moleculare (SLM). Furthermore, α4β2* nicotinic EPSPs were largest in the SLM. Thus, our data suggest that nicotinic EPSPs in hippocampal CA1 interneurons are predominantly mediated by α4β2* nicotinic receptors and their activation may preferentially affect extrahippocampal inputs in SLM of hippocampal CA1.

Topics: 2-Amino-5-phosphonovalerate; Acetylcholine; Aconitine; Animals; Biophysics; CA1 Region, Hippocampal; Channelrhodopsins; Cholinergic Fibers; Diagonal Band of Broca; Dihydro-beta-Erythroidine; Electric Stimulation; Excitatory Amino Acid Antagonists; Excitatory Postsynaptic Potentials; In Vitro Techniques; Interneurons; Light; Mice; Neural Pathways; Nicotine; Nicotinic Antagonists; Optics and Photonics; Patch-Clamp Techniques; Quinoxalines; Receptors, Nicotinic; Transduction, Genetic; Voltage-Sensitive Dye Imaging

alpha7 Nicotinic receptors have been suggested to play an important role in hippocampal learning and memory. However, the direct action of this receptor subtype on hippocampal pyramidal neurones in vivo has not yet been fully investigated. The availability of selective agonists for alpha7 receptors [AR-R17779 and (R)-(-)-5'-phenylspiro[1-azabicyclo[2.2.2] octane-3,2'-(3'H)furo[2,3-b]pyridine (PSAB-OFP)] has now allowed this role to be investigated.. Single-cell extracellular recordings were made from hippocampal CA3 pyramidal neurones in anaesthetized rats. The effects of nicotine, AR-R17779 and PSAB-OFP, applied either systemically or iontophoretically, were studied on the activity of these neurones.. Intravenous injection of cumulative doses of nicotine and PSAB-OFP induced dose-related, significant increases in neuronal firing in the majority of neurones tested. This excitation could be inhibited by intravenous administration of methyllycaconitine (MLA), a selective alpha7 nicotinic receptor antagonist. Furthermore, iontophoretic application of nicotine, AR-R17779 and PSAB-OFP each evoked current-dependent excitation of most CA3 pyramidal neurones studied, and this excitation was antagonized by co-iontophoretic application of MLA. In addition, the excitation induced by iontophoretic application of nicotine, AR-R17779 or PSAB-OFP was also blocked by co-iontophoretic application of either 6,7-dinitroquinoxaline-2,3-dione (DNQX) or D(2)-2-amino-5-phosphonopentanoate (D-AP5), selective N-methyl-D-aspartic acid (NMDA) and non-NMDA receptor antagonists respectively.. CA3 pyramidal neurones are modulated by activation of presynaptic alpha7 nicotinic receptors, which, at least in part, enhances glutamate release onto post-synaptic (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole proprionic acid and NMDA receptors on these CA3 neurones. This mechanism probably contributes to the effects of nicotine on hippocampal learning and memory.

Topics: 2-Amino-5-phosphonovalerate; Aconitine; Animals; Bridged Bicyclo Compounds, Heterocyclic; Bridged-Ring Compounds; D-Aspartic Acid; Glutamic Acid; Hippocampus; Male; N-Methylaspartate; Neurons; Nicotine; Nicotinic Antagonists; Pyridines; Quinoxalines; Rats; Rats, Sprague-Dawley; Receptors, Nicotinic; Receptors, Presynaptic; Spiro Compounds

Anatomical studies indicate that synaptic inputs from many cortical and subcortical structures converge on neurons of the ventral tegmental area (VTA). Although in vitro electrophysiological studies have examined synaptic inputs to dopamine (DA) and non-DA neurons in the VTA, they have largely relied upon local electrical stimulation to activate these synapses. This provides little information regarding the distinct properties of synapses originating from different brain areas. Using whole-cell recordings in parasagittal rat brain slices that preserved subcortical axons from the pedunculopontine nucleus (PPN) to the VTA, we compared these synapses with those activated by intra-VTA stimulation. PPN-evoked currents demonstrated longer latencies than intra-VTA-evoked currents, and both VTA and PPN responses were mediated by GABA(A) and AMPA receptors. However, unlike VTA-evoked currents, PPN currents were exclusively mediated by glutamate in 25-40% of the VTA neurons. Consistent with a cholinergic projection from the PPN to the VTA, nicotinic acetylcholine receptors (nAChR) were activated by endogenous acetylcholine released during PPN, but not VTA, stimulation. This was seen as a reduction of PPN-evoked, and not VTA-evoked, synaptic currents by the alpha7-nAChR antagonist methyllycaconitine (MLA) and the agonist nicotine. The beta2-nAChR subunit antagonist dihydro-beta-erythroidine had no effect on VTA- or PPN-evoked synaptic currents. The effects of MLA on PPN-evoked currents were unchanged by the GABA(A) receptor blocker picrotoxin, indicating that alpha7-nAChRs presynaptically modulated glutamate and not GABA release. These differences in physiological and pharmacological properties demonstrate that ascending PPN and presumed descending inputs to VTA utilize distinct mechanisms to differentially modulate neuronal activity and encode cortical and subcortical information.

Topics: Aconitine; Afferent Pathways; alpha7 Nicotinic Acetylcholine Receptor; Animals; Bicuculline; Dihydro-beta-Erythroidine; Electric Stimulation; Excitatory Amino Acid Antagonists; GABA Antagonists; In Vitro Techniques; Nicotine; Nicotinic Antagonists; Patch-Clamp Techniques; Pedunculopontine Tegmental Nucleus; Picrotoxin; Quinoxalines; Rats; Rats, Sprague-Dawley; Reaction Time; Receptors, Nicotinic; Strontium; Synapses; Synaptic Transmission; Ventral Tegmental Area

The effects of activation of nicotinic acetylcholine receptors (nAChRs) on glutamatergic transmission in the ventral lateral geniculate nucleus (LGNv) were examined in chick brain slices. Whole cell recordings showed that monosynaptic postsynaptic currents (PSCs) evoked in LGNv neurons by optic tract stimulation were blocked by glutamate receptor antagonists. Exogenously applied nicotine (0.5 microM), choline (1 mM), or acetylcholine (ACh, 100 microM) markedly increased (>3-fold) these evoked PSCs. Potentiation by ACh was dose-dependent and did not desensitize during a 5-min application. In a second set of experiments, the effect of releasing endogenous ACh by stimulating the lateral portion of the LGNv through a separate conditioning electrode before optic tract stimulation was examined. Conditioning stimulation trains increased PSCs by an average of 5.2-fold, an effect dependent on both the intensity and number of conditioning pulses. This increase in PSC amplitude was most likely caused by released ACh activating alpha6- and/or alpha3-containing nAChRs because it was blocked by 100 nM alpha-conotoxin MII, 100 nM dihydro-beta-erythroidine (DHbetaE), and 0.1-1.0 microM methyllycaconitine (MLA). In contrast, exogenously applied ACh increased PSC amplitude by activating a pharmacologically different population of nAChRs because this effect was inhibited by 100 nM alpha-bungarotoxin, 50 nM MLA, and a high concentration (30 microM) of DHbetaE, indicating that alpha7- and/or alpha8-containing receptors were involved. The results are consistent with a model whereby alpha6- and/or alpha3-containing nAChRs on retinal ganglion cell nerve terminals are located preferentially at cholinergic synapses, whereas alpha7- and/or alpha8-containing receptors are primarily extrasynaptic.

Topics: 2-Amino-5-phosphonovalerate; Acetylcholine; Aconitine; Analysis of Variance; Animals; Bicuculline; Bungarotoxins; Calcium; Calcium Channel Blockers; Chick Embryo; Conotoxins; Dihydro-beta-Erythroidine; Dose-Response Relationship, Drug; Drug Interactions; Electric Stimulation; Evoked Potentials; Excitatory Amino Acid Antagonists; GABA Antagonists; Geniculate Bodies; Glutamic Acid; In Vitro Techniques; Magnesium; Membrane Potentials; Neurons; Nicotinic Antagonists; Patch-Clamp Techniques; Quinoxalines; Receptors, Nicotinic; Synapses; Synaptic Transmission; Time Factors

Nicotine facilitates the induction of long-term potentiation (LTP) in the hippocampal CA1 region. The present study reveals the potential mechanisms underlying this effect of nicotine. Timed ACh-mediated activation of alpha7 nicotinic acetylcholine receptors (nAChRs) on pyramidal cells is known to promote LTP induction. Nicotine could suppress this timing-dependent mechanism by desensitizing nAChRs. Timed ACh-mediated activation of alpha7 nAChRs on feedforward interneurons can prevent LTP induction by inhibiting pyramidal cells. Nicotine diminished this ACh-mediated inhibition by desensitizing alpha7 nAChRs, thereby reducing the inhibitory influence on pyramidal cells. In addition to these desensitizing effects, nicotine activated presynaptic non-alpha7 nAChRs on feedforward interneurons to decrease the evoked release of gamma-aminobutyric acid (GABA) onto pyramidal cells. Furthermore, nicotine increased the frequency of spontaneous inhibitory postsynaptic currents (IPSCs) in pyramidal cells, and concomitantly caused a reduction in the size of responses to focal GABA application onto the dendrites of pyramidal cells, suggesting that the nicotine-induced increase in interneuronal activity leads ultimately to a use-dependent depression of evoked IPSCs in pyramidal cells. These nicotine-induced suppressions of inhibition of pyramidal cells were accompanied by enhanced N-methyl-D-aspartate (NMDA) responses in pyramidal cells. Thus, our results suggest that nicotine promotes the induction of LTP by diminishing inhibitory influences on NMDA responses while suppressing the ACh-mediated mechanisms. These ACh-independent mechanisms probably contribute to the nicotine-induced cognitive enhancement observed in the presence of cholinergic deficits, such as those in Alzheimer's disease patients.

Topics: Acetylcholinesterase; Aconitine; Animals; Animals, Newborn; Antibodies, Monoclonal; Cholinergic Fibers; Denervation; Dihydro-beta-Erythroidine; Dose-Response Relationship, Drug; Dose-Response Relationship, Radiation; Electric Stimulation; Excitatory Amino Acid Antagonists; GABA Antagonists; gamma-Aminobutyric Acid; Hippocampus; Immunohistochemistry; Immunotoxins; In Vitro Techniques; Long-Term Potentiation; N-Glycosyl Hydrolases; Neural Inhibition; Neurons; Nicotine; Nicotinic Agonists; Nicotinic Antagonists; Patch-Clamp Techniques; Phosphinic Acids; Picrotoxin; Propanolamines; Quinoxalines; Rats; Receptors, Nicotinic; Ribosome Inactivating Proteins, Type 1; Saporins; Valine

Segregation of ion channels and neurotransmitter receptors is an important mechanism for determining the functionality of the nervous system. In the case of nicotinic acetylcholine receptors, electrophysiological and anatomical studies have demonstrated that these receptors can be located at the somatodendritic and the axon terminal portions of neurons. Functionally, somatodendritic nicotinic receptors mediate fast excitatory transmission and possibly regulate other cell functions, while presynaptic nicotinic receptors enhance the release of neurotransmitters from axon terminals. Neurons in the mesencephalic lateral spiriform nucleus of the chick do not appear to restrict the localization of nicotinic receptors to specific membrane compartments, since receptors containing alpha5 and/or beta2 subunits are found both on the cell bodies and on the axonal projections of these neurons [Torrao A. S. et al. (1996) Brain Res. 743, 154-161]. We report here that, in contrast to lateral spiriform neurons, neurons in the nucleus semilunaris do appear to compartmentalize nicotinic receptors. The cholinergic nucleus semilunaris neurons express a high density of alpha7-containing nicotinic receptors on their somas [Britto L. R. G. et al. (1992) J. comp. Neurol. 317, 325-340]. However, when we examined the projections of these neurons in the lateral spiriform nucleus, we found no evidence for expression of alpha7-containing receptors on the cholinergic fibers from nucleus semilunaris neurons. Furthermore, patch-clamp electrophysiological recording from lateral spiriform neurons indicated an absence of presynaptic alpha7-containing nicotinic receptors capable of modulating the release of acetylcholine. We conclude that neurons are capable of segregating alpha7-containing nicotinic receptors to specific areas of their plasma membrane. Such targeting of nicotinic receptors would play an important role in determining their functional role in neurons.

Topics: Aconitine; alpha7 Nicotinic Acetylcholine Receptor; Animals; Brain Chemistry; Chickens; Choline O-Acetyltransferase; Dendrites; Excitatory Amino Acid Antagonists; Excitatory Postsynaptic Potentials; Neurons; Nicotinic Antagonists; Quinoxalines; Receptors, Nicotinic; Superior Colliculi

1. The whole-cell configuration of the patch-clamp technique was used to study the modulation of giant depolarizing potentials (GDPs) by nicotinic acetylcholine receptors (nAChRs) in CA3 hippocampal neurons in slices from postnatal day (P) 2-6 rats. 2. Bath application of nicotine increased GDP frequency in a concentration-dependent manner. For example, nicotine (0.5-1 microM) enhanced GDP frequency from 0.05 +/- 0.04 to 0.17 +/- 0.04 Hz. This effect was prevented by the broad-spectrum nicotinic receptor antagonist dihydro-beta-erythtroidine (DHbetaE, 50 microM) and partially antagonized by methyllycaconitine (MLA, 50 nM) a competitive antagonist of alpha7 nAChRs. GDP frequency was also enhanced by AR-17779 (100 microM), a selective agonist of alpha7 nAChRs. 3. The GABA(A) receptor antagonist bicuculline (10 microM) and the non-NMDA glutamate receptor antagonist DNQX (20 microM) blocked GDPs and prevented the effects of nicotine on GDPs. In the presence of DNQX, nicotine increased GABA-mediated synaptic noise, indicating that this drug may have a direct effect on GABAergic interneurons. 4. Bath application of edrophonium (20 microM), a cholinesterase inhibitor, in the presence of atropine (1 microM), increased GDP frequency, indicating that nAChRs can be activated by ACh released from the septo-hippocampal fibres. This effect was prevented by DHbetaE (50 microM). 5. In the majority of neurons tested, MLA (50 nM) and DHbetaE (50 microM) reduced the frequency of GDPs with different efficacy: a reduction of 98 +/- 11 and 61 +/- 29 % was observed with DHbetaE and MLA, respectively. In a subset of cells (40 % in the case of MLA and 17 % in the case of DHbetaE) these drugs induced a twofold increase in GDP frequency. 6. It is suggested that, during development, nAChRs modulate the release of GABA, assessed as GDPs, through distinct nAChRs. The rise of intracellular calcium via nAChRs would further strengthen GABA-mediated oscillatory activity. This can be crucial for consolidation of synaptic contacts and for the fine-tuning of the developing hippocampus.

Topics: Acetylcholine; Aconitine; alpha7 Nicotinic Acetylcholine Receptor; Animals; Animals, Newborn; Bicuculline; Bridged-Ring Compounds; Dihydro-beta-Erythroidine; Excitatory Amino Acid Antagonists; GABA Antagonists; gamma-Aminobutyric Acid; Glutamic Acid; Hippocampus; Insecticides; Membrane Potentials; Nicotine; Nicotinic Agonists; Organ Culture Techniques; Patch-Clamp Techniques; Pyramidal Cells; Quinoxalines; Rats; Rats, Wistar; Receptors, Nicotinic; Spiro Compounds