A novel nicotinic acetylcholine receptor subtype in basal forebrain cholinergic neurons with high sensitivity to amyloid peptides.

Nicotinic acetylcholine receptors (nAChRs) containing alpha7 subunits are thought to assemble as homomers. alpha7-nAChR function has been implicated in learning and memory, and alterations of alpha7-nAChR have been found in patients with Alzheimer's disease (AD). Here we report findings consistent with a novel, naturally occurring nAChR subtype in rodent, basal forebrain cholinergic neurons. In these cells, alpha7 subunits are coexpressed, colocalize, and coassemble with beta2 subunit(s). Compared with homomeric alpha7-nAChRs from ventral tegmental area neurons, functional, presumably heteromeric alpha7beta2-nAChRs on cholinergic neurons freshly dissociated from medial septum/diagonal band (MS/DB) exhibit relatively slow kinetics of whole-cell current responses to nicotinic agonists and are more sensitive to the beta2 subunit-containing nAChR-selective antagonist, dihydro-beta-erythroidine (DHbetaE). Interestingly, presumed, heteromeric alpha7beta2-nAChRs are highly sensitive to functional inhibition by pathologically relevant concentrations of oligomeric, but not monomeric or fibrillar, forms of amyloid beta(1-42) (Abeta(1-42)). Slow whole-cell current kinetics, sensitivity to DHbetaE, and specific antagonism by oligomeric Abeta(1-42) also are characteristics of heteromeric alpha7beta2-nAChRs, but not of homomeric alpha7-nAChRs, heterologously expressed in Xenopus oocytes. Moreover, choline-induced currents have faster kinetics and less sensitivity to Abeta when elicited from MS/DB neurons derived from nAChR beta2 subunit knock-out mice rather than from wild-type mice. The presence of novel, functional, heteromeric alpha7beta2-nAChRs on basal forebrain cholinergic neurons and their high sensitivity to blockade by low concentrations of oligomeric Abeta(1-42) suggests possible mechanisms for deficits in cholinergic signaling that could occur early in the etiopathogenesis of AD and might be targeted by disease therapies.

GABA(A) receptor-mediated excitation in dissociated neurons from human hypothalamic hamartomas.

The cellular mechanisms underlying intrinsic epileptogenesis in human hypothalamic hamartoma (HH) are unknown. We previously reported that HH tissue is composed predominantly of GABAergic neurons, but how GABAergic-neuron-rich HH tissue is intrinsically epileptogenic is unclear. Here, we tested the hypotheses that some HH neurons exhibit immature features and that GABA excites these neurons via activation of GABA(A) receptors (GABA(A)Rs). Gramicidin-perforated and cell-attached patch-clamp recordings were performed using freshly-dissociated HH neurons to evaluate GABA(A)R-mediated currents, Cl(-) equilibrium potentials, and intracellular Cl(-) concentrations. Single-cell RT-PCR and immunocytochemical techniques were used to examine cation-Cl(-) co-transporter (NKCC1 and KCC2) gene and KCC2 protein expression and molecular markers of maturation. From a total of 93 acutely-dissociated HH neurons from 34 patients, 76% were small (soma: 6-9 microm) and 24% were large (soma: >20 microm) in size. Under gramicidin-perforated patch recording conditions, GABA(A)R activation depolarized/excited large but hyperpolarized/inhibited small HH neurons in most cases. Compared to small HH neurons, large HH neurons exhibited more positive Cl(-) equilibrium potentials, higher intracellular Cl(-) concentrations, lower KCC2 expression, and an immature phenotype, consistent with GABA(A)R-mediated excitation. Taken collectively, we provide novel evidence for and mechanistic insights into HH epileptogenicity: GABA(A)R-mediated excitation.

Electrophysiological properties and subunit composition of GABAA receptors in patients with gelastic seizures and hypothalamic hamartoma.

Abnormalities in GABA(A) receptor structure and/or function have been associated with various forms of epilepsy in both humans and animals. Whether this is true for patients with gelastic seizures and hypothalamic hamartoma (HH) is unknown. In this study, we characterized the pharmacological properties and native subunit composition of GABA(A) receptors on acutely dissociated single neurons from surgically resected HH tissues using patch-clamp, immunocytochemical, and RT-PCR techniques. We found that 1) GABA induced an inward current (I(GABA)) at a holding potential of -60 mV; 2) I(GABA) was mimicked by the GABA(A) receptor agonist muscimol and blocked by the GABA(A) receptor antagonist bicuculline, suggesting that I(GABA) was mediated principally through the GABA(A) receptor; 3) the EC(50) and Hill coefficient derived from the I(GABA) concentration-response curve were 6.8 muM and 1.9, respectively; 4) the current-voltage curve was linear at a reversal potential close to zero; and 5) I(GABA) exhibited low sensitivity to zinc and diazepam but higher sensitivity to pentobarbital and pregnanolone. Additionally, using Xenopus oocytes microtransplanted with normal human hypothalamic tissue, we confirmed that the functional properties of GABA(A) receptors were similar to those seen in small isolated HH neurons. Finally, the expression profile of GABA(A) receptor subunits obtained from normal control human hypothalamic tissue was identical to that from surgically resected human HH tissue. Taken together, our data indicate that GABA(A) receptors on small HH neurons exhibit normal pharmacosensitivity and subunit composition. These findings bear relevance to a broader understanding of inhibitory neurotransmission in human HH tissue.

Iptakalim modulates ATP-sensitive K(+) channels in dopamine neurons from rat substantia nigra pars compacta.

Iptakalim, a novel cardiovascular ATP-sensitive K(+) (K(ATP)) channel opener, exerts neuroprotective effects on dopaminergic (DA) neurons against metabolic stress-induced neurotoxicity, but the mechanisms are largely unknown. Here, we examined the effects of iptakalim on functional K(ATP) channels in the plasma membrane (pm) and mitochondrial membrane using patch-clamp and fluorescence-imaging techniques. In identified DA neurons acutely dissociated from rat substantia nigra pars compacta (SNc), both the mitochondrial metabolic inhibitor rotenone and the sulfonylurea receptor subtype (SUR) 1-selective K(ATP) channel opener (KCO) diazoxide induced neuronal hyperpolarization and abolished action potential firing, but the SUR2B-selective KCO cromakalim exerted little effect, suggesting that functional K(ATP) channels in rat SNc DA neurons are mainly composed of SUR1. Immunocytochemical staining showed a SUR1-rather than a SUR2B-positive reaction in most dissociated DA neurons. At concentrations between 3 and 300 microM, iptakalim failed to hyperpolarize DA neurons; however, 300 microM iptakalim increased neuronal firing. In addition, iptakalim restored DA neuronal firing during rotenone-induced hyperpolarization and suppressed rotenone-induced outward current, suggesting that high concentrations of iptakalim close neuronal K(ATP) channels. Furthermore, in human embryonic kidney 293 cells, iptakalim (300-500 microM) closed diazoxide-induced Kir6.2/SUR1 K(ATP) channels, which were heterologously expressed. In rhodamine-123-preloaded DA neurons, iptakalim neither depolarized mitochondrial membrane nor prevented rotenone-induced mitochondrial depolarization. These data indicate that iptakalim is not a K(ATP) channel opener in rat SNc DA neurons; instead, iptakalim is a pm-K(ATP) channel closer at high concentrations. These effects of iptakalim stimulate further pharmacological investigation and the development of possible therapeutic applications.