| beta-amyloid (Abeta), a 39-43 amino acid peptide deposited in the brains of Alzheimer's patients, disrupts synaptic function through mechanisms that are not fully understood. Nicotinic acetylcholine receptors (nAChRs) are potential targets for the actions of Abeta in the brain. This thesis examines (1) the role of nAChRs on basal forebrain neurons in Ca 2+ signaling and excitatory neurotransmission and (2) if Abeta influences these functions via nAChRs.; Intracellular calcium levels ([Ca2+]i) in acutely dissociated rat basal forebrain neurons were measured with the ratiometric dye fura-2. Focal applications of nicotine, evoked increases of [Ca 2+]i mediated via Ca2+ entry via nAChRs and intracellular Ca2+ release. Nicotine-evoked [Ca 2+]; rises were inhibited by alpha-bungarotoxin, and dihydro-beta-erythroidine (DHbetaE). Nicotine-evoked increases were irreversibly potentiated by Abeta 1-42, while focal application of Abeta1-42 alone did not alter [Ca2+]i. Abeta1-42 also potentiated caffeine-mediated, but not KCl-evoked rises of [Ca2+] i. Abeta1-42 potentiation of nicotine-mediated rises of [Ca2+]i was blocked by either the SERCA inhibitor thapsigargin or CGP-37157, an inhibitor of the mitochondrial Na+/Ca 2+ exchanger. Thus Abeta1-42 potentiation of repeated release of Ca2+ from intracellular stores by multiple nicotine challenges is due to overfilling of caffeine sensitive stores by mitochondria.; The effects of Abeta1-42, nicotine and acetylcholine (ACh) on cholinergic basal forebrain neurons prelabeled with Cy3-IgG192 or identified by electrophysiological criteria were examined by recording miniature excitatory synaptic currents (mEPSCs) using whole-cell patch clamp recordings in brain slice preparations. In 54% of neurons, Abeta1-42 significantly increased mEPSC frequency while in 32% of neurons, Abeta1-42 significantly decreased mEPSC frequency. DHbetaE blocked Abeta1-42 mediated increases of mEPSC frequency, but did not block Abeta 1-42 mediated decreases of mEPSC frequency. Nicotine or ACh in the presence of atropine, increased the frequency of mEPSCs in a dose-dependent manner. No changes in peak amplitude were observed in any treatments. These data suggest that in a subset of cholinergic neurons, Abeta1-42 increases mEPSC frequency by activating pre-synaptic alpha4beta2 nAChRs, while another unknown pre-synaptic mechanism mediated the decrease of mEPSC frequency. As the regulation of [Ca2+]i and synaptic transmission are important for cognitive functioning, these results may be important to understanding the pathophysiological mechanisms underlying Alzheimer's disease. |
