Neurons of the tuberomammillary (TM) hypothalamic nucleus represent the major source of histamine in the brain and project their axons to most brain regions to control attention and arousal, as evidenced by the sedation caused by anti-histamines. Alpha-bungarotoxin-sensitive nicotinic acetylcholine receptors (nAChRs), whose kinetics and pharmacology are similar to those of highly Ca2+-permeable homomeric a7 nAChRs, are the only nAChRs expressed in TM neurons in high densities. Our experiments have shown that activation of TM nAChRs does not elevate cytosolic Ca2+ concentrations ([Ca2+]i) in TM somata in the absence of voltage-gated Ca2+ channels (VGCCs). Therefore, the kinetics of a7-like nAChRs and functional links among a7-like nAChRs and other TM Ca2+ sources become the key factors determining the impact of a7-like nAChR activation on [Ca2+]i. This implication is important because the physiological role of a7 nAChRs remains unknown, and may involve cytoprotection and regulation of TM histaminergic function by endogenous and therapeutic nicotinic agents, such as choline and nicotine.
Aim 1 will test the hypothesis that TM a7-like nAChRs are heteromeric (involve non-a7 subunits), whose permeability to Ca2+ ions is lower than that of homomeric a7 nAChRs. Accordingly, Aim 2 will test the hypothesis that a7-like nAChRs regulate TM [Ca2+]i indirectly, via functional links to key Ca2+ sources: VGCCs; Ca2+ stores; and Na+-Ca2+ exchangers. We will use Ca2+ imaging and patch-clamp; reverse transcription polymerase chain reaction; Western blot and immunohistochemistry to determine the nature of a7-like nAChRs; mechanisms of [Ca2+]i regulation, and links among a7-like nAChRs and other key Ca2+ sources in TM neurons. The proposed study will provide important information regarding the nature of a7-like nAChRs in histaminergic TM neurons; and a7-like nAChR-driven mechanisms of regulation of Ca2+ homeostasis implicated in cytoprotection and regulation of histaminergic function, linking cholinergic and histaminergic brain systems. Intriguingly, the behavioral and cognitive effects of nicotine and histamine are similar: both agents are anti-nociceptive; improve attention; promote alertness and arousal; and inhibit food intake. Some of these effects of nicotine and therapeutic nicotinic agents may arise from their ability to modulate histaminergic function. ? ?
