Somatic Ca²⁺ dynamics in response to choline-mediated excitation in histaminergic tuberomammillary neurons

Histaminergic tuberomammillary (TM) neurons of the posterior hypothalamus have been implicated in cognition, alertness and sleep-wakefulness cycles. Spontaneous firing of TM neurons has been associated with histamine release and wakefulness. The expression of α7 nicotinic acetylcholine receptors (nAChRs) in TM neurons suggests a role for endogenous choline and for nicotinic drugs in the regulation of intracellular Ca²⁺ metabolism, normal TM neuronal activity and histamine release. First, we established the link between TM neuronal spontaneous firing frequency and cytosolic free Ca²⁺ concentration ([Ca²⁺]_i). A strong correlation was observed: an onset of spontaneous firing (3-4Hz) was accompanied by a 20-fold increase in [Ca²⁺]_i from 56±18nM to 1.0±0.6μM. The same range of firing frequencies has been observed in TM neurons in vivo and is associated with wakefulness. Secondly, choline-induced activation of α7 nAChRs did not elevate [Ca²⁺]_i directly, i.e. in the absence of high-threshold voltage-gated Ca²⁺ channel (HVGCC) activation. Cd²⁺ (200μM) completely blocked all Ca²⁺ signals, but inhibited only 37±16% of α7 nAChR-mediated currents. Thirdly, the responsiveness of [Ca²⁺] _i to choline-mediated excitation was inhibited by hyperpolarization and enhanced by depolarization, sensitizing [Ca²⁺]_i at membrane voltages associated with normal TM neuronal activity. These properties of [Ca²⁺]_i define the ability of TM neurons to translate cholinergic stimuli of identical strengths into different cytosolic Ca ²⁺ effects, providing the physiological substrate for state-specific modulation of incoming cholinergic information and would be expected to play a very important role in determining activity profiles of TM neurons exposed to elevated concentrations of cholinergic agents, such as choline and nicotine.