Nicotine induces intracellular Ca<sup>2+</sup> increases in cultured hippocampal astrocytes by nAChR-dependent and -independent pathways

Abstract

Nicotine, the major addictive substance in tobacco, interacts with nicotinic acetylcholine receptors (nAChRs) located in neuronal and glial cells, modulating synaptic transmission and memory. Here, we show that nAChRs agonists, including nicotine, acetylcholine, and choline, increase the intracellular Ca 2+ concentration ([Ca 2+ ] i) in cultured hippocampal astrocytes, indicating the involvement of nAChRs. Interestingly, inhibition of nAChRs, with a cocktail of antagonists (mecamylamine, methyllycaconitine plus dihydro-βerythroidine), does not prevent the astrocytic [Ca 2+ ] i increases generated by nicotine. This last effect would be attributable to inhibition of K + currents by nicotine in these cells, as previously we showed using patchclamp recordings. Furthermore, the application of tetraethylammonium, an inhibitor of K + currents, also increases the [Ca 2+ ] i. Together, these results indicate that nicotine increases [Ca 2+ ] i in hippocampal astrocytes through two pathways: by activation of nAChRs, and likely by direct inhibition of K + currents.

Nicotine induces morphological and functional changes in astrocytes via nicotinic receptor activity

“…Mecamylamine (20 μM), a nonselective nAChR blocker (McKee, Weinberger, Harrison, Coppola, & George, 2009), was added 30 min prior to the nicotine treatment. Preincubation with the blocker was performed to ensure receptor inhibition before adding drugs of interest (Galloway et al, 2018; Hernández‐Morales & García‐Colunga, 2014; Yu et al, 2011). Mecamylamine effectively abolished the observed nicotine‐induced changes in the length and number of astrocytic processes and we saw no difference between astrocytes treated with mecamylamine and nicotine, just mecamylamine, and controls without any drugs (Figure 2, Supplementary Figure 3e–h).…”

“…Nicotine was previously found to increase intracellular Ca 2+ concentration by activating nAChRs and inhibiting non‐decaying potassium currents (Hernández‐Morales & García‐Colunga, 2014; Sharma & Vijayaraghavan, 2001; Shen & Yakel, 2012). Most of these studies applied relatively high concentrations of nicotine rather than a physiologically relevant concentration and they characterize events occurring on a time scale of a few seconds or min in the form of Ca 2+ bursts.…”

“…While some studies have shown α and β nAChR subunits (Gahring, Persiyanov, & Rogers, 2004; Oikawa, Nakamichi, Kambe, Ogura, & Yoneda, 2005) in astrocytes, the only functional subtype found either in vitro and in vivo in rodent brain astrocytes are homomeric α7 nAChRs (Gotti & Clementi, 2004; Graham et al, 2003; Patel, McIntire, Ryan, Dunah, & Loring, 2017; Shen & Yakel, 2012; Teaktong et al, 2003). Nicotine has been shown to increase calcium activity in astrocytes (Hernández‐Morales & García‐Colunga, 2014), and unlike their neuronal counterparts astrocytic nAChRs are found to be involved in calcium‐induced calcium release from intracellular store (Sharma & Vijayaraghavan, 2001). Nicotine is also found to protect astrocytes from apoptosis by playing a role in anti‐inflammatory action against LPS and IL‐1β induced reactive astrocytosis (Liu et al, 2015; Revathikumar et al, 2016).…”