Muscarinic receptor subtypes in bovine adrenal medulla

Characterization of Ca2+ signaling pathways in mouse adrenal medullary chromaffin cells

In the present study, we characterized the Ca2+ responses and secretions induced by various secretagogues in mouse chromaffin cells. Activation of the acetylcholine receptor (AChR) by carbachol induced a transient intracellular Ca2+ concentration ([Ca2+](i)) increase followed by two phases of [Ca2+](i) decay and a burst of exocytic events. The contribution of the subtypes of AChRs to carbachol-induced responses was examined. Based on the results obtained by stimulating the cells with the nicotinic receptor (nAChR) agonist, 1,1-dimethyl-4-phenylpiperazinium iodide, high K(+) and the effects of thapsigargin, it appears that activation of nAChRs induces an extracellular Ca2+ influx, which in turn activate Ca(2+)-induced Ca2+ release via the ryanodine receptors. Muscarine, a muscarinic receptor (mAChRs) agonist, was found to induce [Ca2+](i) oscillation and sustained catecholamine release, possibly by activation of both the receptor- and store-operated Ca2+ entry pathways. The RT-PCR results showed that mouse chromaffin cells are equipped with messages for multiple subtypes of AChRs, ryanodine receptors and all known components of the receptor- and store-operated Ca2+ entry. Furthermore, results obtained by directly monitoring endoplasmic reticulum (ER) and mitochondrial Ca2+ concentration and by disabling mitochondrial Ca2+ uptake suggest that the ER acts as a Ca2+ source, while the mitochondria acts as a Ca2+ sink. Our results show that both nAChRs and mAChRs contribute to the initial carbachol-induced [Ca2+](i) increase which is further enhanced by the Ca2+ released from the ER mediated by Ca(2+)-induced Ca2+ release and mAChR activation. This information on the Ca2+ signaling pathways should lay a good foundation for future studies using mouse chromaffin cells as a model system.

Muscarinic excitation-secretion coupling in chromaffin cells

Excitation-secretion coupling in adrenomedullary chromaffin cells physiologically commences when acetylcholine molecules released from splanchnic nerve terminals bind to cholinergic receptors located at the cell's plasma membrane. While nicotinic acetylcholine receptors ensure a rapid and efficacious transmission of preganglionic impulses, muscarinic acetylcholine receptors are considered to play a subsidiary role mostly by facilitating the nicotinic responses. Nevertheless, the variety of effects brought about by muscarinic stimulation in chromaffin cells (release of intracellular Ca2+, activation of Ca2+ entry through non-selective cation channels and voltage-dependent Ca2+ channels, impairment and/or enhancement of action potential firing, etc.) and the long-lasting nature of many of them suggests that muscarinic receptors might contribute to the fine tuning of the catecholamine secretory response upon graded preganglionic stimulation and prolonged periods of time. Such a variety of effects probably reflects not only the diversity of muscarinic receptors expressed in chromaffin cells but also the existence of differences among the animal species employed in the reported investigations. Accordingly, we first review on an animal species-based approach the most relevant features of the muscarinic response in chromaffin cells from a set of mammals, and finally present a unified picture of the mechanisms of muscarinic excitation-secretion coupling in chromaffin cells.

Nicotinic and muscarinic acetylcholine receptors are essential for the long-term response of tyrosine hydroxylase gene expression to chronic nicotine treatment in rat adrenal medulla

Nicotine induces tyrosine hydroxylase (TH) mRNA by interacting with nicotinic acetylcholine receptors (nAChRs) in cultured adrenal medullary cell systems; however, the mechanisms responsible for the induction of adrenal TH in response to systemically administered nicotine under in vivo conditions are more complex. In the present study, we tested whether nAChRs and muscarinic acetylcholine receptors (mAChRs) participate in the induction of adrenal TH observed after long-term treatment with nicotine. Chronic nicotine treatment (1.6 mg/kg, two daily injections spaced 12 h apart for 7 days) induced TH mRNA, TH protein and TH activity in rat adrenal medulla. This induction of TH gene expression was totally blocked when an antagonist of either nAChRs or mAChRs was administered prior to each nicotine injection. Repeated injections of the mAChR agonist bethanechol (5 mg/kg injected twice per day for 7 days) also produced increases in TH mRNA levels; however, TH protein levels and TH activity did not increase in response to bethanechol. In denervated adrenal glands chronic nicotine treatment did not lead to induction of either TH mRNA, TH protein or TH activity, whereas chronic bethanechol treatment led to induction of TH mRNA, but not TH protein or activity. These results suggest that agonist occupation of both nAChRs and mAChRs are essential for the complete response of TH gene expression to chronic nicotine treatment in rat adrenal medulla, but that stimulation of either cholinergic receptor by itself is not sufficient to elicit a full response. The results also suggest that both transcriptional and post-transcriptional mechanisms may potentially need to be regulated to induce TH protein in response to some stimuli.