Amperometric Study of the Kinetics of Exocytosis in Mouse Adrenal Slice Chromaffin Cells: Physiological and Methodological Insights

Monitoring the Secretory Behavior of the Rat Adrenal Medulla by High-Performance Liquid Chromatography-Based Catecholamine Assay from Slice Supernatants

Catecholamine (CA) secretion from the adrenal medullary tissue is a key step of the adaptive response triggered by an organism to cope with stress. Whereas molecular and cellular secretory processes have been extensively studied at the single chromaffin cell level, data available for the whole gland level are much scarcer. We tackled this issue in rat by developing an easy to implement experimental strategy combining the adrenal acute slice supernatant collection with a high-performance liquid chromatography-based epinephrine and norepinephrine (NE) assay. This technique affords a convenient method for measuring basal and stimulated CA release from single acute slices, allowing thus to individually address the secretory function of the left and right glands. Our data point that the two glands are equally competent to secrete epinephrine and NE, exhibiting an equivalent epinephrine:NE ratio, both at rest and in response to a cholinergic stimulation. Nicotine is, however, more efficient than acetylcholine to evoke NE release. A pharmacological challenge with hexamethonium, an α3-containing nicotinic acetylcholine receptor antagonist, disclosed that epinephrine- and NE-secreting chromaffin cells distinctly expressed α3 nicotinic receptors, with a dominant contribution in NE cells. As such, beyond the novelty of CA assays from acute slice supernatants, our study contributes at refining the secretory behavior of the rat adrenal medullary tissue, and opens new perspectives for monitoring the release of other hormones and transmitters, especially those involved in the stress response.

Gap junction signalling is a stress-regulated component of adrenal neuroendocrine stimulus-secretion coupling in vivo

Elucidating the mechanisms whereby neuroendocrine tissues coordinate their input and output signals to ensure appropriate hormone secretion is currently a topical issue. In particular, whether a direct communication mediated by gap junctions between neurosecretory cells contributes to hormone release in vivo still remains unknown. Here we address this issue using a microsurgical approach allowing combined monitoring of adrenal catecholamine secretion and splanchnic nerve stimulation in anaesthetised mice. Pharmacological blockade of adrenal gap junctions by the uncoupling agent carbenoxolone reduces nerve stimulation-evoked catecholamine release in control mice and to a larger extent in stressed mice. In parallel, the gap junction-coupled cell network is extended in stressed mice. Altogether, this argues for a significant contribution of adrenomedullary gap junctions to catecholamine secretion in vivo. As such, gap junctional signalling appears to be a substantial component for neuroendocrine function in the adrenal medulla, as it may represent an additional lever regulating hormone release.

Distinguishing splanchnic nerve and chromaffin cell stimulation in mouse adrenal slices with fast-scan cyclic voltammetry

Electrical stimulation is an indispensible tool in studying electrically excitable tissues in neurobiology and neuroendocrinology. In this work, the consequences of high-intensity electrical stimulation on the release of catecholamines from adrenal gland slices were examined with fast-scan cyclic voltammetry at carbon fiber microelectrodes. A biphasic signal, consisting of a fast and slow phase, was observed when electrical stimulations typically used in tissue slices (10 Hz, 350 μA biphasic, 2.0 ms/phase pulse width) were applied to bipolar tungsten-stimulating electrodes. This signal was found to be stimulation dependent, and the slow phase of the signal was abolished when smaller (≤250 μA) and shorter (1 ms/phase) stimulations were used. The slow phase of the biphasic signal was found to be tetrodotoxin and hexamethonium independent, while the fast phase was greatly reduced using these pharmacological agents. Two different types of calcium responses were observed, where the fast phase was abolished by perfusion with a low-calcium buffer while both the fast and slow phases could be modulated when Ca²(+) was completely excluded from the solution using EGTA. Perfusion with nifedipine resulted in the reduction of the slow catecholamine release to 29% of the original signal, while the fast phase was only decreased to 74% of predrug values. From these results, it was determined that high-intensity stimulations of the adrenal medulla result in depolarizing not only the splanchnic nerves, but also the chromaffin cells themselves resulting in a biphasic catecholamine release.

Real-time monitoring of chemical transmission in slices of the murine adrenal gland

The real-time electrochemical detection of catecholamine secretion from murine adrenal slices using fast-scan cyclic voltammetry (FSCV) and amperometry at carbon fiber microelectrodes is described. Bright-field and immunofluorescent microscopy supported that chromaffin cells in the adrenal medulla are organized into clusters and positively stain for tyrosine hydroxylase confirming that they are catecholaminergic. Spontaneous exocytotic catecholamine events were observed inside chromaffin cell clusters with both FSCV and amperometry and were modulated by the nicotinic acetylcholine receptor antagonist hexamethonium and low extracellular calcium. Reintroduction of extracellular calcium and pressure ejection of acetylcholine caused the frequency of spikes to increase back to predrug levels. Electrical stimulation caused the synchronous secretion from multiple cells within the gland, which were modulated by nicotinic acetylcholine receptors but not muscarinic receptors or gap junctions. Furthermore, electrically stimulated release was abolished with perfusion of low extracellular calcium or tetrodotoxin, indicating that the release requires electrical excitability. An extended waveform was used to study the spontaneous and stimulated release events to determine their chemical content by FSCV. Consistent with total content analysis and immunohistochemical studies, about two thirds of the cells studied spontaneously secreted epinephrine, whereas one third secreted norepinephrine. Whereas adrenergic sites contained mostly epinephrine during electrical stimulation, noradrenergic sites contained a mixture of the catecholamines showing the heterogeneity of the adrenal medulla.

A physiological view of the central and peripheral mechanisms that regulate the release of catecholamines at the adrenal medulla

Here we review the tight neural control of the differential secretion into the circulation, of the adrenal medullary hormones adrenaline and noradrenaline. One or the other catecholamines are differentially released on various stress conditions. This is specifically controlled by central nervous system nuclei at the cortex, hypothalamus and spinal cord. Different firing patterns of splanchnic nerves and nicotinic or muscarinic receptors cause the selective release of noradrenaline or adrenaline, to adapt the body to the 'fight or flight' reaction, or during severe hypoglycaemia, haemorrhage, cold, acute myocardial infarction or other severe stressful conflicts. Endogenously acetylcholine (ACh) released at the splanchnic nerve-chromaffin cell synapse, acting on muscarinic and nicotinic receptors, causes membrane depolarization and action potentials (AP) in chromaffin cells. These changes vary with the animal species, the cell preparation (intact bisected adrenal, adrenal slices, or isolated fresh or cultured cells) or the recording technique (intracellular microelectrodes, patch-clamp, perforated-patch, cell-attached). Conflicting results leave many open questions concerning the actions of ACh on chromaffin cell excitability. The use of adrenal slices and field electrical stimulation will surely provide new insights into these mechanisms. Chromaffin cells have been thoroughly used as models to study the relationship between Ca2+ entry, cytosolic Ca2+ signals, exocytosis and endocytosis, using patch-clamp and amperometric techniques. Cells have been stimulated with single depolarizing pulses (DPs), DP trains and with simulated AP waveforms. These approaches have provided useful information but we have no data on APs generated by pulsatile secretory quanta of ACh, trying to mimic the intermittent and repetitive splanchnic nerve discharge of the neurotransmitter. We present some recent experiments using ultrashort ACh pulses (25 ms), that cause non-desensitizing repetitive APs with each ACh pulse, at low ACh concentrations (30 microM). Ultrashort pulses of a high ACh concentration (1000 microM) causes a single AP followed by a prolonged depolarization. It could be interesting trying to correlate these 'patterns of splanchnic nerve discharge' with Ca2+ signals and exocytosis. This, together with the use of adrenal slices and transmural electrical stimulation of splanchnic nerves will provide new physiologically sound data on the regulation of adrenal medullary secretion.