Desensitization to nicotinic cholinergic agonists and K+, agents that stimulate catecholamine secretion, in isolated adrenal chromaffin cells

Adaptive remodeling of the stimulus-secretion coupling: Lessons from the 'stressed' adrenal medulla

Stress is part of our daily lives and good health in the modern world is offset by unhealthy lifestyle factors, including the deleterious consequences of stress and associated pathologies. Repeated and/or prolonged stress may disrupt the body homeostasis and thus threatens our lives. Adaptive processes that allow the organism to adapt to new environmental conditions and maintain its homeostasis are therefore crucial. The adrenal glands are major endocrine/neuroendocrine organs involved in the adaptive response of the body facing stressful situations. Upon stress episodes and in response to activation of the sympathetic nervous system, the first adrenal cells to be activated are the neuroendocrine chromaffin cells located in the medullary tissue of the adrenal gland. By releasing catecholamines (mainly epinephrine and to a lesser extent norepinephrine), adrenal chromaffin cells actively contribute to the development of adaptive mechanisms, in particular targeting the cardiovascular system and leading to appropriate adjustments of blood pressure and heart rate, as well as energy metabolism. Specifically, this chapter covers the current knowledge as to how the adrenal medullary tissue remodels in response to stress episodes, with special attention paid to chromaffin cell stimulus-secretion coupling. Adrenal stimulus-secretion coupling encompasses various elements taking place at both the molecular/cellular and tissular levels. Here, I focus on stress-driven changes in catecholamine biosynthesis, chromaffin cell excitability, synaptic neurotransmission and gap junctional communication. These signaling pathways undergo a collective and finely-tuned remodeling, contributing to appropriate catecholamine secretion and maintenance of body homeostasis in response to stress.

Chronic stress alters adrenal clock function in a sexually dimorphic manner

Glucocorticoid production is gated at the molecular level by the circadian clock in the adrenal gland. Stress influences daily rhythms in behavior and physiology, but it remains unclear how stress affects the function of the adrenal clock itself. Here, we examine the influence of stress on adrenal clock function by tracking PERIOD2::LUCIFERASE (PER2::LUC) rhythms in vitro Relative to non-stressed controls, adrenals from stressed mice displayed marked changes in PER2::LUC rhythms. Interestingly, the effect of stress on adrenal rhythms varied by sex and the type of stress experienced in vivo To investigate the basis of sex differences in the adrenal response to stress, we next stimulated male and female adrenals in vitro with adrenocorticotropic hormone (ACTH). ACTH shifted phase and increased amplitude of adrenal PER2::LUC rhythms. Both phase and amplitude responses were larger in female adrenals than in male adrenals, an observation consistent with previously described sex differences in the physiological response to stress. Lastly, we reversed the sex difference in adrenal clock function using stress and sex hormone manipulations to test its role in driving adrenal responses to ACTH. We find that adrenal responsiveness to ACTH is inversely proportional to the amplitude of adrenal PER2::LUC rhythms. This suggests that larger ACTH responses from female adrenals may be driven by their lower amplitude molecular rhythms. Collectively, these results indicate a reciprocal relationship between stress and the adrenal clock, with stress influencing adrenal clock function and the state of the adrenal clock gating the response to stress in a sexually dimorphic manner.

Direct effects of recurrent hypoglycaemia on adrenal catecholamine release

In Type 1 and advanced Type 2 diabetes mellitus, elevation of plasma epinephrine plays a key role in normalizing plasma glucose during hypoglycaemia. However, recurrent hypoglycaemia blunts this elevation of plasma epinephrine. To determine whether recurrent hypoglycaemia affects peripheral components of the sympatho-adrenal system responsible for epinephrine release, male rats were administered subcutaneous insulin daily for 3 days. These recurrent hypoglycaemic animals showed a smaller elevation of plasma epinephrine than saline-injected controls when subjected to insulin-induced hypoglycaemia. Electrical stimulation of an adrenal branch of the splanchnic nerve in recurrent hypoglycaemic animals elicited less release of epinephrine and norepinephrine than in controls, without a change in adrenal catecholamine content. Responsiveness of isolated, perfused adrenal glands to acetylcholine and other acetylcholine receptor agonists was also unchanged. These results indicate that recurrent hypoglycaemia compromised the efficacy with which peripheral neuronal activity stimulates adrenal catecholamine release and demonstrate that peripheral components of the sympatho-adrenal system were directly affected by recurrent hypoglycaemia.

Pituitary adenylate cyclase-activating peptide enhances electrical coupling in the mouse adrenal medulla

Neuroendocrine adrenal medullary chromaffin cells receive synaptic excitation through the sympathetic splanchnic nerve to elicit catecholamine release into the circulation. Under basal sympathetic tone, splanchnic-released acetylcholine evokes chromaffin cells to fire action potentials, leading to synchronous phasic catecholamine release. Under elevated splanchnic firing, experienced under the sympathoadrenal stress response, chromaffin cells undergo desensitization to cholinergic excitation. Yet, stress evokes a persistent and elevated adrenal catecholamine release. This sustained stress-evoked release has been shown to depend on splanchnic release of a peptide transmitter, pituitary adenylate cyclase-activating peptide (PACAP). PACAP stimulates catecholamine release through a PKC-dependent pathway that is mechanistically independent of cholinergic excitation. Moreover, it has also been reported that shorter term phospho-regulation of existing gap junction channels acts to increase junctional conductance. In this study, we test if PACAP-mediated excitation upregulates cell-cell electrical coupling to enhance chromaffin cell excitability. We utilize electrophysiological recordings conducted in adrenal tissue slices to measure the effects of PACAP stimulation on cell coupling. We report that PACAP excitation increases electrical coupling and the spread of electrical excitation between adrenal chromaffin cells. Thus PACAP acts not only as a secretagogue but also evokes an electrical remodeling of the medulla, presumably to adapt to the organism's needs during acute sympathetic stress.

Nicotine tolerance to PC12 cell line: acute and chronic exposures modulate dopamine D2 receptor and tyrosine hydroxylase expression

PC12 is a clonal cell line from chromaffin tumor of rat adrenal pheochromocytoma that releases catecholamine including dopamine, which via interaction with its receptor (D(1) and D(2) receptor), is known to be involved in reward and reinforcement properties of many addictive drugs like nicotine. Nicotine tolerance is the key aspect of nicotine addiction. However, nicotine tolerance on dopamine receptors in PC12 cell line is poorly understood. In this paper, we have demonstrated the tolerance to acute and chronic nicotine administrations on PC12 cell line on the basis of the expressions of dopamine receptors and tyrosine hydroxylase, the rate-limiting enzyme of dopamine biosynthesis, by Western blot, immunohistochemistry and in situ hybridization. In vitro treatment of nicotine resulted in similar expressional changes of dopamine D(2) receptor and tyrosine hydroxylase at protein and mRNA levels in dose- and time-dependent manner, whereas dopamine D(1) receptor did not reveal any positive output. Moreover, moderate to strong signals were obtained from 0.1 to 10 microM of nicotine concentrations and the signals were gradually decreased at 100 and 1000 microM nicotine concentrations relative to the untreated control cell line. Therefore, this study implied a new approach towards nicotine tolerance which is likely to be related to the modulation of dopamine D(2) receptor and tyrosine hydroxylase expressions by chronic and acute nicotine exposures in PC12 cell line.

Nicotinic Cholinergic Regulation of Tyrosine Hydroxylase Gene Expression and Catecholamine Synthesis in Isolated Bovine Adrenal Chromaffin Cells

Isolated bovine adrenal chromaffin cells were used to study the nicotinic regulation of tyrosine hydroxylase (TH) gene expression. Continuous exposure of the cells to carbachol or the nicotinic receptor agonist 1,1-dimethyl-4-phenylpiperazinium (DMPP) produces a time- and concentration-dependent increase in TH enzyme activity, whereas muscarine has no effect. DMPP at 1 microM (EC50 = 0.3 microM) elicits a two- to threefold elevation of both TH activity and TH immunoreactive protein level after 3-5 days in the presence of 2.5 mM calcium; the increase in enzyme levels is significantly less at lower extracellular calcium levels. The rate of hydroxylation of tyrosine to dopamine (DA) in intact cells, an index of endogenous TH activity, increases in parallel with the rise in TH levels. The TH mRNA level is elevated before the increase in protein levels. As determined by nuclear run-on assays, TH gene transcription is stimulated two- to threefold within 30 min of addition of 1 microM DMPP to the cells; transcription returns to basal levels by 2 h. Nitrendipine (20 microM) blocks the stimulation of transcription by DMPP. Pretreatment of the cells with cycloheximide (5 microM) does not prevent the DMPP stimulation of transcription. Forskolin (10 microM) also increases TH transcription (fourfold in 15 min) by a mechanism that is not blocked by cycloheximide. These results show that nicotinic receptor stimulation increases TH mRNA synthesis, TH protein levels, and TH activity in a calcium-dependent manner. Furthermore, the nicotinic influence on TH gene expression does not appear to require the synthesis of a protein factor for its effects. That in situ DA synthesis rates are elevated consequent to the rise in TH levels demonstrates that TH induction serves as a mechanism for enhancing the catecholamine-synthesizing capacity of the chromaffin cell on a long-term basis.

Regulation of the neuronal nicotinic acetylcholine receptor by SRC family tyrosine kinases

Src family kinases (SFKs) are abundant in chromaffin cells that reside in the adrenal medulla and respond to cholinergic stimulation by secreting catecholamines. Our previous work indicated that SFKs regulate acetylcholine- or nicotine-induced secretion, but the site of modulatory action was unclear. Using whole cell recordings, we found that inhibition of SFK tyrosine kinase activity by PP2 (4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo(3,4-d)pyrimidine) treatment or expression of a kinase-defective c-Src reduced the peak amplitude of nicotine-induced currents in chromaffin cells or in human embryonic kidney cells ectopically expressing functional neuronal alpha3beta4alpha5 acetylcholine receptors (AChRs). Conversely, the phosphotyrosine phosphatase inhibitor, sodium vanadate, or expression of mutationally activated c-Src resulted in enhanced current amplitudes. These results suggest that SFKs and putative phosphotyrosine phosphatases regulate the activity of AChRs by opposing actions. This proposed model was supported further by the findings that SFKs physically associate with the receptor and that the AChR is tyrosine-phosphorylated.

The role of angiotensin II in regulating catecholamine secretion during hypoxia in rainbow troutOncorhynchus mykiss

Experiments were performed in vivo on chronically cannulated adult rainbow trout (Oncorhynchus mykiss) to assess the involvement of serotonergic or muscarinic receptor stimulation or activation of the renin–angiotensin system in eliciting catecholamine release during acute hypoxia during periods of nicotinic receptor desensitisation.Despite nicotinic receptor desensitisation induced by intravenous infusion of nicotine (1.3×10–5 mol kg–1 h–1), plasma catecholamine levels were increased to levels (adrenaline plus noradrenaline 125–200 nmol l–1) similar to those in control fish during severe hypoxia (40–45 mmHg; 5.3–6.0 kPa). Blockade of serotonergic receptors using methysergide or of muscarinic receptors using atropine did not affect the ability of fish to elevate circulating catecholamine levels during hypoxia. However, selective blockade of the renin–angiotensin system, using lisinopril to inhibit angiotensin-converting enzyme, prevented the elevation of both angiotensin II and circulating catecholamine levels in acutely hypoxic fish experiencing nicotinic receptor desensitisation. In fish possessing functional nicotinic receptors, angiotensin-converting enzyme blockade attenuated but did not prevent the elevation of plasma catecholamine levels during hypoxia. The results of this study indicate that the renin–angiotensin system is activated during hypoxia and plays a role in eliciting catecholamine release that is secondary to activation of nicotinic receptors. However, under conditions of nicotinic receptor desensitisation, activation of the renin–angiotensin system during hypoxia is a prerequisite for catecholamine release.

Pituitary adenylate cyclase-activating polypeptide stimulates secretoneurin release and secretogranin II gene transcription in bovine adrenochromaffin cells through multiple signaling pathways and increased binding of pre-existing activator protein-1-like transcription factors

Secretoneurin (SN) is a novel bioactive peptide that derives from the neuroendocrine protein secretogranin II (SgII) by proteolytic processing and participates in neuro-immune communication. The neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP-38) dose-dependently stimulates (EC(50) approximately 3 nM) SN release (up to 4-fold) and SgII gene expression (up to 60-fold) in cultured bovine adrenochromaffin cells. The effect of PACAP on both SN secretion and SgII mRNA levels is rapid and long lasting. We analyzed in this neuroendocrine cell model the transduction pathways involved in both SN secretion and SgII gene transcription in response to PACAP. The cytosolic calcium chelator BAPTA-AM and the nonselective calcium channel antagonist NiCl(2) equally inhibited both secretion of the peptide and transcription of the SgII gene, indicating a major contribution of calcium influx in PACAP-induced SN biosynthesis and release in chromaffin cells. Inhibition of protein kinase A (PKA) or C (PKC) also reduced PACAP-evoked SN release but did not alter the stimulatory effect of PACAP on SgII mRNA levels. Conversely, application of mitogen-activated protein kinase inhibitors suppressed PACAP-induced SgII gene expression. The effect of PACAP on SgII mRNA levels, like the effect of the PKC stimulator 12-O-tetradecanoylphorbol-13-acetate (TPA), was not affected by cycloheximide, whereas the effects of the PKA stimulator forskolin or cell-depolarization by high K(+) were significantly reduced by the protein synthesis inhibitor. PACAP and TPA both increased the binding activity of the SgII cAMP response element to trans-acting factors present in chromaffin cell nuclear extracts, which are recognized by antibodies to activator protein-1-related proteins. These data indicate that SN biosynthesis is regulated by PACAP in chromaffin cells through complex signaling cascades, suggesting that SN may play a function during trans-synaptic stimulation of the adrenal medulla.

Desensitisation of chromaffin cell nicotinic receptors does not impede catecholamine secretion during acute hypoxia in rainbow trout (Oncorhynchus mykiss)

Experiments were performed on adult rainbow trout (Oncorhynchus mykiss) in vivo using chronically cannulated fish and in situ using a perfused posterior cardinal vein preparation (i) to characterise the desensitisation of chromaffin cell nicotinic receptors and (ii) to assess the ability of fish to secrete catecholamines during acute hypoxia with or without functional nicotinic receptors. Intra-arterial injection of nicotine (6.0×10(−)(7)mol kg(−)(1)) caused a rapid increase in plasma adrenaline and noradrenaline levels; the magnitude of this response was unaffected by an injection of nicotine given 60 min earlier. Evidence for nicotinic receptor desensitisation, however, was provided during continuous intravenous infusion of nicotine (1.3×10(−)(5)mol kg(−)(1)h(−)(1)) in which plasma catecholamine levels increased initially but then returned to baseline levels. To ensure that the decline in circulating catecholamine concentrations during continuous nicotine infusion was not related to changes in storage levels or altered rates of degradation/clearance, in situ posterior cardinal vein preparations were derived from fish previously experiencing 60 min of saline or nicotine infusion. Confirmation of nicotinic receptor desensitisation was provided by demonstrating that the preparations derived from nicotine-infused fish were unresponsive to nicotine (10(−)(5)mol l(−)(1)), yet remained responsive to angiotensin II (500 pmol kg(−)(1)). The in situ experiments demonstrated that desensitisation of the nicotinic receptor occurred within 5 min of receptor stimulation and that resensitisation was established 40 min later. The ability to elevate plasma catecholamine levels during acute hypoxia (40–45 mmHg; 5.3-6.0 kPa) was not impaired in fish experiencing nicotinic receptor desensitisation. Indeed, peak plasma adrenaline levels were significantly higher in the desensitised fish during hypoxia than in controls (263+/−86 versus 69+/−26 nmol l(−)(1); means +/− s.e.m., N=6-9). Thus, the results of the present study demonstrate that activation of preganglionic sympathetic cholinergic nerve fibres and the resultant stimulation of nicotinic receptors is not the sole mechanism for eliciting catecholamine secretion during hypoxia.

Cotinine and nicotine inhibit each other's calcium responses in bovine chromaffin cells

Cotinine is the major metabolite of nicotine. It has some biological activity, but its pathophysiological effects are largely unclear. We studied whether cotinine initiates calcium transients or affects those induced by nicotine. In bovine adrenal chromaffin cells labeled with the fluorescent calcium indicator Fura 2, cotinine (0. 32-3.2 mM) concentration-dependently increased the intracellular Ca(2+) concentration ([Ca(2+)](i)). The effect was abolished by omitting extracellular Ca(2+) during the stimulations. Also nicotinic receptor channel blockers hexamethonium (10 microM-1 mM) and chlorisondamine (100 microM), as well as a competitive nicotinic receptor antagonist dihydro-beta-erythroidine (10-100 microM), inhibited the response. Cotinine (0.32-3.2 mM) preincubation for 2 min inhibited both the nicotine-induced and the cotinine-induced increases in [Ca(2+)](i). Also nicotine (3.2-10 microM) inhibited the cotinine-induced increase in [Ca(2+)](i). Tetrodotoxin (1 microM) and thapsigargin (1 microM) pretreatments did not affect the responses to cotinine, while 300 nM nimodipine partially inhibited the cotinine-induced increase in [Ca(2+)](i). The results indicate that cotinine has nicotine-like effects on chromaffin cells. It may also desensitize the nicotinic cholinergic receptors, possibly by acting as a low-affinity agonist at these receptors.

Stimulus-specific signaling pathways in rabbit carotid body chemoreceptors

The carotid body is an arterial chemosensory organ which responds to multiple natural and pharmacological stimuli, including hypoxia and nicotine. Numerous studies have investigated the initial molecular events which activate chemosensory type I cells in the carotid body, but less attention has been focused on later steps in the transduction cascade, which mediate neurotransmitter release from type I cells and excitation of chemoreceptor afferent fibers in the carotid sinus nerve. In the present study, we examined the effects of a highly specific inhibitor of calcium/calmodulin-dependent kinase II, KN-62, and a calmodulin inhibitor, trifluoperazine, on carotid sinus nerve activity and catecholamine release evoked from rabbit carotid bodies superfused in vitro. KN-62 did not alter sinus nerve activity and catecholamine release evoked by hypoxia, but this agent significantly reduced nerve activity and neurotransmitter release evoked by 100 microM nicotine. Trifluoperazine (10 microM), likewise inhibited activity evoked by nicotine, as well as hypoxia. Basal levels of nerve activity and catecholamine release (established in superfusate equilibrated with 100% O2) were unaffected by all drug treatments. Separate biochemical experiments showed that Ca2+/calmodulin-dependent incorporation of 32P into carotid body particulate proteins is significantly reduced following incubation of intact carotid bodies in nicotine, but not following exposure to hypoxia. Our observations suggest that excitation of the carotid body by diverse stimuli may involve the activation of distinct, stimulus-specific transduction pathways. Furthermore, these data correlate with our previous findings which showed that hypoxia, on the one hand, and nicotine on the other, evoke the preferential release of either dopamine or norepinephrine, respectively, from carotid bodies incubated in vitro.

Time-dependent effects of the neuropeptide PACAP on catecholamine secretion : stimulation and desensitization

Pituitary adenylyl cyclase-activating polypeptide (PACAP) is a potent endogenous secretagogue for chromaffin cells. We previously reported that PACAP coupled to the PAC1 receptor to evoke dihydropyridine-sensitive early (15 to 20 minutes) catecholamine secretion and cAMP response element binding protein-mediated trans-activation of the secretory protein chromogranin A promoter in PC12 pheochromocytoma cells. In this report, we studied whether the secretory and transcriptional responses elicited by PACAP were subject to desensitization. We found that PACAP evoked distinct immediate (initial, 0 to 20 minutes) and long-lasting (20 to 180 minutes) effects on catecholamine secretion. Initial secretory and chromogranin A trans-activation responses induced by PACAP were desensitized in a dose-dependent fashion after preexposure of cells to PACAP, and the IC(50) doses of PACAP for desensitization were approximately 18- to approximately 32-fold lower than the EC(50) activating doses for secretion or transcription. Desensitization of the initial secretion response was associated with decreased Ca(2+) influx through L-type voltage-operated Ca(2+) channels. Acute exposure to PACAP also triggered long-lasting (up to 3 hours), extracellular Ca(2+)-dependent, pertussis toxin-insensitive catecholamine secretion; indeed, even after short-term (20 minutes) exposure to PACAP and removal of the secretagogue, PC12 cells continued to secrete norepinephrine up to 76.9+/-0.22% of cellular norepinephrine content after 3 hours. A phospholipase C-beta inhibitor (U-73122) blocked this extended secretory response, which was dependent on low-magnitude Ca(2+) influx resistant to several L-, N-, P/Q-, or T-type Ca(2+) channel antagonists, but sensitive to Zn(2+), Ni(2+), Cd(2+), or to the store-operated Ca(2+) channel blocker SKF96365. A less than additive effect of the sarco-endoplasmic reticulum Ca(2+)-ATPase inhibitor thapsigargin plus PACAP on this sustained secretion also supported a contribution of store-operated Ca(2+) entry to the sustained secretory response. We propose that PACAP-evoked secretion and transcription are subject to homologous desensitization in PC12 cells; however, PACAP also induces long-lasting secretion, even under dose and time circumstances in which acute, dihydropyridine-sensitive secretion has been desensitized. Although initial secretion is mediated by an L-type voltage-operated Ca(2+) channel, extended secretion may involve a store-operated Ca(2+) channel that is activated through a G(q/11)/phospholipase C-beta/phosphoinositide signaling pathway.

Neuronal control of catecholamine secretion from chromaffin cells in the rainbow trout (Oncorhynchus mykiss)

The goal of the present investigation was to assess the relative involvement of nicotinic and muscarinic cholinergic receptors in the neuronal control of catecholamine secretion from the chromaffin tissue of rainbow trout (Oncorhynchus mykiss). This was accomplished by first developing and validating a nerve-stimulating technique able specifically to activate the nerve fibres innervating the chromaffin cells in order to elicit secretion of catecholamines. Using an in situ saline-perfused posterior cardinal vein preparation, it was demonstrated that whole-body field stimulation caused specific voltage-dependent neuronal stimulation of adrenaline and noradrenaline secretion. The contribution of non-specific depolarization was negligible. Several experimental results confirmed the specificity of the field stimulation technique. First, pre-treatment with neostigmine (an anticholinesterase) prolonged and more than doubled the amount of adrenaline secreted in response to electrical stimulation. Second, pre-treatment with the nicotinic receptor antagonist hexamethonium inhibited the electrically evoked secretion of adrenaline and noradrenaline. Third, perfusion with Na+-free saline or removal of the spinal cord abolished secretion of both catecholamines in response to the electrical stimulus. By using the field stimulation technique, this study is the first to demonstrate conclusively a role for muscarinic receptors in catecholamine secretion from trout chromaffin cells. Specifically, muscarinic cholinergic stimulation enhances nicotinic-evoked secretion of catecholamines and, under intense stimulation, may directly cause secretion. The results of the present study suggest the presence of muscarinic receptors on rainbow trout chromaffin cells with a functional role in the cholinergic control of catecholamine secretion.

Proadrenomedullin N-terminal 20 peptide: minimal active region to regulate nicotinic receptors

Proadrenomedullin N-terminal 20 peptide (PAMP-[1-20]; ARLDVASEFRKKWNKWALSR-amide) is a potent hypotensive and catecholamine release-inhibitory peptide released from chromaffin cells. We studied the mechanism of PAMP action and how its function is linked to structure. We tested human PAMP-[1-20] on catecholamine secretion in PC12 pheochromocytoma cells and found it to be a potent, dose-dependent (IC50 approximately 350 nmol/L) secretory inhibitor. Inhibition was specific for nicotinic cholinergic stimulation since PAMP-[1-20] failed to inhibit release by agents that bypass the nicotinic receptor. Nicotinic cationic (22Na+,45Ca2+) signal transduction was disrupted by this peptide, and potencies for inhibition of 22Na+ uptake and catecholamine secretion were comparable. Even high-dose nicotine failed to overcome the inhibition, suggesting noncompetitive nicotinic antagonism. N- and C-terminal PAMP truncation peptides indicated a role for the C-terminal amide and refined the minimal active region to the C-terminal 8 amino acids (WNKWALSR-amide), a region likely to be alpha-helical. PAMP also blocked (EC50 approximately 270 nmol/L) nicotinic cholinergic agonist desensitization of catecholamine release, as well as desensitization of nicotinic signal transduction (22Na+ uptake). Thus, PAMP may exert both inhibitory and facilitatory effects on nicotinic signaling, depending on the prior state of nicotinic stimulation. PAMP may therefore contribute to a novel, autocrine, homeostatic (negative-feedback) mechanism controlling catecholamine release.

Peptidergic activation of transcription and secretion in chromaffin cells. Cis and trans signaling determinants of pituitary adenylyl cyclase-activating polypeptide (PACAP)

Pituitary adenylyl cyclase-activating polypeptide (PACAP) is a potent endogenous secretagogue for chromaffin cells. Chromogranin A is the major soluble core component in secretory vesicles. Since chromogranin A is secreted along with catecholamines, we asked whether PACAP regulates expression of the chromogranin A gene in PC12 rat chromaffin cells, so as to resynthesize the just-secreted protein, and whether such biosynthetic regulation is coupled mechanistically to catecholamine secretion. PACAP activated the endogenous chromogranin A gene by four- to fivefold. Proportional results (seven- to eightfold activation) were obtained with a transfected 1,200-bp mouse chromogranin A promoter/luciferase reporter construct. A series of chromogranin A promoter 5' deletion mutant/luciferase reporter constructs narrowed down the PACAP response element to a proximal region containing the cAMP response element (CRE box), at (-71 bp)5'-TGACGTAA-3'(-64 bp). Site-directed point mutations of the CRE site suppressed PACAP-induced trans-activation of the promoter. Thus, the proximal CRE box is entirely necessary for the chromogranin A promoter response to PACAP. Transfer of the CRE box to a neutral, heterologous promoter also conferred activation by PACAP, suggesting that the CRE domain is also sufficient to mediate the transcriptional response to PACAP. Expression of a dominant-negative mutant (KCREB) of the CRE-binding factor CREB markedly diminished trans-activation of the chromogranin A promoter by PACAP. Cotransfection of expression plasmids encoding the protein kinase A inhibitor, or an inactive protein kinase A (PKA) catalytic beta subunit, inhibited both forskolin and PACAP activation of chromogranin A transcription, revealing that PACAP-induced trans-activation is highly dependent on PKA. By contrast, inhibition of protein kinase C (by chronic exposure to phorbol ester) had no effect on transcriptional activation by PACAP. The potent PACAP/vasoactive intestinal peptide (VIP) type I receptor antagonist PACAP6-38 impaired both chromogranin A transcription or catecholamine secretion triggered by PACAP38, while the PACAP/VIP type II receptor antagonist (p-Chloro-D-Phe6, Leu17)-VIP had little or no ability to antagonize the PACAP38 effect. The agonist VIP was approximately 100- to 1,000-fold less potent than PACAP in stimulating either secretion or transcription. Thus, PACAP-evoked chromogranin A transcription and catecholamine secretion are likely mediated by the PACAP/VIP type I receptor isoform. Although the calcium channel antagonists Zn2+ (100 microM), nifedipine (10 microM), or ruthenium red (10 microM), or the cytosolic calcium chelator BAPTA-AM (50 microM) each strongly impaired PACAP-induced secretion, transcriptional activation of chromogranin A remained unaltered. Therefore, we propose that PACAP signals to chromogranin A transcription through the CRE in cis, and through PKA and CREB in trans. By contrast, a pathway involving cytosolic calcium entry through L-type voltage-dependent channels is required for PACAP to evoke catecholamine secretion.

Desensitization and resensitization of norepinephrine release in the rat hippocampus with repeated nicotine administration

Desensitization of norepinephrine release was investigated with repeated intravenous (i.v.) infusions of nicotine and in vivo microdialysis of the hippocampus. At 100 min intervals, rats received three infusions of one of the following doses of nicotine: 0.045, 0.09 or 0.135 mg/kg. Doses of 0.09 mg/kg or higher increased norepinephrine release (F= 2.41, P < 0.05). However, the norepinephrine response to the second or third infusion was significantly reduced, compared to the first. The extent of desensitization and rate of resensitization was investigated further by administering consecutive infusions of nicotine (0.135 mg/kg) 40, 60, 100 or 200 min apart. Less norepinephrine was released after a second nicotine infusion given 40 to 100 min later, but this was not reduced further by a third infusion. Norepinephrine release was unchanged with a 200 min inter-infusion interval. Therefore, in the hippocampus, maximal desensitization of nicotine-stimulated norepinephrine release occurs as early as 40 min and persists for at least 100 min; thereafter, resensitization becomes the dominant process.

Substance P and epibatidine-evoked catecholamine release from fractionated chromaffin cells

Bovine chromaffin cells were separated by density gradient centrifugation into subfractions enriched with either > 90% adrenaline- or 70-80% noradrenaline-producing cells. Concentrations of epibatidine (an alkaloid with nicotinic receptor activity) as low as 10 nM released adrenaline and noradrenaline from both fractions of cells maintained as monolayer cultures. The maximal effect was evoked by 30 nM epibatidine and was comparable to that evoked by 10 microM nicotine. The catecholamine release from the noradrenaline fraction was 30-40% higher than from the adrenaline fraction. Initial exposure to 50 nM epibatidine reduced release induced by a second exposure to the drug. There was cross-desensitization between epibatidine and nicotine. Substance P inhibited the epibatidine-evoked catecholamine release from both fractions by up to 85% (IC50 = 3-5 microM). The release of noradrenaline was inhibited more than that of adrenaline. In addition, substance P protected the chromaffin cells against desensitization of the nicotinic receptor by epibatidine. The C-terminal heptapeptide sequence of substance P was 10 x less active, two N-terminal sequences did not modulate the catecholamine release.

Ibogaine and the dopaminergic response to nicotine

There is increasing evidence that the rewarding effect of nicotine is mediated by the mesolimbic dopamine system. The first objective of this study was to examine the dopamine response to repeated i.v. infusions of nicotine. Using in vivo microdialysis in awake and freely moving male Sprague-Dawley rats, we demonstrated that i.v. nicotine infusions (0.16 mg/kg or 0.32 mg/kg per infusion) produced increases in extracellular dopamine levels that were dose- and infusion order-dependent. Acute tolerance was evidenced by the smaller dopamine response produced by a second infusion of nicotine, administered 1 h after the first one. Tolerance was reversible, since the dopamine response to a second infusion of nicotine was unchanged when the interval between the infusions was increased to 3 h. Ibogaine, an alkaloid found in Tabernanthe iboga, is claimed to decrease smoking and to have an anti-nicotinic action. The second objective of this study was to establish whether this claim has any neurochemical basis. Pretreatment with ibogaine (40 mg/kg, i.p.) 19 h prior to the first nicotine infusion (0.32 mg/kg per infusion) significantly attenuated the increase in extracellular dopamine levels induced by-the nicotine infusions, suggesting that ibogaine may decrease the rewarding effect of nicotine.

Nicotine and sympathetic neurotransmission

Nicotine increases heart rate, myocardial contractility, and blood pressure. These nicotine-induced cardiovascular effects are mainly due to stimulation of sympathetic neurotransmission, as nicotine stimulates catecholamine release by an activation of nicotine acetylcholine receptors localized on peripheral postganglionic sympathetic nerve endings and the adrenal medulla. The nicotinic acetylcholine receptor is a ligand-gated cation channel with a pentameric structure and a central pore with a cation gate, which is essential for ion selectivity and permeability. Binding of nicotine to its extracellular binding site leads to a conformational change of the central pore, which results in the influx of sodium and calcium ions. The resulting depolarization of the sympathetic nerve ending stimulates calcium influx through voltage-dependent N-type calcium channels, which triggers the nicotine-evoked exocytotic catecholamine release. In the isolated perfused guinea-pig heart, cardiac energy depletion sensitizes cardiac sympathetic nerves to the norepinephrine-releasing effect of nicotine, as indicated by a leftward shift of the concentration-response curve, a potentiation of maximum transmitter release, and a delay of the tachyphylaxis of nicotine-evoked catecholamine release. This sensitization was also shown to occur in the human heart under in vitro conditions. Through the intracardiac release of norepinephrine, nicotine induces a beta-adrenoceptor-mediated increase in heart rate and contractility, and an alpha-adrenoceptor-mediated increase in coronary vasomotor tone. The resulting simultaneous increase in oxygen demand and coronary resistance has a detrimental effect on the oxygen balance of the heart, especially in patients with coronary artery disease. Sensitization of the ischemic heart to the norepinephrine-releasing effect of nicotine may be a trigger for acute cardiovascular events in humans, such as acute myocardial infarction and/or life-threatening ventricular tachyarrhythmias.

Activation of Fyn tyrosine kinase upon secretagogue stimulation of bovine chromaffin cells

Adrenal medullary chromaffin cells derive from the neural crest during embryogenesis and differentiate into dedicated secretory cells that release catecholamines in response to acetylcholine in vivo or nicotinic agonists in vitro. Previous studies have indicated that tyrosine kinases participate in early secretagogue-induced events in these cells and are required for exocytosis. Abundant levels of the cytoplasmic tyrosine kinases, c-Src and c-Yes, have been detected in chromaffin cells, thereby implicating them as kinases relevant to these events. However, c-Src has been found to undergo a decrease in activity following secretagogue-stimulation, and c-Yes appears to exist in a constituitively low activity state, suggesting that other tyrosine kinases are involved. Furthermore, other members of the Src family of tyrosine kinases have been implicated as playing roles in secretion in a variety of cell types. Therefore, we sought to determine if other Src family members were present in chromaffin cells, and if so, to examine them for subcellular localization and changes in activity following treatment with nicotinic agonists. To this end, antibodies for Fyn, Lck, Lyn, and Fgr were assembled and used in immunoprecipitation, in vitro autokinase, and Western immunoblotting assays. Of these four kinases, only Fyn was found to be expressed at detectable levels. Differential centrifugation studies revealed that Fyn resides predominantly (> 95%) in the crude plasma membrane fraction and undergoes nicotinic-and carbachol-induced activation. This activation is reduced by the nicotinic antagonist, mecamylamine, is not elicited by muscarine, and is dependent upon the presence of extracellular Ca2+. These results suggest that Fyn is involved in signalling through the nicotinic receptor and may be one of the relevant kinases responsible for at least some of the tyrosine phosphorylations detected after stimulation.

Pressor and tachycardic responses to intravenous substance P in anesthetized rats

Intravenous injection of 3-33 nmol/kg of substance P (SP) caused pressor and tachycardic responses in anesthetized rats. The responses were not blocked by a ganglion nicotinic receptor antagonist or by pithing. Pretreatment with reserpine blocked both responses. beta-Adrenoceptor blockade attenuated only the tachycardic response, and alpha-adrenoceptor blockade attenuated only the pressor response. These findings indicated that the effects of SP to increase blood pressure and heart rate are due to sympathetic ganglion stimulation. Studies with adrenalectomized rats showed that stimulation of the adrenals by SP contributes to both responses but makes a greater contribution to the tachycardic response. These observations raise the possibility that the tachykinin innervation of sympathetic ganglia and the adrenal medulla may be involved in the local regulation of blood pressure and heart rate.

Functional studies with substance P analogues: effects of N-terminal, C-terminal, and C-terminus-extended analogues of substance P on nicotine-induced secretion and desensitization in cultured bovine adrenal chromaffin cells

Substance P (SP) and SP analogues, including C-terminal, N-terminal, and C-terminus-extended analogues, have been investigated for their ability to modulate nicotine-induced secretion from bovine adrenal chromaffin cells in culture. Secretion was monitored by measuring the release of endogenous catecholamines by electrochemical detection following separation on HPLC and the release of endogenous ATP with an on-line luciferin-luciferase bioluminescence technique. SP is known to have the following two effects on nicotine-induced secretion of catecholamines (see Livett and Zhou, 1991): inhibition of the nicotinic response and protection against nicotinic desensitization. Secretion induced by 10(-5) M nicotine was inhibited 70-80% by SP, SP-methyl ester, and the C-terminus-extended analogue SP-Tyr12-NH2, 65% by (Ala3)SP-NH2, 45% by the C-terminal analogue SP(4-11), and 20 and 5% by the N-terminal analogues SP(1-7) and SP(1-5), respectively, when these peptides were present at 3 x 10(-5) M concentrations. The order of potency was SP = SP-methyl ester = SP-Tyr12-NH2 > (Ala3)SP-NH2 > SP(4-11) > SP(1-7) > SP(1-5). SP, SP-methyl ester, and (Ala3)SP-NH2 protected against nicotinic desensitization by 40-55%, and SP(4-11) protected by 20% (all at 3 x 10(-5) M). In contrast, the N-terminal analogues SP(1-7) and SP(1-5) and the C-terminus-extended analogue SP-Tyr12-NH2 at 3 x 10(-5) M did not protect against nicotinic desensitization. Cyclo-SP(3-9), Ac-SP(3-9)-NH2, SP(3-9), and SP(3-6) had neither inhibitory nor facilitatory effects on secretion.(ABSTRACT TRUNCATED AT 250 WORDS)

Absence of appreciable tolerance and morphine cross-tolerance in rats with adrenal medullary transplants in the spinal cord

Adrenal medullary transplants in the spinal subarachnoid space may be a means of achieving sustained local delivery of pain-reducing neuroactive substances on a continually renewable basis. However, a potential limitation of this approach is tolerance development to agents released from the transplanted cells. In particular, since adrenal medullary chromaffin cells release opioid peptides, reduced antinociceptive efficacy of opioids is possible. To determine this, alterations in the dose-effectiveness of morphine were assessed in animals with adrenal medullary transplants. Results indicated that, not only was there no apparent tolerance, but that adrenal medullary transplants could potentiate the analgesic efficacy of morphine. An additional goal of these studies was to determine whether chronic or intermittent nicotine could produce increased antinociception, since stimulation of cell surface nicotinic receptors increases release of neuroactive substances from chromaffin cells. This was assessed using subcutaneously implanted nicotine pellets or repeated systemic administration of nicotine. Findings indicated that exposure to nicotine results in an acute tolerance, or tachyphylaxis, to nicotine which is rapidly reversed following cessation of nicotinic stimulation. Together, these results suggest that adrenal medullary transplants may provide a constant source of opioid peptides, augmentable by intermittent nicotinic stimulation, without the development of appreciable tolerance to these pain-reducing neuroactive substances.

Nicotine tolerance in chromaffin cell cultures: acute and chronic exposure to smoking-related nicotine doses

Nicotine tolerance and dependence are key aspects of tobacco addiction; however, the cellular mechanisms underlying these phenomena are poorly understood. Adrenal chromaffin cells release catecholamines upon exposure to nicotine and with repeated exposure this response exhibits nicotine tolerance. Using bovine adrenal chromaffin cells in culture, we have demonstrated acute and chronic nicotine tolerance at doses relevant to that in the blood and tissues of smokers (10(-7) M to 10(-6) M). Chromaffin cells are preexposed to low doses of nicotine for time periods ranging from 10 min to 7 days and then subsequently challenged with a maximally stimulating dose of nicotine (10(-5) M) for 10 min, all at 37 degrees C. Preexposure to nicotine results in a depression of 45Ca uptake and catecholamine release upon subsequent nicotine challenge. Acute tolerance or desensitization of nicotine-stimulated catecholamine release begins to occur in minutes after preexposure to 10(-6) M nicotine at 37 degrees C. The depression of catecholamine release upon preexposure to nicotine is both dose and temperature dependent and is not seen with potassium-evoked release. Chronic exposure to 10(-7) M nicotine for 3 days led to a depression of the secretory response to approximately 70% of control responses. There was a trend toward recovery of full response by days 5 and 7 of 10(-7) M nicotine preexposure. Nearly complete depression of the nicotine-evoked release occurs within the first day of exposure to 10(-6) M nicotine and persists for at least a week of nicotine exposure at 37 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)

Desensitization of nicotine-stimulated [3H]dopamine release from mouse striatal synaptosomes

Potential desensitization of brain nicotinic receptors was studied using a [3H]dopamine release assay. Nicotine-stimulated [3H]dopamine release from mouse striatal synaptosomes was concentration-dependent with an EC50 of 0.33 +/- 0.13 microM and a Hill coefficient of 1.44 +/- 0.18. Desensitization by activating concentrations of nicotine had a similar EC50 and a half-time of 35 s. Concentrations of nicotine that evoked little release also induced a concentration-dependent desensitization (EC50 = 6.9 +/- 3.6 nM, t1/2 = 1.6-2.0 min, nH = 1.02 +/- 0.01). Both types of desensitization produced a maximum 75% decrease in [3H]dopamine release. Recovery from desensitization after exposure to low or activating concentrations of nicotine was time-dependent with half-times of 6.1 min and 12.4 min, respectively. Constants determined for binding of [3H]nicotine to striatal membrane at 22 degrees C included a KD of 3.7 +/- 0.5 nM, Bmax of 67.5 +/- 2.2 fmol/mg, and Hill coefficient of 1.07 +/- 0.06. Association of nicotine with membrane binding sites was biphasic with half-times of 9 s and 1.8 min. The fast rate process contributed 37% of the total reaction. Dissociation was a uniphasic process with a half-time of 1.6 min. Comparison of constants determined by the release and binding assays indicated that the [3H]-nicotine binding site could be the presynaptic receptor involved in [3H]dopamine release in mouse striatal synaptosomes.

Nicotine-induced intracellular calcium changes are not antagonized by alpha-bungarotoxin in adrenal medullary cells

The snake toxin alpha-bungarotoxin distinguishes between neuronal nicotinic receptor subtypes. In chick ciliary ganglion neurons, activation of alpha-bungarotoxin-sensitive nicotinic receptors has been proposed to produce elevations in intracellular calcium levels. In the present study we show that prolonged treatment with alpha-bungarotoxin did not affect the nicotine-evoked calcium response in suspended chromaffin cells. On the other hand, the classical nicotinic receptor blocker d-tubocurarine potently blocked nicotinic receptor mediated effects. The degree of inhibition of the nicotinic response observed with d-tubocurarine was not modified by prior treatment with alpha-bungarotoxin. These results suggest that nicotinic alpha-bungarotoxin receptors are not primarily involved in nicotine-mediated increases in intracellular calcium in bovine adrenal medullary cells.

Exocytosis from a single rat chromaffin cell by cholinergic and peptidergic neurotransmitters

Secretion of catecholamines from chromaffin cells is mediated by cholinergic and peptidergic neurotransmitters. The cholinergic transmitter acetylcholine activates both nicotinic and muscarinic receptors to trigger catecholamine secretion in rat adrenal medulla. Vasoactive intestinal polypeptide (VIP) has been identified as the peptidergic transmitter in rat adrenal medulla and may also be the non-cholinergic transmitter in bovine adrenal. Pituitary adenylate cyclase activating polypeptide (PACAP), a VIP-like secretin peptide, is also found in the adrenal, and is a potent secretagogue. Thus, PACAP may be another peptidergic transmitter at the adrenal synapse. A most intriguing property of rat chromaffin cells is that stimulation of nicotinic, muscarinic, VIP or PACAP receptors are each able to produce robust catecholamine secretion on their own. This raises the question of whether a single chromaffin cell can respond to each of the above agonists or whether the secretion is due to subpopulations of chromaffin cells. This issue was addressed by using electrochemical techniques to monitor exocytosis from individual chromaffin cells in culture. We demonstrate that acetylcholine, nicotine, muscarine, VIP and PACAP are each able to evoke catecholamine secretion from a single chromaffin cell. Some cells only responded to acetylcholine. Furthermore, each agonist produced a distinct pattern of exocytosis. Muscarine-evoked secretion exhibited a latency of 0.5-2 s, but exocytosis persisted up to 30 s following 500 ms stimulation. Nicotine produced an immediate response which usually ended within 10 s. The secretory pattern following acetylcholine appeared to be the sum of the nicotinic and muscarinic patterns, showing both rapid onset and longer duration.(ABSTRACT TRUNCATED AT 250 WORDS)

pp60c-src enhances the acetylcholine receptor-dependent catecholamine release in vaccinia virus-infected bovine adrenal chromaffin cells

Secretion of catecholamines by adrenal chromaffin cells is a highly regulated process that involves serine/threonine and tyrosine phosphorylations. The nonreceptor tyrosine kinase pp60c-src is expressed at high levels and localized to plasma membranes and secretory vesicle membranes in these cells, suggesting an interaction of this enzyme with components of the secretory process. To test the hypothesis that pp60c-src is involved in exocytosis, we transiently expressed exogenous c-src cDNA using a vaccinia virus vector in primary cultures of bovine adrenomedullary chromaffin cells. Chromaffin cells infected with a c-src recombinant virus restored the diminished secretory activity accompanying infection by wild type virus alone or a control recombinant virus. The level of enhanced catecholamine release correlated directly with the time and level of exogenous c-src expression. These results could not be attributed to differences in cytopathic effects of wild type versus recombinant viruses as assessed by cell viability assays, nor to differences in norepinephrine uptake or basal release, suggesting that pp60c-src is involved in stimulus-secretion coupling in infected cells. Surprisingly, exogenous expression of an enzymatically inactive mutant c-src also restored catecholamine release, indicating that regions of the introduced c-src protein other than the kinase domain may affect catecholamine release. Secretory activity was elevated by both forms of c-src in response to either nicotine or carbachol (which activate the nicotinic and the nicotinic/muscarinic receptors, respectively). In contrast, release of catecholamines upon membrane depolarization (as elicited by 55 mM K+) or by treatment with the calcium ionophore A23187 was unaffected by either vaccinia infection or increased levels of pp60c-src. These results suggest that pp60c-src affects secretory processes in vaccinia-infected cells that are activated through ligand-gated, but not voltage-gated, ion channels.

An amyloid peptide, beta A4 25-35, mimics the function of substance P on modulation of nicotine-evoked secretion and desensitization in cultured bovine adrenal chromaffin cells

The amyloid protein (beta A4) is found in the CNS of patients with Alzheimer's disease; however, the pathogenic role of this protein is not known. In the present study, a peptide fragment of beta A4 (beta A4 25-35; Gly-Ser-Asn-Lys-Gly-Ala-Ile-Ile-Gly-Leu-Met-NH2), which contains the conserved C-terminal sequence of substance P (X-Gly-Leu-Met-NH2), and the neuropeptide substance P (SP) were examined for their ability to modulate nicotine-evoked secretion from cultured bovine adrenal chromaffin cells. Secretion of the released endogenous catecholamines was monitored by electrochemical detection after separation by HPLC. Secretion induced by 10(-5) M nicotine was inhibited by SP and beta A4 25-35. The IC50 of SP and beta A4 25-35 was 3 x 10(-6) and 3 x 10(-5) M, respectively. SP and beta A4 25-35 both protected against nicotine receptor desensitization. However, beta A4 25-35 was approximately 10-fold less effective than SP in its protective effect. The present work shows that beta A4 25-35 can mimic the modulatory actions of SP on the nicotinic response of cultured bovine chromaffin cells, i.e., inhibition of the nicotinic response and protection against nicotinic desensitization. These modulatory actions may be associated with changes in nicotinic receptor levels reported to occur in Alzheimer's disease.

Inhibition of voltage-dependent Ca2+ channels via alpha 2-adrenergic and opioid receptors in cultured bovine adrenal chromaffin cells

Adrenal chromaffin cells secrete catecholamines and opioids. The effects of these agents on whole-cell Ca2+ channel currents were studied, using bovine adrenal chromaffin cells kept in short term culture. Ca2+ channel currents recorded during voltage-clamp pulses from a holding potential of -80 mV to 0 mV were reversibly reduced by 10 microM epinephrine (in the presence of 1 microM propranolol) or 5 microM of the synthetic opioid, d-Ala2-d-Leu5-enkephalin (DADLE) by approximately 35% and 25%, respectively. The inhibitory action of epinephrine was mimicked by clonidine, reduced by yohimbine but not affected by prazosin. The DADLE-induced reduction of the Ca2+ channel current was antagonized by naloxone. The dihydropyridine (+)PN 200-110 (5 microM) reduced the Ca2+ channel current by approximately 40%; the Ca2+ channel current inhibited by (+)PN 200-110 was not further reduced by epinephrine. Intracellular infusion of guanosine-5'-O-(2-thiodiphosphate) and pretreatment of cells with pertussis toxin abolished the inhibitory effect of both epinephrine and DADLE. In membranes of adrenal chromaffin cells, four pertussis-toxin-sensitive G-proteins were identified, including Gi1, Gi2, Go1 and another Go subtype, possibly Go2. The data show that activation of alpha 2-adrenergic and opioid receptors causes an inhibition of dihydropyridine-sensitive Ca2+ channels in adrenal chromaffin cells. These inhibitory modulations are mediated by pertussis-toxin-sensitive G-proteins and may represent a mechanism for a negative feedback signal by agents released from the adrenal medulla.

Nicotine stimulates adrenergic terminals and inhibits contractions of mouse uterine horns

1. Nicotine (1-100 microM) stimulated both basal and electrically evoked release of 3H-norepinephrine and also caused a transient inhibition of contractions in an in vitro preparation of mouse uterine horns. 2. The inhibitory effect of nicotine on electrically evoked contractions was potentiated by aminophylline (89 micrograms/ml), and overcome by both propranolol (1 microM) and by omitting magnesium from the physiological solution. Acetylcholine (10 microM), in the presence of atropine (10 microM) was able to reproduce the inhibitory effect of nicotine. 3. These pharmacological findings suggest that the inhibitory action of nicotine on electrically evoked contractions in mouse uterus could be indirect, i.e. mediated through the action of this compound on presynaptic nicotine receptors located on adrenergic terminals.

Activation of tyrosine hydroxylase by nicotine in rat adrenal gland

The administration of nicotine activates tyrosine hydroxylase in the rat adrenal gland. This activation is apparently maximal 25 min after a single subcutaneous injection of nicotine at 2.3 mg/kg. Repeated injections of nicotine (seven injections once every 30 min) are associated with a persistent activation of adrenal tyrosine hydroxylase for at least 3 h. The nicotinic receptor antagonist hexamethonium does not significantly inhibit the nicotine-mediated activation of tyrosine hydroxylase in innervated adrenal glands. However, hexamethonium completely blocks the activation of adrenal tyrosine hydroxylase by nicotine in denervated adrenal glands. Furthermore, even though a single injection of nicotine activates tyrosine hydroxylase in both innervated and denervated adrenal glands, repeated injections of nicotine do not activate tyrosine hydroxylase in denervated adrenal glands. Our results suggest that the systemic administration of nicotine activates adrenal tyrosine hydroxylase by two mechanisms: (1) via direct interaction with adrenal chromaffin cell nicotinic receptors; and (2) via stimulation of the CNS leading to the release from the splanchnic nerve of substances that interact with adrenal chromaffin cell receptors other than the nicotinic receptor.

Substance P modulates the time course of nicotinic but not muscarinic catecholamine secretion from perfused adrenal glands of rat

1. Substance P (SP) and acetylcholine (ACh) are contained within the splanchnic nerve terminals in the adrenal gland and can be released in response to stress. In the rat, the release of aCh brings about secretion of catecholamines (CA) by acting on nicotinic and muscarinic receptors on the adrenal chromaffin cells. 2. In the present study, we have used a rat isolated adrenal gland preparation to investigate the effects of SP, perfused at different concentrations, on CA secretion evoked by 10(-5) M nicotine and 10(-4) M muscarine. 3. In the first 10 min stimulation period (S1), in the absence of SP, nicotine (10(-5) M) evoked substantial and equal secretion of noradrenaline (NA) and adrenaline (Ad). In a second 10 min stimulation period (S2), carried out 18 min after S1, the nicotinic response was desensitized. In contrast, the muscarinic response, which preferentially evoked Ad secretion in S1 (Ad/NA: 8.7/1), was well maintained in S2. 4. SP present in S1 had no effect on desensitization of the subsequent nicotinic response in S2. 5. At low concentrations (10(-7)-10(-10) M), SP changed the time course of nicotine-induced CA secretion during S1 by enhancing CA secretion in the first 4 min and inhibiting CA secretion thereafter. The maximal effect occurred at 10(-9) M SP. 6. At a higher concentration (10(-5) M), SP inhibited total nicotinic CA secretion throughout S1 and produced a biphasic secretion of CA (depressed in the presence of SP and enhanced after wash out of SP). Pre-exposure of adrenal glands to SP (10-' to 10- M) for 10min produced marked inhibition of the nicotine-induced CA secretion. 7. In contrast to the effect of SP on the nicotinic response, SP from 10- to 10-SM had no effect on muscarinic CA secretion. 8. This difference in sensitivity of the nicotinic and muscarinic responses to SP points to a diversity of mechanisms available for control of adrenal catecholamine secretion. In addition to the ability of SP to increase or decrease the total amount of adrenal CA secretion, dependent on the concentration of SP, the present study shows that SP can change the time-course of nicotinic CA secretion. These results with the rat adrenal gland perfused in vitro suggests both a quantitative and temporal role for SP as a novel modulator of adrenal CA secretion.

Electrically-evoked catecholamine release from cat adrenals. Role of cholinergic receptors

Catecholamine (CA) release from perfused cat adrenal glands was continuously monitored using an on-line system coupled to an electrochemical detector. This highly sensitive procedure allowed the detection of small changes in the rate of secretion, even using short trains of electrical stimulation or brief acetylcholine (ACh) pulses. CA release was linear with increasing strength of ACh, transmural or splanchnic nerve stimulation. By using specific blockers, the contribution of nicotinic or muscarinic receptors to the overall secretory response to various stimuli could be established. That nicotinic receptors play a major role in mediating the secretory response to all stimuli is shown by the clear inhibition of the response with mecamylamine (10 microM). In contrast, atropine (1 microM) halved secretion evoked by ACh or nerve stimulation but had little effect on the response to trains of transmural electrical stimulation. When transmural electrical stimulation was applied continuously (instead of in trains), increasing the frequency in a step-wise manner, a bell-shaped curve was obtained; secretion reached a peak at 8 Hz and then declined sharply at 16 and 32 Hz. With this stimulation pattern, atropine decreased by 50% the secretion response at the higher frequencies (4-32 Hz). Very few studies are available which define the role of receptors and ionic channels in mediating electrically-evoked CA release. These stimulation patterns have not been used previously and are likely to mimic more closely than those used in earlier studies the physiologic firing pattern of splanchnic nerves innervating adrenomedullary cells.

Mixed nicotinic and muscarinic features of cholinergic receptor coupled to secretion in bovine chromaffin cells

Acetylcholine evokes release from cultured bovine chromaffin cells by a mechanism that is believed to be classically nicotinic. However, we found that the full muscarinic agonist oxotremorine-M (Oxo-M) induced a robust catecholamine (CA) secretion. By contrast, muscarine, pilocarpine, bethanechol, and McN-A-343 did not elicit any secretory response. Desensitization of the response to nicotine by Oxo-M and desensitization of the response to Oxo-M by nicotine suggest that both nicotine and Oxo-M were acting at the same receptor. Additional experiments supporting this conclusion show that nicotine-induced secretion and Oxo-M-induced secretion were similarly blocked by various muscarinic and nicotinic antagonists. Moreover, secretion induced by nicotine and Oxo-M were Ca2+ dependent, and both agonists induced 45Ca2+ uptake. Equilibrium binding studies showed that [3H]Oxo-M bound to chromaffin cell membranes with a Kd value of 3.08 x 10(-8) M and a Hill coefficient of 1.00, suggesting one binding site for this ligand. Nicotine inhibited Oxo-M binding in a noncompetitive manner, suggesting that both ligands bind at two different sites on the same receptor. We propose that the receptor on bovine chromaffin cells that is coupled to secretion represents an unusual cholinergic receptor that has both nicotinic and muscarinic features.

Regulation of the secretory response in bovine chromaffin cells

The nicotine-induced current and the Ca2+ current were studied in cultured bovine chromaffin cells using the whole cell patch-clamp technique. The dose-response curve for the nicotinic current gave a dissociation constant of 53 microM and a Hill coefficient of 1.3. Desensitization of the nicotinic current was rapid, with time constants of 22 and 155 ms at 10 microM nicotine. At higher concentrations of nicotine, both time constants decreased somewhat, but the most prominent effect was on the ratio of the two components. Recovery from desensitization was fitted by a single exponential with a time constant of approximately 6 s. Ca2+ current and catecholamine secretion were highly sensitive to changes in extracellular H+ concentration ([H+]o), such that small increases in [H+]o markedly decreased both. The Ca2+ current measured in a chromaffin cell located within a cluster of cells, but not in a single isolated cell, was markedly suppressed when KCl or nicotine was used to induce secretion, suggesting possible local feedback of secretory agents. Among agents secreted by chromaffin cells, ATP, enkephalins, epinephrine, and protons, only protons significantly suppressed the Ca2+ current. Our findings suggest that the secretory response of chromaffin cells may be modulated by rapid desensitization of the nicotinic receptor and a secretion-dependent suppression of the Ca2+ current.

Capsaicin-sensitive sensory neurons are involved in the plasma catecholamine response of rats to selective stressors

1. The effect of capsaicin pre-treatment on adrenal catecholamine (CA) secretion in response to stress is controversial. In earlier experiments performed under pentobarbitone anaesthesia, the release of CA in response to stress was complicated by the effects of the barbiturate anaesthesia. 2. In the present study we have used conscious freely moving rats with indwelling cannulae to study the effect of neonatal capsaicin pre-treatment on the plasma CA response to different types of stressors (swimming stress, hypovolaemic stress, immobilization stress and cold stress). 3. After swimming for 20 min, plasma noradrenaline (NA) levels increased by 8-fold and adrenaline by 2-fold in control rats. The increase in plasma NA levels in the capsaicin group was attenuated at 10 min of swimming compared with the vehicle group (P < 0.05). 4. With hypovolaemic stress, there were no differences in plasma CA levels, blood pressure and heart rate between the capsaicin group and the vehicle group. There were also no differences in plasma CA levels after immobilization stress between the two groups. 5. With cold stress, plasma NA levels increased 5-fold and adrenaline levels by 3-fold over basal at 45 min in the vehicle pre-treated rats. This increase was not observed in the capsaicin group. 6. Immunoreactive substance P was depleted by only 68% in the splanchnic nerve following capsaicin pre-treatment. If the remaining 32% was biologically active substance P then it could account for the maintenance of the response to hypovolaemic and immobilization stress. However, it might be possible that the responses to hypovolaemic and immobilization stresses could be attenuated if a more complete depletion were achieved. 7. These results in conscious rats indicate that capsaicin-sensitive sensory neurons are required for plasma CA response to selective stressors. They are required for CA output in response to cold stress and to the early phase of swimming stress, but not to hypovolaemic stress and immobilization stress.

Coordinate and differential regulation of proenkephalin A and PNMT mRNA expression in cultured bovine adrenal chromaffin cells: responses to secretory stimuli

The expression of proenkephalin A (ProEnk A) mRNA and phenylethanolamine N-methyltransferase (PNMT) mRNA in response to nicotine and to a number of secretagogues was examined in cultured bovine adrenal chromaffin cells. Prolonged incubation with nicotine (10 microM) resulted in a 2-fold increase in ProEnk A mRNA but had no significant effect on the level of PNMT mRNA. Similarly, prolonged stimulation with high K+ (56 mM) induced a time-dependent elevation in the level of ProEnk A mRNA reaching 4-fold basal level after 24 h incubation. By contrast, the level of PNMT mRNA was not changed by treatment with high K+. The increase in the level of ProEnk A mRNA by high K+ was abolished by the presence of 10 microM D600, a calcium channel blocker. Unlike the effects of high K+, treatment of the cells with the sodium channel activator veratridine significantly elevated the levels of both ProEnk A and PNMT mRNA. This increase in ProEnk A and PNMT mRNA levels was however less affected by D600. Stimulation of the cells with Ba2+ (1.1 mM) also stimulated the levels of ProEnk A and PNMT mRNA and this action required the presence of extracellular Ca2+. This was in contrast to the effect of Ba2+ in stimulating catecholamine secretion, which was inhibited by Ca2+ and enhanced in Ca2(+)-free buffer. The results of the present study indicate that membrane depolarization and entry of extracellular Ca2+ play an important role on the regulation of ProEnk A and PNMT mRNAs, in addition to their well-known actions on hormone secretion. Furthermore, these results suggest that the expression of ProEnk A mRNA and PNMT mRNA are under independent regulation in response to secretory stimulation.

Inhibition by opioids of phagocytosis in peritoneal macrophages

We have tested the effect of prototypic opioid agonists on phagocytosis of sheep erythrocytes by mouse peritoneal macrophages. It was found that morphine and all the opioid peptides tested inhibited phagocytosis by a biphasic, naloxone-reversible mechanism. Delta agonists were the most effective inhibitors, suggesting that the response is mediated by a delta receptor. Chronic exposure to morphine apparently results in the development of tolerance since under these conditions the inhibitory effect of the opiate is abolished. These results are similar to previously reported effects of opioids on endocytosis in other systems, which suggests that this inhibition is part of a basic regulatory mechanism that has been conserved in evolution.

Substance P enhances desensitization of the nicotinic response in bovine chromaffin cells but enhances secretion upon removal

A fundamental process in neurosecretion is desensitization, or a declining response to a stimulus. The response of chromaffin cells to continuous nicotinic stimulation, secretion of catecholamines, desensitizes within a few minutes. The neuropeptide substance P (SP) has been reported to prevent desensitization in culture dish experiments and to enhance desensitization in patch clamp studies. In the present study, these contradictory responses have been demonstrated and the apparent contradictions resolved. We have measured catecholamine secretion by on-line electrochemical detection in a constant-pressure flow system. Isolated chromaffin cells cultured on quartz plates were stimulated with the nicotinic agonist 1,1-dimethyl-4-phenylpiperazinium (DMPP) in the presence and absence of SP. SP inhibited secretion and increase the rate of desensitization compared with stimulation by DMPP alone. However, when the cells were stimulated a second time with DMPP alone immediately after 5-min stimulation with SP + DMPP, the rate of desensitization was markedly lower than the control. Removal of SP after a desensitizing stimulation with SP + DMPP caused a slow secondary release of catecholamine in response to the continued stimulation with DMPP. The kinetic analysis of the secretory response shows that the primary response to SP is enhanced desensitization, but that upon removal of SP the response to DMPP desensitizes less rapidly. We suggest that SP protects some receptors from nicotinic desensitization while holding them in an inactive state, and that upon removal of SP these receptors can slowly respond to DMPP.

Agonist-dependent patterns of cytosolic Ca2+ changes in single bovine adrenal chromaffin cells: relationship to catecholamine release

The patterns of agonist-induced elevations of cytosolic free Ca2+ ([Ca2+]i) were characterized and compared by the use of single adrenal chromaffin cells. Initial histamine- or angiotensin II (AII)-induced elevations of [Ca2+]i were equal in magnitude (peaks 329 +/- 20 [SE] and 338 +/- 46 nM, respectively). These initial increases of [Ca2+]i were transient, insensitive to either Gd3+ or removing external Ca2+, and were primarily the result of Ca2+ release from intracellular stores. After the initial peak(s) of [Ca2+]i, a second phase of moderately elevated [Ca2+]i was observed, and this response was sensitive to either Gd3+ or removing external Ca2+, supporting a role for Ca2+ entry. In most cases, the second phase of elevated [Ca2+]i was sustained during histamine stimulation but transient during AII stimulation. Maintenance of the second phase was a property of the agonist rather than of the particular cell being stimulated. Thus, individual cells exposed sequentially to histamine and AII displayed distinct patterns of [Ca2+]i changes to each agonist, regardless of the order of addition. Histamine also stimulated twice as much [3H]catecholamine release as AII, and release was completely dependent on external Ca2+. Therefore, the ability of histamine and AII to sustain (or promote) Ca2+ entry appears to underlie their efficacy as secretagogues. These data provide evidence linking agonist-dependent patterns of [Ca2+]i changes in single cells with agonist-dependent functional responses.

Effect of capsaicin-sensitive sensory nerves on plasma glucose and catecholamine levels during 2-deoxyglucose-induced stress in conscious rats

1. Sensory fibres innervate the adrenal medulla but their function is not known. In this paper we have studied the effect of capsaicin-sensitive sensory fibres on the adrenal catecholamine (CA) response and blood glucose response to 2-deoxyglucose (2-DG)-induced glucopenic stress in conscious rats. 2. 2-DG at 500 mg kg-1 (i.v.) induced a 2.5 fold increase in plasma glucose levels, a 3.5 fold increase in inferior vena caval (i.v.c.) plasma noradrenaline (NA) levels and a 7 fold increase in i.v.c. plasma adrenaline (Ad) levels over 60 min. The hyperglyaemia in response to 2-DG was attenuated by pentolinium and by left splanchnicotomy plus right adrenalectomy. These procedures also caused a complete inhibition of the increase in plasma CA. 3. The hyperglycaemia in response to 2-DG was attenuated by pretreatment of rats with capsaicin as neonates, suggesting that capsaicin-sensitive sensory fibres are required for regulation of plasma glucose in response to glucopenic stress. 4. The increase in i.v.c. plasma CA levels in response to 2-DG during the early phase of glucopenia (first 30 min) in the conscious rats pretreated with capsaicin was the same as in the rats pretreated with vehicle alone. During the later phase of glucopenia (after 45 min), the increase in plasma CA levels in rats pretreated with capsaicin was higher than in the rats pretreated with vehicle alone. 5. In vehicle-pretreated rats and capsaicin-pretreated rats the tissue NA and Ad levels in the adrenal medulla after 8 h of stress were depleted to the same extent. However, tissue CA levels in the capsaicin group recovered faster over 24 h than in the vehicle group. 6. These results indicate that capsaicin-sensitive sensory fibres are not required to maintain adrenal CA secretion during glucopenic stress in the conscious rat but are required for maintenance of blood glucose levels.

Substance P increases catecholamine secretion from perfused rat adrenal glands evoked by prolonged field stimulation

1. We have studied the effect of substance P (SP) on catecholamine (CA) secretion evoked by prolonged field stimulation from the perfused rat adrenal gland in vitro. 2. Previous studies show that SP has an inhibitory effect on the nicotinic response in a number of different tissue preparations. In the present study, we found that SP at concentrations from 10(-7) to 10(-6) M markedly increased CA secretion evoked by prolonged high-frequency field stimulation but not that evoked by low-frequency field stimulation. 3. In the absence of field stimulation SP by itself had no direct effect on CA secretion. This indicates that SP acts as a neuromodulator rather than a neurotransmitter to increase CA secretion from the adrenal medulla. 4. The nicotinic receptors were still functional following a period of prolonged field stimulation (120 min) because CA secretion was markedly increased in response to the nicotinic agonist dimethylphenylpiperazinium (DMPP). 5. Capsaicin pre-treatment reduced CA secretion evoked by prolonged field stimulation and the facilitatory action of SP on CA secretion lasted longer in these capsaicin-pre-treated rats than in controls, indicating that SP-containing capsaicin-sensitive fibres innervating the adrenal medulla may be involved in the regulation of CA secretion. 6. In parallel with the increase in CA secretion, 3H overflow from the splanchnic nerve pre-labelled with [3H]choline was also increased by SP. The increase in CA secretion by SP lasted longer than the increase in 3H overflow. These results suggest that SP facilitates CA secretion from the adrenal gland at two levels: (1) pre-synaptically by facilitating ACh release from splanchnic nerve terminals, and (2) post-synaptically by modulating the nicotinic secretory response by protection against nicotinic desensitization of secretion. 7. The present studies provide further evidence that endogenous SP in the splanchnic nerve may modulate CA secretion during stress.

Cholinergic regulation of hamster pulmonary neuroendocrine cell calcitonin

The lung content of the peptide hormone calcitonin (iCT) has been localized to discrete and clustered pulmonary neuroendocrine cells. We have undertaken a study of the effect of the autonomic ganglionic agent nicotine on the iCT of hamster lung. The acute subcutaneous administration of nicotine raised serum iCT, while lung tissue concentration of the hormone changed reciprocally to that of the serum. One week following right-sided vagotomy, total (left plus right side) iCT levels decreased following nicotine; the concentration of iCT on the denervated right lung was more markedly decreased than that from the left lung. Hamster serum levels of iCT were significantly reduced following thyroid ablation. As in the intact animals, the subsequent subcutaneous injection of nicotine to these animals raised serum iCT. Further pharmacological evaluations revealed that subcutaneously administered nicotinic cholinergic antagonists, but not muscarinic cholinergic antagonists, prevented the nicotine-evoked increase in serum iCT. Furthermore, the subcutaneous injection of acetylcholine was found to mimic the effect noted for nicotine; this was also abolished by prior administration of a nicotinic antagonist, but not by a muscarinic antagonist. The present study suggests that the nicotinic effect on pulmonary iCT secretion is ganglionically mediated, presumably via vagal innervation of the pulmonary neuroendocrine cells.

Kinetic characteristics of calcium-dependent, cholinergic receptor controlled ATP secretion from adrenal medullary chromaffin cells

Adrenal chromaffin cells secrete catecholamines (CA) and ATP in response to acetylcholine (ACh) and high [K+]o. The release process is relatively fast making it difficult to measure the early phase of the secretory response. Recently we were able to resolve the time course of the secretory response by measuring the release of ATP using luciferin-luciferase included in the extracellular medium. For the three secretagogues studied, ACh, nicotine and high [K+]o, the early phase of release followed a complex kinetics. Allowing for an initial delay of the secretory response, the kinetics could be described as the sum of two power exponential processes. Increasing the temperature from 23 to 37 degrees C induced a marked decrease in the two time constants needed to fit the early time course of the ATP secretion. The activation energies, estimated from Arrhenius plots, were approx. 20 and 16 kcal/mol for both phases of ATP release induced by either cholinergic agonists or high [K+]o. These results suggest that cholinergic receptor activation and membrane depolarization induce ATP (and CA) secretion through a common pathway. The initial delay in the onset of the secretory response decreased with increasing doses of secretagogue and with temperature. We propose that the delay preceding the actual onset of ATP release represents the time required for generation of intracellular second messengers. The effective concentration attained by these messengers depend apparently on both receptor occupancy by the agonist and the ensuing Ca2+ channel activation.

Dopamine release from bovine adrenal medullary cells in culture

The present study tested whether release of dopamine from isolated bovine adrenal medullary cells in culture could be stimulated or inhibited by secretagogues and modulators known to affect noradrenaline and adrenaline release from adrenal medullary chromaffin cells. K+ depolarization or activation of voltage-sensitive Na+ channels by veratridine both stimulated dopamine release. Ca2+-dependent dopamine release was also stimulated by the mixed nicotinic-muscarinic agonist, carbachol. Carbachol-induced dopamine release was inhibited by a nicotinic but not by a muscarinic antagonist and dopamine release was also stimulated by a selective nicotinic agonist, 1,1-dimethyl-4-phenyl-piperazinium. Carbachol-induced dopamine release was inhibited by substance P and by neuropeptide Y. Histamine also stimulated dopamine release, while angiotensin II and glutamate produced no significant stimulation of dopamine release. Noradrenaline and adrenaline were released in response to the above agents with a profile almost identical to that of dopamine. The results indicate that dopamine can be directly released from adrenal medullary cells in response to stimulation of those cells and suggest that the dopamine release originates from chromaffin cells similar or identical to those storing noradrenaline and adrenaline. A possible role for dopamine, released from adrenal chromaffin cells, in modulating catecholamine release from the chromaffin cells and/or contributing to circulating plasma dopamine is discussed.

A 42-kD tyrosine kinase substrate linked to chromaffin cell secretion exhibits an associated MAP kinase activity and is highly related to a 42-kD mitogen-stimulated protein in fibroblasts

The localization of the protein tyrosine kinase pp60c-src to the plasma membrane and to the membrane of secretory vesicles in neurally derived bovine chromaffin cells has suggested that tyrosine phosphorylations may be associated with the process of secretion. In the present study we have identified two cytosolic proteins of approximately 42 and 45 kD that become phosphorylated on tyrosine in response to secretagogue treatment. Phosphorylation of these proteins reached a maximum (3 min after stimulation) before maximum catecholamine release was observed (5-10 min after stimulation). Both secretion and tyrosine phosphorylation of p42 and p45 required extracellular Ca2+. Tyrosine-phosphorylated proteins of similar Mr have previously been identified in 3T3-L1 adipocytes stimulated with insulin (MAP kinase; Ray, L. B., and T. W. Sturgill. 1987. Proc. Natl. Acad. Sci. USA. 84:1502-1506) and in avian and rodent fibroblasts stimulated with a variety of mitogenic agents (Cooper, J. A., D. F. Bowen-Pope, E. Raines, R. Ross, and T. Hunter. 1982. Cell. 31:263-273; Nakamura, K. D., R. Martinez, and M. J. Weber. 1983. Mol. Cell. Biol. 3:380-390). Comparisons of the secretion-associated 42-kD protein of chromaffin cells with the 42-kD protein of Swiss 3T3 fibroblasts and 3T3-L1 adipocytes provide evidence that these three proteins are highly related. This evidence includes comigration during one-dimensional SDS-PAGE, cochromatography using ion exchange and hydrophobic matrices, similar isoelectric points, identical cyanogen-bromide peptide maps, and cochromatography of MAP kinase activity with the tyrosine-phosphorylated form of pp42. This protein(s), which appears to be activated in a variety of cell types, may serve a common function, perhaps in signal transduction involving a cascade of kinases.