Distinct preganglionic neurons innervate noradrenaline and adrenaline cells in the cat adrenal medulla

Neural influences on cardiovascular variability: possibilities and pitfalls

Altered variability in the cardiovascular system is associated with a range of cardiovascular diseases and increased mortality. Because blood pressure and heart rate show distinct low-frequency oscillations that appear to be affected by either vagal or sympathetic activity, it has been hoped that measurement of the strength of these oscillations could be used as an index of autonomic tone and thus form the basis of a diagnostic test. This review focuses on recent research that has examined the fundamental origin of variability associated with respiration and a slow oscillation at 0.1 Hz in the human. A new hypothesis is proposed to account for the slow oscillation in heart rate and blood pressure that incorporates components of the central nervous system, other reflex pathways regulating sympathetic activity, and resonance in the baroreflex control of blood pressure. Whereas it is clear that sympathetic activity and arterial baroreflexes are critical elements in producing cardiovascular variability, there is also evidence that other factors, including the ability of the vasculature to respond to sympathetic activity, appear to play a role in determining the strength of oscillations. Given the potential impact of other nonbaroreflex or nonautonomic pathways in affecting cardiovascular variability, it is proposed that one must use care in relating changes in the strength of an oscillation in blood pressure and heart rate as definitively due to a change in autonomic control.

Differential chemoreceptor reflex responses of adrenal preganglionic neurons

Adrenal sympathetic preganglionic neurons (ADR SPNs) regulating the chromaffin cell release of epinephrine (Epi ADR SPNs) and those controlling norepinephrine (NE ADR SPNs) secretion have been distinguished on the basis of their responses to stimulation in the rostral ventrolateral medulla, to glucopenia produced by 2-deoxyglucose, and to activation of the baroreceptor reflex. In this study, we examined the effects of arterial chemoreceptor reflex activation, produced by inhalation of 100% N(2) or intravenous injection of sodium cyanide, on these two groups of ADR SPNs, identified antidromically in urethane-anesthetized, artificially ventilated rats. The mean spontaneous discharge rates of 38 NE ADR SPNs and 51 Epi ADR SPNs were 4.4 +/- 0.4 and 5.6 +/- 0.4 spikes/s at mean arterial pressures of 98 +/- 3 and 97 +/- 3 mmHg, respectively. Ventilation with 100% N(2) for 10 s markedly excited all NE ADR SPNs (+222 +/- 23% control, n = 36). In contrast, the majority (40/48; 83%) of Epi ADR SPNs were unaffected or slightly inhibited by ventilation with 100% N(2) (population response: +6 +/- 10% control, n = 48). Similar results were obtained after injection of sodium cyanide. These observations suggest that the network controlling the spontaneous discharge of NE ADR SPNs is more sensitive to brief arterial chemoreceptor reflex activation than is that regulating the activity of Epi ADR SPNs. The differential responsiveness to activation of the arterial chemoreceptor reflex of the populations of ADR SPNs regulating epinephrine and norepinephrine secretion suggests that their primary excitatory inputs arise from separate populations of sympathetic premotor neurons and that a fall in arterial oxygen tension is not a major stimulus for reflex-mediated adrenal epinephrine secretion.

Differential control of sympathetic outflow

With advances in experimental techniques, the early views of the sympathetic nervous system as a monolithic effector activated globally in situations requiring a rapid and aggressive response to life-threatening danger have been eclipsed by an organizational model featuring an extensive array of functionally specific output channels that can be simultaneously activated or inhibited in combinations that result in the patterns of autonomic activity supporting behavior and mediating homeostatic reflexes. With this perspective, the defense response is but one of the many activational states of the central autonomic network. This review summarizes evidence for the existence of tissue-specific sympathetic output pathways, which are likely to include distinct populations of premotor neurons whose target specificity could be assessed using the functional fingerprints developed from characterizations of postganglionic efferents to known targets. The differential responses in sympathetic outflows to stimulation of reflex inputs suggest that the circuits regulating the activity of sympathetic premotor neurons must have parallel access to groups of premotor neurons controlling different functions but that these connections vary in their ability to influence different sympathetic outputs. Understanding the structural and physiological substrates antecedent to premotor neurons that mediate the differential control of sympathetic outflows, including those to noncardiovascular targets, represents a challenge to our current technical and analytic approaches.

Role of calcium channels and adenylate cyclase in the PACAP-induced adrenal catecholamine secretion

We elucidated the functional contribution of voltage-dependent calcium channels (VDCCs) and adenylate cyclase to epinephrine (Epi) and norepinephrine (NE) secretion induced by pituitary adenylate cyclase-activating polypeptide (PACAP) in the isolated perfused rat adrenal gland. PACAP increased Epi and NE output, which was inhibited by perfusion with calcium-free solution or by nifedipine, an L-type VDCC blocker. However, the PACAP-induced responses were resistant to omega-conotoxin GVIA, an N-type VDCC blocker, or omega-conotoxin MVIIC, a P/Q-type VDCC blocker. MDL-12330A, an adenylate cyclase inhibitor, inhibited the PACAP-induced increase in Epi, but not NE, output. Treatment with nifedipine and MDL-12330A caused additive inhibition of the PACAP-induced catecholamine responses. These results suggest that opening of L-type VDCCs is responsible for adrenal catecholamine secretion induced by PACAP and that activation of adenylate cyclase is involved in the PACAP-induced Epi, but not NE, secretion. These pathways may act independently of each other.

Different adrenal sympathetic preganglionic neurons regulate epinephrine and norepinephrine secretion

Brain stimulation or activation of certain reflexes can result in differential activation of the two populations of adrenal medullary chromaffin cells: those secreting either epinephrine or norepinephrine, suggesting that they are controlled by different central sympathetic networks. In urethan-chloralose-anesthetized rats, we found that antidromically identified adrenal sympathetic preganglionic neurons (SPNs) were excited by stimulation of the rostral ventrolateral medulla (RVLM) with either a short (mean: 29 ms) or a long (mean: 129 ms) latency. The latter group of adrenal SPNs were remarkably insensitive to baroreceptor reflex activation but strongly activated by the glucopenic agent 2-deoxyglucose (2-DG), indicating their role in regulation of adrenal epinephrine release. In contrast, adrenal SPNs activated by RVLM stimulation at a short latency were completely inhibited by increases in arterial pressure or stimulation of the aortic depressor nerve, were unaffected by 2-DG administration, and are presumed to govern the discharge of adrenal norepinephrine-secreting chromaffin cells. These findings of a functionally distinct preganglionic innervation of epinephrine- and norepinephrine-releasing adrenal chromaffin cells provide a foundation for identifying the different sympathetic networks underlying the differential regulation of epinephrine and norepinephrine secretion from the adrenal medulla in response to physiological challenges and experimental stimuli.

Adrenal medullary catecholamine secretion patterns in rats evoked by reflex and direct neural stimulation

Epinephrine (EPI) and norepinephrine (NE), secretion patterns evoked by reflex (to hypotension and hypoglycemia) and direct neural stimulation of the adrenal medulla were measured in pentobarbital anesthetized male Sprague-Dawley rats. Secretion rates were determined by collecting adrenal venous blood. Baseline catecholamine secretion was similar in innervated and denervated glands indicating that there was little tonic sympathetic neural drive to the medulla. Both hydralazine-induced hypotension and insulin-induced hypoglycemia significantly increased catecholamine secretion with the secretion of EPI predominating. Similarly, in response to stimulation of the splanchnic nerve, frequency-related increments in EPI and NE were elicited with EPI release being greater than NE at all frequencies. However, the magnitude of the increase in secretion during splanchnic stimulation at frequencies above 1 hz greatly exceeded the release achieved by reflex stimulation. The results indicate that despite the fact that the stimuli of hypotension and hypoglycemia are integrated by different centers in the brain, the pattern of adrenal release is similar.

Physiological aspects of exocytosis in chromaffin cells of the adrenal medulla

The adrenal medulla is composed principally of groups of adrenergic and noradrenergic chromaffin cells, with minor populations of small intensely fluorescent cells and ganglionic neurones. Different molecular stimuli evoke distinct secretory events in the gland, involving the release of either adrenaline or noradrenaline together with various neuroactive peptides. The nature of the secretory response can be controlled at a central level or regulated locally within the gland. Specific innervation patterns to the different types of chromaffin cell have been implicated in central regulatory mechanisms, while several explanations for regulating secretion locally have been proposed. The differential distribution of various types of receptors between cell phenotypes, such as muscarinic or nicotinic acetylcholine receptors, histamine receptors, angiotensin receptors and different classes of opiate receptors between the two principal chromaffin cell populations could be involved in local control. In addition exocytosis parameters could be modulated differently in adrenergic and noradrenergic cells by phenotype-specific mechanisms, possibly involving molecules like Growth Associated Protein-43, Synaptosomal Associated Protein-25 isoforms or the p11 annexin subunit. The distribution of the various types of calcium channels is also known to vary between chromaffin cell subtypes. This short review examines possible ways in which specific innervation patterns in the adrenal gland could be programmed and discusses exocytosis mechanisms that could differ between chromaffin cell phenotypes. Data reviewed here suggest that the adrenal medulla should no longer be viewed as a homogeneous entity but as consisting of an ensemble of individual cell subpopulations each with a distinct secretory response that could in part reflect its local history.

Adrenal epinephrine secretion is not regulated by sympathoinhibitory neurons in the caudal ventrolateral medulla

By providing the principal inhibitory regulation of the discharge of sympathetic premotor neurons in the rostral ventrolateral medulla (RVLM), neurons in the caudal ventrolateral medulla (CVLM) play a major role in regulating the level of sympathetic nerve activity (SNA) to cardiovascular targets. To determine whether adrenal medullary secretion of epinephrine (EPI) is also regulated by sympathoinhibitory inputs from the CVLM to the RVLM, we compared levels of plasma EPI obtained after disinhibition of RVLM neurons with levels obtained after inhibition of CVLM neurons, both of which result in sustained elevations in arterial blood pressure (AP), SNA, and heart rate (HR). Plasma norepinephrine (NE) concentrations were significantly elevated following bilateral microinjection either of bicuculline (BIC) into the RVLM or of muscimol into the CVLM of urethane/chloralose-anesthetized, artificially-ventilated rats. In sharp contrast, although plasma EPI concentrations were significantly elevated following disinhibition of neurons in the RVLM, they were unchanged by inhibition of neurons in the CVLM. These results demonstrate that the discharge of sympathetic premotor neurons in the RVLM regulating adrenal secretion of EPI is modulated by a tonic, GABA-ergic inhibition that arises from a source that is different from the sympathoinhibitory neurons in the CVLM that project to RVLM sympathetic premotor neurons controlling vasoconstrictor and cardiac targets.

Adrenal epinephrine and norepinephrine release to hypoglycemia measured by microdialysis in conscious rats

Experiments were conducted in conscious male rats to determine whether hypoglycemia induced by insulin administration preferentially stimulated epinephrine (Epi) or norepinephrine (NE) adrenal medullary chromaffin cells. The release of Epi and NE from the adrenal medulla was continuously monitored using a microdialysis probe of novel design that had been inserted in the adrenal medulla approximately 16 h before the administration of insulin. Following insulin, 3 U/kg i.v., blood glucose declined and dialysate Epi levels rose. No measurable increment in dialysate NE was obtained. Similarly, plasma Epi increased with no detectable change in NE. Patterns of dialysate and plasma catecholamine changes were similar in two groups of animals that had been fed or fasted overnight before insulin treatment. However, the magnitude of the Epi increase was greater in the fasted animals. After recovery of the blood glucose concentration to preinsulin levels, dialysate and plasma catecholamine concentrations returned to control values. These experiments clearly demonstrate that adrenal medullary chromaffin cells that produce Epi are preferentially stimulated in response to insulin-induced hypoglycemia.

Neurokinin-1 receptor-immunoreactive sympathetic preganglionic neurons: target specificity and ultrastructure

Substance P is involved in cardiovascular control at the spinal cord level, where it acts through neurokinin-1 receptors. In this study we used immunocytochemistry and retrograde tracing to investigate the presence of the neurokinin-1 receptor and its ultrastructural localization in rat sympathetic preganglionic neurons that project to the superior cervical ganglion or the adrenal medulla. Immunofluorescence for the neurokinin-1 receptor outlined the somatic and dendritic surfaces of neurons in autonomic subnuclei of spinal cord segments T1-T12, whereas immunofluorescence for the tracer, cholera toxin B subunit, filled retrogradely labelled cells. There was a significant difference in the proportion of neurokinin-1 receptor-immunoreactive sympathetic preganglionic neurons supplying the superior cervical ganglion and the adrenal medulla. Thirty-eight percent of the neurons that projected to the superior cervical ganglion were immunoreactive for the neurokinin-1 receptor compared to 70% of neurons innervating the adrenal medulla. Of neurons projecting to the superior cervical ganglion, significantly different proportions showed neurokinin-1 receptor immunoreactivity in spinal cord segment T1 (15%) versus segments T2 T6 (45%). At the ultrastructural level, neurokinin-1 receptor staining occurred predominantly on the inner leaflets of the plasma membranes of retrogradely labelled sympathetic preganglionic neurons. Deposits of intracellular label were often observed in dendrites and in the rough endoplasmic reticulum and Golgi apparatus of cell bodies. Neurokinin-1 receptor immunoreactivity was present at many, but not all, synapses as well as at non-synaptic sites, and occurred at synapses with substance P-positive as well as substance P-negative nerve fibres. Only 37% of the substance P synapses occurred on neurokinin-1-immunoreactive neurons in the intermediolateral cell column. These results show that presence of the neurokinin-1 receptor in sympathetic preganglionic neurons is related to their target. The ultrastructural localization of the receptor suggests that sympathetic preganglionic neurons may be affected (i) by substance P released at neurokinin-1 receptor-immunoreactive synapses, (ii) by other tachykinins (e.g., neurokinin A), which co-localize in substance P fibres in the intermediolateral cell column, acting through other neurokinin receptors, and (iii) by substance P that diffuses to neurokinin-1 receptors from distant sites.

Immunohistochemical localization of serotonin, galanin, cholecystokinin, and methionine-enkephalin in adrenal medullary cells of the chicken

The identification of adrenaline- (A) and noradrenaline- (NA) containing cells in the adrenal medulla of the chicken and colocalization of serotonin and neuropeptides with A or NA in medullary cells were investigated with the use of immunohistochemical methods. Antisera against tyrosine hydroxylase and phenylethanolamine-N-methyltransferase were used as markers for catecholamine- and A-synthesizing cells, respectively. About 70% of catecholamine-synthesizing cells also exhibited immunoreactivity for phenylethanolamine-N-methyltransferase antiserum. Therefore, these cells are A-containing ones and the rest of cells seem to be NA-containing cells. Immunoreactivity with serotonin antiserum was observed in almost all medullary cells. Galanin-immunoreactivity was also found throughout the adrenal medulla, but was stronger in A-containing cells than in NA-containing ones. Cholecystokinin-immunoreactivity was restricted to A-containing cells. Methionine-enkephalin-immunoreactivity was seen in both A- and NA-containing cells, but in about half of medullary cells. From these results, it is suggested that serotonin, galanin, cholecystokinin, and methionine-enkephalin may be co-released with A and/or NA from adrenal medullary cells of the chicken.

Different contributions of L- and Q-type Ca2+ channels to Ca2+ signals and secretion in chromaffin cell subtypes

In this study, we investigated the contribution of different subtypes of voltage-dependent Ca2+ channels to changes in cytosolic free Ca2+ ([Ca2+]i) and secretion in noradrenergic and adrenergic bovine chromaffin cells. In single immunocytochemically identified chromaffin cells, [Ca2+]i increased transiently during high K+ depolarization. Furnidipine and BAY K 8644, L-type Ca2+ channel blocker and activator, respectively, affected the [Ca2+]i rise more in noradrenergic than in adrenergic cells. In contrast, the Q-type Ca2+ channel blocker omega-conotoxin MVIIC inhibited the [Ca2+]i rise more in adrenergic cells. omega-Agatoxin IVA (30 nM), which blocks P-type Ca2+ channels, had little effect on the [Ca2+]i signal. The N-type Ca2+ channel blocker omega-conotoxin GVIA similarly inhibited the [Ca2+]i rise in both cell types. The effects of furnidipine, BAY K 8644, and omega-conotoxin MVIIC on K+-evoked norepinephrine and epinephrine release paralleled those effects on [Ca2+]i signals. However, omega-conotoxin GVIA and 30 nM omega-agatoxin IVA did not affect the secretion of either amine. The data suggest that, in the bovine adrenal medulla, the release of epinephrine and norepinephrine are preferentially controlled by Q- and L-type Ca2+ channels, respectively. P- and N-type Ca2+ channels do not seem to control the secretion of either catecholamine.

Distribution of immunoreactivity for the NK1 receptor on different subpopulations of sympathetic preganglionic neurons in the rat

The distribution of immunoreactivity to the receptor for substance P, the neurokinin 1 (NK1) receptor, was examined in preganglionic sympathetic neurons of the rat by using immunohistochemistry and retrograde neuronal tracing. About one-third of all sympathetic preganglionic neurons were NK1 receptor immunoreactive, and most of the NK1 receptor-immunoreactive neurons were also nitric oxide synthase immunoreactive. The proportions of sympathetic preganglionic neurons projecting to the superior and inferior mesenteric ganglia, adrenal gland, and lumbar sympathetic chain which were NK1 receptor-immunoreactive were determined. Most (89%) of the preganglionic neurons projecting to the adrenal glands were NK1 receptor immunoreactive. Few (17%) of the preganglionic neurons projecting to the L5 sympathetic chain ganglion were immunoreactive for the receptor, while preganglionic neurons projecting to the prevertebral ganglia were NK1 receptor immunoreactive at intermediate frequencies (61-64%). Thus, substance P acting on NK1 receptors is likely to be important in the preganglionic pathways to the adrenal medulla and viscera via the prevertebral ganglia, but is unlikely to be important in pathways to the lumbar sympathetic chain. The co-localisation of the NK1 receptor with the enzyme nitric oxide synthase was also examined. The majority of NK1 receptor-immunoreactive neurons were also nitric oxide synthase immunoreactive. Thus NK1 receptors occur on preganglionic neurons over many spinal segments and in a range of preganglionic pathways, as well as in a range of combinations with nitric oxide synthase. The heterogeneity of preganglionic neurons showing NK1 receptor immunoreactivity may reflect the involvement of NK1-mediated transmission in a variety of functional pathways, most notably the preganglionic projections to the adrenal medulla and to the viscera.

Selective neural regulation of epinephrine and norepinephrine cells in the adrenal medulla -- cardiovascular implications

The innervation of the adrenal medulla regulates the release of catecholamines from the two, epinephrine (EPI) and norepinephrine (NE), populations of chromaffin cells. Adjustments in the neural output to the adrenal medulla are made by centers in the brain that integrate the sensory input arising from a variety of challenges and the resulting changes in secretion assist in the restoration of homeostasis. Interestingly, the adrenal medullary secretory responses do not simply reflect increments a fixed ratio of EPI to NE as might be expected if release was proportional to the number EPI and NE cells. Instead, the ratio of EPI to NE changes depending on the magnitude and type of stimulus that initiates neural activation of the medulla. The variability in the EPI:NE release ratio implies that the EPI and NE cells can be differentially stimulated. Although the underlying mechanisms are not fully characterized, this review presents an emerging view that the selective control of EPI and NE cells is accounted for, first, by the existence of separate neural circuits between brain centers and the chromaffin cells, and second, through neuromodulation that selectively influences EPI and NE cells. The presence of mechanisms that allow for separate control of the EPI and NE cells may significantly augment the range of cardiovascular and metabolic responses mediated through activation of the adrenal medulla.