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

Differences in the regulatory mechanism of blood flow in the orofacial area mediated by neural and humoral systems

Marked blood flow (BF) changes mediated by the autonomic neural and humoral systems may be important for orofacial hemodynamics and functions. However, it remains questionable whether differences in the autonomic vasomotor responses mediated by neural and humoral systems exist in the orofacial area. This study examined whether there are differences in changes in the BF and vascular conductance (VC) between the masseter muscle and lower lip mediated by autonomic neural and humoral systems in urethane-anesthetized rats. Electrical stimulation of the central cut end of the lingual nerve elicited BF increases in the masseter (mainly cholinergic) and lower lip (mainly non-cholinergic), accompanied by an increase in arterial blood pressure (ABP), while cervical sympathetic trunk stimulation consistently decreased BF at both sites. The lingual nerve stimulation induced a biphasic change in the VC in the masseter, consisting of an initial decrease and a successive increase. This decrease in VC was positively correlated with changes in ABP and diminished by guanethidine. Cervical vagus nerve stimulation also induced BF increases at both sites; the increases were greater in the masseter than in the lower lip. Adrenal nerve stimulation and isoproterenol administration induced BF increases in the masseter but not in the lower lip. These results indicate that cholinergic parasympathetic-mediated hemodynamics evoked by trigeminal somatosensory inputs are closely related to ABP changes. The sympathetic nervous system, including the sympathoadrenal system and visceral inputs, may be more involved in hemodynamics in the muscles than in epithelial tissues in the orofacial area.

Insulin-responsive autonomic neurons in rat medulla oblongata

Low blood glucose activates brainstem adrenergic and cholinergic neurons, driving adrenaline secretion from the adrenal medulla and glucagon release from the pancreas. Despite their roles in maintaining glucose homeostasis, the distributions of insulin-responsive adrenergic and cholinergic neurons in the medulla are unknown. We fasted rats overnight and gave them insulin (10 U/kg i.p.) or saline after 2 weeks of handling. Blood samples were collected before injection and before perfusion at 90 min. We immunoperoxidase-stained transverse sections of perfused medulla to show Fos plus either phenylethanolamine N-methyltransferase (PNMT) or choline acetyltransferase (ChAT). Insulin injection lowered blood glucose from 4.9 ± 0.3 mmol/L to 1.7 ± 0.2 mmol/L (mean ± SEM; n = 6); saline injection had no effect. In insulin-treated rats, many PNMT-immunoreactive C1 neurons had Fos-immunoreactive nuclei, with the proportion of activated neurons being highest in the caudal part of the C1 column. In the rostral ventrolateral medulla, 33.3% ± 1.4% (n = 8) of C1 neurons were Fos-positive. Insulin also induced Fos in 47.2% ± 2.0% (n = 5) of dorsal medullary C3 neurons and in some C2 neurons. In the dorsal motor nucleus of the vagus (DMV), insulin evoked Fos in many ChAT-positive neurons. Activated neurons were concentrated in the medial and middle regions of the DMV beneath and just rostral to the area postrema. In control rats, very few C1, C2, or C3 neurons and no DMV neurons were Fos-positive. The high numbers of PNMT-immunoreactive and ChAT-immunoreactive neurons that express Fos after insulin treatment reinforce the importance of these neurons in the central response to a decrease in glucose bioavailability.

Perifornical hypothalamic pathway to the adrenal gland: Role for glutamatergic transmission in the glucose counter-regulatory response

Adrenaline is an important counter-regulatory hormone that helps restore glucose homeostasis during hypoglycaemia. However, the neurocircuitry that connects the brain glucose sensors and the adrenal sympathetic outflow to the chromaffin cells is poorly understood. We used electrical microstimulation of the perifornical hypothalamus (PeH) and the rostral ventrolateral medulla (RVLM) combined with adrenal sympathetic nerve activity (ASNA) recording to examine the relationship between the RVLM, the PeH and ASNA. In urethane-anaesthetised male Sprague-Dawley rats, intermittent single pulse electrical stimulation of the rostroventrolateral medulla (RVLM) elicited an evoked ASNA response that consisted of early (60±3ms) and late peaks (135±4ms) of preganglionic and postganglionic activity. In contrast, RVLM stimulation evoked responses in lumbar sympathetic nerve activity that were almost entirely postganglionic. PeH stimulation also produced an evoked excitatory response consisting of both preganglionic and postganglionic excitatory peaks in ASNA. Both peaks in ASNA following RVLM stimulation were reduced by intrathecal kynurenic acid (KYN) injection. In addition, the ASNA response to systemic neuroglucoprivation induced by 2-deoxy-d-glucose was abolished by bilateral microinjection of KYN into the RVLM. This suggests that a glutamatergic pathway from the perifornical hypothalamus (PeH) relays in the RVLM to activate the adrenal SPN and so modulate ASNA. The main findings of this study are that (i) adrenal premotor neurons in the RVLM may be, at least in part, glutamatergic and (ii) that the input to these neurons that is activated during neuroglucoprivation is also glutamatergic.

Interactions between β-adrenergic vasodilation and cervical sympathetic nerves are mediated by α2-adrenoceptors in the rat masseter muscle

Neural and humoral autonomic mechanisms may be important in the maintenance of blood flow in the masseter muscle (MBF). However, their interactions remain unclear. In this study, we examined interactions between neural and humoral regulation of MBF and investigated the mechanisms mediating these interactions in urethane-anesthetized rats. Stimulation of the adrenal nerve (AN) projecting to the adrenal medulla increased MBF, and this increase was mediated by β-adrenoceptors. Sectioning of the superior cervical sympathetic trunk (CST) significantly inhibited increases in MBF induced by AN stimulation during high activity in the CST, but not during low activity. AN stimulation with clonidine after CST sectioning induced a significant increased in MBF, however phenylephrine had no observable effect. Pretreatment with yohimbine or propranolol significantly inhibited the increase in the MBF. Our results suggest an interaction between β-adrenergic vasodilation evoked by circulating adrenaline and the cervical sympathetic nerves that is mediated by α₂-adrenoceptors in the masseter muscle.

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.

Partial blockade of nicotinic acetylcholine receptors improves the counterregulatory response to hypoglycemia in recurrently hypoglycemic rats

Recurrent exposure to hypoglycemia can impair the normal counterregulatory hormonal responses that guard against hypoglycemia, leading to hypoglycemia unawareness. This pathological condition known as hypoglycemia-associated autonomic failure (HAAF) is the main adverse consequence that prevents individuals with type 1 diabetes mellitus from attaining the long-term health benefits of tight glycemic control. The underlying molecular mechanisms responsible for the progressive loss of the epinephrine response to subsequent bouts of hypoglycemia, a hallmark sign of HAAF, are largely unknown. Normally, hypoglycemia triggers both the release and biosynthesis of epinephrine through activation of nicotinic acetylcholine receptors (nAChR) on the adrenal glands. We hypothesize that excessive cholinergic stimulation may contribute to impaired counterregulation. Here, we tested whether administration of the nAChR partial agonist cytisine to reduce postganglionic synaptic activity can preserve the counterregulatory hormone responses in an animal model of HAAF. Compared with nicotine, cytisine has limited efficacy to activate nAChRs and stimulate epinephrine release and synthesis. We evaluated adrenal catecholamine production and secretion in nondiabetic rats subjected to two daily episodes of hypoglycemia for 3 days, followed by a hyperinsulinemic hypoglycemic clamp on day 4. Recurrent hypoglycemia decreased epinephrine responses, and this was associated with suppressed TH mRNA induction (a measure of adrenal catecholamine synthetic capacity). Treatment with cytisine improved glucagon responses as well as epinephrine release and production in recurrently hypoglycemic animals. These data suggest that pharmacological manipulation of ganglionic nAChRs may be promising as a translational adjunctive therapy to avoid HAAF in type 1 diabetes mellitus.

PACAP controls adrenomedullary catecholamine secretion and expression of catecholamine biosynthetic enzymes at high splanchnic nerve firing rates characteristic of stress transduction in male mice

The neuropeptide PACAP (pituitary adenylate cyclase-activating polypeptide) is a cotransmitter of acetylcholine at the adrenomedullary synapse, where autonomic regulation of hormone secretion occurs. We have previously reported that survival of prolonged metabolic stress in mice requires PACAP-dependent biosynthesis and secretion of adrenomedullary catecholamines (CAs). In the present experiments, we show that CA secretion evoked by direct high-frequency stimulation of the splanchnic nerve is abolished in native adrenal slices from male PACAP-deficient mice. Further, we demonstrate that PACAP is both necessary and sufficient for CA secretion ex vivo during stimulation protocols designed to mimic stress. In vivo, up-regulation of transcripts encoding adrenomedullary CA-synthesizing enzymes (tyrosine hydroxylase, phenylethanolamine N-methyltransferase) in response to both psychogenic and metabolic stressors (restraint and hypoglycemia) is PACAP-dependent. Stressor-induced alteration of the adrenomedullary secretory cocktail also appears to require PACAP, because up-regulation of galanin mRNA is abrogated in male PACAP-deficient mice. We further show that hypoglycemia-induced corticosterone secretion is not PACAP-dependent, ruling out the possibility that glucocorticoids are the main mediators of the aforementioned effects. Instead, experiments with bovine chromaffin cells suggest that PACAP acts directly at the level of the adrenal medulla. By integrating prolonged CA secretion, expression of biosynthetic enzymes and production of modulatory neuropeptides such as galanin, PACAP is crucial for adrenomedullary function. Importantly, our results show that PACAP is the dominant adrenomedullary neurotransmitter during conditions of enhanced secretory demand.

Incidence and cause of hypertension during adrenal radiofrequency ablation

PURPOSE

To evaluate the incidence and cause of hypertension prospectively during adrenal radiofrequency ablation (RFA).

METHODS

For this study, approved by our institutional review board, written informed consent was obtained from all patients. Patients who received RFA for adrenal tumors (adrenal ablation) and other abdominal tumors (nonadrenal ablation) were included in this prospective study. Blood pressure was monitored during RFA. Serum adrenal hormone levels including epinephrine, norepinephrine, dopamine, and cortisol levels were measured before and during RFA. The respective incidences of procedural hypertension (systolic blood pressure >200 mmHg) of the two patient groups were compared. Factors correlating with procedural systolic blood pressure were evaluated by regression analysis.

RESULTS

Nine patients underwent adrenal RFA and another 9 patients liver (n = 5) and renal (n = 4) RFA. Asymptomatic procedural hypertension that returned to the baseline by injecting calcium blocker was found in 7 (38.9%) of 18 patients. The incidence of procedural hypertension was significantly higher in the adrenal ablation group (66.7%, 6/9) than in the nonadrenal ablation group (11.1%, 1/9, P < 0.0498). Procedural systolic blood pressure was significantly correlated with serum epinephrine (R (2) = 0.68, P < 0.0001) and norepinephrine (R (2) = 0.72, P < 0.0001) levels during RFA. The other adrenal hormones did not show correlation with procedural systolic blood pressure.

CONCLUSION

Hypertension occurs frequently during adrenal RFA because of the release of catecholamine.

Neuropeptide coding of sympathetic preganglionic neurons; focus on adrenally projecting populations

Chemical coding of sympathetic preganglionic neurons (SPN) suggests that the chemical content of subpopulations of SPN can define their function. Since neuropeptides, once synthesized are transported to the axon terminal, most demonstrated chemical coding has been identified using immunoreactive terminals at the target organ. Here, we use a different approach to identify and quantify the subpopulations of SPN that contain the mRNA for pituitary adenylate cyclase activating polypeptide (PACAP) or enkephalin. Using double-labeled immunohistochemistry combined with in situ hybridization (ISH) we firstly identified the distribution of these mRNAs in the spinal cord and determined quantitatively, in Sprague-Dawley rats, that many SPN at the T4-T10 spinal level contain preproPACAP (PPP+, 80 ± 3%, n=3), whereas a very small percentage contain preproenkephalin (PPE+, 4 ± 2%, n=4). A similar neurochemical distribution was found at C8-T3 spinal level. These data suggest that PACAP potentially regulates a large number of functions dictated by SPN whereas enkephalins are involved in few functions. We extended the study to explore those SPN that control adrenal chromaffin cells. We found 97 ± 5% of adrenally projecting SPN (AP-SPN) to be PPP+ (n=4) with only 47 ± 3% that were PPE+ (n=5). These data indicate that adrenally projecting PACAPergic SPN regulate both adrenal adrenaline (Ad) and noradrenaline (NAd) release whereas the enkephalinergic SPN subpopulation must control a (sub) population of chromaffin cells - most likely those that release Ad. The sensory innervation of the adrenal gland was also determined. Of the few adrenally projecting dorsal root ganglia (AP-DRG) observed, 74 ± 12% were PPP+ (n=3), whereas 1 ± 1% were PPE+ (n=3). Therefore, if sensory neurons release peptides to the adrenal medulla, PACAP is most likely involved. Together, these data provide a neurochemical basis for differential control of sympathetic outflow particularly that to the adrenal medulla.

Circulating adrenaline released by sympathoadrenal activation elicits acute vasodilatation in the rat masseter muscle

The present study was designed to examine the effects of circulating catecholamines released by sympathoadrenal system on the haemodynamics of the masseter muscle in deeply urethane-anaesthetized, artificially ventilated, cervically vagotomized and sympathectomized rats. Intravenous administration of adrenaline induced a biphasic change of blood flow in the masseter muscle (MBF). The change of blood flow showed an initial marked increase and successive slight decrease in a dose-dependent manner (0.01-1 microg/kg). The administration of noradrenaline had no significant effect on the MBF. The increase in the MBF evoked by exogenously applied adrenaline was markedly reduced by the intravenous administration of propranolol (100 microg/kg), whereas pretreatment with either hexamethonium (10 mg/kg), atropine (100 microg/kg), or phentolamine (1 mg/kg) failed to affect the MBF increase. Electrical stimulation of splanchnic nerve (SPLN) preganglionic neurones projecting to the adrenal medulla elicited frequency-dependent (1-20 Hz) increases in the MBF. The intravenous administration of the beta(2)-adrenergic receptor selective antagonist, ICI 118551 (0.5 mg/kg), almost abolished the MBF increase induced by SPLN stimulation, but pretreatment with the beta(1)-adrenergic receptor selective antagonist, atenolol (1 mg/kg), had no effect on this response. The results of the present study indicate that circulating adrenaline elicits acute vasodilatation through a beta-adrenergic mechanism in the rat masseter muscle. Vascular beta(2)-adrenergic receptors in the masseter muscle may be activated preferentially by adrenaline released from the adrenal medulla, suggesting that the sympathoadrenal system is involved in the marked MBF increase during sympathoexcitation.

Centrally administered neuromedin U elevates plasma adrenaline by brain prostanoid TP receptor-mediated mechanisms in rats

Neuromedin U is a hypothalamic peptide involved in energy homeostasis and stress responses. The peptide, when administered intracerebroventricularly (i.c.v.), decreases food intake and body weight while increasing body temperature and heat production. We examined the effect of i.c.v. administered neuromedin U on plasma catecholamines with regard to the brain prostanoid using anesthetized rats. Neuromedin U (0.1, 0.5 and 1 nmol/animal, i.c.v.) effectively elevated plasma adrenaline (a maximal response was obtained at 0.5 nmol/animal), but had little effect on plasma noradrenaline. However, intravenously administered neuromedin U (0.5 nmol/animal) had no effect on plasma catecholamines. Neuromedin U (0.5 nmol/animal, i.c.v.)-induced elevation of plasma adrenaline was effectively reduced by intracerebroventricular pretreatments with indomethacin (an inhibitor of cyclooxygenase) (0.6 and 1.2 micromol/animal), furegrelate (an inhibitor of thromboxane A2 synthase) (0.9 and 1.8 micromol/animal) and (+)-S-145 (a blocker of prostanoid TP receptors) (250 and 625 nmol/animal), respectively. The neuromedin U-induced adrenaline response was also abolished by acute bilateral adrenalectomy. These results suggest that centrally administered neuromedin U evokes the secretion of adrenaline from the adrenal medulla by brain prostanoid TP receptor-mediated mechanisms in rats.

Adrenal adrenaline- and noradrenaline-containing cells and celiac sympathetic ganglia are differentially controlled by centrally administered corticotropin-releasing factor and arginine-vasopressin in rats

The adrenal glands and sympathetic celiac ganglia are innervated mainly by the greater splanchnic nerves, which contain preganglionic sympathetic nerves that originated from the thoracic spinal cord. The adrenal medulla has two separate populations of chromaffin cells, adrenaline-containing cells (A-cells) and noradrenaline-containing cells (NA-cells), which have been shown to be differentially innervated by separate groups of the preganglionic sympathetic neurons. The present study was designed to characterize the centrally activating mechanisms of the adrenal A-cells, NA-cells and celiac sympathetic ganglia with expression of cFos (a marker for neural excitation), in regard to the brain prostanoids, in anesthetized rats. Intracerebroventricularly (i.c.v.) administered corticotropin-releasing factor (CRF) induced cFos expression in the adrenal A-cells, but not NA-cells, and celiac ganglia. On the other hand, i.c.v. administered arginine-vasopressin (AVP) resulted in cFos induction in both A-cells and NA-cells in the adrenal medulla, but not in the celiac ganglia. Intracerebroventricular pretreatment with indomethacin (an inhibitor of cyclooxygenase) abolished the CRF- and AVP-induced cFos expression in all regions described above. On the other hand, intracerebroventricular pretreatment with furegrelate (an inhibitor of thromboxane A2 synthase) abolished the CRF-induced cFos expression in the adrenal A-cells, but not in the celiac ganglia, and also abolished the AVP-induced cFos expression in both A-cells and NA-cells in the adrenal medulla. These results suggest that centrally administered CRF activates adrenal A-cells and celiac sympathetic ganglia by brain thromboxane A2-mediated and other prostanoid than thromboxane A2 (probably prostaglandin E2)-mediated mechanisms, respectively. On the other hand, centrally administered AVP activates adrenal A-cells and NA-cells by brain thromboxane A2-mediated mechanisms in rats.

Centrally administered histamine evokes the adrenal secretion of noradrenaline and adrenaline by brain cyclooxygenase-1- and thromboxane A2-mediated mechanisms in rats

Plasma adrenaline is originated from adrenal medulla, while plasma noradrenaline reflects the release from sympathetic nerves in addition to the secretion from adrenal medulla. The present study was designed to characterize the source of plasma catecholamines induced by centrally administered histamine, with regard to the brain prostanoids. Intracerebroventricularly (i.c.v.) administered histamine (1, 5 and 10 microg/animal) elevated plasma noradrenaline and adrenaline (noradrenaline<adrenaline) in a dose-dependent manner. Ketoprofen (a selective inhibitor of cyclooxygenase-1) (100, 250 and 500 microg/animal, i.c.v.) dose-dependently reduced the histamine (5 microg/animal, i.c.v.)-induced elevation of both catecholamines, while NS-398 (a selective inhibitor of cyclooxygenase-2) (250 and 500 microg/animal, i.c.v.) had no effect. The histamine-induced response was dose-dependently attenuated by furegurelate (an inhibitor of thromboxane A(2) synthase) (250 and 500 microg/animal, i.c.v.), and abolished by acute bilateral adrenalectomy. These results suggest that centrally administered histamine evokes plasma noradrenaline and adrenaline from adrenal medulla by brain cyclooxygenase-1- and thromboxane A(2)-mediated mechanisms in rats.

Brain prostanoid TP receptor-mediated adrenal noradrenaline secretion and EP3 receptor-mediated sympathetic noradrenaline release in rats

Sympathetic nerves release noradrenaline, whereas adrenal medullary chromaffin cells secrete noradrenaline and adrenaline. Therefore, plasma noradrenaline reflects the secretion from adrenal medulla in addition to the release from sympathetic nerves, however the exact mechanisms of adrenal noradrenaline secretion remain to be elucidated. The present study was designated to characterize the source of plasma noradrenaline induced by intracerebroventricularly (i.c.v.) administered bombesin and prostaglandin E2 in urethane-anesthetized rats. Bombesin (1.0 nmol/animal, i.c.v.) elevated plasma noradrenaline and adrenaline, while prostaglandin E2 (0.3 nmol/animal, i.c.v.) elevated only plasma noradrenaline. The bombesin-induced elevations of both catecholamines were attenuated by pretreatments with furegrelate (an inhibitor of thromboxane A2 synthase) [250 and 500 microg (0.9 and 1.8 micromol)/animal, i.c.v.)] and [(+)-S-145] [(+)-(1R,2R,3S,4S)-(5Z)-7-(3-[4-3H]-phenylsulphonyl-aminobicyclo[2.2.1]hept-2-yl)hept-5-enoic acid sodium salt] (an antagonist of prostanoid TP receptors) [100 and 250 microg (250 and 625 nmol)/animal)], and abolished by acute bilateral adrenalectomy. On the other hand, the prostaglandin E2-induced elevation of plasma noradrenaline was not influenced by acute bilateral adrenalectomy. These results suggest that adrenal noradrenaline secretion and sympathetic noradrenaline release are mediated by differential central mechanisms; brain prostanoid TP receptors activated by bombesin are involved in the adrenal noradrenaline secretion, while brain prostanoid EP (probably EP3) receptors activated by prostaglandin E2 are involved in the sympathetic noradrenaline release in rats. Brain prostanoid TP receptors activated by bombesin are also involved in the adrenal adrenaline secretion.

Different Effects of Novel Stressors on Sympathoadrenal System Activation in Rats Exposed to Long-Term Immobilization

Activation of the sympathoadrenal system, evaluated by plasma levels of epinephrine (E) and norepinephrine (NE) after exposure of rats to various stressors, is well documented. However, response of rats exposed long-term to a homotypic stressor and then exposed once to a heterotypic novel stressor is poorly understood. In the present study, we examined changes in plasma levels of catecholamines (CA) and corticosterone (COR) of rats after a single (2-h) or long-term repeated immobilization (41 times, 2 h daily) and in rats adapted to long-term immobilization exposed once to the novel stress of cold exposure or insulin or 2-deoxy-D-glucose (2DG) administration. Long-term immobilization produced a significant elevation of basal plasma COR but not NE and E levels. Long-term immobilized rats exposed to insulin or 2DG showed significant elevation of plasma CA and COR levels in comparison to the administration to control rats. Exposure of long-term immobilized and control rats to cold stress increased plasma NE and COR, whereas plasma E was not significantly changed. The exposure of long-term immobilized rats to a further single immobilization (2 h) increased plasma CA levels, but, in naive control rats, the single immobilization produced more pronounced increases. These data suggest that rats exposed to homotypic long-term immobilization are able to respond to heterotypic stressors by higher activation of the sympathoadrenal system as compared with the control, previously unstressed rats. Reduced plasma CA levels in long-term immobilized rats exposed to homotypic stressor are most probably due to an adaptation at the level of brain regulatory centers.

Effect of sympathetic denervation on the rate of protein synthesis in rat skeletal muscle

Rates of protein synthesis were investigated in skeletal muscles from rats submitted to chemical and surgical sympathectomy. Three models of sympathetic denervation were used: 1) treatment with guanethidine (100 mg.kg(-1).day(-1) sc); 2) lumbar sympathetic denervation (surgical excision of the second and third lumbar ganglia of the sympathetic chain, from which arises the postganglionic fibers to the skeletal muscles of rat hindlimb); and 3) adrenodemedullation. Protein synthesis was estimated in isolated soleus muscle by the rate of incorporation of [(14)C]tyrosine (0.1 mM, 0.05 microCi/ml) into total protein. Soleus isolated after 2 and 4 days of chemical sympathectomy or after 3 days of lumbar denervation showed a 17-20% statistically significant decrease in in vitro rates of protein synthesis. These effects were reverted by addition of 10(-5) M isoproterenol or epinephrine in vitro. Neither clenbuterol nor isoproterenol (10(-7), 10(-6), or 10(-5) M) in vitro affected the rate of protein synthesis in soleus from normal rats. On the other hand, clenbuterol or epinephrine (10(-5) M) increased by 20% the rate of protein synthesis in soleus muscles from adrenodemedullated rats and prevented its decrease in muscles from fasted rats. The data suggest that the sympathetic nervous system stimulates protein synthesis in oxidative muscles, probably through the activation of beta(2)-adrenoceptors, especially in situations of hormonal or nutritional deficiency.

Cardiovascular regulation after destruction of the C1 cell group of the rostral ventrolateral medulla in rats

To evaluate the role of C1 neurons in the rostral ventrolateral medulla (RVLM) in cardiovascular regulation, we studied rats in which this cell group was destroyed by the injection of anti-dopamine-beta-hydroxylase-saporin into the RVLM. These immunotoxin injections resulted in 32-99% depletion of the RVLM-C1 neurons and approximately 50% depletion of the A5 cell population. In conscious rats with large (>80%) depletion of the RVLM-C1 cell population, resting arterial pressure was approximately 10 mmHg lower than in control injected rats, although heart rate was not significantly different. Similar results were observed when arterial pressure was recorded in urethan-anesthetized rats, although under anesthesia, heart rate was also reduced in rats with large (>80%) depletion of the RVLM-C1 neuronal population. Sympathoexcitatory responses to baroreceptor unloading, chemoreceptor activation, and electrical stimulation of sciatic nerve afferent fibers were attenuated in rats with >80% depletion of the RVLM-C1 cell population. These effects of RVLM-C1 plus A5 cell populations were not mimicked by either smaller lesions of the RVLM-C1 population or by selective destruction of the A5 cell population with 6-hydroxydopamine. Sympathoinhibitory responses such as decreases in arterial pressure and heart rate evoked by injection of GABA into the RVLM or by intravenous phenylbiguanide administration were not altered by RVLM-C1 plus A5 cell depletion. These data suggest that RVLM-C1 cells contribute to the maintenance of baseline arterial pressure and play an integral role in sympathoexcitatory responses.

Role of brain thromboxane A2 in the release of noradrenaline and adrenaline from adrenal medulla in rats

Plasma noradrenaline reflects the release from adrenal medulla and sympathetic nerves; however, the exact mechanisms of adrenal noradrenaline release remain to be elucidated. The present study was designed to characterize the source of plasma noradrenaline induced by centrally administered vasopressin and corticotropin-releasing hormone (CRH) in urethane-anesthetized rats. Intracerebroventricularly administered vasopressin (0.2 nmol/animal) and CRH (1.5 nmol/animal) elevated plasma levels of noradrenaline and adrenaline. Intracerebroventricularly administered indomethacin [1.2 micromol (500 microg)/animal] (an inhibitor of cyclooxygenase) abolished the elevations of both noradrenaline and adrenaline induced by vasopressin and CRH. Intracerebroventricularly administered furegrelate [1.8 micromol (500 microg)/animal] (an inhibitor of thromboxane A(2) synthase) abolished the elevations of both noradrenaline and adrenaline induced by vasopressin, while the reagent only attenuated the elevation of plasma adrenaline evoked by CRH. Acute bilateral adrenalectomy abolished the elevation of both noradrenaline and adrenaline induced by vasopressin, while the procedure reduced only the elevation of adrenaline induced by CRH. These results suggest that the release of noradrenaline from adrenal medulla and sympathetic nerves is mediated by different central mechanisms. The vasopressin-induced noradrenaline release from adrenal medulla is mediated by brain thromboxane A(2)-mediated mechanisms, while the CRH-induced noradrenaline release from sympathetic nerves is mediated by brain prostanoid (other than thromboxane A(2))-mediated mechanisms. The vasopressin- and CRH-induced adrenaline release from adrenal medulla is also mediated by brain thromboxane A(2)-mediated mechanisms in rats.

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.