Effects of activating spinal alpha-adrenoreceptors on sympathetic nerve activity in the rat

Chemogenetic activation of noradrenergic A5 neurons increases blood pressure and visceral sympathetic activity in adult rats

In mammals, the pontine noradrenergic system influences nearly every aspect of central nervous system function. A subpopulation of pontine noradrenergic neurons, called A5, are thought to be important in the cardiovascular response to physical stressors, yet their function is poorly defined. We hypothesized that activation of A5 neurons drives a sympathetically mediated increase in blood pressure (BP). To test this hypothesis, we conducted a comprehensive assessment of the cardiovascular effects of chemogenetic stimulation of A5 neurons in male and female adult rats using intersectional genetic and anatomical targeting approaches. Chemogenetic stimulation of A5 neurons in freely behaving rats elevated BP by 15 mmHg and increased cardiac baroreflex sensitivity with a negligible effect on resting HR. Importantly, A5 stimulation had no detectable effect on locomotor activity, metabolic rate, or respiration. Under anesthesia, stimulation of A5 neurons produced a marked elevation in visceral sympathetic nerve activity (SNA) and no change in skeletal muscle SNA, showing that A5 neurons preferentially stimulate visceral SNA. Interestingly, projection mapping indicates that A5 neurons target sympathetic preganglionic neurons throughout the spinal cord and parasympathetic preganglionic neurons throughout in the brainstem, as well as the nucleus of the solitary tract, and ventrolateral medulla. Moreover, in situ hybridization and immunohistochemistry indicate that a subpopulation of A5 neurons coreleases glutamate and monoamines. Collectively, this study suggests A5 neurons are a central modulator of autonomic function with a potentially important role in sympathetically driven redistribution of blood flow from the visceral circulation to critical organs and skeletal muscle.

Modulation of sympathetic preganglionic neuron activity via adrenergic receptors

The sympathetic preganglionic neurons (SPNs) play a key role in the sympathetic nervous system. Previous reports have suggested that norepinephrine (NE) directly affects SPNs via both inhibitory hyperpolarization interactions mediated by α2 receptors and excitatory depolarization interactions mediated by α1 receptors. It remains poorly understood, however, whether the excitability of SPNs can be inhibited indirectly (presynaptically) as well as directly (postsynaptically). We intracellularly recorded 41 SPNs using the whole-cell patch-clamp technique in spinal cord slice preparations of neonatal rats. We examined the effects of NE or dexmedetomidine hydrochloride (Dxm) (α2-adrenergic receptor agonist) on SPNs by analyzing the excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs). EPSPs were dominant in 15 SPNs (EPSP-SPNs) and IPSPs were dominant in 7 SPNs (IPSP-SPNs) at baseline. We were unable to analyze the postsynaptic potentials in the other 19 SPNs, due to high frequency of action potential firings (firing-SPNs). At baseline, the membrane potentials and resistances of each type of SPN were similar. NE (1  μM) gradually depolarized the EPSP-SPNs and IPSP-SPNs (P   < 0.001) and NE significantly increased the EPSP frequency of the EPSP-SPNs (P   < 0.05). Dxm (10  nM) after application of NE decreased the EPSP frequency of the EPSP-SPNs (P  <  0.001) and the EPSP voltage and IPSP voltage of the IPSP-SPNs (P  <  0.05). In 5 of the 19 firing-SPNs, NE induced membrane hyperpolarization (P   < 0.05) and completely inhibited firings. Dxm had no effect in these neurons. The SPNs received inhibitory modulation through α2-adrenergic receptors. Some SPNs can be directly inhibited via effects independent of the α2 receptors.

Orexin A in rat rostral ventrolateral medulla is pressor, sympatho-excitatory, increases barosensitivity and attenuates the somato-sympathetic reflex

BACKGROUND AND PURPOSE

The rostral ventrolateral medulla (RVLM) maintains sympathetic nerve activity (SNA), and integrates adaptive reflexes. Orexin A-immunoreactive neurones in the lateral hypothalamus project to the RVLM. Microinjection of orexin A into RVLM increases blood pressure and heart rate. However, the expression of orexin receptors, and effects of orexin A in the RVLM on splanchnic SNA (sSNA), respiration and adaptive reflexes are unknown.

EXPERIMENTAL APPROACH

The effect of orexin A on baseline cardio-respiratory variables as well as the somato-sympathetic, baroreceptor and chemoreceptor reflexes in RVLM were investigated in urethane-anaesthetized, vagotomized and artificially ventilated male Sprague-Dawley rats (n= 50). orexin A and its receptors were detected with fluorescence immunohistochemistry.

KEY RESULTS

Tyrosine hydroxylase-immunoreactive neurones in the RVLM were frequently co-localized with orexin 1 (OX(1) ) and orexin 2 (OX(2) ) receptors and closely apposed to orexin A-immunoreactive terminals. Orexin A injected into the RVLM was pressor and sympatho-excitatory. Peak effects were observed at 50 pmol with increased mean arterial pressure (42 mmHg) and SNA (45%). Responses to orexin A (50 pmol) were attenuated by the OX(1) receptor antagonist, SB334867, and reproduced by the OX(2) receptor agonist, [Ala(11) , D-Leu(15) ]orexin B. Orexin A attenuated the somato-sympathetic reflex but increased baroreflex sensitivity. Orexin A increased or reduced sympatho-excitation following hypoxia or hypercapnia respectively.

CONCLUSIONS AND IMPLICATIONS

Although central cardio-respiratory control mechanisms at rest do not rely on orexin, responses to adaptive stimuli are dramatically affected by the functional state of orexin receptors.

Differential regulation of the central neural cardiorespiratory system by metabotropic neurotransmitters

Central neurons in the brainstem and spinal cord are essential for the maintenance of sympathetic tone, the integration of responses to the activation of reflexes and central commands, and the generation of an appropriate respiratory motor output. Here, we will discuss work that aims to understand the role that metabotropic neurotransmitter systems play in central cardiorespiratory mechanisms. It is well known that blockade of glutamatergic, gamma-aminobutyric acidergic and glycinergic pathways causes major or even complete disruption of cardiorespiratory systems, whereas antagonism of other neurotransmitter systems barely affects circulation or ventilation. Despite the lack of an 'all-or-none' role for metabotropic neurotransmitters, they are nevertheless significant in modulating the effects of central command and peripheral adaptive reflexes. Finally, we propose that a likely explanation for the plethora of neurotransmitters and their receptors on cardiorespiratory neurons is to enable differential regulation of outputs in response to reflex inputs, while at the same time maintaining a tonic level of sympathetic activity that supports those organs that significantly autoregulate their blood supply, such as the heart, brain, retina and kidney. Such an explanation of the data now available enables the generation of many new testable hypotheses.

Imidazoline I(2)-receptors and spinal reflexes in the decerebrated rabbit

Idazoxan potentiates spinal reflexes in the decerebrated rabbit, an effect that has been attributed to antagonism of tonic noradrenergic inhibition. It is now known that this drug has a higher affinity for I(2)-imidazoline receptors than alpha(2)-receptors. The roles of I(2)-receptors in modulating transmission in spinal reflex pathways have been investigated using the selective ligands RX 821029 and RS-45041-190, and, as I(2)-receptors are closely associated with monoamine oxidase (MAO), the MAO inhibitors pargyline and clorgyline. In decerebrated rabbits with an intact spinal cord, intrathecal doses of 5-365 microg (cumulative) of the I(2)-ligands augmented, to 150-180% of pre-drug levels, the reflex responses of medial gastrocnemius motoneurones to electrical stimulation of the sural nerve, and significantly increased mean arterial blood pressure by approximately 10 mmHg over pre-drug values. Neither MAO inhibitor had significant effects on reflexes, but the highest dose of the MAO-A selective agent clorgyline (365 microg cumulative) caused a significant rise in blood pressure of 6 mmHg. Neither the I(2)-ligands nor the MAO inhibitors prevented the further enhancement of reflexes or blood pressure by subsequent administration of the selective alpha(2)-adrenoceptor antagonist RX 821002. In decerebrated, spinalized rabbits, intrathecal RS-45041-190 (60 microg, single dose) increased spinal reflex responses to 109% of pre-drug values, a significantly smaller effect than that seen in non-spinal preparations, and had no effect on blood pressure. These data show that imidazoline I(2)-receptors can influence somatic and autonomic motor outflows. These effects should be taken into account when interpreting the spinal effects of imidazoline-based adrenoceptor-active drugs.

Influence of hypergravity on the development of monoaminergic systems in the rat spinal cord

We have investigated in this study the influence of a moderate hypergravity (1.8 G) on the development of monoaminergic projections to the spinal cord in the rat. Pregnant dams and their offspring were submitted to hypergravity from day 11 of gestation to postnatal day 15. Some animals were sacrificed at birth, other at postnatal day 15 and other after 8 months of normal gravity. In newborn animals, a substantial delay of the development of monoaminergic projections to the spinal cord was evidenced. In 15 days and 8 months animals, the pattern of innervation appeared anarchic, with numerous dystrophic profiles, mainly of serotonergic system. Ultrastructural examination of serotonergic projections revealed a paucity of synapses, and the frequent enveloping of serotonergic boutons by thin astrocytic profiles. We conclude that rats submitted to hypergravity during the critical period of onset of monoaminergic projections to the spinal cord are affected durably in the organization and the ultrastructure of these projections. Future studies are directed to the functional analysis of hypergravity animals, and to the influence of microgravity on the same system.

Recovery of locomotion following transplantation of monoaminergic neurons in the spinal cord of paraplegic rats

Severe traumatic lesions of the spinal cord yield a permanent deficit of motricity in adult mammals and specifically a loss of locomotor activity of hindlimbs when the lesion is located at the lower thoracic level. To restore this function, we have developed a paradigm of transplantation in rats based on a transection model of the spinal cord and the subsequent injection at the sublesional level of a suspension of embryonic brainstem monoaminergic neurons which play a key role in the modulation of locomotion. A genuine locomotion was characterized in transplanted animals by electromyographic and electroneurographic recordings. This correlated with a specific reinnervation pattern of targets, where typical synapses were found, and with the normalization of biochemical parameters.

Influence of arterial baroreceptors and intracerebroventricular guanabenz on synchronized renal nerve activity

The contributions of changes in the number of active fibres and the peak interval of synchronized neural discharges to arterial baroreflex regulated alterations in renal sympathetic nerve activity were examined in intact conscious rats. Stimulation of central nervous system alpha 2 adrenoreceptors with intracerebroventricular guanabenz (10, 20, 40 micrograms) was used to alter renal sympathetic nerve activity by a non-reflex mechanism in both intact and sinoaortic denervated (SAD) rats. Synchronized renal sympathetic nerve discharge was analysed with the sympathetic peak detection algorithm. When arterial pressure was increased from 50 mmHg to 150 mmHg in intact rats, the peak height (number of simultaneously active fibres) of synchronized discharges decreased in a sigmoidal fashion while the peak interval remained unchanged. Guanabenz produced a dose dependent inhibition of renal sympathetic nerve activity due to both a decrease in peak height and an increase in peak interval of synchronized discharges in both intact and SAD rats. Arterial baroreflex mediated changes in renal sympathetic nerve activity are due to changes in the number of simultaneously active nerve fibres. Central nervous system alpha 2 adrenoreceptor stimulation decreases renal sympathetic nerve activity by decreasing the number of active fibres and increasing the peak interval, acting on additional neural pathways not involved in buffering acute arterial pressure changes.

Cuneiform nucleus stimulation-induced sympathoexcitation: role of adrenoceptors, excitatory amino acid and serotonin receptors in rat spinal cord

Stimulation of the midbrain cuneiform nucleus has previously been shown to produce increases in arterial blood pressure and lumbar sympathetic nerve activity. While this sympathoexcitatory effect is, in part, due to excitation of premotor sympathoexcitatory neurons in the rostral ventrolateral medulla, the specific spinal neurotransmitter systems recruited by cuneiform nucleus stimulation remains to be elucidated. In this study, mean arterial pressure, resting and cuneiform nucleus stimulation-evoked lumbar sympathetic nerve activity were analysed following intrathecal injections of an excitatory amino acid antagonist (kynurenic acid), alpha1-adrenoceptor antagonist (prazosin) and a serotonin receptor antagonist (methiothepin) in anesthetized, paralysed male Sprague-Dawley rats. Mean arterial pressure and resting sympathetic nerve discharge were decreased by all treatments (n = 6/group) compared to the vehicle control group. Intermittent electrical stimulation of the cuneiform nucleus produced a bimodal sympathoexcitatory response, of which the short latency peak was significantly attenuated (43% reduction) by intrathecal kynurenate whereas the long latency peak was reduced by intrathecal prazosin (decrease of 21%) and methiothepin (38% attenuation). These results are consistent with the significant roles of excitatory amino acid, alpha1-adrenergic and serotonin receptors in modulating the activity of sympathetic vasomotor preganglionic neurons supplying the lumbar sympathetic nerve trunk, and suggest the existence of at least three neuronal groups and/or pathways associated with the sympathoexcitatory response to cuneiform nucleus stimulation.

Hypotensive and hypertensive effects of catecholamines intrathecally injected in anesthetized rats

The cardiovascular effects of catecholamines intrathecally (i.t.) injected at the T12-L1 level were analyzed in pentobarbital anesthetized rats. Volumes of injection were not greater than 3 microliters. Noradrenaline in doses ranging from 0.03 to 0.3 micrograms (i.t.) did not alter the mean blood pressure (MBP) while higher doses (1, 3 and 10 micrograms, i.t.) caused a dose-dependent increase in MBP. Adrenaline induced hypotensive effects at low doses (0.03-0.3 micrograms i.t.) and pressor effects at high doses (3 and 10 micrograms, i.t.). Neither adrenaline nor noradrenaline modified the heart rate. The pressor responses to both catecholamines were antagonized by the alpha 1-adrenoceptor blocker prazosin (0.05-1 microgram, i.t.) and by the selective alpha 1A-adrenoceptor antagonist 5-methyl urapidil (10 and 15 micrograms, i.t.). In contrast, these pressor effects were not modified by the alpha 1B-adrenoceptor antagonist chloroethylclonidine (90 micrograms i.t.). In animals pretreated with 1 microgram prazosin (i.t.), low doses of noradrenaline (0.03 and 0.1 microgram, i.t.) caused a hypotensive effect. Prazosin (1 microgram i.t.) failed to alter the hypotension caused by 0.1 microgram adrenaline. The hypotensive response induced by either 0.1 microgram noradrenaline (in the presence of prazosin) or 0.1 microgram adrenaline was blocked by the alpha 2-adrenoceptor antagonist yohimbine (1 mg/kg, i.v.), by the GABA-A antagonists bicuculline (3.2 micrograms, i.t.) and picrotoxin (2.7 micrograms, i.t.), and by the GABA-B antagonist 2-hydroxy saclofen (30 micrograms, i.t.). The glycine-receptor antagonist strychnine (25 micrograms, i.t.) did not modify the hypotension induced by either noradrenaline (in the presence of prazosin) or adrenaline. These findings suggest that in the low thoracolumbar spinal cord of pentobarbital-anesthetized rats, noradrenaline and adrenaline have excitatory as well as inhibitory effects on the control of the BP. The pressor responses of high doses of i.t. injected catecholamines could be mediated by the activation of spinal alpha 1A-adrenoceptors, although the participation of alpha 1B-adrenoceptors cannot be rule out entirely. The hypotensive responses induced by low doses of i.t. injected catecholamines seem to involve the activation of spinal alpha 2A-adrenoceptors and the stimulation of an inhibitory GABAergic neuron in the spinal cord.

Barosensitive neurons in the rostral ventrolateral medulla of the rat in vivo: morphological properties and relationship to C1 adrenergic neurons

The aim of this study, conducted in anaesthetized rats, was to examine the morphology of barosensitive neurons in the rostral ventrolateral medulla and their immunoreactivity for a catecholamine synthesizing enzyme, tyrosine hydroxylase. Thirty neurons displaying inhibitory postsynaptic potentials following stimulation of the aortic depressor nerve were intracellularly labelled with Lucifer Yellow or Neurobiotin. Some of these neurons could be excited antidromically from the second thoracic segment of the spinal cord, with conduction velocities of spinal axons ranging from 1.9 to 7.2 m/s. The filled somas were found immediately caudal to the facial nucleus and ventral or ventromedial to compact formation of the nucleus ambiguus. Some dendrites reached the ventral medullary surface. Axons usually projected dorsomedially and then made a sharp rostral and/or caudal turn. The caudally projecting axon could, in some cases, be followed to the first cervical segment of the spinal cord. Seven cells issued fine axon collaterals on the ipsilateral side. These were identified mainly in two areas: in the rostral ventrolateral medulla (or immediately dorsomedial to that region), and within the dorsal vagal complex. Seven of 27 examined cells (26%) were tyrosine hydroxylase-immunoreactive and were classified as C1 adrenergic neurons. No clear relationship was found between the presence or absence of adrenergic phenotype and the morphology of filled cells. However, the amplitude of aortic nerve-evoked inhibitory postsynaptic potentials was significantly larger in tyrosine hydroxylase-positive neurons. Possible reasons for the low percentage of barosensitive cells with tyrosine hydroxylase immunoreactivity found in this study, in comparison with previously published estimates, are discussed. This is the first study describing the morphology of neurons in this part of the medulla identified as barosensitive in vivo, and directly demonstrating adrenergic phenotype in a subset of these neurons.

Intraspinal noradrenergic-rich implants reverse the increase of alpha 1 adrenoceptors densities caused by complete spinal cord transection or selective chemical denervation: a quantitative autoradiographic study

This study examined, in the adult rat, whether the intraspinal transplantation of a cell suspension of embryonic day (ED)13 rat locus coeruleus primordia was able to normalize the lesion-induced increase of spinal alpha 1-adrenoceptors. Two experimental models of spinal denervation were studied. The first model consisted of a complete spinal cord transection (thoracic vertebrae level T8-T9) and 1 week later, the cell suspension was transplanted below the section; the second one was obtained by a selective chemical lesion of the noradrenergic (NA) system and one month later, the cell suspension was implanted at the same level as in transected rats. Five weeks after grafting, all animals were sacrificed and spinal cord tissue sections were processed for immunohistochemical detection of noradrenaline or for quantification of alpha 1-adrenoceptors binding sites densities using [3H]prazosin as a ligand. After 6-OHDA lesion, as well as caudally to the transection, a significant increase by 21% (P < 0.01) to 68% (P < 0.001) of alpha 1-adrenoceptors densities was detected. The implantation of embryonic NA neurons into the denervated spinal cord led to a reversal of the lesion-induced increase of spinal alpha 1-adrenoceptors, five weeks later. Moreover, this reversal seems to be more effective after mechanical than after chemical denervation.

Role of alpha 1-adrenergic receptors in the intermediolateral column in mediating the pressor responses elicited by the stimulation of ventrolateral medullary pressor area

Microinjections of alpha 1-adrenergic receptor agonists into the intermediolateral cell column of the spinal cord (IML) elicit sympathoexcitatory responses. This observation, together with the identification of projections of epinephrine-containing cells in the rostral ventrolateral medullary pressor area (VLPA) to the IML, has prompted speculation that epinephrine may mediate pressor responses to the stimulation of the VLPA. This hypothesis was tested in pentobarbital-anesthetized, artificially ventilated, male Wistar rats. A mesenteric arterial branch was cannulated for monitoring blood pressure. Pressor responses were elicited predominantly from T8-T10 by injections (1.7 nmol/20 nl) of L-glutamate into the IML; maximum pressor responses (29.3 +/- 4 mmHg) were elicited from T9. Pressor responses were also elicited by injections of epinephrine into the IML at T9; maximum pressor effect (16.3 +/- 1.2 mmHg) was elicited by a dose of 0.05 pmol/20 nl. This effect of epinephrine at T9 was blocked by prior injections of prazosin (a selective alpha 1-adrenergic receptor blocker; 0.125 pmol/20 nl) at the same site. Stimulation of the VLPA by unilateral microinjections of glutamate elicited pressor responses (56 +/- 12 mmHg). Bilateral injections of prazosin at T8-T10, in the dose (0.125 pmol) that blocked a maximally effective dose of epinephrine, did not block the pressor responses to subsequent injections of glutamate into the VLPA. On the other hand, bilateral microinjections of AP-7 (an NMDA receptor blocker; 1 nmol/20 nl), but not DNQX (10 pmol; a non-NMDA receptor blocker), into the IML at T8-T10 blocked the pressor effects of the subsequent injections of glutamate into the VLPA.(ABSTRACT TRUNCATED AT 250 WORDS)

Motor and cardiovascular effects of selective alpha 2-adrenoceptor antagonists in the decerebrated rabbit

In the decerebrated rabbit, intrathecal administration of the alpha 2-adrenoceptor antagonists idazoxan and yohimbine increases reflex responses of gastrocnemius motoneurones to electrical stimulation of the sural nerve. The maximum effect of idazoxan is about twice that of yohimbine, suggesting that part of the effect of idazoxan may be due to an action at non-adrenergic idazoxan or imidazoline binding sites. RX811059 and RX821002 are selective alpha 2-adrenoceptor antagonists with minimal affinity for imidazoline binding sites. In the present study it was found that intrathecal administration of these compounds in doses of 5-200 micrograms increased the sural-gastrocnemius reflex to the same level as idazoxan, confirming that this reflex is subject to powerful adrenergic inhibition mediated through spinal alpha 2-adrenoceptors. RX811059 and RX821002 also increased arterial blood pressure and heart rate. A retrospective analysis of data from earlier studies showed that whereas intrathecal yohimbine had increased blood pressure, idazoxan had not. It is possible that idazoxan acts at non-adrenergic idazoxan binding sites to cancel out the hypertensive effects which are associated with alpha 2-adrenoceptor blockade.

Normal distribution of alpha-1-adrenoceptors in the rat spinal cord and its modification after noradrenergic denervation: a quantitative autoradiographic study

The distribution of alpha 1 (alpha 1)-adrenoceptors along the different segments of the spinal cord (cervical, thoracic, lumbar, and sacral) of normal rats has been studied by quantitative autoradiography using the specific alpha 1-antagonist [3H]Prazosin as a ligand. In addition, the influence of noradrenergic (NA) denervation [obtained either by complete transection of the spinal cord at vertebrae level T8-T9 or by selective lesion of NA spinal cord system carried out by intracisternal injection of 6-hydroxydopamine (6-OHDA)] on eventual variations of alpha 1-adrenoceptor density at spinal cord target cells was studied in parallel. In control rats, the quantitative analysis of alpha 1-adrenoceptor densities revealed a widespread distribution of these receptors along all segments of the spinal cord with a similar pattern in the various subregions of gray matter studied. This distribution of alpha 1-adrenoceptors was quite well correlated with the distribution of NA terminals, when referring to previous descriptions by immunohistochemistry. After 6-OHDA lesion, as well as caudally to the transection, a significant increase of alpha 1-adrenoceptor densities was observed in all spinal subregions thus evidencing supersensitivity. These results suggest that NA may act in the spinal cord, at least partly, via alpha 1-adrenoceptors and that the expression of these receptors could be influenced by NA dysfunction, as demonstrated here through the effects observed in lesioned animals.

Microinjections of norepinephrine into the intermediolateral cell column of the spinal cord exert excitatory as well as inhibitory effects on the cardiac function

Cardiac responses to microinjections of norepinephrine (NE) into the intermediolateral column of the spinal cord (IML) at T2 level were studied in pentobarbital-anesthetized, immobilized and artificially ventilated, male Wistar rats. For describing the effects of NE conveniently, the doses of NE were divided into two ranges. The small dose-range consisted of 20 nl volumes of 50, 75 and 100 micromolar (microM) solutions (i.e. 1, 1.5 and 2 pmole in 20 nl, respectively). The larger dose-range consisted of 20 nl volumes of 2.5, 25, 40 and 50 millimolar (mM) solutions (i.e. 0.05, 0.5, 0.8 and 1 nmole in 20 nl, respectively). Injections of small doses of NE (1-2 pmole) into the IML increased heart rate (HR); intravenous injections of these doses did not alter either blood pressure (BP) or HR. Larger doses of NE (0.05-1 nmole) elicited a decrease in HR; intravenous injections of these doses increased HR and BP. Maximum increase in HR was produced by injections of 1.5 pmole of NE into the IML; this effect was blocked by prior injections of prazosin (an alpha 1 adrenergic receptor antagonist; 50 pmole) but not idazoxan (an alpha 2 adrenergic receptor blocker; 10 pmole) into the IML. Maximum decrease in HR was elicited by injections of 0.8 nmole of NE into the IML; this effect was blocked by idazoxan (10 pmole) but not prazosin (50 pmole). Microinjections of idazoxan (10 pmole) alone increased HR while prazosin (50 pmole) alone was ineffective. Intravenous injections of chlorisondamine (a ganglion blocker) blocked the increase in HR elicited by injections of 1.5 pmole of NE into the IML.(ABSTRACT TRUNCATED AT 250 WORDS)

Central noradrenergic neurons: the autonomic connection

Most CNS noradrenergic (NE) cell groups reside in portions of the medulla oblongata primarily involved in autonomic control (A1, A2, A5) and even the pontine locus coeruleus (A6) receives a major innervation from these medullary areas. This review examines the neuroanatomical and neurophysiological literature relevant to the issue of the role of CNS NE neurons in central autonomic control (with emphasis on cardiovascular control). It is concluded that NE cells, with the possible exception of certain A5 and A1 neurons, have relatively weak or no inputs from visceral cardiovascular afferents but provide a complex "open loop" control over non-aminergic circuits which are more specialized in the processing of cardiovascular and other autonomic reflexes. The question of whether the C1 "adrenergic" cells of the rostral medulla oblongata actually use noradrenaline as a neurotransmitter is also briefly addressed.

Excitation and inhibition of rat sympathetic preganglionic neurones by catecholamines

The actions of microiontophoretically applied catecholamines on antidromically identified sympathetic preganglionic neurones (SPN) in the upper thoracic spinal cord of the anaesthetized rat were investigated. Noradrenaline (NA) excited the majority of neurones (50/71), however, a significant number were inhibited by the catecholamine (17/71). Adrenaline excited 4/9 SPN and inhibited 2/9. Dopamine had excitatory actions on SPN (3/3). Dual actions of NA on the same SPN were demonstrated, with the actions of the catecholamine being modulated by excitatory amino acids. NA was also shown to induce burst firing in 21% of SPN.

Alpha 1-adrenergic receptor mediates arachidonic acid release in spinal cord neurons independent of inositol phospholipid turnover

The alpha 1-adrenergic receptor has been shown to mediate the release of arachidonic acid in FRTL5 thyroid cells and MDCK kidney cells. In primary cultures of spinal cord cells, norepinephrine stimulated release of arachidonic acid (from neurons only) and turnover of inositol phospholipids (from neurons and glia) via alpha 1-adrenergic receptors. These two responses were dissociated by treatment with phorbol ester and pertussis toxin, which inhibited production of inositol phosphates with no appreciable effect on release of arachidonic acid. Extracellular calcium was required for release of arachidonic acid, but not for production of inositol phosphates. The calcium channel blockers nifedipine and verapamil inhibited release of arachidonic acid only. However, 8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate (TMB-8), a compound that blocks intracellular calcium release, diminished production of inositol phosphates, but had little effect on release of arachidonic acid. These results suggest that alpha 1-adrenergic receptors couple to release of arachidonic acid in primary cultures of spinal cord cells by a mechanism independent of activation of phospholipase C, possibly via the activation of phospholipase A2.

The influence of 5-hydroxytryptamine agonists and antagonists on identified sympathetic preganglionic neurones in the rat, in vivo

1. 5-Hydroxytryptamine (5-HT) was applied by microiontophoresis in the vicinity of identified sympathetic preganglionic neurones in the upper thoracic spinal cord of the rat, in vivo. 2. Sympathetic preganglionic neurones responded in one of three ways to 5-HT: by (a) excitation (76%), (b) inhibition (4%) or (c) in a biphasic manner (5%). 3. The excitatory responses evoked by 5-HT were mimicked by 5-carboxamidotryptamine (5-CT) and alpha-methyl-5-hydroxytryptamine (alpha-Me-5-HT). The inhibitory and biphasic responses evoked by 5-HT were mimicked by 2-methyl-5-hydroxytryptamine (2-Me-5-HT). The observed responses evoked by 5-HT and selective agonists may be different on the same cell. In several instances a single neurone excited by one agonist was inhibited by another agonist. 4. The 5-HT2-receptor antagonists, ketanserin and LY 53857, failed to abolish selectively the excitatory responses evoked by 5-HT and alpha-Me-5-HT, when applied by microiontophoresis. The antagonists non-selectively reduced the excitatory responses evoked by 5-HT, 5-CT, alpha-Me-5-HT, D,L-homocysteic acid (DLH) and noradrenaline (NA). A reduction in synaptically evoked activity was also observed. 5. The 5-HT3-receptor antagonist, ICS 205-930, failed to abolish the inhibitory responses evoked by 5-HT. 6. It was concluded that the excitatory responses evoked by 5-HT are mediated by a receptor that is neither 5-HT2 or 5-HT3, but shows similarities to the 5-HT1-like receptor profile. The inhibitory actions of 5-HT are mimicked by 2-Me-5-HT, but the receptor is not 5-HT3, or 5-HT1-like or 5-HT2.

Role of neurotransmitters in the central regulation of the cardiovascular system

The last decade has seen tremendous progress in determining the nature of the neurotransmitters which regulate central nervous system pathways involved in the regulation of blood pressure. Investigations are now pursuing the identity and functional importance of neurotransmitters contained within pathways shown to be important in cardiovascular regulation. In addition, several key components of the brain stem networks involved in the control of sympathetic activity have been identified. For example, numerous studies indicate the importance of neurons located in the rostral ventrolateral medulla in the regulation of SPN. Indeed, this area contains medullospinal sympathoexcitatory neurons which represent the final site of integration of many brain stem and reflex pathways involved in the regulation of sympathetic nerve activity. The neurotransmitter which is utilized by this medullospinal pathway remains unknown. Epinephrine, substance P and glutamate have all been hypothesized as primary chemical mediators in the descending pathway from the brain stem to SPN. Interestingly, lesions of, or antagonists to, epinephrine, substance P, glutamate and 5-HT neurons all abolish sympathetic activity and reduce blood pressure to a level similar to that in a spinal animal. Clearly, not all these transmitters are primary mediators of sympathetic information carried from the brain stem to the spinal cord. It is likely that monoamines and neuropeptides act in the IML, as in other area of the central nervous system, as neuromodulators to set the level of excitability of SPN rather than relaying sympathetic information over a functionally specific medullospinal pathway. This conclusion is supported by the observation that midline medullary 5-HT neurons provide a tonic excitatory input to SPN, but receive no afferent inputs from other central sympathetic or baroreceptor pathways. However, the firing of 5-HT neurons appears to relate to the state of vigilance of the animal. This suggests that 5-HT neurons may lower the threshold of SPN to sympathetic inputs during states of wakefulness. In addition, the time course of the norepinephrine-mediated slow EPSPs and IPSPs in SPN is consistent with a gain-setting function. By analogy, epinephrine is likely to act as a neuromodulator in the IML rather than to serve as the primary mediator of sympathetic information descending from the rostral ventrolateral medulla.(ABSTRACT TRUNCATED AT 400 WORDS)

Excitatory and inhibitory effects of epinephrine on neonatal rat sympathetic preganglionic neurons in vitro

Current and voltage recordings were made from antidromically identified sympathetic preganglionic neurons (SPNs) in transverse thoracolumbar spinal cord slices removed from neonatal rats. When applied by either pressure ejection or superfusion, epinephrine (Epi) caused a slow depolarization or an inward current in 62 SPNs (42%) and a slow hyperpolarization or an outward current in 21 SPNs (14%). The responses persisted in low calcium- or tetrodotoxin-containing media. The Epi-induced depolarization or inward current was associated with increased membrane resistance; it was reduced by membrane hyperpolarization and nullified at a membrane potential of about -100 mV; a clear reversal however was not observed at more negative potential levels. In a number of SPNs the Epi-induced depolarization was accompanied by small inhibitory postsynaptic potentials. The latter were eliminated by a low calcium solution and by the glycine antagonist strychnine, suggesting that they were caused by glycine or a glycine-like substance released from interneurons subsequent to activation by Epi. The Epi-induced hyperpolarization or outward current was associated with decreased membrane resistance, and nullified around -100 mV. The alpha-adrenergic antagonist, dihydroergotamine, and alpha 1-antagonist, prazosin, reversibly blocked the excitatory, whereas the alpha 2-antagonist, yohimbine, abolished the inhibitory response, respectively. It is concluded that Epi acting on alpha 1- and alpha 2-adrenergic receptors depolarizes and hyperpolarizes the rat SPNs by decreasing or increasing membrane conductances to potassium ions.