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

State-dependent modulation of sympathetic firing by α1-adrenoceptors requires constitutive PKC activity in the neonatal rat spinal cord

The central adrenergic and noradrenergic neurotransmitter systems diffusively affect the operation of the spinal neural network and dynamically gauge central sympathetic outflow. Using in vitro splanchnic nerve-thoracic spinal cord preparations as an experimental model, this study examined the intraspinal α₁-adrenoceptor-meidated modulation of sympathetic firing behaviors. Several sympathetic single-fiber activities were simultaneously recorded. Application of phenylephrine (Phe, an α₁-adrenoceptor agonist) increased, decreased or did not affect spontaneous firing. A log-log plot of the change ratios of the average firing rates (AFR) versus their basal AFR displays a linear data distribution. Thus, the heterogeneity in α₁-adrenoceptor-mediated responses is well described by a power law function. Phe-induced power-law firing modulation (plFM) was sensitive to prazosin (Prz, an α₁-adrenoceptor antagonist). Heparin (Hep, a competitive IP₃ receptor blocker) and chelerythrine (Che, a protein kinase C inhibitor) also caused plFM. Phe-induced plFM persisted in the presence of Hep; however, it was occluded by Che pretreatment. Pair-wise analysis of single-fiber activities revealed synchronous sympathetic discharges. Application of Phe, Hep or Che suppressed synchronous discharges in fiber pairs with apparent correlated firing (ACF) and induced or potentiated synchronous discharges in those without or with minimal ACF. Thus, the basal activities of the sympathetic preganglionic neurons participate in determining the responses mediated by the activation of α₁-adrenoceptors. This deterministic factor, which is intrinsic to spinal neural networks, helps the supraspinal adrenergic and noradrenergic systems differentially control their widely distributed neural targets.

Activation of melanocortin receptors in the intermediolateral cell column of the upper thoracic cord elicits tachycardia in the rat

Melanocortin receptors (MCRs) are present in the intermediolateral cell column of the spinal cord (IML). We tested the hypothesis that activation of MCRs in the IML elicits cardioacceleratory responses and the source of melanocortins in the IML may be the melanocortin-containing neurons in the hypothalamic arcuate nucleus (ARCN). Experiments were done in urethane-anesthetized, artificially ventilated adult male Wistar rats. Microinjections (50 nl) of α-melanocyte stimulating hormone (α-MSH) (0.4-2 mM) and adrenocorticotropic hormone (ACTH) (0.5-2 mM) into the right IML elicited increases in heart rate (HR). These tachycardic responses were blocked by microinjections of melanocortin receptor 4 (MC4R) antagonists [SHU9119 (0.25 mM) or agouti-related protein (AGRP, 0.1 mM)] into the right IML. Stimulation of right ARCN by microinjections (30 nl) of N-methyl-d-aspartic acid (NMDA, 10 mM) elicited increases in HR. Blockade of MC4Rs in the ipsilateral IML at T1-T3 using SHU9119 (0.25 mM) attenuated the tachycardic responses elicited by subsequent microinjections of NMDA into the ipsilateral ARCN. ARCN neurons retrogradely labeled by microinjections of Fluoro-Gold into the right IML showed immunoreactivity for proopiomelanocortin (POMC), α-MSH, and ACTH. Fibers immunoreactive for POMC, α-MSH, and ACTH were present in the IML at T1-T3. These results indicated that activation of MC4Rs in the right IML elicited tachycardia and one of the sources of melanocortins in the IML is the ARCN. Melanocortin levels are elevated in stress and ARCN neurons are activated during stress. Our results allude to the possibility that cardiac effects of stress may be mediated via melanocortin containing ARCN neurons that project to the IML.

Control of sympathetic, respiratory and somatomotor outflow by an intraspinal pattern generator

1. Sympathetic and somatic motor outflow results from the summation of excitatory and inhibitory inputs arising from intra- and supra-spinal origins. Here we determined the contribution of intra- and supra-spinal GABAergic inputs, utilizing GABA-A receptors, in producing sympathetic and somatic motor outflow. 2. Spinal GABA-A receptor blockade, with bicuculline or picrotoxin injected intrathecally at T9, increased the level and lability of arterial pressure, sympathetic (splanchnic and cervical sympathetic) and motor (phrenic) nerve activity. Bursts of activity occurring irregularly, at low frequency were seen in all nerves. 3. C1 spinal transection abolished phrenic nerve activity and reduced sympathetic nerve activities and arterial pressure. Intrathecal bicuculline-induced bursting in sympathetic and motor (phrenic, sciatic and brachial) nerves was similar to that seen prior to C1 transection. Thus supraspinal control of sympathetic and somatomotor outflow is not dependent on GABA-A receptors. 4. Bicuculline-induced effects on phrenic nerve activity were eliminated after C8 spinal cord transection and regular phrenic rhythm resumed indicating that bicuculline was not acting directly on phrenic motoneurons. 5. Bicuculline evoked similar bursting characteristics in both sympathetic and motor nerves attributable to increased excitability of spinal cord neurons. The bursting patterns evoked were often coincident in sympathetic and motor nerves suggesting a common site of origin. 6. These data suggest there is intraspinal coupling between multiple sympathetic and motor outflows in the adult rat spinal cord in vivo. Cervicothoracic spinal cord generator/s perhaps in the form of interneuronal networks, utilizing GABA-A and glutamate receptors, can simultaneously drive functionally independent nerves.

Role of inferior olive and thoracic IML neurons in nonshivering thermogenesis in rats

Removal of the midbrain tonic inhibitory mechanism on nonshivering thermogenesis (NST) results in increased temperatures of the interscapular brown adipose tissue (IBAT) and rectum (T(IBAT) and T(rec), respectively) via an enhanced central sympathetic output. Because it is unlikely that neurons (primary) of the midbrain inhibitory mechanism tonically inhibit the IBAT monosynaptically, there must be secondary or tertiary neurons posterior to the midbrain. Such neurons, therefore, may increase their activity during enhanced NST after removal of the midbrain tonic inhibition. The aim of the present experiments was to localize these secondary or tertiary neurons and establish descending neuronal pathway(s) that may project to the major NST effector IBAT. T(IBAT) and T(rec) increases induced by removal of the tonic inhibition by midbrain procaine microinjections were accompanied with appearance of c-Fos-positive neurons in the inferior olive (IO) and the intermediolateral (IML) cell column of the thoracic spinal cord. Electrical stimulation of and L-glutamate microinjections into the IO increased T(IBAT) and T(rec). Midbrain procaine-induced T(IBAT) and T(rec) increases were blocked by electrolytic IO lesions. These results suggest that central thermal signals produced from the lower midbrain are transmitted to IBAT through the IO and IML and that the IO has a role in the central sympathetic functions.

Catecholamine systems in the brain of vertebrates: new perspectives through a comparative approach

A comparative analysis of catecholaminergic systems in the brain and spinal cord of vertebrates forces to reconsider several aspects of the organization of catecholamine systems. Evidence has been provided for the existence of extensive, putatively catecholaminergic cell groups in the spinal cord, the pretectum, the habenular region, and cortical and subcortical telencephalic areas. Moreover, putatively dopamine- and noradrenaline-accumulating cells have been demonstrated in the hypothalamic periventricular organ of almost every non-mammalian vertebrate studied. In contrast with the classical idea that the evolution of catecholamine systems is marked by an increase in complexity going from anamniotes to amniotes, it is now evident that the brains of anamniotes contain catecholaminergic cell groups, of which the counterparts in amniotes have lost the capacity to produce catecholamines. Moreover, a segmental approach in studying the organization of catecholaminergic systems is advocated. Such an approach has recently led to the conclusion that the chemoarchitecture and connections of the basal ganglia of anamniote and amniote tetrapods are largely comparable. This review has also brought together data about the distribution of receptors and catecholaminergic fibers as well as data about developmental aspects. From these data it has become clear that there is a good match between catecholaminergic fibers and receptors, but, at many places, volume transmission seems to play an important role. Finally, although the available data are still limited, striking differences are observed in the spatiotemporal sequence of appearance of catecholaminergic cell groups, in particular those in the retina and olfactory bulb.

Involvement of GABA and glutamate receptors in the blood pressure responses to intrathecally injected sodium nitroprusside in anesthetized rats

In pentobarbital-anesthetized rats the intrathecal (i.t.) injection of the nitric oxide (NO) donor, sodium nitroprusside (125, 250 and 500 nmol), induced a dose-dependent hypotensive response followed by a dose-dependent pressor effect. The pressor response to sodium nitroprusside (250 nmol) was reduced to 30% of the control value by the selective antagonist for AMPA/kainate receptors, 6.7-dinitroquinoxaline-2,3-dione (50 nmol, i.t.), whereas it was not modified by the selective NMDA receptor antagonist, 2-amino-5-phosphono-valeric acid (30 nmol, i.t.). The hypotensive effect of sodium nitroprusside was antagonized by the GABA(A) receptor antagonists, bicuculline (4.4 nmol, i.t.) and picrotoxin (4.4 nmol, i.t.), and also by the GABA(B) receptor antagonist, 2-hydroxy saclofen (113 nmol, i.t.). The blood pressure responses to sodium nitroprusside were not modified by blockade of muscarinic receptors with methyl atropine (164 nmol, i.t.), or of nicotinic receptors with hexamethonium (211 nmol, i.t.), of alpha1-adrenoceptors with prazosin (3.1 nmol, i.t.), of alpha2-adrenoceptors with yohimbine (2.8 micromol/kg, i.v.), of 5-HT receptors with methysergide (5.1 micromol/kg, i.v.), or of glycine receptors with strychnine (65 nmol, i.t.). It is concluded that NO generated from sodium nitroprusside in the spinal cord exerts inhibitory and excitatory effects on blood pressure probably through the release of GABA and glutamate, respectively. The inhibitory action on blood pressure involves the stimulation of spinal GABA(A) and GABA(B) receptors whereas the excitatory response to glutamate appears to be mediated through the activation of spinal AMPA/kainate receptors.

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.

A comparison of the effects of doxazosin and terazosin on the spontaneous sympathetic drive to the bladder and related organs in anaesthetized cats

The effects of i.v. infusion of the alpha1-adrenoceptor antagonists doxazosin and terazosin (2 mg kg-1 h-1) on spontaneous hypogastric, renal and inferior cardiac nerve activity, spontaneous bladder contractions, blood pressure, heart rate and femoral arterial flow were investigated separately in alpha-chloralose-anaesthetized cats. Both drugs caused a reduction in hypogastric nerve activity associated with no overt changes in spontaneous bladder contractions. Doxazosin was more potent than terazosin, in that there was a significant reduction in hypogastric nerve activity after 20 min (0.67 mg kg-1) of infusion, while for terazosin this occurred after 40 min (1.33 mg kg-1). Both drugs also caused significant falls in blood pressure of 34 +/- 3 mm Hg and 33 +/- 4 mm Hg after 60 min. This was associated with no change in heart rate for doxazosin while terazosin caused an initial and significant increase in heart rate of 20 +/- 3 beats min-1 by 5 min, declining by 30 min to 1 +/- 5 beats min-1. This terazosin-induced tachycardia was associated with a significant increase in cardiac nerve activity of 128 +/- 22%. Both drugs caused increases in renal nerve activity however only for doxazosin was this increase significant. Femoral arterial conductance was also increased by both drugs, however, for doxazosin this increase was immediate and larger over the infusion period. These results demonstrate that alpha1-adrenoceptor antagonists can reduce sympathetic drive to the bladder and related organs.

Intrathecal kynurenate but not benextramine blocks hypoxic sympathoexcitation in chemodenervated anesthetized rats

In peripherally chemodenervated anesthetized and ventilated rats, inhalation of 100% N2 for 20 s rapidly excites reticulospinal vasomotor neurons of the rostroventrolateral reticular nucleus (RVL) and sympathetic nerves. After an initial fall, arterial pressure is also elevated. The sympathoexcitatory response and secondary increases in arterial pressure were abolished by microinjections of muscimol (250 pmol) into the RVL or by intrathecal administration of kynurenate (500 nmol), a broad-spectrum antagonist of excitatory amino acid receptors, but not by benextramine, an alpha-adrenoceptor antagonist. The results indicate that excitation of pre-ganglionic sympathetic neurons in the spinal cord by hypoxia results from excitation of RVL-spinal vasomotor neurons, probably the glutamatergic neurons.

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)

Renal sympathetic preganglionic neurons demonstrated by herpes simplex virus transneuronal labelling in the rabbit: close apposition of neuropeptide Y-immunoreactive terminals

Renal sympathetic preganglionic neurons in the spinal cord of rabbits were transneuronally retrogradely labelled by injection of Herpes simplex virus type 1 into the renal nerve and immunohistochemical demonstration of viral antigen. The morphology of the labelled neurons was examined, particularly with respect to the shape and extent of their dendritic trees. Double-labelling immunohistochemical studies were performed to determine the relationship of neuropeptide Y-immunoreactive axons to virus-labelled perikarya and dendrites. The shape of the renal sympathetic preganglionic neurons differed according to whether the neurons were located in the intermediolateral cell column or in other sympathetic areas. The neurons in the intermediolateral cell column had very long dendrites, extending in the rostrocaudal and mediolateral directions. The medially oriented processes extended towards and beyond the central canal. The laterally oriented dendritic processes projected within the dorsolateral funiculus, towards the edge of the spinal cord. Neuropeptide Y-immunoreactive fibres were concentrated in regions containing renal sympathetic preganglionic neurons of the spinal segments examined (T7-L2). Immunoreactive varicose terminals were closely opposed to individual preganglionic neurons, especially to the dendritic processes of these neurons. Our findings indicate that neurotransmitter candidates such as neuropeptide Y are likely to influence renal preganglionic neurons by an input to dendritic processes at some distance from the perikarya. Electrophysiological and other functional studies utilizing applications of neurotransmitter candidates onto these neurons should take this into account.

Effect on cardiac sympathetic nerve activity of phenylephrine microinjected into the cat intermediolateral cell column

1. In anaesthetized cats the effect of the alpha 1-adrenoceptor agonist phenylephrine, microinjected into the left intermediolateral cell column of the spinal cord at the third thoracic level, was studied on left inferior cardiac nerve activity. 2. Microinjection of 100 nl of 10 or 40 mM-phenylephrine caused increases in inferior cardiac nerve activity in fifteen out of seventeen experiments. 3. The microinjection of the alpha 1-adrenoceptor antagonist alfuzosin (100 nl of 10 mM) into the intermediolateral cell column antagonized the excitatory response elicited by phenylephrine. 4. Increases in inferior cardiac nerve activity produced by glutamate and 5-hydroxytryptamine microinjected into the intermediolateral cell column were not antagonized by alfuzosin. 5. It is concluded that activation of alpha 1-adrenoceptors in the region of the intermediolateral cell column can cause an increase in the firing rate of sympathetic preganglionic neurones which innervate postganglionic neurones projecting into the inferior cardiac nerve.