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

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.

Brain alpha 1-adrenergic neurotransmission is necessary for behavioral activation to environmental change in mice

Terazosin, a water-soluble alpha 1 antagonist that can be administered in high doses intraventricularly was used to study the relationship between brain alpha 1 adrenoceptor neurotransmission and behavioral activation in the mouse. The antagonist was found to produce a dose-dependent, complete inhibition of motor activity and catalepsy which were reversed preferentially by coinfusion of an alpha 1 agonist (phenylephrine) compared to a D1 (SKF38393) or a D2 agonist, (quinpirole). Blockade of central beta-1 (betaxolol), alpha-2 (RX821002) or beta-2 (ICI 118551) adrenoceptors had smaller or non-significant effects. Terazosin's selectivity for alpha 1 receptors versus dopaminergic receptors was verified under the present conditions by showing that the intraventricularly administered antagonist protected striatal and cerebral cortical alpha 1 receptors but not striatal or cortical D1 receptors from in vivo alkylation by N-ethoxycarbonyl-2-ethoxy-1, 2-dihydroxyquinoline. That its effect was due to blockade of brain rather than peripheral receptors was shown by the finding that intraperitoneal doses of terazosin three to 66 times greater than the maximal intraventricular dose produced less behavioral inhibition. Intraventricular terazosin also produced hypothermia and a reduced respiratory rate suggestive of a reduced sympathetic outflow. However, external heat did not affect the inactivity, and captopril, a hypotensive agent, did not mimic it. Terazosin did not impair performance on a horizontal wire test or the ability to make co-ordinated movements in a swim test suggesting that its activity-reducing effect was not due to sedation and may have a motivational or sensory gating component. It is concluded that central alpha 1-noradrenergic neurotransmission is required for behavioral activation to environmental change in the mouse and may operate on sensorimotor and motivational processes.

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.

Medullary-evoked EPSPs in neonatal rat sympathetic preganglionic neurones in vitro

1. Whole-cell patch clamp recordings were made from twenty-three sympathetic preganglionic neurones (SPNs) in the upper thoracic segments of a neonatal rat brainstem-spinal cord preparation to study their synaptic responses to stimulation of the rostral ventrolateral medulla (RVLM) and the receptors involved. 2. SPNs were identified by their antidromic activation following stimulation of a ventral root, their morphology and their location in the spinal cord. 3. Electrical stimulation within the RVLM elicited EPSPs in all SPNs tested (n = 23). These EPSPs consisted of one or more components that had different time courses, voltage relationships and pharmacological sensitivities. 4. All SPNs responded to RVLM stimulation with a constant-latency fast EPSP that increased in size as the membrane was hyperpolarized. This EPSP was reduced in amplitude by the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (10-20 microM). 5. In thirteen SPNs the response to RVLM stimulation was a complex EPSP consisting of a fast EPSP and a slow EPSP that either followed or summed with the fast EPSP. The amplitude of the slow EPSP was (i) either reduced in size or not affected as the membrane was hyperpolarized, and (ii) reduced by the NMDA receptor antagonist, D, L-2-amino-5-phosphonovaleric acid (50 microM). 6. Selective activation of neuronal cell bodies in the RVLM by chemical stimulation elicited slow depolarizations and increases in synaptic activity in SPNs. 7. These results provide evidence that an excitatory amino acid is involved in transmitting sympathoexcitatory drive from the RVLM, partly via a monosynaptic pathway. Both non-NMDA and NMDA receptors play a role in mediating this drive.

Annual review prize lecture. Central nervous mechanisms contributing to cardiovascular control

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Inhibition of uptake 1 by (+)-oxaprotiline reveals a differential central regulation of noradrenaline and adrenaline release

Inhibition of uptake 1 in the central nervous system leads to a decrease of sympathetic outflow to many tissues; central alpha 2-adrenoceptors are involved in this decrease. The aim of the present study was to compare the effects of the selective uptake 1 inhibitor (+)-oxaprotiline on the plasma kinetics of noradrenaline and adrenaline in anaesthetized and in conscious rabbits. [3H]Noradrenaline and [3H]adrenaline were infused i.v. The arterial plasma concentrations of endogenous and radiolabelled noradrenaline and adrenaline were measured, and the clearance from and spillover into the plasma of noradrenaline and adrenaline were calculated. Results obtained in conscious and anaesthetized rabbits were similar. (+)-Oxaprotiline 0.2, 0.6 and 1.8 mg kg-1 i.v. dose-dependently reduced the clearance of [3H]noradrenaline from the plasma. The clearance of [3H]adrenaline was reduced less. The spillover of endogenous noradrenaline was decreased by up to 35%. In contrast, the spillover of adrenaline tended to be enhanced. Prazosin 0.1 and 1 mg kg-1 was injected i.v. in a second part of each experiment. It lowered the blood pressure and caused a marked increase in noradrenaline spillover but no increase or even a decrease in adrenaline spillover. The results are compatible with the following hypothesis. The sympathetic outflow from the central nervous system is subject to a twofold alpha-adrenoceptor-mediated modulation: alpha 2-adrenoceptor-mediated inhibition and alpha 1-adrenoceptor-mediated excitation. In the control of the sympathetic outflow to many extra-adrenal tissues, the alpha 2-adrenergic inhibition prevails. Uptake 1 inhibitors depress sympathetic outflow to such tissues by enhancing the alpha 2-adrenergic inhibition.(ABSTRACT TRUNCATED AT 250 WORDS)