Influence of impulse pattern on noradrenaline release from sympathetic nerves in cerebral and some peripheral vessels

Dynamic resistance exercise-induced pressor response does not alter hypercapnia-induced cerebral vasodilation in young adults

Excessive arterial pressure elevation induced by resistance exercise (RE) attenuates peripheral vasodilatory function, but its effect on cerebrovascular function is unknown. We aimed to evaluate the effect of different pressor responses to RE on hypercapnia-induced vasodilation of the internal carotid artery (ICA), an index of cerebrovascular function. To manipulate pressor responses to RE, 15 healthy young adults (11M/4F) performed two RE: high intensity with low repetitions (HL) and low intensity with high repetitions (LH) dynamic knee extension. ICA dilation, induced by 3 min of hypercapnia, was measured before and 10 min after RE using Doppler ultrasound. HL exercise elicited a greater pressor response than LH exercise. In relaxation phases of RE, ICA blood velocity increased in both HL and LH trials. However, ICA shear rate did not significantly increase in either trial (P = 0.06). Consequently, neither exercise altered post-exercise hypercapnia-induced ICA dilation (HL, 3.9 ± 1.9% to 5.1 ± 1.7%; LH, 4.6 ± 1.4% to 4.8 ± 1.8%; P > 0.05 for all). When viewed individually, the changes in ICA shear rate were positively correlated with changes in end-tidal partial pressure of carbon dioxide (P_ETCO₂) (r = 0.46, P < 0.01) than with mean arterial pressure (r = 0.32, P = 0.02). These findings suggest that the effects of RE-induced pressor response on cerebrovascular function may be different from peripheral arteries. An increase in P_ETCO₂ during the relaxation phase may play a more crucial role than elevated pressure in increasing cerebral shear during dynamic RE.

The sympathetic innervation of the heart: Important new insights

Autonomic control of the heart has a significant influence over development of life threatening arrhythmias that can lead to sudden cardiac death. Sympathetic activity is known to be upregulated during these conditions and hence the sympathetic nerves present a target for treatment. However, a better understanding of the anatomy and physiology of cardiac sympathetic nerves is required for the progression of clinical interventions. This review explores the organization of the cardiac sympathetic nerves, from the preganglionic origin to the postganglionic innervations, and provides an overview of literature surrounding anti-arrhythmic therapies including thoracic sympathectomy and dorsal spinal cord stimulation. Several features of the innervation are clear. The cardiac nerves differentially supply the nodal and myocardial tissue of the heart and are dependent on activity generated in spinal neurones in the upper thoracic cord which project to synapse with ganglion cells in the stellate complex on each side. Networks of spinal interneurones determine the pattern of activity. Groups of spinal neurones selectively target specific regions of the heart but whether they exhibit a functional selectivity has still to be elucidated. Electrical or ischemic signals can lead to remodeling of nerves in the heart or ganglia. Surgical and electrical methods are proving to be clinically beneficial in reducing atrial and ventricular arrhythmias, heart failure and severe cardiac pain. This is a rapidly developing area and we need more basic understanding of how these methods work to ensure safety and reduction of side effects.

Respiratory modulated sympathetic activity: a putative mechanism for developing vascular resistance?

KEY POINTS

Sympathetic activity exhibits respiratory modulation that is amplified in hypertensive rats. Respiratory modulated sympathetic activity produces greater changes in vascular resistance than tonic stimulation of the same stimulus magnitude in normotensive but not hypertensive rats. Mathematical modelling demonstrates that respiratory modulated sympathetic activity may fail to produce greater vascular resistance changes in hypertensive rats because the system is saturated as a consequence of a dysfunctional noradrenaline reuptake mechanism. Respiratory modulated sympathetic activity is an efficient mechanism to raise vascular resistance promptly, corroborating its involvement in the ontogenesis of hypertension.

ABSTRACT

Sympathetic nerve activity (SNA) exhibits respiratory modulation. This component of SNA is important - being recruited under cardiorespiratory reflex conditions and elevated in the spontaneously hypertensive (SH) rat - and yet the exact influence of this modulation on vascular tone is not understood, even in normotensive conditions. We constructed a mathematical model of the sympathetic innervation of an arteriole, and used it to test the hypothesis that respiratory modulation of SNA preferentially increases vasoconstriction compared to a frequency-matched tonic pattern. Simulations supported the hypothesis, where respiratory modulated increases in vasoconstriction were mediated by a noradrenergic mechanism. These predictions were tested in vivo in adult Wistar rats. Stimulation of the sympathetic chain (L3) with respiratory modulated bursting patterns, revealed that bursting increases vascular resistance (VR) more than tonic stimulation (57.8 ± 3.3% vs. 44.8 ± 4.2%; P < 0.001; n = 8). The onset of the VR response was also quicker for bursting stimulation (rise time constant = 1.98 ± 0.09 s vs. 2.35 ± 0.20 s; P < 0.01). In adult SH rats (n = 8), the VR response to bursting (44.6 ± 3.9%) was not different to tonic (37.4 ± 3.5%; P = 0.57). Using both mathematical modelling and in vivo techniques, we have shown that VR depends critically on respiratory modulation and revealed that this pattern dependency in Wistar rats is due to a noradrenergic mechanism. This respiratory component may therefore contribute to the ontogenesis of hypertension in the pre-hypertensive SH rat - raising VR and driving vascular remodelling. Why adult SH rats do not exhibit a pattern-dependent response is not known, but further modelling revealed that this may be due to dysfunctional noradrenaline reuptake.

Electrophysiological impact of trazodone on the dopamine and norepinephrine systems in the rat brain

Previous study has documented the long-term effects of the antidepressant trazodone on the serotonin (5-HT) system. The present work examined the impact of sustained trazodone on ventral tegmental area (VTA) dopamine (DA) and locus ceruleus (LC) norepinephrine (NE) neurons firing activity, and characterized its effects at 5-HT(2C), 5-HT(2A) receptors and α₁- and α₂-adrenoceptors. Electrophysiological recordings were carried out in anesthetized rats. Subcutaneously implanted minipumps delivered vehicle or trazodone (10 mg/kg/day) for 2 or 14 days. Administration of trazodone for 2 and 14 days did not alter the firing activity of DA neurons. Systemic injection of trazodone, however, reversed the inhibitory effect of the 5-HT(2C) receptor agonist Ro 60,0175 on the DA neuronal firing, suggesting an antagonistic action of trazodone at this receptor. Administration of trazodone for 2 days significantly enhanced the NE neurons firing. Despite a return of the NE neurons firing rate to the baseline following 14-day trazodone, the percentage of neurons discharging in burst was increased by this regimen. Administration of trazodone for 14 days enhanced the tonic activation of postsynaptic α₂-adrenoceptors, as indicated by the disinhibitory effect of the α₂-adrenoceptor antagonist idazoxan on hippocampus pyramidal neurons firing. The inhibitory effect of acute trazodone on dorsal raphe (DR) 5-HT neurons firing was shown to be through the 5-HT(1A) receptor. Systemic injection of trazodone reversed the inhibitory action of 5-HT(2A) agonist DOI on the NE neurons firing rate, indicating its antagonistic action at 5-HT(2A) receptors. The enhancement in α₂-adrenergic transmission by trazodone, and its 5-HT(2A) and 5-HT(2C) receptor antagonism may contribute to its therapeutic action in major depression.

Shining light on wakefulness and arousal

Alterations in arousal states are associated with multiple neuropsychiatric disorders, including generalized anxiety disorders, addiction, schizophrenia, and depression. Therefore, elucidating the neurobiological mechanisms controlling the boundaries between arousal, hyperarousal, and hypoarousal is a crucial endeavor in biological psychiatry. Substantial research over several decades has identified distinct arousal-promoting neural populations in the brain; however, how these nuclei act individually and collectively to promote and maintain wakefulness and various arousal states is unknown. We have recently applied optogenetic technology to the repertoire of techniques used to study arousal. Here, we discuss the recent results of these experiments and propose future use of this approach as a way to understand the complex dynamics of neural circuits controlling arousal and arousal-related behaviors.

Angiotensin type 1A receptors in C1 neurons of the rostral ventrolateral medulla modulate the pressor response to aversive stress

The rise in blood pressure during an acute aversive stress has been suggested to involve activation of angiotensin type 1A receptors (AT(1A)Rs) at various sites within the brain, including the rostral ventrolateral medulla. In this study we examine the involvement of AT(1A)Rs associated with a subclass of sympathetic premotor neurons of the rostral ventrolateral medulla, the C1 neurons. The distribution of putative AT(1A)R-expressing cells was mapped throughout the brains of three transgenic mice with a bacterial artificial chromosome-expressing green fluorescent protein under the control of the AT(1A)R promoter. The overall distribution correlated with that of the AT(1A)Rs mapped by other methods and demonstrated that the majority of C1 neurons express the AT(1A)R. Cre-recombinase expression in C1 neurons of AT(1A)R-floxed mice enabled demonstration that the pressor response to microinjection of angiotensin II into the rostral ventrolateral medulla is dependent upon expression of the AT(1A)R in these neurons. Lentiviral-induced expression of wild-type AT(1A)Rs in C1 neurons of global AT(1A)R knock-out mice, implanted with radiotelemeter devices for recording blood pressure, modulated the pressor response to aversive stress. During prolonged cage-switch stress, expression of AT(1A)Rs in C1 neurons induced a greater sustained pressor response when compared to the control viral-injected group (22 ± 4 mmHg for AT(1A)R vs 10 ± 1 mmHg for GFP; p < 0.001), which was restored toward that of the wild-type group (28 ± 2 mmHg). This study demonstrates that AT(1A)R expression by C1 neurons is essential for the pressor response to angiotensin II and that this pathway plays an important role in the pressor response to aversive stress.

Enhancement of serotonergic and noradrenergic neurotransmission in the rat hippocampus by sustained administration of bupropion

RATIONALE

Previous studies reported that bupropion, an effective antidepressant, exerts modulatory actions on serotonin (5-HT) and norepinephrine (NE) neurons.

OBJECTIVES

This study examined effects of bupropion administration on 5-HT and NE neurotransmission in hippocampus.

METHODS

Electrophysiological recordings were obtained from anesthetized Sprague-Dawley rats. Subcutaneously implanted minipumps delivered saline or bupropion (30 mg/kg/day) for 2 and 14 days.

RESULTS

Although sustained bupropion administration did not alter the sensitivity of 5-HT(1A) and α₂-adrenergic receptors, the tonic activation of postsynaptic 5-HT(1A) receptors by endogenous 5-HT was enhanced in 14-day bupropion-treated rats to a greater extent than in the 2-day and control rats, as revealed by the greater disinhibitory action of the 5-HT(1A) antagonist WAY-100635 on hippocampus pyramidal neurons. The function of terminal 5-HT(1B) autoreceptors was not changed as determined by the unaltered effectiveness of different frequencies of stimulation of the ascending 5-HT fibers. The function of α₂-adrenergic receptors on 5-HT terminals was, however, diminished, as indicated by the lesser effect of the α₂-adrenoceptor agonist clonidine. Tonic activation of postsynaptic α₂- and α₁-adrenoceptors by endogenous NE was also increased in 14-day bupropion-treated rats, as indicated by the greater effect of the α₂- and α₁-adrenoceptor antagonists idazoxan and prazosin, respectively, on pyramidal firing. The function of terminal α₂-adrenergic autoreceptors was attenuated since increasing frequency of stimulation of the ascending NE pathway produced a lesser degree of suppression of pyramidal neurons in rats administered bupropion than the control.

CONCLUSION

Enhancement of 5-HT and NE transmissions in hippocampus by prolonged bupropion may account for its effectiveness in major depression.

Spontaneous rhythmogenic capabilities of sympathetic neuronal assemblies in the rat spinal cord slice

Neuronal networks generating rhythmic activity as an emergent property are common throughout the nervous system. Some are responsible for rhythmic behaviours, as is the case for the spinal cord locomotor networks; however, for others the function is more subtle and usually involves information processing and/or transfer. An example of the latter is sympathetic nerve activity, which is synchronized into rhythmic bursts in vivo. This arrangement is postulated to offer improved control of target organ responses compared to tonic nerve activity. Traditionally, oscillogenic circuits in the brainstem are credited with generating these rhythms, despite evidence for the persistence of some frequencies in spinalized preparations. Here, we show that rhythmic population activity can be recorded from the intermediolateral cell column (IML) of thoracic spinal cord slices. Recorded in slices from 10- to 12-day-old rats, this activity was manifest as 8–22 Hz oscillations in the field potential and was spatially restricted to the IML. Oscillations often occurred spontaneously, but could also be induced by application of 5-HT, α-methyl 5-HT or MK212. These agents also significantly increased the strength of spontaneous oscillations. Rhythmic activity was abolished by TTX and attenuated by application of gap junction blockers or by antagonists of GABA_A receptors. Together these data indicate that this rhythm is an emergent feature of a population of spinal neurons coupled by gap junctions. This work questions the assumption that sympathetic rhythms are dependent on supraspinal pacemaker circuits, by highlighting a surprisingly strong rhythmogenic capability of the reduced sympathetic networks of the spinal cord slice.

Sympathetic nervous system overactivity and its role in the development of cardiovascular disease

This review examines how the sympathetic nervous system plays a major role in the regulation of cardiovascular function over multiple time scales. This is achieved through differential regulation of sympathetic outflow to a variety of organs. This differential control is a product of the topographical organization of the central nervous system and a myriad of afferent inputs. Together this organization produces sympathetic responses tailored to match stimuli. The long-term control of sympathetic nerve activity (SNA) is an area of considerable interest and involves a variety of mediators acting in a quite distinct fashion. These mediators include arterial baroreflexes, angiotensin II, blood volume and osmolarity, and a host of humoral factors. A key feature of many cardiovascular diseases is increased SNA. However, rather than there being a generalized increase in SNA, it is organ specific, in particular to the heart and kidneys. These increases in regional SNA are associated with increased mortality. Understanding the regulation of organ-specific SNA is likely to offer new targets for drug therapy. There is a need for the research community to develop better animal models and technologies that reflect the disease progression seen in humans. A particular focus is required on models in which SNA is chronically elevated.

Electrophysiological effects of the co-administration of escitalopram and bupropion on rat serotonin and norepinephrine neurons

Clinical studies indicate that addition of bupropion to selective serotonin (5-HT) reuptake inhibitors (SSRIs) provides incremental benefit over SSRI monotherapy in depression. This study was designed to investigate the effects of co-administration of bupropion with escitalopram on the firing rate of 5-HT and norepinephrine (NE) neurons in anesthetized rats. Escitalopram (10 mg/kg/day x 2 days), given via subcutaneously (s.c.) implanted minipumps, decreased the firing of 5-HT and NE neurons by 70% and 55%, respectively. The firing of 5-HT neurons, unlike that of NE neurons, recovered after the 14-day escitalopram regimen. Bupropion, injected once daily (30 mg/kg/day, s.c. x 2 days), did not increase 5-HT firing but decreased that of NE by 55%. After 14 days of repeated bupropion administration, 5-HT firing was increased by 50%, and NE firing was back to baseline. Co-administration of escitalopram and bupropion doubled 5-HT firing after 2 and 14 days, whereas NE neurons were inhibited by 60% after 2 days, but partially recovered after 14 days. The responsiveness of 5-HT(1A) autoreceptors was significantly attenuated in the combination-treated rats after 2 days, indicating an early desensitization. These results provide support for contributions from 5-HT and NE mechanisms for enhanced effectiveness of combination of SSRI and bupropion treatment.

Electrophysiological characterization of the effects of asenapine at 5-HT(1A), 5-HT(2A), alpha(2)-adrenergic and D(2) receptors in the rat brain

Asenapine is a psychopharmacologic agent being developed for schizophrenia and bipolar disorder. This study electrophysiologically characterized the in vivo effects of asenapine at dorsal raphe nucleus (DRN) and hippocampus serotonin-1A (5-HT(1A)), ventral tegmental area D(2), locus coeruleus 5-HT(2A,) and alpha(2)-adrenergic receptors in anesthetized rats. Asenapine displayed potent antagonistic activity at alpha(2)-adrenoceptors (ED(50), 85+/-2 microg/kg), 5-HT(2A) (ED(50), 75+/-2 microg/kg) and D(2) receptors (ED(50), 40+/-2 microg/kg) as evidenced by its reversal of clonidine-, DOI-, and apomorphine-induced inhibition of norepinephrine and dopamine neurons. In contrast, asenapine acted as a partial agonist at 5-HT(1A) receptors in DRN and hippocampus, as indicated by blockade of its inhibitory effect on neuronal firing by the 5-HT(1A) antagonist WAY 100635 and the partial inhibition of the suppressant action of 5-HT when co-applied by microiontophoresis. These results confirm that asenapine displays potent antagonistic activity at 5-HT(2A), D(2), alpha(2)-adrenergic receptors and provide evidence to support its 5-HT(1A) partial agonistic activity.

Amplified respiratory-sympathetic coupling in the spontaneously hypertensive rat: does it contribute to hypertension?

Sympathetic nerve activity (SNA) is elevated in established hypertension. We tested the hypothesis that SNA is elevated in neonate and juvenile spontaneously hypertensive (SH) rats prior to the development of hypertension, and that this may be due to augmented respiratory-sympathetic coupling. Using the working heart-brainstem preparation, perfusion pressure, phrenic nerve activity and thoracic (T8) SNA were recorded in male SH rats and normotensive Wistar-Kyoto (WKY) rats at three ages: neonates (postnatal day 9-16), 3 weeks old and 5 weeks old. Perfusion pressure was higher in SH rats at all ages reflecting higher vascular resistance. The amplitude of respiratory-related bursts of SNA was greater in SH rats at all ages (P < 0.05). This was reflected in larger Traube-Hering pressure waves in SH rats (1.4 +/- 0.8 versus 9.8 +/- 1.5 mmHg WKY versus SH rat, 5 weeks old, n = 5 per group, P < 0.01). Recovery from hypocapnic-induced apnoea and reinstatement of Traube-Hering waves produced a significantly greater increase in perfusion pressure in SH rats (P < 0.05). Differences in respiratory-sympathetic coupling in the SH rat were not secondary to changes in central or peripheral chemoreflex sensitivity, nor were they related to altered arterial baroreflex function. We have shown that increased SNA is already present in SH rats in early postnatal life as revealed by augmented respiratory modulation of SNA. This is reflected in an increased magnitude of Traube-Hering waves resulting in elevated perfusion pressure in the SH rat. We suggest that the amplified respiratory-related bursts of SNA seen in the neonate and juvenile SH rat may be causal in the development of their hypertension.

Frequency and pattern dependence of adrenergic and purinergic vasoconstriction in rat skeletal muscle arteries

Sympathetic nerves fire in bursts followed by brief periods of quiescence. Periods of quiescence may be a valuable part of coding for different neurotransmitters. We compared adrenergic- and non-adrenergic-mediated vasoconstriction with repeating burst patterns versus constant frequency stimulation. Seventeen rats were killed, and the femoral arteries dissected out and mounted in organ tissue baths at 37 degrees C and pH 7.4. Field stimulation was applied to artery rings from five rats at constant frequencies of 2-6 Hz for 144 impulses. In 12 rats, artery rings were stimulated with two burst pattern protocols consisting of repeating pairs, triplets, quadruplets or sextuplets performed using either 8 or 30 Hz as the instantaneous frequency for a total of 144 impulses. All protocols were repeated with the P2 purinergic antagonist pyridoxal-phosphate-6-azophenyl-2'4'-disulphonic acid (PPADs; 0.42 m) or the alpha(1)-antagonist prazosin (1.59 microM). Tension was decreased by the addition of the P2 antagonist PPADs (P < 0.05). Prazosin abolished tension at all constant frequencies (P < 0.05). P2 and alpha(1)-antagonism decreased tension with 8 and 30 Hz burst pattern field stimulation. However, the magnitude of decrease in tension with prazosin was less with burst patterns compared to the same average constant frequencies (P < 0.05). It appears that P2X receptors and alpha(1)-receptors in the femoral artery are sensitive to frequency and patterns of electrical stimulation.

T (tail)-rhythm oscillators: principles of nervous control of the vasculature revealed?

This review focuses on the nervous control of the caudal ventral artery of the rat tail, and aims to convince the reader that sympathetic control of the vasculature can be mediated via neural oscillators intrinsic to the sympathetic nervous system. The definitive functional significance of these oscillators is unknown at present. However, it is expected that through dynamic relationships with modulating and driving inputs, such oscillators would permit graded vascular responses.

Mechanisms of blockade by the novel migraine prophylactic agent, dotarizine, of various brain and peripheral vessel contractility

The novel antimigraineur, dotarizine, inhibited 5-HT (5 hydroxytryptamine)-evoked contractions of rabbit vertebral, aorta, femoral and mesenteric arteries, with IC(50)s of 1.35, 1.40, 0.52 and 1.09 microM, respectively. Flunarizine had little effect on these contractions, while ketanserin was more potent (IC(50)s of 0.17 microM for vertebral, 0.22 microM for aorta, 0.05 microM for femoral and 0.03 microM for mesenteric arteries). At 10 microM, dotarizine caused 40% blockade of K(+)-evoked contractions of rabbit aorta, and 70% inhibition of 5-HT-evoked responses; these values were 30% and 20% for 10 microM flunarizine. Contractions of rabbit aorta elicited by noradrenaline, angiotensin II or prostaglandin F(2alpha) were not affected by 10 microM dotarizine or flunarizine. Ketanserin shifted to the right, in parallel, the concentration-response curves for 5-HT in rabbit aorta; however, dotarizine caused a non-competitive type of blockade, increasing the maximum 5-HT contraction at 30 nM and decreasing it at 3 and 30 microM. K(+)-evoked contractions of rabbit aorta were halved by 3 microM dotarizine in a voltage-independent manner; flunarizine caused a delayed-type, non-reversible post-drug blockade, and exhibited some voltage-dependence. Blockade by nifedipine was voltage-dependent and fully reversible. Ca(2+)-evoked contractions of depolarised bovine middle cerebral arteries were blocked by 1--3 microM dotarizine in a non-surmountable manner. Contraction of these vessels evoked by electrical stimulation was blocked 50% and 70% by 1 and 3 microM dotarizine, respectively. Dotarizine (1--3 microM) also inhibited to a similar extent the K(+)-evoked [(3)H]noradrenaline release from cultured rat sympathetic neurones. These data suggest that the mechanism of blockade by dotarizine of cerebral vessels contractility has three components: (i) presynaptic inhibition of noradrenaline release; (ii) blockade of postsynaptic vascular 5-HT receptors; (iii) blockade of Ca(2+)entry into the vascular smooth muscle cell cytosol. The compound does not affect the vascular receptors for noradrenaline, angiotensin II or prostaglandin F(2alpha).

Multiple oscillators, dynamic synchronization and sympathetic control

1. Intermittent bursts of activity are a robust feature of the discharges of sympathetic nerves. There are at least two major mechanisms producing such discharges: (i) phasic inputs influencing sympathetic circuits; and (ii) oscillators embedded within sympathetic networks. The functional significance of patterned and synchronized activity underlying bursts of population activity may reside in their influence on information transfer between excitable cells. At the level of the single neuron, firing pattern appears to be an important determinant of synaptic/neuroeffector function (e.g. the probability of transmitter release, the types of transmitter released, the types of receptor activated and plasticity). Synchronization of inputs at a target favours summation and, therefore, may influence response (short term and long term). 2. In the present paper, I review the work from my laboratory that has focused on furthering understanding of the potential functional importance of pattern and synchrony coding in sympathetic nervous control of cardiovascular function. Because the rat tail artery has been used extensively as a model for studying neuroeffector transmission, in our investigations we have recorded from its sympathetic innervation. 3. In the anaesthetized preparation, under steady state conditions, we have established that the discharges of these sympathetic neurons have a distinct rhythm (frequency approximately 0.8 Hz). This can be detected both at single neuron and population levels. 4. A family of oscillators appears to control their discharge such that under some conditions all neurons do not have the same frequency of rhythmical activity. However, these weakly coupled or uncoupled oscillators can be synchronized dynamically by various inputs, such as central respiratory drive, lung inflation cycle-related inputs and inputs arising from visceral and somatic afferents. 5. The potential functional significance of dynamic synchronization of sympathetic oscillators in relation to sympathetic pattern generation and neuroeffector transmission is discussed.

Effects of maturation on adrenergic neurotransmission in ovine cerebral arteries

The present studies examine the hypothesis that multiple adrenergic neuroeffector mechanisms are not fully developed in fetal, compared with adult, ovine middle cerebral arteries. In arteries denuded of endothelium and pretreated with 1 microM atropine to block involvement of muscarinic receptors, 10 microM capsaicin to deplete sensory peptidergic neurons, and 10 microM nitro-L-arginine methyl ester (L-NAME) to block possible influences from nitric oxidergic innervation, transmural stimulation at 16 Hz increased contractile tensions to 9.5 +/- 3.7% (n = 6) of the potassium maximum in adult arteries. Corresponding values in fetal arteries, however, were significantly less and averaged only 1.1 +/- 0.6% (n =10). However, postsynaptic sensitivity to norepinephrine (NE) was similar in the two age groups; NE pD(2) values (-log EC(50)) averaged 6.11 +/- 0.12 (n = 6) and 6.33 +/- 0.09 M (n = 9) in fetal and adult arteries, respectively. Similarly, NE content measured via HPLC was also similar in the two age groups and averaged 32.4 +/- 5.0 (n = 17) and 32.5 +/- 3.9 ng/ng wet wt (n = 13) in fetal and adult middle cerebral arteries, respectively. In contrast, stimulation-induced NE release was greater in fetal than in adult arteries, whether calculated as total mass released [883 +/- 184 (n = 17) vs. 416 +/- 106 pg NE/mg wet wt (n = 13)] or as fractional release [51.1 +/- 5.3 (n = 17) vs. 22.8 +/- 3.8 pg/pg NE content per pulse x 10(-6)]. Measured as an index of synaptic density, neuronal cocaine-sensitive NE uptake was similar in fetal and adult arteries [1.55 +/- 0.40 (n = 10) and 1.84 +/- 0.51 pmol/mg wet wt (n = 7), respectively]. Overall, age-related differences in postsynaptic sensitivity to NE, NE release, and NE uptake capacity cannot explain the corresponding age-related differences in response to stimulation. The data thus suggest that total synaptic volume and cleft width, in particular, are probably greater and/or that adrenergic corelease of vasoactive substances other than NE is altered in fetal compared with adult middle cerebral arteries.

Functional significance of the pattern of renal sympathetic nerve activation

To assess the renal functional significance of the pattern of renal sympathetic nerve activation, computer-generated stimulus patterns (delivered at constant integrated voltage) were applied to the decentralized renal sympathetic nerve bundle and renal hemodynamic and excretory responses determined in anesthetized rats. When delivered at the same integrated voltage, stimulus patterns resembling those observed in in vivo multifiber recordings of renal sympathetic nerve activity (diamond-wave patterns) produced greater renal vasoconstrictor responses than conventional square-wave patterns. Within diamond-wave patterns, increasing integrated voltage by increasing amplitude produced twofold greater renal vasoconstrictor responses than by increasing duration. With similar integrated voltages that were subthreshold for renal vasoconstriction, neither diamond- nor square-wave pattern altered glomerular filtration rate, whereas diamond- but not square-wave pattern reversibly decreased urinary sodium excretion by 25 +/- 3%. At the same number of pulses per second, intermittent stimulation produced faster and greater renal vasoconstriction than continuous stimulation. At the same number of pulses per second, increases in rest period during intermittent stimulation proportionally augmented the renal vasoconstrictor response compared with that observed with continuous stimulation; the maximum augmentation of 55% occurred at a rest period of 500 ms. These results indicate that the pattern of renal sympathetic nerve stimulation (activity) significantly influences the rapidity, magnitude, and selectivity of the renal vascular and tubular responses.

Functional significance of the 10 Hz rhythmic discharges in sympathetic nerves

1. By measuring renal vascular conductance in anaesthetized and immobilized rabbits, the functional significance of the 10 Hz rhythmic discharges in the sympathetic nerves was assessed by electrical stimulation of the renal nerve. This stimulation mimicked the intermittently occurring 10 Hz rhythmic discharges. 2. Comparison of high-frequency (10-20 Hz) intermittent electrical stimulation with continuous low-frequency (5 Hz) stimulation showed that the intermittent high-frequency stimulation evoked faster (smaller time constant) and larger responses of the renal vascular conductance if the total number of stimulus pulses was the same. 3. Therefore, the intermittently occurring 10 Hz rhythmic discharges in the sympathetic nerve produces faster and larger effects on peripheral organs than effects produced by continuous discharges if the number of spikes is equal.

Neural regulation of kidney function by the somatosensory system in normotensive and hypertensive rats

This investigation examined the renal sympathetic nerve and renal excretory responses to somatosensory stimulation in normotensive and stroke-prone spontaneously hypertensive rats (SHRSP). Somatosensory activation was achieved by either subcutaneous capsaicin administration or exposure of the airways tract to irritant fumes from acetic acid in chloralose-urethan-anesthetized animals. In Wistar rats, blood pressure increased between 10 and 20% (P < 0.001-0.01), renal perfusion pressure was maintained unchanged, renal hemodynamics were unaltered, and urine flow and sodium excretion were decreased by 25 to 50% (P < 0.001-0.05). In the SHRSP, the somatosensory-induced increases in blood pressure were slightly larger (approximately 15-20% P < 0.05) than those of the Wistar rats, whereas the excretory responses were one-half those of the normotensive animals (P < 0.05). The somatosensory challenges reflexly increased integrated renal sympathetic nerve activity in both normotensive and hypertensive rats. The power spectral analysis demonstrated that the increases in percentage power at heart rate frequency and total power were two to three times more (P < 0.05) in the Wistar rats compared with the SHRSP. The reduced ability of the SHRSP to modulate the energy in the renal sympathetic nerve signal at heart rate frequency might explain in part the attenuated functional responses to the somatosensory challenges.

Action of ryanodine on neurogenic responses in rat isolated mesenteric small arteries

1. Rat mesenteric (approximately 250 microns) were set up in a single-channel isometric myograph designed to allow with 6 microM fura-2AM for 2 h and simultaneous recordings of neurogenic contraction (force) and intracellular calcium [Ca2+]i were obtained. In other experiments, arteries were loaded with 1 microCi ml-1 [3H]-noradrenaline (NA) for 30 min in order to measure release of [3H]-NA in response to field stimulation to examine whether ryanodine directly inhibited neuronal release of NA. 2. Arteries were activated by single intermittent field stimulation or continuously to excite intrinsic sympathetic nerves, or by cumulative addition of noradrenaline (1 nM-10 microM) to the bathing solution. 3. Pre-incubation with ryanodine markedly inhibited the contraction and [Ca2+]i release in response to single-pulse nerve stimulation. Ryanodine also inhibited an early phasic component of the response to continuous field stimulation and reduced the rate of rise in force in response to continuous field stimulation. However, stable maximal contraction and [Ca2+]i in response to continuous field stimulation as well as maximal responses to exogenous NA were unaffected. Release of [3H]-NA in response to single intermittent field stimulation was not affected by ryanodine when compared to vehicle. 4. Our results suggest that brief intermittent activation of intramural sympathetic nerves increases [Ca2+]i and contracts small arteries primarily by releasing Ca2+ from a ryanodine-sensitive intracellular store. In contrast, the stable rise in tone and [Ca2+]i resulting from continuous nerve stimulation may largely depend on sources of Ca2+ other than the ryanodine-sensitive intracellular store.

Neural control of renal function

The renal nerves are the communication link between the central nervous system and the kidney. In response to multiple peripheral and central inputs, efferent renal sympathetic nerve activity is altered so as to convey information to the major structural and functional components of the kidney, the vessels, glomeruli, and tubules, each of which is innervated. At the level of each of these individual components, information transfer occurs via interaction of the neurotransmitter released at the sympathetic nerve terminal-neuroeffector junction with specific postjunctional receptors coupled to defined intracellular signaling and effector systems. In response to normal physiological stimuli, changes in efferent renal sympathetic nerve activity contribute importantly to homeostatic regulation of renal blood flow, glomerular filtration rate, renal tubular epithelial cell solute and water transport, and hormonal release. Afferent input from sensory receptors located in the kidney participates in this reflex control system via renorenal reflexes that enable total renal function to be self-regulated and balanced between the two kidneys. In pathophysiological conditions, abnormal regulation of efferent renal sympathetic nerve activity contributes significantly to the associated abnormalities of renal function which, in turn, are of importance in the pathogenesis of the disease.

Renal nerve responses to somatic nerve activation in stroke-prone spontaneously hypertensive rats

In chloralose/urethane anaesthetised stroke-prone spontaneously hypertensive rats, blood pressure and integrated renal nerve activity were higher whereas heart rate was lower than in Wistar rats by 37, 146 and 11%, respectively (all P < 0.001). The renal nerve signal was subjected to fast Fourier transformation to generate power spectra. In the hypertensive rats, total spectral power was 400% (P < 0.01) and power at the heart rate frequency was 50% (P < 0.01) greater while phase and time differences were shorter (both P < 0.001) than in Wistar rats. Brachial nerve stimulation increased total power in Wistar and hypertensive rats (P < 0.05), but importantly, power at the heart rate frequency was decreased by 80% in Wistar whereas there was a 20% (P < 0.05) increase in hypertensive rats, while phase and time differences were raised only in hypertensive rats (P < 0.05). Bilateral cervical vagotomy of the hypertensive rats had minimal actions on most variables but phase and time differences were doubled compared to intact hypertensive animals, but brachial nerve stimulation decreased power at the heart rate frequency (P < 0.05) which was a very different response from intact rats. Resting blood pressure, heart rate, total power and power at the heart rate frequency in the carotid sinus denervated animals were lower than in intact hypertensive rats, between 17 and 71%, respectively, but increased during brachial nerve stimulation. These experiments demonstrated that whereas somatic sensory input can modulate the pattern of sympathetic nerve activity to the kidney under normal conditions, this does not occur in the hypertensive rat. This appears to be related to afferent information carried by the vagus which suppresses the normal response; the carotid sinus baroreceptors are devoted to organising the nerve activity in relation to the blood pressure pulse wave.

On the dominant rhythm in the discharges of single postganglionic sympathetic neurones innervating the rat tail artery

1. In anaesthetized rats, using a focal recording technique, activity was recorded from single sympathetic postganglionic neurones innervating the caudal ventral artery of the tail. The following hypotheses were tested: (i) that the frequency of the dominant rhythmic discharge of the neurones can be different from the frequency of the central respiratory rhythm (as indicated by rhythmic phrenic discharge); and (ii) that the dominant sympathetic rhythm is not reliant on afferent feedback carried in aortic, sinus and vagus nerves. 2. Four types of preparation were used: spontaneously breathing (group 1), artificially ventilated (group 2), artificially ventilated with vagi cut (group 3), and artificially ventilated with vagus and sino-aortic denervation (group 4). 3. The frequencies of the dominant sympathetic rhythm under control conditions were: group 1, 0.91 +/- 0.12 Hz (mean +/- S.E.M., n = 5); group 2, 0.81 +/- 0.04 Hz (n = 18); group 3, 0.83 +/- 0.03 Hz (n = 17); group 4, 0.95 +/- 0.06 Hz (n = 11). The frequency of the dominant sympathetic rhythm was different from that of the phrenic rhythm in thirty-five out of fifty-one cases. 4. The mean frequency of the dominant sympathetic rhythm was not influenced significantly by hypocapnic apnoea. 5. Hyperthermia increased the frequency of the phrenic rhythm whilst decreasing that of the dominant sympathetic rhythm. 6. In all cases the frequency of the dominant sympathetic rhythm was different from that of the artificial ventilation cycle. 7. It is concluded that the frequency of the dominant sympathetic rhythm can be different from that of central respiratory drive and that it is not "driven' by afferent feedback relayed via sinus, aortic and vagus nerves. 8. It is proposed that the dominant sympathetic rhythm is unlikely to be generated by a central respiratory oscillator.

Sequential secretion of rat submandibular kallikrein and peroxidase during intermittent sympathetic stimulation

Secretion of peroxidase from acinar cells, true tissue kallikrein (rK1) from granular tubules and total protein have been assessed in sequential samples of rat submandibular saliva formed during intermittent periods of sympathetic stimulation in bursts of 50 Hz 1 s every 10 s at 5 V, for 1 or 2 min each with 2 min intervals between periods. This protocol was repeated twice after 1 h rest pauses. The salivary flow remained greater throughout than when using ongoing burst stimulation. Protein secretion was considerable in the first 2 samples (1 min followed by 2 min) reaching the remarkable concentration of 285 +/- 14.4 mg/ml, then it gradually decreased with little recovery after 1 h pauses. rK1 outputs followed a similar pattern. Peroxidase, however, showed no greater output in initial samples and continued steadily in similar amounts throughout. When intermittent stimulations were used for 1 sequence only (total stimulation time = 9 min) the glands showed 84 +/- 2.6% depletion of rK1 compared to the control glands. A correspondingly large depletion of granules occurred from the granular tubules on the stimulated side, which was greater than with ongoing burst stimulation for 1 h (Garrett et al., 1991). Thus, secretion of rK1 from the granules in granular tubules occurs most efficiently with short sharp bursts of high frequency sympathetic stimulation but soon diminishes. In contrast, peroxidase secretion from acinar cells continues steadily and more modestly for long periods of time undiminished.

The 10-Hz rhythm in the sympathetic nerve activity of cats, rats and rabbits

Since the 10-Hz rhythmic activity in the sympathetic nerves was reported only in cats, we examined whether the activity of the same frequency could be observed in rats and rabbits as well as in the cats. Histograms of inter-burst-peak intervals of discharges of the renal nerves revealed that the 100 ms interval activity, the reverse of the 10-Hz, was observed in all the three mammals, of which the baroreceptor afferents were intact or inactivated. Further, the same frequency activity could be evoked by intermittent electrical stimulation of the dorsolateral funiculus of the cervical cord in spinal animals. It was suggested that the 10-Hz rhythmic activity in the sympathetic nerves was a common phenomenon throughout mammals and this activity was produced in the spinal cord. The physiological significance of the 10-Hz activity in the sympathetic nerves was discussed.

Effects of patterns of sympathetic nerve stimulation on vasoconstricting responses in the hindquarter of rabbits

It is now well known that sympathetic nerve discharges (SND) of animals as well as humans oscillate at low frequencies. To determine effects of the oscillation or burst on vasoconstriction, we applied two different kinds of electrical stimulation of the lumbar sympathetic nerve, and examined the magnitude and rate of vasoconstriction in the autoperfused hindquarter of alpha-chloralose anesthetized rabbits (n = 6). In the first protocol, we obtained power spectra of lumbar SND of rabbits with sinoaortic denervation and vagotomy. The power resided over the frequency range of 0.5-5 Hz with a broad peak at 1 Hz. In the second protocol, we modulated the basal stimulus trains 5 Hz on an average with slower rhythms of 0.5, 1.0 and 2.0 Hz (frequency modulation). This experiment revealed that, compared with the results with constant stimulation, the frequency modulation of stimulation did not affect the magnitude of the maximal vasoconstriction but augmented the rate of vasoconstriction at 0.5 and 1.0 Hz (P < 0.01). In the third protocol, we examined effects of stimulation on vasoconstriction while changing the intra-burst frequency at a fixed inter-burst interval. Since the power spectra of lumbar SND showed a peak at 1 Hz in the first protocol, we fixed the inter-burst interval at 1 Hz and varied the intra-burst frequency at 10, 20, and 40 Hz while the total number of stimuli were kept constant at 5 impulses per second. This experiment showed that the 10 Hz burst was most effective in augmenting the rate of vasoconstriction, though the magnitude of maximal vasoconstriction was not affected by any of them.(ABSTRACT TRUNCATED AT 250 WORDS)