Activation of prejunctional beta-adrenoceptors in rat atria by adrenaline applied exogenously or released as a co-transmitter.

1 Adrenaline (10 nM) significantly enhanced the stimulation-induced efflux of radioactivity from rat atria previously incubated with [3H]-noradrenaline ([3H]-NA). This effect was abolished by metoprolol (.01 muM). 2 Adrenaline in a higher concentration (1 muM) and NA (1 muM) significantly reduced the stimulation-induced efflux of radioactivity. However, in the presence of phenoxybenzamine (10 muM), adrenaline (1 muM) enhanced the efflux, whereas NA (1 muM) had no effect. 3 In rat isolated atria pre-incubated with adrenaline and then incubated with NA, both catecholamines were taken up and were released by field stimulation. When pre-incubation was with adrenaline and incubation was with [3H]-NA, metoprolol decreased the stimulation-induced efflux of radioactivity. This effect did not occur if the atria were pre-incubated with NA instead of adrenaline, suggesting tht neuronally released adrenaline activates prejunctional beta-adrenoceptors. 4 In conscious rats, intravenously administered adrenaline (6.0 and 0.6 nmol/kg) was taken up and retained in the atria and could be released by field stimulation. The release was calcium-dependent from these rats up to 24 h after administration.     

Neuropeptide Y (NPY) reduces field stimulation-evoked release of noradrenaline and enhances force of contraction in the rat portal vein

The effect of neuropeptide Y (NPY) on fractional tritium-noradrenaline (3H-NA) release and contractile activity was studied in the isolated portal vein of SHR and WKY rats. NPY (5 X 10(-7) M) enhanced the force of the spontaneous contractile activity by about 40%. The fractional 3H-release elicited by transmural nerve stimulation (TNS), which mainly reflects 3H-NA, was reduced by about 40% after preincubation with 5 X 10(-7) M NPY in portal veins from both SHR and WKY rats. The inhibitory effect of NPY on TNS-evoked 3H-release was more slowly reversed by washout than the facilitatory action on spontaneous contractile force. The contractile response to field stimulation was not reduced by NPY, but rather tended to be increased. It is concluded that NPY exerts a dual action in the SHR and WKY portal vein, thus enhancing the smooth muscle contractions and inhibiting sympathetic neurotransmission. The inhibitory effect of NPY on TNS-evoked NA efflux, which is present in both SHR and WKY rats, is most likely due to a presynaptic site of action.     

Adrenaline hypothesis: effect of formoterol on noradrenaline release

1 Originally, the so-called 'adrenaline hypothesis' related the release of noradrenaline (NA) to stimulation of presynaptic beta2-receptors in nerve endings; now it confers a possible role to adrenaline taken up then released by nerves endings. It represents a potentially useful therapeutic pathway. The present study aims to investigate the effects of formoterol, a highly selective beta2-adrenoceptor agonist. 2 It was carried out in freely moving rats, the isotope dilution technique being used to measure the NA spill over rate (NA-SOR) and metabolic clearance rate (MCR). 3 A series of three results are reported. (a) When compared with adrenaline on equimolar basis, formoterol (2.3 micro kg-1 min-1) increased NA-SOR while mean arterial blood pressure was decreased and heart rate increased. Thus, it was difficult to separate a direct presynaptic effect from indirect baroreflex-dependent activation of the sympathetic system. (b) When formoterol was infused at 1 ng kg-1 min-1, a dose empirically defined to induce no haemodynamic effect, NA-SOR was significantly increased, while NA-MCR remained unchanged. (c) The NA-SOR response to formoterol was not amplified by the presynaptic alpha2-adrenoceptor blocker, yohimbine, in contrast to the NA-SOR response to adrenaline. 4 In conclusion, formoterol, a beta2-adrenoceptor agonist, is shown to increase the release and plasma concentration of NA while its clearance was not changed.     

Adrenaline hypothesis: effect of formoterol on noradrenaline release

1 Originally, the so‐called `adrenaline hypothesis' related the release of noradrenaline (NA) to stimulation of presynaptic β₂‐receptors in nerve endings; now it confers a possible role to adrenaline taken up then released by nerves endings. It represents a potentially useful therapeutic pathway. The present study aims to investigate the effects of formoterol, a highly selective β₂‐adrenoceptor agonist. 2 It was carried out in freely moving rats, the isotope dilution technique being used to measure the NA spill over rate (NA‐SOR) and metabolic clearance rate (MCR). 3 A series of three results are reported. (a) When compared with adrenaline on equimolar basis, formoterol (2.3 μg kg^?1 min^?1) increased NA‐SOR while mean arterial blood pressure was decreased and heart rate increased. Thus, it was difficult to separate a direct presynaptic effect from indirect baroreflex‐dependent activation of the sympathetic system. (b) When formoterol was infused at 1 ng kg^?1 min^?1, a dose empirically defined to induce no haemodynamic effect, NA‐SOR was significantly increased, while NA‐MCR remained unchanged. (c) The NA‐SOR response to formoterol was not amplified by the presynaptic α₂‐adrenoceptor blocker, yohimbine, in contrast to the NA‐SOR response to adrenaline. 4 In conclusion, formoterol, a β₂‐adrenoceptor agonist, is shown to increase the release and plasma concentration of NA while its clearance was not changed.     

Release of noradrenaline from cat spleen slices by potassium.

1 When cat spleen slices were exposed to a potassium-enriched (140 mM) Krebs solution, 367 +/- 31 ng g-1 5 min-1 of noradrenaline (NA) was released into the bathing medium. 2 Phenylephrine and clonidine (10(-7) to 10(-3) M) did not significantly modify the potassium-evoked NA release; acetylcholine decreased it in a dose-dependent manner. 3 Phenoxybenzamine increased NA release by 50% but phentolamine did not alter it; high concentrations of this drug greatly decreased NA release. Cocaine increased the NA release by about 30%. 4 It is suggested that the failure of sympathomimetic amines to depress, and of alpha-adrenoceptor blocking agents to enhance the release of NA by high potassium concentrations may be related to prolonged depolarization of the nerve terminals, which may desensitize presynaptic alpha-receptors. The fact that the same drugs are able to modify NA release during electrical nerve stimulation may be ascribed to the much shorter periods of depolarization occurring under these conditions.     

A cumulative dose-response technique for the characterization of presynaptic receptors modulating [3H]noradrenaline release from rat brain slices

A cumulative dose-response technique was developed for the characterization of presynaptic receptors involved in the modulation of [³H]noradrenaline (NA) release from rat hippocampus slices, using continuous K⁺ (20 mM) depolarization. The results obtained with this technique were compared with those obtained using a repetitive K⁺ stimulation procedure. The release of [³H]NA induced by continuous K⁺ stimulation as well as that caused by repetitive K⁺ stimulation was strongly Ca²⁺-dependent and consisted for more than 90% of unmetabolized [³H]NA. Using continuous K⁺ stimulation it was demonstrated that the presynaptic inhibition of ³H-NA release by exogenous NA reached a maximum 10 min after addition of NA. The inhibitory effect of NA appeared to be independent of the time of addition, suggesting that the sensitivity of the presynaptic α-adrenoceptors remained unchanged during the experiment. Cumulative dose-response curves were recorded by the successive addition, at 10 min intervals, of increasing concentrations of NA. It was shown that continuous stimulation and repetitive K⁺ stimulation were basically similar with regard to the characteristics of the resulting [³H]NA release as well as its presynaptic α-adrenoceptor-mediated modulation by exogeneous NA. However, the cumulative dose-response technique, which can be carried out only using continuous K⁺ stimulation, makes it possible to determine more rapidly and also more accurately the apparent affinities and intrinsic activities of drugs towards receptors involved in the modulation of neurotransmitter release from brain slices.     

Differential actions of desipramine on sympathoadrenal release of noradrenaline and adrenaline.

Spillovers noradrenaline (NA) and adrenaline (A) into plasma were examined before and after i.v. administration of 0.3-0.5 mg kg-1 desipramine (DMI) in 19 normal volunteers. DMI decreased the total body spillover of NA by 20 +/- 4%, but increased that of A by 45 +/- 10%. The results indicate differential sympathoadrenal actions of DMI, characterized by inhibition of NA release from sympathetic nerves and stimulation of A secretion from the adrenals. These effects of DMI provide an explanation for some of the cardiovascular complications of tricyclic antidepressant therapy and may also help to explain how sympathoadrenal function is differentially regulated.     

Effects of GABA on noradrenaline release and vasoconstriction induced by renal nerve stimulation in isolated perfused rat kidney.

We examined effects of gamma-aminobutyric acid (GABA) on vasoconstriction and noradrenaline (NA) release induced by electrical renal nerve stimulation (RNS) in the isolated pump-perfused rat kidney. RNS (1 and 2 Hz for 2.5 min each, 0.5-ms duration, supramaximal voltage) increased renal perfusion pressure (PP) and renal NA efflux. GABA (3, 10 and 100 microM) attenuated the RNS-induced increases in PP by 10-40% (P<0.01) and NA efflux by 10-30% (P<0.01). GABA did not affect exogenous NA (40 and 60 nM)-induced increases in PP. The selective GABA(B) agonist baclofen (3, 10 and 100 microM) also attenuated the RNS-induced increases in PP and NA efflux, whereas the RNS-induced responses were relatively resistant to the selective GABA(A) agonist muscimol (3, 10 and 100 microM). The selective GABA(B) antagonist 2-hydroxysaclofen (50 microM), but not the selective GABA(A) antagonist bicuculline (50 microM), abolished the inhibitory effects of GABA (10 microM) on the RNS-induced responses. The selective alpha2-adrenoceptor antagonist rauwolscine (10 nM) enhanced the RNS-induced responses. GABA (3, 10 and 100 microM) potently attenuated the RNS-induced increases in PP by 40-60% (P<0.01) and NA efflux by 20-50% (P<0.01) in the presence of rauwolscine. Prazosin (10 and 30 nM) suppressed the RNS-induced increases in PP by about 70-80%. Neither rauwolscine (10 nM) nor GABA (10 microM) suppressed the residual prazosin-resistant PP response. These results suggest that GABA suppresses sympathetic neurotransmitter release via presynaptic GABA(B) receptors, and thereby attenuates adrenergically induced vasoconstriction in the rat kidney.     

Effects of clorgyline on the release of noradrenaline from rat vas deferens.

Release of endogenous noradrenaline (NA) and functional activity of presynaptic alpha 2-adrenoceptors were measured in isolated rat vas deferens following acute (2 mg/kg) or chronic (21 * 1 mg/kg daily, i.p.) treatment with clorgyline. Noradrenaline tissue content was higher in rats treated chronically with clorgyline than in those treated acutely. In vitro experiments done in the presence of 1 microM desipramine and 0.5 microM yohimbine showed that NA release, elicited by electrical field stimulation, was higher in chronic clorgyline rats than in controls, while no significant difference was found between acute clorgyline and control rats. Yohimbine enhanced evoked release of NA in all treatment groups, provided that desipramine was present in the Krebs solution, and the enhancement was non significantly higher in chronic clorgyline than in acute clorgyline and control rats. Efflux of 3,4-dihydroxyphenylglycol was lower in chronic clorgyline rats than in other groups. No difference was found between the treatment groups in a 1 min [3H]NA uptake into the tissue, nor in the ability of desipramine (1 microM) to block the uptake. The results indicate that following chronic treatment with clorgyline, evoked release of NA increased, and there was no reduction in the ability of the presynaptic alpha 2-adrenoceptor inhibitory mechanism to reduce nerve stimulation induced release of NA.     

Some mechanism which may modulate noradrenaline release in guinea-pig isolated ileum

Guinea-pig isolated ileum pre-incubated with [3H]noradrenaline, released 3H-label during coaxial stimulation at 3.2-12.8 Hz. This evoked 3H-overflow seems due to nerve stimulation, and may be frequency-dependent. The 3H-overflow evoked by coaxial stimulation was increased with 2.6 muM phentolamine, reduced at threshold stimulation by 0.56 muM prostaglandin E1, but the tendency for 2.8 muM indomethacin to increase the evoked 3H-overflow was not statistically significant. The results indicate that the release of noradrenaline from guinea-pig ileum may be modulated by activation of presynaptic alpha-adrenoceptors, whereas prostaglandins act mainly at a postsynaptic site to modulate the sympathetic response.     

Pharmacological characterization of presynaptic alpha-adrenoceptors in the nucleus tractus solitarii and the cerebral cortex of the rat

The presynaptic alpha-adrenoceptors mediating inhibition of K+-induced release of [3H]noradrenaline (NA) from superfused slices and synaptosomes obtained from the nucleus tractus solitarii (NTS) region were characterized pharmacologically and compared to those in the cerebral cortex. Clonidine concentration-dependently (pD2 7.80) reduced the K+-induced [3H]NA release from slices and synaptosomes from the nucleus tractus solitarii of both normotensive and spontaneously hypertensive rats; 10(-6) M clonidine inhibited the [3H]NA release induced by 10 mM K+ to 40% of control. This inhibitory effect of clonidine was antagonized by the alpha2-adrenoceptor antagonist yohimbine (10(-7) M), but not by the alpha1-adrenoceptor antagonist prazosin (10(-7) M). The rank order of apparent affinities of agonists for the presynaptic alpha-adrenoceptors mediating inhibition of [3H]NA release from slices of nucleus tractus solitarii induced by 20 mM K+, was: oxymetazoline greater than clonidine greater than adrenaline greater than alpha-methylnoradrenaline greater than noradrenaline much greater than methoxamine. A similar rank order was found for cerebral cortex slices. The rank order of intrinsic activities in both brain regions was found to be: noradrenaline approximately equal to alpha-methylnoradrenaline approximately equal to adrenaline greater than clonidine greater than oxymetazoline much greater than methoxamine. The data indicate that presynaptic alpha-adrenoceptors mediating inhibition of NA release both in the nucleus tractus solitarii and the cerebral cortex belong to the alpha2-type. The presynaptic alpha2-adrenoceptors in the two brain regions appear to be similar both functionally and pharmacologically. It is suggested that these receptors in the nucleus tractus solitarii may play a role in the hypotensive effect of clonidine.     

Prolonged treatment with (+/-) propranolol induces supersensitivity to (L)noradrenaline in mesenteric arteries in the rat

In the isolated perfused mesenteric arteries of the rat, neither (+/-) propranolol (0.1 microM) nor (+/-)isoproterenol (0.05 microM) modified the overflow of DL-[3H]noradrenaline (DL-[3H]NA) induced by sympathetic nerve stimulation at either 5 or 10 Hz. The blockade of alpha presynaptic receptors with phentolamine (4.7 microM) increased the 3H-transmitter overflow at 5 and 10 Hz. (+/-)Propranolol (0.1 microM) failed to modify this effect. Vasoconstrictor responses to exogenous NA or sympathetic nerve stimulation were not modified by (+/-)propranolol (0.1 microM). Prolonged treatment with (+/-)propranolol (7 mg/kg) for 15 days potentiates responses to both exogenous NA and sympathetic nerve stimulation; however, the fractional release per pulse of DL-[3H]NA was not modified at either 5 or 10 Hz. These results provide no evidence to support the hypothesis that the release of NA is regulated by presynaptic beta-adrenoreceptors in the mesenteric arteries of the rat. The enhancement of vascular responses after prolonged treatment with propranolol could be caused by postsynaptic supersensitivity.     

Characterization of presynaptic beta-adrenoceptors facilitating endogenous noradrenaline release in the portal vein of permanently cannulated, freely moving rats

We investigated the nature of presynaptic beta-adrenoceptors and the possible heterogeneity of these receptors in the portal vein nervous plexus of freely moving unanesthetized rats using the differential blockade technique with CGP 20712A as a highly beta 1-selective antagonist, ICI 118,551 as a very beta 2-selective antagonist and fenoterol and endogenous NA as beta 2- and beta 1-selective agonists respectively. The fenoterol (0.25 mg/kg)-induced increase of the basal NA level (290%) was dose dependently decreased by 0.1, 0.3 and 1.0 mg/kg ICI 118,551, but was not affected by a high dose (3.0 mg/kg) of CGP 20712A. During electrical stimulation (2 Hz, 3 ms, 5 mA) of the portal vein nervous plexus, 0.25 mg/kg fenoterol induced a 2.1-fold increase in NA overflow compared to the control stimulation value. ICI 118,551 was also able to decrease the fenoterol-induced enhancement during stimulation. During stimulation in the presence of CGP 20712A and fenoterol, the control stimulation value was not significantly decreased. Pretreatment with yohimbine (0.5 mg/kg) was used to create a strong beta 1-stimulus by raising the intra-synaptic NA level through blockade of the inhibitory alpha 2-adrenoceptors. Under these conditions, CGP 20712A did not deminish the yohimbine-induced enhancement of the stimulus evoked NA overflow, clearly indicating the absence of the beta 1-adrenoceptor subtype. ICI 118,551 (0.1 mg/kg) was also unable to influence the evoked NA overflow under these conditions, implying that, even at high concentrations, NA is not able to facilitate its own release by stimulation of the presynaptic beta 2-adrenoceptor.(ABSTRACT TRUNCATED AT 250 WORDS)     

Dual effect of adrenaline on noradrenaline release in the pithed rabbit

We examined the effects of adrenaline on the noradrenaline release rate and plasma catecholamine levels in the pithed rabbit with electrically stimulated sympathetic outflow (3 Hz). Adrenaline (0.06 micrograms/kg/min) increased the rate of noradrenaline release into the plasma. This increase was prevented by propranolol (0.2 mg/kg + 0.1 mg/kg/h) and probably involves activation of presynaptic beta-adrenoceptors. A higher dose of adrenaline (1.0 micrograms/kg/min) significantly reduced the noradrenaline release rate. The reduction was "reversed" to a facilitatory effect by phenoxybenzamine (4 mg/kg). Propranolol alone slightly inhibited the noradrenaline release rate. After pretreatment with desipramine (1.0 mg/kg + 0.2 mg/kg/h), the inhibitory effect of propranolol on noradrenaline release was more pronounced and blood pressure was also lowered. However, in rabbits pretreated with captopril (1 mg/kg) in addition to desipramine, the sympathoinhibitory effect of propranolol was not observed. These results suggest that adrenaline can activate either presynaptic beta-adrenoceptors to increase noradrenaline release or, in higher doses, presynaptic alpha-adrenoceptors to inhibit noradrenaline release in vivo. The decrease in the noradrenaline release rate produced by propranolol alone may not be due to blockade of facilitatory presynaptic beta-adrenoceptors, but rather to depression of renin secretion. This would decrease angiotensin II formation and hence decrease the presynaptic release-enhancing effect of angiotensin II.     

Effect of flow-stop on noradrenaline release from normal spleens and spleens treated with cocaine, phentolamine or phenoxybenzamine

1. Cat spleens were perfused with Krebs-bicarbonate solution, using a constant-flow pump at a rate of about 7 ml/min at 33-35 degrees C. Noradrenaline (NA) overflow by nerve stimulation at 10 Hz for 20 s was determined with or without flow-stop before and after treatment with cocaine, phentolamine or phenoxybenzamine. In order to determine the effect of flow-stop on overflow, the arterial and the venous flows were occluded by clamping the inflow and outflow tubes during the period of stimulation plus 30, 60 or 120 seconds.2. Without flow-stop, NA output was 0.93+/-0.25 ng/stimulus, which was significantly increased after cocaine (123+/-6.6%), phentolamine (415+/-93%) and phenoxybenzamine (578+/-107%). Phentolamine and phenoxybenzamine were much more effective than cocaine in enhancing overflow.3. Before treatment with drugs, flow-stops of 30, 60 and 120 s reduced NA outputs to 70+/-6.6, 27.5+/-2 and 7%, respectively, of the control outputs without flow-stop. None of the drugs significantly influenced the percentage reductions in NA outputs during a 30 s flow-stop. However, the percentage outputs after cocaine or phenoxybenzamine treatment during a 60 s flow-stop significantly increased to 45+/-2.5% and 57+/-6%, respectively, as compared to the percentage output of 27.5+/-2% from untreated spleens during a corresponding flow-stop period. During flow-stop, there was no appreciable metabolism of the released transmitter.4. Diffusion of the released transmitter from the site of liberation plays only a minor role in the removal of the released NA.5. It is suggested that the NA released by nerve stimulation acts on the presynaptic alpha sites to inhibit its own release by a negative feedback mechanism. Adrenoceptor blocking agents enhance the NA overflow from spleen because they remove this autoinhibition by blocking the presynaptic alpha sites.     

Effects of presynaptic alpha-adrenoceptors and neuronal reuptake on noradrenaline overflow and cardiac response.

Using an in situ perfused, innervated rat heart model, we studied the effects of presynaptic alpha-adrenergic and neuronal reuptake inhibition on evoked noradrenaline (NA) overflow and the postsynaptic response by sympathetic ganglion stimulation. NA overflow was significantly increased by neuronal reuptake inhibitors (desipramine and (+)-oxaprotiline) or by alpha-adrenoceptor antagonists (phentolamine and yohimbine), but the inotropic response was augmented only by alpha-antagonists. In the presence of desipramine, nerve stimulation induced a frequency-dependent increase in NA overflow and postsynaptic response, both were enhanced by yohimbine. In the absence of desipramine, however, postsynaptic response was potentiated by yohimbine despite an unchanged (at 2 and 5 Hz) or even reduced NA overflow (at 10 Hz). Thus, this study suggests that NA release and cardiac response are modulated by presynaptic alpha-adrenoceptors, and that the neuronal reuptake modifies the amount of NA overflow but has little effect on the postsynaptic response.     

Influence of reserpine-induced depletion of noradrenaline on the negative feed-back mechanism for transmitter release during nerve stimulation

1. The effects of depletion of endogenous noradrenaline by reserpine-pretreatment on [(3)H]-noradrenaline overflow elicited by nerve stimulation were determined in the isolated nerve-muscle preparation of the cat's nictitating membrane.2. Reserpine pretreatment (0.3 mg/kg, s.c., 4 days prior to the experiment) reduced the noradrenaline levels in the smooth muscle of the nictitating membrane to about 10% of the control values while granular retention of [(3)H]-noradrenaline had recovered to nearly 40% of the controls.3. In the reserpine-pretreated tissue the fraction release per shock induced by nerve stimulation was 2.2-fold higher than the value obtained in the untreated tissues. This effect was correlated with the degree of depletion of the noradrenaline stores rather than with the decrease in the response of the effector organ.4. Phenoxybenzamine, 2.9 muM reduced the responses to nerve stimulation to the same extent in control and in reserpine-pretreated tissues. Yet, this concentration of phenoxybenzamine increased by 13-fold the overflow of the labelled transmitter in the controls and only by 3-fold in reserpine-pretreated tissues.5. The decrease in effectiveness of phenoxybenzamine in enhancing transmitter overflow after reserpine-pretreatment appears to be due to the decrease in the total release of the transmitter.6. The results obtained support the view that in reserpine-pretreated tissues decreased transmitter output reduces the activation of the presynaptic alpha-adrenoceptors which mediate the negative feed-back mechanism that regulates transmitter release by nerve stimulation.     

Phorbol ester-mediated enhancement of hippocampal noradrenaline release: which ion channels are involved?

Enhancement of neurotransmitter release following phorbol ester-induced activation of protein kinase C (PKC) may be mediated by changes in ion conductance through the presynaptic membrane. This question was studied with rabbit hippocampal slices preincubated with [3H]noradrenaline ([3H]NA). NA release was evoked by pulses of either high K+ or Ca2+ (in the presence of high K+), or by electrical field stimulation. 4 beta-Phorbol 12,13-dibutyrate (PDB) increased and polymyxin B (PMB) reduced the K+-evoked NA release independent of the K+ concentration used for depolarization. The effects of PDB and PMB were not reduced by tetrodotoxin. PDB still enhanced the NA release triggered by short Ca2+ pulses in depolarized, axon terminal membranes (30 mM K+ and no Ca2+). The electrically evoked NA release was markedly enhanced by PDB even in the absence of Cl- in the medium or in the presence of the K+ channel blockers, tetraethylammonium, 4-amino- and 3,4-diaminopyridine. The inhibitory effect of the Ca2+ channel blocker, Cd2+, remained almost unchanged in the presence of PDB. It is concluded that PKC activation facilitates NA release in the hippocampus but not via presynaptic changes in Na+, K+ or Cl- currents. Whether phorbol ester mediates an increased intracellular Ca2+ availability, or whether a triggering 'normal' Ca2+ influx simply initiates, and synergistically supports, the PKC-mediated reactions leading to enhanced exocytosis, cannot be decided from the results of the present experiments.     

Release of neuropeptide Y and noradrenaline during afferent nerve stimulation.

The present study was carried out to investigate the possibility that noradrenaline (NA) and neuropeptide Y (NPY) are co-released after afferent vagal or saphenous stimulation (1, 5, 10 and 20 Hz) in chloralose-anaesthetized dogs. Electrical stimulation of the vagus elicited an increase in plasma NA levels for the 5, 10 and 20 (but not 1) Hz frequencies. Blood pressure only increased after a 20-Hz stimulation. In contrast, no change in plasma NPY levels was observed whatever the frequency of stimulation. Electrical stimulation of the saphenous nerve failed to change plasma NA and NPY levels. The present data suggest that (1) the release of NA varies according to the frequency of stimulation of nociceptive fibres, (2) NPY release does not seem to be involved in the pressor effect elicited by the stimulation of nociceptive-sensitive fibres, and (3) NPY and NA release are not necessarily linked.