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

[3H]-amezinium is taken up selectively into noradrenergic axons and their transmitter-storing vesicles and is released from these axons by action potentials. We used it as a non-a-adrenergic marker in order to study the a-adrenergic autoinhibition of noradrenaline release.2 Rat occipitocortical slices were preincubated with [3H]-amezinium 0.03 tLM and then superfused and stimulated electrically (3 Hz for 3 min). The stimulation-evoked overflow of tritium was measured in six groups of slices: from saline-pretreated rats; from saline-pretreated rats, the slices being exposed to exogenous noradrenaline before preincubation with [3H]-amezinium; from saline-treated rats, slices from which were exposed simultaneously to noradrenaline and cocaine before preincubation with [3H]-amezinium; from rats in which noradrenaline stores had been depleted by pretreatment with a-methyltyrosine (a-NMT); from a-MT-treated rats, the slices being exposed to noradrenaline before preincubation with [3H]-amezinium; and from a-MT-treated rats, slices from which were exposed to noradrenaline plus cocaine before preincubation with [3H]-amezinium. 3 The stimulation-evoked overflow of tritium, expressed as a percentage of the tritium content of the tissue, was 1.15% in slices from saline-pretreated rats, and was similar in slices from salinepretreated rats after exposure to noradrenaline or noradrenaline plus cocaine. It was 2.56 % in slices from a-MT-treated rats, 1.20% from a-MT-treated rats after exposure to noradrenaline, and 2.88% from a-MT-treated rats after exposure to noradrenaline plus cocaine. 4 Yohimbine 0.1 and 1 piM increased the stimulation-evoked overflow of tritium in slices from all groups of saline-pretreated rats and in those slices from a-MT rats that had been in contact with exogenous noradrenaline. Yohimbine did not change the evoked overflow in slices from a-MT rats that had not been exposed to noradrenaline, or had been exposed to noradrenaline plus cocaine. 5 Clonidine 0.01-1 LM decreased the stimulation-evoked overflow of tritium moderately in slices from saline-pretreated rats, markedly in slices from a-MT-treated rats, and moderately again when the latter slices had been exposed to noradrenaline.6 It is concluded that the action potential-evoked release of [3H]-amezinium as well as the modulation of this release by yohimbine and clonidine depend on the presence or absence of a-adrenergic autoinhibition caused by the co-secretion of noradrenaline. When there is co-secretion of noradrenaline, the evoked release of [3H]-amezinium is relatively small, yohimbine increases the release, and clonidine can cause only moderate inhibition. When there is no or very little cosecretion of noradrenaline, the evoked release of [3H]-amezinium is at least doubled, yohimbine causes no further increase and clonidine produces strong inhibition.

Section: Discussionsupporting

confidence: 79%

Section: Introductionmentioning

confidence: 99%

Release of [³H]‐amezinium from cortical noradrenergic axons: a model for the study of the α‐autoreceptor hypothesis

Hedler¹,

Starke²,

Steppeler³

1983

“…As to the reason for the small differences at 4, 8 and 16 Hz in the neurogenic response in reserpine-treated vessels compared with the prazosin-resistant neurogenic response of control vessels, further investigation is required. It could be due to the absence of a modulatory effect ofnoradrenaline (Enero & Langer, 1973) or to some direct effect of the reserpine treatment on the purinergic component of the neurogenic response.…”

Section: Discussionmentioning

confidence: 99%

Effects of reserpine and 6‐hydroxydopamine on the adrenergic and purinergic components of sympathetic nerve responses of the rabbit saphenous artery

Warland

Burnstock

1987

1 The effects of reserpine and of 6-hydroxydopamine on the contractions of the rabbit isolated saphenous artery produced by stimulation of the sympathetic nerves were studied. 2 In vessels exposed to reserpine, substantial contractions to nerve stimulation were recorded despite a 95.7% reduction in the noradrenaline content of the tissue. These responses of the vessel were not significantly affected by the t,-antagonist, prazosin, whereas after desensitization of the P2-purinoceptor with a,-methylene ATP, no response to nerve stimulation remained.3 In vessels exposed to 6-hydroxydopamine, no nerve-mediated responses were observed. 4 Noradrenaline-containing nerves were observed by fluorescence histochemistry in control tissues, but were not observed in tissues treated with reserpine or 6-hydroxydopamine. 5 The potencies of ATP and histamine were not significantly affected by reserpine or 6-hydroxydopamine treatment. However, there was a slight supersensitivity to noradrenaline in reserpine-treated and 6-hydroxydopamine-treated vessels compared with that of control vessels. Prazosin was selective for a-adrenoceptors, while a,p-methylene ATP was selective for P2-purinoceptors.6 These results substantiate the finding that ATP and noradrenaline are sympathetic cotransmitters in the rabbit isolated saphenous artery, and demonstrate that ATP can act as a transmitter independently of noradrenaline in this vessel.

“…The total increase in outflow of tritium elicited by nerve stimulation was expressed as the fractional release per shock (FR), i.e. the total nCi released per shock divided by the total nCi remaining in the tissue at the onset of stimulation (Enero & Langer, 1973). The radioactivity in the bathing solution and in the tissue was measured by scintillation counting as described by AdlerGraschinsky et al (1972).…”

Section: Guinea-pig Atriamentioning

confidence: 99%

POSSIBLE ROLE OF a Β‐ADRENOCEPTOR IN THE REGULATION OF NORADRENALINE RELEASE BY NERVE STIMULATION THROUGH a POSITIVE FEED‐BACK MECHANISM

Adler‐Graschinsky

Langer

1975

328

The effects of isoprenaline, propranolol and phentolamine, were studied on tritiated noradrenaline overflow elicited by postganglionic nerve stimulation in guinea‐pig isolated atria. Isoprenaline (1.2 × 10−8) increased while propranolol (1.0 × 10‐7 M) reduced the overflow of tritiated noradrenaline evoked by nerve stimulation. These effects were less than those of phentolamine (3.1 × 10‐6 M), which increased by approximately three‐fold the overflow of [3 H] ‐noradrenaline elicited by nerve stimulation. Neuronal accumulation of tritiated noradrenaline in guinea‐pig atria was not affected by isoprenaline, propranolol or phentolamine at the concentrations employed in this study. Isoprenaline (1.2 × 10‐8 M) induced a positive chronotropic effect of about 80% of the maximum. On the other hand, propranolol produced a shift to the right in the frequency‐response curve to nerve stimulation and in the concentration‐response curve to exogenous noradrenaline in guinea‐pig atria. In the isolated nictitating membrane of the cat, the frequency‐response curve to nerve stimulation was not modified by propranolol, while in the presence of 3.9 × 10−6 m of N,‐2‐(2,6‐dimethylphenoxy)propyl‐N,N,N ‐trimethylammonium (β‐methyl‐TM 10) there was a shift to the right and a depression of slope. Neither propranolol nor β‐methyl‐TM 10 affected responses to exogenous noradrenaline. The effects of isoprenaline and of propranolol on transmitter release are compatible with the view that in addition to the presynaptic negative feed‐back mechanism for noradrenaline release by nerve stimulation mediated via α‐adrenoceptors a positive feed‐back mechanism exists in adrenergic nerve endings which is triggered through the activation of presynaptic β‐adrenoceptors.