Site(s) and ionic basis of alpha-autoinhibition and facilitation of "3H'noradrenaline secretion in guinea-pig vas deferens

Quantification of formaldehyde induced fluorescence and its application in neurobiology

The methods used for measurements of fluorescence intensity in histochemical preparation will be described. Under certain conditions, the FIF intensity of biological objects shows a linear relationship with the local monoamine concentration. Microfluorometry enables determination of changes in monoamine concentrations in cell bodies, terminal axons and extraneuronal structures. In certain neuronal systems, FIF intensity of cell bodies appears to be correlated with neuronal activity. FIF intensity measurements of terminal axons can provide information on turnover of monoamine in structures as small as individual varicosities. Finally, the significance of extraneuronal FIF intensity in relationship to neurotransmission will be discussed. This paper aims to show that determinations of FIF provide information on the functional activity of neuronal systems.

Local regulation of transmitter release from rodent sympathetic nerve terminals?

1. Electrophysiological techniques were used to observe the release of transmitter from single release sites in the sympathetic neuro-effector junction of the rodent vas deferens. Transmitter release produces transient peaks in the rate of depolarization of the smooth muscle cells, known as 'discrete events'. 2. The amplitude distributions of stimulus-evoked discrete events in both mouse and guinea-pig vas are multi-modal. In the mouse, the distribution fits a Poisson with a 'quantal content' of about two. There are too many zeros in the amplitude distributions of guinea-pig discrete events to fit a Poisson distribution, and it is likely that in this species there is a mechanism preceding the transmitter release process which may occasionally prevent it operating. 3. alpha-adrenoreceptor agonists and antagonists produce, respectively, left and right shifts in the amplitude distributions of discrete events at a single latency, with no change in the amplitudes at the modes. 4. There is, however, no evidence of any inhibitory relationship between either discrete events evoked by successive stimuli, or early and late discrete events following a given stimulus. 5. Transmitter release at this junction is therefore packeted, with few quanta released by each stimulus. Release from single sites is affected in ways compatible with the 'alpha-feed-back' hypothesis by alpha-adrenoreceptor agonists and antagonists, but in the absence of drugs we can find no evidence of any local feed-back inhibition of transmitter release.

The effects of atropine on [3H]acetylcholine secretion from guinea-pig myenteric plexus evoked electrically or by high potassium

1. The myenteric plexus-longitudinal muscle preparation of the guinea-pig ileum was incubated with [(3)H]choline (1.125-1.5 muM), and then superfused with Tyrode solution containing hemicholinium-3 (10 muM). Secretion of [(3)H]acetylcholine ([(3)H]ACh) was evoked either (a) by electrical field stimulation (0.5-15 Hz, 150 shocks per period, 0.5 msec), used to ;indirectly' depolarize the varicosities of nerve terminals, or (b) by high potassium (40 mM with 1 muM-tetrodotoxin, for 6 min, or 80 mM without tetrodotoxin, for 1 min), to ;directly' depolarize varicosities.2. With these stimulation parameters, which yielded about the same fractional secretion of [(3)H]ACh, and with eserine (10 muM) present in the medium, atropine (1 muM) enhanced the ;indirectly', electrically evoked secretion 3.65+/-0.34 (n = 6) fold, and that caused by 40 mM or 80 mM-potassium 1.82+/-0.06 (n = 6) or 1.55+/-0.09 (n = 10) fold, respectively. Atropine thus enhanced ;indirectly', electrically evoked secretion 4-fold more than that caused by ;direct' depolarization of varicosities with high potassium (P < 0.001).3. This difference is not likely to be caused by depression of the sensitivity of the presynaptic muscarinic receptors to ACh released by nerve stimulation, caused by the hypertonicity of the medium in the potassium stimulation experiments. The medium made hypertonic by addition of Tris-HEPES (80 mM) did lower the binding affinity of membrane preparations of (pre- and post-synaptic) muscarinic receptors, to carbamylcholine, and also the contractile responsiveness of the longitudinal muscle to this agent, in both cases to about one half. But it did not appear to alter the responsiveness of either pre- or post-synaptic muscarinic receptors to endogenous ACh, released by nerve stimulation.4. The results support a dual-mode model for the muscarinic negative feed-back control of ACh secretion from the nerve terminals of this preparation, mainly operating by restriction of the invasion of terminals, and only secondarily by depression of the efficiency of depolarization-secretion coupling in invaded varicosities.5. Since this model has earlier been proposed to apply for the control of secretion of [(3)H]noradrenaline from the micro-anatomically similar nerve terminals of noradrenergic nerves, the present findings suggest that the model may have a wider biological significance, and possibly apply to the control of the secretory activity of boutons-en-passant nerve terminals in general.