Pattern of ongoing discharge of single renal sympathetic neurons in the rabbit

Ongoing discharge in single renal sympathetic neurons was first studied in vagotomized rabbits without baroreceptor information (60 min after section of the aortic nerves). Under urethane + chloralose anaesthesia interspike-interval histograms were compiled and discharge rates were measured in 79 neurons. The following parameters were analysed: (a) the shortest, (b) the preferred and (c) the longest interspike-intervals, (d) discharge rate, (e) spread of a histogram, (f) coefficient of symmetry, and (g) coefficient of variability. The type of distribution of histograms and 9 correlations between some parameters were also assessed. These parameters were considered to make up the pattern of the ongoing discharge. The shortest, preferred and longest interspike-intervals of the ongoing discharge were: 14.1 +/- 0.9, 30.4 +/- 3.5 and 1672 +/- 82 ms. The mean rate of discharge amounted to 1.78 +/- 0.08 spikes/s. Three out of 9 correlation coefficients between the above parameters were statistically significant. In a second part, the effect of section of the aortic nerves and of 4-aminopyridine (a drug known to enhance synaptic transmission) on the pattern of ongoing discharge were also studied. Ten minutes after section of the aortic nerves the rate of discharge significantly increased, the shortest interspike-interval diminished and coefficient of variability was not changed. The number of significant correlations rose from 3 to 9.4-Aminopyridine significantly increased the discharge rate, did not alter the shortest interspike-interval and increased the coefficient of variability. These data show that assessing several parameters of ongoing discharge making up its pattern may differentiate between the excitatory effects of section of the aortic nerves and administration of 4-aminopyridine and in this way help to elucidate the mechanisms of action of various factors affecting renal sympathetic discharge.