Activity dependence of action potential duration in rat supraoptic neurosecretory neurones recorded in vitro.

Abstract

SUMMARY1. Action potential durations, measured at one-third peak amplitude, were examined during intracellular recordings in 134 supraoptic nucleus neurones maintained in vitro in perfused hypothalamic explants.2. Spike durations ranged between 1-2 and 3-9 ms and were dependent on firing frequency. Shortest measurements (1 P74 + 0-03 ms; mean + S.E. ofmean) were obtained during relative quiescence, i.e. < 0'5 Hz. A gradual increase in firing frequency through continuous injection of depolarizing current prolonged spike duration, with maximum levels (2-68 + 0-05 ms) achieved at 20 Hz. When interspike interval variability was eliminated and firing was more precisely regulated by brief 15-20 ms intracellular current pulses given at pre-determined frequencies, a proportional relationship between increasing spike duration and firing frequency was retained but the change in spike duration at frequencies between 2 and 10 Hz was less pronounced.3. Once action potentials had achieved the long duration configuration, their return to the shorter duration took place gradually during any succeeding silent interval with a time constant of 4-9 s. 4. Action potential broadening occurred progressively and was most pronounced at the onset of spontaneous or current-induced bursts. In thirty-six phasically active neurones, spike broadening at the onset of a burst was concurrent with the presence of 5-10 consecutive short (< 100 ms) interspike intervals; thereafter, despite a greater than 50 % reduction in firing frequency, action potential durations remained prolonged throughout the burst.5. In all of nineteen cells tested, frequency-dependent changes in spike duration were reversibly decreased or blocked by Cd2+, Co2+ and Mn2+, or when CaCl2 was exchanged for equimolar amounts of EGTA in the perfusion medium.6. These observations indicate that a Ca2+ conductance contributes to frequencyand firing-pattern-dependent changes in spike duration in rat supraoptic nucleus neurones.

Action potential broadening and frequency-dependent facilitation of calcium signals in pituitary nerve terminals.

“…An example of such a terminal, filled with the fluorescent dye Lucifer yellow, is shown in Fig. 1 (10,11,(14)(15)(16)(18)(19)(20)(21)(22). As reported previously (14,23), action potentials of pituitary terminals broadened during high-frequency stimulation (Fig.…”

“…Peptide secretion from these terminals also exhibits a striking form of use-dependent plasticity: secretion is a sensitive function of action potential frequency (11,(13)(14)(15)(16)(17). This frequency dependence is essential to the input-output properties of the hypothalamichypophyseal axis (18) and has been proposed to be due to frequency-dependent modulation of action potential duration (10,11,13,14,(19)(20)(21)(22)(23) (Fig. 1).…”

Changes in the Electrical Properties of Supraoptic Nucleus Oxytocin and Vasopressin Neurons during Lactation

“…The number of neurons is in parentheses for all figures. ( Andrew and Dudek, 1985;Bourque and Renaud, 1985b). Frequency-dependent changes in K ϩ and Ca 2ϩ conductances have been implicated in the expression of spike broadening (O'Regan and Cobbett, 1993).…”

“…9B). Another characteristic of MNCs during repetitive firing is the progressive increment in spike duration (spike broadening) (Andrew and Dudek, 1985;Bourque and Renaud, 1985b). Figure 9C shows a plot of the mean spike duration for the first nine spikes during a 180 msec depolarizing pulse (0.4 nA) for OT and VP neurons from lactating and diestrous rats.…”

C-type Natriuretic Peptide Inhibits L-type Ca2+Current in Rat Magnocellular Neurosecretory Cells by Activating the NPR-C Receptor

“…MNCs exhibit a significant broadening of successive action potentials in a train due to the activation of voltage-gated calcium channels (Bains and Ferguson 1999;Bourque and Renaud 1985a). This phenomenon is thought to be an important factor contributing to the frequency and pattern-dependent potentiation of hormone release in the nerve terminals (Fisher and Bourque 2001;Renaud and Bourque 1991).…”