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Bursting properties of caudal neurosecretory cells in the flounder Platichthys flesus, in vitro

M. J. Brierley, A. J. Ashworth, J. R. Banks, R. J. Balment and C. R. McCrohan^*

School of Biological Sciences, University of Manchester, Manchester, UK

View larger version (18K): [in a new window]   Fig.1. Bursting patterns in type 1 Dahlgren cells. (A) Spontaneous bursting pattern. (B) A single burst (bottom trace) and instantaneous spike frequency (top trace) revealing acceleration and deceleration phases with a peak firing frequency of approx. 4Hz. (C) Action potential duration (measured at half-maximal amplitude) increases with instantaneous spike frequency. At maximal firing rate, spike duration increases (approximately 30%) compared to the first spike of the burst.

View larger version (10K): [in a new window]   Fig.2. Spontaneous depolarising input to type 1 cells. Prior to the onset of a burst, a depolarising waveform is seen, consisting of at least two excitatory components. Underlying the fast unitary events is a slower depolarisation, which reaches spike threshold after approximately 35s.

View larger version (38K): [in a new window]   Fig.3. Burst triggering by current injection. (A) Injection of a 5s depolarising pulse via the recording electrode (during the first half of an interburst interval) triggered a burst, which lasted 128s (end of burst not shown). (B) Following termination of a 500ms hyperpolarising pulse, a burst of action potentials, lasting 55s, occurred. The same hyperpolarising pulse and first six spikes (highlighted in the box) are shown on an expanded scale in (C). During the pulse, the membrane potential waveform does not follow that of the square wave hyperpolarisation (deviation from dashed line) and upon termination of the pulse, an action potential is immediately triggered. Succeeding this action potential, but prior to the onset of the next, is a depolarising after potential (DAP; arrow).

View larger version (19K): [in a new window]   Fig.4. Voltage- and time-dependence of depolarising after potentials. (A) The membrane potential (EM) of a type 1 cell was held at three different potentials by constant current injection. Constant amplitude and duration hyperpolarizing pulses elicit different membrane potential waveforms, depending on EM. A sag potential (dashed line) occurs during the pulse and a spike is triggered on pulse termination only at the more depolarised EM. A DAP (arrowed) follows the action potential. Membrane sag and DAP are thus dependent on EM. (B) Hyperpolarising pulses of constant duration but differing amplitude were injected into a depolarised type 1 cell. DAP amplitude increases with increased hyperpolarising pulses. (C) Constant amplitude hyperpolarizing current pulses of increasing duration were applied to a type 1 cell. DAP amplitude increase with increased pulse duration. (D) The first spike of a spontaneous burst is succeeded by a DAP (arrow).

View larger version (28K): [in a new window]   Fig.5. Extracellular recordings of CNSS activity. (A) Individual units (arrows) can be distinguished by spike amplitude. (B) Bursts of activity (approximately 120s duration) are seen in an individual unit. (C) Activity from several units contributes to a long-duration superburst.

View larger version (16K): [in a new window]   Fig.6. Burst termination by 5-HT. (A) A spontaneous burst recorded intracellularly in a type 1 Dahlgren cell is prematurely terminated by 10s focal application of 5-HT (100µmoll-1; bar), which also hyperpolarised the cell by 25mV. (B) A 15min bath application of 5-HT (100µmoll-1; bar) abolishes ongoing activity. Spontaneous activity returns following a 25min washout.

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