CALCIUM-CONCENTRATION CHANGES IN THE CALYCIFORM NERVE-TERMINAL OF THEAVIAN CILIARY GANGLION AFTER TETANIC STIMULATION

A study has been made of the changes in calcium concentration in the c alyciform nerve terminal ([Ca](c)) and in-the neurone soma ([Ca](s)) o f avian ciliary ganglion cells following tetanic stimulation of the ne rve terminal. Dissociated ciliary neurones were loaded with the calciu m indicator Fura-2 and digital imaging techniques used to determine th e spatial and temporal distribution of calcium in the cells during pos t-tetanic potentiation (PTP) and long-term potentiation (LTP). Stimula tion of the calyciform terminal with an extracellular electrode at 10 Hz for 2 s increased both [Ca](c) and [Ca](s) over 3-fold, with the [C a] increasing for each impulse in the facilitatory train. The increase in [Ca](s) could be prevented by allowing the terminal to degenerate in culture before stimulation. Stimulation of the calyciform terminal with a long tetanus of 30 Hz for 20 s gave an over 4-fold increase in both [Ca](c) and [Ca](s) by the end of the train. Analysis of the decl ine in [Ca](c) after the train showed that it disappeared from the cal yx along a double exponential time course with time constants of about 1 min and 50 min, respectively. These times are similar to those of P TP and LTP in the ganglia, and are almost independent of the extracell ular calcium level. Tn order to determine whether the influx of calciu m ions during a tetanus was through N-type calcium channels, these wer e blocked with adenosine (100 mu M). Adenosine blocked the increase in both [Ca](s) and [Ca](c) that normally accompanies a tetanus. Thapsig argin (200 nM) did not affect [Ca](c) or [Ca](s), but blocked transien t increases in [Ca] caused by caffeine (10 mM) in both 3 mM and Ca2+ f ree bath solutions. These results are discussed in relation to the rol e of intracellular calcium in initiating LTP after a tetanus to the ne rve terminals.