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1. Synaptic transmission was studied in visually identified parasympathetic ganglion cells that modulate the heart beat of the mudpuppy Necturus maculosus).

2. The brief pulse of acetylcholine (ACh) released from terminals of the vagus nerve after each impulse can produce two distinct post-synaptic responses in individual principal cells of the ganglion: (i) within a milli-second of release, ACh generates a rapid and strong excitatory post-synaptic potential (e.p.s.p.) that normally initiates a post-synaptic impulse; (ii) this excitation is usually followed by a slow hyperpolarizing inhibitory post-synaptic potential (i.p.s.p.) that lasts for several seconds. The magnitude and time course of the i.p.s.p. depends on the frequency and number of vagal stimuli. When the hydrolysis of ACh is inhibited by prostigmine, a train of nerve stimuli may be followed by an i.p.s.p. lasting half a minute or longer.

3. The rapid e.p.s.p. and slow i.p.s.p. result from the direct action of ACh on two different types of chemoreceptors in the post-synaptic membrane of the principal cell. The e.p.s.p. can be preferentially blocked by the nicotinic antagonist dihydro--erythroidine (5 x 10^-7 M), while the i.p.s.p. is selectively blocked by the muscarinic antagonist atropine (5 x 10^-9 M).

4. Potentials resembling nerve-evoked e.p.s.p.s and i.p.s.p.s can be produced by iontophoretic release of ACh from micropipettes onto the post-synaptic membrane. Application of the muscarinic agonist bethanechol generates exclusively inhibitory responses.

5. The reversal potential for the i.p.s.p. is about -105 mV, which is approximately the equilibrium potential for potassium (E_K). When the external K⁺ concentration is altered, the reversal potential for inhibition is shifted to the new value of E_K as expected from the Nernst equation. Changes in the external Na⁺ and Cl^- concentrations have no appreciable effect on the reversal potential. Thus, the i.p.s.p. is the result of a conductance increase for K⁺.

6. The conductance change producing the i.p.s.p. is voltage sensitive. When the membrane potential is shifted from -40 to -60 mV, the i.p.s.p becomes larger and longer. Beyond -60 mV the inhibitory response decreases in proportion to the driving force on K⁺ without any further change in time course.

7. The inhibitory response produced by an iontophoretically applied pulse of bethanechol has a delayed onset of about 150 msec at 24 °C. The early portion of this response, including the delay, is proportional to t³, where t is time. The proportionality factor (the apparent rate constant) decreases elevenfold when the temperature is lowered by 10 °C. This suggests that a multi-step process is involved in the activation of the conductance increase that leads to the inhibitory response. Inhibitory responses with similar kinetics were produced in heart muscles of the mudpuppy upon application of ACh.

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