Figure 6. Synchronous and asynchronous release in the IO under near-physiological conditions (35°C in 1.5 mM Ca²⁺/1.0 mM Mg²⁺).

(A) IPSCs evoked by single electrical stimuli in the IOB/A showing individual trials (gray) and average across trials (black). (B) Same as in A, but for the PIO. (C) Jitter (Left), decay time (middle) and rise time (right) of IPSCs in the IOB/A (empty markers) and PIO (filled markers) evoked by single stimuli in near-physiological conditions. (D) Averaged IPSCs evoked by trains of electrical stimuli (50 Hz x 10 stimuli) for two different IO neurons in the IOB/A. (E) Same as in D, but for two IO neurons in the PIO. (F) Average normalized charge transfer during a 50 Hz train (left) and average decay time of the last IPSC in the 50 Hz train (right) for neurons in the IOB/A (empty markers) and PIO (filled markers). Data is represented as mean ± SEM for charge, and individual cells for decay time.

Figure 6—figure supplement 1. bursting and oscillation of electrically-coupled neurons in the IO can obscure synaptic responses.

(A) Neurons in the IO are electrically-coupled and as a result fire and oscillate synchronously and spontaneously. Oscillations and bursting are more common at near-physiological temperature (35°C). Bursting from neighboring IO neurons can be seen in a neuron in the PIO held at −60 mV despite the presence of intracellular blockers of Na⁺, K⁺, and Ca²⁺ currents (by QX-314, TEA, and D-600, respectively). Bursts have waveforms distinguished by large inward currents followed by a slow outward component (middle) that reflects low-pass filtered synchronous firing of neighboring coupled neurons. Small inward synaptic events can be seen between bursts (right). Recordings were obtained at near-physiological conditions (35°C in 1.5 mM Ca²⁺ / 1.0 mM Mg²⁺). (B) Same cell as in A, but held at +60 mV. Currents generated by spontaneous electrically-coupled bursts are attenuated (Left, middle), but do not reverse. Small events between bursts mediated by chemical synapses reverse (right). (C) Current evoked by a single stimulus in a PIO neuron held at −60 mV in the presence of blockers of excitatory (NBQX, CPP) and inhibitory transmission (Gabazine, Strychnine). Evoked currents did not reverse, even when held at +60 mV (not shown). (D) Current evoked by a 50 Hz train of 10 stimuli in a PIO neuron held at −60 mV in the presence of synaptic blockers as shown in C. The train consistently initiated a burst from neighboring IO neurons, followed by a subthreshold sinusoidal oscillation that is truncated after a single phase. Evoked currents did not reverse and were not eliminated when the cell was held at +60 mV (not shown).