Helix B2-B2 neuron pairs form unidirectional and bidirectional chemical synapses and exhibit short-term plasticity in vitro. Helix B2-B2 neuron pairs form unidirectional and bidirectional chemical synapses and exhibit short-term plasticity in vitro. (A) Phase-contrast micrograph of a soma-to-soma pair of cultured Helix B2 neurons. Scale bar: 50 μm. (B) Sample electrophysiological recordings of a bidirectional synapse between two B2 neurons (labeled ‘a’ and ‘b’ in panel A). (C) Occurrence of unidirectional or bidirectional synapses between chemically interconnected Helix B2-B2 neurons cultured in the presence of ganglia-derived trophic factor. The majority of B2-B2 pairs show bidirectional connections. (D) Representative induction and decay of PTP recorded from B2-B2 synapses in culture. The presynaptic neuron was electrically stimulated to fire an action potential five times at a basal frequency of 0.05 Hz and the corresponding EPSPs recorded in the postsynaptic cell kept hyperpolarized at −85 mV to prevent firing. Next, a 10 Hz presynaptic stimulation was applied for 10 seconds (arrowhead). Thirty seconds after the tetanus, the presynaptic stimulation was brought back to the basal rate of 0.05 Hz. Carlo Natale Giuseppe Giachello et al. J Cell Sci 2010;123:881-893 © 2010.