Secondary nicotinic synapses on sympathetic B neurons and their putative role in ganglionic amplification of activity

The strength and number of nicotinic synapses that converge on secretomotor B neurons were assessed in the bullfrog by recording intracellularly from isolated preparations of paravertebral sympathetic ganglia 9 and 10. One in put to every B neuron invariably produced a suprathreshold EPSP and was def ined as the primary nicotinic synapse. In addition, 93% of the cells receiv ed one to four subthreshold inputs that were defined as secondary nicotinic synapses. This contradicts the prevailing view, which has long held that a mphibian B neurons are singly innervated. More important, the results revea led that B cells provide the simplest possible experimental system for exam ining the role of secondary nicotinic synapses on sympathetic neurons. Comb ining the convergence data with previous estimates of divergence indicates that the average preganglionic B neuron forms connections with 50 ganglioni c B neurons and that the majority of these nicotinic synapses are secondary in strength. Secondary EPSPs evoked by low-frequency stimulation ranged fr om 0.5 to 10 mV in amplitude and had an average quantal content of 1. Nonet heless, secondary synapses could trigger action potentials via four mechani sms: spontaneous fluctuations of EPSP amplitude, two-pulse facilitation, co activation with other secondary synapses, and coactivation with a slow pept idergic EPSP. The data were used to formulate a stochastic theory of integr ation, which predicts that ganglia function as amplifiers of the sympatheti c outflow. In this two-component scheme, primary nicotinic synapses mediate invariant synaptic gain, and secondary nicotinic synapses mediate activity -dependent synaptic gain. The model also provides a common framework for co nsidering how facilitation, metabotropic mechanisms, and preganglionic osci llators regulate synaptic amplification in sympathetic ganglia.