An intracellular study of synaptic transmission and dendritic morphology in sympathetic neurons of the chick embryo.

The characteristics of synaptic transmission in whole embryonic avian sympathetic ganglia have been examined by intracellular recording. Neurons in lumbar paravertebral ganglia of chick embryos exhibit both fast nicotinic excitatory postsynaptic potentials (EPSPs) and non-cholinergic slow EPSPs. Fast nicotinic transmission is mediated by at least 3-5 convergent preganglionic inputs and can be detected at the earliest embryonic stage examined (Stage 38; 12 days of incubation). Two types of non-cholinergic slow EPSPs have been observed and distinguished by their time course and the resulting changes in input resistance. One of these slow synaptic potentials is mimicked by exogenously applied substance P, but not by exogenous luteinizing hormone-releasing hormone (LH-RH). Muscarinic agonists also evoke slow depolarizations in the ganglia, mediated at least in part by inhibition of the M-current. Intracellular labeling with horseradish peroxidase reveals cells with 5-10 primary dendrites at Stage 42 (16 days of incubation), the earliest stage to exhibit slow EPSPs. The active and passive membrane properties of avian sympathetic neurons, including the presence of the M-current, generally resemble those of adult mammalian and amphibian sympathetic neurons. Functional activity in chick sympathetic neurons is present at a developmental stage where both biochemical and morphological indices of synapse maturation are at low levels. Since this progression has also been observed in the avian ciliary ganglion, it may be of general relevance to neuronal development.