Electrophysiological studies of fetal mouse olfactory bulb explants during development of synaptic functions in culture

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Abstract
Explants of 18‐ or 19‐day fetal mouse olfactory bulb have been maintained in culture for periods up to 5 weeks. Compound action potentials can be evoked in the bulb explants by 1 day in vitro, and by 3–4 days, synaptically mediated slow wave discharges can be demonstrated in bicuculline (10−5M). The capability of the bulb explants to generate these slow‐wave discharges has also been revealed by the introduction of picrotoxin (10−5M), d‐tubocurarine (10−4M) and chloride‐free medium, but not of strychnine (up to 3 × 10−5M). The data indicate early functional development of inhibitory, as well as excitatory, synaptic systems. In addition, the selective and reversible depression of these slow wave potentials by GABA (1–5 × 10−4M), but not by glycine (up to 3 × 10−3M), indicates a GABA‐ergic component in the inhibitory network. Single unit extracellular recordings have been obtained from the presumptive mitral cells which, in culture, are spontaneously active even as early as 1–2 days after explantation. Correlative Bodian silver‐impregnations demonstrate the presence of neurons in these explants which resemble typical mitral cells. Studies of mitral cells using paired stimuli suggest the development in vitro of an inhibitory system analogous to that known to suppress the excitability of their in situ counterparts following orthodromic or antidromic activation. These data, as well as the pharmacological sensitivities of the mitral cells in culture to GABA (5 × 10−4M) and bicuculline (10−5M), indicate that granule‐to‐mitral synapses may develop characteristic functions in olfactory bulb explants.