GABAERGIC NEURONS IN THE EMBRYONIC OLFACTORY PIT VOMERONASAL ORGAN - MAINTENANCE OF FUNCTIONAL GABAERGIC SYNAPSES IN OLFACTORY EXPLANTS/

In previous work, we showed a robust gamma-aminobutyric acid (GABAergi c) synaptic input onto embryonic luteinizing hormone-releasing hormone (LHRH) neurons maintained in olfactory explants. In this study, we id entify GABAergic neurons in olfactory pit (OF) of embryonic mice in vi vo and study, using patch-pipet whole-cell current and voltage clamp t echniques, synaptic interactions of these neurons in explant cultures. In vivo, glutamate decarboxylase (GAD, the enzyme which synthesizes G ABA) mRNA was first detected in nasal regions on Embryonic Day (E) 11. 5. From E12.5 to E13.5, robust GAD expression was localized to cells p rimarily in the ventral aspect of the OF. GAD mRNA was not detected ov er dorsally located cells in olfactory sensory or respiratory epitheli um. In addition, GAD mRNA was not observed in cells along olfactory ax ons. GAD mRNA was dramatically reduced in the OP/vomeronasal organ by E16.5. Using antibodies against both GABA and GAD, immunopositive axon al-like tracts were detected in the nasal septum on E12.5. GABAergic s taining decreased by E13.5. To examine synaptic interactions of these GABAergic cells, embryonic olfactory explants were generated and maint ained in serum-free media. As explants spread, neuron-like cells migra ted into the periphery, sometimes forming ganglion-like clusters. Cell s were recorded, marked intracellularly with Lucifer Yellow and postfi xation, immunocytochemically examined. forty-six cells, typically mult ipolar, were GABAergic, had resting potentials around -50 mV, and exhi bited spontaneous action potentials which were generated by spontaneou s depolarizing GABAergic (GABA(A)) synaptic activity. OP neurons depol arized in response to GABA by increasing Cl- conductance. The biophysi cal properties of OF-derived GABAergic neurons were distinct from thos e reported for olfactory receptor neurons but similar to embryonic LHR H neurons. However, unlike LHRH neurons, GABAergic neurons did not mig rate large distances in olfactory explants or appear to leave the olfa ctory pit in vivo. (C) 1996 Academic Press, Inc.