Synaptic terminals immunolabelled against glutamate in the lobus parolfactorius of domestic chicks (Gallus domesticus) in relation to afferents from the archistriatum

The lobus parolfactorius (LPO) has been implicated in memory formation associated with passive avoidance training of young posthatch domestic chicks. The anatomical circuitry underlying memory formation in the chick is likely to involve the intermediate medial hyperstriatum ventrale-archistriatum-LPO are. In the present work, we attempted to combine an ultrastructural characterisation of archistriatal afferent terminals in LPO with a description of the synaptic structure of LPO, in particular those elements that are immunoreactive to glutamate and GABA. Ventral archistriatal regions of 7-day-old domestic chicks were iontophoretically injected with Phaseolus vulgaris leucoagglutinin and the anterograde transport of the tracer was detected in the LPO. Selected samples from these birds, and also from other day-old chicks, were resin-embedded and reacted for L-glutamate or GABA, using the postembedding immunocytochemical method. Glutamate was abundant in the neuropil of LPO and typically seen in axodendritic or axospinous terminals with asymmetrical junctions, often multiple or perforated postsynaptic appositions. Conversely, GABA was often present in aspinous dendrites, probably representing GABAergic local circuit neurons or (putative striatonigral) projection neurons. Archistriatal efferents terminating in LPO formed small en passant or terminal varicosities, with infrequent asymmetrical axospinous synapses. Glutamate was not detected in these boutons. The findings imply that the functional state of LPO, based on powerful glutamatergic excitation, may be modified by a non-glutamatergic archistriatal input.