SYNAPTIC TERMINALS IMMUNOLABELED 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 domesti c chicks. The anatomical circuitry underlying memory formation in the chick is likely to involve the intermediate medial hyperstriatum ventr ale-archistriatum-LPO are. In the present work, we attempted to combin e an ultrastructural characterisation of archistriatal afferent termin als in LPO with a description of the synaptic structure of LPO, in par ticular those elements that are immunoreactive to glutamate and GABA. Ventral archistriatal regions of 7-day-old domestic chicks were iontop horetically injected with Phaseolus vulgaris leucoagglutinin and the a nterograde transport of the tracer was detected in the LPO. Selected s amples from these birds, and also from other day-old chicks, were resi n-embedded and reacted for L-glutamate or GABA, using the postembeddin g immunocytochemical method. Glutamate was abundant in the neuropil of LPO and typically seen in axodendritic or axospinous terminals with a symmetrical junctions, often multiple or perforated postsynaptic appos itions. Conversely, GABA was often present in aspinous dendrites, prob ably representing GABAergic local circuit neurons or (putative striato nigral) projection neurons. Archistriatal efferents terminating in LPO formed small en passant or terminal varicosities, with infrequent asy mmetrical axospinous synapses. Glutamate was not detected in these bou tons. The findings imply that the functional state of LPO, based on po werful glutamatergic excitation, may be modified by a non-glutamatergi c archistriatal input.