Striato-telencephalic and striato-tegmental circuits: relevance to learning in domestic chicks

Memory formation for a passive avoidance task in the domestic chick is like ly to involve a hyperstriatum ventrale (IMHV)-archistriatum-lobus parolfact orius (LPO) are. The present study summarises previous findings, relevant t o this neural system, and is also supplemented with some recent data from o ur laboratory. Projections from the IMHV on the archistriatum, as well as f rom the archistriatum on the LPO, have been characterised using a combinati on of anterograde pathway tracing (Phaseolus lectin), and post-embedding GA BA and glutamate immunocytochemistry. The majority of IMHV efferents have b een found to synapse with dendritic spine heads and necks of densely spiny projection neurons of the ventral archistriatum, and the ultrastructure of synapses suggested a potent excitatory input. Similar synaptic connections of the excitatory type were ultrastructurally verified between ventral arch istriatal afferent terminals and dendrites or spines of the LPO, suggesting an involvement of the medium sized spiny neurons, which are typical of the striatum. Although some of the IMHV boutons terminating in the archistriat um were immunoreactive to glutamate, this was not observed in the archistri atal-LPO pathway. Tegmental connections of the basal ganglia, in particular LPO, are also likely to play a role in processing of the avoidance respons e. We have demonstrated reciprocal connections between the LPO and dopamine rgic (TH-positive) neurons of the substantia nigra and ventral tegmentum. D opamine D1 receptors were upregulated bilaterally in the LPO following avoi dance learning and this response was not accompanied by significant changes in the level of dopamine or its metabolites (HVA, DOPAC), as revealed by H PLC chromatography of brain samples dissected from the LPO of control and t rained chicks. The dopamine receptor-related phosphoprotein DARPP-32 was lo calised in dendritic elements of the LPO, often forming asymmetric synapses with glutamate immunoreactive axon terminals. The findings are consistent with a scenario in which the striatum acts as a suppressor of natural pecki ng behaviour. Learned visual association with the target (bead) occurs in t he IMHV and is relayed to the basal ganglia via the limbic archistriatum (a mygdala equivalent) the latter introducing a motivational element (aversion , fear). Suppression of a brainstem pecking centre is likely to involve act ivation of the nigrostriatal (tegmentostriatal) dopaminergic circuit. (C) 1 999 Elsevier Science B.V. All rights reserved.