SEROTONINERGIC INNERVATION OF NONPRINCIPAL CELLS IN THE CEREBRAL-CORTEX OF THE LIZARD PODARCIS-HISPANICA

The mechanism of serotoninergic transmission in the neo- and archicort ex of mammals is complex, including both synaptic and nonsynaptic comp onents, direct actions on principal cells, and indirect effects mediat ed by GABAergic interneurons. Here we studied the termination pattern and synaptic-organization of the serotoninergic afferents in the cereb ral cortex of the lizard, Podarcis hispanica, which is considered to c orrespond in part to the mammalian hippocampal formation, with the aim of unraveling basic, phylogenetically preserved rules in the connecti vity of this pathway. We demonstrate that serotoninergic afferents, vi sualized by immunostaining for serotonin itself, establish multiple sy naptic contacts with different subpopulations of nonprincipal cells co ntaining parvalbumin, neuropeptide Y, and opioid peptides. The former two subpopulations contain GABA, whereas the opioid-immunoreactive neu rons are most likely GABA-negative cells. Evidence is provided at the electron microscopic level that serotonin-immunoreactive varicosities establish conventional asymmetric synaptic contacts with their nonprin cipal targets, but nonsynaptic varicosities also exist. We conclude th at, similarly to mammals, a selective synaptic innervation of nonprinc ipal, possibly inhibitory, neurons is among the mechanisms of serotoni nergic modulation of cerebral cortical activity in the lizard. (C) 199 4 Wiley-Liss, Inc.