DOPAMINERGIC MODULATION OF MITRAL CELL-ACTIVITY IN THE FROG OLFACTORY-BULB - A COMBINED RADIOLIGAND BINDING ELECTROPHYSIOLOGICAL STUDY

Dopamine content in the amphibian olfactory bulb is supplied by intern eurons scattered among mitral cells in the external plexiform/mitral c ell layer. In mammals, dopamine has been found to be involved in vario us aspects of bulbar information processing by influencing mitral cell odour responsiveness. Dopamine action in the bulb depends directly on the localization of its receptor targets, found to be mainly of the D -2 type in mammals. The present study assessed, in the frog, both the anatomical localization of D-2-like, radioligand-labelled receptors of dopamine and the in vivo action of dopamine on unitary mitral cell ac tivity in response to odours delivered over a wide range of concentrat ions. The [I-125]iodosulpride-labelled D-2 binding sites were visualiz ed on frozen sagittal sections of frog brains by film radioautography. The sites were found to be restricted to the external plexiform/mitra l cell layer; other layers of the olfactory bulb were devoid of specif ic labelling. Electrophysiological recordings of mitral unit activity revealed that dopamine or its agonist apomorphine induced a drastic re duction of spontaneous firing rate of mitral cells in most cases witho ut altering odour intensity coding properties of these cells. Moreover , pre-treatment with the D-2 antagonist eticlopride blocked the dopami ne-induced reduction of mitral cell spontaneous activity. In the frog olfactory bulb, both anatomical localization of D-2-like receptors and functional data on dopamine involvement in information processing dif fer from those reported in mammals. This suggests a phylogenetic evolu tion of dopamine action in the olfactory bulb. In the frog, anatomical data perfectly corroborate electrophysiological results, together str ongly suggesting a direct action of dopamine on mitral cells. In a phy siologically operating system, such an action would result in a global improvement of signal-to-noise ratio. (C) 1997 IBRO.