GABA(B)-mediated Action in the Frog Olfactory Bulb Makes Odor Responses More Salient

In the olfactory bulb, GABA(B) receptors are selectively located in the glomerular layer. A current hypothesis is that GABAergic inhibition mediated through these receptors would be, at least partly, presynaptic and would exerted by decreasing the release of the olfactory receptor neuron excitatory neurotransmitter. Here, we assessed, in the frog, the in vivo action of baclofen, a GABA(B) agonist, on single-unit mitral cell activity in response to odors. Local application of baclofen in the glomerular region of the olfactory bulb was shown to drastically affect mitral cell spontaneous activity, since they became totally silent. Moreover, under baclofen, mitral cells still responded to odors and still specified odor concentration increases through their temporal response patterns. The pharmacological specificity of the GABA(B) agonist action was confirmed by showing that saclofen, a GABA(B) antagonist, partly prevented the inhibitory action of baclofen and restored the initial rate of mitral cell spontaneous activity. The results show that GABA(B)-mimicked inhibition suppressed mitral cell spontaneous activity while odor responses were maintained. This suggests that olfactory receptor neurons partly drive spontaneous mitral cell activity. Moreover, the effect of GABA(B)-mediated inhibition was seen to be very close to that described previously for dopamine D(2) receptor-mediated inhibition. In conclusion, we propose that these two inhibitory mechanisms would offer the possibility to reduce or suppress mitral cell spontaneous activity so as to make their responses to odor especially salient.