The Source of Spontaneous Activity in the Main Olfactory Bulb of the Rat

In vivo, most neurons in the main olfactory bulb exhibit robust spontaneous activity. This paper tests the hypothesis that spontaneous activity in olfactory receptor neurons drives much of the spontaneous activity in mitral and tufted cells via excitatory synapses.Single units were recorded in vivo from the main olfactory bulb of a rat before, during, and after application of lidocaine to the olfactory nerve. The effect of lidocaine on the conduction of action potentials from the olfactory epithelium to the olfactory bulb was assessed by electrically stimulating the olfactory nerve rostral to the application site and monitoring the field potential evoked in the bulb.Lidocaine caused a significant decrease in the amplitude of the olfactory nerve evoked field potential that was recorded in the olfactory bulb. By contrast, the lidocaine block did not significantly alter the spontaneous activity of single units in the bulb, nor did it alter the field potential evoked by electrical stimulation of the lateral olfactory tract. Lidocaine block also did not change the temporal patters of action potential or their synchronization with respiration.Spontaneous activity in neurons of the main olfactory bulb is not driven mainly by activity in olfactory receptor neurons despite the extensive convergence onto mitral and tufted cells. These results suggest that spontaneous activity of mitral and tufted is either an inherent property of these cells or is driven by centrifugal inputs to the bulb

The effects of olfactory nerve block on field potentials.

<p>Field potentials recorded in the main olfactory bulb were evoked by stimulating the olfactory nerve (ON field potential, A and B) and the lateral olfactory tract (LOT field potential, C and D) before (control, A and C) and after (ON block, B and D) applying lidocaine to the olfactory nerve.</p

Histological evaluation of the extent of the olfactory nerve block.

<p>DiI was applied to the cut ends of the exposed olfactory nerve. (A) shows a whole mount, bright field view of the main olfactory bulb. (B) shows the same view as A under fluorescence. (C) shows a section through the olfactory bulb showing the cell bodies with DAPI staining; the olfactory nerve layer (ONL), the glomerular (GL), external plexiform (EPL) and mitral cell (MCL) and granule cell (Gr) layers of the bulb are indicated. (D) shows the DiI staining in the same section.</p

Effects of ON block on MOB units.

<p>Effects of ON block on MOB units.</p

The effects of olfactory nerve block on mean rate.

<p>The mean change in the amplitude of the ON-evoked field potential (A) caused by lidocaine application is compared to the mean change in the rate of spontaneous activity (B).</p