Centrifugal Input Modifies Spontaneous Activity of Olfactory Bulb Neurons

Abstract

The specific aim of this thesis was to examine the effects of centrifugal input on the spontaneous activity of main olfactory bulb (MOB) neurons. Spontaneous activity is the ability of a neuron to generate action potentials in the absence of external stimuli. In order to study the effects of centrifugal fibers on the spontaneous activity of bulbar neurons, impulse conduction of these fibers was blocked using a reversible chemical blockade, while simultaneously recording spontaneous activity in the MOB. Our results show that blocking centrifugal input does modulate the levels of spontaneous activity, indicating that these fibers release neurotransmitters in the steady state. The two pathways by which centrifugal fibers reach the MOB are through the lateral olfactory tract (LOT) and the anterior olfactory nucleus (AON). Lidocaine was applied topically to the LOT or injected into the AON. Electrically evoked field potentials were recorded in the MOB or the AON to demonstrate the efficacy of the lidocaine block and to assess the specificity of the lidocaine application. Single unit activity was recorded from neurons in all layers of the MOB before, during and after lidocaine application.

Topical application of a small cotton ball containing 2% lidocaine significantly reduced the LOT-evoked field potential without affecting the field potentials recorded in the same location in the MOB evoked by olfactory nerve stimulation or AON stimulation. Topical application of lidocaine significantly and reversibly changed the spontaneous activity of neurons in all layers of the MOB. In the mitral cell layer, after the lidocaine blockade of the LOT, some cells decreased their rate of spontaneous activity, and some cells increased their activity. Similar results were obtained in the external plexiform layer, and the granule cell layer. Only a decrease in rate was observed in the glomerular cell layer. Topical application of lidocaine to the LOT had no significant effect on the coefficient of variation of the interspike intervals of spontaneous action potentials and no obvious effect on the entrainment of spontaneous action potentials with respiration.

Preliminary experiments were performed to examine the effects of blocking the AON. Injection of 2% lidocaine into the AON significantly reduced the olfactory nerve and LOT-evoked field potentials recorded in the AON without significantly affecting the field potential recorded in the same location in the MOB. Injection of lidocaine into the AON significantly decreased the spontaneous activity of neurons in the external plexiform layer and the mitral cell layer, no other cell layer was recorded from in these preliminary experiments.

In summary, the data presented in this thesis indicate that centrifugal fibers do indeed release neurotransmitters into the MOB in the steady state. This tonic release modifies the levels of spontaneous activity of bulbar neurons. These results help to elucidate the physiological role of centrifugal input to olfaction.