Abstract:
Odorant molecules stimulate olfactory receptor neurons, and axons of these neurons
project into the main olfactory bulb where they synapse onto mitral and tufted
cells. These project to the primary olfactory cortex including the anterior olfactory
nucleus (AON), the piriform cortex, amygdala, and the entorhinal cortex. The properties
of mitral cells have been investigated extensively, but how odor information
is processed in subsequent brain regions is less well known. In the present study, we
recorded the electrical activity of AON neurons in anesthetized rats. Most AON cells
fired in bursts of 2–10 spikes separated by very short intervals (<20 ms), in a period
linked to the respiratory rhythm. Simultaneous recordings from adjacent neurons
revealed that the rhythms of adjacent cells, while locked to the same underlying
rhythm, showed marked differences in phase. We studied the responses of AON cells
to brief high-frequency stimulation of the lateral olfactory tract, mimicking brief
activation of mitral cells by odor. In different cells, such stimuli evoked transient
or sustained bursts during stimulation or, more commonly, post-stimulation bursts
after inhibition during stimulation. This suggests that, in AON cells, phase shifts
occur as a result of post-inhibitory rebound firing, following inhibition by mitral
cell input, and we discuss how this supports processing of odor information in the
olfactory pathway. Cells were tested for their responsiveness to a social odor (the
bedding of a strange male) among other simple and complex odors tested. In total, 11
cells responded strongly and repeatedly to bedding odor, and these responses were
diverse, including excitation (transient or sustained), inhibition, and activation after
odor presentation, indicating that AON neurons respond not only to the type of complex odor but also to temporal features of odor application.
