Landmark discoveries made in the olfactory bulb have formed the basis of
much of our understanding of other brain regions. In fact, it was in the olfactory bulb
that the first example of a dendrodentritic synapse in the mammalian central nervous
system was found. The olfactory bulb is rich in a diverse selection of
neurotransmitters, and with the bonus that the bulb is relatively easy to access, it
provides an excellent model in which to study neural networks.The aim of this PhD project was to study the neural pathway that is thought to
connect the olfactory bulb to the supraoptic nucleus of the hypothalamus. To
understand the input to the supraoptic nucleus from the olfactory system we sought
to determine the discharge frequency and firing pattern of the output neurones, mitral
cells. The electrical activity of single neurones was recorded extracellularly from the
olfactory bulb of anaesthetised rats. Mitral cells were identified electrophysiologically by antidromic activation following stimulation of the lateral olfactory tract
and observation of bulbar field potentials. It became apparent that the mitral cells
consistently showed a spontaneous patterned discharge that has not been previously
reported and we have described this pattern in terms of three separate levels of
bursting behaviour.What we have termed the 'gross' phasic pattern was displayed by all mitral cells
and consisted of a characteristic slow, cyclic firing pattern with peaks of activity
occurring with a constant periodicity, burst lengths lasted for approximately two
minutes with equal periods of quiescence separating the bursts. Some mitral cells
(53 %) show distinct silent periods between bursts of high activity others (47 %)
simply show a reduced rate of activity between bursts. Auto -correlation plots show
that within this overall phasic pattern is a respiratory driven bursting activity, the
activity of mitral cells increases during the inspiratory phase as air is drawn over the
olfactory receptors in the nasal mucosa. Plotting the instantaneous frequency of
mitral cell activity reveals the third bursting pattern, exhibited by 57% of mitral cells
recorded. This shows that during each long burst of activity the mitral cell fires at
two distinct frequencies, the lower frequency is in the range 0 -50Hz and the high
frequency firing is in the range 100- 250Hz. In 84% of the bursts that showed two
distinct firing frequencies there was a delay in the onset of the higher frequency
mode, at the start of each peak of activity. Mitral cells have been shown to be
capable of initiating and propagating action potentials from their distal dendrites, as
well as from the conventional initiation site at the soma -axon hillock region. It is
proposed in this thesis that the high frequency firing mode described might be
generated in the mitral cell dendrites.The mitral cell is involved in complex interactions with both neighbouring
mitral cells and granule cells that provide for lateral and reciprocal inhibition
respectively. Granule cells are the most numerous of the various types of
interneurone in the bulb and their firing pattern was found to be non-phasic and at
only one frequency mode. Following stimulation of the lateral olfactory tract mitral
cells exhibited a period of inhibition following the stimulus pulse. This is consistent
with the general consensus that upon activation, mitral cells activate granule cells,
which in turn feedback to inhibit the mitral cells (reciprocal inhibition). Extracellular
recordings of mitral cell activity were also made in a slice preparation of the
olfactory bulb. It was discovered that in vitro the mitral cells did not discharge in a
slow, phasic pattern and the high frequency firing seen in vivo was not evident.
During the slice preparation many of the long lateral dendrites of the mitral cells are
unavoidably removed and this may disturb the local interactions and thereby alter the
discharge pattern.Once the discharge pattern of olfactory neurones was determined these
parameters were then used as a basis for the stimulation of the lateral olfactory tract
and the effect on supraoptic neurone activity determined by studying the distribution
of Fos -positive cells. Two stimulation protocols were used both were strong stimuli
applied unilaterally, in different formats. The first was a short burst at a high
frequency to mimic an acute, strong output from the olfactory bulb and the second
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was a prolonged stimulation used to disrupt the output discharge pattern. The
literature suggests that the connection between the olfactory bulb and the supraoptic
nucleus is unilateral, monosynaptic and terminates in the ventro -lateral dendritic
region of the supraoptic nucleus. Following prolonged stimulation of the lateral
olfactory tract there was a significant increase (p <0.01) in Fos expression in the
supraoptic nucleus on both the ipsilateral and contralateral sides which suggests that
the pathway between the olfactory bulb and the supraoptic nucleus may be more
complex than initially thought. Areas of the brain known to receive strong olfactory
input, such as the piriform cortex, showed a unilateral increase in Fos expression
following the brief pulse of stimulation.Administration of morphine during parturition interrupts the progress of
parturition by inhibiting oxytocin release. The olfactory bulb is highly active at the
time of parturition and shows dense expression of mu- and kappa opioid receptors,
and so the possibility that morphine may impair oxytocin release in part by blocking
the input from the olfactory bulb was considered. The effect of morphine and its
antagonist, naloxone on the discharge pattern of mitral cells was studied in both the
in vivo and in vitro preparations. In vivo morphine was seen to have a subtle effect in
that it inhibited the high frequency firing but did not significantly alter the overall
firing rate or periodicity of bursts, this effect was irreversible. However, in vitro
morphine fully inhibited mitral cell activity which returned to pre -morphine rates
following the administration of naloxone. The discrepancy between the two sets of
data may be a dose issue. In vivo the drugs were administered via an intravenous
route which may have led to a reduced concentration of the drug evoking a response
from the mitral cells compared to the concentration of the drug that the mitral cells in
vitro were exposed to. It may also be due to the reduced local circuitary of the mitral
cell in the in vitro preparation, causing the mitral cell to become more susceptible to
the effects of morphine
