C .J.H. Elliott, A. Vehovszky Brain Research 887 2000 63 –69
65
The effects of tonically stimulating the OC is much min. In the isolated CNS, the fictive feeding pattern was
smaller than the repetitive stimulation Fig. 1B. This monitored and each N2 rasp phase input on the SO
could be because the OC →
SO and OC →
N1L connections counted as 1 bite. The N2 inputs are the only central
are biphasic, with both inhibitory and excitatory com- pattern generator inputs that inhibit the SO interneuron
ponents [20]. In Fig. 1B, the inputs to the SO and N1L do strongly enough to stop it firing.
not lead to activation of the feeding pattern. Note that the firing rate of the OC in the tonic and repetitive stimuli
Fig. 1A and b are nearly the same 25–30 spikes s.
4. Results
3.2. Design of prepulse OC experiment 4.1. Prepulse activation of an OC interneuron modulates
the feeding system Since repetitive OC stimulation has quick outputs on its
followers in the feeding system Fig. 1A, we used a SO-driven fictive feeding is enhanced when it is pre-
prepulse experiment to separate these intra-cyclic effects ceded by OC prepulse stimuli. Fig. 2A shows a control
from the long-term, polycyclic modulation. experiment — in which the stimulus current into the SO
We compare the fictive feeding produced by steady protraction phase interneuron triggers SO action potentials
stimulation of the SO interneuron with the response to the and two inhibitory rasp N2 phase inputs, making the
same SO current after prepulse stimulation to the OC feeding rate 5.4 bites min. In the experimental panel Fig.
interneuron. Three aspects of the experiment are important. 2B, a set of prepulse stimuli to the OC interneuron for 20
First, we allowed 4 s between the end of the prepulse s frequency about 10 cycles min, is followed 4 s later by
stimulation of OC interneuron and the start of the injection the same depolarising current to the SO as in the control.
of depolarising current into the SO interneuron. This The feeding rate evoked by SO stimulation is much higher,
approach ensures that any intra-cyclic effect will have 17 bites min and the whole central pattern generator
decayed away because 4 s is the average time for a network is much more intensely activated, as indicated by
complete feeding cycle. the strong bursts in the OC interneuron. The third panel
Secondly, the current injected into the OC neuron during Fig. 2C is a second control, 10 min later, without OC
the prepulse was set between the level required to evoke stimulation. The feeding rate evoked by the SO is again
postsynaptic potentials onto the follower feeding inter- low 3.8 bites min, and no burst of activity is seen on OC
neurons and the level at which the PSPs summated to neuron.
produce spikes in the follower cells which activate the Our results from 26 trials in five preparations are
feeding rhythm. In the experiments analysed here, the summarised in Fig. 2D. This shows the increase in feeding
mean firing rate of the OC interneurons is less than in Fig. rate produced by stimulating the SO in the two situations:
1, with a mean of 17 Hz. This rate provides a suitable with and without OC prepulses. Before SO stimulation, the
stimulus, because the maximal-firing rate of OC inter- mean feeding rate was 2.060.37 bites min mean6S.E..
neurons we have seen in the isolated CNS is 15 Hz. When only the SO is stimulated, the fictive feeding rate
Furthermore, we only stimulate one of the OC interneurons more than doubles, increasing on average by 5.462.6
though the microelectrode. Although the other two OC bites min. However, if the OC stimulus precedes the SO
cells are electrically coupled, they are unlikely to fire stimulus the increase in fictive feeding is much bigger,
because the electrical coupling is not strong enough to 10.861.6 bites min. This difference is significant at the
evoke action potentials on any of the coupled neurons [21]. 2 level paired t-test, N55.
Thirdly, we used a small current to depolarize the SO, The histogram in Fig. 3 shows the time course of the
so that the feeding rhythm is not activated too strongly in changes in fictive feeding rate before, during and after SO
the control traces. Because the SO was less depolarized stimulation. The shaded and open bars show the data with
than in many previous publications, e.g. [5,16,23], the N2 and without OC stimulation, respectively. Although the
and N3 inputs seen in the SO, N1L and OC interneurons OC stimulation ends 3.861.7 s before the SO stimulation
appear smaller. begins, the main increase in fictive feeding rate takes place
Taking these aspects together, we can be sure that any during the first 12 s of SO stimulation Fig. 3. The fastest
change in the SO driven fictive feeding is due to the feeding rate recorded following the OC prepulse is 20
prepulse activation of the OC interneuron, prior to SO bites min and this occurs 6–12 s after the start of SO
stimulation. depolarization i.e. 10–16 s after the end of the OC
Twenty-six experiments 14 with, 12 without the OC prepulse stimuli. In the period before SO stimulation, the
stimuli, from five preparations were analysed. For statisti- fictive feeding rate is not significantly different between
cal tests, the control and OC prepulse feeding rate from control and OC prepulse trials 2.360.55 and 1.860.5
each preparation were averaged, and the means compared bites min, respectively.
using a paired t-test. Trials were separated by at least 3 Fig. 4 shows results from a similar experiment on
66 C
Fig. 2. OC prepulse protocol enhances SO driven fictive feeding A Initial control, injection of depolarising current into the SO interneuron evokes a weak fictive feeding pattern, with just cycles as shown by the two inhibitory N2, rasp inputs asterisks. B When the OC interneuron is stimulated with four
prepulses, and 4 s later the same current is injected into the SO, the SO evokes a faster and stronger feeding pattern, with five feeding cycles N2 inputs marked by asterisks and the recruitment of the OC interneuron into the pattern. C A repeat control after 10 min showing that the SO alone with the same
current cannot activate the feeding system. D Summary of 24 experiments. SO stimulation increases the feeding rate by 5.4 bites min, but when it is preceded by OC stimulation, the SO induced increase is 10.8 bites min. The bars represent the standard error. The difference is significant at the 2 level
paired t-test on five preparations.
another preparation. This time the stimulus current to the OC and SO interneurons is not required for the long-term
SO was set to a level in which the SO fired at a rate too modulatory effect of the OC interneurons.
slow to produce rhythmic feeding activity in any of the neurons recorded Fig. 4A. After the OC prepulse with
4.2. Specificity of the modulation to the OC interneuron parameters similar to those in the previous experiment 12
cycles min, mean pulse duration 1.9 s, SO stimulation We also tested to see if the modulation is specific to the
produces a rhythmic pattern; the inhibitory inputs to the octopamine containing OC interneurons, or if it can be
SO are clearly visible Fig. 4C. This indicates that the OC evoked by the electrically coupled N3p interneurons,
prepulse has enhanced the ability of the SO to generate which fire in the same phase of the feeding cycle. In the
fictive feeding, even though the OC interneuron does not controls, weak current injection into the SO interneuron
fire during the SO stimulus unlike Fig. 2B. In 20 out of does not trigger the feeding pattern Fig. 4A and D, while
the 26 experiments the OC interneuron did not fire during prepulse stimulation of OC neurons enables the SO to
the SO depolarization, so that synchronous activity of the produce a slow feeding pattern Fig. 4C. However, the
C .J.H. Elliott, A. Vehovszky Brain Research 887 2000 63 –69
67
Fig. 3. A histogram showing the time course of the effect of the OC prestimulus on SO driven fictive feeding. Summary of 26 trials 14 with OC, 12 without in five preparations. Each bar shows the mean feeding rate in each 6-s period the minimum time for two feeding cycles with time measured
relative to the start of SO stimulation. The open bars represent control trials, with no OC stimulation, while the filled bars indicate the trials in which the OC prepulse stimulation was applied. The dotted line under the graph represents the average time of OC stimulation, while the solid bar shows the average
period of SO stimuli. The OC stimulation ended 3.7 s before SO stimulation. During the SO depolarization, the trials with an OC prepulse gave a significantly higher feeding rate, but there is no significant difference in feeding rate beforehand.
facilitatory modulation of OC neuron cannot be replicated presence of octopamine facilitates the SO-activation of
by stimulating the N3p interneurons in the same way. In feeding pattern.
Fig. 4B, the same prepulse protocol is used, but now stimulating the N3p interneuron instead of the OC inter-
neuron before depolarizing the SO interneuron. The
5. Discussion
