CENTRAL PATTERN GENERATOR INTERNEURONS ARE TARGETS FOR THE MODULATORYSEROTONERGIC CEREBRAL GIANT-CELLS IN THE FEEDING SYSTEM OF LYMNAEA

1. The objective of the experiments was to explore the medulatory func tions of the serotonergic cerebral giant cells (CGCs) of the Lymnaea f eeding system by examining their synaptic connections with the central pattern generator (CPG) interneurons and the modulatory slow oscillat or (SO) interneuron. 2. One type of modulatory function, ''gating,'' r equires that the CGCs fire tonically at a minimum of 7 spikes/min. Abo ve this minimum level the CGCs control the frequency of CPG interneuro n oscillation-''frequency control,'' a second type of modulation. In a n SO-driven fictive feeding rhythm, an increase in the frequency of th e rhythm, with increased CGC firing rate, resulted from a reduction in the duration of the N1 (protraction) and N2 (rasp) phases of the feed ing cycle with little effect on the N3 (swallow) phase. 3. The CGCs ex cited the N1 phase interneurons SO and N1M (N1 medial) cells but had n o consistent effects on the N1 lateral cells. The CGC --> SO postsynap tic response was probably monosynaptic (less than or equal to 200 ms i n duration) with unitary 1:1 excitatory postsynaptic potentials (EPSPs ) following each CGC spike. The CGC --> N1M excitatory response was sl ow and nonunitary, and a burst of CGC spikes evoked a depolarization o f the N1M cells that lasted up to 10 s and triggered N1M cell bursts. Both CGC --> SO and CGC --> N1M excitatory responses could be mimicked by the focal application of serotonin (5-HT). 4. Both CGC --> SO and CGC --> N1M excitatory connections systematically increased the N1M ce ll firing rate within the CGCs' physiological firing range (0-40 spike s/min). This was due to both the direct (CGC --> N1M) and indirect (CG C --> SO --> N1M) excitatory synaptic pathways. The CGC-induced increa se in N1M cell firing rate probably accounted for the reduced duration of the N1M cell feeding burst by causing a more rapid reversal of the feeding cycle from the N1 phase to the N2 phase. This phase reversal was due to the previously described recurrent inhibitory pathway (N1 - -> N2 excitation followed by N2 --> N2 inhibition). 5. The CGCs' abili ty to provide a depolarizing drive to the N1M cells meant that this ex citatory connection was also likely to be important for gating. 6. Act ivity in the CGCs produced nonunitary, long-lasting, excitatory postsy naptic responses on the N2 ventral (N2v) CPG interneurons, and these w ere likely to be involved in both the gating and the frequency control by the CGCs on the N2 phase of the feeding rhythm. Suppressing CGC to nic firing initially increased the duration of the N2v plateau (which determines the duration of the N2 phase of the feeding cycle, frequenc y function) but eventually led to a loss of N2v plateauing (gating fun ction). 7. Nonunitary, weakly inhibitory CGC --> N2 dorsal responses w ere recorded that could be mimicked by the application of 5-HT. 8. Spi kes in the CGCs evoked 1:1 monosynaptic EPSPs in the N3 tonic (N3t) CP G interneurons. This excitatory effect could be mimicked by the applic ation of 5-HT. Within the physiological range of CGC firing, this exci tation did not appear to influence the firing rate of the N3t cells. 9 . N3 phasic (N3p) CPG interneurons showed biphasic (hyperpolarizing fo llowed by depolarizing) unitary responses to spikes evoked in the CGCs . The inhibitory synaptic response was maintained in a high-Ca2+/high- Mg2+ (Hi-Di) saline and was mimicked by the focal application of 5-HT, indicating that it was probably monosynaptic. The excitatory componen t was, however, reduced in a Hi-Di saline, indicating that it was prob ably polysynaptic. Suppressing the CGCs during an SO-driven feeding rh ythm caused the N3p cells to fire less, suggesting that the removal of the excitatory component of the response might be significant.10. We conclude that the general depolarizing effects of the CGCs on a number of the CPG cell types may provide one explanation of the CGCs' abilit y to gate the feeding CPG. These excitatory responses fell into two cl asses. Either the recorded responses were unitary, lasting similar to- 200 ms (cf. SO and N3p and N3t cells), or the responses lasted much lo nger, similar to 10 s (cf. N1M and N2v cell types). Gating may also in volve affects on the endogenous properties of a subset of the CPG inte rneurons. These include the N1M cells (triggering bursts) and N2v cell s (necessary for plateauing). Several of the CGCs' effects on the CPG interneurons could explain their ability to control the frequency of a n SO-driven rhythm. First, by exciting the SO, N1M, and N2v cell types , they could control the duration of the N1 phase of the feeding cycle . Second, by influencing the duration of the N2v plateau they could de termine the duration of the N2 phase of the feeding cycle. 11. CGC sti mulation or the application of 5-HT causes responses on the CPG intern eurons that last a maximum of 10 s. However, the effects seen after su ppression of CGC activity take many minutes to have complete effect. I t is possible therefore that these prolonged effects may be due to som e other unknown effect of 5-HT, or to the presence of a cotransmitter.