Download An octopaminergic system in the CNS of the snails, Lymnaea

The octopamine-containing buccal neurons represent a new group of interneurons in the feeding
network of the pond snail Lymnaea stagnalis
Ágnes Vehovszky and C.J.H. Elliott,
In the intact pond snail Lymnaea stagnalis, octopaminergic antagonists block the feeding
response to sucrose (Vehovszky et al 1998). The paired buccal ganglia contain 3 octopamineimmunoreactive neurons (OC cells) (Elekes et al 1996, Vehovszky et al 1998). To establish the role of
OC neurons in building up the feeding pattern, we studied the individual synaptic connections between
OC neurons and known members of the buccal network using conventional electrophysiological
methods.
All 3 OC cells are electrically coupled together and make the same synaptic effects on their
followers. They interact with all the known buccal feeding motoneurons, and with all the buccal
modulatory and central pattern generating interneurons.
N1 (Protraction) phase: Motoneurons firing in this phase of the feeding cycle receive either
single excitatory (depolarising) synaptic inputs (B1, B6 neurons) or a biphasic response
(hyperpolarisation followed by depolarisation) (B5, B7 motoneurons). Protraction phase feeding
interneurons receive the same biphasic synaptic input after OC stimulation. These interneurons
all inhibit the firing activity of the OC neurons.
N2 (Retraction) phase: These motoneurons (B2, B3, B9, B10) and N2 interneurons are
hyperpolarised by OC stimulation. N2 interneurons have a variable (probably polysynaptic)
effect on the activity of the OC neurons.
N3 (swallowing) phase: OC neurons are strongly coupled to both N3 phase (B4, B4cluster, B8)
motoneurons and N3p interneurons by electrical synaptic connections. Moreover the OC-N3p
connections have an additional, chemical component in both (OC>N3p, N3p>OC) directions of
the neuronal transmission. However, the synaptic connections formed by either OC neurons or
N3p interneurons are not identical, as they make different synaptic connections with both
motoneurons (B3) and feeding interneurons (N2).
CGC: The cerebral, serotonergic CGC neurons excite the OC cells, but the OC neurons have no
effect on the CGC activity.
In addition to these direct synaptic effects, OC neurons evoke longer-lasting changes in the activity of
feeding neurons: in a silent preparation OC stimulation may evoke feeding pattern, but when the
neurons are regularly firing, OC neurons decrease the rate of the fictive feeding.
These results suggest that the OC neurons form a sub-population of N3 phase feeding interneurons.
However, they differ from the N3p and N3t neurons described previously in morphology, and
physiology. The long lasting synaptic/modulatory effects of OC neurones on the other members of the
feeding network suggest that these neurons straddle the boundary between central pattern generator and
modulatory neurons. Thus OC interneurons are part of the cellular basis of the octopaminergic
modulation in the buccal feeding system.