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PHOTOINACTIVATION OF CARDIORESPIRATORY INTERNEURONS IN THE POND SNAIL, LYMNAEA-STAGNALIS
Abstract
Photoinactivation of cardio-respiratory inter-neurons in the pond snail, Lymnaea stagnalis
D.R.Skingsley and C.J.H.Elliott
School of Sciences (Biology), Staffordshire University, College Road, Stoke-on-rent ST42DE and Department of Biology, University of York, York Y015DD
In Lymnaea, the large interneurons RPeD1 and VWI are located in the pedal and visceral ganglia respectively. These cells have wide-ranging effects on a large number of respiratory and cardiac interneurons and motoneurons (Buckett et al.1990; Skingsley et al.1993). The RPeD1 interneuron inhibits the VWI while the VWI has a dual excitatory-inhibitory connection to the RPeD1. The synaptic connections of these cells are particularly interesting because Syed et al.(1990) suggested that they, together with a third interneuron, could account for the snail's respiratory rhythm and showed that the surgical ablation of VWI caused a severe reduction in respiratory behaviour (Syed et al.1992).
We have isolated the CNS from laboratory-reared Lymnaea and filled one or other of these interneurons with the fluorescent dye 5-(6)carboxyfluorescein (-1to-2-5nA for 30-120min). While recording from the other interneuron and an A-group or B-group follower motoneuron, the filled cell was irradiated with blue light from a helium-cadmium laser.
When the cell body of RPeD1 was irradiated, the cell gradually depolarized, firing action potentials tonically. Eventually, the membrane potential declined to less than 10mV (mean time to kill was 7+1-7(S.D.) min, n=3). During the burst of action potentials the follower cells showed transient synaptic inputs. If the cell body of the VWJ was irradiated, a similar pattern was seen, but instead of tonic action potentials the cell fired rhythmic bursts of gradually declining size before tonic action potentials were recorded (the mean time to kill was 3-2+1P3min, n=11).
The VWI has branches to the pedal ganglia through both thel eft and right connectives. Irradiating the left and right branches blocked all synaptic transmission from the VWI to RPeD1 but did not affect transmission from VWI to motoneurons in the parietal ganglia (n=3). However, if only one of the right or left branches were irradiated then the VWI to RPeD1 connection remained intact. In two preparations in which the RPeD1 was quiescent, irradiating the left connective changed the VWI to RPeD1 connection from a fast + slow biphasic connection to a fast connection only. Irradiating only the right axon changed the VWI's own membrane properties, so that it no longer was rhythmically active in response to tonic current injection, but fired tonically.
We conclude that our more precise lesions show that the cardio-respiratory network is not so simple as suggested by the cell culture and surgical ablations.
We would like to thank Andries ter Maat for the gift of some of the laboratory-reared snails.
REFERENCES
Buckett,K.J., Peters,M. & Benjamin,P.R.(1990). J.Neurophysiol. 63,1436-1447.
Skingsley,D.R., Bright,K., Santama,N., vanMinnen, J.,Brierley, M.J.,Burke, J.F.& Benjamin,P.R. (1993). J.Neurophysiol. 69,915-927.
Syed,N.I., Bulloch,A.G.M. & Lukowiak,K. (1990). Science 250,282-285.
Syed,N.I., Ridgway,R.L., Lukowiak,K. & Bulloch,A.G.M. (1992). Neuron 8,767-774.
https://physoc.onlinelibrary.wiley.com/doi/epdf/10.1113/jphysiol.1995.sp020641
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Most rhythmic behaviors such as respiration, locomotion, and feeding are under the control of networks of neurons in the central
nervous system known as central pattern generators (CPGs). The respiratory rhythm of the pond snail Lymnaea stagnalis is a
relatively simple, CPG-based behavior for which the underlying neural elements have been identified. A three-neuron network
capable of generating the respiratory rhythm of this air-breathing mollusk has been reconstructed in culture. The intrinsic
and network properties of this neural ensemble have been studied, and the mechanism of postinhibitory rebound excitation was
found to be important for the rhythm generation. This in vitro model system enables a better understanding of the neural basis
of rhythm generation.
The possibility that damaged neural circuitries can be repaired through grafting has raised questions regarding the cellular mechanisms required for functional integration of transplanted neurons. Invertebrate models offer the potential to examine such mechanisms at the resolution of single identified neurons within well-characterized neural networks. Here it is reported that a specific deficit in the respiratory behavior of a pulmonate mollusc, caused by the ablation of a solitary interneuron, can be restored by grafting an identical donor interneuron. The transplanted interneuron not only survives and extends neurites within the host nervous system, but under specific conditions forms synapses with appropriate target neurons and is physiologically integrated into the host's circuitry, thereby restoring normal behavior.