Behavioral changes induced by GABA-receptor agonists in Lymnaea stagnalis L

1. GABA, the GABAA receptor agonist, muscimol, and the GABAB receptor agonist, baclofen, were tested to study the involvement of the GABA neurotransmitter system in control of behaviour in the freshwater pulmonate snail, Lymnaea stagnalis L. Single injections of GABA (1-10 micrograms/ gbw) into the haemocoel of intact snails elicits a sequence of behavioural changes subsequently affecting feeding, locomotion, escape reactions, male mating behaviour and respiration. 2. Both muscimol and baclofen mimic distinct aspects of GABA action implying that the GABA action is mediated by both types of receptors. 3. The modulatory actions of GABA, muscimol, and baclofen on feeding were manifested as opening of the mouth and triggering of specific radular movements (e.g., protraction, retraction, rasping). 4. Baclofen (1-10 micrograms/gbw) evoked the full erection of the penis for a time long enough for natural copulation, while GABA itself caused only partial eversion of the preputium. Muscimol was less effective than GABA. The latency to penis eversion varied in a dose-dependent manner. The data emphasize the participation of baclofen-sensitive receptors in control of reproduction. 5. GABA and baclofen induced withdrawal of the head and anterior part of the foot accompanied by arrest of locomotion and respiration could be considered to be an escape reaction. 6. Muscimol (1-5 micrograms/gbw) failed to elicit withdrawal reactions but caused the loss of normal orientation and longitudinal contractions of the foot. 7. All the agents tested inhibited locomotion in a dose-dependent manner, for a substantial period of time. A decrease in total locomotor activity lead to the failure of animals to attach to the underlying surface, to have foot contractions and also to diminish locomotor velocity. 8. The majority of animals maintained the stereotyped complex reactions of respiration, but with restricted clockwise turning of the shell after drug treatment. 9. All the substances were shown to shift the background behavioural state characterised by high BSSs to a state similar to "rest" which was awarded a low BSS. It is concluded that the GABA neurotransmitter system can play an important role in activation and coordination of neuronal ensembles underlying behaviour and behaviour selection in Lymnaea stagnalis L.