The molluscan neuropeptide, small cardioactive peptide B (SCPB; Lloyd, 1978; Morris et al. 1982), has been shown to have excitatory effects on both central neurones and peripheral effectors associated with feeding behaviour in gastropod molluscs (Krajniak et al. 1985; Willows & Lloyd, 1983; Murphy et al. 1985; see Lloyd, 1988; Prior & Welsford, 1988, for reviews).
In the terrestrial slug, Limax maximus, application of exogenous SCPB to an isolated CNS preparation can increase the responsiveness of the neuronal network underlying feeding, the feeding motor programme (Gelperin et al. 1978; Prior & Watson, 1988). Furthermore, SCPB-like immunoreactive material has been localized to several neurones within the buccal ganglia of Limax (Prior & Watson, 1988). Activation of one of these neurones (Bl) has been shown to mimic the excitatory effects of exogenous SCPB on other central neurones (Prior & Delaney, 1986; Prior & Welsford, 1988) and on peripheral effectors (Welsford & Prior, 1987; Prior & Welsford, 1988). Although these observations suggest an involvement of SCPB in the control of feeding in Limax, they are derived from in vitro experiments on isolated CNS preparations.
In intact Limax, feeding involves a stereotyped behavioural sequence which includes: (1) cessation of locomotion in the presence of food; (2) partial retraction of the superior tentacles; (3) eversion of the lips; (4) initiation of rhythmic lip movements; and (5) cyclical protraction and retraction of the radula against the food (see Gelperin et al. 1978).
The only effects of SCPB reported have been from in vitro preparations. We have examined the effects of injection of SCPB on the behaviour of intact slugs, and report that SCPB can elicit specific behavioural responses that are associated with feeding.
Specimens of Limax maximus (0·5–1·5g) were kept at 18°C in natural L:D conditions in large Petri plates and were fed rat chow ad libitum. Prior to experimentation, animals were fasted for 2–3 days. The fasted animals exhibited no feeding-related responses when presented with food. Control slugs received 25/rl injections of saline into the posterior haemocoel, whereas the test slugs received injections of SCPB (Peninsula) prepared in slug saline (Prior & Gelperin, 1977). Peptide injections were adjusted so that final haemolymph concentrations could be reported (see Prior et al. 1983). Observations were made between 10.00 h and 13.00 h or 14.00 h and 17.00 h and the order of treatments was randomized for each animal within each trial so that each slug was injected with each concentration of SCPB and the control saline. The injections were counterbalanced over the two observation periods. Over the course of the experiments, each slug received a total of five injections, but an individual animal received only one injection per day. No significant difference in the behavioural responses of slugs was observed between the two observation periods or between different orders of injections. Observations were made by viewing the ventral side of the slugs through a clear Petri plate with the aid of an illuminated magnifying glass.
Preliminary studies indicated that feeding-related behaviour could be initiated by injection of SCPB, but that the response was observed only within the 10 min following injection. Thus, in the present experiments, behavioural observations were limited to this 10 min period. The following responses were recorded: (1) the presence or absence of locomotor activity; (2) withdrawal of the superior tentacles (to at least half their fully extended length); (3) lip eversion (for a minimum of 45 s); (4) rhythmic lip movement (for a minimum of 45 s); (5) cyclical protraction and retraction of the radula. These data were analysed for significance using a nonparametric analogue to a two-way ANOVA, Friedman’s test (Friedman, 1937), followed by a non-parametric multiple comparisons procedure (Zar, 1974). Since Friedman’s test made comparisons between the overall behavioural scores of each treatment group, comparisons of the expression of individual responses (e.g. tentacular retraction) between groups was not possible in this analysis. In all comparisons, a probability of 0–05 was considered significant.
Animals injected with 10−5moll-1 SCPB (final haemolymph concentration) displayed: (1) reduced locomotor activity compared with controls; (2) retraction of the superior tentacles; (3) lip eversion; and (4) rhythmic lip movements (Table 1). Thus, when exposed to 10−5moll-1 SCPB, slugs displayed the initial sequence of responses known to be associated with feeding. However, cyclical radular movements were not observed. It is important to note that these responses were elicited even in the absence of food stimuli. Thus, SCPB alone is capable of initiating the sequence of responses that are normally initiated by chemosensory cues.
In an effort to determine a threshold concentration for the effect of SCPB on intact Limax, animals were injected with either 10−6 or 10−7moll-1 SCPB. Injection of 10−6moll-1 SCPB caused increases in tentacular retraction, lip eversion and lip movements but no decrease in locomotion when compared with controls (Table 1). Although injection of 10−7moll-1 SCPB resulted in a slight increase in both tentacular retraction and lip eversion, there was no observable effect on locomotion or lip movement (Table 1). Highly significant differences existed between the experimental groups in the expression of feeding-related responses (X20·5, 3 = 49·35; P < 0·005) (Table 1).
Since SCPB is known to increase the responsiveness of the feeding motor programme in Limax, we tested the possibility that, together with exposure to an appropriate food stimulus, injections of SCPB might initiate the entire feeding sequence, including cyclical radular movements. Slugs that had been injected with either 10−6 or 10−5moll-1 SCPB(N = 12) moved towards the food (5 cm), ceased locomotion, withdrew their tentacles, everted their lips and moved them rhythmically. None of these animals, however, initiated cyclical radular movements. Control animals, which had been injected with saline, exhibited no feeding-related responses.
As an initial test of the specificity of SCPB in initiating feeding-related behaviour, we examined the effects of injections of SCPA, a neuropeptide with close sequence homology to SCPB. Injections of 10−6 or IO-5 moll-1 SCPA had only slight behavioural effects, causing lip eversion and lip movements in only 25 % of the trials. Thus, in Limax, as in other molluscan systems, SCPA has effects similar to those of SCPB, albeit less marked (see Lloyd, 1988).
The present observations demonstrate that, regardless of the presence of food, SCPB can elicit a sequence of feeding-related responses in Limax that include: (1) decrease in locomotor activity; (2) partial retraction of the superior tentacles; (3) lip eversion; and (4) rhythmic lip movements. That cyclical radular movements were not observed is not totally unexpected and is consistent with the observation that SCPB does not directly elicit a feeding motor programme in Limax. Rather, SCPB has been shown to increase the responsiveness of the feeding motor programme to chemosensory stimulation (Prior & Watson, 1988).
The expression of a behaviour as complex as feeding must require the interaction of multiple modulatory systems (e.g. opiates, Kavaliers et al. 1984, 1986; FMRFamide, Cooke et al. 1985; dopamine, Weiland & Gelperin, 1983). The present results are particularly striking in that a single neuropeptide is capable of eliciting an entire series of feeding-related responses.
The apparent threshold concentration of SCPB was 10−6moll-1, but the actual haemolymph concentration of SCPB resulting from injections has yet to be measured. If there is degradation or non-specific binding of the injected SCPB, the haemolymph concentration may be lower than calculated. Consistent with this is the observation that the behavioural effects of SCPB were observed only in the initial 10min following injection, whereas the effects of SCPB on isolated CNS preparations persist for up to 20–30 min (Prior & Watson, 1988).
This study has shown that, in intact Limax, a sequence of feeding-related responses can be initiated by SCPB. These results, together with those from isolated CNS preparations, suggest that an SCPB-like peptidergic system may be involved in the control of feeding behaviour in Limax.
This research was supported in part by a grant from the Whitehall foundation to DJP and a Summer Research Fellowship awarded by The University of Kentucky to IGW. KAS is a recipient of the Funkhouser Research Award at the University of Kentucky and PAB is the recipient of the Bayless Research Award from Northern Arizona University. This is contribution no. 282 from the Tallahassee, Sopchoppy and Gulf Coast Marine Biological Association.