The Effect of Blood Perception on Decision-Making in the Leech

IntroductionThere are many factors that influence the behavioral response of an animal to a given stimulus. In the medicinal leech, Hirudo medicinalis, chemical stimuli such as blood are known to increase locomotor activity - swimming and crawling. Recently, the Brodfuehrer lab demonstrated that the perception of artificial blood in the leeches’ environment changes their behavioral responses elicited by electrical stimulation of the body wall. After foraging for artificial blood, the number of times that body wall stimulation leads to swimming significantly increases compared to control experiments. Here, we extend these studies to test whether artificial blood affects the ability of peripheral nerve stimulation to initiate swimming in a mouth-isolated cord preparation.

Materials and MethodsLeeches were dissected in cold saline, and a preparation consisting of the isolated nerve cord extending from the tail to the intact prostomial lip (mouth) was removed from the animal. The mouth and nerve cord were pinned ventral side up in a recording chamber and separated by a Vaseline bridge (Fig. 1). Suction electrodes were attached to dorsal posterior (DP) nerve 10 and DP(11). Each leech was subjected to three wash treatments over the mouth: pond water, either 10% or 100% artificial blood, and a second pond water wash, respectively. Following each wash, either DP(10) or DP(11) was stimulated ten times over the course of ten minutes. The electrical response in the DP nerve was recorded after each stimulation and the number of stimulus trials that led to swimming (percent swimming), as indicated by rhythmic bursts of spikes in the DP, and the number of cycles per swim episode (average swim length) were calculated.

Results

Fig. 2. Effect of 100 % Artificial Blood on % Swimming and Average Swim Length

Fig. 1. Ventral view of mouth-isolated cord preparation.

Fig. 3. Swim length following DP stimulation. Pond water 1 - 14 cycles, 100% artificial blood- 0 cycles, and pond water 2 - 25 cycles.

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Fig. 5. Swim length following DP stimulation. Pond water 1 - 12 cycles, 10% artificial blood- 16 cycles, and pond water 2 -24 cycles.

•The presence of artificial blood greatly altered the response of the leech to electrical stimulation of a peripheral nerve.

•Application of 10% artificial blood led to an overall enhancement of percent swimming and average swim of swim bursts that remained following its removal.

•Application of 100% artificial blood led to a dramatic, but transient decrease in percent swimming and average swim length. Both swim parameters substantially increased when 100% artificial blood was replaced by pond water.

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Ghazal Zekavat, Maureen Convery, and Peter Brodfuehrer,Department of Biology, Bryn Mawr College

Vaseline Bridge

Mouth Isolated ventral nerve cord

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Fig. 4. Effect of 10 % Artificial Blood on % Swimming and Average Swim Length

Conclusions

Application of 100% artificial blood decreased percent swimming compared to the initial pond water condition from 41.4% to 17.1%. Upon reapplication of pond water, percent swimming increased to 57.1%. The same trend was observed with respect to average swim length - 12.3 in pond water 1, 4.2 in 100% artificial blood, and 22.9 pond water 2 (Figs. 2 & 3). Conversely, in the trials in which 10% artificial blood followed initial pond water, percent swimming increased from 38.6% to 62.9%, and further increased upon reapplication of pond water to 80.0%. Average swim length also increased in each subsequent treatment - 8.2 in pond water 1, 13.9 in 10% artificial blood, and 13.9 pond water 2 (Figs. 4 & 5).

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