Injected just below first swimmeret(s) with water. Subject was anesthetized using ice bath. Recovered nicely after about 30 minutes.
However, once an animal has been dominant long enough for its response to serotonin to take on the typical dominant profile, this profile is retained even if the animal later becomes subordinate (Yeh et al., 1997). Thus, some experience-caused changes in neuromodulation are readily reversible, but others are not.
Social dependence of 5-HT effectsThere is yet another, and rather extraordinary layer of complexity to this story. Crayfish have long been known to form social hierarchies (Bovbjerg, 1953; Lowe, 1956). When two crayfish are brought together, one generally becomes dominant and the other subordinate after a short period of interaction (see below). Quite remarkably, social experience alters the effects of serotonin on transmission to the LGs: Whereas in social isolates low concentrations of serotonin facilitate transmission to the LGs (as discussed above), after a crayfish has lived for 1–2 wk as a subordinate, serotonin comes to inhibit transmission to the LGs (Fig. 6; Yeh et al., 1996, 1997). This is not the depolarizing, chloride conductance-increasing type of inhibition produced by high 5-HT, but a hyperpolarizing, presumably potassium conductance-increasing inhibition. Thus, whereas in isolates low concentrations of 5-HT decrease potassium ion conductance, in subordinates serotonin increases potassium ion conductance—a directly opposite effect (Fig. 2, line 8). Living as a dominant causes a more subtle change: Whereas in isolates prolonged exposure to serotonin causes facilitation that persists after washout of serotonin, dominants do not show this persistence of facilitation.
However, once an animal has been dominant long enough for its response to serotonin to take on the typical dominant profile, this profile is retained even if the animal later becomes subordinate (Yeh et al., 1997). Thus, some experience-caused changes in neuromodulation are readily reversible, but others are not. Each response type is associated with a giant command neuron. The medial giants (MGs) sum input from anterior sensory channels and make output connections with giant flexor motor neurons (motor “giants”—MoGs in Fig. 1) that cause a dart backwards when the excitation produces even one spike in the MGs. The lateral giants (LGs) sum input from posterior channels and cause an upward rotation that distances the hind end of the animal from the disturbing stimulus. MG and LG responses are often referred to as “reflex” responses or as “giant fiber (GF)” responses for the giant axons of the MGs and LGs, which run the length of the nerve cord. GF responses are very prompt (muscle potentials can begin within 3 msec) and are good at getting the crayfish moving away from the source of stimulation rapidly.
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