Gastric mill activity in the lobster. II. Proctolin and octopamine initiate and modulate chewing.

1. The neuromodulators proctolin and octopamine were injected into the circulatory system of lobsters, and the subsequent reaction of their gastric mill was analyzed with the help of an endoscope. 2. Injections of proctolin into the dorsal heart sinus elicited chewing with period durations of 5-60 s after a latency of 53 +/- 42 s (n = 32 injections). The threshold dose was 1 ml of 1.5 X 10(-7) M, which results in an estimated concentration of 3 X 10(-9) M in the blood. 3. The effects of proctolin on the coordination of the three teeth of the gastric mill is dose dependent. Proctolin injections of 1 ml of 1.5 X 10(-6) M elicited chewing in the squeeze mode, 1 ml of 1.5 X 10(-4) M triggered chewing in the cut-and-grind mode. Both modes are different in terms of coordination and usage of functionally different parts of the teeth. 4. An increase in the proctolin dose causes an increase of the duty cycle (ratio of closing duration to period duration) of the chewing from 0.19 to 0.51. The corresponding period duration shortens (from 30.8 to 9.9 s) at intermediate doses, but lengthens to 16.6 s at high doses because the closing duration goes up. 5. Chewing following a single injection can last between 2 and 30 min. Besides more or less stereotypic chewing in one of the basic modes, variations occurred, such as chewing of just the lateral teeth, cycle-by-cycle switching between different modes, or double bites of either the lateral teeth or the medial tooth. 6. Proctolin increased the strength of reflex bites, which could be elicited by mechanical stimulation of the cardiac sac. 7. Octopamine elicited not only irregular chewing, but also other reactions such as struggling, only if high doses, between 1 ml of 1.5 X 10(-4) and 1.5 X 10(-3) M were given, which correspond to an estimated concentration in the blood of between 3 X 10(-6) and 3 X 10(-5) M. 8. The proctolin effects on the gastric mill match the spontaneously occurring behavioral repertoire of the gastric mill, and they are explainable with known properties of the gastric central pattern generator and its sensitivity to proctolin.