Octopamine enhances neuromuscular transmission in developing and adult moths, Manduca sexta

The effect of octopamine on neuromuscular transmission was examined in developing and adult Manduca sexta. Intracellular recordings were made from the dorsal longitudinal muscle (DLM), superfused with solutions containing DL‐octopamine or other amines. In untreated adult moths and pharate adults nearly ready to eclose (stage Day 19), stimulation of the motor nerve evokes a large excitatory junction potential (EJP), an active membrane respons, and a twitch. In adults and Day 19 animals DL‐octopamine (10−7 to 10−4 M) has no effect on the amplitude and rise‐time of the electrical response in normal saline, but 10−6 to 10−4 M DL‐octopamine increases the amplitude of the excitatory junction potential recorded in saline containing one‐third the normal calcium concentration. Immature (Day 16) muscle, which normally produces only small EJPs following stimulation of its motor nerve, responds to 10−6 to 10−4 M DL‐octopamine by an increase in the EJP above threshold for an active membrane response and a contraction. When the muscle has developed sufficiently to spike and contract in response to nerve stimulation in the absence of exogenous octopamine (Days 17 and 18), application of DL‐octopamine increases the maximum rate at which the muscle contracts in response to each stimulus in a train (designated the maximum following frequency, MFF). The threshold dose for an effect on the MFF of Day 18 immature moths is less than 10−10 M. At this stage 10−8 M DL‐octopamine increases the MFF four‐fold. The effect on the MFF is dose‐dependent over the range 10−10 M to 10−6 M. The biogenic amines DL‐epinephrine, DL‐norepinephrine, tyramine, DL‐phenylethanolamine, 2‐phenylethylamine, and dopamine, applied at concentrations of 10−8 or 10−4 M, do not change the MFF. Both DL‐synephrine (10−8 M) and serotonin (10−7 M) mimic the action of 10−10 M DL‐octopamine on the MFF. The action of DL‐octopamine (10−7 M) is blocked by phentolamine (10−4 M) but not by propranolol (10−4 M). The octopamine content of hemolymph was determined with a radioenzymtic assay. The concentration of octopamine in the hemolymph increases 3.6‐fold, from 5 × l0−8 M on Day 18 (duration of adult development is 19 days) to 1.85 × 10−7M one day following eclosion. The concentration of octopamine in the hemolymph on Days 14 and 16 is not significantly different from that on Day 18. This concentration is sufficient to increase the MFF of Day 18 preparations. These data provide evidence for a modulatory role of octopam