Motor neuronal function in old Aplysia is improved by long-term stimulation of the siphon/gill reflex

As Aplysia age, motor neuronal (L7) elicited gill-pinnule contractions are significantly decreased, as is transmission at pinnule junctions. To determine whether this reduced function of L7 with increased age could be altered, the siphon/gill reflex, which involves the L7-pinnule pathway, was stimulated regularly in unrestrained old animals. Aplysia, more than 240 days old, were assigned randomly into trained and untrained groups. For more than 3 weeks, a 1-s, 25-g water jet stimulus was administered to the siphon of the trained animals 10 times per day at 20-min intervals. The duration of siphon withdrawal increased significantly during training. In semi-intact preparations, pinnule contractions and junctional transmissions were then measured during 3-s depolarized pulses to L7 (frequency range = 1-44 per 3-s interval). The trained animals had (a) significantly higher pinnule contractions, (b) a significantly greater increase in pinnule contractions elicited by increasing L7 spike rates, (c) significantly higher double-spike facilitation, and (d) significantly higher facilitation per spike across the four spike trains below or at pinnule contraction threshold. Long-term stimulation of the siphon/gill reflex in old Aplysia did not produce the same functional efficiency observed in mature Aplysia; nevertheless, the significant training-induced increases in both pinnule contraction and junction transmission indicate that a considerable level of plasticity still exists in an aging nervous system. Whether this long-term training of Aplysia retards the same processes responsible for the age-related decline in neuromuscular transmission or produces a compensatory change in other neuronal processes is discussed.