Reversal of age-related learning deficiency by the vertebrate PACAP and IGF-1 in a novel invertebrate model of aging: the pond snail (Lymnaea stagnalis)

Abstract

With the increase of life span, nonpathological age-related memory decline is affecting an increasing number of people. However, there is evidence that age-associated memory impairment only suspends, rather than irreversibly extinguishes, the intrinsic capacity of the aging nervous system for plasticity (1). Here, using a molluscan model system, we show that the age-related decline in memory performance can be reversed by administration of the pituitary adenylate cyclase activating polypeptide (PACAP). Our earlier findings showed that a homolog of the vertebrate PACAP38 and its receptors exist in the pond snail (Lymnaea stagnalis) brain (2), and it is both necessary and instructive for memory formation after reward conditioning in young animals (3). Here we show that exogenous PACAP38 boosts memory formation in aged Lymnaea, where endogenous PACAP38 levels are low in the brain. Treatment with insulin-like growth factor-1, which in vertebrates was shown to transactivate PACAP type I (PAC1) receptors (4) also boosts memory formation in aged pond snails. Due to the evolutionarily conserved nature of these polypeptides and their established role in memory and synaptic plasticity, there is a very high probability that they could also act as "memory rejuvenating" agents in humans.

Keywords: Learning; Memory.; PACAP.

Figure 1.

Age-related impairment of both intermediate-term memory (ITM) and long-term memory (LTM) in Lymnaea. Time line of experiment is shown above the graph. ITM was tested 1 hour, whereas LTM was tested 24 hours, after single-trial food-reward classical conditioning. Means ± SEM are shown. Asterisks indicate statistical significance.

Figure 2.

Endogenous pituitary adenylate cyclase activating polypeptide (PACAP) is required for both ITM and LTM in young Lymnaea. Time line of experiment is shown above the graph. Injection with the PAC1 antagonist PACAP6-38 (anti-PACAP) 60 minutes before single-trial food-reward classical conditioning caused significant (asterisks) impairment of both ITM and LTM. Means ± SEM are shown.

Figure 3.

Exogenous PACAP reverses memory impairment in aged Lymnaea. Time line of experiment is shown above the graph. Injection with synthetic PACAP38 rescued both ITM and LTM from age-related impairment. Means ± SEM are shown. Asterisks indicate statistical significance.

Figure 4.

Age-related reduction of endogenous PACAP levels in the “learning ganglia” of Lymnaea. (A) Western blot bands of synthetic PACAP38 and endogenous Lymnaea PACAP at approximately 9kDa. In both young and aged Lymnaea, endogenous actin (~40kDa) was also assayed in the same samples that were used for PACAP detection. The level of endogenous PACAP was below detection threshold in the sample from aged snail brains shown here. (B) Endogenous PACAP expression is significantly (asterisk) lower in aged versus young brain samples. Means ± SEM are shown.

Figure 5.

Exogenous insulin-like growth factor-1 reverses memory impairment in aged Lymnaea. Time line of experiment is shown above the graph. Similar to PACAP38 (see Figure 3), injection with synthetic insulin-like growth factor-1 rescued LTM from age-related impairment. ITM was not tested in this experiment. For comparison, the conditioned feeding response in vehicle-injected young animals in the same experiment at 24 hours after single-trial classical conditioning is also shown. Means ± SEM are shown. Asterisks indicate statistical significance.