Nitric Oxide Signals ThatAplysiaHave Attempted to Eat, a Necessary Component of Memory Formation After Learning That Food Is Inedible

Abstract

To gain insight into the function of NO transmission during learning we tested whether blocking NO synthesis affects aspects of feeding that are expressed both in a nonlearning context and during learning. Inhibiting NO synthesis with L-NAME and blocking guanylyl cyclase with methylene blue decreased the efficacy of ad libitum feeding. D-NAME had no effect. L-NAME also decreased rejection responses frequency, but did not affect rejection amplitude. The effect of L-NAME was explained by a decreased signaling that efforts to swallow are not successful, leading to a decreased rejection rate, and a decreased ability to reposition and subsequently consume food in ad libitum feeding. Signaling that animals have made an effort to swallow is a critical component of learning that food is inedible. Stimulation of the lips with food alone did not produce memory, but stimulation combined with the NO donor SNAP did produce memory. Exogenous NO at a concentration causing memory also excited a key neuron responding to NO, the MCC. Block of the cGMP secondmessenger cascade during training by methylene blue also blocked memory formation after learning. Our data indicate that memory arises from the contingency of three events during learning that food is inedible. One of the events is efforts to swallow, which are signaled by NO by cGMP.

Nitric oxide and histamine signal attempts to swallow: A component of learning that food is inedible inAplysia

“…It was therefore hypothesized that the role of NO in learning that food is inedible is signaling efforts to swallow (Katzoff et al 2006). However, NO is now known to also affect additional aspects of Aplysia feeding.…”

“…When lip stimulation is paired with injection of a NO donor, animals learn that food is inedible, indicating that NO substitutes for attempts to swallow (Katzoff et al 2006). However, memory arising from pairing a NO donor and stimulating the lips with food could be caused by NO initiating egg-laying, perhaps because egg-laying causes inhibition of feeding (Stuart and Strumwasser 1980;Ram 1982Ram , 1983Teyke et al 1991), and lip stimuli become associated with inhibition of feeding.…”

“…Additional operant-like training paradigms also affect Aplysia feeding Susswein et al 1986;Schwarz et al 1988Schwarz et al , 1991Chiel and Susswein 1993;Botzer et al 1998;Katzoff et al 2002Katzoff et al , 2006Cohen-Armon et al 2004;Lyons et al 2005). In these paradigms Aplysia are exposed to a palatable food that is difficult or impossible to consume.…”

“…In contrast, successful consumption of a difficult food causes an increase in subsequent feeding responses ). Previous studies provided evidence that learning with inedible foods requires the contingency during training of three separable events: (1) chemical and mechanical stimuli signaling the food touching the lips (Schwarz et al 1988); (2) stimuli signaling attempts to consume the food (Katzoff et al 2006); (3) stimuli from the gut sensing that the attempts to consume the food have failed .…”

“…It was hypothesized that NO is required for learning that food is inedible because its release signals attempts to swallow, the second of the three contingent stimuli required for memory formation. Data that support this hypothesis include: (1) exogenous NO substituted for attempts to swallow during training with inedible food (Katzoff et al 2006);and (2) blocking NO transmission made ad libitum feeding less efficient and slowed rejection responses, presumably because animals are unable to monitor their efforts to swallow (Katzoff et al 2006). Other than affecting rejection, neither blocking NO nor treatment with a NO donor was found to affect feeding or other behaviors (Katzoff et al 2002(Katzoff et al , 2006.…”