Role of the protein kinase G in the behavioral and neuronal plasticity induced by aversive stimuli in the marine invertebrate Aplysia
Authors: Chatterji Ruma , Wainwright Marcy , Mozzachiodi Riccardo 
Published: November 2023

The ability to make decisions when in a state of trepidation is a crucial component of survival for many organisms. While there have been studies regarding adaptations to fear, what is less achievable is to investigate these behavioral adaptations and their underlying cellular mechanisms in a parallel fashion. Therefore, the project goal is to examine both behavioral and neuronal plasticity using the marine mollusk, Aplysia. In Aplysia, exposure to aversive stimuli causes a learned enhancement of a defensive response (sensitization) in the tail‐siphon withdrawal reflex (TSWR), and the suppression of a non‐defensive behavior (i.e., feeding). Correspondingly, at the cellular level, exposure to the aversive stimuli causes an increase in excitability of tail sensory neurons (SNs) responsible for TSWR and a decrease in excitability of neuron B51, controlling feeding. The Cyclic Guanosine Monophosphate (cGMP) pathway dependent on protein kinase (PKG) is involved in learning‐dependent behavioral and neuronal plasticity associated with defensive and non‐defensive responses in Aplysia. The specific project aim is to investigate the role of PKG in sensitization and suppression of feeding induced in Aplysia by a training protocol consisting of aversive electrical stimuli, both in in vivo and in in vitro experiments. The selective inhibitor KT5823 was used to block PKG activity. For the behavioral component, animals were injected in vivo with either KT5823 or vehicle solution. Four groups were used: trained/injected with KT5823, untrained/injected with KT5823, trained/injected with vehicle, and untrained/injected with vehicle. Feeding was investigated by measuring the number of bites elicited in response to a food source at three time points: before training (pre‐test), 15 min after training, and 2 hr after training. The differences in bites between pre‐test and 15 min and between pre‐test and 2 hr time points were calculated. The TSWR was also measured at the same three time points. These TSWR durations were then calculated as a ratio (post/pre) for both 15 min and 2 hr time points. Statistical comparisons of feeding and TSWR among the four groups were determined using the Kruskal‐Wallis test followed by the Student‐Newman‐Keuls post‐hoc test. Current findings indicate that KT5823 did not prevent either feeding suppression or sensitization. Therefore, these results suggest that PKG may not contribute to the plasticity in these behaviors induced by aversive stimuli in Aplysia. In order to further validate the negative result obtained at the behavioral level, the next step in my project focuses on the contribution of PKG on the cellular plasticity in B51 and tail SNs. This series of experiments is currently being done through in vitro preparations by isolating the nervous system containing the feeding and TSWR neural circuits. In order to mimic the in vivo sensitization training, peripheral nerves are electrically stimulated (i.e., training) to induce reduced excitability for neuron B51 and an increased excitability for tail SNs. The same selective inhibitor, KT5823, is being used to block PKG activity in vitro. A bath application of either KT5823 or vehicle solution are being applied to trained and untrained preparations. Using the current‐clamp technique, excitability of B51 and SN neurons are being measured before and 15 minutes after training. Data on the effects of KT5823 on decreased B51 excitability and increased SNs excitability are currently in progress.

Support or Funding Information

Funding provided by NIH‐NIGMS Grant SC3GM111188 to Riccardo Mozzachiodi