cGMP mediates short- and long-term modulation of excitability in a decision-making neuron in Aplysia

Abstract

In elementary neural circuits, changes in excitability can have a strong impact in the expression of a given behavior. One example is provided by B51, a neuron with decision-making properties in the feeding neural circuit of the mollusk Aplysia. The excitability of B51 is bidirectionally modulated by external and internal stimuli in a manner that is consistent with the corresponding induced changes in feeding behavior. For example, in operant reward learning, which up-regulates feeding, B51 excitability is increased via a cAMP-dependent mechanism. Conversely, following training protocols with aversive stimuli, which down-regulate feeding, B51 excitability is decreased. In this study, we tested the hypothesis that B51 decreased excitability may be mediated by another cyclic nucleotide, cGMP. Our results revealed that iontophoretic injection of cGMP was capable of inducing both short-term (45 min) and long-term (24 h) reduction of B51 excitability. We next investigated which biochemical trigger could increase cGMP cytosolic levels. The neurotransmitter nitric oxide was found to decrease B51 excitability through the activation of the soluble guanylyl cyclase. These findings indicate that a cGMP-dependent pathway modulates B51 excitability in a manner opposite of cAMP, indicating that distinct cyclic-nucleotide pathways bidirectionally regulate the excitability of a decision-making neuron.

Keywords: Aplysia; Excitability; Neuronal plasticity; Second messengers.

Fig. 1.

Time course of the effects of single injection of cGMP on B51 excitability. (A) Placements of recording and iontophoretic electrodes in B51 during pre-test (A1) and post-tests (A2), and timeline of B51 measurements prior to and after the single-injection (A3). (B) Sample traces of B51 burst threshold from cGMP-injected (B1) and vehicle-injected cells (B2). (C) Summary data illustrate that the single cGMP injection significantly increased B51 burst threshold for up to 45 min after treatment. In this and in the following figure, statistical significance is indicated by an asterisk.

Fig. 2.

A four-cGMP injection protocol induced a decrease of B51 excitability 24 h after treatment. (A) Placement of recording and iontophoretic electrodes in B51 during pre-test (A1) and 24-h post-test (A2), and timeline of B51 measurements prior to and 24 h after the four-injections (A3). (B) Sample traces of B51 burst threshold from cGMP-injected (B1) and vehicle-injected cells (B2). (C) Summary data illustrate that four cGMP injections significantly increased B51 burst threshold 24 h after treatment.

Fig. 3.

Inhibition of sGC activity increased B51 excitability. (A) Protocol of B51 measurements prior to and 15 min after ODQ/DMSO treatment. (B) Sample traces of B51 burst threshold from ODQ-treated (B1) and DMSO-treated cells (B2). (C) Summary data illustrate that ODQ significantly decreased B51 burst threshold 15 min after treatment.

Fig. 4.

B51 excitability is modulated by NO signaling. (A) Protocol of B51 measurements prior to and 3 min after treatments with either SNAP/DMSO or L-NAME/ASW. (B) Sample traces of B51 burst threshold from SNAP-treated (B1) and DMSO-treated cells (B2). (C) Summary data illustrate that SNAP significantly increased B51 burst threshold 3 min after treatment. (D) Sample traces of B51 burst threshold from L-NAME-treated (D1) and ASW-treated cells (D2). (E) Summary data illustrate that L-NAME significantly decreased B51 burst threshold 3 min after treatment.

Fig. 5.

The NO-dependent decrease of B51 excitability is mediate by the sGC. (A) Protocol of incubation with ODQ/DMSO and subsequent measurements of B51 properties prior to and 3 min after SNAP treatment. (B) Sample traces of B51 burst threshold from SNAP-treated cells incubated with ODQ (B1) and SNAP-treated cells incubated with DMSO (B2). (C) Summary data illustrate that ODQ incubation prevented the SNAP-induced increase of B51 threshold.