Persistent effects of cyclic adenosine monophosphate are directly responsible for maintaining a neural network state

Persistent modulatory actions and task switching in the feeding network of Aplysia

The activity of multifunctional networks is configured by neuromodulators that exert persistent effects. This raises a question, does this impact the ability of a network to switch from one type of activity to another? We review studies that have addressed this question in the Aplysia feeding circuit. Task switching in this system occurs "asymmetrically." When there is a switch from egestion to ingestion neuromodulation impedes switching (creates a "negative bias"). When there is a switch from ingestion to egestion the biasing is "positive." Ingestion promotes subsequent egestion. We contrast mechanisms responsible for the two types of biasing and show that the observed asymmetry is a consequence of the fact that there is more than one set of egestive circuit parameters.

Variable task switching in the feeding network of Aplysia is a function of differential command input

Command systems integrate sensory information and then activate the interneurons and motor neurons that mediate behavior. Much research has established that the higher-order projection neurons that constitute these systems can play a key role in specifying the nature of the motor activity induced, or determining its parametric features. To a large extent, these insights have been obtained by contrasting activity induced by stimulating one neuron (or set of neurons) to activity induced by stimulating a different neuron (or set of neurons). The focus of our work differs. We study one type of motor program, ingestive feeding in the mollusc Aplysia californica, which can either be triggered when a single projection neuron (CBI-2) is repeatedly stimulated or can be triggered by projection neuron coactivation (e.g., activation of CBI-2 and CBI-3). We ask why this might be an advantageous arrangement. The cellular/molecular mechanisms that configure motor activity are different in the two situations because the released neurotransmitters differ. We focus on an important consequence of this arrangement, the fact that a persistent state can be induced with repeated CBI-2 stimulation that is not necessarily induced by CBI-2/3 coactivation. We show that this difference can have consequences for the ability of the system to switch from one type of activity to another.NEW & NOTEWORTHY We study a type of motor program that can be induced either by stimulating a higher-order projection neuron that induces a persistent state, or by coactivating projection neurons that configure activity but do not produce a state change. We show that when an activity is configured without a state change, it is possible to immediately return to an intermediate state that subsequently can be converted to any type of motor program.

New insights from small rhythmic circuits

Small rhythmic circuits, such as those found in invertebrates, have provided fundamental insights into how circuit dynamics depend on individual neuronal and synaptic properties. Degenerate circuits are those with different network parameters and similar behavior. New work on degenerate circuits and their modulation illustrates some of the rules that help maintain stable and robust circuit function despite environmental perturbations. Advances in neuropeptide isolation and identification provide enhanced understanding of the neuromodulation of circuits for behavior. The advent of molecular studies of mRNA expression provides new insight into animal-to-animal variability and the homeostatic regulation of excitability in neurons and networks.

Convergent effects of neuropeptides on the feeding central pattern generator of Aplysia californica

Modulators that induce distinct motor programs act divergently on neural networks to specify output. We study a situation where modulators that act divergently also act convergently. We focus on an interneuron (B63) that is part of the feeding central pattern generator (CPG) in Aplysia californica. Previous work has established that B63 is critical for program initiation regardless of the type of evoked activity. B63 receives input from a number of different elements of the feeding circuit. Program initiation occurs reliably when some are activated, but we show it does not occur reliably with activation of others. When program initiation is reliable, modulatory neuropeptides are released. For example, previous work has established that an ingestive input to the feeding CPG, cerebral buccal interneuron 2 (CBI-2), releases feeding circuit activating peptide (FCAP) and cerebral peptide 2 (CP-2). Afferents with processes in the esophageal nerve (EN) that trigger egestive motor programs release small cardioactive peptide (SCP). Previous studies have described divergent effects of FCAP/CP-2 and SCP on the feeding circuit that specify motor activity. Here, we show that FCAP/CP-2 and SCP increase the B63 excitability. Thus, we show that peptides that have well characterized divergent effects on the feeding circuit additionally act convergently at the level of a single neuron. Since convergent effects of neuromodulators are not necessary for specifying network output, we ask why they might be important. Our data suggest that they have an impact during a task switch.

The Complement of Projection Neurons Activated Determines the Type of Feeding Motor Program in Aplysia

Multiple projection neurons are often activated to initiate behavior. A question that then arises is, what is the unique functional role of each neuron activated? We address this issue in the feeding system of Aplysia. Previous experiments identified a projection neuron [cerebral buccal interneuron 2 (CBI-2)] that can trigger ingestive motor programs but only after it is repeatedly stimulated, i.e., initial programs are poorly defined. As CBI-2 stimulation continues, programs become progressively more ingestive (repetition priming occurs). This priming results, at least in part, from persistent actions of peptide cotransmitters released from CBI-2. We now show that in some preparations repetition priming does not occur. There is no clear seasonal effect; priming and non-priming preparations are encountered throughout the year. CBI-2 is electrically coupled to a second projection neuron, cerebral buccal interneuron 3 (CBI-3). In preparations in which priming does not occur, we show that ingestive activity is generated when CBI-2 and CBI-3 are coactivated. Programs are immediately ingestive, i.e., priming is not necessary, and a persistent state is not induced. Our data suggest that dynamic changes in the configuration of activity can vary and be determined by the complement of projection neurons that trigger activity.