| Animals make feeding and avoidance decisions weighing the cost of foraging with potential metabolic gains based on the their experience and internal state (i.e. hunger level). Here, I describe the potential roles and mechanisms of two endogenous neuromodulators, the signal molecule nitric oxide (NO) and the biogenic amine serotonin (5-HT), in regulating the activity within the feeding neural network in Pleurobranchaea californica, a predatory sea slug that makes stereotypic feeding and avoidance decisions based on its hunger state within the structure of a relatively simple nervous system. NO selectively depolarizes key feeding elements in the CNS in part through enhancement of specific cyclic nucleotide-gated currents (Type 1 I Na,cAMP). Neurons not exhibiting Type 1 INa,cAMP, such as locomotory pedal neurons, are not depolarized by NO. Potentiation of I Na,cAMP by NO occurs through elevations in intracellular calcium which in turn may perturb intracellular pH through Ca2+/H + ATPase homeostatic regulation of Ca2+. Type 1 I Na,cAMP amplitudes are sensitive to pH. Additionally, NO enhancement Of INa,cAMP is suppressed with injection of either Ca2+ chelators or pH buffers. 5-HT positively modulates feeding in Pleurobranchaea through both enhancement of Type 1 INa,cAMP as well as by directly gating a depolarizing current I5-HT with a reversal potential of -15 mV. Within the feeding network, 5-HT can globally modulate the feeding motor program increasing the activity and reactivity to sensory stimulation. These effects are paralleled in intact animals whereby injection of 5-HT or its precursor 5-HTP lowers feeding sensory thresholds sufficient to elicit biting. Hunger state is thought to increase feeding arousal, observed as decreased sensory thresholds in the intact animal, through the potentiation of the feeding network. NO and 5-HT actions in regulating the feeding network activity state suggest that these neuromodulators may be acting as neural correlates for hunger in Pleurobranchaea. 5-HT concentrations measured in single identified serotonergic feeding neurons are directly correlated with increased hunger state in these animals being significantly higher than paired satiated animals. Thus, the regulation of the expression of feeding versus avoidance behaviors by hunger state may involve dynamic NO and 5-HT regulation of the feeding neural network. |
