LONG-TERM EXPRESSION OF 2 INTERACTING MOTOR PATTERN-GENERATING NETWORKS IN THE STOMATOGASTRIC SYSTEM OF FREELY BEHAVING LOBSTER

Rhythmic movements of the gastric mill and pyloric regions of the crus tacean foregut are controlled by two stomatogastric neuronal networks that have been intensively studied in vitro. By using electromyographi c recordings from the European lobster, Homarus gammarus, we have moni tored simultaneously the motor activity of pyloric and gastric mill mu scles for less than or equal to 3 mo in intact and freely behaving ani mals. Both pyloric and gastric mill networks are almost continuously a ctive in vivo regardless of the presence of food. In unfed resting ani mals kept under ''natural-like'' conditions, the pyloric network expre sses the typical triphasic pattern seen in vitro but at considerably s lower cycle periods (2.5-3.5 s instead of 1-1.5 s). Gastric mill activ ity occurs at mean cycle periods of 20-50 s compared with 5-10 s in vi tro but may suddenly stop for up to tens of minutes, then restart with out any apparent behavioral reason. When conjointly active, the two ne tworks express a strict coupling that involves certain but not all mot or neurons of the pyloric network. The posterior pyloric constrictor m uscles, innervated by a total of 8 pyloric (PY) motor neurons, are inf luenced by the onset of each gastric mill medial gastric/lateral gastr ic (MG/LG) neuron powerstroke burst, and for one cycle, PY neuron burs ts may attain >300% of their mean duration. However, the duration of a ctivity in the lateral pyloric constrictor muscle, innervated by the u nique lateral pyloric (LP) motor neuron, remains unaffected by this pe rturbation. During this period after gastric perturbation, LP neuron a nd PY neurons thus express opposite burst-to-period relationships in t hat LP neuron burst duration is independent of the ongoing cycle perio d, whereas PY neuron burst duration changes with period length. In vit ro the same type of gastro-pyloric interaction is observed, indicating that it is not dependent on sensory inputs. Moreover, this interactio n is intrinsic to the stomatogastric ganglion itself because the relat ionship between the two networks persists after suppression of descend ing inputs to the ganglion. Intracellular recordings reveal that this gastro-pyloric interaction originates from the gastric MG and LG neuro ns of the gastric network, which inhibit the pyloric pacemaker ensembl e. As a consequence, the pyloric PY neurons, which are inhibited by th e pyloric dilator (PD) neurons of the pyloric pacemaker group, extend their activity during the time that PD neuron is held silent. Moreover , there is evidence for a pyloro-gastric interaction, apparently recti fying, from the pyloric pacemakers back to the gastric MG/LG neuron gr oup.