Neural circuit regulating drinking behavior in the eel acclimated to sea water

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Although the circumventricular organs(CVOs) in the eel is not identified yet, we identified 3 CVOs in the eel, magnocellular preoptic nucleus (PM), anterior tuberal nucleus (NAT) and area postrema (AP). The PM and the AP were immuno- reactive to vaosotocin (AVT)- and tyrosine hydroxylase (TH)-antibody, respectively (Mukuda et al., 2005).Swallowing is due to relaxation of the upper esophageal sphincter(UES) muscle. After injection of Evans blue into the UES, the glossopharyngeal-vagal motor complex (GVC) was labeled retrogradely, and these neurons were immunoreactive to choline acetyltransfetase-antibody (Mukuda & Ando, 2003). Indeed, the UES muscle (striated muscle) was innervated by a cholinergic neuron and constricted by ACh (Kozaka & Ando, 2003). The vagus nerve was divided to 10 branches (X1-X10) behind the gill arcs. Among these vagal branches, only X5 situated on the internal jugular vain innervated the UES, with optimal frequency of 20 Hz (Ogawa & Ando, in preparation). The existence of optimum frequency seems to be due to Ca^<2+>-ATPase on the sarcoplasmic reticulum (Tamura & Ando, in preparation). On the other hand, the UES was constricted by AVT and relaxed by isotocin via cAMP (Watanabe & Ando, in preparation).Neuronal activity of the GVC was inhibited by catecholamines in a dose-dependent manner, and this inhibitory effects were blocked by yohimbine. Since the vagal lobe (LX), the commissural nucleus of Cajal (NCC), the reticular formation (RF) and the AP in the medulla are immunoreactive to TH-antibody, some of these nuclei may innervate the GVC (Ito, Mukuda & Ando, submitted). Indeed, the GVC activity was inhibited by an electrical stimulation on the LX, but the inhibitory effect was not blocked by yohimbine (Ito & Ando, in preparation). Therefore, it is likely that the GVC activity is regulated by many inputs and the neuronal circuit on the GVC seems to be complicated.