Research reportOral-motor patterns of rhythmic trigeminal activity generated in fetal rat brainstem in vitro
Introduction
Mammalian fetuses form neural networks to prepare for birth and post-natal life, in order to develop fundamental rhythmical behaviors such as respiration, locomotion, and sucking. Many ultrasonographic studies have reported that swallowing amniotic fluid and finger sucking occur even during the fetal stage [2], [4]. Conversely, the neural mechanisms, circuitry network and neural activity for generation of respiration or locomotion have been investigated through in vitro studies using fetal brainstem spinal-cord preparations that retain functional respiratory and locomotor networks [5], [7], [16], [17], [18]. Evidence from these studies suggests that the basic mechanisms required for respiratory pattern generation are already operational in fetuses. However, few in vitro studies have investigated neural circuits and mechanisms related to generation of rhythmical jaw movements such as sucking during the fetal stage. The neural mechanisms for oral-motor activities involved with mastication and sucking have undergone detailed analysis in neonates and adults using both in vivo and in vitro studies [1], [13], [14], [15], [23], revealing that rhythmogenesis for jaw movements originates from central pattern-generating circuits within the brainstem.
In the present study, rhythmical trigerminal activity (RTA) was analyzed in vitro using isolated fetal rat brainstem block preparations. Differences in patterns of RTA were identified between fetal days and may be attributable to development of oral-motor neural circuitry.
Section snippets
Materials and methods
All animal-use procedures were reviewed and approved by the Osaka University Faculty of Dentistry Intramural Animal Care and Use Committee.
Timed-pregnant Sprague–Dawley rats were deeply anesthetized using halothane (2-bromo-2-chloro-1,1,1-trifluoroethane) and the fetus (E16–E21) partial brainstem preparation was isolated as previously described [8]. The brainstem was transected from between the level of the inferior and superior colliculi to the level of the obex, for confirmation of conditions
Results
For E20–E21, bath application of NMA (20 μM) combined with BIC (10 μM) induced RTA in trigeminal motor nerves (extracellular recordings) at a high frequency of 5–8 Hz, similar to either the pattern or rhythm of neonatal RTA (Fig. 1A). Cyclic duration of RTA in these samples (>30 consecutive cycles) was 155.07±30.17 ms (n=12).
For E18–E19, the same bath application (NMA 20 μM; BIC 10 μM) induced a complex pattern of rhythmic discharges in trigeminal motor nerves. The trigeminal motor nerve
Discussion
The present study demonstrated discharge patterns of oral-motor rhythmical activities for in vitro fetal stages for the first time.
For E20–E21, both cycle duration and pattern of RTA resembled those seen in neonatal rat brainstem preparations [3], [8], [9], [10], [19]. The present results suggest that the basic functional networks for generating patterns of oral-motor rhythmical activity are already formed by E20. In E18–E19, the trigeminal motor nerve can induce RTA, although amplitude of
Acknowledgements
Part of this study was supported by Grant-in-Aid for Scientific Research (A) No. 10307050 from the Ministry of Education, Science, Sports and Culture of Japan.
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