Increased in vivo levels of neurotransmitters to trigeminal motoneurons: Effects on craniofacial bone and TMJ

The results of chronic, in vivo delivery of excitatory and inhibitory neuro transmitter substances upon the craniofacial skeleton are of ongoing intere st to clinician and basic scientist alike. Our purpose was to document and compare the effects of biodegradable glycine, glutamate, and thyrotropin-re leasing hormone (TRH) microspheres upon the craniofacial skeleton and TMJ o f actively growing rats. Glycine, glutamate, TRH, and blank microspheres we re stereotactically implanted in proximity to motoneurons within the trigem inal motor nucleus in order to test the following null hypotheses: (1) neur otransmitter microspheres implanted near trigeminal motoneurons of growing rats have no significant effect on the craniofacial skeleton and temporoman dibular joints of implanted animals, and (2) there are no significant diffe rences between the relative effects of glutamate, TRH (excitatory to trigem inal motoneurons), and glycine (inhibitory to trigeminal motoneurons) impla nts upon the craniofacial skeleton and temporomandibular joint. Fifty male Sprague-Dawley rats underwent stereotactic neurosurgery at 35 days; five ra ts each were killed at 14 and 21 days postoperative for data collection and comparison between glycine-, glutamate-, TRH-, blank-microsphere, and sham -surgery rats. Glycine rats had significantly (P less than or equal to 0.05 , 0.01) smaller implant-side cranial dimensions and mandibular condyles, al l glycine rats showed increased gracility of implant-side bones, and deviat ion of their facial skeleton away from the implant-side; this was in contra st to the generally larger implant-side bony structures in both glutamate a nd TRH rats. The two null hypotheses were both rejected. Due to their inhib itory and excitatory effects upon trigeminal motoneurons, masticatory muscl es, and their neuromuscular generation of biomechanical forces that affect bone, the neurotransmitter substances glycine, glutamate, and TRH appear to play an important role in the growth and development of the mammalian cran iofacial skeleton and TMJ. Anat Rec 258: 369-383, 2000. (C) 2000 Wiley-Liss , Inc.