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

Abstract

The results of chronic, in vivo delivery of excitatory and inhibitory neurotransmitter substances upon the craniofacial skeleton are of ongoing interest to clinician and basic scientist alike. Our purpose was to document and compare the effects of biodegradable glycine, glutamate, and thyrotropin-releasing hormone (TRH) microspheres upon the craniofacial skeleton and TMJ of actively growing rats. Glycine, glutamate, TRH, and blank microspheres were stereotactically implanted in proximity to motoneurons within the trigeminal motor nucleus in order to test the following null hypotheses: (1) neurotransmitter microspheres implanted near trigeminal motoneurons of growing rats have no significant effect on the craniofacial skeleton and temporomandibular joints of implanted animals, and (2) there are no significant differences between the relative effects of glutamate, TRH (excitatory to trigeminal motoneurons), and glycine (inhibitory to trigeminal motoneurons) implants upon the craniofacial skeleton and temporomandibular joint. Fifty male Sprague-Dawley rats underwent stereotactic neurosurgery at 35 days; five rats 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 < or = 0.05, 0. 01) smaller implant-side cranial dimensions and mandibular condyles, all glycine rats showed increased gracility of implant-side bones, and deviation of their facial skeleton away from the implant-side; this was in contrast to the generally larger implant-side bony structures in both glutamate and TRH rats. The two null hypotheses were both rejected. Due to their inhibitory and excitatory effects upon trigeminal motoneurons, masticatory muscles, 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 craniofacial skeleton and TMJ.

Effects of Post-natal Serotonin Levels on Craniofacial Complex

“…Poly(fatty acid dimer-co-sebacic acid) P(FAD-SA), with a copolymer ratio of 50:50 (Mw 38,000), was prepared as described earlier (Domb and Maniar, 1993;Byrd et al, 1997Byrd et al, , 2000. 5-hydroxytryptamine creatinine sulfate complex (serotonin) was purchased from Sigma (St. Louis, MO, USA).…”

“…1997, 20)0). Ahtci-idcictification ot'thc implant-side trigceminal motoi nLicCLISx thc StimuilatilIg clectrode was xithdiaxx n and replaced wixih a mircosplhcic delivciy system (Byrid c al., 1992, 2000( 13v id and Hamilton-Byrd, 1994 .-Figure 1 (A,B,C). Superior views of 1 5% serotonin implant rat heads at 21 days post-implantation.…”

“…TenI perceilt sci-otoili-n15(,lO Sciotolill-, blank-riCrosphere;an1d shamxli-iigery rats Acrc kiIleCd at 14 and 2I post-opcrati\c days by trianscardiac pCeILiSion ().9",, phYSiologiCal Saline and 1(",, bu'f'eicid forrualin) ai'tcr PrOfOI1ini aInCStlCtI/etiiOIl. ThlC as per Byrd et al (1997Byrd et al ( , 2000. The implantand non-implantside trigeminal motoneurons of 5877 10-pm sections were studied in this investigation.…”

“…The temporalis, masseter, medial pterygoid, anterior digastric, and lateral pterygoid muscles were carefully dissected out intact and weighed to the nearest mg by Sartorius (Westbury, NY, USA) elec-tronic scales (Byrd et al, 1997(Byrd et al, , 2000. A series of 4 cranial dimension and 2 TMJ condylar measurements was taken from dry skull preparations of the 40 rats after brainstem removal and muscle dissection (Byrd et al, 1990(Byrd et al, , 1992(Byrd et al, , 1997(Byrd et al, , 2000); all measurements were taken and recorded to the nearest 0.01 mm. Details for the cranial dimension and condylar measurements actually taken are provided in the Appendix (www.dentalresearch.org).…”

“…We have earlier demonstrated that neurotransmitter substances that affect motoneurons in the trigeminal motor nucleus of the brainstem-glutamate, thyrotropin-releasing hormone, and glycine-all play important roles in postnatal growth and development of the craniofacial complex (Byrd et al, 1992(Byrd et al, , 1997(Byrd et al, , 1998(Byrd et al, , 2000Byrd and Hamilton-Byrd, 1994). Accordingly, we became interested in better defining those interactions among the craniofacial skeleton, masticatory muscles, TMJ, and motoneurons within the trigeminal motor nucleus following post-natal implantation of serotonin as delivered by biodegradable microspheres (Byrd et al, 1997(Byrd et al, , 1998(Byrd et al, , 2000.…”