Effects of a liquid diet on the response properties of temporomandibular joint nociceptive neurons in the trigeminal subnucleus caudalis of growing rats.

OBJECTIVE: To investigate whether low mechanical loading on the temporomandibular joint (TMJ) when ingesting a liquid diet affects the response properties of neurons in the trigeminal spinal tract subnucleus caudalis (Sp5C) in growing rats. MATERIALS AND METHODS: Shortly after weaning, 2-week-old male rats were fed chow pellets (control) or a liquid diet (experimental). Firing activities of single sensory units were recorded from the Sp5C at 4, 5, 7, and 9 weeks. Neurons were functionally classified by their responsiveness to TMJ stimuli. The responses of Class II and III neurons to TMJ stimuli were investigated. RESULTS: In both neuron classes, the firing threshold in the experimental group was significantly lower than in the control group at all time points, but remained static in the control group throughout the experimental period, whereas it peaked in the experimental group at 4 weeks, decreased at 5 weeks, and remained stable thereafter until 9 weeks. Similarly, the initial firing frequency was significantly higher in the experimental group than in the control group, but remained static in the control group throughout the experimental period, whereas in the experimental group, it was at its lowest at 4 weeks, increased at 5 weeks, and stayed stable thereafter until 9 weeks. CONCLUSION: Differences in TMJ loading arising from variable diet consistency during growth may affect the functional characteristics of Sp5C neurons.

Differences in maturation of the jaw-opening reflex between rats that received early-and late-masticatory stimulation.

The jaw-opening reflex (JOR) plays an important role in the regulation of jaw movement during mastication. Previous study showed that altered masticatory function during growth impedes JOR maturation and thus may affect masticatory performance in adults. However, no studies have compared the benefit of early and delayed correction in terms of functional development. Therefore, this study tested the hypothesis that early-stimulation of masticatory function during growth can promote JOR maturation better than late-stimulation during adulthood. Soon after weaning, 120 female Wistar rats were divided into two groups and fed either solid (control group) or liquid (experimental group) diets. The experimental group was further divided into early-, late-, and non-stimulation subgroups. Early- and late-stimulation groups were fed a solid diet instead of a liquid diet at 5- and 11-week-old, respectively, whereas non-stimulation group was fed only a liquid diet until the end of the experiment. At 3, 5, 7, 9, 11 and 13 weeks, JOR recordings were conducted in anaesthetised rats of all groups. Latency and peak-to-peak amplitude of the JOR were compared between the groups. From 7 to 13 weeks, early-stimulation group showed a JOR with short latency and high amplitude similar to that of control group. In contrast, late- and non-stimulation groups showed significantly longer latency and smaller amplitude of the JOR than in control group. We demonstrated that early masticatory stimulation within the critical period for programming mastication may have greater potential to restore JOR maturation to values close to those in normal adults.

Functional adaptability of jaw-muscle spindles after bite-raising.

In a previous experiment, we found that masseter muscle spindles show functional plasticity after 5 to 15 days under increased occlusal vertical dimension (iOVD). In the present study, we hypothesized that spindle function would eventually recover if longer observation periods were allowed. Therefore, in this study we investigated changes in masseter muscle spindle function over periods of 1 day to 8 weeks. Masseter muscle-spindle responses to ramp-and-hold jaw stretches were recorded from the mesencephalic trigeminal nucleus in 35 barbiturate-anesthetized female Wistar rats. The rats were previously divided into Control and iOVD groups, and those in the iOVD group received a 2.0-mm composite resin build-up to the maxillary molars. In this condition, there were no statistically significant differences in masseter muscle spindle sensitivity between Control and iOVD in the six- and eight-week subgroups. Our results further indicate a high degree of adaptability in masseter muscle spindle function following changes in OVD.

Role of occlusal vertical dimension in spindle function.

Several studies have suggested the jaw-muscle spindle as the receptor responsible for regulating and maintaining the occlusal vertical dimension (OVD). However, to challenge this assumption, we hypothesized that long-term changes in OVD could affect the sensory inputs from jaw-muscle spindles. In this study, we investigated changes in masseter muscle spindle function under an increased OVD (iOVD) condition. Responses of primary and secondary endings of masseter muscle spindles to cyclic sinusoidal stretches were investigated. Twenty barbiturate-anesthetized female Wistar rats were divided into control and iOVD groups. Rats in the iOVD group received a 2.0-mm composite resin build-up to the maxillary molars. After iOVD, masseter muscle spindle sensitivity gradually decreased. Primary and secondary spindle endings were affected differently. We conclude that iOVD caused reduction in masseter muscle spindle sensitivity. This result suggests that peripheral sensory plasticity may occur following changes in OVD. Such changes may provide a basis for physiological adaptation to clinical occlusal adjustments.

Structure of the rice glutaredoxin (thioltransferase) gene.

We have isolated the gene encoding a glutaredoxin in rice (Oryza sativa L.) and determined the nucleotide (nt) sequence of about a 4.2 kb long. The cloned gene (gRASC8) was found to contain four exons interrupted by three introns. The first exon begins the ATG translation start codon and the four exons code for a protein composed of 112 amino acids. The tetrapeptide -Cys-Pro-Phe-Cys- [-Cys-Pro-Phe(Tyr)-Cys-] which constitutes an active site of Escherichia coli and mammalian glutaredoxins, was conserved. The nt sequence contained consensus TATA and CAAT boxes, and two polyadenylation signals. Southern blot analysis of rice genomic DNA suggests that there are two copies of the glutaredoxin genes in rice.

Cloning and sequence analysis of a cDNA encoding rice glutaredoxin.

A full-length cDNA clone (RASC8) encoding glutaredoxin (thioltransferase) was isolated from a cDNA library of an aleurone layer prepared from a developing seed of rice (Oryza sativa L.). RASC8, 568bp in length, contained an ATG codon and two possible polyadenylation signals, and encoded 112 amino acid residues. Cys-Pro-Phe-Cys, which is the active site and a highly conserved sequence among thioltransferases, was found in the deduced amino acid sequence. RASC8 was introduced into an expression vector pMALc2 and the translated product possessed thioltransferase activity.

Functional reversibility of temporomandibular joint mechanoreceptors.

OBJECTIVE: Previous studies have reported that the maturation of temporomandibular joint (TMJ) mechanoreceptors occurs during the early stages of mastication, and indicated that TMJ mechanoreceptors lose their function when masticatory loading is decreased. The purpose of the present study was to investigate whether the resumption of proper TMJ loading during the early growth period could restore TMJ mechanoreceptor function. DESIGNS: Ninety-nine 2-week-old male Wistar rats were divided into two groups and fed either pellets [control group (n=33)] or a liquid diet [experimental group (n=66)]. At 5 weeks of age, the experimental group was split into changing-diet (n=33) and liquid-diet (n=33) groups; the former was fed pellets instead of a liquid diet. TMJ mechanoreceptor activities were recorded from the trigeminal ganglion at 5, 7 and 9 weeks. The firing threshold and maximum instantaneous firing frequency of single TMJ units were measured in each group. RESULTS: In the changing-diet group, the firing properties of TMJ units were recovered at 7 weeks. CONCLUSIONS: Proper TMJ loading during the early growth period can lead to the restoration of TMJ mechanoreceptor function.

Increased occlusal vertical dimension induces cortical plasticity in the rat face primary motor cortex.

Previous studies have demonstrated that functional plasticity in the primary motor cortex (M1) is related to motor-skill learning and changes in the environment. Increased occlusal vertical dimension (iOVD) may modulate mastication, such as in the masticatory cycle, and the firing properties of jaw-muscle spindles. However, little is known about the changes in motor representation within the face primary motor cortex (face-M1) after iOVD. The purpose of the present study was to determine the effect of iOVD on the face-M1 using intracortical microstimulation (ICMS). In an iOVD group, the maxillary molars were built-up by 2mm with acrylic. The electromyographic (EMG) activities from the left (LAD) and right (RAD) anterior digastric (AD), masseter and genioglossus (GG) muscles elicited by ICMS within the right face-M1 were recorded 1, 2 and 8 weeks after iOVD. IOVD was associated with a significant increase in the number of sites within the face-M1 from which ICMS evoked LAD and/or GG EMG activities, as well as a lateral shift in the center of gravity of the RAD and LAD muscles at 1 and 2 weeks, but not at 8 weeks. These findings suggest that a time-dependent neuroplastic change within the rat face-M1 occurs in association with iOVD. This may be related to the animal's ability to adapt to a change in the oral environment.

Effects of a liquid diet on temporomandibular joint mechano-receptors.

Load during mastication is an important factor for the development and maintenance of mechano- receptor properties. The purpose of this study was to examine property changes in the rat TMJ mechano-receptors under conditions of liquid diet feeding and low articular load during the growth period. The hypothesis was that alterations in mastication of liquid diet might increase TMJ mechano-receptor sensitivity. Sixty-six two-week-old male Wistar rats were divided into two groups: a control group that was fed on whole pellets, and an experimental group that was fed a liquid diet. Electrophysiological recordings from the TMJ units were obtained from the trigeminal ganglion when the rats were 5, 7, and 9 weeks old. In the experimental group, TMJ mechanoreceptor sensitivity increased, because the firing threshold gradually decreased and the maximum instantaneous frequency gradually increased. In conclusion, functional properties of TMJ mechano-receptors under low articular loading conditions cannot mature normally within the growth period.