Periaqueductal gray matter and nucleus raphe magnus involvement in anterior pretectal nucleus-induced inhibition of jaw-opening reflex in rats

Abstract

In previous studies we have shown that electrical stimulation of the cortex or anterior pretectal nucleus (APT) inhibits the jaw-opening reflex (JOR). In the present study we investigated whether these effects are mediated by a relay in the periaqueductal gray matter (PAG) or rostroventromedial medulla (RVM). Experiments were performed on chloralose-urethane anesthetized rats. The JOR which was elicited by electrical stimulation of the mandibular incisor tooth was monitored by recording the evoked digastric muscle activity. Conditioning stimulation (20 ms-train of 0.2 pulses at 400 Hz) was delivered to the facial area of the sensorimotor cortex, APT, PAG or nucleus raphe magnus (NRM) 50 ms prior to the test stimulus to the tooth that evoked the JOR. In addition, the effects of microinjections of glutamate into APT, PAG and NRM on the tooth-evoked JOR were also evaluated. The inhibition of the JOR by electrical and glutamate conditioning stimulation was found to be most potent for activation of the NRM and least potent for the APT. Local anesthetic (2% Iidocaine, 0.3–0.6 μl) block of the PAG could partially, significantly (P < 0.05) and reversibly reduce both the APT and cortical-induced depression of the JOR. Lidocaine block of the ventromedial pons reversible reduced the PAG, APT and cortical-induced inhibition of the JOR (P < 0.05). Lidocaine block of the lateral RVM had powerfully (P < 0.01) and reversibly reduced the PAG-induced inhibition, but had onaly a small effect (P < 0.05) on the APT-induced inhibition and no significant effect on the cortical-induced inhibition. Lidocaine block of NRM produced a small reversible reduction in the PAG- and APT-induced inhibition but had no significant effect on cortical-induced inhibition of the JOR. These findings suggest that the inhibitory effects of APT and cortical stimulation on the JOR are mediated in part by a relay in the PAG and that in addition the APT-induced inhibition may also be partly mediated by a relay in the RVM.