Atonia-related regions in the rodent pons and medulla

Electrical stimulation of circumscribed areas of the pontine and medullary reticular formation inhibits muscle tone in cats. In this report, we presen t an analysis of the anatomical distribution of atonia-inducing stimulation sites in the brain stem of the rat. Muscle atonia could be elicited by ele ctrical stimulation of the nuclei reticularis pontis oralis and caudalis in the pons as well as the nuclei gigantocellularis, gigantocellularis alpha, gigantocellularis ventralis, and paragigantocellularis dorsalis in the med ulla of decerebrate rats. This inhibitory effect on muscle tone was a funct ion of the intensity and frequency of the electrical stimulation. Average l atencies of muscle-tone suppressions elicited by electrical stimulation of the pontine reticular formation were 11.02 +/- 2.54 and 20.49 +/- 3.39 (SD) ms in the neck and in the hindlimb muscles, respectively. Following medull ary stimulation, these latencies were 11.29 +/- 2.44 ms in the neck and 18. 87 +/- 2.64 ms in the hindlimb muscles. Microinjection of N-methyl-D-aspart ate (NMDA, 7 mM/0.1 ml) agonists into the pontine and medullary inhibitory sites produced muscle-tone facilitation, whereas quisqualate (10 mM/ 0.1 ml ) injection induced an inhibition of muscle tone. NMDA-induced muscle tone change had a latency of 31.8 +/- 35.3 s from the pons and 10.5 +/- 0.7 s fr om the medulla and a duration of 146.7 +/- 95.2 s from the pons and 55.5 +/ - 40.4 s from the medulla. The latency of quisqualate (QU)-induced reductio n of neck muscle tone was 30.1 +/- 37.9 s after pontine and 39.5 +/- 21.8 s after medullary injection. The duration of muscle-tone suppression induced by QU injection into the pons and medulla was 111.5 +/- 119.2 and 169.2 +/ - 145.3 s. Smaller rats (8 wk old) had a higher percentage of sites produci ng muscle-tone inhibition than larger rats (16 wk old), indicating an age-r elated change in the function of brain stem inhibitory systems. The anatomi cal distribution of atonia-related sites in the rat has both similarities a nd differences with the distribution found in the cat, which can be explain ed by the distinct anatomical organization of the brain stem in these two s pecies.