Effects of chemical and electrical stimulation of the midbrain on feline T 2-T 6 spinoreticular and spinal cell activity evoked by cardiopulmonary afferent input

Abstract

Responses to electrical and chemical stimulation of the periaqueductal gray (PAG) and midbrain reticular formation (RF) were examined on extracellular activity of 28 spinoreticular tract (SRT) and 56 spinal neurons in T 2–T 6 segments of anesthetized cats. All cells received excitatory viscerosomatic convergent input from the left forelimb triceps region and from cardiopulmonary sympathetic afferents. Evoked activity that resulted from pinching the left triceps was reduced by PAG and midbrain RF stimulation (100 Hz, 100 μs, 50–500 μA). Visceral afferent input to the neurons was elicited during electrical stimulation of the PAG and adjacent RF decreased A-δ and C-fiber activation of these neurons produced during electrical stimulation of cardiopulmonary afferents and decreased the activity of cells during excitatory responses to intracardiac bradykinin. Electrical stimulation of the PAG or midbrain RF similarly decreased spontaneous and evoked cell activity. Selective activation of cell bodies with microinjection of glutamate into the PAG reduced the activity of 6 to 8 cells whereas glutamate injections into midbrain RF reduced the activity of only 3 of 13 neurons. This difference in the effectiveness of chemically stimulating the PAG vs midbrain RF was significant (P < 0.05). These data demonstrate that (1) neuronal activity evoked by visceral afferent input from the heart was decreased by electrical stimulation of the PAG and midbrain RF and (2) a portion of this descending inhibition may be mediated by cell bodies in the PAG.