Pharmacology of the brachium conjunctivum: Red nucleus synaptic system in the baboon

Acetylcholine, biogenic amines, and certain amino acids were applied by microintophoresis to parvicellular and magnocellular red nucleus (RN) neurons of baboon while recording brachium conjunctivum (BC)‐evoked and amino acid‐evoked unit discharge from these neurons. Glycine, γ‐aminobutyric acid, and β‐alanine were potent depressants of BC‐RN synaptic transmission, amino acid‐evoked firing, and spontaneous activity of all RN neurons studied. Glycine was clearly more potent than the other 2 depressant amino acids. L‐Glutamic and DL‐homocysteic acid were strong excitants of all RN neurons tested. Dopamine, noradrenaline, and 5‐hydroxytryptamine depressed the excitability of both parvicellular and magnocellular RN neurons; no excitatory effects were observed with these biogenic amines on RN neurons. Acetylcholine increased the rate of firing of spontaneously discharging parvicellular RN neurons and facilitated the amino acid‐induced firing of these same neurons. Acetylcholine did not facilitate BC‐RN synaptic transmission nor could this transmission be blocked by cholinergic antagonists. Unlike parvicellular RN neurons, the responsiveness of magnocellular neurons was either unaltered by acetylcholine or slightly decereased. These experiments demonstrate a difference in the pharmacologic responsiveness of parvicellular and magnocellular RN neurons to acetylcholine but do not provide evidence for a cholinergic input to RN via the brachium conjunctivum.