Cholinergic responses in crossed tecto-reticular neurons of rat superior colliculus

Neurons in the intermediate gray layer (SGI) of mammalian superior colliculus (SC) receive cholinergic innervation from the brain stem parabrachial region, which seems to modulate the signal processing in the SC. To clarify its role particularly in orienting behaviors, we studied cholinergic effects on the major output neuron group of the SGI, crossed tecto-reticular neurons (cTRNs), identified by retrograde labeling from the contralateral brain stem gaze center in SC slices obtained from rats (PND 17-22) by whole cell patch-clamp techniques. Bath application of carbachol induced either 1) nicotinic inward (nIN) + muscarinic inward (mIN) (11/24) or 2) nIN + mIN + muscarinic outward (mOUT) (13/24) current responses. Transient pressure application of 1 mM acetylcholine elicited nIN in all neurons tested (n = 58). In a majority of these neurons (52/58), the nIN was completely suppressed by dihydro-beta-erythroidine, a specific antagonist for alpha4beta2 nicotinic receptor subtype. The remaining 6/58 neurons exhibited not only the slower alpha4beta2 receptor-mediated component but also a faster component that was inhibited by a specific antagonist for alpha7 nicotinic receptor, alpha-bungarotoxin. cTRNs expressing alpha7 nicotinic receptors tended to be smaller in size than those lacking alpha7 receptors. Bath application of muscarine induced two response patterns: mIN only (17/38) and mIN+ mOUT (21/38). The mIN and mOUT were mediated by M3 (plus M1) and M2 muscarinic receptors, respectively. These results suggest that a major response to cholinergic inputs to cTRNs is excitatory. This would indicate the facilitatory role of the brain stem cholinergic system in the execution of orienting behaviors including saccadic eye movements.