Functional changes in ventrobasal thalamic neurones responsive to noxious and non-noxious cutaneous stimuli after chloralose treatment: New evidence for the presence of pre-existing “silent connections” in the adult nervous system?

Abstract

Neurons (25) of the thalamic VB [ventrobasal] complex of the rat, first characterized under moderate volatile anesthesia (1/3 O2-2/3 N2O, 0.5% halothane) were subsequently studied under deep chloralose anesthesia. Neurons were classified during the control period as noxious (N), non-noxious (Nn) or convergent (NnN) according to their responsiveness to cutaneous mechanical stimulations. For all the N (n = 15) and NnN (n = 4) neurons, the responses induced by noxious stimuli (mechanical electrical or thermal) progressively disappeared after chloralose administration. Simultaneous with disappearance of the responses elicited by noxious stimuli, responses to non-noxious stimuli appeared for 12 N neurons: 5 responded to a brisk tap applied to any part of the body and 7 to brushing of light touch from a new receptive field (RF), at a variable distance from the initial RF, but always contralateral to the recording site. The Nn neurons (n = 6) continued to be activated by light touch or brushing but there was a consistent concentric enlargement of their RF. The NnN neurons also presented an enlarged RF to the non-noxious stimuli. The depressive and unmasking effects of chloralose are discussed and compared to the various indications of pre-existing connections in the CNS.