Cryogenic blockade of the visual cortico-thalamic projection in the rat

A simple procedure is described for rapidly inactivating areas 17 and 18 in the rat by superfusion of the immediately overlying dura mater with chilled, physiological saline solution. Unit recording indicates deep depression of cortical activity within 20 s or less, and equally rapid restoration upon rewarming. Repetition does not appear to be deleterious. During such inactivation of the "visual" cortex (VC) essentially all neurons in the visual portion of the thalamic reticular nucleus (vTRN) are significantly depressed in their background activity and/or in their response to photically or electrically elicited input over the optic tract (Table 1). Activity of neurons in the dorsal portion of the lateral geniculate nucleus (LGNd), on the other hand, is much less likely to be affected, although in occasional neurons the effects should be profound. No evidence of disinhibition was apparent. It is concluded that the vTRN in the rat is highly dependent upon the VC in its activity, and is, thus, likely to be primarily a tool of the VC in the modulation of thalamic events. Extensive work of others shows that the vTRN provides inhibitory input to the LGNd, but in the present experiments the loss of inhibition via this route seems to be balanced by a corresponding loss of general excitatory input when the VC is inactivated.