CHOLINERGIC RESPONSIVENESS OF NEURONS IN THE VENTROPOSTERIOR THALAMUSOF THE ANESTHETIZED RAT

Acetylcholine has been implicated as an important neurotransmitter in the mechanisms of thalamic activation. Cholinergic mechanisms are thou ght to directly underlie the high level of excitability observed in th alamic relay neurons during waking and rapid eye movement sleep. We so ught to determine if the cholinergic responsiveness of neurons in the ventroposterior nuclei of the thalamus in rat is consistent with this view. Neurons in the chloral hydrate-anesthetized rat were studied wit h extracellular recording and microiontophoretic application of cholin ergic agents. In most cases (63% of 63 cells), the ejection of the ago nist, carbachol, had no observable effect on spontaneous activity. Fac ilitation (25%), inhibition (8%) and inhibition followed by facilitati on (3%) were also observed. Carbachol ejections that by themselves wer e ineffective in altering spontaneous activity proved capable, in 93% of 28 cells, of antagonizing the uniformly facilitatory responses prod uced by glutamate ejection. The putative M1-selective, cholinergic ago nist, McN-A-343, was also ineffective alone in altering spontaneous ac tivity in the majority of cases (74% of 27 cells) and produced only in hibitory responses in the remaining seven neurons studied. Interacting applications of McN-A-343 and glutamate resulted, in all cases, in an tagonism of glutamate facilitation (N = 12). The various responses to applied cholinergic agonists were all capable of being antagonized by muscarinic receptor-blocking agents. Both the high proportion of inhib itory responses and the antagonism of glutamate facilitatory responses suggest that ventroposterior neurons in the rat differ from other tha lamocortical relay neurons in the rat and cat with regard to cholinerg ic responsiveness. Additionally, the lack of predominantly facilitator y responding renders it unlikely that cholinergic mechanisms directly underlie increases in excitability of ventroposterior neurons observed during waking and rapid eye movement sleep.