Many neurones in the thalamus, like neurones in other parts of the CNS are excited by iontophoretlcally applied acetylcholine (ACh), and it has been suggested that ACh may be involved in synaptic transmission in the thalamus. In these experiments, the iontophoretlc technique was employed to investigate the location, and the neurophysiological and pharmacological properties of neurones in the ventrolateral nucleus of the thalamus (VL), which responded to electrical stimulation of the ipsilateral nucleus of the posterior commisure (NPC). Stimulation of the contralateral brachium conjunetivum (BC) was used to confirm the presence of the recording micropipette in VL. Four drugs were applied in various sequences to most neurones encountered above, in and below VL. DL-homocysteate (DLH) was used for the activation and localisation of quiescent neurones; ACh was applied to test whether the neurones were cholinoceptivej eserine (physostigmine) was used as an anticholinesterase; and atropine was used as a muscarinic blocking agent. Choiinoceptive cells were found above (3.0-6.0 mm below the fornix), and In VL (0.0-10.0mm below the fornix), the highest proportion being located in VL. The majority of cholinoceptive cells in VL responded to NPC and to BC stimulation. Cholinoceptive neurones located above VL were not evoked either by NPC or BC stimulation, while those in the lowest part of VL were evoked by NPC stimulation only. Although atropine blocked the effect of iontophoretically applied ACh, It did not affect synaptic responses evoked by stimulation of NPC and/or BC fibers. Eserine excited some cells and potentiated the actions of ACh. These results indicate that a pathway arising from the NPC projects to the ipsilateral VL and that this fiber tract is non-cholinergic. / Medicine, Faculty of / Cellular and Physiological Sciences, Department of / Graduate