Abstract
1. The responses of neurones in the anterior pretectal nucleus (APTN) to electrical stimulation of the dorsal columns at twice the threshold for A fibres were studied in the rat anaesthetized with urethane. 2. APTN cells were excited by dorsal column stimulation. Forty-six discharged phasically in response to a single stimulus. Sixteen cells did not respond phasically but slowly increased the discharge rate with repeated stimulation. 3. Electrical stimulation of the contralateral gracile fasciculus caused neurones in the APTN to discharge with a variable latency of 2-22 ms. Stimulations of the ipsilateral gracile and contralateral cuneate fasciculi had weaker effects. 4. Microinjection of DL-homocysteic acid into the contralateral gracile nucleus increased the discharge rate of APTN neurones. Microinjection of gamma-aminobutyric acid into the contralateral gracile nucleus blocked the gracile fasciculus evoked excitation of APTN neurones. 5. On thirteen occasions cells in the gracile nucleus were driven antidromically by electrical stimulation of the APTN. 6. It is concluded that electrical stimulation of the gracile fasciculus activates a monosynaptic excitatory input to the APTN.