Analgesia produced by morphine when acting from the liquor space

1 In cats analgesia was produced by morphine sulphate introduced into different parts of the liquor space in doses too small to be effective on intravenous injection. Analgesia was measured with the tail pinch method of Russell & Tate (1975). 2 On infusion into the fourth ventricle or into the subarachnoid space beneath the ventral surface of the brain stem caudal to the pons, doses of 100 to 200 mug of morphine sulphate were sufficient to produce strong long-lasting analgesia. On injection into the cisterna magna somewhat larger doses (400 to 800mug) were required. 3 It is concluded that the site where morphine acts when producing analgesia in all three circumstances is at the ventral surface of the brain stem. 4 The possibility is discussed that the structures acted upon are tryptaminergic nerve fibres. They arise from the raphe nuclei, belong to a descending inhibitory pathway, and on their way to the spinal cord, reach the ventral surface of the brain stem lateral to each pyramid, where they could be reached and acted upon by the morphine. This theory postulates a morphine sensitivity of tryptaminergic nerve fibres.