Analgesia and autonomic function following intrathecal administration of morphine and norepinephrine to the rat

Abstract
The acute intrathecal (i.t.) administration of 10, 25, 50 and 100 .mu.g morphine and 7.5, 10, 15 and 30 .mu.g (-)norepinephrine (NE) to the rat produced dose-dependent, long-lasting analgesia as assessed by the tail-flick and hot-plate tests. For i.t. morphine, maximum analgesia was observed 30-60 min after drug administration. The duration of analgesia in the tail-flick test ranged from 30-150 min; the duration of analgesia in the hot-plate test ranged from 60-120 min. For i.t. NE, maximum analgesia was observed 15-60 min after drug infusion. The duration of NE-induced analgesia in the hot-plate test ranged from 45-120 min and was 120 min in the tail-flick test. The effects of acute i.t. and i.v. infusions of morphine (10 .mu.g) and NE (15 .mu.g) on heart rate, blood pressure, arterial pH, partial pressure of O2 (PO2), partial pressure of CO2 (PCO2), and standard bicarbonate were determined over 45 min in rats anesthetized with .alpha.-chloralose (70 mg/kg). Morphine significantly decreased PO2 throughout the experiment but did not affect blood pressure, heart rate, pH, PCO2 and standard bicarbonate. A significant increase in blood pressure (137% of control) was observed 2.5 min after i.t. administration of NE. I.v. NE produced a marked increase in blood pressure (246% of control) followed by a compensatory decrease in heart rate. There were no significant changes in blood gases with i.t. and i.v. NE. I.t. morphine and NE can produce effective analgesia with minimal effects on cardiovascular and respiratory function.