THE EFFECTS OF MORPHINE, PENTOBARBITAL, PENTAZOCINE AND NALORPHINE ON BIOELECTRICAL POTENTIALS EVOKED IN THE BRAIN STEM OF THE CAT BY ELECTRICAL STIMULATION OF THE TOOTH PULP

¹ Department of Pharmacology, College of Medicine, Stale University of Iowa, Iowa City, Iowa

The purpose of this investigation was to compare the effects of morphine, pentobarbital, pentazocine and nalorphine on 1) the EEG activity, 2) the threshold for electrocortical recruitment, and 3) bioelectrical responses in the brain stem elicited by tooth pulp stimulation in the cat.

Following morphine (1, 2 and 4 mg/kg) and nalorphine (2, 4 and 8 mg/kg) the EEG activity was essentially unchanged. Typical spontaneous spindle bursts appeared in the EEG after Pentobarbital (2.5, 5 and 10 mg/kg). A shift to a higher frequency and a lower voltage was observed after pentazocine (2, 4 and 8 mg/kg).

The effects of morphine and pentobarbital on the threshold for recruitment were variable. Nalorphine had no effect, while pentazocine produced a significant increase in the threshold.

Only pentobarbital significantly depressed the evoked response in the ventralis posteromedialis to tooth pulp stimulation. In the periaqueductal gray matter, the response was depressed by all doses of all drugs except for nalorphine at 2 and 4 mg/kg. The evoked response in the ventral tegmentum of the mesencephalon was also depressed by all doses of all of the agents.

It was concluded that the observed effects of these agents could not be directly correlated with mechanisms of analgesia. However, it was suggested that the approach employed in this investigation, when extended to other areas of the brain might be of value in the elucidation of such mechanisms.