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Abstract
Both electrical stimulation and injection of morphine into the midbrain periaqueductal
gray (PAG) produce analgesia in the rat. There is evidence that this analgesic effect
is mediated by a descending system that involves nucleus raphe magnus (NRM) and adjacent
reticular formation. In the studies reported here, the activity of the cells in the
PAG was increased by microinjection of glutamate in this area and its effect on both
the activity of single cells in the NRM and on a flexion reflex elicited by noxious
heat was measured. It is shown that an increase in the firing rate of the cells in
the PAG is associated with a raised threshold for flexion and is also correlated with
an increase in the firing rate of a majority of the cells in the NRM. This effect
on the flexion reflex can be abolished by (a) lesion of the nucleus raphe magnus and
a small area of the reticular formation surrounding this nucleus and (b) by nalazone
20 min after its i.v. injection. It is concluded that there is an excitatory connection
between the periaqueductal gray and the nucleus raphe magnus and that activation of
this system can cause analgesia.