854
DI PIRRO, THOMPSON AND KRISTAL
from baseline was observed regardless of the fluid ingested,
F(l,20)<1.0.
Experiment 2B: Central Morphine Administration and Blocked
Peripheral Receptors
METHOD
Subjects
The subjects were 18 nulliparous, Long-Evans (hooded) rats,
90-150 days of age, with a mean body weight of 282 ±4.1 g.
The rats were morphine-naive, but had previously experienced a
single TFL test. The subjects were housed and maintained iden-
tically to those in Experiment 2A.
Apparatus
Measurement of TFL, infusion of fluids, and intracerebro-
Ventricular administration of drugs were conducted as in Experi-
ment 2A.
Procedure
Each rat was assigned to one of two Enhancer groups [AF,
BB (0.25 ml, PO)]. All rats received a central injection of mor- .
phine sulfate, MS (2.5 μg in 2.0 μl, ICV), and a systemic in-
jection of QN (8 mg∕kg, SC). Pain threshold was measured 20
and 40 min after the orogastric infusion of AF or BB.
The sequence of treatments was identical to that used in Ex-
periment 2A, except that a systemic injection of QN was given
20 min prior to determination of baseline TFL, to block periph-
eral but not central opioid receptors.
All surgical and histological procedures were the same as
those in Experiment 2A. Four rats were excluded from the anal-
ysis because of inaccurate cannula placement.
RESULTS AND DISCUSSION
The results of Experiment 2B are depicted in Fig. 3.
A 1-way ANOVA comparing TFL baselines showed that they
did not differ between the AF and BB groups, F(l,12)<1.0.
Analysis by 2-way ANOVA did not yield statistically signifi-
cant results. Inspection of the data revealed that the variability
of responses at 40 min was greater than at 20 min (primarily
due to the change in response of one rat). Since our main goal
was to determine whether AF treatment enhanced analgesia at
any time, the 20-min time point was analyzed separately by
1-way ANOVA. This analysis revealed that at 20 min after in-
fusion, AF ingestion did significantly elevate pain threshold in
QN-treated rats receiving ICV MS, F(l,12) = 5.33, p<0.05.
Since peripheral opioid receptors were blocked in this experi-
ment, the results clearly show that amniotic-fluid ingestion, and
therefore POEF ingestion, enhances the analgesic effect of cen-
trally administered morphine by action on central opioid recep-
tors. The combined results of Experiments 1 and 2 do not
exclude the possibility of an effect of POEF on the peripheral
action of morphine in addition to enhancement of the central ac-
tion of morphine.
EXPERIMENT 3
The third experiment examined whether eliminating the cen-
tral action of morphine also prevented enhancement of analgesia
by ingestion of amniotic fluid. Enhancement of systemically ad-
ministered morphine was measured while central opioid recep-
tors were blocked by centrally administered QN.
METHOD
Subjects
The subjects were 18 nulliparous, female, Long-Evans (hood-
ed) rats, 90-150 days of age, with a mean body weight of
284±5.9 g. All rats had experienced a single TFL test in a prior
study, but none had been exposed to morphine. The rats were
housed and maintained in a manner identical to those in Experi-
ment 2.
Apparatus
TFL assessment, fluid infusion, and Intracerebroventricular
drug administration were conducted as in Experiment 2.
Procedure
Each subject was assigned to one of two Enhancer groups
[AF, BB (0.25 ml, PO)]. The rats in both groups received a
systemic injection of MS (3 mg∕kg, IP) and a central injection
of QN (8.0 μg in 2.0 μl, ICV). Analgesia was assessed 15 min
after orogastric infusion of Enhancer.
The sequence of treatments was similar to that employed in
Experiment 1, except that the QN was injected intracerebroven-
tricularly to block the central action of morphine.
The procedures used in surgery for implantation of indwell-
ing Intracerebroventricular cannulae and verification of cannula
placement were identical to those employed in Experiment 2.
Histological examination of brain sections revealed that four
cannula placements did not fall within the right lateral ventricle;
those rats were therefore excluded from statistical analysis.
RESULTS AND DISCUSSION
The baseline TFLs did not differ significantly between the AF
and BB Groups, F(l,12)<1.0.
The 1-way ANOVA showed that AF ingestion did not ele-
vate pain threshold in rats treated with systemic morphine and
central QN. The mean % change from baseline TFL for rats
treated with AF was —1.84 ±2.39 s and for rats treated with
BB was +1.27±1.50 s, F(l, 12)= 1.34, p>0.05. Since cen-
trally administered QN restricts morphine to its peripheral ac-
tion, this result in combination with the results of Experiment 1
suggests that AF ingestion does not enhance the peripheral anal-
gesic effects of morphine.
GENERAL DISCUSSION
The results of this study clearly show that ingestion of POEF
in amniotic fluid enhances opiate-mediated analgesia by affect-
ing the central action of morphine.
In Experiment 1, a blockade of peripheral opiate receptor
sites attenuated, but did not eliminate, enhancement of system-
ic-morphine-mediated analgesia by amniotic-fluid ingestion. This
result is consistent with an effect on a central opioid mechanism.
In Experiment 2, AF ingestion enhanced the analgesic effect of
central morphine administration both with and without blockade
of peripheral opioid receptors. The combined results of Experi-
ments 1 and 2 unequivocally demonstrate that ingestion of am-
niotic fluid, and therefore POEF, enhances the central locus of
opiate action, but do not preclude the possibility that it also en-
hances the peripheral locus. The results of Experiment 3 suggest
that a blockade of central opiate receptors prevents the enhance-
ment of systemic-morphine-mediated analgesia by amniotic-fluid