J.M. DiPirro, M.B. Kristal / Brain Research 1014 (2004) 22-33
29
with vehicle (v2[1, n=24] = 0, p>0.05), on the very slight
level of antinociception produced by the moderate (150
nmol) spiradoline dose (v2[1, n =22] = 0.09, p>0.05), or on
the modest antinociception produced by the high (200 nmol)
spiradoline dose (v2[1, n =22] = 0.18, p>0.05). Spiradoline
is considered to be a selective agonist for the n1 receptor
subtype [67]. Therefore, these data indicate that the opioid-
enhancing action of POEF, already demonstrated at the y1
receptor, extends to the n1 receptor.
POEF enhancement, although to a small degree, is
clearly evident statistically at the low dose of spiradoline,
but disappears at the high dose. The reason for the dose
selectivity may be that spiradoline contains both (+) and
(—) enantiomers, and the (+) enantiomer shows some weak
A-agonist activity [54,92]. Therefore, the effect of POEF on
spiradoline antinociception may represent a narrow range of
dose effectiveness: enhancement of n activity only at low
doses (100 nmol).
Prior to the execution of Experiment 3, pilot studies were
performed using a n agonist that is more selective than
spiradoline: U-50,488 [46,93]. However, hotplate-test
results indicated that this n ligand does not produce anti-
nociception when injected i.c.v. This finding, together with
reports that spiradoline, although slightly less selective than
U-50,488, displays greater antinociceptive potency in the rat
warmplate (49.5 jC) and hotplate (55.0 jC) assays, after
systemic injection [60,92], and in the mouse hotplate test,
after intracranial injection [70], led to the decision to use
spiradoline in Experiment 3.
That A agonism does not contribute appreciably to
spiradoline antinociception is an important consideration
in the interpretation of the present results, and in the
conclusion that POEF enhances n receptor activity. Note
that spiradoline, although less selective than U-50,488,
exhibits considerable n receptor binding preference, as
mentioned [46]. Furthermore, the n-receptor-specific nature
of spiradoline antinociception can be asserted on the basis of
its reversibility by n-selective antagonism. This has been
demonstrated both with conventional pharmacological strat-
egies and with molecular (antisense) techniques [4,8,60].
These results indicate that the antinociceptive effects of
spiradoline are mediated exclusively at the n receptor, and
together with evidence that POEF decreases the A antinoci-
ception generated by DAMGO (Experiment 2), make it very
likely that the enhancing action of POEF on antinociception
induced by the low dose of spiradoline represents a n, and
not a A, effect.
6. General discussion
The present studies provide evidence that POEF, ingested
as a component of placenta, exerts a complex modulatory
influence on antinociception produced by the central phar-
macological activation of different opioid receptors. POEF
ingestion enhanced the antinociceptive efficacy of i.c.v. y-
opioid receptor agonist DPDPE and i.c.v. n-opioid receptor
agonist spiradoline, but decreased the antinociceptive effi-
cacy of i.c.v. A-opioid receptor agonist DAMGO. These
results strongly suggest that the antinociception-modulating
properties of POEF are positive for y and/or n activity, and
negative for A activity. Furthermore, the absence of a POEF
effect on nociceptive threshold when no exogenous opioid
was given, in each of the three experiments reported here,
confirms all previous reports [39] that POEF acts, not as an
analgesic itself, but rather as a potent modulator of opioid
antinociceptive activity.
Previous observations indicate that the modulatory nature
of POEF on nonspecific opioid receptor-induced antinoci-
ception is biphasic: antinociception-enhancing at low opiate
doses, and antinociception-inhibiting at high opiate doses
(Kristal laboratory, unpublished observations). The present
data, which indicate a receptor-specific mode of action of
POEF, suggest that such a biphasic effect may be due to
differential action at different opioid receptors. In the
present study, in no case was POEF ingestion linked to
both an elevation and a reduction of nociceptive-response
levels at different doses of the same agonist.
The results of this series of studies indicate a role for both
y and n receptors in the antinociception-enhancing effects of
ingested POEF. However, the data also suggest that the
degree of enhancement, presumably a reflection of the
effectiveness of POEF to modify activity at a particular
opioid site, is different for these two receptors. It is likely
that potency differences in enhancement observed at these
receptors reflect, in part, the different maximal analgesic
effect of each agonist and the different physicochemical
properties of each agonist (which determine, for example,
rate and distance of drug diffusion), and that these differ-
ences might not be apparent if POEF ingestion were tested
with other selective agonists.
The present strategy, the use of individual receptor-
preferring agonists, allowed for a straightforward test of
the ability of POEF, as ingested placenta, to modulate
antinociceptive activity induced at each individual receptor
type. However, this strategy does not rule out the small
possibility of a non-receptor-selective action by each ago-
nist. A more definitive test might be to measure the effect of
POEF on antinociception induced by an opioid receptor-
selective agonist during simultaneous blockade of alterna-
tive receptors (i.e., other opioid receptors). Furthermore, the
strategy we used does not provide information about the
influence of POEF on antinociception induced by simulta-
neous activation of more than one receptor, which may be
different—quantitatively or qualitatively—from the simple
sum of antinociceptive activity at each receptor type
[63,75,77,81]. Such information could be obtained by
testing POEF with combinations of opioid receptor-selective
agonists.
It seems unlikely that non-specific explanations (e.g.,
thermoregulatory or motoric effects) can account for the
POEF modulation of hotplate paw-lick/jump latency