Participation of Placental Opioid-Enhancing Factor in Opioid-Modulated Events at Parturition
Mark B. Kristal
Behavioral Neuroscience Program, Department of Psychology
University at Buffalo, Buffalo, NY 14260, USA
kristal @ acsu.buffalo.edu
Endogenous opioids play a significant role in mammalian parturition as well as in the behaviors associated with delivery itself and toward the emerging young. Distention of the uterus, and distention and mechanical stimulation of the vaginal/cervical area are critical stimuli in the positive feedback loop associated with expulsion of the fetus, and in the onset of maternal behavior upon emergence of the fetus [9, 11, 25]. These stimuli are sufficiently "stressful", "painful", or "aversive" so that they occur during a period of elevated endogenous opioid levels and elevated pain threshold [8, and see 12 for review]. Furthermore, the elevation of endogenous opioids in certain brain areas at delivery, namely the ventral tegmental area, is associated with the rapid onset of appropriate maternal caretaking behavior at that time [24]. One might hypothesize that if endogenous opioids facilitate maternal behavior and delivery, that the level of opioid present would be the same for the optimization of both dimensions of parturition. In fact, increasing pain threshold beyond that observed immediately prior to delivery, in rats, by administration of exogenous opioid (morphine), interferes with the onset of proper maternal care [1, 10, 19, 21]. How then is antinociception increased, at parturition, in a way that does not interfere with the expression of proper maternal behavior?
Placenta and amniotic fluid contain a molecule(s) that potentiates the antinociceptive action of opioids. We refer to this substance as POEF, for Placental Opioid-Enhancing Factor. In an extensive series of experiments, we elucidated many of the features of POEF [see 12 for review]. The following is a summary of the findings of the early phase of POEF research:
More recent research has focused on the differential effects of POEF administration on various opioid-mediated phenomena (e.g., hyperthermia, locomotor activation, antinociception, maternal retrieval). These differential effects have led us to speculate that either POEF differentially modifies different classes of opioid receptors, or has location-specific effects, or both.
In an attempt to determine the extent to which POEF is opioid-receptor specific, DiPirro has conducted a series of studies in which the effect of ingested placenta was tested on rats receiving an intraventricular injection of a specific opioid-receptor agonist [2, 3, 6]. The agonists used were DPDPE for d-opioid receptors; DAGO for m-opioid receptors; and U-62066 (spiradoline) for k-opioid receptors. Different doses of agonist were injected in a constant volume through indwelling intracerebroventricular cannulae, and pain thresholds were assessed at peak agonist effect, using a 52° C hotplate, after ingestion of 1g of placenta or meat control. No repeated measures were used.
DPDPE -- The effect of placenta (and presumably POEF) ingestion on d-opioid antinociception produced by an intracerebroventricular injection of DPDPE is presented in the next figure (Fig. 3):
Clearly, placenta ingestion potentiated the effect of d-opioid mediated antinociception produced by DPDPE. Without placenta, DPDPE did not produce a significant elevation of pain threshold at a dose of 62 nmoles. Even at 70 nmoles, the increase in pain threshold was minor. With placenta ingestion, however, DPDPE produced a 250% increase in pain threshold at a dose of 62 nmoles.
DAGO -- The effect of placenta (and presumably POEF) ingestion on m-opioid-mediated antinociception produced by the administration of DAGO can be seen in the following figure (Fig. 4):
The antinociceptive effect of DAGO alone (along with ingestion of meat control) appears at the 0.29-nmole dose. At 0.39 nmoles, the antinociception is quite pronounced. However, in combination with placenta ingestion, a significant attenuation of DAGO-induced antinociception becomes apparent at the 0.29-nmole dose, and the elevation of pain threshold produced by 0.39 nmoles DAGO is all but eliminated. Therefore, placenta (and presumably, therefore, POEF) ingestion, blocks antinociception mediated by m-opioid receptors.
U-62066 -- The effect of placenta (and presumably POEF) ingestion on k-opioid-mediated antinociception produced by central administration of U-62066 (spiradoline) is illustrated in the following figure (Fig. 5):
At the lowest dose of U-62 used, 100 nmoles, U-62 alone was clearly ineffective, whereas U-62 in conjunction with placenta ingestion, produced a significant elevation of pain threshold.
The effect of POEF on central opioid processes is almost
certainly location specific as well as receptor specific; very recent data
indicate that antinociception produced by morphine injected directly into
the periaqueductal gray matter is unaffected by placenta ingestion [4].
Opioid receptors are differentially distributed, and clearly show different
effects at different locations. Bridges' work, for instance, has demonstrated
that increased opioid levels in the medial preoptic area interfere with
maternal behavior [1, 10, 19]. In contrast, Thompson's recent work has
shown that increasing the opioid activity of the ventral tegmental area
(by injecting morphine), which also increases motivated behavior, facilitated
the onset of maternal behavior in inexperienced, nonpregnant rats. Conversely,
blocking the effect of morphine injection in the ventral tegmental area,
by pretreating the rats with the opiate antagonist naltrexone, blocked
the facilitative effect on maternal behavior of the intra-tegmental morphine
injection. Furthermore, interfering with the effect of endogenous
opioids in the ventral tegmental area at the end of parturition
by intra-tegmental injection of naltrexone methobromide, severely
inhibited the naturally-occurring onset of maternal behavior
at that time [24]. The following figure (Fig 6) shows the deleterious effect
on the rate of onset of maternal behavior, at parturition, of naltrexone
methobromide (quaternary naltrexone) injected into the ventral tegmental
area.
Although POEF ingestion can modify the effects of intra-tegmental morphine on locomotor behavior [24], we do not yet know if POEF ingestion potentiates the effect of intra-VTA morphine on maternal behavior; POEF, ingested in amniotic fluid, however, did not increase the effect of a subthreshold systemic dose of morphine on maternal retrieval to the point where it disrupted the retrieval of young by mother rats (see Fig. 2).
The mechanical stimulation and distention of the vaginal/cervical area during expulsion of the fetus is well above the minimum amount necessary to trigger pseudopregnancy in nonpregnant females experiencing elevated estrogen levels. Yet the parturient rat does not enter pseudopregnancy, but rather experiences a postpartum estrus and, if inseminated, a postpartum pregnancy. We hypothesized, based on some supportive pilot data, that afterbirth ingestion would decrease the likelihood that relatively intense vaginal/cervical stimulation would induce pseudopregnancy. We tested this hypothesis in a study [23] in which groups of nonpregnant rats received vaginal/cervical stimulation of pressures of either 75, 125, 175 or 225 g. The stimulation applied to the vaginal/cervical area, as expected [14], produced a dose-dependent level of antinociception (measured as lengthening of tail-flick latency or hot-water tail-withrawal latency). In addition, orogastric infusion of amniotic fluid, shortly before the application of vaginal/cervical stimulation, enhanced the level of antinociception produced by 125 g pressure. Interestingly, the 225-g level of stimulation induced pseudopregnancy in 44% of the rats receiving an orogastric infusion of saline, but only in 10% of the rats infused with amniotic fluid. A detailed series of follow-up studies on the effect of amniotic-fluid ingestion on pseudopregnancy induction is currently being conducted by Patricia Abbott.
Mechanical stimulation during delivery, resulting from
expulsion and activity of the fetus, produces major activation of the pelvic,
hypogastric [15], and pudendal nerves. This afferent information, including
nociceptive, results in a central release of endogenous opioids. The opioids,
in turn, orchestrate a variety of parturitional phenomena including an
elevation of pain threshold, the onset of maternal caretaking behaviors,
and even perhaps a reduction in the likelihood that this vaginal/cervical
stimulation will induce pseudopregnancy, a condition that would eliminate
the postpartum estrus. Paradoxically, a higher level of endogenous opioids,
despite producing an apparently advantageous additional rise in pain threshold,
would counteract some other beneficial effects of opioids, especially those
relating to the emergence or performance of the complex of behaviors necessary
for caretaking of the young. However, it is apparent that a substance is
available in afterbirth material (POEF) that, when ingested by the mother,
potentiates the antinociceptive actions of endogenous opioids (raising
pain threshold above that level produced solely by the opioids), but not
the actions that would, if increased, have a deleterious effect on maternal
behavior. This differential effect on certain opioid functions is the result
of selective action on some classes of opioid receptor (as well as some
degree of location specificity). Mu-opioid receptor activity, largely associated
with negative "side" effects of opioid activity, is suppressed by POEF,
whereas k- and d-opioid-mediated
phenomena are enhanced. This is particularly interesting, and important,
in light of recent studies showing that k-opioid
analgesics (and to a lesser extent d-opioid
analgesics) are particularly effective in females [e.g., 7, 20].
