HYPOTHALAMIC CUTS AND MATERNAL BEHAVIOR
635
hypothalamic area (A 6.8-7.0), severing most of the
mediolateral connections of the MPO between the anterior
commissure and the optic chiasm (see Fig. 2). In the other
two rats, the cuts began in the middle of the anterior-
posterior extent of the MPO (A 7.4-7.6) and extended well
into the anterior hypothalamic area (A 6.6-6.8).
MFB cuts were Iocatedjust posterior to the posterior tips
of the ventromedial nuclei in a coronal plane that varied from
A 4.5 to 5.0 [47]. The MFB was cut bilaterally in all 12 rats
from L 0.7 to 1.7 and from just ventral to the mammil-
Iothalamic tract (H -3.0) to the base of the brain. Thus, the
fornix and parafornical MFB were severed bilaterally in all
12 rats with MFB cuts (see Fig. 2). In four rats the cuts
extended bilaterally into the medial hypothalamus almost to
the third ventricle (L 0.3); in two of these rats the cuts ex-
tended bilaterally as far lateral as L 2.0.
Four of the rats in the ASYM Group had unilateral MPO
cuts that were similar to those described above for rats with
bilateral MPO cuts. The other four rats in this group had
slightly longer cuts, which extended caudally to A 6.4. The
location and extent of unilateral MFB cuts in rats in the
ASYM Group were the same as described above for rats
with bilateral MFB cuts.
The rat with MPO cuts that cannibalized some of its
young was the only one with cuts that extended rostrally into
the diagonal band of Broca (A 8.2). This rat was also among
the four with MPO cuts that ate all their delivered placentas.
However, comparison of the knife-cut damage in these rats
with that in rats that did not eat all their delivered placentas
revealed no relationship between knife-cut placement and
Parturitional placentophagia. Similarly, no relationship was
found between placement of MPO cuts and the incidence of
nestbuilding (two rats with MPO cuts had built nests by Test
U).
DISCUSSION
All three knife cuts reduced the likelihood that a rat would
eat all of her delivered placentas, and prevented the display
of retrieving, crouching, pup-licking, and nestbuilding at 6 hr
postpartum. These findings suggest that connections be-
tween the MPO and the posterior MFB constitute a func-
tional longitudinal neural system that is involved in produc-
ing the rapid onset of both the ingestive and caretaking com-
ponents of Periparturitional behavior at delivery in primipar-
ous rats. That the knife-cut-induced deficits in the onset of
placentophagia, maternal behavior, and nestbuilding were
caused directly by disruption of a neural system specifically
involved in producing the rapid onset of these behaviors is
supported by the finding of no differences between the
SHAM Group and the three knife-cut groups with respect to
postoperative food and water intake, timing of parturition, or
condition of offspring.
It might be that the observed deficits in placentophagia,
retrieving, pup-licking, and nestbuilding were merely an in-
direct result of a general disruption of oral behaviors. How-
ever, this seems unlikely, since cuts disrupted crouching, a
non-oral caretaking behavior, but did not disrupt feeding and
drinking, oral non-caretaking behaviors.
The conclusion that the onset of parturitionaɪ behaviors is
controlled by a longitudinal neural system was based on the
criterion that cuts at all three placements must produce the
same behavioral deficits. This criterion was also applied in
determining whether deficits observed during the subsequent
5-day test period could be attributed to the disruption of a
longitudinal system.
By the end of testing on Day 5 postpartum, many of the
rats with asymmetrical cuts had begun to display full mater-
nal responsiveness, including retrieving, crouching, pup-
licking, and nestbuilding. This suggests that the eventual
display of maternal behavior and nestbuilding does not de-
pend on the longitudinal system found to be necessary for
the rapid onset of these behaviors at parturition. The even-
tual display of maternal responsiveness in rats with asym-
metrical cuts, and the eventual display of some but not all
maternal behaviors by the other knife cut groups (e.g., pup-
licking but not nestbuilding) are thought to be similar to the
eventual display of maternal responsiveness in virgin rats in
response to pup stimuli. Although it is possible that within-
group differences in latency to respond to pups could be
explained by differential rates of recovery from surgical
trauma, other behavioral measures indicate satisfactory re-
covery from surgery, and the observed latency differences
are also consistent with the variability in rates of maternal
sensitization typically observed in virgin rats.
In contrast to the temporary effects of ASYM cuts,
both MPO cuts and MFB cuts produced behavioral deficits
that appeared to be permanent. Consistent with the findings
of previous investigations [39,57], MPO cuts eliminated re-
trieving and nestbuilding for the entire 5-day test period.
Since these effects were not duplicated by ASYM cuts, they
cannot be attributed to the disruption of a longitudinal sys-
tem. That is, the lateral connections of the MPO that are
important for retrieving and nestbuilding apparently are not
those which pass through the posterior MFB. There is, in
fact, evidence suggesting that MPO projections that descend
in the region of the supraoptic commissures, and that were
spared in rats with ASYM cuts, are critical for the display of
retrieving and nestbuilding [29-31]. Despite these long-term
deficits in retrieving and nestbuilding, however, most of the
rats with MPO cuts eventually began to lick and crouch over
pups. These results provide support for the idea that differ-
ent components of maternal behavior are controlled by sepa-
rate neural systems [57].
MFB cuts eliminated nestbuilding for the entire 5-day test
period and produced hyperphagia on the high-fat diet.
Neither of these effects was duplicated by ASYM cuts, and
therefore cannot be attributed to the disruption of a longitud-
inal system. The production of hyperphagia by MFB cuts is
not surprising, since MFB cuts that extend into the medial
hypothalamic area produce hyperphagia [29,52]. Although
the production of hyperphagia by MFB cuts casts some
doubt on the specificity of the deficits in parturitional behav-
ior exhibited by rats with these cuts, this problem of inter-
pretation does not have any serious implications for the
conclusions that a longitudinal system mediates the onset of
parturitional behaviors; rats with ASYM cuts exhibited
deficits in these behaviors but were not hyperphagic.
On the basis of observations made at 6 hr postpartum that
rats with knife cuts had not eaten all their delivered placen-
tas, were not maternally responsive toward foster pups, and
did not build nests, we have concluded that a longitudinal
system participates in the control of both the ingestive and
caretaking components of parturitional behavior.
In some respects, the parturitional behavior of rats with
knife cuts was similar to that of SHAM controls. During
delivery of individual pups, they adopted the head-between-
the-heels position and pulled at pups and placentas as they
were delivered, licking them as they emerged. They some-