Antinociceptive and behavioral activation responses elicited by D-Pro(2)-Endomorphin-2 in the ventrolateral periaqueductal gray are sensitive to sex and gonadectomy differences in rats
Ek. Krzanowska et al., Antinociceptive and behavioral activation responses elicited by D-Pro(2)-Endomorphin-2 in the ventrolateral periaqueductal gray are sensitive to sex and gonadectomy differences in rats, PEPTIDES, 21(5), 2000, pp. 705-715
Sex differences have been observed in antinociception after morphine admini
stered into either the lateral ventricles, rostral ventromedial medulla, or
ventrolateral periaqueductal gray such that male rats exhibit significantl
y greater antinociception than female rats. Adult gonadectomy produced smal
l, but significant changes in morphine antinociception relative to same-sex
sham-operated controls. The present study examined whether sex and adult g
onadectomy differences were observed in antinociceptive responses after D-P
ro(2)-Endomorphin-2 (1-50 mu g) elicited from the ventrolateral periaqueduc
tal gray (vIPAG) on the tail-flick and jump tests in rats, and compared the
se effects with morphine antinociception. D-Pro(2)-Endomorphin-2 antinocice
ption in the vIPAG was significantly greater in estrous-phase, sham-operate
d and ovariectomized female rats relative to sham-operated and castrated ma
le rats on the tail-flick, but not jump test that differed markedly from th
e greater magnitude of morphine antinociception noted for male rats on both
tests. In testing whether D-Pro(2)-Endomorphin-2's antinociceptive sex dif
ferences were secondary to alterations in activity, similar decreases in th
e pattern of total activity were observed after D-Pro(2)-Endomorphin-2 in t
he vIPAG in male and female rats. In evaluating whether male and female rat
s differed in their behavioral activation responses after D-Pro(2)-Endomorp
hin-2 in the vIPAG, significantly more excessive grooming, seizures, barrel
rolls and explosive running behaviors were observed after D-Pro(2)-Endomor
phin-2 in male, but not female rats during the precise periods of time when
they were failing to display robust antinociceptive responses on the tail-
flick test. Thus, the different patterns of sex differences after D-Pro(2)-
Endomorphin-2 in the vIPAG appear to be attributable to sex-dependent alter
ations in behavioral activation rather than nociceptive processing per se.
(C) 2000 Elsevier Science Inc. All rights reserved.