Cyclic ovarian hormone modulation of supraspinal Δ⁹-tetrahydrocannabinol-induced antinociception and cannabinoid receptor binding in the female rat

Highlights

• Cyclic estradiol enhanced females' supraspinal Δ⁹-THC-induced antinociception.

• Ovarian hormones do not modulate supraspinal Δ⁹-THC-induced sedation.

• Estradiol produces time-dependent changes in CB₁ receptor density and affinity in the brain.

Abstract

Estrous cycle-related fluctuations in delta-9-tetrahydrocannabinol (THC)-induced antinociception have been observed in the rat. The aim of this study was to determine which major ovarian hormone modulates the antinociceptive effects of i.c.v. THC, and whether hormone modulation of THC's behavioral effects could be due to changes in brain cannabinoid receptors (CBr). Vehicle (oil) or hormones (estradiol or progesterone, or both) were administered to female rats on days 3 and 7 post-ovariectomy. On the morning or afternoon of day 8 or day 9, vehicle or THC (100 μg) was administered i.c.v. Paw pressure, tail withdrawal, locomotor activity and catalepsy tests were conducted over a 3-h period. Estradiol (with and without progesterone) enhanced THC-induced paw pressure antinociception only. Ovarian hormones time-dependently modulated CBr in brain structures that mediate antinociception and locomotor activity, but the changes observed in CBr did not parallel changes in behavior. However, the time course of CBr changes must be further elucidated to determine the functional relationship between receptor changes and antinociceptive sensitivity to THC.

Introduction

Behavioral effects of cannabinoids are sexually dimorphic, with female rats being more sensitive than males to the reinforcing, antinociceptive, sedative and cognition-impairing effects of cannabinoids (Tseng and Craft, 2001, Romero et al., 2002, Cha et al., 2007, Fattore et al., 2007, Craft and Leitl, 2008, Wakley and Craft, 2011, Craft et al., 2012). Sex differences in antinociceptive response to cannabinoids such as delta-9-tetrahydrocannabinol (THC) may be explained by ovarian hormones. Estradiol (E2) enhanced systemic THC-induced antinociception in ovariectomized (OVX) females compared to OVX females without hormone replacement (Craft and Leitl, 2008). This enhancement was similar to that observed in gonadally intact females tested in estrus vs. those tested in diestrus (Craft and Leitl, 2008). Estrous stage also modulates antinociception following supraspinal THC administration: females in late proestrus showed greater paw pressure antinociception than females in estrus and males (Wakley and Craft, 2011). These studies demonstrate that ovarian hormones likely contribute to sex and estrous stage-related differences in cannabinoid antinociception.

Ovarian hormones also influence endocannabinoid signaling. E2 decreased pituitary cannabinoid receptor (CBr) mRNA levels in OVX females (González et al., 2000). Similarly, acute E2 decreased Gα protein activation in the cortex and hippocampus of prepubescent female rats (Mize and Alper, 2000). In the limbic forebrain, ovariectomy decreased, and E2 increased CBr density (Bonnin et al., 1993, Rodriguez De Fonseca et al., 1994). Similarly, ovariectomy decreased CBr density in amygdala but increased it in the hypothalamus and hippocampus; E2 reversed these changes (Riebe et al., 2010). The other major ovarian hormone, progesterone (P4), increased CBr density in the limbic forebrain and midbrain but decreased it in the striatum of OVX females; however, when P4 was given with E2, hypothalamic CBr density was increased (Rodriguez De Fonseca et al., 1994), suggesting that P4 can reverse E2's effects. Although hormone modulation of the brain endocannabinoid system is a likely mechanism underlying ovarian hormone enhancement of the behavioral effects of cannabinoids in females, hormone effects appear to be brain structure-dependent and hormone-specific.

Previous studies examining ovarian hormone modulation of the endocannabinoid system and the behavioral effects of cannabinoids have used hormone regimens that do not mimic natural hormone fluctuations in a gonadally intact female. Typically, a single hormone was administered, and effects were examined at a single time point thereafter (Bonnin et al., 1992, Bonnin et al., 1993, Rodriguez De Fonseca et al., 1994, Craft and Leitl, 2008, Kalbasi Anaraki et al., 2008, Riebe et al., 2010). During the 4- to 5-day estrous cycle of a rat, E2 and P4 peak in early and late proestrus, respectively (Smith et al., 1975, Feder, 1981, Freeman, 1988). Thus, in the present study a modified cyclic hormone regimen (Asarian and Geary, 2002) was used to determine which ovarian hormone is responsible for the enhanced supraspinal THC-induced antinociception previously observed in gonadally intact, late proestrous females (Wakley and Craft, 2011). The goals of the present research were to determine if: 1) E2, P4 or both are responsible for increased supraspinal THC-induced antinociception; and 2) cyclic ovarian hormones modulate CBr density/affinity in brain areas known to mediate antinociception.