Functional interactions between estrogen and insulin-like growth factor-I in the regulation of alpha 1B-adrenoceptors and female reproductive function

Abstract

The ovarian hormone estradiol (E(2)) and insulin-like growth factor-I (IGF-I) interact in the CNS to regulate neuroendocrine function and synaptic remodeling. Previously, our laboratory showed that 2 d E(2) treatment induces alpha(1B)-adrenoceptor expression and promotes IGF-I enhancement of alpha(1)-adrenoceptor potentiation of cAMP accumulation in the preoptic area (POA) and hypothalamus (HYP). This study examined the hypothesis that E(2)-dependent aspects of female reproductive function, including alpha(1B)-adrenoceptor expression and function in the POA and HYP, are mediated by brain IGF-I receptors (IGF-IRs) in female rats. Ovariohysterectomized rats were implanted with a guide cannula aimed at the third ventricle and treated in vivo with vehicle or E(2) daily for 2 d before experimentation. Intracerebroventricular infusions of JB-1, a selective IGF-IR antagonist, were administered every 12 hr beginning 1 hr before the first E(2) injection. Administration of JB-1 during E(2) priming completely blocks hormone-induced luteinizing hormone release and partially inhibits hormone-dependent reproductive behavior. Reproductive behavior is restored by intracerebroventricular infusion of 8-bromo-cGMP, the second messenger implicated in alpha(1)-adrenergic facilitation of lordosis. In addition, blockade of IGF-IRs during E(2) priming prevents E(2)-induced increases in alpha(1B)-adenoceptor binding density and abolishes acute IGF-I enhancement of NE-stimulated cAMP accumulation in HYP and POA slices. These data document the existence of a novel mechanism by which IGF-I participates in the remodeling of noradrenergic receptor signaling in the HYP and POA after E(2) treatment. These events may help coordinate the timing of ovulation with the expression of sexual receptivity.

Fig. 1.

Effect of JB-1 infused into the third ventricle on E₂-induced α_1B-adrenoceptor (AR) density in the POA and HYP. Membranes from POA and HYP were prepared from OVX control and E₂-treated (EB) female rats infused with saline or JB-1 as described in Materials and Methods. A, Total³H-prazosin binding corrected for nonspecific binding reflects both α₁-AR subtypes. B, The α_1A-AR population is measured after CEC inactivation.C, The α_1B-AR population is determined by subtracting the binding of ³H-prazosin after CEC inactivation from total ³H-prazosin binding.B_max values were obtained by Scatchard analysis. The data presented are the means ± SEM from four independent replications. *p < 0.05.

Fig. 2.

Effect of JB-1 infused into the third ventricle on hormone-induced LH secretion. Serum for LH radioimmunoassay was prepared as described in Materials and Methods. OVX rats were given one of the following treatments: OIL, EB, or EB + progesterone (P). Multiple intracerebroventricular infusions of vehicle saline (SAL) or JB-1 were given 1 hr before the first EB injection and every 12 hr thereafter. The data presented are the means ± SEM from four to eight independent replications. *OIL + SAL, OIL + JB-1, EB + SAL, and EB + P + SAL (p < 0.05); **all other groups (p < 0.05), #OIL + SAL, OIL + JB-1, and EB + P + SAL (p < 0.05).

Fig. 3.

Effect of JB-1 infused into the third ventricle on lordosis behavior of E₂- and progesterone-primed female rats. A, Lordosis quotient (LQ) after chronic infusions of saline (SAL) or JB-1 alone or JB-1 followed by intracerebroventricular infusion of 1 μg of 8-bromo-cGMP 4 hr before behavior testing. B, Quality of lordosis (QL) in same animals shown in A. Values presented are the means ± SEM (n = 9). *p < 0.05. C, LQ after acute infusion of 4 μg of JB-1 or SAL 12 hr before behavior testing. Values presented are the means ± SEM (n = 6).

Fig. 4.

Effect of JB-1 infused into the third ventricle on E₂-dependent, IGF-I enhancement of NE-stimulated cAMP. POA and HYP slices from E₂-treated female rats were prepared from animals infused chronically with JB-1 or saline (SAL) as described in Materials and Methods. Slices were incubated for 15 min with 10 nm IGF-I followed by a 20 min incubation with 0.01 N HCl vehicle (VEH) or 100 μm NE. The PDE inhibitor 1 mm IBMX was included. The data presented are the means ± SEM from four independent replications. *VEH; **all other groups (p < 0.05).

Fig. 5.

Effect of JB-1 infused into the third ventricle on acute IGF-I activation of extracellular receptor activated kinase 2 (ERK2). HYP slices from E₂-treated female rats were prepared from animals infused chronically with JB-1 or saline (Sal) as described in Materials and Methods and incubated for 15 min with 10 nm IGF-I or vehicle (Veh). Immunoblots were done using a monoclonal antibody for phosphorylated ERK1/2 (P-ERK1/2) and for total ERK2. Phosphotyrosine immunoblots were quantitatively analyzed by taking the ratio of the OD of the P-ERK2 band to the OD of the total ERK2 band. The immunoblot shown is representative of two independent experiments.