Ovarian hormone dependence of alpha(1)-adrenoceptor activation of the nitric oxide-cGMP pathway: relevance for hormonal facilitation of lordosis behavior

Abstract

The ovarian hormones estradiol (E(2)) and progesterone (P) facilitate rat lordosis behavior in part by regulating the expression of and signal transduction by adrenoceptors in the hypothalamus (HYP) and preoptic area (POA). The major adrenoceptor subtype mediating E(2) and P facilitation of lordosis is the alpha(1)-adrenoceptor. In the present studies, we tested the hypotheses that (1) alpha(1)-adrenoceptors in the HYP enhance lordosis responses by activating the nitric oxide (NO)-cGMP signaling pathway, and (2) coupling of alpha(1)-adrenoceptors to this signal transduction pathway is hormone-dependent. Basal levels of cGMP were significantly higher in HYP and POA slices from animals treated with E(2) and P when compared with slices from ovariectomized controls or females treated with only E(2) or P. When slices of HYP and POA from ovariectomized female rats were incubated with norepinephrine or the selective alpha(1)-adrenoceptor agonist phenylephrine, cGMP accumulation was observed only if slices had been derived from females treated with both E(2) and P before experimentation. Moreover, alpha(1)-adrenoceptor stimulation of cGMP synthesis was blocked by an inhibitor of NO synthase, confirming that these receptors act by NO-mediated stimulation of soluble guanylyl cyclase. Behavioral studies demonstrated further that the cell-permeable cGMP analog 8-bromoadenosine-cGMP reverses the inhibitory effects of the alpha(1)-adrenoceptor antagonist prazosin on lordosis behavior in E(2)- and P-treated female rats. Thus, the NO-cGMP pathway mediates the facilitatory effects of alpha(1)-adrenoceptors on lordosis behavior in female rats, and previous exposure of the HYP and POA to both E(2) and P are required to link alpha(1)-adrenoceptors to this pathway.

Fig. 1.

Priming with EB plus P increases basal cGMP levels in HYP–POA slices of ovariectomized female rats. Animals were injected subcutaneously with 2 μg of EB or 0.1 ml of vehicle (oil) at 24 and 48 hr before being killed. Some of the oil- and EB-treated rats were also injected subcutaneously with 200 μg of P 4 hr before being killed. The data are shown as mean ± SEM (n = 5–7). *p < 0.05 versus oil, EB, and P; Tukey test.

Fig. 2.

NE and phenylephrine (PHE) stimulate cGMP production only in EB plus P-treated HYP–POA slices. Hormone treatments were the same as described in Figure 1. Either 100 μm NE or 10 μm phenylephrine was administered in vitro for 20 min. The ratio was calculated as the cGMP level in a drug-treated slice/the cGMP level in an adjacent, vehicle-treated slice. The data are shown as mean ± SEM (n = 5–7). *p < 0.05 versus oil, EB, and P; Tukey test.

Fig. 3.

Prazosin (Praz) blocks the stimulatory effect of phenylephrine (PHE) on cGMP production in HYP–POA slices from EB plus P-treated animals. NE (100 μm), phenylephrine (10 μm), or vehicle (0.01N HCl) was administered in vitro for 20 min after 20 min preincubation with prazosin or vehicle (0.25% propylene glycol). Control slices were exposed to both vehicles. Ratios were calculated as described in Figure 2. The data are shown as mean ± SEM (n = 7). *p < 0.05 versus control; Tukey test.

Fig. 4.

The NO synthase inhibitors NAME and NOARG block cGMP production in slices from EB plus P-treated animals. NAME (100 μm ), NOARG (300 μm), or vehicle (distilled water) was administered 20 min before stimulation with 100 μm NE or vehicle (0.01N HCl) for 20 min. Control slices were exposed to both vehicles. Ratios were calculated as described previously. The data are shown as mean ± SEM (n = 8). *p < 0.05 versus control; #p < 0.05 versus control and NE; Tukey test.

Fig. 5.

Facilitation of lordosis behavior by 8-br-cGMP and 8-br-cGMP reversal of prazosin inhibition of lordosis behavior. InA, 8-br-cGMP (1 μm) or saline was infused into the third ventricle of rats primed only with EB for 44 hr; hourly behavior testing began 4 hr later. In B, EB-primed rats were injected with 0.5 mg/kg prazosin 1 hr before infusion of 1 μm 8-br-cGMP or saline into the third ventricle. P (200 μg) was given subcutaneously to all the animals at the same time as 8-br-cGMP or saline infusion. (▵ indicates the mean LQ ± SEM from animals receiving the same hormone priming without prazosin or 8-br-cGMP; n = 13) (Chu and Etgen, 1997). The data are shown as mean ± SEM (n = 3–5).p < 0.05 versus saline; Tukey test.