Serotonin receptor involvement in effects of restraint on female rat lordosis behavior

Abstract

Ovariectomized Fischer (CDF-344) rats, with bilateral cannulae in the mediobasal hypothalamus (MBH) near the ventromedial nucleus of the hypothalamus (VMN), were used to test the hypothesis that serotonin receptors in the VMN contribute to the lordosis-inhibiting effects of mild restraint. Rats were hormonally primed with 10 microg estradiol benzoate (EB) followed 48 h later with sesame seed oil. Four to six hours later (during the dark portion of the light-dark cycle), rats were pretested for sexual behavior. Thereafter, they were infused with saline, 2 microg of the serotonin (5-HT) 2 receptor agonist, (+/-)-2,5-dimethoxy-4-iodophenyl-2-aminopropane HCl (DOI), or 1 microg of the 5-HT(1A) receptor antagonist, N-{2[4-(2-methoxyphenyl)-1-piperazinyl]ethyl}-N-(2-pyridinyl) cyclohexanecarboxamide trihydrochloride (WAY100635). After a 5 min restraint, rats were tested for sexual receptivity. Rats infused with saline showed a significant decline in lordosis behavior after restraint. Infusion with either DOI or WAY100635 attenuated these effects of restraint. These findings extend earlier observations that the lordosis-disruptive effects of mild restraint include activation of 5-HT(1A) receptors in the VMN and are the first to implicate VMN 5-HT(2) receptors in protection against mild restraint.

Figure 1. Effects of DOI and WAY100635 on sexual receptivity after 5 min restraint

Ovariectomized rats with bilateral cannulae near the VMN were hormonally primed with 10 μg estradiol benzoate followed by sesame seed oil. After a pretest for sexual receptivity, rats were infused with saline, DOI, or WAY100635 (Ns respectively are 16, 12 and 6) and restrained for five min as described in the Methods. Data in 1A and 1B are the mean ± S.E. lordosis to mount (L/M) ratios and lordosis quality scores, respectively, for the pretest interval (PRE) and 6 consecutive five min intervals after restraint. * indicates intervals where the L/M ratio was significantly different from the pretest. # indicates intervals where rats infused with DOI or WAY100635 differed significantly from the saline group.

Figure 2. Location of cannulae in rats infused with saline or DOI

In the figure are shown cannulae locations for rats used in this experiment. Coronal sections are taken from König and Klippel (1963). Numbers in the figure refer to atlas locations. Cannula location was assigned to the brain section containing the most ventral location of the cannula track. In 2A and 2B, filled circles on the right side of each section indicate bilateral locations from rats that were infused with saline. Filled circles on the left of 2A indicate bilateral locations from rats that were infused with DOI. Filled circles on the left of 2B indicate bilateral locations from rats that were infused with WAY100635.

Figure 3. A representative section of cannulae locations in the VMN

The figure represents a section through the VMN where infusion cannulae were located. The section was treated as described in the Methods.