ICV galanin-like peptide stimulates non-contact erections but not touch-based erections in adult, sexually experienced male rats

Highlights

• GALP stimulates ex copula erections.

• GALP increases olfactory investigations.

• GALP does not increase penile reflexive erections.

• GALP increase male rat appetitive sex behaviors.

Abstract

Galanin-like peptide (GALP) is a neuropeptide transcribed only within the arcuate nucleus of the hypothalamus and is thought to be a mediator between energetics and reproductive function. Intracerebroventricular (ICV) injection of GALP is known to have effects on feeding, and to significantly increase gonadotropin releasing hormone- (GnRH-) mediated luteinizing hormone (LH) secretion. Furthermore, ICV GALP is known to stimulate fos production in the medial pre-optic area (mPOA) and to a lesser extent, the paraventricular nucleus (PVN). ICV injection of 5.0 nmol GALP profoundly stimulates male rat sexual behavior. It is not known if GALP's effects on sex behavior are due to an increase in appetitive or mechanical (erectile) aspects of male sexual behavior. To determine this, sexually experienced male rats were cannulated in the lateral ventricle and injected with 5.0 nmol GALP or vehicle. Immediately after injections, male rats were placed in an arena connected to a second arena via a tube with a fan. The second arena contained a steroid-primed female and her bedding. The male rat had olfactory but not visual or tactile contact with the female. We analyzed the amount of time the male rats spent investigating the air intake and the number of non-contact erections (NCEs) in a 30 minute test. ICV GALP significantly (p < 0.05) increased both the amount of time of olfactory investigations and NCEs compared to vehicle. In a second set of animals, we tested if ICV GALP could stimulate touch-based erections. GALP had no significant effect on touch-based erections compared to vehicle. These data suggest that GALP's activation of fos within the mPOA is indicative of its action to stimulate the appetitive aspects of male sexual behavior.

Introduction

Galanin-like peptide (GALP) is a 60-amino-acid neuropeptide; GALP9-21 are identical to galanin's first thirteen N-terminal amino acids (Ohtaki et al., 1999). While galanin is expressed widely throughout the central nervous system (Gaymann and Martin, 1989, Melander et al., 1986a, Melander et al., 1986b, Newton, 1992, Tatemoto et al., 1983) immunostaining for GALP mRNA reveals neuronal bodies expressing GALP are located primarily in the hypothalamic arcuate nucleus (ARC) as well as areas nearby including the infundibular stalk and median eminence (Cunningham et al., 2002, Gundlach et al., 2001, Kerr et al., 2000, Ohtaki et al., 1999, Takatsu et al., 2001). GALP is an important regulator of feeding and reproductive behavior, as it responds to satiety hormone levels to ultimately affect important aspects of sexual arousal (reviewed in Gottsch et al., 2004, Gundlach, 2002, Kageyama et al., 2005, Lawrence and Fraley, 2010).

The expression of GALP is critically dependent on the amount of fuel available for metabolism as GALP is up-regulated by both leptin and insulin. A link between the leptin and GALP was apparent when ≤ 85% of GALP-expressing neurons in the ARC also were shown to express leptin receptors (Jureus et al., 2001, Juréus et al., 2000). In a similar manner to leptin, insulin up-regulates the expression of GALP mRNA in STZ-induced diabetic rats (Fraley et al., 2004a). In diabetic rats displaying sexual dysfunction, GALP has been shown to restore reproductive behavior to a similar extent that exogenous injections of leptin and insulin do (Fraley et al., 2004a, Stoyanovitch et al., 2005). Notably, immunoneutralization of GALP attenuated the restorative effects of leptin and insulin on sexual behavior in diabetic rats (Stoyanovitch et al., 2005).

Extensive study into the arousal states of animals has found the medial preoptic area (mPOA) to be a site of integration of external stimuli, and it is essential to the appetitive aspects of sex (reviewed in Bitran and Hull, 1987, Sachs, 2000). Sexual motivation is quantified by non-contact erections (NCE) as a means to observe the culmination of input and processing of the central pathways and neuroendocrinology (Sachs, 2000). One telling study of the mPOA's role in reproductive behavior showed mPOA-lesioned rats have diminished copulatory behavior yet the NCE count was unaffected. These findings allude to an extra-mPOA pathway by which sexual motivation is mediated, with some evidence that lesions to the paraventricular nucleus (PVN) attenuated the latency to the first NCE (Liu et al., 1997). GALP-containing fibers extend to the PVN, mPOA, and the bed nucleus of the striatum terminalis (BST) (Jureus et al., 2001). However, intracerebroventricular (ICV) injections of GALP brought about increased c-fos expression only within the mPOA (Fraley et al., 2003) and ARC while an increase in c-fos was elicited to a much lesser extent within the PVN (Fraley et al., 2003, Lawrence et al., 2003).

Our study aimed to distinguish the pathway through which GALP acts to affect sexual arousal. To accomplish this, two experiments were run using male Long-Evans rats. The first experiment elucidated the appetitive effects of central GALP administration in response to an estrous female rat's scent. During a second experiment we observed the number of touch-based, erections achieved by rats restrained in the supine position after ICV injections of GALP. Our findings provide evidence that GALP affects male copulatory behavior through a central appetitive pathway and not a pathway involved in erectile function.