Some new data suggests that Viagra (sildenafil) may have other mechanisms that support sexual function, such as an effect on dopamine levels in the brain.

Here is an interesting article (study) on this: LINK HERE from the Journal of Sexual Medicine. Here is the abstract:
J Sex Med. 2012 Nov 15. doi: 10.1111/j.1743-6109.2012.03000.x.

Experimental Evidence for Sildenafil’s Action in the Central Nervous System:
Dopamine and Serotonin Changes in the Medial Preoptic Area and Nucleus Accumbens During Sexual Arousal.

Source

Department of Pharmacology, Medical School, University of Athens, Athens,
Greece First Department of Psychiatry, Eginition Hospital, Medical School,
University of Athens, Athens, Greece Andrology Institute of Athens, Athens,
Greece.

Abstract

Introduction.  Sildenafil is the first effective oral treatment for male
erectile dysfunction. Although it is generally accepted that its action is
peripheral, it has been suggested that it influences central neural pathways
that are involved in male sexual arousal. Recently, it was shown that local
sildenafil administration enhances extracellular dopamine (DA) in the nucleus
accumbens (NAcc). Aim.  The aim of this study was to determine whether
sildenafil administration alters dopaminergic and serotonergic activity in the
NAcc and the medial preoptic area (mPOA) during a model of sexual arousal.
Methods.  An acute (2 days) or chronic (21 days) sildenafil regimen (1 mg/kg)
was administered intraperitoneally to male rats. Thirty minutes after the last
sildenafil injection, all males were exposed to noncontact erection sessions by
the presentation of inaccessible estrous females. Half of the males had previous
experience of noncontact sexual encounter and the other half were exposed for
the first time. Main Outcome Measures.  Tissue levels of DA and its metabolites,
3,4-Dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), as well as
serotonin (5-HT) and its metabolite 5-HIAA, were measured in the mPOA and NAcc
with high-performance liquid chromatography with electrochemical detector.
Dopamine ([DOPAC+HVA]/DA) and serotonin (5-HIAA/5-HT) turnovers were also
calculated as indices of neurotransmission. Results.  In nontrained males, acute
and chronic sildenafil treatment increased DA and 5-HT turnover rates in the
mPOA and NAcc. In trained rats, acute sildenafil also increased DA and 5-HT
turnover rates in both structures, whereas chronic treatment enhanced 5-HT
turnover rate only in the mPOA and DA turnover rate only in the NAcc.
Conclusions.  Our data confirm that sildenafil enhances dopaminergic activity in
the NAcc, extend these findings to the mPOA and furthermore, reveal
sildenafil-induced effects on serotonergic activity in these brain regions as
well. Therefore, present findings support an effect of sildenafil on central
neural pathways that are involved in the control of sexual arousal.