Our gallant friends, The Scientists, have alledgedly bred a mouse that is perpetually happy, as detailed in this
giddy Slashdot post. The technique will of course be sought to be applied to humans at some point, and the
genetic modification of the TREK1 channel is maybe sufficient to achieve this.
Oversimplified, I think the mechanism is similar as with newer antidepressants (SSRIs - selective serotonin uptake inhibitors). The released serotonin doesn't get absorbed and stays floating between the synapses, enabling faster transmission of chemical information in the brain. Mechanisms are of course of little importance - it's the application that counts. For me, this would be the biggest ethical question human sciences have yet faced. The fundamental question is "Should we be allowed to breed happy people?"
This begs definitions. Happiness is to a large extent relative, I think we can agree. If every day were Christmas, Santa Claus wouldn't be special at all. Thus, it can be argued that this would in fact make no difference to the person modified in this manner, because they wouldn't know the difference - arguably, they wouldn't know what being depressed feels like. The same argument can be used from the other perspective as well - if there is no change, then the only negative effect (other things equal) would be the effort invested in modifying an organism this way.
Moving on, would this be something that would be made available to people on a case-by-case basis (to children born into demographics with higher likelyhood of depression, for example) or to all equally? In the long run, provided that the modification makes a difference, only the latter option would be genuinely equal.
One of the effects of SSRIs such as Prozac is that they diminish sexual drive. For me, this makes immediate sense - I bet if every depressed person got more sex, we'd all be happier. Moving on (quickly indeed), the reports that link the antidepressant trace contents in rivers to reproductive problems in fish (and the feminization of young male fish) would suggest that to this armchair neurobiologist (I'm armchair everything, really).
Before suggesting permanently high serotonin levels to signal the extinction of the human race, let's take a step back and look at the whole philosophical picture. This invention would at first seem to be in conflict with evolution - but making such a statement would be to assume that evolution itself had a goal, a meaning, a destination. And even if it did, maybe intelligence negating and erasing the instinctual animalistic behaviour - this burden our race carries from our past millenia in the oceans, the jungles, the savannas - would be in the "interest of evolution". Intelligent Design fans, rejoice (yes, both of you). I read too much Nietzsche as a kid, but this scenario does ring to me like a part of the human development from the camel into the lion into the child.
But man's search for meaning has been the driving force behind many innovations, advances of science and discoveries about the principles governing our existence. Arguably, this same search for meaning has also stagnated us, bound us to books some believe to be the word of god and stopped others from inquiring beyond these ultimate truths. Depending on what the balance turns out to be, perpetually happy people would perhaps not strive for such discoveries: they would be complacent, happy with the status quo. There is certainly evidence that depressed people treated with SSRIs are able to remain in the productive workforce better than depressed people not treated with comparable medication. But are there studies about the effect of serotonin levels on creativity, scientific inquiry or innovation? What if such aspirations are culled by higher serotonin levels resulting from antidepressants?
I tried to find a quotation to end this on high note, but the all the ones I found about achievement, happiness, suffering or innovation assumed that overcoming difficulties is a virtue in itself, not because of the end result. That is a leap of faith I cannot take - so there's no quote here, only the notion of a singular, absolutely fulfilled and complacent being in the void, going nowhere.