Leigh Brasington's tentative hypothesis

First, from Emily Yoffe in Slate Magazine:

Mammals stimulating the lateral hypothalamus seem to be caught in a
loop, Panksepp writes, "where each stimulation evoked a reinvigorated
search strategy". [...] 'Wanting' is Berridge's equivalent for
Panksepp's 'seeking' system.

[However,] it is the 'liking' system that Berridge believes is the brain's
reward center. When we experience pleasure, it is our own opioid
system, rather than our dopamine system, that is being stimulated.
This is why the opiate drugs induce a kind of blissful stupor so
different from the animating effect of cocaine and amphetamines.

[In short,] wanting and liking are complementary. The former
catalyzes us to action; the latter brings us to a satisfied pause.
Seeking needs to be turned off, if even for a little while, so that
the system does not run in an endless loop. When we get the object of
our desire (be it a Twinkie or a sexual partner), we engage in
consummatory acts that Panksepp says reduce arousal in the brain and
temporarily, at least, inhibit our urge to seek.

Now, from http://www.leighb.com/jhananeuro.htm:

Quite secluded from the dopamine-based wanting system (substantia
nigra), secluded from any other disturbing mental activity -- one
enters & remains in the first jhana: dopamine-based liking system
(ventral tegmental area) activation [...] accompanied by background mental
activity.

With the stilling of background mental activity, one enters & remains
in the second jhana: dopamine-based liking system activation &
opioid-based liking system activation born of concentration,
unification of awareness free from background mental activity --
internal tranquility due to opioids flooding the nervous system.

With the fading of dopamine-based liking system activation, remaining
equanimous, mindful & clearly aware -- with continued opioid-based
liking system activation, one is physically sensitive of pleasure. One
enters & remains in the third jhana, of which the Noble Ones declare,
'Equanimous & mindful due to opioid system activation, one has a
pleasurable abiding.'

With the abandoning of liking generated by opioid system activation --
as with the earlier disappearance of liking generated by dopamine
system activation -- one enters & remains in the fourth jhana:
mindfulness fully purified by the equanimity remaining from the prior
opioid activation, a neutral state now that the liking systems have
calmed down.

(N.B.: both dopamine-based liking and opioid-based liking are housed
in the Nucleus Accumbens, cf. this recent post on Gathatoulie,
http://gathatoulie.blogspot.com/2011/10/self-agency-reward-and-anxiety.html)