Lorenz's "motivation" & upregulation

[B.]

	I am trying to find out if there has been any study of receptor 
regulation among neurons concerned with certain behavioral repertoires.  
There is an old and now defunct metaphor proposed by Konrad Lorenz that
certain behaviors are controlled by "motivation" similar to an old-style
flush toilet.  Such that as time progresses since the last time the 
behavior was produced, the motivation toward performing that behavior
increases.  Such that, the longer the period between behaviors the 
smaller the stimulus necessary to elicit that behavior, via the increased
motivation.  This argument was used as an attempt to describe why some
animals appear to be "behaving out of context".  The classic e.g. is of a
captive squirrel attempting to bury a nut that doesn't exist.  With 
receptor upregulation in mind, is it possible that Lorenz may not have 
been that far off, that "motivation" could be the amount of stimulating
neurotransmitter neccessary to elicit a response?  So with time an 
upregulation of the receptors requires less stimulus (normally insufficient
or marginal) to get the behavior.  Does anyone know of any such research 
concerning receptor regulation among "behaving" neural networks (as 
opposed to say the sensory-motor system where this has been rather well 
studied)?  I was thinking regions of the lymbic system such the amygdala
would be a relevant site of study.