Is there a "foreplay" equivalent, priming activity for thinking or conversation?
Yes, there is such an activity and it is called priming.  It is the same technique that involves mental, sexual foreplay. There has been a fair bit of research about the psychological effects of priming. I have limited the discussion, purely to priming, but there are many other interesting, associations with priming that could be discussed within another post.

I find the following points provide a good summary of the various modes of priming.


  Priming is providing a stimulus that influences their near-term future
  thoughts and actions, even though they may not seem to be connected.
  Conceptual priming occurs where related ideas are used to prime the
  response, for example 'hat' may prime for 'head'.
  Semantic priming
  occurs where the meaning created influences later thoughts. Semantic
  and conceptual priming are very similar and the terms may be used
  interchangeably.
  Non-associative semantic priming refers to related
  concepts but where one is less likely to trigger thoughts of the
  other, for example 'Sun' and 'Venus'.
  Perceptual priming, is based
  on the form of the stimulus, for example where a part-picture is
  completed based on a picture seen earlier.
  Associative priming
  happens when a linked idea is primed, for example when 'bread' primes
  the thought of 'butter'. This particularly applies to 'free
  association' word pairs.
  Masked priming occurs where a word or image
  is presented for a very short time but is not consciously recognized.
  Repetitive priming occurs where the repetition of something leads to
  it influencing later thoughts.
  Reverse priming occurs where people
  realize they are being primed and, feeling they have been biased,
  over-respond in their choices which are now biased in the reverse
  direction


Meyer and Schvanveldt (1971), Neely (1977), Bargh and Pietromonaco (1982), Marcel, (1983), Draine and Greenwald (1998), Sherman and Kim (2002)

On the down side:
I have been unable to find studies investigating the neurotransmitters involved in priming (at this stage). One thought occurred to me from when I studied Neuroscience (many years ago). That arousal (as in attention) is required to LTP cells. Although, this specific example may not apply to priming, it is a parallel between the effects of psychological priming and physiological priming. I am sure there are better articles, I, currently, have limited access to journals. 

Learning-facilitated long-term depression and long-term potentiation at mossy fiber-CA3 synapses requires activation of β-adrenergic receptors.
Hagena H, Manahan-Vaughan D.
Source
Medical Faculty,
 Department of Neurophysiology,
Ruhr University
Bochum Bochum, Germany.