"WHY "DO" WE, HAVE these receptors, for compouds we are'nt going to "run into""

I dont think we do ..... i think that some substances are chemicaly similar and fit like a key in a lock to the receptors we have ?. < -------- thank god or the morphine and co and  THC wouldnt beam us ?

Exactly. TLS, exactly.

In most cases it is prescisely that, the lock was designed for the endogenous key to fit in, but it wasn't 'designed' to be a perfect fit for that, and only that neurotransmitter(s), the drugs simply look similar to the neurotransmitters, structurally speaking. Not nescessarily in terms of what appears by eye in the skeletal molecular representation, but in terms of (bioiso)steric arrangement, and electron topology, and if something mimics a neurotransmitter, then it can slide into the 'keyhole' and bind in the receptor, if its sufficiently good as a structural mimic then one can have all sorts of funny stuff happening, as it isn't identical, and can activate different functionally selective second messenger pathways within the cell, or happen to a larger extent than would normally happen with the small quantities of neurotransmitter present/released at any one time, activate the receptor at different times, or stabilise different conformations of a receptor/transport protein etc. Or not quite fit well enough to activate the receptor, yet still sufficiently well to bind, which results in an antagonist (inverse agonists are of course different here in that respect)

It all depends how, and where, quite likely when, to a lesser extent also.

Cocaine would be a good example, compared with other tropanes having a different binding mode at DAT or different pharmacokinetics, and compared with direct agonists at dopamine receptor subtypes.

Some other of the tropane-core cocaine analogs stabilize a different conformation of the DA transporter than coke, and are less recreational for instance. And as for where....if DA reuptake inhibition is going on in the nigrostriatal tract then its likely to be pleasurable, if its going on in the chemoreceptor trigger-zone then the result is making an semi-burnt offering of stomach contents, bile and bits of spleen to the almighty porcelain god.

And how...going from experience, the qualitative effects of a dopamine receptor agonist are different from those of a reuptake inhibitor/releaser, I've taken a combined D2/D3 receptor agonist, and the results were nothing like those of a highly selective DARI, although useful, and pleasant nevertheless. I wouldn't call it reinforcing though, at least not to me (although it should be said for the sake of disclosure, while making this point, that I do not find stimulants reinforcing, you wouldn't catch me pressing the lever in my cage until I starved to death)

     
      "A Question,, a VERY GOOD question, IMHO,,, TLS

     The answer being "Both",,, in that "both of these effects",,, 'can and do occur',, depending on the substance one has consumed--However "many" of these "neurotransmitters, And their 'components," are Naturally Recycled, so as to maintain Homeostasis,        most especially, in the Brain.

  However, they can become "depleted" due to dietary imbalances, or by the, repeated use, of
  "various drugs",    without giving the body, "adequate time to re-acquire a State of Equilibrium, aka Homeostasis"
       I think many of this forums members Would be familiar with them
 
    Hence Why, when Drugz that were ABLE TO,,  "Block the brains natural,, "old as Can Be",, System of

  "re balancing Neurotransmitters!?!!?"    SHEEIT! NO!!! thats like giving the 15yo kid next door, the Keys to a Porsche "to look after" for the weekend....."And REMEMBER to DRIVE SAFELY....xox...Yeah right!   "No worries mom, I'll be REALLY CAREFUL" (when I'm doing 150mph he he !!)

     E.G Prozac and its friends and family that was backin the early 90's, ans I can remember the doctors "talking it up, BIG time"

  What A Great Way to "Establish a Whole NEW bunch of Victims,,"To Cure"

     And I was one of its "first Victims', as it had just become approved by our Govt,and I was quickly put on 20mg p/d soon increasing to 40mg p/d,,for anxiety/depression, from chronic pain!!!
   
       however, luckily, I put together,that, "the way I had begun to Feel, and Think",,, was NOT,' becoz of me', but "felt more like becoz of one of, the number of Prescribed medications,,, I was taking at the time,, maybe 6 different drugz daily,,,
   
      ONE  being Prozac,, 20mg twice a day....well for the first few days it was like having 30-50mg of MDMA for a couple of days, and then i started to feel decidely 'unusual, and Not Myself', which then proceeded into this Weird,,, "manic/depressive-bi-polar type state"-Which I really Didnt Like

   Now, at the time, the Dr who was prescribing my medications,( including, "Prozac"...Fluoxetine) at the time, was a dude around my age at the time, late 20's--early 30's,  and seemed to be an "Open minded" type of guy,  e.g.--- he knew that I smoked cigarettes, so would often "Ask if we could start our consult with a ciggy in the Laneway beside the building,coz he wanted one too!!"  I never said No!!!

        However, for all his "I'm all over this stuff, and it works SO well hype"..  I challenged him on it, suggesting that maybe "playing with endogenous neurotranmitter, Re-uptake Systems", could be much like opening a Type of  Mental,,"Pandora's Box",,, and then told him that, 'I had "Already halved my dose to 20mg p/d, WAS feeling Much Better',,, and was continuing to down to Nothing;  as I had found 'The Drug' to have a Number of "Unpleasant side affects", which I did not care for....

   HE FLIPPED!!!     It was one of the MOST UNEXPECTED REACTIONS I have EVER Had From ANYONE--especially a Doctor!!!
 
        Needless to say we Parted company, that  date, and I was "managed by Another Partner until I could find Another suitable doctor,for my conditions management........

   Nowadays, various neurotransmitter "adjustment compounds" are available and ubiquitous!
   
  I find this a major reason for concern, Fuckin Hell,,,these Big Pharmaceutical Companies

   'Invent Illnesses" for "The drugz they have in stock/patented/tested and ready  to sell" to people who   have been
    indocrinated to "Believe,,, "Doctor=Good" therfore What the Doctor TELLS Me IS Good...Aaaaaaargh! 
    Its fuckin scary ,,from MY pov

  Think about it,,,we HAVE THESE RECEPTORS HERE for REASONS we both DO and DONT fully understand, as Sedit, for example was able to "suggest,"for example Ideas,, about the possible functions of/for Some of the Hundreds of Neurotransmitting compounds which are currently being discovered 'regularly'!! And no doubt "More to come!"
     
         As well as the remarkably sophisticated relationship they have with each  other, of which  we really Know nothing!!!! These are Uncharted waters, which I find quite Fascinating

 Its an exceptionally "Complex and Involved  System"   and in my opinion, one that Drs shouldnt have

    the "Option to Play with it'.....particularly with bizarre synthetic compounds, 'found to have certain

      effects", and then some "Type of Condition is Dis/un covered that Drug x looks like just the ticket!!"
 
     I think its playing with fire

   Hypnos