the brain

Opioid Tolerance has been shown to be regulated by NMDA receptors. When somebody takes an opioid glutamate is released in the brain and binds to the NMDA receptors causing an increase in opioid tolerance. There have been several studies that show that taking and opioid with a NMDA antagonist (a drug that blocks NMDA receptors, effectively "turning them off") prevents the development of opioid tolerance. An accessible NMDA antagonist is DXM or Dextromethorphan which is found in many different cold medicine preparations. Be careful if taking a multiple ingredient cold medicine as many contain acetaminophin and other ingredients that can be dangerous if taken in high doses. Hope this helps, I included a link to a study if your interested.

http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=2045628

Na+

The main role of the NMDA receptor in the body is in ensuring neural plasticity. Further research is on-going to further refine understanding of this receptor, and this conclusion may change in future as more studies may find further roles for this receptor.

Yes. This is mediated by ciprofloxacin's antagonism of the GABA(a) receptor, leading to NMDA receptor over-excitation and tinnitus.