Opiate mechanisms: Evaluation of research involving neuronal action potentials

• 1.

  1. Various conclusions about the opiate effects in the central nervous system can be drawn from studies involving the recording of nerve impulses (action potentials). There are, for example, regional differences in the kind and character of opiate effects, both in acutely treated animals and during the development of tolerance to repeated exposure to opiates.

• 2.

  2. Generally speaking, opiates depress neuronal activity with pharmacological stereospecificity. Excitatory effects (some of which are stereospecific and due to disinhibition) do occur in several brain regions. Excitatory effects can become more prominent during tolerance development.

• 3.

  3. Opiates interact with major neurotransmitter systems in a variety of ways that include not only post-synaptic agonist/antagonist phenomena but also pre- and post-synaptic modulatory functions; mechanisms of such interactions are poorly understood and are a focus of current research in many laboratories.

• 4.

  4. Opiates have profound effects in various brain structures that are known to be associated with the more prominent behaviors that are induced by opiates, as reflected in such phenomena as catalepsy, euphoria, reward and analgesia. There is, however, no clear understanding or unified theory to explain how these effects are produced. In the case of analgesia, the opiates are known to act simultaneously within nociceptive pathways at several levels: spinal, brainstem, and thalamic.

• 5.

  5. The widespread distribution of cells that are sensitive to both the stereospecific and non-stereospecific effects of opiates makes it likely that the various behavioral effects of opiates are mediated through several receptor types and several levels of the nervous system.