CHRONIC MORPHINE--REGULATION OF ION CONDUCTANCES

The experiments in this proposal are aimed at uncovering adaptive
changes in neuronal activity that result from chronic opioid treatment.
Behaviorally it can be observed that a high degree of tolerance to and
dependence on morphine develops with chronic use. It is curious that,
at the cellular and biochemical level, the degree of tolerance and
dependence is orders of magnitude smaller. The disparity in the
observations may suggest that there are multiple mechanisms arranged
in series that could account for the high level of behavioral
tolerance. Another possibility is that regulation of ion conductances
other than those directly affected by opioids are altered with chronic
treatment and they may account for the increased level of tolerance and
dependence. Opioids activate potassium channels and/or the inactive calcium
channels to perturb cell excitability and function. In the continued
presence of opioids, tolerance develops to these direct actions of
opioids. There are, in addition, other adaptive changes in cell
function and excitability not directly related to the specific ion
channels gated by opioids. The regulation of those channels
'indirectly' gated by opioids is the subject of this proposal.
Intracellular and whole cell recordings from neurons in the locus
coeruleus and substantia gelatinosa of the spinal trigeminal nucleus
will be made in brain slice preparations. With these two preparations,
two opioid mediated responses will be investigated, the increase in
potassium conductance and the presynaptic inhibition of glutamate
release. One consistent observation made with chronic opioid treatment
is an increase in the basal and stimulated level of cAMP. In this
proposal, the effects of cAMP-dependent processes on transmitter
release and ion conductances gated by voltage, transmitters and G-
proteins will be identified and characterized. These cAMP-dependent
processes will be studied after acute application (min or hours) of
opioid agonists and in animals treated chronically (days) with
morphine. By investigating the alteration in the regulation of ion
conductances not directly affected by opioids, it may be possible to
clarify the mechanisms by which the high levels of tolerance found at
the behavioral level are generated.