John T. Williams

Neuromodulatory systems exert profound influences on brain function. Understanding how these systems modify the operating mode of target circuits requires spatiotemporally precise measurement of neuromodulator release. We developed dLight1, an intensity-based genetically encoded dopamine indicator, to enable optical recording of dopamine dynamics with high spatiotemporal resolution in behaving mice. We demonstrated the utility of dLight1 by imaging dopamine dynamics simultaneously with pharmacological manipulation, electrophysiological or optogenetic stimulation, and calcium imaging of local neuronal activity. dLight1 enabled chronic tracking of learning-induced changes in millisecond dopamine transients in mouse striatum. Further, we used dLight1 to image spatially distinct, functionally heterogeneous dopamine transients relevant to learning and motor control in mouse cortex. We also validated our sensor design platform for developing norepinephrine, serotonin, melatonin, and opioid neuropeptide indicators.

Although opioids are highly effective for the treatment of pain, they are also known to be intensely addictive. There has been a massive research investment in the development of opioid analgesics, resulting in a plethora of compounds with varying affinity and efficacy at all the known opioid receptor subtypes. Although compounds of extremely high potency have been produced, the problem of tolerance to and dependence on these agonists persists. This review centers on the adaptive changes in cellular and synaptic function induced by chronic morphine treatment. The initial steps of opioid action are mediated through the activation of G protein-linked receptors. As is true for all G protein-linked receptors, opioid receptors activate and regulate multiple second messenger pathways associated with effector coupling, receptor trafficking, and nuclear signaling. These events are critical for understanding the early events leading to nonassociative tolerance and dependence. Equally important are associative and network changes that affect neurons that do not have opioid receptors but that are indirectly altered by opioid-sensitive cells. Finally, opioids and other drugs of abuse have some common cellular and anatomical pathways. The characterization of common pathways affected by different drugs, particularly after repeated treatment, is important in the understanding of drug abuse.

The effects of agonists at mu and delta opioid receptors were compared by measuring membrane currents under voltage clamp from neurons of the rat nucleus locus coeruleus and guinea pig submucous plexus. In each tissue, the appropriate selective agonist (Tyr-D-Ala-Gly-MePhe-Gly-ol for mu receptors in locus coeruleus or Tyr-D-Pen-Gly-Phe-D-Pen for delta receptors in submucous plexus) increased the conductance of an inwardly rectifying potassium conductance and strongly hyperpolarized the membrane. The properties of the potassium conductance affected by the two opioids could not be distinguished. Experiments with intracellular application of guanosine 5'-[gamma-thio]triphosphate indicated that a guanine nucleotide-binding regulatory protein was involved in the coupling between opioid receptor and potassium channel, but there was no evidence for activation of either cAMP-dependent protein kinase or protein kinase C. It is noted that a number of vertebrate neurotransmitter receptors are coupled to potassium channels. The potassium conductance associated with these channels has properties similar to the conductance activated by mu and delta opioids; this family includes the following receptors: acetylcholine M2, norepinephrine alpha 2, dopamine D2, 5-hydroxytryptamine 5-HT1, adenosine A1, gamma-aminobutyric acid GABAB, and somatostatin. It is suggested that this conductance is a conserved neuronal effector coupled to one of the receptor types that mediates the effects of each of several major transmitters. The mu and delta opioid receptors appear to be unusual in that both utilize this same effector mechanism.