Acute opioid administration causes hyperpolarization of locus ceruleus (LC) neurons. A G-protein-gated, inwardly rectifying potassium (GIRK/K(G)) conductance and a cAMP-dependent cation conductance have both been implicated in this effect; the relative contribution of each conductance remains controversial. Here, the contribution of K(G) channels to the inhibitory effects of opioids on LC neurons was examined using mice that lack the K(G) channel subunits Kir3.2 and Kir3.3. Resting membrane potentials of LC neurons in brain slices from Kir3.2 knock-out, Kir3.3 knock-out, and Kir3.2/3.3 double knock-out mice were depolarized by 15-20 mV relative to LC neurons from wild-type mice. [Met](5)enkephalin-induced hyperpolarization and whole-cell current were reduced by 40% in LC neurons from Kir3.2 knock-out mice and by 80% in neurons from Kir3.2/3.3 double knock-out mice. The small opioid-sensitive current observed in LC neurons from Kir3.2/3.3 double knock-out mice was virtually eliminated with the nonselective potassium channel blockers barium and cesium. We conclude that the acute opioid inhibition of LC neurons is mediated primarily by the activation of G-protein-gated potassium channels and that the cAMP-dependent cation conductance does not contribute significantly to this effect.

The transcription factor cAMP-responsive element binding protein (CREB) has been shown to regulate different physiological responses including drug addiction and emotional behavior. Molecular changes including adaptive modifications of the transcription factor CREB are produced during drug dependence in many regions of the brain, including the locus coeruleus (LC), but the molecular mechanisms involving CREB within these regions have remained controversial. To further investigate the involvement of CREB in emotional behavior, drug reward and opioid physical dependence, we used two independently generated CREB-deficient mice. We employed the Cre/loxP system to generate mice with a conditional CREB mutation restricted to the nervous system, where all CREB isoforms are lacking in the brain (Creb1 NesCre ). A genetically defined cohort of the previously described hypomorphic Creb1 aD mice, in which the two major transcriptionally active isoforms (a and D) are disrupted throughout the organism, were also used. First, we investigated the responses to stress of the CREB-deficient mice in several paradigms, and we found an increased anxiogenic-like response in the both Creb1 mutant mice in different behavioral models. We investigated the rewarding properties of drugs of abuse (cocaine and morphine) and natural reward (food) using the conditioned place-preference paradigm. No modification of motivational responses of morphine, cocaine, or food was observed in mutant mice. Finally, we evaluated opioid dependence by measuring the behavioral expression of morphine withdrawal and electrophysiological recordings of LC neurons. We showed an important attenuation of the behavioral expression of abstinence and a decrease in the hyperactivity of LC neurons in both Creb1 mutant mice. Our results emphasize the selective role played by neuronal CREB in emotional-like behavior and the somatic expression morphine withdrawal, without participating in the rewarding properties induced by morphine and cocaine.

A close relationship between desensitization and internalization of -opioid receptors (MORs) has been proposed based on differential actions of series of agonists. The role that these two processes have in the development of tolerance and dependence to opioids has been a controversial subject that has been studied in a variety of model systems. Here, we examine desensitization and internalization of endogenous MORs simultaneously in primary cultures of locus ceruleus neurons using fluorescently tagged peptide agonists. With the use of two fluorescent opioid peptides, dermorphin-Bodipy Texas Red and dermorphin-Alexa594 (Derm-A594), desensitization was measured electrophysiologically and trafficking was followed by the accumulation of intracellular fluorescent puncta. Blocking endocytosis with concanavalin A eliminated the accumulation of fluorescent puncta but desensitization induced by Derm-A594 was unaffected. Likewise, after treatment with concanavalin A, there was no change in either desensitization or recovery from desensitization induced byenkephalin. The results demonstrate that desensitization and the recovery from desensitization are not dependent on receptor internalization and suggest that the activity of endogenous MORs in primary neurons can be modulated at the level of the plasma membrane.

The neurotransmitter serotonin (5-HT) has a multifaceted function in the modulation of information processing through the activation of multiple receptor families, including G-protein-coupled receptor subtypes (5-HT1, 5-HT2, 5-HT4–7) and ligand-gated ion channels (5-HT3). The largest population of serotonergic neurons is located in the midbrain, specifically in the raphe nuclei. Although the medial and dorsal raphe nucleus (DRN) share common projecting areas, in the basal ganglia (BG) nuclei serotonergic innervations come mainly from the DRN. The BG are a highly organized network of subcortical nuclei composed of the striatum (caudate and putamen), subthalamic nucleus (STN), internal and external globus pallidus (or entopeduncular nucleus in rodents, GPi/EP and GPe) and substantia nigra (pars compacta, SNc, and pars reticulata, SNr). The BG are part of the cortico-BG-thalamic circuits, which play a role in many functions like motor control, emotion, and cognition and are critically involved in diseases such as Parkinson's disease (PD). This review provides an overview of serotonergic modulation of the BG at the functional level and a discussion of how this interaction may be relevant to treating PD and the motor complications induced by chronic treatment with L-DOPA.