Daniel S. McGehee

Long-Term Potentiation of Excitatory Inputs to Brain Reward Areas by Nicotine

Nicotine reinforces smoking behavior by activating nicotinic acetylcholine receptors (nAChRs) in the midbrain dopaminergic (DA) reward centers, including the ventral tegmental area (VTA). Although nicotine induces prolonged excitation of the VTA in vivo, the nAChRs on the DA neurons desensitize in seconds. Here, we show that activation of nAChRs on presynaptic terminals in the VTA enhances glutamatergic inputs to DA neurons. Under conditions where the released glutamate can activate NMDA receptors, long-term potentiation (LTP) of the excitatory inputs is induced. Both the short- and the long-term effects of nicotine required activation of presynaptic alpha7 subunit-containing nAChRs. These results can explain the long-term excitation of brain reward areas induced by a brief nicotine exposure. They also show that nicotine alters synaptic function through mechanisms that are linked to learning and memory.

Synaptic mechanisms underlie nicotine-induced excitability of brain reward areas.

A single nicotine exposure increases dopamine levels in the mesolimbic reward system for hours, but nicotine concentrations experienced by smokers desensitize nAChRs on dopamine neurons in seconds to minutes. Here, we show that persistent modulation of both GABAergic and glutamatergic synaptic transmission by nicotine can contribute to the sustained increase in dopamine neuron excitability. Nicotine enhances GABAergic transmission transiently, which is followed by a persistent depression of these inhibitory inputs due to nAChR desensitization. Simultaneously, nicotine enhances glutamatergic transmission through nAChRs that desensitize less than those on GABA neurons. The net effect is a shift toward excitation of the dopamine reward system. These results suggest that spatial and temporal differences in nicotinic receptor activity on both excitatory and inhibitory neurons in reward areas coordinate to reinforce nicotine self-administration.

Botulinum toxin type A inhibits sensory neuropeptide release in rat bladder models of acute injury and chronic inflammation

To determine the effect of botulinum toxin type A (BTX‐A) on the release of the neuropeptides substance P (SP) and calcitonin gene‐related peptide (CGRP) from isolated bladder preparations after acute injury with HCl and the induction of cyclophosphamide (CYP)‐induced cystitis, as neurogenic inflammation has been increasingly identified in urological disorders such as interstitial cystitis.

Enhanced striatal cholinergic neuronal activity mediates l-DOPA–induced dyskinesia in parkinsonian mice

Treatment of Parkinson disease (PD) with l-3,4-dihydroxyphenylalanine (l-DOPA) dramatically relieves associated motor deficits, but l-DOPA–induced dyskinesias (LID) limit the therapeutic benefit over time. Previous investigations have noted changes in striatal medium spiny neurons, including abnormal activation of extracellular signal-regulated kinase1/2 (ERK). Using two PD models, the traditional 6-hydroxydopamine toxic lesion and a genetic model with nigrostriatal dopaminergic deficits, we found that acute dopamine challenge induces ERK activation in medium spiny neurons in denervated striatum. After repeated l-DOPA treatment, however, ERK activation diminishes in medium spiny neurons and increases in striatal cholinergic interneurons. ERK activation leads to enhanced basal firing rate and stronger excitatory responses to dopamine in striatal cholinergic neurons. Pharmacological blockers of ERK activation inhibit l-DOPA–induced changes in ERK phosphorylation, neuronal excitability, and the behavioral manifestation of LID. In addition, a muscarinic receptor antagonist reduces LID. These data indicate that increased dopamine sensitivity of striatal cholinergic neurons contributes to the expression of LID, which suggests novel therapeutic targets for LID.

Mechanism of extracellular Ca2+ receptor-stimulated hormone release from sheep thyroid parafollicular cells

1 Expression of receptors to extracellular calcium enables parafollicular cells of the thyroid gland (PF cells) to release calcitonin (CT) and serotonin (5‐HT) in response to increased external Ca2+. Recently, a calcium‐sensing receptor (CaR), similar to the G protein‐coupled receptor for external Ca2+ cloned from parathyroid gland, was shown to be expressed in PF cells. Using a highly purified preparation of sheep PF cells, we have examined the electrical and biochemical processes coupling CaR activation to hormone release. 2 Whole‐cell recordings in the permeabilized‐patch configuration show that elevated extracellular Ca2+ concentration ([Ca2+]o) depolarizes these cells and induces oscillations in membrane potential. In voltage clamp, high [Ca2+]o activates a cation conductance that underlies the depolarization. This conductance is cation selective, with a reversal potential near −25 mV indicating poor ion selectivity. 3 The CaR expressed in these cells is activated by other multivalent cations with a rank order potency of Gd3+ > Ba2+ > Ca2+≫ Mg2+. The insensitivity of these cells to high external Mg2+ contrasts with the reported sensitivity of the cloned CaR from parathyroid. 4 Elevation of [Ca2+]o also stimulates increases in intracellular Ca2+ concentration ([Ca2+]o) and this effect is largely inhibited by the Ca2+ channel blocker nimodipine, indicating that L‐type voltage‐gated Ca2+ channels contribute to the response to elevated [Ca2+]o. 5 Elevated [Ca2+]o induces an inward current under conditions where the only permeant external cation is Ca2+, indicating that influx via the cation conductance is another source of the increases in [Ca2+]i. 6 Extracellular Ca2+ stimulates 5‐HT release with an EC50 of 1.5 mm. Nimodipine blocks 90% of the Ca2+ ‐induced 5‐HT release, while other inhibitors of voltage‐gated calcium channels had no effect. These data support an important role for L‐type Ca2+ channels in CaR‐induced hormone secretion. Although earlier studies indicate that high [Ca2+]o induces release of Ca2+ from intracellular stores, thapsigargin‐induced depletion of these stores did not affect secretion from these cells, indicating that Ca2+ influx is necessary and sufficient for the Ca2+ ‐induced 5‐HT secretion. 7 Inhibition of protein kinase C (PKC) using chelerythrine, staurosporine, or calphostin C inhibited Ca2+ ‐induced 5‐HT release by 50% while phorbol ester‐induced 5‐HT secretion was completely inhibited. Thus, PKC is an important component of the pathway linking CaR activation to hormone release. However, another as yet unknown second messenger also contributes to this pathway. 8 We tested the contribution of two different phospholipases to the CaR responses to determine the source of the PKC activator diacylglycerol (DAG). Selective inhibition of phosphatidylinositol‐specific phospholipase C (PI‐PLC) with U73122 had no effect on the response to elevated [Ca2+]o. However, pretreatment with D609, a selective inhibitor of phosphatidylcholine‐specific phospholipase C (PC‐PLC), inhibited Ca2+ ‐induced 5‐HT release to 50% of control indicating that phosphatidylcholine is a likely source of DAG in the response of PF cells to elevated [Ca2+]o.

Molecular diversity of neuronal nicotinic acetylcholine receptors.

ABSTRACT: The potent behavioral and cognitive effects of nicotine highlight the physiological importance of nicotinic acetylcholine receptors (nAChRs). These receptors are part of the superfamily of neurotransmitter‐gated ion channels that are responsible for rapid intercellular communication. Molecular cloning of the protein subunits that make up these receptors has led to greater understanding of the pharmacology and physiology of nAChRs. This review outlines our current understanding of the molecular constituents of these receptors and some of the recent studies of the structural determinants of receptor function.

Dopamine Scales Performance in the Absence of New Learning

Learning and motivation are integral in shaping an organisms adaptive behavior. The dopamine system has been implicated in both processes; however, dissociating the two, both experimentally and conceptually, has posed significant challenges. We have developed an animal model that dissociates expression or scaling of a learned behavior from learning itself. An inducible dopamine transporter (DAT) knockdown mouse line has been generated, which exhibits significantly slower reuptake of released dopamine and increased tonic firing of dopamine neurons without altering phasic burst firing. Mice were trained in experimental tasks prior to inducing a hyperdopaminergic tone and then retested. Elevated dopamine enhanced performance in goal-directed operant responses. These data demonstrate that alterations in dopaminergic tone can scale the performance of a previously learned behavior in the absence of new learning.

Heteromeric Complexes of α5 and/or α7 Subunits: Effects of Calcium and Potential Role in Nicotine‐Induced Presynaptic Facilitation

ABSTRACT: Nicotine alters a broad spectrum of behaviors, including attention, arousal, anxiety, and memory. The cellular physiology of nicotine is comparably diverse: nicotine interacts with an array of ionotropic receptors whose gating can lead to direct depolarization of neurons or to an indirect modulation of neuronal excitability by presynaptic facilitation. Furthermore, as many laboratories have shown, the α‐ and β‐type subunits that comprise neuronal nicotinic acetylcholine receptors (nAChRs) are encoded by multiple, homologous genes, yielding at least seven α and three β subunits, distinct in primary sequence. nAChRs that differ in subunit composition differ in pharmacology, conductance, and kinetics as well as in their permeability to and modulation by calcium. We will first discuss recent studies on the biophysics of a special (peculiar?) subset of nAChRs, focusing on heteromeric nAChRs comprised of α4β2 ±α5 or α7 ±β2 and α5. These nAChR channel subtypes are potently and differentially modulated by changes in intracellular calcium ([Ca]). Thus, the Po, τo, and desensitization kinetics of α4β2 channels are altered by changes in [Ca]int from 0 to 50 μM; nAChRs that include the α5 subunit are oppositely regulated. Mutagenesis of specific residues within the M1 to M2 domain of α4, β2, and α5 suggest a possible Ca binding “pocket.” The assembly of functional nAChRs that include α5 and/or α7 and the potential role of these novel heteromeric complexes in presynaptic facilitation will also be presented.

Presynaptic opioid and nicotinic receptor modulation of dopamine overflow in the nucleus accumbens.

Behaviorally relevant stimuli prompt midbrain dopamine (DA) neurons to switch from tonic to burst firing patterns. Similar shifts to burst activity are thought to contribute to the addictive effects of opiates and nicotine. The nucleus accumbens DA overflow produced by these drugs is a key element in their pathological effects. Using electrochemical techniques in brain slices, we explored the effects of opioids on single-spike and burst stimuli-evoked DA overflow in the dorsal and ventral striatum. In specific subregions of the nucleus accumbens, μ-opioids inhibit DA overflow elicited with single-spike stimuli while leaving that produced by burst stimuli unaffected. This is similar to published effects of nicotinic receptor blockade or desensitization, and is mediated by opioid receptor-induced inhibition of cholinergic interneurons. Whereas δ-opioids have similar effects, κ-opioids inhibit evoked DA overflow throughout the striatum in a manner that is not overcome with high-frequency stimuli. These observations reveal remarkable mechanistic overlap between the effects of nicotine and opiates within the dopamine reward pathway.

Cholinesterase Inhibition by Potato Glycoalkaloids Slows Mivacurium Metabolism

Background The duration of action for many pharmaceutical agents is dependent on their breakdown by endogenous hydrolytic enzymes. Dietary factors that interact with these enzyme systems may alter drug efficacy and time course. Cholinesterases such as acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE) hydrolyze and inactivate several anesthetic drugs, including cocaine, heroin, esmolol, local ester anesthetics, and neuromuscular blocking drugs. Natural glycoalkaloid toxins produced by plants of the family Solanaceae, which includes potatoes and tomatoes, inhibit both AChE and BuChE. Here the authors assess the extent to which two solanaceous glycoalkaloids (SGAs), &agr;-solanine and &agr;-chaconine, can alter the effects of neuromuscular blocking drugs and cholinesterase inhibitors in vivo and in vitro. Methods Inhibition of purified human AChE and BuChE by SGAs, neuromuscular blocking drugs, and cholinesterase inhibitors was assessed by an in vitro colorimetric cholinesterase assay. In vivo experiments were carried out using anesthetized rabbits to test whether SGAs affect recovery from mivacurium-induced paralysis. Results SGAs inhibited human BuChE at concentrations similar to those found in serum of individuals who have eaten a standard serving of potatoes. Coapplication of SGAs (30–100 nm) with neuromuscular blocking drugs and cholinesterase inhibitors produced additive cholinesterase inhibition. SGA administration to anesthetized rabbits inhibited serum cholinesterase activity and mivacurium hydrolysis. In addition, SGA prolonged the time needed for recovery from mivacurium-induced paralysis (149 ± 12% of control; n = 12). Conclusions These findings support the hypothesis that inhibition of endogenous enzyme systems by dietary factors can influence anesthetic drug metabolism and duration of action. Diet may contribute to the wide variation in recovery time from neuromuscular blockade seen in normal, healthy individuals.