Jeffery D. Steketee

Sensing the environment: regulation of local and global homeostasis by the skin's neuroendocrine system.

Skin, the bodys largest organ, is strategically located at the interface with the external environment where it detects, integrates, and responds to a diverse range of stressors including solar radiation. It has already been established that the skin is an important peripheral neuro-endocrine-immune organ that is tightly networked to central regulatory systems. These capabilities contribute to the maintenance of peripheral homeostasis. Specifically, epidermal and dermal cells produce and respond to classical stress neurotransmitters, neuropeptides, and hormones. Such production is stimulated by ultraviolet radiation (UVR), biological factors (infectious and noninfectious), and other physical and chemical agents. Examples of local biologically active products are cytokines, biogenic amines (catecholamines, histamine, serotonin, and N-acetyl-serotonin), melatonin, acetylocholine, neuropeptides including pituitary (proopiomelanocortin-derived ACTH, beta-endorphin or MSH peptides, thyroid-stimulating hormone) and hypothalamic (corticotropin-releasing factor and related urocortins, thyroid-releasing hormone) hormones as well as enkephalins and dynorphins, thyroid hormones, steroids (glucocorticoids, mineralocorticoids, sex hormones, 7-delta steroids), secosteroids, opioids, and endocannabinoids. The production of these molecules is hierarchical, organized along the algorithms of classical neuroendocrine axes such as hypothalamic-pituitary-adrenal axis (HPA), hypothalamic-thyroid axis (HPT), serotoninergic, melatoninergic, catecholaminergic, cholinergic, steroid/secosteroidogenic, opioid, and endocannbinoid systems. Dysregulation of these axes or of communication between them may lead to skin and/ or systemic diseases. These local neuroendocrine networks are also addressed at restricting maximally the effect of noxious environmental agents to preserve local and consequently global homeostasis. Moreover, the skin-derived factors/systems can also activate cutaneous nerve endings to alert the brain on changes in the epidermal or dermal environments, or alternatively to activate other coordinating centers by direct (spinal cord) neurotransmission without brain involvement. Furthermore, rapid and reciprocal communications between epidermal and dermal and adnexal compartments are also mediated by neurotransmission including antidromic modes of conduction. In conclusion, skin cells and skin as an organ coordinate and/or regulate not only peripheral but also global homeostasis.

Cocaine increases medial prefrontal cortical glutamate overflow in cocaine-sensitized rats: a time course study

Excitatory amino acid transmission within mesocorticolimbic brain pathways is thought to play an important role in behavioural sensitization to psychomotor stimulants. The current studies evaluated a time course of the effects of cocaine on extracellular glutamate levels within the medial prefrontal cortex (mPFC) following increasing periods of withdrawal from repeated cocaine exposure. Male Sprague–Dawley rats underwent stereotaxic surgeries and were pretreated daily with saline (1 mL/kg/day × 4 days, i.p.) or cocaine (15 mg/kg/day × 4 days, i.p.) and withdrawn for 1, 7 or 30 days. After withdrawal rats were challenged with the same dose of saline or cocaine and in vivo microdialysis of the mPFC was conducted with concurrent analysis of locomotor activity. Animals that were withdrawn from repeated daily cocaine for 1 day and 7 days displayed an augmentation in cocaine‐induced mPFC glutamate levels compared to saline and acute control subjects, which were similarly unaffected by cocaine challenge. At the 7 day time point, a subset of animals that received repeated cocaine did not express behavioural sensitization, nor did these animals exhibit the enhancement in mPFC glutamate in response to cocaine challenge. In contrast to these early effects, 30 days of withdrawal resulted in no significant changes in cocaine‐induced mPFC glutamate levels regardless of the pretreatment or behavioural response. These data suggest that repeated cocaine administration transiently increases cocaine‐induced glutamate levels in the mPFC during the first week of withdrawal, which may play an important role in the development of behavioural sensitization to cocaine.

Effects of repeated inhalation of toluene on ionotropic GABAA and glutamate receptor subunit levels in rat brain

Toluene is a commonly abused solvent found in many industrial and commercial products. The neurobiological effects of toluene remain unclear, but many of them, like those of ethanol, may be mediated by gamma-aminobutyric acid (GABA) and glutamate receptors. Chronic ethanol administration has been shown to alter levels of specific subunits for GABA type A (GABA(A)), N-methyl-d-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors. However, little is known about the effects of toluene on subunit levels of these receptors. To examine this, rats were exposed to toluene vapors (8000 ppm) or air for 10 days (30 min/day), and afterwards GABA(A) alpha1, NR1 and NR2B (NMDA) and GluR1 and GluR2/3 (AMPA) receptor subunit levels were determined in discrete brain regions of these animals by Western blotting. Toluene increased GABA(A) alpha1, NR1, NR2B and GluR2/3 subunits in the medial prefrontal cortex and decreased GABA(A) alpha1 and NR1 subunits in the substantia nigra compacta. Toluene inhalation produced modest increases in GABA(A) alpha1 subunits in the striatum, as well as slight decreases in this subunit in the ventral tegmental area. NR2B subunit levels were also slightly increased within the nucleus accumbens by toluene. These studies show that toluene differentially alters the levels of specific GABAergic and glutamatergic receptor subunits in a regionally selective manner.

Time-dependent effects of repeated cocaine administration on dopamine transmission in the medial prefrontal cortex.

The medial prefrontal cortex (mPFC) has been implicated in the development of behavioral sensitization, which is the progressive enhancement of locomotor activity that occurs with repeated administration of psychostimulants. Previous data suggest that mPFC dopamine (DA) transmission may be attenuated in cocaine-sensitized animals, but the onset and duration of this effect have not been investigated. After recovery from stereotaxic surgeries, animals were given four daily injections of saline (1 ml/kg, i.p.) or cocaine (15 mg/kg, i.p.) and were subsequently challenged with saline or cocaine after 1, 7 or 30 d of withdrawal, on which days in vivo microdialysis of the mPFC was conducted simultaneously with monitoring of locomotor activity. Compared to acutely administered controls, the results in cocaine-pretreated animals were as follows: 1d of withdrawal was associated with a significant attenuation in cocaine-induced locomotion and mPFC DA overflow; after 7d, behavioral sensitization was accompanied by a significant attenuation in cocaine-induced elevations in mPFC DA levels; 30 d of withdrawal led to the expression of sensitized behaviors paralleled by an augmentation in cocaine-induced mPFC DA. These data suggest that repeated cocaine produces temporally distinct behavioral effects associated with alterations in mPFC DA responsiveness to cocaine that may be involved in the development of behavioral sensitization.

Repeated exposure to cocaine alters the modulation of mesocorticolimbic glutamate transmission by medial prefrontal cortex Group II metabotropic glutamate receptors

Repeated cocaine exposure enhances glutamatergic output from the medial prefrontal cortex to subcortical brain regions. Loss of inhibitory control of cortical pyramidal neurons may partly account for this augmented cortical glutamate output. Recent research indicated that repeated cocaine exposure reduced the ability of cortical Group II metabotropic glutamate receptors to modulate behavioral and neurochemical responses to cocaine. Thus, experiments described below examined whether repeated cocaine exposure alters metabotropic glutamate receptor regulation of mesocorticolimbic glutamatergic transmission using in vivo microdialysis. Infusion of the Group II metabotropic glutamate receptor antagonist LY341495 into the medial prefrontal cortex enhanced glutamate release in this region, the nucleus accumbens and the ventral tegmental area in sensitized animals, compared to controls, following short‐term withdrawal but not after long‐term withdrawal. Additional studies demonstrated that vesicular (K+‐evoked) and non‐vesicular (cystine‐evoked) glutamate release in the medial prefrontal cortex was enhanced in sensitized animals, compared to controls, that resulted in part from a reduction in Group II metabotropic glutamate receptor modulation of these pools of glutamate. In summary, these findings indicate that the expression of sensitization to cocaine is correlated with an altered modulation of mesocorticolimbic glutamatergic transmission via reduction of Group II metabotropic glutamate receptor function.

Inactivation of the central nucleus of the amygdala reduces the effect of punishment on cocaine self-administration in rats

Continued cocaine use despite the negative consequences is a hallmark of cocaine addiction. One such consequence is punishment, which is often used by society to curb cocaine use. Unfortunately, we know little about the mechanism involved in regulation by punishment of cocaine use. The fact that cocaine addicts continue to use cocaine despite potentially severe punishment suggests that the mechanism may be impaired. Such impairment is expected to critically contribute to compulsive cocaine use. This study was aimed at testing the hypothesis that the central nucleus of the amygdala (CeN) plays a critical role in such regulation. To this end, rats were trained to press a lever to self‐administer cocaine under a chained schedule: a response on one lever (cocaine‐seeking lever) led to access to the other lever (cocaine‐taking lever), on which a response was reinforced by cocaine and cues. Thereafter, responses on the seeking lever were punished by footshock with a probability of 0.5. Cocaine self‐administration (SA) was significantly suppressed by punishment in an intensity‐dependent manner. Interestingly, rats trained with daily 6‐h (extended access) but not 2‐h (limited access) sessions showed resistance to the lower intensity of punishment. Inactivation of the CeN induced a robust anti‐punishment effect in both groups. These data provided evidence that the CeN is a critical neural substrate involved in regulation by punishment of cocaine SA. Rats with a history of extended cocaine SA appeared to be less sensitive to punishment. The decreased sensitivity could result from the neuroplastic changes induced by extended cocaine SA in the CeN.

Repeated exposure to cocaine alters medial prefrontal cortex dopamine D2‐like receptor modulation of glutamate and dopamine neurotransmission within the mesocorticolimbic system

J. Neurochem. (2011) 119, 332–341.

Activation of D2-like receptors in rat ventral tegmental area inhibits cocaine-reinstated drug-seeking behavior

Relapse is a hallmark of cocaine addiction. Cocaine‐induced neuroplastic changes in the mesocorticolimbic circuits critically contribute to this phenomenon. Pre‐clinical evidence indicates that relapse to cocaine‐seeking behavior depends on activation of dopamine neurons in the ventral tegmental area. Thus, blocking such activation may inhibit relapse. Because the activity of dopamine neurons is regulated by D2‐like autoreceptors expressed on somatodendritic sites, this study, using the reinstatement model, aimed to determine whether activation of D2‐like receptors in the ventral tegmental area can inhibit cocaine‐induced reinstatement of extinguished cocaine‐seeking behavior. Rats were trained to self‐administer i.v. cocaine (0.25 mg/infusion) under a modified fixed‐ratio 5 schedule. After such behavior was well learned, rats went through extinction training to extinguish cocaine‐seeking behavior. The effect of quinpirole, a selective D2‐like receptor agonist microinjected into the ventral tegmental area, on cocaine‐induced reinstatement was then assessed. Quinpirole (0–3.2 μg/side) dose‐dependently decreased cocaine‐induced reinstatement and such effects were reversed by the selective D2‐like receptor antagonist eticlopride when co‐microinjected with quinpirole into the ventral tegmental area. The effect appeared to be specific to the ventral tegmental area because quinpirole microinjected into the substantia nigra had no effect. Because D2‐like receptors are expressed on rat ventral tegmental area dopamine neurons projecting to the pre‐frontal cortex and nucleus accumbens, our data suggest that these dopamine circuits may play a critical role in cocaine‐induced reinstatement. The role of potential changes in D2‐like receptors and related signaling molecules of dopamine neurons in the vulnerability to relapse was discussed.

Characterization of dopamine transport in crude synaptosomes prepared from rat medial prefrontal cortex

Accumulating evidence suggests that dopamine (DA) uptake into mesocortical neurons may be regulated through mechanisms that are markedly different from those observed in nigrostriatal or mesoaccumbens systems. The current studies were conducted to develop a rapid and sensitive DA uptake assay in crude synaptosomes prepared from the medial prefrontal cortex (mPFC) of a single animal. Uptake of DA into the mPFC was saturable, linear with respect to protein concentration, time dependent, and sensitive to the effects of monoamine transport inhibitors. Saturation analysis revealed the K(m) and V(max) values for DA transport in the mPFC were approximately 60 nM and 6.5 pmol/min mg protein, respectively. A significant amount of DA uptake in the mPFC was more sensitive to inhibition by nisoxetine compared to GBR12909, fluoxetine (FLX), and cocaine (COC), suggesting the norepinephrine transporter (NET) plays an important role in the clearance of DA within this region. The described assay conditions would be useful in examining DA uptake within specific brain regions obtained from a single animal.

Repeated injections of sulpiride into the medial prefrontal cortex induces sensitization to cocaine in rats

RationaleRecent studies have suggested that the medial prefrontal cortex (mPFC) plays an important role in the development of sensitization to cocaine. In particular, a recent report proposed that sensitization is associated with a decreased dopamine D2 receptor function in the mPFC. The present study was designed to further examine the involvement of mPFC dopamine D2 receptors in cocaine sensitization.ObjectivesThe experiments described below sought to determine the effects of acute or repeated intra-mPFC injections of the dopamine D2 antagonist sulpiride on subsequent motor-stimulant and nucleus accumbens dopamine responses to cocaine.MethodsRats received bilateral cannulae implants above the ventral mPFC for microinjections and above the nucleus accumbens for in vivo microdialysis. Initial studies examined the effects of intra-mPFC sulpiride pretreatment on the acute motor-stimulant and nucleus accumbens dopamine responses to cocaine. Follow-up studies determined the effects of repeated intra-mPFC sulpiride injections on subsequent behavioral and nucleus accumbens dopamine responses to a cocaine challenge.ResultsIntra-mPFC sulpiride enhanced the cocaine-induced increases in motor activity and dopamine overflow in the nucleus accumbens. Repeated intra-mPFC sulpiride induced behavioral and neurochemical cross-sensitization to cocaine.ConclusionsThe data support previous findings that sensitization is associated with a decrease in dopamine D2 receptor function in the mPFC.