Morphine blood levels, dependence, and regulation of hippocampal subgranular zone proliferation rely on administration paradigm

Chronic morphine, administered via s.c. pellet, decreases the number of proliferating cells in the dentate gyrus subgranular zone (SGZ) in both rats and mice. This robust morphine-induced decrease could be used to better understand mechanisms regulating adult hippocampal neurogenesis, as well as to explore the relationship between neurogenesis and drug dependence, withdrawal, and relapse behaviors. Such research would benefit enormously from identifying a route of morphine administration that produces addiction-relevant blood levels of morphine, results in a high degree of dependence, translates to both rat and mouse, and is free of the behavioral confounds of s.c. pellets. Therefore, we examined a classic chronic morphine pellet paradigm (two s.c. pellets over 5 days) versus three chronic morphine injection paradigms (escalating dose i.p. injections over 2, 5, or 10 days) for their effect in adult male C57BL/6J mice. We assessed blood morphine levels, SGZ proliferation, and drug dependence as assessed by tolerance to locomotion sensitization and naloxone-precipitated withdrawal. The pellet paradigm produced high and relatively stable blood levels of morphine, a high degree of dependence, and a significant decrease in SGZ proliferation. In contrast, the three injection paradigms produced transient spikes in morphine blood levels, significantly less dependence than the pellet paradigm, and no significant decrease in SGZ proliferation. These data show that regulation of mouse SGZ proliferation requires high and relatively stable blood levels of morphine, and provide critical knowledge for the design of future studies to probe the relationship between addiction and neurogenesis.

Distinct roles for spinophilin and neurabin in dopamine-mediated plasticity

Protein phosphatase 1 plays a major role in the governance of excitatory synaptic activity, and is subject to control via the neuromodulatory actions of dopamine. Mechanisms involved in regulating protein phosphatase 1 activity include interactions with the structurally related cytoskeletal elements spinophilin and neurabin, synaptic scaffolding proteins that are highly enriched in dendritic spines. The requirement for these proteins in dopamine-related neuromodulation was tested using knockout mice. Dopamine D1-mediated regulation of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptor activity was deficient in both striatal and prefrontal cortical neurons from neurabin knockout mice; in spinophilin knockout mice this deficit was manifest only in striatal neurons. At corticostriatal synapses long-term potentiation was deficient in neurabin knockout mice, but not in spinophilin knockout mice, and was rescued by a D1 receptor agonist. In contrast, long-term depression was deficient in spinophilin knockout mice but not in neurabin knockout mice, and was rescued by D2 receptor activation. Spontaneous excitatory post-synaptic current frequency was increased in neurabin knockout mice, but not in spinophilin knockout mice, and this effect was normalized by D2 receptor agonist application. Both knockout strains displayed increased induction of GluR1 Ser(845) phosphorylation in response to D1 receptor stimulation in slices, and also displayed enhanced locomotor activation in response to cocaine administration. These effects could be dissociated from cocaine reward, which was enhanced only in spinophilin knockout mice, and was accompanied by increased immediate early gene induction. These data establish a requirement for synaptic scaffolding in dopamine-mediated responses, and further indicate that spinophilin and neurabin play distinct roles in dopaminergic signal transduction and psychostimulant response.

Neuronal nicotinic acetylcholine receptor subunit knockout mice: physiological and behavioral phenotypes and possible clinical implications

Nicotinic acetylcholine receptors (nAChRs) in the muscle, autonomic ganglia, and brain are targets for pharmacologically administered nicotine. Several of the subunits that combine to form neuronal nicotinic receptors have been deleted by knockout or mutated by knockin in mice using homologous recombination. We will review the biochemical, pharmacological, anatomical, physiological, and behavioral phenotypes of mice with genetically altered neuronal nAChR subunits. Clinically relevant mutations in nAChR genes will also be discussed. In addition, some of the signal transduction pathways activated through nAChRs will be described in order to delineate the longer-term changes that might result from persistent activation or inactivation of nAChRs. Genetically manipulated mice have greatly increased our understanding of the subunit composition and physiological properties of nAChRs in vivo. In addition, these mice have provided a model system to determine the molecular basis for many of the pharmacological actions of nicotine on neurotransmitter release and behavior. Genetic manipulations in mice have also elucidated the role of nAChR subunits in various disease states, and suggest several avenues for drug development.

Nicotine receptor inactivation decreases sensitivity to cocaine

The reinforcing properties of nicotine and psychomotor stimulants are thought to be mediated through the mesolimbic dopamine (DA) system. This study investigates the role of high affinity nicotinic acetylcholine receptors (nAChRs) in cocaine place preference and examines some neurochemical changes in the mesolimbic DA system that might account for the interaction between nicotine and cocaine. 5 mg/kg is the lowest dose of cocaine able to condition a place preference in C57Bl/6 mice. Co-treatment with the nicotinic antagonist mecamylamine (1.0 mg/kg) disrupted place preference to 5 mg/kg cocaine. In addition, mice lacking the high affinity nAChR containing the beta2 subunit showed decreased place preference to 5 mg/kg cocaine, although higher doses of cocaine could condition a place preference in these knock out animals. In contrast, co-administration of a low dose of nicotine (0.2 mg/kg) potentiated place preference to a subthreshold dose of cocaine (3 mg/kg). DA turnover was monitored in several brain regions using tissue levels of DA and its primary metabolite DOPAC as an indication of DA release. Wild type mice showed decreased DA turnover following treatment with 5 mg/kg cocaine; whereas, this response was not seen in mice lacking the beta2 subunit of the nAChR. Induction of chronic fos-related antigens by cocaine was also reduced in mutant mice as compared to their wild type siblings, implying that downstream actions of cocaine were also affected by inactivation of the high affinity nAChR. These data indicate that activation of the high affinity nAChR may contribute to cocaine reinforcement.

Galanin receptor 1 gene expression is regulated by cyclic AMP through a CREB-dependent mechanism

The galanin receptor-1 (GalR1) protein belongs to a family of G protein-coupled receptors for the neuropeptide galanin (GalR1, GalR2 and GalR3) distributed throughout the central and peripheral nervous system. Activation of galanin receptors by their ligands results in increased feeding, impaired learning, enhanced opiate analgesia and decreased opiate place preference. We have shown that opiate withdrawal, which is known to increase levels of cAMP in the locus coeruleus (LC), results in an increase in the number of galanin binding sites and the level of GalR1 mRNA in the LC. We have isolated a 3.6-kb fragment 5' of the inititiation codon of the mouse GalR1 gene and generated a series of deletion mutations of this fragment driving expression of luciferase for use in transient transfection assays in PC12 and Cath.a cell lines. Treatment with forskolin, but not dideoxyforskolin, up-regulates GalR1 transcription, likely through elevation of cAMP levels. The region between - 1050 and - 700 base pairs upstream of exon one is necessary both for basal activity of the GalR1 promoter and for forskolin-mediated increases in transcription. The forskolin effect can be blocked by simultaneous mutation of a CRE-like site and a CRE/DRE-like site, but not mutation of either site alone. Gel shift and super-shift experiments demonstrate that the transcription factor CREB can bind to both sites and is likely to be responsible for the cAMP-mediated increase in GalR1 promoter activity. This study provides a molecular mechanism for the increased GalR1 expression in the LC seen following opiate withdrawal.

Gender differences in learned helplessness behavior are influenced by genetic background

Learned helplessness behavior was examined in female and male C57BL/6J (B6), 129/J (129) and (B6 x 129)F1 mice, common genetic backgrounds for the generation of knockout models, as well as in mice of a mixed genetic background (outbred mice). Both genotype and gender differences were observed in learned helplessness. Outbred males showed increased shuttle escape latencies following 60, 120, or 360 inescapable shocks compared to nonshocked controls, but outbred females showed no increase in escape latencies following inescapable shock pretreatment. B6 females showed increased escape latencies following 60, 120, or 360 inescapable shocks, whereas B6 males showed increased latencies only after 360 shocks. Female and male 129 and B6129F1 mice did not show an increase in escape latencies following inescapable shock, but this was most likely due to poor escape performance in nonshocked control mice. Differences in escape performance could not be explained by differences in pain thresholds between genotypes. These results support the idea that genetic background and gender are important to consider when using the learned helplessness model in genetically manipulated mice.

Upregulation of galanin binding sites and GalR1 mRNA levels in the mouse locus coeruleus following chronic morphine treatments and precipitated morphine withdrawal

The neuropeptide galanin and its receptors are expressed in the locus coeruleus (LC), a brain area associated with drug dependence and withdrawal. Although galanin peptide mRNA levels do not change during withdrawal, it is not known whether galanin receptor levels are regulated following opiate withdrawal. This study demonstrates that galanin binding in the LC is upregulated by chronic-intermittent morphine administration or by precipitated withdrawal, but not by acute morphine treatment, suggesting that increased activity in the LC may be able to regulate galanin binding sites. Moreover, the increase in galanin binding sites seems to be caused by increased transcription or stabilization of the galanin receptor 1 (GalR1) gene, because there is a dramatic increase in mRNA levels following withdrawal in the LC. It is, therefore, possible that the increase in GalR1 could be an adaptive mechanism that leads to regulation of cAMP levels and possibly firing rate of LC neurons.

Nicotinic receptors in the brain. Links between molecular biology and behavior

Molecular cloning has elucidated the sequence of a family of acetylcholine receptor subunits that are activated by nicotine. Subsequent studies on the localization of individual subunits and the physiological properties of nicotinic subunit combinations in vitro, have led to identification of subunit compositions of nicotinic receptors that may function in vivo, as the native receptor. A particular challenge for the field has been to use these molecular data to determine which individual nicotinic receptor subtype is responsible for mediating each of the behavioral effects of nicotine. Human and animal studies have shown that nicotine is reinforcing and likely responsible for the addictive properties of tobacco. In addition, nicotine has been shown to have effects on locomotion, cognition, affect, and pain sensitivity. Recent studies combining the techniques of molecular biology, pharmacology, electrophysiology, and behavioral analysis to analyze knock out mice that lack individual subunits of the nicotinic acetylcholine receptor, have helped identify the role of specific nicotinic subunits in some of these complex behaviors. These studies could ultimately be useful in designing specific nicotinic receptor agonists and antagonists that may have uses in the clinic.

Polymorphisms of the human apolipoprotein E promoter and bleomycin hydrolase gene: risk factors for Alzheimer's dementia?

In addition to the apolipoprotein E (ApoE) tri-allele polymorphism, genetic variants of the apolipoprotein E promoter (-491A/T) and the bleomycin hydrolase (BH-PEN) gene have also been proposed as genetic risk factors for Alzheimer's dementia (AD). Since reports about the relevance of these polymorphisms for the pathogenesis of AD have been contradictory, we performed an association study with some modifications. First, the control group in this study was made up of non-demented psychiatric inpatients, rather than healthy subjects. This procedure allows the specificity of the relationship between a given genotype and AD (as opposed to other psychiatric disorders) to be determined. Second, as an alternative to preexisting relatively time consuming techniques, BH-PEN polymorphism was determined using a simplified method based on PCR genotyping. We found a significant linkage disequilibrium between the -491A/T and ApoE polymorphisms. However, no direct association was observed between the -491A/T or BH-PEN polymorphism and AD.

Centrally administered galanin blocks morphine place preference in the mouse

Galanin is a neuropeptide with appetitive, antinociceptive and neuroendocrine functions. Galanin and galanin binding sites are present in brain areas that mediate reinforcement, such as nucleus accumbens and ventral tegmental area, as well as locus coeruleus, an area known to be involved in development of drug dependence and withdrawal. This localization, coupled with the observation that there is a strong interaction between morphine and galanin in spinal cord, made it of interest to study whether galanin might have effects on morphine reinforcement. Using the place preference paradigm we found that galanin (1 microg i.c.v.) alone does not possess reinforcing or aversive properties but attenuates the preference conditioned by peripheral administration of morphine (5 mg/kg s.c.). Quantitative receptor autoradiography showed that morphine treatment that could condition a place preference decreased galanin binding in the nucleus accumbens and increased galanin binding in the locus coeruleus. In contrast, acute naltrexone administration increased galanin binding in the nucleus accumbens, suggesting that levels of galanin binding are tonically regulated by opioid receptors in that area. Contrary to what is seen in the spinal cord, these results indicate that galanin and morphine have an antagonistic interaction in the brain that results in attenuation of morphine reinforcement by activation of the galaninergic system.

Hypotension and inflammatory cytokine gene expression triggered by factor Xa-nitric oxide signaling

The signaling pathway initiated by factor Xa on vascular endothelial cells was investigated. Factor Xa stimulated a 5- to 10-fold increased release of nitric oxide (NO) in a dose-dependent reaction (0.1-2.5 microG/ml) unaffected by the thrombin inhibitor hirudin but abolished by active site inhibitors, tick anticoagulant peptide, or Glu-Gly-Arg-chloromethyl ketone. In contrast, the homologous clotting protease factor IXa or another endothelial cell ligand, fibrinogen, was ineffective. A factor Xa inter-epidermal growth factor synthetic peptide L (83)FTRKL(88) (G) blocking ligand binding to effector cell protease receptor-1 inhibited NO release by factor Xa in a dose-dependent manner, whereas a control scrambled peptide KFTGRLL was ineffective. Catalytically active factor Xa induced hypotension in rats and vasorelaxation in the isolated rat mesentery, which was blocked by the NO synthase inhibitor L-N(G)-nitroarginine methyl ester (L-NAME) but not by D-NAME. Factor Xa/NO signaling also produced a dose-dependent endothelial cell release of interleukin 6 (range 0.55-3.1 ng/ml) in a reaction inhibited by L-NAME and by the inter-epidermal growth factor peptide Leu(83)-Leu(88) but unaffected by hirudin. Maximal induction of interleukin 6 mRNA required a brief, 30-min stimulation with factor Xa, unaffected by subsequent addition of tissue factor pathway inhibitor. These data suggest that factor Xa-induced NO release modulates endothelial cell-dependent vasorelaxation and cytokine gene expression. This pathway requiring factor Xa binding to effector cell protease receptor-1 and a secondary step of ligand-dependent proteolysis may preserve an anti-thrombotic phenotype of endothelium but also trigger acute phase responses during activation of coagulation in vivo.

Capsaicin's effect on rat nasal mucosa substance P release: experimental basis for vasomotor rhinitis treatment

Substance P (SP) is one of several neuropeptides found in nasal mucosa. It exists primarily in sensory afferent neurons, which are best demonstrated by immunohistochemical staining. These substance P-like immunoreactive (SPLI) nerve fibers are unmyelinated C fibers, which connect to the trigeminal ganglion and the spinal trigeminal nucleus. They are found around vessels and seromucinous glands in the submucosa. When the nasal mucosa receives a noxious stimulus, SP is released and acts orthodromically as a comediator of nasal pain, while antidromically it induces vasodilatation, plasma extravasation, mucosal edema, and rhinorrhea. Its antidromic effects have been implicated in vasomotor rhinitis and can be blocked by topical intranasal capsaicin application. Wolf and others have demonstrated in human subjects that vasomotor rhinitis can be blocked up to 1 year by a series of intranasal capsaicin applications. It has not yet been demonstrated that nasal mucous SP levels following noxious nasal stimulus change after intranasal capsaicin pretreatment. Consequently a project was designed to determine whether intranasal capsaicin pretreatment would affect nasal substance P release measured in nasal secretion. Nasal secretion SP levels were measured before and after noxious nasal stimulus in controls and in capsaicin pretreated rats. The difference in measured nasal secretion SP levels were significant (p < 0.05).

Dynorphin-(1-8) inhibits the release of substance P-like immunoreactivity in the spinal cord of rats following a noxious mechanical stimulus

This study was conducted to determine the effect of the opioid peptide dynorphin-(1-8) on the release of substance P-like immunoreactivity in the dorsal horn during mechanical activation of peripheral nociceptors. A push-pull cannula was used to perfuse the dorsal horn of decerebrate/spinal transected rats before, during and following the application of a noxious mechanical stimulus to the ipsilateral hindpaw and lower limb. The collected perfusates were assayed for substance P-like immunoreactivity using radioimmunoassay. Dynorphin-(1-8) applied to the spinal cord at a concentration of 1 microM reduced the basal release of substance P-like immunoreactivity by 28 +/- 11% and prevented the mechanically evoked release of substance P-like immunoreactivity. This effect of dynorphin-(1-8) was reversed by 2 microM of the selective kappa-opioid receptor antagonist nor-binaltorphimine. Moreover, blockade of the kappa-opioid receptors by nor-binaltorphimine resulted in a 33 +/- 5% increase in the basal release of substance P-like immunoreactivity. These data show that activation of nor-binaltorphimine-sensitive sites by dynorphin-(1-8) results in inhibition of the release of substance P-like immunoreactivity in the dorsal horn of the rat.

Low but not high rate noxious radiant skin heating evokes a capsaicin-sensitive increase in spinal cord dorsal horn release of substance P

Some kinds of nociception appear to be partially mediated by the release of substance P (SP) in the spinal cord dorsal horn from terminals of primary afferent nociceptors. Only some nociceptors contain and release SP however. Specifically, SP appears to be released by unmyelinated (C) nociceptive afferents when activated by noxious stimulation to the skin, but does not appear to be contained in cutaneous myelinated (A delta) nociceptive afferents. We have proposed a model of nociception in rats that uses different rates of noxious skin heating to allow for differential assessment on behavioral responses mediated by the activation of A delta or C fiber nociceptors. As one means of testing the validity of this model we have examined the effects of using high and low rate noxious skin heating on the dorsal horn release of substance P-like immunoreactivity (SPLI) in decerebrate/spinal transected animals. Consistent with the model, low rate skin heating evokes a significant increase in dorsal horn SPLI release indicating C fiber mediation, whereas high rate skin heating did not evoke SP release, indicating mediation by afferents other than C afferents, i.e. A delta nociceptive afferents. Also consistent with behavioral effects, topical application of capsaicin, which sensitizes C nociceptors, increased the SPLI release evoked by low but not high rate skin heating. These data provide additional evidence that foot withdrawals evoked by low rate skin heating are mediated by C fiber activation, whereas foot withdrawals evoked by high rate skin heating are evoked by A delta fiber activation.

Delta-Opioid receptor modulation of the release of substance P-like immunoreactivity in the dorsal horn of the rat following mechanical or thermal noxious stimulation

The present study was undertaken to investigate the effects of the opioid peptide Met-enkephalin (met-enk) on the release of substance P-like immunoreactivity (SPLI) in the lumbar dorsal horn during the application of a noxious mechanical or thermal stimulus to the ipsilateral hind paw and lower limb of the rat. A push-pull cannula was introduced to the lumbar dorsal horn in non-anesthetized decerebrate/spinal transected rats. The dorsal horn was perfused with artificial CSF and the collected perfusates were assayed for SPLI using radioimmunoassay. A noxious mechanical or thermal stimulus was applied to different areas of the ipsilateral hind paw and lower limb. Met-enk (500 nM) applied to the dorsal horn through the perfusate reduced the basal release of SPLI by 29 +/- 9% and prevented the increase in the release of SPLI evoked by the noxious mechanical or thermal stimulus. The effect of met-enk was blocked by the selective delta-opioid receptor antagonist naltrindole (500 nM). Naltrindole (NTD) alone elicited a 75 +/- 30% increase in the basal release of SPLI. These data show that met-enk inhibits the thermally or mechanically evoked release of SPLI in the dorsal horn by activating the delta opioid receptors. These receptors are also involved in the tonic spinal regulation of the release of SPLI.

Kappa-opioid receptor modulation of the release of substance P in the dorsal horn

Substance P (SP), a member of the tachykinin peptide family, has been found in high concentrations in the superficial laminae of the dorsal horn and it is thought to play a major role in the transmission of nociceptive information. Dynorphin(1-8), an opioid peptide with high selectivity for the kappa-opioid receptor subtype, is also found in the dorsal horn of the spinal cord. The aim of this study was to determine the effect of dynorphin(1-8) on the release of SP-like-immunoreactivity (SPLI) in the dorsal horn before and during the activation of peripheral nociceptors by a thermal stimulus. A push-pull canula was used to perfuse the dorsal horn of non-anesthetized decerebrate/spinal transected rats and the collected perfusates were assayed for SPLI by using radioimmunoassay. Dynorphin(1-8) applied to the spinal cord at a concentration of 1 microM elicited a 27 +/- 8% decrease in the basal release of SPLI and prevented the increase in the release of SPLI evoked by the application of a noxious thermal stimulus to the ipsilateral hind paw and lower limb. The effect of dynorphin(1-8) was reversed by 2 microM of nor-binaltorphimine (nor-BNI), a selective kappa opioid receptor antagonist. Application of nor-BNI alone to the perfusate resulted in a 62 +/- 23% increase in the basal release of SPLI. In conclusion, dynorphin(1-8) reduces the basal release of SPLI and prevents the increase in the release of SPLI elicited by the application of a noxious cutaneous thermal stimulus. This effect is mediated through the kappa-opioid receptor, which appears to tonically regulate the release of SPLI in the dorsal horn.