Presynaptic Dopaminergic Function in the Nucleus Accumbens Following Chronic Opiate Treatment and Precipitated Withdrawal

Naloxone treatment at three days following implantation of pellets containing morphine base increased uptake of tritiated dopamine by the nucleus accumbens but did not alter efflux of tritiated dopamine by the nucleus accumbens or tritiated norepinephrine by the hippocampus. At six days following placement of pellets containing morphine base, withdrawal score was increased after treatment with either saline or naloxone, indicating that animals were undergoing spontaneous opiate withdrawal. Fractional efflux of tritiated dopamine was decreased at this time point following intermittent stimulation with 317 and 1000 M 4-aminopyridine, for striatal slices obtained from animals pretreated with either saline or naloxone. For the nucleus accumbens at six days after placement of morphine pellets, similar decreases in the efflux of tritiated dopamine were only observed in slices obtained from naloxone treated animals. Fractional dopamine efflux was also diminished after in vitro exposure to rising concentrations of 4-aminopyridine, amphetamine, or cocaine for tissue obtained from the nucleus accumbens, but not for slices from the striatum at six days following morphine pellet implantation. In conclusion, deficits in dopamine efflux by the nucleus accumbens occur at a time when animals are undergoing spontaneous opiate withdrawal at six days following morphine pellet implantation, but do not occur at an earlier time point when withdrawal is precipitated by naloxone treatment. These deficits are apparent for brain slices obtained from the striatum or nucleus accumbens after exposure to rising concentrations of different in vitro treatments, with tissue obtained from the nucleus accumbens being more sensitive than the striatum to dopamine efflux produced by a wider range of treatments.     

Biphasic Effects of Selegiline on Striatal Dopamine: Lack of Effect on Methamphetamine-Induced Dopamine Depletion

We tested the hypothesis that selegiline can attenuate dopamine depletion if administered following high doses of methamphetamine that cause neurotoxicity in the striatum. Methamphetamine produced decreases of 50% or greater in both striatal concentrations of dopamine and combined concentrations of homovanillic acid and DOPAC in mice. For animals not exposed to methamphetamine, chronic treatment with selegiline over 18 days caused biphasic effects on striatal dopamine content, with decreases, no effect, or increases observed for mice receiving treatment with 0.02, 0.2, and 2.0 mg/kg, respectively. Selegiline failed to modify methamphetamine-induced reductions in striatal dopamine content or combined concentrations of homovanillic acid and DOPAC. Significant increases in mortality following the onset of selegiline treatment (24 hours after the initial dose of methamphetamine) occurred in methamphetamine-treated mice that received saline or 2.0 mg/kg of selegiline, but not for mice treated with 0.02 or 0.2 mg/kg of selegiline. These results indicate that selegiline fails to attenuate dopamine depletion when administered chronically following exposure to methamphetamine, but may attenuate methamphetamine-induced mortality. In control animals that did not receive methamphetamine, low doses of selegiline produced decreases the concentration of striatal dopamine, while high dose treatment caused increases in striatal dopamine content.     

Protein Aggregation in Retinal Cells and Approaches to Cell Protection

1. Retinal dystrophies (RD) comprise a group of clinically and genetically heterogeneous retinal disorders, which typically result in the degeneration of photoreceptors followed by the impairment or loss of vision. Although age-related macular degeneration (AMD) and retinitis pigmentosa (RP) are among the most common forms of RD, currently, there is no effective treatment for either disorder. 2. Recently, abnormal protein accumulation and aggregation due to protein misfolding and proteasome inhibition have been implicated in the pathogenesis of RD. In this paper we describe effects of several factors on protein aggregation and survival of photoreceptor cells. 3. Expression of rhodopsin carrying P23H mutation causes its accumulation in intracellular inclusion bodies in a perinuclear area of photoreceptor cells. beta- and gamma-synucleins and heat shock protein Hsp-70, but not alpha-synuclein, protect cultured ocular cells from mutant opsin accumulation. This effect might be explained by their chaperonic activity. 4. Knock-out of alpha- and gamma-synucleins does not affect gross retinal morphology, but induces tyrosine hydroxylase in the inner prexiform layer of the retina. Selegiline-a monoamine oxidase inhibitor used for the treatment of Parkinson's disease, reduces apoptosis and increases viability in cultured retinal pigment epithelium cells (APRE-19). 5. These results suggest that chaperones and selegiline may be considered promising candidates for the protection of ocular cells from the accumulation of misfolded and aggregated proteins.     

Different Effects of Opiate Withdrawal on Dopamine Turnover,Uptake, and Release in the Striatum and Nucleus Accumbens

The purpose of these experiments was to further characterize changes in dopaminergic function that follow withdrawal from chronic opiate treatment. Withdrawal after treatment to a maximum dose of 120 mg/kg of morphine did not alter dopamine concentrations in the substantia nigra, ventral tegmental area, striatum, or nucleus accumbens; but did decrease concentrations of DOPAC and the ratio of DOPAC to dopamine in the lateral striatum and nucleus accumbens. Uptake of tritiated dopamine was diminished for withdrawn slices obtained from the striatum with no effect observed for tissue from the nucleus accumbens. Deficits of in vitro release of tritiated dopamine also occurred following withdrawal, with the nucleus accumbens being sensitive to dependence produced by a lower dose of morphine. In conclusion, opiate withdrawal produces a complex pattern of effects on dopaminergic function that is specific for the striatum and nucleus accumbens.     

Opiate Modulation of Striatal Dopamine and Hippocampal Norepinephrine Release Following Morphine Withdrawal

When opiates are abruptly withdrawn after chronic treatment, increases in hippocampal noradre-nergic function are observed which are accompanied by decreases in striatal dopamine release. The latter effects have to shown to persist for several weeks following the onset of opiate withdrawal. We examined the long-term effects of opiate withdrawal on 4-aminopyridine and potassium stimulated release of striatal dopamine and hippocampal norepinephrine. Tissue samples were obtained either from rats that had been exposed to opiate withdrawal following a seven day morphine infusion or sham treated control subjects. At 48 hours after the onset of withdrawal (cessation of morphine infusions), slices were loaded with [³H] neurotransmitter, washed extensively, and exposed to different drug treatments. 4-aminopyridine induced concentration related increases in striatal dopamine release, which was 36% calcium independent. Similar values for fractional release of striatal dopamine were obtained in morphine withdrawn and control subjects, for both potassium and 4-aminopyridine induced release. In addition, thresholds for 4-aminopyridine or potassium induced release of striatal dopamine did not differ between control and morphine withdrawn subjects. Treatment with 1.0 M morphine sulfate potentiated potassium evoked release of norepinephrine to an equal extent in both morphine withdrawn and sham treated hippocampal tissue. Exposure to a threshold concentration of potassium (8.0 mM), stimulated increased release of hippocampal norepinephrine in a significantly greater fraction of tissue samples obtained from morphine withdrawn animals. Although these results do not support changes in striatal dopamine release following opiate withdrawal, opiate mechanisms appear to be important determinants of in vitro hippocampal norepinephrine release.     

Self-administration of morphine contingent on heart rate in the rat

The tendency to associate a given response-reinforcement combination reflects the adaptive significance of the association. Biologically relevant reinforcement can be much more effective in modifying certain responses. For example, treatments that result in various types of illness readily condition aversions to novel flavors, but electric shock is relatively ineffective. While opioid self-administration contingent on lever pressing has been extensively studied, the potential for opioids to reinforce visceral responses remains to be determined. An approach to reinforcing changes in heart rate with drug infusions is described. Methods to control for unconditioned drug effects include reversing the direction of change in heart rate required for infusions and addition of a yoked control subject. In several instances, rats exposed to .1 mg/kg infusions of morphine sulfate contingent on tachycardia showed trends for elevated heart rate, with increased locomotor and grooming activity preceding infusions. Increases in heart rate were most pronounced during daytime, normally inactive periods.     

GRK5 deficiency accelerates {beta}-amyloid accumulation in Tg2576 mice via impaired cholinergic activity

Membrane G protein-coupled receptor kinase 5 (GRK5) deficiency is linked to Alzheimer disease, yet its precise roles in the disease pathogenesis remain to be delineated. We have previously demonstrated that GRK5 deficiency selectively impairs desensitization of presynaptic M2 autoreceptors, which causes presynaptic M2 hyperactivity and inhibits acetylcholine release. Here we report that inactivation of one copy of Grk5 gene in transgenic mice overexpressing β-amyloid precursor protein (APP) carrying Swedish mutations (Tg2576 or APPsw) resulted in significantly increased β-amyloid (Aβ) accumulation, including increased Aβ(+) plaque burdens and soluble Aβ in brain lysates and interstitial fluid (ISF). In addition, secreted β-APP fragment (sAPPβ) also increased, whereas full-length APP level did not change, suggesting an alteration in favor of β-amyloidogenic APP processing in these animals. Reversely, perfusion of methoctramine, a selective M2 antagonist, fully corrected the difference between the control and GRK5-deficient APPsw mice for ISF Aβ. In contrast, a cholinesterase inhibitor, eserine, although significantly decreasing the ISF Aβ in both control and GRK5-deficient APPsw mice, failed to correct the difference between them. However, combining eserine with methoctramine additively reduced the ISF Aβ further in both animals. Altogether, these findings indicate that GRK5 deficiency accelerates β-amyloidogenic APP processing and Aβ accumulation in APPsw mice via impaired cholinergic activity and that presynaptic M2 hyperactivity is the specific target for eliminating the pathologic impact of GRK5 deficiency. Moreover, a combination of an M2 antagonist and a cholinesterase inhibitor may reach the maximal disease-modifying effect for both amyloid pathology and cholinergic dysfunction.