Repeated Cocaine Administration Alters Extracellular Glutamate in the Ventral Tegmental Area

Abstract: The present study determined if repeated cocaine injections alter the effect of cocaine on extracellular glutamate in the ventral tegmental area (VTA). All rats were treated with daily cocaine (15 mg/kg i.p. × 2 days, 30 mg/kg i.p. × 5 days) or saline for 7 days. At 21 days after discontinuing the daily injections, a dialysis probe was placed into the VTA and the extracellular levels of glutamate were estimated. A systemic injection of cocaine (15 mg/kg i.p.) elevated extracellular glutamate in the VTA of rats pretreated with daily cocaine but not in the daily saline-pretreated subjects. No significant change in glutamate was produced by a saline injection in either pretreatment group. In a group of rats pretreated with daily cocaine, the D₁ antagonist SCH-23390 (30 µ M ) was infused through the dialysis probe prior to the acute injections of saline and cocaine. SCH-23390 prevented the increase in extracellular glutamate associated with the acute administration of cocaine. Behavioral data were collected simultaneously with the measures of extracellular glutamate. The behavioral stimulant effect of cocaine was greater in cocaine-pretreated than saline-pretreated subjects, and the behavioral augmentation in cocaine-pretreated rats was partly blocked by SCH-23390. These data support the hypotheses that repeated cocaine administration produces an increase in the capacity of D₁ receptor stimulation to release glutamate in the VTA and that this mechanism partly mediates behavioral sensitization produced in rats treated with daily cocaine injections.     

Effect of chronic cocaine administration and cocaine withdrawal on coronary flow rate and heart rate responses to epinephrine and cocaine in isolated perfused rat hearts

The acute dose-dependent effects of epinephrine and cocaine on heart rate and coronary flow rate (CFR) were examined in isolated, perfused (Langendorff) rat hearts from animals: i) pretreated with daily cocaine injections (20 mg/kg/day) for 8 weeks; ii) after 2-day withdrawal from 8-week cocaine pretreatment; iii) vehicle-treated controls. Chronic cocaine (CC) hearts were significantly less sensitive to the chronotropic effects of epinephrine than control (C) or withdrawal (CW) hearts. CW hearts exhibited significantly higher heart rates in response to epinephrine than C and CC hearts. Epinephrine alone (2.5 x 10(-7) M) decreased CFR 11% (C), 9%(CC), 14%(CW) from respective baseline levels. Cocaine alone had no significant effect on CFR in C hearts but produced slight dose-dependent decrements in CFR in CC and particularly CW hearts at higher doses. Cocaine plus epinephrine markedly decreased CFR in all groups, particularly in CW hearts. The results indicate that chronic daily cocaine administration produces a functional tolerance of the heart to the chronotropic actions of epinephrine but a 2-day withdrawal from chronic cocaine results in a rebound supersensitivity to adrenergic stimulation and cocaine's sympathomimetic effects. In addition, cocaine produces only minor decrements in coronary flow in the rat heart, while cocaine acts synergisticallly with epinephrine to produce a marked decrease in CFR.     

Liver damage from cocaine in mice

Four daily injections of 20 mg per kg cocaine hydrochloride into B6AF₁/J mice produced focal necrosis of liver parenchymal cells in the midzonal region. Massive liver damage and marked elevation of serum glutamic-oxaloacetic transaminase was observed after a single injection of 50 mg per kg cocaine-HC1. This damage led to increased sleep time after pentobarbital, and a decreased rate of pentobarbital metabolism $\text{in vivo}$.     

Repeated cocaine administration changes the function and subcellular distribution of adenosine A1 receptor in the rat nucleus accumbens

Adenosine A1 receptor (A1) protein and mRNA is increased in the nucleus accumbens following repeated cocaine treatment. In spite of this protein up-regulation, A1 agonist-stimulated [35S]GTPgammaS binding was attenuated in accumbens homogenates of rats withdrawn for 3 weeks from 1 week of daily cocaine injections. Cellular subfractionation revealed that the discrepancy between total A1 protein and G protein coupling resulted from a smaller proportion of receptors in the plasma membrane. The decrease in functional receptor in the plasma membrane was further indicated by diminished formation of heteromeric receptor complex consisting of A1 and dopamine D1A receptors. To explore the functional significance of the altered distribution of A1 receptors, at 3 weeks after discontinuing repeated cocaine or saline, animals were injected with cocaine and 45 min later the subcellular distribution of A1 receptors quantified. Whereas a cocaine challenge in repeated saline-treated animals induced a marked increase in membrane localization of the A1 receptor, the relative distribution of receptors in repeated cocaine rats was not affected by acute cocaine. These data suggest that the sorting and recycling of A1 receptors is dysregulated in the nucleus accumbens as the consequence of repeated cocaine administration.     

Circadian discrimination of reward: evidence for simultaneous yet separable food- and drug-entrained rhythms in the rat

A unique extra-suprachiasmatic nucleus (SCN) oscillator, operating independently of the light-entrainable oscillator, has been hypothesized to generate feeding and drug-related rhythms. To test the validity of this hypothesis, sham-lesioned (Sham) and SCN-lesioned (SCNx) rats were housed in constant dim-red illumination (LL(red)) and received a daily cocaine injection every 24 h for 7 d (Experiment 1). In a second experiment, rats underwent 3-h daily restricted feeding (RF) followed 12 d later by the addition of daily cocaine injections given every 25 h in combination with RF until the two schedules were in antiphase. In both experiments, body temperature and total activity were monitored continuously. Results from Experiment 1 revealed that cocaine, but not saline, injections produced anticipatory increases in temperature and activity in SCNx and Sham rats. Following withdrawal from cocaine, free-running temperature rhythms persisted for 2-10 d in SCNx rats. In Experiment 2, robust anticipatory increases in temperature and activity were associated with RF and cocaine injections; however, the feeding periodicity (23.9 h) predominated over the cocaine periodicity. During drug withdrawal, the authors observed two free-running rhythms of temperature and activity that persisted for >14 d in both Sham and SCNx rats. The periods of the free-running rhythms were similar to the feeding entrainment (period = 23.7 and 24.0 h, respectively) and drug entrainment (period = 25.7 and 26.1 h, respectively). Also during withdrawal, the normally close correlation between activity and temperature was greatly disrupted in Sham and SCNx rats. Taken together, these results do not support the existence of a single oscillator mediating the rewarding properties of both food and cocaine. Rather, they suggest that these two highly rewarding behaviors can be temporally isolated, especially during drug withdrawal. Under stable dual-entrainment conditions, food reward appears to exhibit a slightly greater circadian influence than drug reward. The ability to generate free-running temperature rhythms of different frequencies following combined food and drug exposures could reflect a state of internal desynchrony that may contribute to the addiction process and drug relapse.     

Circadian Discrimination of Reward: Evidence for Simultaneous Yet Separable Food- and Drug-Entrained Rhythms in the Rat

A unique extra-suprachiasmatic nucleus (SCN) oscillator, operating independently of the light-entrainable oscillator, has been hypothesized to generate feeding and drug-related rhythms. To test the validity of this hypothesis, sham-lesioned (Sham) and SCN-lesioned (SCNx) rats were housed in constant dim-red illumination (LL_red) and received a daily cocaine injection every 24?h for 7 d (Experiment 1). In a second experiment, rats underwent 3-h daily restricted feeding (RF) followed 12 d later by the addition of daily cocaine injections given every 25?h in combination with RF until the two schedules were in antiphase. In both experiments, body temperature and total activity were monitored continuously. Results from Experiment 1 revealed that cocaine, but not saline, injections produced anticipatory increases in temperature and activity in SCNx and Sham rats. Following withdrawal from cocaine, free-running temperature rhythms persisted for 2–10 d in SCNx rats. In Experiment 2, robust anticipatory increases in temperature and activity were associated with RF and cocaine injections; however, the feeding periodicity (23.9?h) predominated over the cocaine periodicity. During drug withdrawal, the authors observed two free-running rhythms of temperature and activity that persisted for >14 d in both Sham and SCNx rats. The periods of the free-running rhythms were similar to the feeding entrainment (period?=?23.7 and 24.0?h, respectively) and drug entrainment (period?=?25.7 and 26.1?h, respectively). Also during withdrawal, the normally close correlation between activity and temperature was greatly disrupted in Sham and SCNx rats. Taken together, these results do not support the existence of a single oscillator mediating the rewarding properties of both food and cocaine. Rather, they suggest that these two highly rewarding behaviors can be temporally isolated, especially during drug withdrawal. Under stable dual-entrainment conditions, food reward appears to exhibit a slightly greater circadian influence than drug reward. The ability to generate free-running temperature rhythms of different frequencies following combined food and drug exposures could reflect a state of internal desynchrony that may contribute to the addiction process and drug relapse. (Author correspondence: heiko@vetmed.wsu.edu)     

Cocaine regulates TRH-related peptides in rat brain

Cocaine administration has previously been reported to alter the levels of prepro-TRH mRNA and TRH (pGlu-His-Pro-NH₂) in the limbic system of rats (J. Neurochem. 60 (1993) 1151). We have now demonstrated that a previously unrecognized family of TRH-like peptides is involved in the actions of cocaine. We treated young adult male Sprague-Dawley rats (five per group, 250 g body weight at sacrifice) for 2 weeks with either twice daily injections of saline (control group), twice daily injections of 15 mg/kg cocaine until sacrifice (chronic group), single injection of 15 mg/kg cocaine 2 h prior to sacrifice (acute group) or chronic cocaine injections replaced by saline injections 72 h prior to sacrifice (withdrawal group (WD)). Twelve different brain regions were dissected and immunoreactivity for TRH (TRH-IR), EEP (pGlu-Glu-Pro-NH₂; EEP-IR) and related peptides were measured by radioimmunoassay (RIA). High pressure liquid chromatography (HPLC) revealed that in many brain regions EEP-IR and TRH-IR consisted of a mixture of TRH, and other TRH-like peptides including EEP, pGlu-Val-Pro-NH₂ (Val²-TRH), pGlu-Tyr-Pro-NH₂ (Tyr²-TRH), pGlu-Leu-Pro-NH₂ (Leu²-TRH), and pGlu-Phe-Pro-NH₂ (Phe²-TRH). Following i.p. injection, these TRH-like peptides readily crossed the blood–brain barrier but cleared very slowly from brain tissues.

Acute cocaine produced a 4.1-fold increase in Val²-TRH level in medulla while Val²-TRH and Tyr²-TRH, increased 6.2- and 2.9-fold, respectively in pyriform cortex PYR. TRH and Leu²-TRH, decreased 47 and 93%, respectively in the nucleus accumbens (AM) while other EEP-IR peaks decreased 50–100% consistent with the significant decrease in total EEP-IR in the AMs following acute cocaine treatment. Because 2 h is too short a time to alter levels of neuropeptides via changes in the rate of biosynthesis, the acute cocaine-induced elevation or reduction in TRH and related peptides is most likely due to suppression or stimulation, respectively, of the corresponding peptide secretion rate. Because TRH and TRH-like peptides have antidepressant, analeptic and euphorigenic properties, we conclude that these endogenous substances are potential mediators of both the cocaine “high” and withdrawal symptoms.     

Extracellular Concentrations of Cocaine and Dopamine Are Enhanced During Chronic Cocaine Administration

Chronic cocaine administration produces significant increases in cocaine-induced locomotor activity and stereotypy. In vivo microdialysis procedures were used to monitor extracellular dopamine (DA) and cocaine concentrations in the nucleus accumbens (N ACC) and cocaine concentrations in plasma of animals that received chronic or acute cocaine treatments. Following a cocaine challenge injection, concentrations of both cocaine and DA increased to significantly higher levels over time in animals that had received daily cocaine injections for 10 or 30 days than in control animals that received daily injections of saline. Concentrations of cocaine and DA in the N ACC reached maximum levels in the first 30 min following a challenge injection of cocaine. The maximum cocaine concentrations of 10- and 30-day chronic animals were, respectively, 186% and 156%, whereas the maximum DA concentrations were 264% and 216% above the maximum values observed in acute control animals. The results indicate that reverse tolerance effects observed following chronic cocaine administration may in part be accounted for by increased cocaine concentrations. Furthermore, chronic cocaine administration (over a 10- or 30-day period) increased the concentration of cocaine detected in plasma above control levels following a challenge injection. The increase in brain concentrations of cocaine in chronic animals is apparently due to increased concentrations of cocaine in plasma. A physiological change occurs in the periphery as a result of chronic cocaine administration that increases cocaine concentrations in plasma, increases extracellular cocaine levels in the brain, and increases the extracellular concentration of DA in the N ACC.     

Drug-environment interaction: Context dependency of cocaine-induced behavioral sensitization

The effect of the environmental context in which rats received cocaine upon subsequent cocaine-induced hyperactivity and stereotypy was explored. “Cocaine test cage” animals were injected with cocaine in the test cage and then received saline upon leaving it 40 minutes later, while in the “saline test cage” group the injections, were in the reverse order. Thus, all animals had identical injections, handling, and environmental exposure, differing only in whether they received cocaine during or after their test cage experience. The cocaine test cage animals displayed increasing response (p < .001) to the 10 daily cocaine injections, i.e., behavioral sensitization. However, in contrast, the “saline-test cage” animals had significantly less cocaine-induced activation upon 2 of 3 challenges with cocaine and showed no significant evidence of behavioral sensitization even though they had received the same dose and number of previous cocaine injections in a different environment. Thus, this study reveals environment- specific drug effects and suggests that environmental context plays a role in the development and manifestation of behavioral sensitization to cocaine.     

Neuroendocrine responses to cocaine do not exhibit sensitization following repeated cocaine exposure.

Cocaine-induced enhancement of motor activity and extracellular dopamine concentrations exhibits sensitization upon repeated exposure. In this study, the neuroendocrine responses to cocaine were examined following cocaine pretreatment regimens which have been shown to produce behavioral sensitization. Adult male rats were injected with cocaine (15 mg/kg, IP) once daily for 14 days, followed by a dose-response challenge with cocaine (1-15 mg/kg, IP) either 18 hours or 7 days after the final pretreatment injection. Blood was collected 15 minutes following injections for radioimmunoassay of ACTH, corticosterone, prolactin, and renin. Cocaine increased plasma ACTH and corticosterone, while it decreased prolactin and renin concentrations. Pretreatment with cocaine for 2 weeks did not alter any of these endocrine responses after either the 18 hour or 7 day interval between pretreatment and challenge injections. In contrast, sensitization to the locomotor stimulant effects of cocaine was observed on the final day of pretreatment injections, and 7 days later. These data suggest that these endocrine effects of cocaine do not exhibit sensitization following repeated cocaine exposure.     

Repeated Cocaine Alters Glutamate Receptor Subunit Levels in the Nucleus Accumbens and Ventral Tegmental Area of Rats that Develop Behavioral Sensitization

Increased glutamate transmission in the nucleus accumbens and ventral tegmental area has been proposed as a mechanism underlying sensitized behavioral responses to repeated cocaine administration. GluR1, GluR2/3, and NMDAR1 subunits of glutamate receptors were quantified from immunoblots in these brain nuclei in rats at 24 h and 3 weeks after discontinuing 1 week of daily cocaine injections. Motor behavior was monitored after the first and last injections of daily cocaine, and those rats that showed >20% increase in motor activity after the last compared with the first injection were considered to have developed behavioral sensitization. The subjects that developed behavioral sensitization showed a significant increase in GluR1 levels in the nucleus accumbens at 3 weeks but not at 24 h of withdrawal. Conversely, sensitized animals showed a significant increase in NMDAR1 and GluR1 levels in the ventral tegmental area at 1 day but not at 3 weeks of withdrawal. None of these increases occurred in the rats exposed to daily cocaine that did not develop behavioral sensitization (<20% increase in motor activity), and no changes were measured in the level of GluR2/3 in any treatment group. The functional importance of the increases in glutamate receptor subunit levels is suggested by the fact that the changes were present only in rats that developed behavioral sensitization to repeated cocaine administration.     

Minocycline suppresses oxidative stress and attenuates fetal cardiac myocyte apoptosis triggered by in utero cocaine exposure

This study investigates the molecular mechanisms by which minocycline, a second generation tetracycline, prevents cardiac myocyte death induced by in utero cocaine exposure. Timed mated pregnant Sprague-Dawley (SD) rats received one of the following treatments twice daily from embryonic (E) day 15–21 (E15–E21): (i) intraperitoneal (IP) injections of saline (control); (ii) IP injections of cocaine (15 mg/kg BW); and (iii) IP injections of cocaine + oral administration of 25 mg/kg BW of minocycline. Pups were killed on postnatal day 15 (P15). Additional pregnant dams received twice daily IP injections of cocaine (from E15–E21) + oral administration of a relatively higher (37.5 mg/kg BW) dose of minocycline. Minocycline treatment continued from E15 until the pups were sacrificed on P15. In utero cocaine exposure resulted in an increase in oxidative stress and fetal cardiac myocyte apoptosis through activation of c-Jun-NH₂-terminal kinase (JNK) and p38 mitogen-activated protein kinase (MAPK)-mediated mitochondria-dependent apoptotic pathway. Continued minocycline treatment from E15 through P15 significantly prevented oxidative stress, kinase activation, perturbation of BAX/BCL-2 ratio, cytochrome c release, caspase activation, and attenuated fetal cardiac myocyte apoptosis after prenatal cocaine exposure. These results demonstrate in vivo cardioprotective effects of minocycline in preventing fetal cardiac myocyte death after prenatal cocaine exposure. Given its proven clinical safety and ability to cross the placental barrier and enter into the fetal circulation, minocycline may be an effective therapy for preventing cardiac consequences of in utero cocaine exposure.     

Synergism between cocaine and atropine at the caudal ventrolateral medulla of cats

We have previously reported that the anticholinergic properties of cocaine may be important in cocaine induced apneusis. We have studied the effects of the cholinergic muscarinic antagonist atropine (ATR) on cocaine induced apneusis at the caudal chemosensitive areas of the ventrolateral medulla oblongata (CVLM). Experiments were performed in urethane anesthetized and tracheotomized cats with the CVLM surgically exposed. Topical application of ATR (44 mM ) to the CVLM produced significant decrements in minute ventilation (V(E)) and mean arterial blood pressure (MABP) (P<0.05) but the effects on tidal volume (V(T)), respiratory frequency (f) and heart rate (HR) were not significant. Administration of cocaine (37 mM) to ATR pretreated animals increased the incidence of cocaine induced respiratory arrest to more than twofold greater than when cocaine was administered in the absence of pretreatment. The ATR pretreated animals that did not experience inspiratory arrest after cocaine were shown to exhibit significant decrements in f and V(E) as a consequence of prolonged inspiratory pauses. The reduction in MABP after cocaine in ATR pretreated animals was also significant. These results suggest that ATR enhances the central respiratory toxicity of cocaine by acting synergistically at CVLM chemosensitive sites.     

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

Repeated exposure to cocaine progressively increases drug-induced locomotor activity, which is termed behavioral sensitization. Previous studies have demonstrated that sensitization to cocaine is associated with a decrease in dopamine D? receptor function in the medial prefrontal cortex. The present report tested the hypothesis that reduced medial prefrontal cortex D? receptor function as a result of repeated cocaine exposure results in augmented excitatory transmission to the nucleus accumbens and ventral tegmental area, possibly as a partial result of enhanced inhibition of local dopamine release. Dual probe microdialysis experiments were conducted in male Sprague-Dawley rats 1, 7 or 30 days following the last of four daily injections of saline (1.0 mL/kg) or cocaine (15 mg/kg). Infusion of quinpirole (0.01, 1.0 and 100 μM), a D?-like receptor agonist, into the medial prefrontal cortex produced a dose-dependent decrease in cortical, nucleus accumbens and ventral tegmental area extracellular glutamate levels in control but not sensitized animals. Quinpirole also reduced basal dopamine levels in the medial prefrontal cortex in sensitized animals following 1 day of withdrawal from cocaine. Following 30 days of withdrawal, quinpirole also reduced dopamine levels in sensitized animals relative to saline controls, but not relative to baseline levels. These findings indicate that the expression of sensitization to cocaine is associated with altered modulation of mesocorticolimbic glutamatergic transmission at the level of the medial prefrontal cortex.     

Second-order schedules of drug injection.

Key-press responding of squirrel monkeys produced intravenous injections of cocaine under two simple types of schedule. Under a fixed ratio schedule, every 30th response produced an injection; steady responding at high rates of over one per second were maintained during each fixed-ratio component. Under a fixed-interval schedule, the first response occurring after a fixed time of 5 min produced an injection; there was a pause at the start of each interval and then progressively increasing responding until cocaine was injected at the end of the interval. Both squirrel monkeys and rhesus monkeys also were studied under second-order schedules of drug injection. Under one type of second-order schedule, studies only in squirrel monkeys, completion of each fixed-interval component produced only a 2 sec light; completion of the 10th fixed-interval component produced the brief light and an intravenous injection of cocaine. Under a second type of second-order schedule, each fixed-ratio component completed during a fixed time interval (5 or 60 min) produced only a 2-sec light; the first fixed-ratio component completed after the interval of time elapsed produced the brief light and an intravenous (squirrel monkeys) or intramuscular (rhesus monkeys) injection of cocaine. Under both types of second-order schedules, repeated sequences of responding were maintained during each session and characteristic fixed-interval or fixed-ratio patterns of responding were controlled by the brief visual stimuli.     

Cocaine fatalities increased by restraint stress

Laboratory rats injected daily with a moderate dose of cocaine hydrochloride (30 mg/kg, i.p.) showed increased fatalities when cocaine injections were followed by 30 min of restraint stress. The 5-day mortality rate was 58% for the cocaine-plus-stress group, while 17% of the animals receiving cocaine without restraint stress died. This finding suggests that stress can augment the toxic effect of cocaine and that minimizing stress may be an important consideration in the clinical management of cocaine overdose.     

Proteoglycans contain a 4.6 A repeat in muscular dystrophy corneas: x-ray diffraction evidence.

Synchrotron x-ray diffraction patterns from macular corneal dystrophy (MCD) corneas contain an unusual reflection that arises because of an undefined ultrastructure with a periodic repeat in the region of 4.6 A. In this study, we compared with wide-angle x-ray diffraction patterns obtained from four normal human corneas and four MCD corneas. Moreover, portions of two of the MCD corneas were pretreated with a specific glycosidase to shed light on the origin of the 4.6 A reflection. None of the normal corneas produced an x-ray reflection in the region of 4.6 A, whereas all four of the MCD corneas did (MCD type I at 4.65 A and 4.63 A, MCD type II at 4.63 A and 4.67 A). This reflection was diminished after incubation of the MCD tissues with either chondroitinase ABC or N-glycanase. The findings indicate that glycosaminoglycans or proteoglycans contribute to the unusual MCD x-ray reflection and hence most likely contain a periodic 4.6 A ultrastructure. Furthermore, the results imply that periodic 4.6 A MCD ultrastructures reside in either intact, unsulfated lumican molecules and regions of the CS/DS-containing molecules or in a region of a hybrid macromolecular aggregate formed by the interaction of the two molecules.     

Serotonin and Dopamine Sensitization in the Nucleus Accumbens, Ventral Tegmental Area, and Dorsal Raphe Nucleus Following Repeated Cocaine Administration

Abstract— The present study was designed to examine the effects of chronic cocaine administration on the extracellular response of serotonin (5-HT) and dopamine (DA) to a peripheral cocaine injection using in vivo brain microdialysis in awake rats. Two different dual probe preparations were used: One group of animals had guide cannulae aimed at the ventral tegmental area (VTA) and nucleus accumbens (N ACC) and a second group of animals had guide cannulae aimed at the dorsal raphe nucleus (DRN) and N ACC. Rats from both groups were given daily injections of either cocaine (20 mg/kg i.p.) or saline (0.9%; 0.05 ml/kg i.p.) for 10 consecutive days. On day 11, baseline dialysate levels of DA, 5-HT, dihydroxyphenylacetic acid, and 5-hydroxyindoleacetic acid were obtained from either the N ACC and VTA or the N ACC and DRN, followed by a 10 mg/kg i.p. cocaine injection and an additional 150 min of dialysate sampling. The percent baseline increases of both 5-HT and DA were significantly higher in the N ACC, VTA, and DRN of animals that received daily injections of cocaine compared with saline controls ( p < 0.05, in each region). Maximum dialysate 5-HT concentrations after cocaine challenge were significantly higher in the N ACC and VTA ( p < 0.05) and DRN ( p < 0.01) of chronically treated animals compared with saline controls. Maximum dialysate DA concentrations were significantly higher in the N ACC and DRN ( p < 0.05) of chronically treated animals compared with saline controls. There was no significant difference between acute and chronic animals in the maximum dialysate DA concentration from the VTA after cocaine challenge. 5-HT was significantly more sensitized in the 5-HT cell body region (DRN) than the N ACC terminal field ( p < 0.05), whereas DA was significantly more sensitized in the N ACC terminal field than the DA cell bodies of the VTA ( p < 0.05).     

Linkage of a gene for macular corneal dystrophy to chromosome 16.

Autosomal recessive macular corneal dystrophy (MCD) is a heterogeneous disorder leading to visual impairment. Sixteen American and Icelandic families (11 type I and 5 type II) were analyzed for linkage, by use of 208 polymorphic microsatellite markers. A significant maximum LOD score Zmax of 7.82 at a maximum recombination fraction (thetamax) of .06 was found with the 16q22 locus D16S518 for MCD type I. In addition, a peak LOD score of 2.50 at a recombination fraction of .00 was obtained for the MCD type II families, by use of the identical marker. These findings raise the possibility that MCD type II may be due to the same genetic locus that is involved in MCD type I.     

Increased Acute Cocaine Sensitivity and Decreased Cocaine Sensitization in GABAA Receptor β3 Subunit knockout Mice

The role of the GABA(A) receptor beta3 subunit in determining acute cocaine sensitivity and behavioral sensitization to repeated cocaine was measured in mice missing both (-/-), one (+/-), or neither (+/+) allele of the beta3 gene. Locomotor stimulation induced by one cocaine injection (20 mg/kg, i.p.) was found to be greater in -/- mice compared with +/+ mice, whereas cocaine-induced behaviors were intermediate in +/- mice. Amphetamine did not cause greater locomotor responses in -/- mice, suggesting that the increased sensitivity of -/- mice to cocaine does not generalize to other psychomotor stimulants. GABA-stimulated chloride uptake was 51% lower in striatum of -/- mice compared with +/+ mice, but only 27% lower in cortex. After 14 daily cocaine injections, the behavioral response to cocaine was increased in +/+ and +/- mice, but was not increased further in -/- mice. Additionally, repeated cocaine exposure decreased striatal GABA(A) receptor function in +/+ and +/- mice. In -/- mice, GABA(A) receptor function was not decreased any further by repeated cocaine injections. Thus, alterations in the beta3 subunit may be responsible for determining the behavioral responses induced by acute and repeated cocaine treatment, as well as mediating the neurochemical adaptation that occurs during sensitization to repeated cocaine.