Transient increases in catecholaminergic activity in medial prefrontal cortex and nucleus accumbens shell during novelty

Voltammetric recordings with electrochemically modified carbon-fiber electrodes were obtained from specific regions of the forebrain in rats given free-choice access to a novel environment. Entry into novelty increased the catechol signal in the medial prefrontal cortex and shell of the nucleus accumbens by more than 100%, but had no consistent effect in either the neostriatum or accumbal core. In both the medial prefrontal cortex and accumbal shell, moreover, the novelty-induced increase in catecholaminergic activity was detectable only during the initial entry into the novel compartment and did not reappear when animals returned to the familiar environment. These results support increasing evidence for a functional distinction between the accumbal core and shell, with the latter having been linked to brain reward mechanisms. The results also indicate that novelty activates, albeit very transiently, some of the same neurochemical systems believed to play a critical role in the reinforcing effects of certain drugs of abuse.

Repeated intra-ventral tegmental area administration of SKF-38393 induces behavioral and neurochemical sensitization to a subsequent cocaine challenge

To assess the role of dopamine receptor subtypes in the initiation of behavioral sensitization to cocaine, the partial D1 agonist SKF-38393 (0.01-1.0 microgram/side), the D2/3 agonist quinpirole (0.1-1.0 microgram/side) or saline was infused once daily over 3 days into either the ventral tegmental area (VTA) or the nucleus accumbens shell. Relative to saline controls, intra-VTA SKF-38393 administration did not influence behavior during the treatment regimen, whereas the higher dose of quinpirole produced a significant suppression of locomotor activity after the third daily administration. Infusion of 1.0 microgram/side SKF-38393 into the nucleus accumbens shell produced marked behavioral hyperactivity after both the first and last daily infusions, with no significant difference between the behavioral responses on these days. In contrast, quinpirole did not influence behavior after its infusion into the nucleus accumbens shell. Two weeks after the last daily intracranial drug infusion, all animals were administered cocaine (15 mg/kg, i.p.) and behavioral activity was monitored. The repeated administration of SKF-38393 into the VTA resulted in sensitization to the locomotor-activating effects of cocaine. No other drug pretreatment influenced the cocaine-induced behavioral response. To determine whether repeated D1 agonist administration influenced dopamine release in the nucleus accumbens shell, in vivo microdialysis was performed in this structure after three daily intra-VTA infusions of saline or 0.1 microgram/side SKF-38393. The increase in dopamine levels in the nucleus accumbens shell induced by cocaine was significantly greater among rats pretreated with repeated SKF-38393 administration. Taken together, these results indicate that the repeated intra-VTA administration of a D1 agonist mimics the sensitization process produced by repeated cocaine administration.

Repeated cocaine augments excitatory amino acid transmission in the nucleus accumbens only in rats having developed behavioral sensitization

Rats were pretreated with daily cocaine or saline injections for 1 week. The rats treated with daily cocaine were separated into two groups: a sensitized group of animals demonstrating > 20% increase in motor activity on the last injection compared with the first injection of daily cocaine, and a nonsensitized group showing < 20% elevation. At 2-3 weeks after the last daily injection, four experiments were performed to assess changes in excitatory amino acid (EAA) transmission in the nucleus accumbens produced by repeated cocaine administration. (1) Rats were challenged with a microinjection of AMPA into the shell or core of the nucleus accumbens. The sensitized rats demonstrated greater motor activity than did the saline-pretreated or nonsensitized animals after AMPA injection into either subnucleus. (2) It was shown that the behavioral distinction between sensitized, nonsensitized, and control rats in behavioral responsiveness to AMPA was not mediated by differences in AMPA-induced dopamine release. (3) The extracellular content of glutamate was measured after a cocaine challenge given at 21 d of withdrawal. Cocaine elevated the levels of glutamate in the core of sensitized rats, but not of nonsensitized or control rats. (4) Microinjection of the non-NMDA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione into the core abolished the augmented motor response to a cocaine challenge in sensitized rats, but was without effect on cocaine-induced motor activity in nonsensitized animals. These results indicate that repeated cocaine administration increases EAA transmission in the nucleus accumbens only in rats that develop behavioral sensitization to cocaine.

Amphetamine produces sensitized increases in locomotion and extracellular dopamine preferentially in the nucleus accumbens shell of rats administered repeated cocaine

Alterations in dopamine transmission in the nucleus accumbens, which is composed of two anatomically distinct compartments termed the shell and core, contribute to the expression of cocaine-induced behavioral sensitization. To test potential presynaptic components of behavioral sensitization, the behavioral and neurochemical response to amphetamine administration in the accumbens shell and core was measured at early (days 1-3) and late (days 20-22) withdrawal in rats pretreated with systemic cocaine (15 mg/kg x 2 days, 30 mg/kg x 5 days) or saline. Behavioral sensitization was observed at late, but not early withdrawal when amphetamine was microinjected into the nucleus accumbens shell of cocaine-pretreated rats. There were no significant differences between cocaine- and saline-pretreated animals when behavior was monitored after amphetamine injections into the core at either withdrawal period. After both withdrawal periods, the amphetamine-induced increase in extracellular dopamine was potentiated among cocaine-pretreated animals in the shell by the local administration of amphetamine (0.03, 0.3, 3.0 and 30 microM through the dialysis probe). In the core at early withdrawal there was tolerance to the amphetamine-induced increase in extracellular dopamine in the cocaine group, whereas there was no difference between the repeated saline and cocaine groups at late withdrawal. In a second experiment designed to evaluate potential postsynaptic influences, the D1 partial agonist, SFK-38393 (0.01 or 0.1 microgram/side), was microinjected into the nucleus accumbens core or shell regions after behavioral sensitization to cocaine. Although there was a motor-stimulant effect of SKF-38393 at both withdrawal periods, there was no difference between rats pretreated with repeated cocaine or saline. Collectively, these results demonstrate that the augmentation in dopamine transmission in the nucleus accumbens that is associated with behavioral sensitization is more robust in the shell than the core.

Ascorbate potentiates amphetamine-induced conditioned place preference and forebrain dopamine release in rats

In order to evaluate the effects of ascorbate, which is known to modulate dopamine neurotransmission, on the reinforcing effects of amphetamine, we coadministered ascorbate and amphetamine during the acquisition of conditioned place preference (CPP) in rats. Our results indicate that 100 mg/kg ascorbate potentiates the CPP induced by 0.5 mg/kg, but not 1.0 mg/kg, amphetamine. A higher dose of ascorbate (500 mg/kg) did not influence the CPP induced by either dose of amphetamine. In vitro release assays revealed that, whereas ascorbate alone (0.01-1.0 mM) did not influence striatal dopamine levels, this vitamin potentiated amphetamine-induced dopamine release in both the nucleus accumbens and neostriatum. Collectively, these results raise the possibility that ascorbate potentiates amphetamine-induced CPP by increasing the ability of this psychostimulant to release dopamine.

Iontophoresis in the neostriatum of awake, unrestrained rats: differential effects of dopamine, glutamate and ascorbate on motor- and nonmotor-related neurons

The neostriatum and its major afferent transmitters, dopamine and glutamate, play a critical role in behavior, but relatively little information is available on their postsynaptic effects in behaving animals. As a first step in addressing this shortcoming, single-unit electrophysiology was combined with iontophoresis in the neostriatum of awake, unrestrained rats. Relative to periods of quiet rest, most neurons (58 of 77) changed discharge rate in close temporal association with movement, while the remainder showed no such relationship. When animals resumed a resting posture, iontophoretic current-response curves were established for dopamine and glutamate as well as for ascorbate, a modulator of neostriatal function released from glutamatergic terminals. Application of either glutamate or ascorbate produced current-dependent increases in activity in all neurons, although this effect was somewhat less pronounced for nonmotor cells. In both types of neurons, the excitatory effect of ascorbate either diminished or shifted to an inhibition at high ejection currents. Dopamine, on the other hand, routinely excited motor-related, but inhibited nonmotor-related neurons. Further assessment of motor-related neurons revealed that in most cases the excitatory effects of either glutamate or dopamine alone were supra-additive when these compounds were either administered together or co-administered with ascorbate. Our results suggest that the response of neostriatal neurons to glutamate or dopamine depends, at least in part, on the motor responsiveness of these cells. Motor-related neurons, moreover, respond to the co-administration of glutamate and dopamine with synergistic increases in firing rate. Ascorbate also influences neostriatal activity, but the postsynaptic action of this substance cannot be explained as a simple interaction with either glutamatergic or dopaminergic mechanisms.

Sensitization to cocaine and dopamine autoreceptor subsensitivity in the nucleus accumbens

It has been suggested that dopamine autoreceptor subsensitivity may play a role in cocaine-induced behavioral sensitization. In order to evaluate this hypothesis, we administered cocaine to rats daily (15 mg/kg ip x 2 days, 30 mg/kg ip x 5 days) and then monitored nucleus accumbens dopamine during the local administration (through the dialysis probe) of the D2/D3 agonist, quinpirole (0, 0.1, 1, and 10 microM). Our results indicate that, relative to saline-pretreated control animals, repeated cocaine administration impaired the ability of quinpirole to decrease extracellular dopamine 1-2 days after the last drug injection. However, quinpirole was equipotent at reducing accumbal dopamine in cocaine- and saline-treated animals following a 21-22 day withdrawal period. These results demonstrate that repeated cocaine produces a short duration functional tolerance in the capacity of autoreceptor stimulation to inhibit accumbal dopamine release.

Amphetamine promotes neostriatal ascorbate release via a nigro-thalamo-cortico-neostriatal loop

In the neostriatum, amphetamine and other dopamine agonists elevate the extracellular level of ascorbate, which is known to modulate neostriatal function. Although both D1 and D2 receptors have been linked to neostriatal ascorbate release, ample evidence suggests it is controlled by areas outside the neostriatum. The present series of experiments used selective lesions and intracerebral drug infusions to probe the involvement of the ventromedial thalamus and substantia nigra pars reticulata. Our results implicate both of these sites in amphetamine-induced increases in the release of neostriatal ascorbate. Thus, whereas unilateral electrolytic lesions of the substantia nigra pars reticulata completely abolished the ability of systemic amphetamine (2.5 mg/kg) to increase extracellular ascorbate in ipsilateral neostriatum, intranigral infusions of this drug (10 and 30 micrograms/microliters) elevated neostriatal ascorbate release. This infusion effect, moreover, was blocked by electrolytic lesions of the ipsilateral ventromedial thalamus, which receives input from the substantia nigra pars reticulata and projects to the cerebral cortex. These results, combined with previous evidence implicating cortical projections to neostriatum as the source of extracellular ascorbate, suggest that neostriatal ascorbate release is regulated, at least in part, by a nigro-thalamo-cortico-neostriatal pathway.

Repeated treatment with ascorbate or haloperidol, but not clozapine, elevates extracellular ascorbate in the neostriatum of freely moving rats

Acute administration of neuroleptic drugs alters the extracellular level of ascorbate in the neostriatum, and increasing evidence suggests a role for this vitamin in the behavioral, and possibly therapeutic, effects of these drugs. To shed further light on this issue, extracellular ascorbate was recorded in the neostriatum and nucleus accumbens of awake, behaving rats following chronic treatment with either classical (haloperidol) or atypical (clozapine) neuroleptics or ascorbate itself. Electrochemically modified, carbon-fiber microelectrodes were lowered in place the day after the last of 21 daily injections of either haloperidol (0.5 mg/kg, SC), clozapine (20 mg/kg, IP), sodium ascorbate (500 mg/kg, IP) or vehicle. Voltammetric measurements were obtained during quiet rest and following administration of d-amphetamine (2.5 mg/kg). Repeated treatment with either haloperidol or ascorbate elevated basal extracellular ascorbate and potentiated the amphetamine-induced increase in ascorbate release in neostriatum but not nucleus accumbens. Both treatment groups also showed a significant increase in amphetamine-induced sniffing and repetitive head movements compared to vehicle-treated animals. In contrast, repeated clozapine had no effect on extracellular ascorbate in either neostriatum or nucleus accumbens, but increased the locomotor response to an amphetamine challenge. Thus, to the extent that increases in neostriatal ascorbate exert neuroleptic-like effects, such effects are likely to parallel haloperidol rather than clozapine.

A vitamin as neuromodulator: ascorbate release into the extracellular fluid of the brain regulates dopaminergic and glutamatergic transmission

Ascorbate is an antioxidant vitamin that the brain accumulates from the blood supply and maintains at a relatively high concentration under widely varying conditions. Although neurons are known to use this vitamin in many different chemical and enzymatic reactions, only recently has sufficient evidence emerged to suggest a role for ascorbate in interneuronal communication. Ascorbate is released from glutamatergic neurons as part of the glutamate reuptake process, in which the high-affinity glutamate transporter exchanges ascorbate for glutamate. This heteroexchange process, which also may occur in glial cells, ensures a relatively high level of extracellular ascorbate in many forebrain regions. Ascorbate release is regulated, at least in part, by dopaminergic mechanisms, which appear to involve both the D1 and D2 family of dopamine receptors. Thus, amphetamine, GBR-12909, apomorphine, and the combined administration of D1 and D2 agonists all facilitate ascorbate release from glutamatergic terminals in the neostriatum, and this effect is blocked by dopamine receptor antagonists. Even though the neostriatum itself contains a high concentration of dopamine receptors, the critical site for dopamine-mediated ascorbate release in the neostriatum is the substantia nigra. Intranigral dopamine regulates the activity of nigrothalamic efferents, which in turn regulate thalamocortical fibers and eventually the glutamatergic corticoneostriatal pathway. In addition, neostriatonigral fibers project to nigrothalamic efferents, completing a complex multisynaptic loop that plays a major role in neostriatal ascorbate release. Although extracellular ascorbate appears to modulate the synaptic action of dopamine, the mechanisms underlying this effect are unclear. Evidence from receptor binding studies suggests that ascorbate alters dopamine receptors either as an allosteric inhibitor or as an inducer of iron-dependent lipid peroxidation. The applicability of these studies to dopamine receptor function, however, remains to be established in view of reports that ascorbate can protect against lipid peroxidation in vivo. Nevertheless, ample behavioral evidence supports an antidopaminergic action of ascorbate. Systemic, intraventricular, or intraneostriatal ascorbate administration, for example, attenuates the behavioral effects of amphetamine and potentiates the behavioral response to haloperidol. Some of these behavioral effects, however, may be dose-dependent in that treatment with relatively low doses of ascorbate has been reported to enhance dopamine-mediated behaviors. Ascorbate also appears to modulate glutamatergic transmission in the neostriatum. In fact, by facilitating glutamate release, ascorbate may indirectly oppose the action of dopamine, though the nature of the neostriatal dopaminergic-glutamatergic interaction is far from settled. Ascorbate also may alter the redox state of the NMDA glutamate receptor thus block NMDA-gated channel function.(ABSTRACT TRUNCATED AT 400 WORDS)

Intraneostriatal administration of glutamate antagonists increases behavioral activation and decreases neostriatal ascorbate via nondopaminergic mechanisms

Behavioral findings suggest that the effects of neostriatal glutamate and ascorbate are opposed to those of neostriatal dopamine. Recent evidence also indicates that glutamate and ascorbate are linked via a carrier-mediated heteroexchange process, suggesting that ascorbate may act through the glutamate system to influence behavior. In order to assess glutamate-ascorbate interactions and their influence on the behavioral output of the basal ganglia, glutamate and homocysteic acid (a glutamate reuptake blocker) as well as NMDA antagonists were infused into the neostriatum of freely moving rats while extracellular neostriatal ascorbate was monitored via electrochemically modified carbon-fiber electrodes. Neostriatal 3,4-dihydroxyphenylacetic acid (DOPAC), a major dopamine metabolite, also was recorded in order to assess the dependency of any drug effect on the nigrostriatal dopamine system. Intraneostriatal infusions of L-glutamate (1 micrograms/microliters), but not L-homocysteic acid (30 micrograms/microliters), elevated extracellular neostriatal ascorbate levels. Neither of these drugs had any effect on neostriatal DOPAC or overt behavioral activity. Intraneostriatal infusion of the noncompetitive NMDA antagonist dizocilpine (MK-801; 3 micrograms/microliters) or the competitive NMDA antagonist 3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonene (CPPene; 5 micrograms/microliters) decreased neostriatal ascorbate but had no effect on neostriatal DOPAC. Both dizocilpine and CPPene activated behavior in intact and sham-lesioned animals as well as in animals with near-total depletions of neostriatal dopamine following a 6-hydroxydopamine lesion. When administered systemically, however, dizocilpine (1.0 mg/kg) significantly increased neostriatal DOPAC. This effect appears to be regulated via midbrain NMDA receptors, in that this effect was completely abolished by electrolytic lesions of the substantia nigra pars reticulata.(ABSTRACT TRUNCATED AT 250 WORDS)

A simple micromanipulator for multiple uses in freely moving rats: electrophysiology, voltammetry, and simultaneous intracerebral infusions

An inexpensive, easily fabricated micromanipulator is described that can be used for single-unit recording or voltammetry in freely moving rats. The basic design is configured around the standard coupling system between a plastic syringe and corresponding needle hub. The device can be used with glass or metal microelectrodes for electrophysiology or carbon-fiber or carbon-disk microelectrodes for voltammetry. With either recording technique, the micromanipulator also can accommodate a 33-ga infusion cannula, which allows drugs to be administered directly to the recording site. The entire assembly is lightweight and can be used with a head-mounted amplifier system for relatively noise-free recording.

Unilateral neostriatal kainate, but not 6-OHDA, lesions block dopamine agonist-induced ascorbate release in the neostriatum of freely moving rats

Unilateral kainate lesions of the neostriatum and 6-hydroxydopamine (6-OHDA) lesions of the medial forebrain bundle were used to assess the role of neostriatal and ascending dopaminergic neurons, respectively, on dopamine-agonist induced release of neostriatal ascorbate as measured voltammetrically in freely moving rats. Electrochemically modified, carbon-fiber electrodes recorded the effects of direct (a combination of 10 mg/kg SKF-38393 and 1.0 mg/kg quinpirole) as well as indirect (2.5 mg/kg D-amphetamine or 20.0 mg/kg GBR-12909) dopamine agonists. Relative to controls, kainate, but not 6-OHDA, lesions abolished the ability of both direct and indirect dopamine agonists to induce neostriatal ascorbate release. These results suggest that unlike dopaminergic afferents, neostriatal output pathways play a critical role in the modulation of neostriatal ascorbate levels.

Dopamine-, NMDA- and sigma-receptor antagonists exert differential effects on basal and amphetamine-induced changes in neostriatal ascorbate and DOPAC in awake, behaving rats

Amphetamine and other dopamine agonists elevate the extracellular level of neostriatal ascorbate, which has been shown to modulate neuronal function. To assess the receptor mechanisms underlying neostriatal ascorbate release, drug-induced changes in both basal and amphetamine-induced ascorbate release were monitored voltammetrically in the neostriatum of freely moving rats. A variety of dopamine receptor antagonists decreased basal ascorbate and reversed the increase induced by 2.5 mg/kg D-amphetamine. Thus, compared to vehicle treatment, administration of classical (haloperidol) and atypical (clozapine) neuroleptics or selective D1 (SCH-23390) and D2 (sulpiride) antagonists completely reversed the amphetamine-induced rise in ascorbate and also lowered basal levels by 20-40%. These same effects occurred following injection of dizocilpine (MK-801), a non-competitive NMDA antagonist, whereas BMY-14802, a sigma ligand, reversed the amphetamine-induced rise without altering basal levels. Simultaneous measurements of extracellular DOPAC, a major dopamine metabolite, revealed that haloperidol, clozapine, sulpiride and BMY-14802 elevated basal levels and reversed the amphetamine-induced decline. Dizocilpine also increased basal DOPAC but failed to alter the DOPAC response to amphetamine, whereas both basal and amphetamine-induced changes in DOPAC were unaffected by SCH-23390. A combination of subthreshold doses of SCH-23390 and sulpiride, however, reversed both the amphetamine-induced release of ascorbate and the corresponding decline in DOPAC. Collectively, these results suggest that whereas dopamine, sigma, and NMDA receptors modulate neostriatal ascorbate release, they exert an opposing influence on extracellular DOPAC. All drugs attenuated at least some components of the amphetamine behavioral response, suggesting a role for multiple mechanisms in the behavioral effects of this drug.

Chronic ascorbate potentiates the effects of chronic haloperidol on behavioral supersensitivity but not D2 dopamine receptor binding

Ample behavioral evidence suggests that ascorbate parallels the action of haloperidol, a widely used neuroleptic. To determine the extent to which this parallel extends to chronic treatment, 21 days of exposure to ascorbate (100 or 500 mg/kg) alone or combined with haloperidol (0.5 mg/kg) were assessed on stereotyped behavior and neostriatal D2 dopamine receptor binding in rats. Our results indicate that when challenged with the dopamine agonist, apomorphine (0.5 mg/kg), animals chronically treated with haloperidol or high-dose ascorbate alone display a supersensitive sniffing response relative to controls, while animals chronically treated with the combination of haloperidol and high-dose ascorbate display a further potentiation of sniffing relative to the haloperidol groups. In addition, [3H]spiperone saturation studies showed, as expected, an up-regulation of striatal D2 dopamine receptors in rats treated with haloperidol as reflected by a change in receptor density (Bmax) but not affinity (KD). Ascorbate treatment, however, had no effect on D2 receptor density or the distribution of [3H]apomorphine in whole brain. Even though chronic treatment with the haloperidol-high-dose-ascorbate combination produced an up-regulation of striatal D2 dopamine receptors, this treatment did not cause a further up-regulation relative to haloperidol alone nor did it have any effect on [3H]apomorphine distribution. Taken together, these findings indicate that although chronic ascorbate produces behavioral supersensitivity to apomorphine through central mechanisms, they appear to differ from those induced by chronic haloperidol.

Acute and long-term amphetamine treatments alter extracellular ascorbate in neostriatum but not nucleus accumbens of freely moving rats

The ability of amphetamine to alter the extracellular level of ascorbate, an apparent modulator of neostriatal function, was assessed voltammetrically in the neostriatum and nucleus accumbens of awake, behaving rats. Whereas acute administration (1.0 and 5.0 mg/kg d-amphetamine) produced a dose-dependent rise in neostriatal ascorbate, there was no change in the nucleus accumbens. Vehicle injections had no significant effect on ascorbate levels in either location. Administration of 5.0 mg/kg d-amphetamine for one week enhanced neostriatal ascorbate release even further, but this effect returned to acute levels when treatment continued for a second week. Multiple amphetamine injections for up to two weeks failed to alter extracellular ascorbate in the nucleus accumbens. The results of these experiments confirm a site-specific action of amphetamine on ascorbate release and suggest complex changes in the extracellular level of this substance in the neostriatum with long-term treatment.

Stimulation of both D1 and D2 dopamine receptors increases behavioral activation and ascorbate release in the neostriatum of freely moving rats

Electrochemically modified carbon-fiber electrodes were used to assess the effects of indirect (amphetamine and GBR-12909) as well as direct D1 (SKF-38393) and D2 (quinpirole) dopamine agonists on extracellular ascorbate in the neostriatum of awake, behaving rats. Relative to controls, 2.5 mg/kg d-amphetamine and 20.0 mg/kg GBR-12909 produced marked behavioral activation concomitant with a significant increase in ascorbate. Comparable effects were observed following the combined administration of 10.0 mg/kg SKF-38393 and 1.0 mg/kg quinpirole, but not after either of these drugs alone. Thus, behavioral activation and release of neostriatal ascorbate were closely related to the concurrent stimulation of both D1 and D2 dopamine receptors.

Effect of forebrain dopamine depletion on novelty-induced place preference behavior in rats

Novelty-induced place preference behavior of rats was studied in two experiments. In the first experiment, separate groups of animals were habituated to a distinct environment 30 min daily for either zero, one, two, four or eight days. On the day following the last habituation day, animals were allowed 15 min free access to both the habituated (familiar) and a distinct novel environment. The results revealed a significant novelty preference in the two-, four- and eight-day habituation groups. In these same animals, the rate of horizontal and vertical activity was lower in the novel environment relative to the familiar environment. The influence of forebrain dopamine (DA) projections on novelty preference behavior was studied in the second experiment. Animals were given an injection of 6-hydroxydopamine (6-OHDA) into the nucleus accumbens or were given sham surgery, and then they were given four habituation days to one environment. Novelty-induced place preference was blocked in the lesioned animals, as the amount of time spent in the novel and familiar environments was not significantly different. Lesioned animals also failed to show a difference in locomotor activity between the novel and familiar environments. Subsequent assay data revealed that the 6-OHDA lesion reduced DA levels in the nucleus accumbens, anterior striatum and olfactory tubercles by over 65% as compared to sham surgery. These results suggest that novelty preference behavior may be mediated by a central DA pathway similar to that involved in other types of reinforcing stimuli, such as food, water and drugs of abuse.

Naloxone enhances the expression of morphine-induced conditioned place preference

The present study examined the effects of naloxone on acquisition and expression of morphine-induced conditioned place preference (CPP). Three groups of rats were given morphine (5 mg/kg, SC), both morphine and naloxone (1 mg/kg, SC), or saline paired with a distinctive environment. On alternating days they were given saline paired with another distinctive environment. After four exposures to each environment, the animals were given a preference test in which they had access to both environments simultaneously while under the influence of either naloxone (1 mg/kg, SC) or saline. Morphine-conditioned animals showed CPP evident as an increased amount of time spent in the drug-associated environment relative to saline controls. Rats given both naloxone and morphine during conditioning, and saline on the test day, did not show CPP. In contrast, morphine-conditioned animals given naloxone on the test day showed stronger CPP than morphine-conditioned animals given saline. These findings indicate that naloxone blocks the acquisition, but enhances the expression of morphine-induced CPP. In a separate experiment, the effects of naloxone on locomotor activity were determined during the CPP test. The results indicated that naloxone decreased locomotor activity. In morphine-conditioned animals only, naloxone also produced an increase in the amount of time per entry in the drug-associated environment. The results suggest that naloxone may enhance morphine-induced CPP by decreasing locomotor activity that may otherwise compete with expression of CPP.

Changes in locomotion and dopamine neurotransmission following amphetamine, haloperidol, and exposure to novel environmental stimuli

Locomotor behavior and dopamine (DA) neurotransmission were assessed in rats exposed to either a novel or familiar stimulus environment while under the influence of amphetamine, haloperidol or saline. The behavioral results indicated that, as expected, amphetamine increased horizontal locomotor activity in a dose-dependent manner. Exposure to novelty also increased horizontal activity, and this behavioral effect was disrupted by both amphetamine and haloperidol. Regardless of whether the animals were exposed to the novel or familiar stimulus environment, amphetamine increased DA synthesis in the nigrostriatal system, but not in the mesolimbic system, whereas haloperidol increased DA synthesis in both the nigrostriatal and mesolimbic systems. Amphetamine also decreased DA metabolism and haloperidol increased DA metabolism in both the nigrostriatal and mesolimbic systems. In contrast, exposure to novelty alone was without effect on DA synthesis or metabolism in any region examined, suggesting that novelty-induced hyperactivity and amphetamine-induced hyperactivity involve different neurochemical mechanisms. However, exposure to novelty while under the influence of haloperidol produced a significant increase in DA metabolism in both the nigrostriatal and mesolimbic systems. These latter results suggest that exposure to novelty may produce a measurable activation of DA systems when the autoreceptors involved in the negative feedback loop are blocked.

Novelty-induced place preference behavior in rats: effects of opiate and dopaminergic drugs

In Experiment 1, adult male rats were given eight 30-min exposures to one of two distinct environments. Control animals received either four exposures to each environment or were not exposed to either environment. When given free-choice access to both environments simultaneously, animals spent significantly more time in the novel environment relative to the familiar environment. In these same animals, horizontal and vertical activity rates were lower in the novel environment than in the familiar environment. In Experiments 2-5, animals were assessed for novelty preference behavior under the influence of either morphine (0, 0.1, 0.3, 1.0 or 3.0 mg/kg), naltrexone (0, 0.1, 0.3 or 1.0 mg/kg), amphetamine (0, 0.1, 0.3 or 1.0 mg/kg) or haloperidol (0, 0.03, 0.1, 0.3 or 1.0 mg/kg). Haloperidol produced a dose-dependent disruption in novelty preference behavior, while all other drugs tested were without effect. Haloperidol also disrupted the novelty-induced decrease in horizontal and vertical activity rates. These results suggest that haloperidol blocks the reinforcing and locomotor-depressant effects of a novel environment in a free-choice preference test.

Antecedents of symptom expression during marital separation

Examined the role of possible antecedents of psychological symptoms among 310 recently separated men and women. Hierarchical set regression analysis indicated that socio-demographic variables generally are unrelated to symptom expression, although age, gender, length of marriage, and length of separation did predict either agitated depressive or stress symptoms. Several dimensions of social stressors contributed to the prediction of one or more of six symptom factors, but symptoms also were associated with current psychological well-being. These results suggest that the symptomatology of divorcing persons can be partitioned into that which is consequent to the disruptive conditions encountered in divorce and that which may reflect characteristic levels of mental health.

The influence of stress upon symptom structure

Evaluated the influence of stress on the factor structure of symptoms in a nonclinical population. A random sample of 299 persons in the process of divorcing were subdivided into those high and low on stressful life events. Separate factor analyses of symptoms for each group provided evidence that stress does affect symptom structure. Ruminative Depression was the only stable factor. Two anxiety factors from the low stress group merged into one factor among the high stressed, while agitated depression appeared only in the low stress condition. A factor suggestive of stress response syndrome was evident among the high stressed.

Dimensions of adult self-concept

A 70-item adjective checklist was administered to four groups of adults as part of a longitudinal study of psychosocial change during four normative transitions. Data gathered at two times with a five-year interval were cluster analyzed separately. Six clusters were extracted from the first set of data, 7 from the second, with minor changes in dimensionality: all 6 original clusters had counterparts in the second analysis, but some dimensional change was seen in all but one cluster. The argument is made that although such dimensional change is often treated as error or completely ignored in longitudinal research, this inattention can lead to serious problems in the interpretation of results.