Movement initiation characteristics in young adult rats in relation to the high- and low-affinity agonist states of the striatal D2 dopamine receptor

Intracellular nonequilibrium fluctuating stresses indicate how nonlinear cellular mechanical properties adapt to microenvironmental rigidity

Living cells are known to be in thermodynamically nonequilibrium, which is largely brought about by intracellular molecular motors. The motors consume chemical energies to generate stresses and reorganize the cytoskeleton for the cell to move and divide. However, since there has been a lack of direct measurements characterizing intracellular stresses, questions remained unanswered on the intricacies of how cells use such stresses to regulate their internal mechanical integrity in different microenvironments. This report describes a new experimental approach by which we reveal an environmental rigidity-dependent intracellular stiffness that increases with intracellular stress - a revelation obtained, surprisingly, from a correlation between the fluctuations in cellular stiffness and that of intracellular stresses. More surprisingly, by varying two distinct parameters, environmental rigidity and motor protein activities, we observe that the stiffness-stress relationship follows the same curve. This finding provides some insight into the intricacies by suggesting that cells can regulate their responses to their mechanical microenvironment by adjusting their intracellular stress.

Pharmacological characterization of the D1- and D2-like dopamine receptors from the brain of the leopard frog, Rana pipiens

The pharmacological profiles of D1- and D2-like dopamine receptors were investigated for native brain receptors in the leopard frog, Rana pipiens, using direct binding assays, which characterize functional receptors rather than assess total receptor protein. We used homogenate assays of R. pipiens fore- and midbrains to determine, via saturation isotherms, that the dissociation constant, Kd, for (3)H-SCH-23390 binding to the D1-like receptors was 0.29 nM, and the maximal receptor density, Bmax, was 40 fmoles/mg protein. This compares with the more than 10-fold higher density of D1 sites in rat striatum. Specific binding for the D2-like receptors was measurable using these methods with (3)H-spiperone as the ligand. However, saturation of binding was not achieved. This contrasts with the > 400 fmoles/mg protein Bmax in rat striatum. Pharmacological profiles (rank order of potency of displacing drugs) for each receptor type were determined. We used non-radioactive SCH-23390, SKF-38393, sulpiride, and spiperone to displace (3)H-SCH-23390 and (3)H-spiperone at D1 and D2 receptors, respectively. Parallel displacement assays were performed with rat striatal controls. Results indicated that the relative rank order displacements in anuran dopamine receptors were characteristic of D1- and D2-like receptors. However, the rank orders were not identical to those in mammals. The rank order for affinity at D1-like receptors in both rats and frogs was SCH-23390 > SKF-38393 > spiperone > sulpiride. The rank order for affinity at D2-like receptors was spiperone > SCH-23390 > sulpiride > SKF-38393 in frogs, and spiperone > sulpiride > SCH-23390 > SKF-38393 in rats. SKF-38393 and spiperone had similar affinities for the 'D1' receptors in both species. SCH-23390 had a slightly lower affinity for the D1-like receptors in Rana, whereas sulpiride had a significantly lower affinity for Rana D1-like receptors compared to rat D1 receptors. In Rana D2-like receptors, spiperone and sulpiride were significantly less potent compared to rat. However, SCH-23390 and SKF-38393 were equally potent for the D2-like receptors in both species. The results indicate that amphibian brain dopamine receptors fall into two classes similar to the mammalian D1 and D2 subfamilies, but with binding characteristics slightly different from those typically described in mammals. This work represents the first pharmacological characterization of native brain dopaminergic receptors in an anuran amphibian. Because direct binding assays measure the initial aspect of the functional interaction between transmitter and receptor, these data provide an important complement to studies using cell expression systems.

Age-related decreases in lymphocyte protein kinase C activity and translocation are reduced by aerobic fitness

This study investigated the effects of advancing age and long-term aerobic fitness on lymphocyte protein kinase C (PKC) activity and translocation. Lymphocytes were obtained from young (20-36 years old) and older (61-78 years old) healthy men who were either aerobically conditioned or deconditioned. Both baseline PKC activity and the response of this enzyme to the direct PKC stimulating agent, phorbol 12-myristate, 13-acetate (PMA) or to the mitogen, phytohaemagglutinin (PHA), were measured in partially purified extracts of cytosolic and membranous fractions of lymphocytes. Basal PKC activity, PMA-induced redistribution of PKC, and PHA-induced enhancement of PKC activity were reduced among older subjects in both lymphocyte cytosolic and membranous fractions. However, the magnitudes of these reductions were smaller among the older subjects who were aerobically fit. Lymphocyte PKC activity and translocation may be biological markers of aging, and the maintenance of aerobic fitness into later life may serve to slow the rate at which activation of this enzyme declines during senescence.

Reaction time responding in rats

The use of reaction time has a great tradition in the field of human information processing research. In animal research the use of reaction time test paradigms is mainly limited to two research fields: the role of the striatum in movement initiation; and aging. It was discussed that reaction time responding can be regarded as "single behavior", this term was used to indicate that only one behavioral category is measured, allowing a better analysis of brain-behavior relationships. Reaction time studies investigating the role of the striatum in motor functions revealed that the initiation of a behavioral response is dependent on the interaction of different neurotransmitters (viz. dopamine, glutamate, GABA). Studies in which lesions were made in different brain structures suggested that motor initiation is dependent on defined brain structures (e.g. medialldorsal striatum, prefrontal cortex). It was concluded that the use of reaction time measures can indeed be a powerful tool in studying brain-behavior relationships. However, there are some methodological constraints with respect to the assessment of reaction time in rats, as was tried to exemplify by the experiments described in the present paper. On the one hand one should try to control for behavioral characteristics of rats that may affect the validity of the parameter reaction time. On the other hand, the mean value of reaction time should be in the range of what has been reported in man. Although these criteria were not always met in several studies, it was concluded that reaction time can be validly assessed in rats. Finally, it was discussed that the use of reaction time may go beyond studies that investigate the role of the basal ganglia in motor output. Since response latency is a direct measure of information processing this parameter may provide insight into basic elements of cognition. Based on the significance of reaction times in human studies the use of this dependent variable in rats may provide a fruitful approach in studying brain-behavior relationships in cognitive functions.

DAD1- and DAD2-like agonist effects on motor activity of C57 mice: differences compared to rats

Studies on rats indicate that DAD1- and DAD2-like agonists produce a biphasic action on motor activity, with low doses reducing activity below control levels, and higher doses initially reducing, then elevating, activity for a prolonged period. Although some of the reported effects of DAD1- and DAD2-like receptor agonists on motor activity of mice are consistent with their effects on rats, the possibility of species differences is also apparent. In the current study the effects of DAD1- and DAD2-like agonists on motor activity of C57BL/6 (C57) mice were determined to establish species consistencies and differences with respect to their effects on rats. The partial DAD1-like agonist SKF 38393 reduced the activity of C57 mice at low doses and elevated activity above control levels at higher doses, if the mice were thoroughly habituated to the test chamber. The full DAD1 agonist SKF 82958 also increased the activity of C57 mice, and along with the SKF 38393 results indicates a response to DAD1 receptor stimulation similar to that reported for rats. In contrast to the species similarity in response to DAD1 stimulation, the DAD2-like agonist quinpirole produced only a dose-responsive monotonic reduction in the activity of C57 mice, whether the animals were nonhabituated or well-habituated to the testing environment, male or female, young or mid-aged, injected intraperitoneally (i.p.) or subcutaneously (s.c.), and with either low or high doses. This apparent species difference in response to quinpirole might reflect distinguishable functional properties of the DA subreceptor systems.

Amphetamine, cocaine, and dizocilpine enhance performance on a lever-release, conditioned avoidance response task in rats

A lever-release version of the conditioned avoidance response (CAR) task was used to assess the behavioral effects of several psychomotor stimulants in rats. The indirect dopamine agonists, d-amphetamine (0.1 and 0.25 mg/kg) and cocaine (7.5 and 15 mg/kg), enhanced performance on this task. Both drugs increased percent avoidance responses and decreased avoidance latency. A higher dose of amphetamine (0.5 mg/kg) also decreased avoidance latency but failed to improve percent avoidance. Similar effects were seen at low (0.01 and 0.025 mg/kg) and high (0.05 mg/kg) doses of dizocilpine (MK-801), a stimulant that acts as a noncompetitive antagonist of N-methyl-d-aspartate (NMDA) glutamate receptors. When combined with haloperidol (0.1 mg/kg), a dopamine antagonist, amphetamine (0.25 mg/kg) and dizocilpine (0.025 mg/kg) had differential effects on the lever-release CAR task. Thus, amphetamine-haloperidol was significantly better than haloperidol alone on percent avoidance but not on avoidance latency, whereas dizocilpine-haloperidol had the opposite effect: significantly better than haloperidol alone on avoidance latency but not on percent avoidance. Taken together, these results provide further support for dopaminergic mechanisms in CAR performance but suggest an opposing glutamatergic influence.

Apomorphine and amphetamine produce differential effects on the speed and success of reaction time responding in the rat

Apomorphine, a nonselective, direct-acting dopamine agonist, and amphetamine, a nonselective indirect-acting dopamine agonist, were compared for their effects on the reaction time response in rats. Animals were shaped to release a lever in response to an auditory/visual stimulus to avoid mild foot shock. The characteristics of the reaction time response of primary interest were percent successful avoidance and response latency. Apomorphine (0, 1, and 5 mg/kg, IP) significantly decreased successful avoidance, but had no effect on response latencies. Thus, the decrease in successful avoidance was not a direct result of longer latencies. Amphetamine (0, 0.5, and 1 mg/kg, IP) produced a different pattern of effects on the reaction time response. Successful avoidance was not affected by amphetamine treatment. However, response latencies were dose-dependently decreased in response to amphetamine. These results demonstrate that dopamine receptor stimulation by different dopamine agonists produces a different pattern of effects on the characteristics of the reaction time response. In addition, these results demonstrate that successful avoidance can be modulated independently of response latencies.

Selective D1 and D2 dopamine receptor antagonists produce differential effects on reaction time in the rat

The purpose of this investigation was to determine whether selectively blocking D1 and D2 dopamine receptors produces a differential effect on the characteristics (speed and success) of the reaction time response in rats. Animals were shaped to release a lever in response to an auditory/visual stimulus to avoid mild foot shock. The selective D1 antagonist SCH 23390 (0, 70, and 100 micrograms/kg, IP) and the selective D2 antagonists spiperone (0, 1, and 10 micrograms/kg, IP) and haloperidol (0, 10, and 100 micrograms/kg, IP) were studied for their effects on successful avoidance and response latency. SCH 23390 impaired successful avoidance and increased response latencies in a dose-dependent manner. Spiperone and haloperidol also produced dose-related decreases in successful avoidance. In contrast to the dose-related increase in response latencies produced by SCH 23990, 1 microgram/kg spiperone and 10 micrograms/kg haloperidol significantly decreased the latencies of successful responses. Spiperone (10 micrograms/kg) had little effect on response latencies, while 100 micrograms/kg haloperidol increased them. The results of these experiments demonstrate that reaction time is differentially affected by selective dopamine receptor blockade and that the speed and success of reaction time responses can be independently modulated by D1 vs. D2 receptor activity.

Characterization of D2 receptors and dopamine levels in the thalamus of the rat

We kinetically characterized D2 receptors in thalami pooled from a group of Sprague-Dawley rats and then determined thalamic levels of dopamine (DA), homovanillic acid (HVA), dihydroxyphenylacetic acid (DOPAC), and norepinephrine (NE) in relation to a measure of thalamic DA D2 receptor densities in another group of rats. The equilibrium dissociation constant (kd) was estimated as 0.1 nM by three independent methods, while the Bmax for thalamic D2 receptors was found to be 6.4 fmol/mg p using 3H-spiperone as ligand and ketanserin to occlude 5HT2 binding. Kinetic constants were in agreement with previously reported kinetic data from rodent caudate-putamen. This suggests that thalamic D2 receptors are similar to D2 receptors from other brain areas. Mean thalamic levels of DA (22.6 ng/mg p), DOPAC (1.19 ng/mg p) and HVA (0.31 ng/mg p) concur with previous reports of a sparse distribution of thalamic DA neurons. D2 receptor densities were positively correlated with DA metabolites DOPAC (P less than .05; r = 0.423) and HVA (P less than .05; r = 0.368), but not DA or NE. These results establish fundamental characteristics of thalamic DA neurotransmission to assist in the investigation of behavioral pharmacology of this area.

Behavioral sensitization following a single apomorphine pretreatment--selective effects on the dopamine release process

Once daily subchronic pretreatments with the dopamine (DA) agonist apomorphine (APO) increase striatal DA synthesis and metabolism. Such changes imply that adaptations to APO do not dissipate completely within 24 h. In the present report we evaluated the effects of a single APO treatment 24 h prior to euthanasia on behavior and on striatal DA synthesis, metabolism, release and receptor binding. The single APO pretreatment reduced DA release from striatal synaptosomes. In contrast, striatal DA synthesis, metabolism, and the high-affinity binding of DA to the D2 receptor were unaltered 24 h after agonist pretreatment. At this time the stereotypic response to a subsequent APO challenge was enhanced. This adaptive pattern is different from that observed 60 min following an acute APO pretreatment, when high-affinity D2 binding is reduced. The pattern 24 h following a single APO pretreatment is also different from that observed following subchronic agonist dosing, when stereotypic behavior is enhanced, while basal DA synthesis and metabolism are increased.

Behavioral sensitization following subchronic apomorphine treatment--possible neurochemical basis

Subchronic treatment with the dopamine agonist apomorphine produces a sensitization to the stereotypic effects of subsequent apomorphine challenge. The present study investigated the effects of this subchronic treatment on apomorphine induced stereotypic behavior and striatal dopamine synthesis, release, metabolism, and D2 receptor binding. The pretreatment, which enhanced the behavioral response to apomorphine challenge, also elevated basal dopamine synthesis and metabolism, but left the ability of a challenge dose of apomorphine to inhibit dopamine synthesis and metabolism unaltered. Thus, ongoing dopamine synthesis and extracellular levels of metabolites would be higher following apomorphine challenge in animals treated subchronically with the agonist. In contrast, neither synaptosomal dopamine release in response to depolarizing stimuli nor the density of D2 dopamine receptors was altered by the treatment. Overall, the results suggest that, while we did not find evidence of autoreceptor desensitization per se, apomorphine treatment may result in enhanced extracellular dopamine levels following dopamine agonist challenge to provide a greater stimulation of an intact dopamine receptor system.

Single apomorphine pretreatment results in a rapid decline in high-affinity dopamine binding to the striatal dopamine D-2 receptor

The formation of a ternary complex of agonist, receptor, and G-protein precedes inhibition of adenylate cyclase and is associated with high-affinity agonist binding. The present experiment was conducted to determine if a single direct dopamine (DA) agonist, apomorphine (APO), pretreatment could produce a rapid uncoupling of the striatal DA D-2 receptor from its G-proteins. APO (30 mg/kg, i.p.) and saline were administered once, with killing 60 or 90 min following the APO or vehicle treatment. APO pretreatment resulted in a reduction in the high-affinity binding of DA to the striatal DA D-2 receptor without altering total agonist binding. The total density of antagonist-defined D-2 receptors (Bmax) was not altered by the treatment. The present results represent, to our knowledge, the first demonstration of changes in high-affinity agonist binding to the DA D-2 receptor following a single in vivo pretreatment of a direct DA agonist.

Reaction time and nigrostriatal dopamine function: the effects of age and practice

Normal aged and Parkinsonian individuals lose the ability to initiate movements rapidly (increased reaction time) in parallel with changes in the nigrostriatal dopamine (DA) system. However, the ability of these individuals to improve their reaction time with practice has not been adequately assessed. We have developed a rodent model of human reaction time in which reaction time performance correlates highly with neurochemical measures of nigrostriatal DA integrity. In the present report, 15 young and 10 old male Sprague-Dawley rats were conditioned in a reaction time task to release a lever quickly in response to external stimuli in order to avoid a mild footshock. In order to examine the effects of practice on this reaction time task, the young animals were tested for 5 days at 3, 6 and 9 months of age and the old animals were tested for 5 days at 18, 21, and 24 months of age. From this well-practiced task, reaction time response latencies were measured and compared to measures of nigrostriatal DA function (steady-state levels of DA and its metabolites, D2DA receptor affinity and density). The old animals were slower in response latencies than the young animals. These age differences in response latencies, however, disappeared after several days of testing at each of the 3 test sessions, so that the old animals were not significantly slower than the young animals on days 4 and 5 of each session. As expected, the old animals showed reduced striatal D2DA receptor density with no age differences in DA receptor affinity.(ABSTRACT TRUNCATED AT 250 WORDS)