Multiple postsynaptic dopamine receptors and behavioral manifestation

Reduction of the rewarding effect of brain stimulation by a blockade of dopamine D1 receptor with SCH 23390

The subtype of dopamine receptor related to the rewarding effect of brain stimulation was determined in 17 rats. The animals were trained to contact a dry spout to receive stimulation through electrodes implanted into the lateral hypothalamic area, ventral tegmental area, or dorsal raphe nucleus. The dopamine D1 blocking agent SCH 23390, 0.08 mg/kg IP, completely suppressed responding. The D2 receptor blocker sulpiride, 50 mg/kg IP, or the serotonin receptor blocker metergoline, 5 mg/kg IP, did not suppress responding. The ED50 for SCH 23390 was 0.022 mg/kg IP. In a runway, rats were trained to run for rewarding goal stimulation consisting of a train of pulses delivered to the lateral hypothalamus. After injection of SCH 23390, 0.01 mg/kg IP, animals showed significantly slower running speed, but their speed returned to normal if the number of pulses in the goal stimulation was increased 2.6 times. These results indicate that blockade of D1 receptors, but not D2 receptors, reduces the rewarding effect of brain stimulation.

Operantly conditioned running: effects on brain catecholamine concentrations and receptor densities in the rat

It was hypothesized that endurance exercise results in an alteration in the brain monoamine systems. Rats were trained to run for food reinforcement on a variable ratio schedule in running wheels. Yoked control rats were also allowed to run but were not specifically reinforced for running. The animals ran 5 days per week for 8 weeks and were sacrificed 48 hours after the last endurance training session. The brains were assayed for norepinephrine and dopamine concentrations and beta-adrenergic (3H-dihydroalprenolol binding) and dopaminergic (3H-spiroperidol binding) receptor densities. Changes in norepinephrine concentration and beta-adrenergic receptor densities were not significantly different between reinforced running and yoked control groups. Dopamine concentrations were significantly higher while dopamine receptor densities were significantly lower in the reinforced running group. These results suggest that chronic running elevates dopamine secretion and consequently produces a compensatory down-regulation of dopaminergic receptor sites. The relationship of these changes to motor activity and to the antidepressant effects of exercise are discussed.

Effects of pertussis toxin on D2-dopamine receptor in rat striatum: evidence for coupling of Ni regulatory protein with D2-receptor

It is well established that a pertussis toxin, islet activating protein (IAP), interacts directly with the Ni regulatory protein involved in the receptor-adenylate cyclase system. In this study we investigated the effect of the toxin on the dopaminergic function of the central nervous system in conjunction with the adenylate cyclase system. Direct bilateral microinjection of the toxin into rat striatum reduced the stereotyped behavior induced by apomorphine. This inhibitory effect was observed even 40 h after administration of the toxin, whereas the inhibition by haloperidol disappeared within 15 h. Toxin administration did not influence either the Bmax or affinity of specific binding of [3H]spiroperidol to the striatal membrane. However, it increased the IC50 value of apomorphine for the specific binding of [3H]spiroperidol. GTP (10(-4) M) had little effect on the apparent affinity of apomorphine to the [3H]spiroperidol binding sites in IAP-treated membrane, though an effect was clearly observed in untreated membrane. [3H]Spiroperidol binding sites were increased 34 +/- 8% (n = 4) on chronic treatment of haloperidol for 2 weeks. On the contrary, on long-term administration of IAP the ligand binding sites were decreased by 25 +/- 10% (n = 4). These results indicate that pertussis toxin can interact with the Ni protein coupled with striatal D2-dopamine receptor. Inhibition of the coupling between Ni protein and the D2-dopamine receptor attenuated the manifestation of rat stereotyped behavior. Furthermore, chronic administration of dopamine antagonist resulted in the up-regulation of the D2-dopamine receptor, while long-term inhibition of coupling between the D2-dopamine receptor and Ni regulatory protein reduced the amount of receptors.