Effects of footshock stress on regional brain monoamine metabolism and the acquisition of conditioned freezing in rats previously exposed to repeated methamphetamine

65 years of research on dopamine's role in classical fear conditioning and extinction: A systematic review

Dopamine, a catecholamine neurotransmitter, has historically been associated with the encoding of reward, whereas its role in aversion has received less attention. Here, we systematically gathered the vast evidence of the role of dopamine in the simplest forms of aversive learning: classical fear conditioning and extinction. In the past, crude methods were used to augment or inhibit dopamine to study its relationship with fear conditioning and extinction. More advanced techniques such as conditional genetic, chemogenic and optogenetic approaches now provide causal evidence for dopamine's role in these learning processes. Dopamine neurons encode conditioned stimuli during fear conditioning and extinction and convey the signal via activation of D1-4 receptor sites particularly in the amygdala, prefrontal cortex and striatum. The coordinated activation of dopamine receptors allows for the continuous formation, consolidation, retrieval and updating of fear and extinction memory in a dynamic and reciprocal manner. Based on the reviewed literature, we conclude that dopamine is crucial for the encoding of classical fear conditioning and extinction and contributes in a way that is comparable to its role in encoding reward.

Ontogeny of the adrenal response to (+)-methamphetamine in neonatal rats: the effect of prior drug exposure

We examined the ontogeny of the corticosterone response to (+)-methamphetamine in neonatal rats. In experiment-1, animals were injected with 10 mg/kg of (+)-methamphetamine or saline and plasma corticosterone levels were examined in separate groups 30 or 105 min later on postnatal day (P) 1, 3, 5, 7, 9, 11, 13, 15, 17, or 19. The adrenal response to methamphetamine was best described by a U-shaped function with the nadir of corticosterone release occurring between P7 and P13. Experiment-2 was similar except that the effect of four consecutive days of exposure to (+)-methamphetamine (four times daily at 2 h intervals with 10 mg/kg) was assessed with a single final dose early on the fifth day (i.e. P1-5, 3-7, 5-9, 7-11, 9-13, 11-15, 13-17, 15-19). The 30 min corticosterone response after multiple methamphetamine doses was augmented compared to single exposures, with the exception of the two earliest dosing intervals ending on P5 and P7, where the responses were lower. In addition, at 105 min, the levels of corticosterone were attenuated relative to a single drug administration. With the exception of animals receiving methamphetamine from P15 to P19, thymus weights were unaffected. The data demonstrate that (+)-methamphetamine is a robust activator of corticosterone release in developing animals and this release is extensively modified by age and previous drug exposure.

Central D-Ala2-Met5-enkephalinamide mu/delta-opioid receptor activation blocks behavioral sensitization to cholecystokinin in CD-1 mice

The present investigation revealed that intraventricular administration of the anxiogenic substance CCK-8S (50 ng) decreased responding for previously rewarding brain stimulation (intracranial self-stimulation; ICSS) and subsequently increased brain stimulation threshold determinations from the dorsal aspects of the ventral tegmental area (VTA) immediately following CCK administration. While central administration of the mixed mu/delta opioid receptor agonist D-Ala(2)-Met(5)-enkephalinamide (DALA; 1 microg) was ineffective in abrogating CCK induced ICSS deficits during the immediate post-stressor interval, DALA restored ICSS brain stimulation thresholds to basal values 24, 48 and 168 h following CCK challenge. At 18 days following the initial 50 ng CCK-8S and/or DALA challenges, mice were exposed to a previously determined non-anxiogenic dose of CCK-8S (5 ng). Among mice which received an intervening dose of saline following the 50 ng CCK-8S challenge, depressed ICSS responding and elevated brain stimulation thresholds were evident during the immediate (Day 18), 24- (Day 19) and 48-h (Day 20) test sessions relative to mice that received an intervening dose of DALA on Day 1. These data imply that while CCK induces relatively protracted and exaggerated behavioral disturbances, mu/delta opioid-receptor activation may block CCK-induced behavioral sensitization and change the course of psychopathology.

Effects of repeated administration of methamphetamine on P3-like potentials in rats

Effects of repeated administration of methamphetamine (MAP) on a component of the cortical event-related potential (ERP), P3-like potential which corresponds to the human P3b, were examined in rats performing an active discrimination task. Rats were trained to press a bar within 1200 ms after cessation of a target tone (1000 Hz) lasting for 800 ms, and to withhold an overt response to the standard tone (2000 Hz). The rats were given intracranial electrical stimulation to the medial forebrain bundle as a reward, only when they correctly responded to the target tone. ERPs before drug administration were recorded after the correct response ratio exceeded 85%. Thereafter, a daily dose of 4 mg/kg of MAP, or the same volume of saline in another group, was administered intraperitoneally 15 times. ERPs were recorded again 7-10 days after the last injection. In the rats which received MAP the amplitude of the P3-like potential decreased with no change in its latency, while the response latency of bar-pressing and the correct response ratio were not altered significantly. These results suggest some changes in catecholaminergic transmission induced by repeated MAP-administration affect a P3 generation mechanism. MAP-treated rats may be useful as an animal model to investigate neural mechanisms of MAP-psychosis and schizophrenia.