Effects of benzodiazepine receptor ligands on simultaneous visual discriminations of variable difficulty

Reduced attention in mice overproducing corticotropin-releasing hormone

Data from several studies suggest that unrestrained secretion of corticotropin-releasing hormone in the CNS produces several signs and symptoms of depression. Recent evidence indicates that blockade of the CRH receptor 1 reduced depression scores in depressed patients. One of the symptoms that occur is depression is impairment in attentional processes. Whether these impairments are due to alterations in the CRH system are so far unknown. In order to investigate whether overproduction of CRH alters attentional process, transgenic mice overproducing CRH were tested on an operant five choice serial reaction time task, a task which taxes sustained and divided attention. Mutants showed impaired autoshaping. During initial discrimination learning, transgenics performed below wildtype level, but with extended training with long stimulus durations, transgenic animals reached similar accuracy levels as wildtype mice. When animals were tested at shortest stimulus duration (0.5s), a mild but significant impairment in accurate responding emerged in transgenics. This was accompanied by longer correct response latencies, while incorrect latencies did not differ between groups, suggesting attentional impairment in CRH transgenics. Because these animals have been reported to also show increased anxiety-related behaviour, animals were treated with the anxiolytic benzodiazepine diazepam. Diazepam failed to affect accuracy, but transgenic mice showed a stronger behavioural disinhibition. This suggests that the attentional impairment seen in CRH overexpressors is independent of alterations in anxiety-like behaviour. These findings may have implications for understanding the pathophysiology of psychiatric disorders such as depression, where it has been suggested that an overactivity of the CRH system accounts for a variety of symptoms, including hyper-arousal and attentional impairment.

Dissociation between the attentional effects of infusions of a benzodiazepine receptor agonist and an inverse agonist into the basal forebrain

The effects of infusions of the benzodiazepine receptor (BZR) full agonist chlordiazepoxide (CDP) or the full inverse agonist beta-CCM into the basal forebrain on behavioral vigilance were tested. Vigilance was measured by using a previously characterized task that requires the animals to discriminate between visual signals of variable length and non-signal events. Measures of performance included hits, misses, correct rejections, false alarms, side bias, and errors of omission. Following the infusion of saline (0.5 microliters/hemisphere), the relative number of hits varied with signal length. In response to shorter signals, the number of hits decreased over time, indicating a vigilance decrement. Infusions of CDP (20, 40 micrograms/hemisphere) initially decreased the relative number of hits in response to shorter signals and, later in the course of the test sessions, to longer signals as well. CDP did not affect the relative number of correct rejections. In contrast, infusions of the inverse agonist beta-CCM (1.5, 3.0 micrograms/hemisphere) did not affect the relative number of hits but decreased the relative number of correct rejections (i.e., increased the number of false alarms). These data suggest that the basal forebrain mediates the attentional effects of BZR ligands. As systemic or intrabasalis administration of BZR agonists and inverse agonists was previously demonstrated to decrease and augment, respectively, activated cortical acetylcholine (ACh) efflux, their effects on behavioral vigilance are hypothesized to be mediated via their effects on cortical ACh.

Effects of nicotinic acetylcholine receptor ligands on behavioral vigilance in rats

The effects of nicotinic receptor ligands on performance in a task measuring sustained attention, or vigilance, were tested. This task required the animals to discriminate between signal and non-signal events. The sequence of signal (central panel light illumination for 500, 50 or 25 ms) and non-signal presentations was randomized over three blocks of 54 trials each (27 signal trials, 9 per length, and 27 non-signal trials). A left lever press following a signal was counted as a hit, and a right lever press following a non-signal event was counted as a correct rejection. Hits and correct rejections were rewarded, whereas misses and false alarms (defined as incorrect right and left lever presses, respectively) were not. Baseline performance was characterized by a signal length dependent ability of the animals to discriminate between signal and non-signal events. Administration of nicotine (0.19, 0.62, 1.9 mumol) or of two novel nicotinic receptor agonists, ABT-418 and A-82695, did not produce main effects on vigilance performance. Lobeline (1.9, 6.2, 19 mumol), a nicotinic receptor ligand with mixed agonist/antagonist activities, impaired the animals' ability to discriminate between signal and non-signal events. The antagonist mecamylamine (5, 15, 50 mumol) potently impaired performance while increasing the number of errors of omission. The lack of effect of nicotine largely corresponds with the findings from previous studies on the acute effects of nicotine in intact subjects and non-smoking humans. While the detrimental effects of lobeline may have been related to the antagonist effects of this compound, the reasons for the differences between the effects of nicotine and lobeline still remain unsettled. These data support the hypothesis that nicotine receptor mechanisms are maximally activated in intact animals performing this task, and suggest that effects of acute nicotinic agonist treatment would not produce further cognitive benefit for these animals.

Behavioral vigilance in rats: task validation and effects of age, amphetamine, and benzodiazepine receptor ligands

An operant task for the measurement of sustained attention or vigilance in rats was characterized. The task requires the animals to respond to the presentation of visual signals (presented for 25, 50, or 500 ms) by operating one lever ("hits") and to the absence of a signal by operating the opposite lever ("correct rejection"). Incorrect responses ("misses" and "false alarms", respectively) were not rewarded. Performance in this task is a function of signal length, i.e., the shorter the signals the higher the number of misses. An increase in "background noise" by flashing the chamber houselight (at 0.5 Hz) impaired the animals' ability to discriminate between signal and non-signal events. Also flashing the houselight augmented the vigilance decrement observed for shortest signals. An increase in the event-rate also resulted in a vigilance decrement. Finally, the inability of the animals to time signals was examined by testing the effects of an increase in event asynchrony. In a second experiment, the performance of differently aged rats (6- and 20 month-old male BNNia/F344 rats) was studied. Compared to young animals, 20-month-old rats showed a decrease in their ability to discriminate between shortest signals (25 ms) and non-signal events but did not differ in their ability to correctly reject non-signal trials. Administration of the benzodiazepine receptor (BZR) agonist chlordiazepoxide (CDP; 3, 5, 8 mg/kg) resulted in an impairment of the animals' ability to discriminate between signal and non-signal events and, similar to the effects of age, this effect was exclusively due to an increase in the number of misses. CDP generally produced potent effects while affecting the aged animals to a greater degree. BZR-ligands with weak or "selective" inverse agonist properties (ZK 93426; beta-CCtB) did not affect vigilance performance. The BZR partial inverse agonist RU 33965 (0.1, 0.5 mg/kg) dose-dependently impaired vigilance performance. The administration of amphetamine (0.4, 0.8 mg/kg) also impaired performance, but these impairments were possibly based on effects unrelated to attentional mechanisms. The finding that performance in this task revealed the interactions between the effects of age and BZR agonists on attentional abilities further supports the validity of measures of performance generated by this task.