Jorge D. Brioni

Involvement of the amygdala GABAergic system in the modulation of memory storage

These experiments examined the involvement of the intrinsic GABAergic system of the amygdaloid complex in the modulation of memory storage. Rats were chronically implanted with bilateral cannulae in the amygdala, trained in an inhibitory avoidance task, and given post-training bilateral intra-amygdala injections of either the GABA receptor antagonist bicuculline methiodide (BMI) (0.1-1.0 nmol) or the GABAA receptor agonist muscimol (0.001-0.1 nmol). As indicated by performance on a 48 h retention test, BMI enhanced retention of the inhibitory avoidance conditioning, while muscimol impaired retention. The memory-enhancement obtained with BMI (0.1 nmol) was produced by a dose lower than that necessary to induce convulsions. Post-training injections of BMI did not affect retention when injected into the caudate-putamen dorsal to the amygdala. These results suggest that the amygdaloid GABAergic system is involved in the modulation of memory storage.

Muscimol injections in the medial septum impair spatial learning

These experiments examined the role of GABAergic systems in modulating septohippocampal cholinergic influences on learning. Microinjections of the GABA(A) agonist muscimol (0.5, 1.0 or 5.0 nmol) or physiological saline were administered (0.5 microliters) into the medial septum of rats via chronically implanted cannulae just prior to daily training in the Morris water maze spatial learning task. The animals received 3 training trials on each of 4 days. The escape latencies of rats trained with a submerged escape platform at a fixed location were significantly shorter than those trained with a randomly located platform. Rate of learning of the fixed location was significantly impaired in rats given pretraining muscimol injections in the medial septum at doses (1.0 and 5.0 nmol) that significantly reduced hippocampal high-affinity choline uptake (HACU). Analyses of responses on a probe trial with no pretraining injections and no platform revealed that, in comparison with controls, animals that had received muscimol prior to each training session were likely to swim in the region where the platform had been located. The finding that muscimol-injected rats were subsequently able to learn the task when trained without muscimol injections indicates that the acquisition impairment was not due to a lasting effect of the drug injections. Our results are consistent with the view that the septal GABAergic modulation of the septohippocampal cholinergic pathway is involved in regulating the acquisition of spatial information.

Post-training administration of GABAergic antagonists enhances retention of aversively motivated tasks

The effect of sub-convulsive doses of GABAergic antagonists on the retention of two aversively motivated tasks, inhibitory avoidance (IA) and Y-maze discrimination (YMD), was investigated in CFW mice. In the IA task, post-training intraperitoneal injections of picrotoxin and bicuculline induced a dose-dependent enhancement of retention measured 24 h after the training, while retention was not affected by bicuculline methiodide (a GABA receptor antagonist that does not readily cross the blood-brain barrier). In the absence of footshock on the training day, post-training administration of picrotoxin and bicuculline did not affect retention test latencies. In the YMD task, the discrimination was reversed on the retention test and errors made on the reversal trials served as the index of retention of the original training. The reversal error scores of mice given post-training injections of picrotoxin or bicuculline, but not bicuculline methiodide, were significantly higher than those of saline-treated controls. These findings extend previous observations that GABAergic antagonists enhance retention of aversively motivated tasks and suggest the involvement of central GABAergic processes on memory consolidation.

Post-training systemic and intra-amygdala administration of the GABA-B agonist baclofen impairs retention.

The effects of the GABA-B receptor agonist baclofen on memory storage were studied in two series of experiments. In the first series, CD-1 mice were trained in two aversively motivated tasks: a one-trial inhibitory avoidance task and a classical conditioning task (conditional emotional response). Immediate post-training ip administration of (+/-)baclofen (10 and 30 mg/kg) impaired retention of animals in both tasks. The effect was time-dependent: Retention was not affected by baclofen administered 120 min after training. In the second series of experiments, which used Sprague-Dawley rats, post-training intra-amygdala administration of baclofen impaired retention of an inhibitory avoidance response. These results support the view that the GABAergic system is involved in the modulation of memory storage and that the amygdaloid complex may be a critical site for effects of drugs affecting the GABAergic system.

Picrotoxin enhances latent extinction of conditioned fear

Male CD1 mice received 20 pairings of tone and footshock (FS) or tone alone in an arm of a Y-maze on Day 1. On Day 2 either extinction (tone alone) or no extinction was followed immediately by saline or picrotoxin (0.5 or 1.0 mg/kg ip). Nonextinguished groups received only saline or picrotoxin (1.0 mg/kg ip) on Day 2. Other groups received saline or picrotoxin (1.0 mg/kg) 2 hr after extinction. On Day 3 all mice were placed in the Y-maze (with doors to all 3 alleys open), and total alley entries during a 2-min test session were recorded. Day 1 FS training resulted in reduced alley entries during the test session. Day 2 extinction session significantly attenuated the effects of the FS training. Day 3 performance of mice given picrotoxin (1.0 but not 0.5 mg/kg) immediately postextinction was comparable to that of mice not given FS on Day 1. The findings suggest that picrotoxin enhanced extinction of conditioned fear.

Amnesia induced by short-term treatment with ethanol: Attenuation by pretest oxotremorine

CD-1 mice were administered a series of tones paired with footshock in the closed arm of a Y maze. On a test session 8 days later the animals were tested for retention of the conditioned emotional response (CER). On the 2-min test session, the three arms of the maze were open and the number of entries into the arms was counted. Retention of the CER was measured by the decrease in the number of entries in comparison with animals trained with no footshock. Starting 24 hr after training, and continuing for the 7 days between training and testing, the animals in different groups received a daily IP injection of saline, 3.6 g/kg of ethanol, 150 micrograms/kg of the cholinergic muscarinic agonist oxotremorine, or ethanol plus oxotremorine. Retention was evaluated 24 hr after the last injection. Ethanol reduced retention of the conditioned emotional response. This effect was attenuated by oxotremorine (150 micrograms/kg) given IP 6 min prior to testing, but not by the same dose of oxotremorine given daily together with the ethanol treatment. Oxotremorine injections administered prior to the retention test also enhanced the retention performance of the control group. Daily oxotremorine administration had no effect. These findings suggest that ethanol weakened retention of the conditioned emotional response, that this effect was unrelated to acquisition or consolidation, and that the deleterious effect of the ethanol treatment can be attenuated by oxotremorine administered prior to the retention test.

Pretest β-endorphin and epinephrine, but not oxotremorine, reverse retrograde interference of a conditioned emotional response in mice

CD-1 mice were trained in a classically conditioned emotional response paradigm and tested 24 hr later. Exposure to an open field 0 or 1, but not 3 hr after training retroactively interfered with retention of the conditioned emotional response. The retroactive interference was counteracted by the pretest IP administration of beta-endorphin (0.05 microgram/mouse) or epinephrine (1 microgram/mouse), but not by that of oxotremorine (5 micrograms/mouse). The three drugs were able to enhance retention test performance in animals not exposed to the open field after training. In view of evidence in the literature that beta-endorphin and epinephrine are released during training in an aversive task like this, it seems likely that these two agents were able to overcome the effect of retroactive interference by reinstating neurohumoral attributes of the conditioned emotional task at the time of testing.

ABT-418: A Novel Cholinergic Channel Activator (ChCA) for the Potential Treatment of Alzheimer’s Disease

Evidence is accumulating to suggest that compounds which activate neuronal nicotinic acetylcholine receptors (nAChRs) may have potential benefit in the treatment of dementia, especially Alzheimer’s disease (AD) (for review: Arneric and Williams, 1994). The focus of this chapter is to summarize the preclinical pharmacology of ABT-418 [(S)-3-methyl-5-(1-methyl-2-pyrrolidinyl)isoxazole], a novel analog of (-)-nicotine that is in clinical development for the treatment of AD. ABT-418 is a cholinergic channel activator (ChCA) with cognitive enhancement and anxiolytic-like activity possessing a substantially reduced side-effect profile compared to (-)-nicotine (Americ et al., 1994; Decker et al., 1994a). The enhanced preclinical safety profile and excellent transdermal delivery across species are consistent with the early Phase I clinical studies (Sebree et al., 1993).

ABT-089: An Orally Effective Cholinergic Channel Modulator (ChCM) with Cognitive Enhancement and Neuroprotective Action

Recent evidence suggests the existence of a diversity of neuronal nicotinic acetylcholine receptor (nAChRs) subtypes with a wide distribution in brain, that each subtype may be involved in mediating specific functions/behaviors, and that these subtypes have a defined pharmacology that may be selectively targeted (Arneric et al., 1995a). The pharmacologic diversity of neuronal nAChR subtypes suggests the possibility of developing selective compounds which would have more favorable side-effect profiles than existing agents which generally exhibit low selectivity. This broader class of agents, collectively called cholinergic channel modulators (ChCMs) selectively activate some subtypes of nAChRs (i.e. Cholinergic Channel Activators, ChCAs) or inhibit the function of other subtypes (Cholinergic Channel Inhibitors, ChCIs).