Influence of post-training intrahippocampally applied oxotremorine on the consolidation of a brightness discrimination

The role of acetylcholine in cocaine addiction

Central nervous system cholinergic neurons arise from several discrete sources, project to multiple brain regions, and exert specific effects on reward, learning, and memory. These processes are critical for the development and persistence of addictive disorders. Although other neurotransmitters, including dopamine, glutamate, and serotonin, have been the primary focus of drug research to date, a growing preclinical literature reveals a critical role of acetylcholine (ACh) in the experience and progression of drug use. This review will present and integrate the findings regarding the role of ACh in drug dependence, with a primary focus on cocaine and the muscarinic ACh system. Mesostriatal ACh appears to mediate reinforcement through its effect on reward, satiation, and aversion, and chronic cocaine administration produces neuroadaptive changes in the striatum. ACh is further involved in the acquisition of conditional associations that underlie cocaine self-administration and context-dependent sensitization, the acquisition of associations in conditioned learning, and drug procurement through its effects on arousal and attention. Long-term cocaine use may induce neuronal alterations in the brain that affect the ACh system and impair executive function, possibly contributing to the disruptions in decision making that characterize this population. These primarily preclinical studies suggest that ACh exerts a myriad of effects on the addictive process and that persistent changes to the ACh system following chronic drug use may exacerbate the risk of relapse during recovery. Ultimately, ACh modulation may be a potential target for pharmacological treatment interventions in cocaine-addicted subjects. However, the complicated neurocircuitry of the cholinergic system, the multiple ACh receptor subtypes, the confluence of excitatory and inhibitory ACh inputs, and the unique properties of the striatal cholinergic interneurons suggest that a precise target of cholinergic manipulation will be required to impact substance use in the clinical population.

Permanent and temporary inactivation of the hippocampus impairs T-maze footshock avoidance acquisition and retention

The hippocampus is widely recognized as playing an important role in learning and memory. Lesions of the hippocampus can disrupt spatial navigational learning and memory and injection of drugs into the hippocampus can affect both spatial navigational and nonspatial tasks. In the current studies we tested the effects of bilateral of electrolytic lesions and reversible inactivation of the hippocampus on acquisition and retention of T-maze footshock avoidance conditioning. Electrolytic lesions, which destroyed 31+/-0.04% of the hippocampus, significantly impaired acquisition and retention for T-maze footshock avoidance. No differences were found in motivation to avoid shock, open field activity, or foot shock sensitivity between lesion and control groups. Temporary inactivation of the hippocampus with lidocaine administered immediately before training disrupted acquisition and retention for T-maze footshock avoidance. Temporary hippocampal inactivation performed just prior to retention testing and post-training inactivation in mice trained to first avoidance had no effect on retention. However, temporary post-training inactivation in 'undertrained' (enough trials to remember 1 week later if treated with saline, but not allowed to make the avoidance response) mice impaired retention. The current findings indicate that the hippocampus plays an important role in learning and memory processing in the aversive T-maze paradigm.

The effect of cholinergic, GABAergic, serotonergic, and glutamatergic receptor modulation on posttrial memory processing in the hippocampus

Though the hippocampus is widely recognized as important in learning and memory, most of the evidence for this comes from animal lesion and human pathological studies. Due to the relatively small number of drugs that have been tested in the hippocampus for their ability to alter posttrial memory processing, there is a general impression that memory processing involves only a few neurotransmitters. We have evaluated the effects of cholinergic, GABAergic, serotonergic, and glutamatergic receptor agonists and antagonists for their ability to facilitate or impair retention. CD-1 mice received acute intrahippocampal drug infusion following footshock avoidance training in a T-maze. Retention was tested 1 week after training and drug administration. The results indicate that receptor agonists of acetylcholine and glutamate improved retention, while antagonists impaired retention. However, scopolamine did not impair retention, but M1 and M2 antagonists did. Receptor agonists of serotonin and GABA impaired retention, while antagonists improved retention. Drugs acting on 5-HT-1 and 5-HT-2 as well as GABA(A) and GABA(B) receptor subtypes did not differentially effect retention.

Modulation of memory processing in the cingulate cortex of mice

To evaluate the possible role of the cingulate cortex in memory processing for training using a noxious stimulus, we trained mice on foot shock avoidance in a T-maze. Cholinergic, GABAergic, serotonergic, and glutamatergic agonists and antagonists were administered into the cingulate cortex immediately after training. Retention for the foot shock avoidance training was tested 1 week later. The results indicate that muscarinic and nicotinic agonists improved retention, while antagonists impaired it. GABA and serotonin agonists impaired retention, while antagonists improved it. Drugs acting on GABA(A) and GABA(B) receptors had similar effects on retention, as did drugs acting on serotonin 1 and 2 receptor subtypes. Glutamate improved retention, and AP5, an antagonist of the excitatory amino acid site of the NMDA receptor, impaired retention. The cingulate cortex, like other parts of the limbic system, is involved in memory processing that occurs shortly after training.

Contribution of cholinergic and gabaergic functions to memory processes in BALB/cANnCrlBR mice

Several lines of evidence indicate that glucose influences on memory depend on interactions between glucose, glucoregulation and hippocampal cholinergic function. We previously demonstrated that glucose and scopolamine differentially affected memory consolidation for an operant bar pressing task in two closely-related BALB/c mouse strains. Whereas glucose normally improves memory in several animal strains, memory consolidation was not effected by systemic glucose injections in BALB/cANnCrlBR mice. Moreover, these mice were relatively insensitive to the normally observed amnestic effects of scopolamine. We therefore sought to determine whether cholinergic mechanisms in the dorsal hippocampus were involved in such atypical drug effects on memory processing in that strain of mice. In Experiment 1, we examined whether post-training oxotremorine would also atypically influence memory consolidation for an appetitively reinforced operant bar pressing task following microinjection in the dorsal hippocampus. In Experiment 2, we examined the effects of intrahippocampal GABAA drugs on memory consolidation. The non-selective muscarinic agonist, oxotremorine, dose-dependently impaired memory and the GABAA antagonist, bicuculline, improved retention in BALB/cANnCrlBR mice. It was concluded that GABA-mediated influences on hippocampal pyramidal output in BALB/cANnCrlBR mice and other strains are similar; but the amnestic effects of oxotremorine from the dorsal hippocampus were opposite to facilitating effects normally observed in other animal strains. Results are discussed relative to possible altered septo-hippocampal cholinergic neurotransmission in BALB/cANnCrlBR mice.

Kindling of the dorsal and the ventral hippocampus: effects on learning performance in rats

The hippocampus represents a heterogeneous structure which has been associated with different functions. It has been suggested that it plays an important role in both learning and memory and epileptogenesis. Thus, it is not surprising that seizure activity generated in the hippocampal formation interferes with memory storage. Little is known about the functional differentiation between the dorsal (DH) and ventral hippocampus (VH). To study this functional differentiation, we kindled Wistar rats either in the DH or in the VH by electrical stimulation. Afterwards, learning performance of these rats was tested in three different models, i.e., response to change (short-term memory), shuttle box (two-way active avoidance), and Y-chamber (brightness discrimination reaction). It was found that VH-kindled rats reached higher seizure scores than DH-kindled rats, but there was no difference in seizure duration. Kindling induced in the VH significantly impaired shuttle box learning, whereas DH-kindled rats showed a dramatically worsened acquisition in the brightness discrimination task. Different anatomical projections probably account, in part, for these differences.

Complex patterns of immediate early gene induction in rat brain following brightness discrimination training and pseudotraining

Following training of rats on a footshock-motivated brightness discrimination task in a Y-maze, different sets of transcription factor encoding immediate early genes (IEGs) were induced in anatomically distinct brain regions. As revealed by Northern analysis, mRNA levels of c-fos, jun-B and zif/268 increased in the hippocampus, while the expression of c-jun remained unchanged over a period of 7 h. In the cerebral cortex, c-jun was induced in addition to the other genes examined. In contrast, only c-fos, but not c-jun or zif/268 mRNAs were increased in the cerebellum. The induction of IEGs was rapid and transient, reaching maximal levels immediately after training and returning to basal levels within 2 h. Similar spatiotemporal expression patterns were observed in rats that received identical, but unpaired, stimuli in a pseudotraining procedure. Our results suggest that the initial prerequisites of learning, such as stimulus novelty, lead to an increased expression of IEG mRNAs after training and pseudotraining as an early necessary but not sufficient precondition for memory consolidation. Additional converging inputs might control at the transcriptional, translational or post-translational level the synthesis and biological effectiveness of proteins necessary to complete the formation of the memory trace in trained animals.

Strain-dependent effects of dopamine agonists on acetylcholine release in the hippocampus: an in vivo study in mice

The effects of selective D1 or D2 dopamine receptor agonists and the indirect dopamine agonist cocaine on hippocampal acetylcholine release in mice of the C57BL/6 and DBA/2 inbred strains were investigated using intracerebral microdialysis. The D1 SKF 38393 (10, 20, 30 mg/kg, i.p.), the D2 agonist LY 171555 (0.5, 1, 2 mg/kg, i.p.) and cocaine (5, 10, 15 mg/kg, i.p.) all increased, dose-dependently, acetylcholine release in the hippocampus of C57BL/6 mice. Both the D1 agonist and cocaine did not produce any significant effect in DBA/2 mice. In the latter strain, however, LY 171555 produced a decrease in acetylcholine release that was evident after 60 min from injection of the doses of 0.5 and 1 mg/kg, but not at the dose of 2 mg/kg. The effects observed in C57BL/6 mice as well as those produced by low doses of LY 171555 in the DBA/2 strain were consistent with previous results obtained in rats. The present results indicate major strain-dependent differences in the effects of dopamine agonists on hippocampal acetylcholine release in mice. Moreover, they suggest a complex genotype-related neural organization of dopamine-acetylcholine interactions in the mesolimbic system. Finally, the strain differences in the effects of the dopamine agonists on hippocampal acetylcholine release parallel previously reported strain differences in the effects of these substances on memory consolidation.

Stimulation of both dopamine D1 and D2 receptors facilitates in vivo acetylcholine release in the hippocampus

The effect of selective D1 and D2 dopamine (DA) receptor agonists and of a mixed D1/D2 agonist on hippocampal acetylcholine (ACh) release was investigated. LY 171555 (0.5 and 1 mg/kg, i.p.), SKF 38393 (1 to 10 mg/kg, i.p.), CY 208-243 (0.2 mg/kg, i.p.) and apomorphine (0.5 to 2 mg/kg, i.p.), at doses stimulating rat behavior, were found to increase the output of ACh in the hippocampus. Maximal increase was observed after LY 171555 1 mg/kg, SKF 38393 10 mg/kg, CY 208-243 2 mg/kg and apomorphine 2 mg/kg (85, 90, 87 and 210%, respectively). The enhancement of ACh release induced by either SKF 38393 (10 mg/kg) or LY 171555 (1 mg/kg) was prevented by the blockade of D1 receptors with SCH 23390 (0.1 mg/kg, s.c.). Co-administration of maximally active doses of LY 171555 (1 mg/kg) and SKF 38393 (10 mg/kg) produced an additive effect (about 200%). In contrast to the findings with high doses, low, presynaptic doses of LY 171555 and apomorphine reduced ACh output. Maximal reduction was observed after 0.05 mg/kg for both drugs, (43 and 52%, respectively). These results show that activation of dopaminergic transmission either at D1 and/or at D2 receptors enhances ACh output in the hippocampus.

Learning-induced changes in D2 receptors of rat brain are sexually dimorphic

Pharmacological agents known to stimulate monoamine systems improve memory, and destruction of the dopaminergic systems or dopamine depletion lead to impairments in various learning-related tasks. These reported effects of the central dopaminergic system imply the involvement of D2 receptors. The aim of the present study was to investigate changes in [3H]spiroperidol binding in seven areas of rat brain following informal and active avoidance learning. Littermate male and female rats were reared until 3 months of age in standard colony conditions and treated as active controls or in enriched environmental conditions and exposed to pole-jump active avoidance trials. Female rats acquired avoidance behavior more rapidly than males. Among the brain regions, only the hippocampus showed significant variations in D2 receptor binding between the groups; sex differences and learning-sex interaction were observed in the corpus striatum. There was an inverse correlation between learning performance and hippocampal D2 receptor binding. Our results show that learning affects hippocampal D2 receptors in a sexually dimorphic pattern.

Kindling and its consequences on learning in rats

To study the learning performance of pentylenetetrazol- and amygdala-kindled Wistar rats we used the following learning tests: short-term memory was tested in the response-to-change model, brightness discrimination was tested in a Y-chamber, and two-way active avoidance learning was tested in a shuttle-box. Short-term memory was not impaired by both kindling procedures. Considering two-way active avoidance learning the performance of pentylenetetrazol (PTZ)-kindled rats was significantly diminished. This effect persists over a period of 4 weeks. However, amygdala (AMY)-kindled rats acquired this task like the controls. In brightness discrimination reaction (BDR) the learning performance of PTZ-kindled animals was not influenced. Although the acquisition of BDR was nearly identical, the 24-h retention was remarkably diminished in AMY-kindled rats. It was hypothesized that the different kindling procedures interfere in different ways and extent with neuronal circuits resulting in different functional impairments.

Amygdala and dorsal hippocampus lesions block the effects of GABAergic drugs on memory storage

These experiments examined the effects of posttraining systemic administration of the GABAergic agonist muscimol and the GABAergic antagonist bicuculline on retention in mice with bilateral lesions of the amygdala, dorsal hippocampus or caudate nucleus. Unoperated male CD1 mice and mice with either sham lesions or electrolytically induced lesions of these 3 brain regions were trained in a one-trial inhibitory avoidance task and, immediately after training, received i.p. injections of either muscimol, (1.0, 2.0 or 3.0 mg/kg), bicuculline, (0.25, 0.5 or 1.0 mg/kg), or control solutions. Retention was tested 24 h after training. Lesions of the 3 brain regions produced comparable impairment of retention. In the unoperated controls and sham controls muscimol and bicuculline produced dose-dependent impairment and enhancement, respectively, of retention. The drug effects on retention were blocked by lesions of the amygdala and hippocampus, but were not blocked by lesions of the caudate nucleus. These findings are consistent with other recent evidence suggesting that the amygdala and hippocampus are involved in mediating posttraining neuromodulatory influences on memory storage.

Post-training injection of the acetylcholine M2 receptor antagonist AF-DX 116 improves memory

The present study examined the effect of systemic post-training administration of the acetylcholine muscarinic M2 receptor antagonist AF-DX 116 on the acquisition of two 8-arm radial maze tasks. On a win-stay visual discrimination task, a light cue signalled the location of food in 4 randomly selected maze arms, and rats were required to visit each of the 4 lit arms twice within a trial. Rats were given one trial per day and injected immediately post-training on day 5. AF-DX 116 (0.5 and 1.0 mg/kg) significantly improved win-stay acquisition relative to vehicle-injected controls. On a win-shift task, rats were allowed to visit 4 randomly selected maze arms, followed by a delay period. After the delay, rats were returned to the maze for a retention test in which only those 4 arms not visited prior to the delay contained food. On the test (i.e. drug) trial, rats were removed from the maze after the first 4 choices and injected with AF-DX 116 or vehicle. The retention test was given following an 18 h delay. AF-DX 116 (2.0 mg/kg) significantly improved retention relative to vehicle controls. When the injections were given 2 h post-training, no effect on retention was observed in either task. The results demonstrate that post-training injection of the selective M2 receptor antagonist AF-DX 116 improves memory in a time-dependent manner. The findings may have implications for the cholinergic pharmacotherapy of Alzheimer's disease.

Memory facilitation produced by dopamine agonists: role of receptor subtype and mnemonic requirements

The role of dopamine (DA) receptor subtypes in the acquisition of two memory tasks in the 8-arm radial maze was examined. The receptors were manipulated with posttraining, subcutaneous injections of an indirect DA receptor agonist (D-amphetamine), a selective D2 receptor agonist (LY171555), and a selective D1 receptor agonist (SKF-38393). On a win-stay task (sensitive to caudate nucleus lesions) a light cue signalled the location of food in 4 randomly selected arms on each trial. Rats were given one trial per day and injected after training on day 5. D-Amphetamine (2.0 mg/kg) and LY171555 (2.0 mg/kg) improved performance relative to controls; however SKF-38393 (1-4 mg/kg) had no effect on the acquisition of win-stay behavior. On a win-shift task (sensitive to fornix/hippocampal lesions) a delay of 18 hr was imposed between the first 4 and second 4 choices; drugs were injected after the first 4 choices. D-Amphetamine (1.0 mg/kg) and LY171555 (2.0 mg/kg) significantly improved retention relative to controls. SKF-38393 (1-4 mg/kg) had no effect on win-shift retention. These results suggest that the memory-improving properties of DA agonists on tasks sensitive to both hippocampal and caudate lesions are mediated by the D2 receptor.

Memory retention: effect of prolonged cholinergic stimulation in mice

The fundamental hypothesis that drugs may affect memory processing by prolonging transmitter action was tested by extending the time of drug action, using repeated administrations of the cholinergic agonist, arecoline hydrobromide (ARE). The ARE was injected intracerebroventricularly into mice immediately after training (T-maze footshock avoidance) and at 90-min intervals thereafter, for a total of 1, 2, or 3 injections. The results indicate that 1 injection had no effect whereas 3 successive injections significantly improved memory retention test performance. The results confirm the hypothesis being tested; six control groups ruled out other plausible interpretations of the results.

Cholinergic receptor interactions and their effects on long-term memory processing

Mice were treated on active avoidance to determine the effect of cholinergic and anticholinergic drugs on retention. All drugs were administered intraventricularly after training and one week prior to testing retention. A dose-response curve was determined for each drug. The results indicate that each of 6 anticholinergics impaired retention. The 8 cholinergic drugs tested all yielded U-shaped dose-response curves with controls showing poor retention, low doses of the drugs yielding good retention and high doses resulting in poor retention.

Effect of n-dipropylacetate on the consolidation of a brightness discrimination

The posttraining intrahippocampal injection of the GABA level enhancing substance n-dipropylacetate revealed an improvement of the retention performance in a brightness discrimination task in rats.