Effects of cholinergic-rich neural grafts on radial maze performance of rats after excitotoxic lesions of the forebrain cholinergic projection system--I. Amelioration of cognitive deficits by transplants into cortex and hippocampus but not into basal forebrain

After ibotenate (10.0 mg/ml) lesions to the nucleus basalis and medial septal regions, at the source of the cortical and hippocampal branches of the forebrain cholinergic projection system, rats displayed long-lasting stable impairment in reference and working memory in both spatial (place) and associative (cue) radial maze tasks. Cell suspension transplants of cholinergic-rich fetal basal forebrain tissue dissected at embryonic day 15 substantially improved all aspects of radial maze performance to a comparable degree whether sited in cortex, hippocampus, or both regions of the host brain. No additive effects were obtained with grafts in both terminal regions, but total graft volume, assessed stereologically, showed a significant negative correlation with error scores. Rats with behaviourally effective grafts, like controls, were disrupted in the place task when tested in dim light which obscured extra-maze spatial cues. Lesioned rats were not affected by change in lighting. Grafts of cholinergic-poor fetal hippocampal tissue did not improve radial maze performance; neither did grafts of cholinergic-rich tissue placed within the host basal forebrain lesion sites. In rats with cholinergic-rich terminal grafts, cortical and hippocampal choline acetyltransferase activity was restored to control level, commensurate with site of transplant, whereas it was significantly reduced in lesioned animals and those with functionally ineffective grafts. The indiscriminate error pattern and insensitivity to changes in lighting shown by lesioned rats suggested that lesioning primarily disrupted attention rather than short- or long-term spatial or associative memory processes. Since rats with cholinergic-rich grafts showed both reduced errors and recovery of stimulus control, the data indicated that grafts affected information processing, rather than changes in motor or motivational processes. Changes in choline acetyltransferase activity and the behavioural efficacy of cholinergic-rich grafts are consistent with the involvement of acetylcholine in the behavioural deficits and recovery displayed by lesioned and grafted groups, but do not rule out contributions from other factors. The equipotency of grafts within each terminal region suggests also that there may be a considerable degree of functional cooperation between the two branches of the forebrain cholinergic projection system. Functional recovery may involve local, nonspecific synaptic or paracrine mechanisms within the target regions, since grafts were efficacious only when placed in the terminal areas, but not when sited homotopically in the basal forebrain, indicating that they did not achieve any functionally significant structural repair to the host brain at that site.

Lever pressing for food reward and changes in dopamine turnover and uric acid in rat caudate and nucleus accumbens studied chronically by in vivo voltammetry

The neurotransmitter dopamine (DA) is widely held to be involved in the maintenance of positively reinforced behaviour, on the basis of self-stimulation experiments and of the effects of drugs which alter DA transmission on self-stimulation, and on behaviours controlled by natural reinforcers. We have used the technique of linear sweep voltammetry at chronically implanted carbon paste electrodes to demonstrate that 30-min sessions of responding on a variable interval schedule for food pellet reward increases first uric acid and then homovanillic acid (HVA, the DA metabolite) in rat caudate. Ascorbic acid levels are reduced, but at later times. We have now established that very similar responses in magnitude and timing are seen in the nucleus accumbens. Furthermore, in individual animals, the response rates, the increases in dopamine turnover, and the changes in uric acid and ascorbic acid remain stable when studied periodically over a 3 month period. Our previous results are broadly consistent with those obtained by others using indirect single-point methods (ex vivo neurochemistry) or in vivo dialysis. However, our present extended observations made possible by the use of paste electrodes (rather than fibre electrodes or dialysis probes) open the possibilities of studying the evolution of neurochemical responses during learning, and the covariation of neurochemical and behavioural effects of drugs within individual animals.

Low dose tetrahydroaminoacridine (THA) improves cognitive function but does not affect brain acetylcholine in rats

Eight days of treatment with two low doses of tetrahydroaminoacridine (THA), given once daily, substantially improved radial maze performance in two groups of rats which showed persistent deficits either after ibotenic acid lesions at the source of forebrain cholinergic projections, or after 28 weeks treatment with alcohol (20% v/v) in drinking water. However, in immature, aged or aged and alcohol-treated rats, acetylcholine content was not significantly affected in any of the brain areas measured, even though the treatment regime had proved behaviourally effective. Inhibition of brain acetylcholinesterase activity was only marginally increased by this treatment regime. Thus, if THA influences behaviour by enhancing cholinergic transmission, its effects do not appear to be related to its activity as a cholinesterase inhibitor, and alternative mechanisms of action should be investigated.

Lever pressing for food reward and in vivo voltammetry: evidence for increases in extracellular homovanillic acid, the dopamine metabolite, and uric acid in the rat caudate nucleus

Linear sweep voltammetry at chronically implanted carbon paste electrodes was used to study extracellular levels of homovanillic acid, a metabolite of dopamine, in the caudate nucleus of freely moving rats. Local infusion of homovanillic acid close to the electrode confirmed that peak 3, at about 550 mV, could be used for this purpose. Thirty minutes of lever pressing for food reward was followed by an increase in homovanillic acid, maximal about 30 min later (+42%). An earlier and larger (+51%) increase in peak 2 (350-400 mV) was seen, highly correlated (r = 0.98) with the rate of lever pressing across individuals. A smaller decrease (-23%) in peak 1 (200 mV; ascorbic acid) was seen 75 min after the behavioural session. Administration of allopurinol, an inhibitor of uric acid production, selectively abolished peak 2. Allopurinol prior to lever pressing reduced the increase in peak 2 very substantially (by 77%), confirming that this too was mainly due to uric acid. These results indicate that linear sweep voltammetry can be used to study the time course of changes in dopamine metabolism, and in other neurochemicals, in individual freely moving rats in relation to behaviour under experimental control. More specifically, they demonstrate directly that rewarded lever pressing increases dopamine metabolism in the caudate, which had previously only been inferred from indirect approaches. The level of uric acid is also increased, and it is suggested that this may serve as an index of metabolic activity.

Are the effects of benzodiazepines on discrimination and punishment dissociable?

Studies have shown that benzodiazepines (BZs) both disrupt discrimination and increase resistance to punishment. Using a delayed response task, we provide evidence that effects of BZs on discrimination cannot be fully explained by deficits in either short or long term memory, or by intolerance for delay of reward. A schedule with rewarded, nonrewarded (Time out: TO) and conflict components was used to investigate effects in rats of compounds active at the BZ receptor on successive discrimination and punished responding in parallel. The GABA transaminase inhibitor ethanolamine-O-sulphate exerted additive effects with chlordiazepoxide (CDP) on punished but not TO responding. Both GABA and CDP injected into the amygdala selectively increased conflict rates, but with peripheral treatment CDP also increased TO rates. Two inverse BZ agonists, CGS 8216 and FG 7142 antagonzied the anti-conflict effects of GABA and CDP, given within the amygdala or peripherally, but the increase in TO rates induced by systemic CDP was counteracted only by peripheral treatments. These compounds also reduced rates of conflict responding below baseline, consistent with anxiogenic activity. Effects of the BZ antagonist Ro 15-1788 were broadly similar to those of the inverse agonists, except that it did not antagonise the anti-conflict action of intra-amygdaloid GABA, nor significantly reduce punished responding at the single dose used. We conclude from these results that the anti-conflict effects of BZs are mediated by a GABAergic amygdaloid mechanism, but that the same mechanism is not involved in BZ effects on discrimination.

Evidence that the amygdala is involved in benzodiazepine and serotonergic effects on punished responding but not on discrimination

Interactions between the benzodiazepines (BZs) chlordiazepoxide (CDP) and midazolam (MDZ), the BZ antagonist R0 15-1788, the inverse BZ receptor agonists CGS 8216 and FG 7142, gamma-aminobutyrate (GABA), serotonin (5-HT), the 5-HT2 antagonist methysergide and the putative 5-HT agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) were investigated using peripheral and intra-amygdaloid treatments. A multiple schedule consisting of rewarded, nonrewarded (Time out: TO) and conflict periods was used to compare in parallel effects on successive discrimination between rewarded and nonrewarded periods and punished responding. The three components were presented in both a fixed order (Experiment 1) and a random order (Experiments 2 and 3). Intra-amygdaloid treatments with GABA and the BZs selectively increased rates of punished responding. CDP given systemically, on the other hand, increased both TO and conflict rates, suggesting an additional impairment of discrimination, which was more marked in the random than the fixed order condition. R0 15-1788, CGS 8216 and FG 7142 given by both routes counteracted the anti-conflict effects of CDP given centrally or systemically. However increases in TO rates induced by IP CDP were antagonized only by IP treatments with these compounds. The two inverse agonists, but not R0 15-1788, also counteracted increases in punished responding which were found after intra-amygdaloid GABA infusions. In Experiments 2 and 3 where baseline rates of pressing in Conflict periods were sufficiently high to detect decreases, CGS 8216 and FG 7142 reduced responding below control level, suggesting a specific anxiogenic activity. Evidence for effects of R0 15-1788 by itself was inconclusive. 5-HT injected into the amygdala also reduced punished responding below control level, whereas methysergide increased it with both central and peripheral treatment. Effects of 8-OH-DPAT varied according to route of administration. With IP treatment Conflict rates were increased, but after amygdaloid infusion both TO and Conflict rates were marginally reduced below control level, with a more consistent depression of punished responding. These results provide evidence that effects of BZs on punished responding are mediated by a GABAergic system which includes the lateral/basolateral amygdala, but which does not participate in BZ-induced disruption of discrimination. They also indicate that the antagonistic effects of CGS 8216 and FG 7142 involve a decrease in GABA transmission, and that these compounds may also be anxiogenic.(ABSTRACT TRUNCATED AT 400 WORDS)

Differential effects of dorsal raphe lesions and intraraphe GABA and benzodiazepines on conflict behavior in rats

Both reductions in brain serotonin activity and injections of benzodiazepine drugs increase punished responding in rats, but the evidence is conflicting on the role of serotonin pathways in the benzodiazepine effect. Therefore a series of studies were carried out using a Geller-Seifter procedure with three components, to examine drug effects on rewarded, nonrewarded, and punished responding. Using male hooded Lister rats and chronic indwelling cannulae, it was found that neither chlordiazepoxide (1.5 and 5.0 micrograms in 0.5 microliter), midazolam (1.0 and 10.0 micrograms in 0.5 microliter), nor GABA (100, 500, 1000, and 5000 ng in 0.5 microliter), exerted significant anticonflict activity when injected into the dorsal raphe. Lesions of the dorsal raphe produced by injections of 5,6-dihydroxytryptamine significantly increased punished responding, and there were significant correlations between lesion size, extent of forebrain serotonin depletion, and increases in punished responding. Peripheral injections of chlordiazepoxide (5.0 and 10.0 mg/kg) and midazolam (1.25, 2.5, and 5.0 mg/kg) significantly increased punished responding both before and after raphe lesions. The increase in lesioned animals was significantly greater than after drug or lesion alone and represented a powerful additive effect which was specific to punished responding. As intraraphe benzodiazepines did not exert significant anticonflict activity, and raphe lesions did not attenuate the anticonflict activity of peripheral benzodiazepines, it is concluded that increases in punished responding seen after serotonin depletion and after benzodiazepine drugs may be dissociable.

Behavioural and pharmacological dissociation of chlordiazepoxide effects on discrimination and punished responding

Effects of chlordiazepoxide (CDP) and ethanolamine-O-sulphate (EOS) alone and in combination were tested on the acquisition and performance of continuous reinforcement - time out (CR-TO) and variable interval reinforcement - time out (VI-TO) operant discriminations in rats. CDP disrupted acquisition of CR-TO discrimination; effects were short lived, and neither CR-TO performance nor its reversal were impaired. Acquisition of VI-TO discrimination was increasingly impaired, and performance disrupted in pre-trained animals. EOS alone was inactive, and with CDP exerted only slight interactive effects. When a conflict component was added to the VI-TO schedule, however, EOS showed substantial additive effects with CDP on punished responding. The results suggest that CDP-induced increases in non-rewarded (TO) responding were related to task difficulty, pointing to a discrimination impairment rather than an anxiolytic effect. In addition, the specificity of EOS potentiation may reflect a pharmacological dissociation between CDP effects on discrimination and on punished responding, and suggest that GABA is selectively involved in resistance to punishment.

Effects of chlordiazepoxide on cued radial maze performance in rats

Effects of chlordiazepoxide (CDP) were examined on the performance of rats in an eight-arm radial maze with four cued and food-baited arms. Two conditions were used; random, with cue location varying over trials, and constant, with the same subset of arms consistently cued. In rats pre-trained to a 60-70% efficiency level (Rewarded entries/Total entries X 100), the effects of CDP (2.5, 5.0 and 10.0 mg/kg, IP) differed according to condition. Efficiency was substantially reduced in the random condition, and types of error were undifferentiated. In the constant cue condition the post-drug drop in efficiency was less marked, and errors were selectively those of re-entry into rewarded arms. In both conditions there was a high incidence of error clusters involving re-entries into both rewarded and non-rewarded arms in the random condition, and rewarded arms in the constant condition. The results suggested that CDP induced a general disruption of information processing rather than a specific impairment of working memory, together with some response perseveration which could occur in the presence or absence of discrimination failure.