Opposite effects of cholinergic agents and benzodiazepine receptor ligands in a passive avoidance task in rats

Increase in cortical endocannabinoid signaling in a rat model of basal forebrain cholinergic dysfunction

The basal forebrain cholinergic pathways progressively degenerate during the progression of Alzheimer's disease, leading to an irreversible impairment of memory and thinking skills. The stereotaxic lesion with 192IgG-saporin in the rat brain has been used to eliminate basal forebrain cholinergic neurons and is aimed at emulating the cognitive damage described in this disease in order to explore its effects on behavior and on neurotransmission. Learning and memory processes that are controlled by cholinergic neurotransmission are also modulated by the endocannabinoid (eCB) system. The objective of the present study is to evaluate the eCB signaling in relation to the memory impairment induced in adult rats following a specific cholinergic lesion of the basal forebrain. Therefore, CB₁ receptor-mediated signaling was analyzed using receptor and functional autoradiography, and cellular distribution by immunofluorescence. The passive avoidance test and histochemical data revealed a relationship between impaired behavioral responses and a loss of approximately 75% of cholinergic neurons in the nucleus basalis magnocellularis (NBM), accompanied by cortical cholinergic denervation. The decrease in CB₁ receptor density observed in the hippocampus, together with hyperactivity of eCB signaling in the NBM and cortex, suggest an interaction between the eCB and cholinergic systems. Moreover, following basal forebrain cholinergic denervation, the presynaptic GABAergic immunoreactivity was reduced in cortical areas. In conclusion, CB₁ receptors present in presynaptic GABAergic terminals in the hippocampus are down regulated, but not those in cortical glutamatergic synapses.

Selective effects of benzodiazepines on the acquisition of conditioned taste aversion compared to attenuation of neophobia in C57BL/6 mice

INTRODUCTION

The effects of pre-conditioning administration of anxiolytic benzodiazepines on the acquisition of a conditioned taste aversion (CTA) and on the acquisition of attenuation of neophobia (AN) were investigated in C57BL/6 mice.

MATERIALS AND METHODS

A CTA was induced by injecting lithium chloride (LiCl; 6 mEq x kg(-1)) 30 min after the animal had imbibed a novel 0.5% saccharin solution. In other animals, neophobia was attenuated by a single access to the novel 0.5% saccharin solution, followed only by injection of saline.

RESULTS AND DISCUSSION

Pre-conditioning administration of chlordiazepoxide (CDZ; 6-24 mg x kg(-1), i.p.) and alprazolam (0.3-1 mg x kg(-1), p.o.) resulted in a CTA that did not differ initially from that observed in vehicle-treated controls, but which showed faster extinction. The acquisition of AN was impaired only after the higher doses of CDZ (12-24 mg x kg(-1), i.p.) or alprazolam (1 mg x kg(-1), i.p.). The results show that in this test, altered acquisition of an aversive CTA memory by anxiolytic benzodiazepines is reflected in more rapid extinction. Moreover, at low doses, these drugs showed selectivity for weakening CTA learning compared to AN learning. Evidence is discussed that selective weakening of aversive memory formation is a clinically relevant effect of anxiolytic benzodiazepines.

GABAA receptor RDL inhibits Drosophila olfactory associative learning

In both mammals and insects, neurons involved in learning are strongly modulated by the inhibitory neurotransmitter GABA. The GABAA receptor, resistance to dieldrin (Rdl), is highly expressed in the Drosophila mushroom bodies (MBs), a group of neurons playing essential roles in insect olfactory learning. Flies with increased or decreased expression of Rdl in the MBs were generated. Olfactory associative learning tests showed that Rdl overexpression impaired memory acquisition but not memory stability. This learning defect was due to disrupting the physiological state of the adult MB neurons rather than causing developmental abnormalities. Remarkably, Rdl knockdown enhanced memory acquisition but not memory stability. Functional cellular imaging experiments showed that Rdl overexpression abolished the normal calcium responses of the MBs to odors while Rdl knockdown increased these responses. Together, these data suggest that RDL negatively modulates olfactory associative learning, possibly by gating the input of olfactory information into the MBs.

Modulation of passive avoidance in mice by the 5-HT1A receptor agonist flesinoxan: comparison with the benzodiazepine receptor agonist diazepam

The effects of the 5-HT(1A) receptor agonist flesinoxan on passive avoidance in mice were compared with those of the benzodiazepine receptor agonist diazepam. In preliminary experiments, the retention latency to enter a dark compartment in mice subjected to single-training sessions with 0.6-mA electric foot shocks for 4, 8, or 16 s slightly increased in all of the test sessions (immediately, 24 h, and 1 week after the training sessions), but none of these changes were significant. In contrast, mice subjected to double-training sessions with 0.6-mA electric foot shocks for 16 s showed a significant increase in retention latency in all of the test sessions. Pretreatment with either flesinoxan or diazepam 30 min before the double-training sessions with 0.6-mA electric foot shocks for 16 s significantly decreased the retention latency in test sessions 24 h and 1 week later. In contrast, mice pretreated with flesinoxan 24 h before the single-training sessions with 0.6-mA electric foot shocks for 4, 8, or 16 s showed a significant increase in retention latency in the test sessions 24 h and/or 1 week later. Similar enhancements of retention latency in the test sessions 24 h and/or 1 week later were observed also in mice pretreated with flesinoxan 24 h before the double-training sessions. However, in this time interval following injection, pretreatment with diazepam did not affect the retention latency of mice in any of the test sessions. Neither flesinoxan nor diazepam, at the same doses and time intervals used in the passive avoidance study, modified the thresholds for flinching and jumping elicited by electrical stimuli. These results suggest that the activation of 5-HT(1A) receptors, but not benzodiazepine receptors, has a dual effect on the formation of learning and memory for an aversive event that depends on the time interval following receptor activation.

Effects of hippocampal injections of a novel ligand selective for the alpha 5 beta 2 gamma 2 subunits of the GABA/benzodiazepine receptor on Pavlovian conditioning

Benzodiazepine pharmacology has led to greater insight into the neural mechanisms underlying learning and anxiety. The synthesis of new compounds capable of modulating responses produced by these receptors has been made possible by the development of an isoform model of the GABA(A)/benzodiazepine receptor complex. In the current experiment, rats were pretreated with several concentrations of the novel ligand RY024 (an alpha 5 beta 2 gamma 2 -selective benzodiazepine receptor inverse agonist) in the hippocampus and were trained in a Pavlovian fear conditioning paradigm. RY024 independently produced fear-related behavior prior to training and, at the highest concentration, decreased the strength of conditioning observed 24 h after training. These data provide further evidence for the involvement of hippocampal GABA(A)/benzodiazepine receptors in learning and anxiety.

Cholinergic modulation of inhibitory avoidance impairment induced by paradoxical sleep deprivation

1. Male Wistar rats were submitted to paradoxical sleep deprivation for 96 hr by a modified multiple platform technique. 2. Training of step-through inhibitory avoidance was performed immediately after the last day of paradoxical sleep deprivation. Twenty-four hr after training the animals were submitted to the retention test. 3. In Experiment 1, pilocarpine (4 mg/kg, i.p.) or atropine (4 mg/kg, i.p.) were administered daily during the paradoxical sleep deprivation period. Pilocarpine, but not atropine, reversed the impairment induced by PS deprivation. 4. In Experiment 2, pilocarpine (4, 8 and 12 mg/kg, i.p.) was injected 1 hr before training in order to verify if the reversal of memory impairment was an effect secondary to residual enhanced blood levels of pilocarpine during training. Acute treatment with pilocarpine, in any dose, did not reverse the impairment produced by paradoxical sleep deprivation 5. Activation of the cholinergic system during the period of deprivation is able to prevent memory deficits induced by paradoxical sleep deprivation.

Pharmacophore/receptor models for GABA(A)/BzR subtypes (alpha1beta3gamma2, alpha5beta3gamma2, and alpha6beta3gamma2) via a comprehensive ligand-mapping approach

Pharmacophore/receptor models for three recombinant GABA(A)/BzR subtypes (alpha1beta3gamma2, alpha5beta3gamma2, and alpha6beta3gamma2) have been established via an SAR ligand-mapping approach. This study was based on the affinities of 151 BzR ligands at five distinct (alpha1-3,5,6beta3gamma2) recombinant GABA(A)/BzR receptor subtypes from at least nine different structural families. Examination of the included volumes of the alpha1-, alpha5-, and alpha6-containing subtypes indicated that region L(2) for the alpha5-containing subtype appeared to be larger in size than the analogous region of the other receptor subtypes. Region L(Di), in contrast, appeared to be larger in the alpha1 subtype than in the other two subtypes. Moreover, region L(3) in the alpha6 subtype is either very small or nonexistent in this diazepam-insensitive subtype (see Figure 16 for details) as compared to the other subtypes. Use of the pharmacophore/receptor models for these subtypes has resulted in the design of novel BzR ligands (see 27) selective for the alpha5beta3gamma2 receptor subtype. alpha5-Selective ligand 27 when injected directly into the hippocampus did enhance memory in one paradigm (Bailey et al., unpublished observations); however, systemic administration of either 9 or 27 into animals did not provide an observable enhancement. This result is in complete agreement with the observation of Liu (1996). It has been shown (Liu, 1996; Wisden et al., 1992) that in the central nervous system of the rat (as well as monkeys and pigeons) there are several native subtypes of the GABA(A) receptor which exhibit different functions, regional distributions, and neuronal locations. Although 27 binds more potently at alpha5beta3gamma2 receptor subtypes and is clearly an inverse agonist (Liu et al., 1996; Liu, 1996), it is possible that this ligand acts as an agonist at one or more subtypes. Liu (1996) clearly showed that a number of imidazobenzodiazepines were negative modulators at one subtype and agonists at another. Therefore, selectivity for a particular subtype at this point is not sufficient to rule out some physiological effect at other GABA(A)/BzR subtypes. The inability of 27 to potentiate memory when given systemically is again in support of this hypothesis, especially since alpha1beta2gamma2 subtypes are distributed throughout the brain (Wisden et al., 1992). A drug delivered systemically is far more likely to interact with all subtypes than one delivered to a specific brain region. This observation (systemic vs intrahippocampal) provides further support for the design of more subtype-specific ligands at the BzR to accurately define their pharmacology, one key to the design of new drugs with fewer side effects.

Olfactory memory in rats, cholinergic agents and benzodiazepine receptor ligands

Drugs and their effects on olfactory learning processes in rats were tested using a modified version of the runway apparatus developed by Ades. Rats were first exposed to a conspecific urine sample and 24 h later were exposed to the same stimulus in the runway. Observations recorded the time spent investigating the urine and the number of sniffs at the site, these being considered to be indices of memory. Diazepam-treated rats (4 or 6 mg/kg) and scopolamine-treated rats (0.5 or 1 mg/kg) showed increases for both parameters. When both drugs were administered simultaneously, the impairing effect was potentiated. However, no changes in learning responses were observed in rats treated with physostigmine (0.125, 0.25, 0.5 mg/kg) or methyl beta-carboline-3-carboxylate (0.3, 0.5, 1 mg/kg), although the administration of physostigmine or methyl beta-carboline-3-carboxylate was shown to antagonize the impairing effect of diazepam or scopolamine respectively. These observations support the hypothesis of interactions existing between cholinergic agents and benzodiazepine receptor ligands and of such interactions affecting olfactory acquisition processes. The runway apparatus appears to be a valid candidate model to be used for the assessment of pharmacological influences on olfactory learning in rats.

The decrease of serotonin release induced by a tryptophan-free amino acid diet does not affect spatial and passive avoidance learning

We assessed whether consumption of a diet lacking in tryptophan (TRP) resulted in alteration in learning and memory performance and hippocampal 5-HT release in rats. Two hours after the acute administration of TRP-free (T) and balanced (B) diet rats were trained in a one-trial passive avoidance task. The two groups of rats showed no significant difference in retention latencies. Two other groups of rats, fed with the above diets during the acquisition of a radial-arm maze task, showed no difference in baseline performance. The acute ingestion of the T diet produced a significant and long lasting decrease of hippocampal and cortical 5-HT release in rats when compared to the B diet, while the 12th day of the T diet, 5-HT was not detectable in the dialysate. These data indicate that the diminished brain release of 5-HT induced by a T diet is not sufficient to impair cognitive processes.

Effect of apamin, a selective blocker of Ca2+-activated K+-channel, on habituation and passive avoidance responses in rats

The effects of apamin, a selective blocker of the small conductance K(Ca) channels were examined in both passive avoidance and habituation of exploratory activity in rats. In the two experiments, animals were subjected to two trials separated by a 24 h interval. Apamin was administered either before or after the first session (acquisition) or before the second session (restitution). In the passive avoidance test, apamin did not alter performance whenever the time of administration. In the habituation task, apamin (0.4 mg/kg) decreased activity (distance travelled, rearing) on the two sessions only when it was injected before acquisition but not when injection took place just after the acquisition session or before the restitution session. Taken together, these findings support the view that the blockade of apamin sensitive K(Ca) channels improved the acquisition in non-stressful task, but not in a stressful situation.

Impulsivity as a confounding factor in certain animal tests of cognitive function

Performance in cognitive tasks which require the subject to wait and/or to process a large amount of information can be disrupted by an increase in impulsive-like behaviour. Accordingly, a decrease in impulsive-like behaviour can improve performance in such tasks. Conversely, impulsive-like behaviour may improve performance in cognitive tasks where simple and fast responses and/or only little information processing is required. Thus, impulsivity constitutes a confounding factor in studies of cognitive function. Impulsive-like behaviour may be modified by serotonergic (5-HT) activity, with underactivity in 5-HT neurotransmission increasing impulsivity and vice versa. Drug- or lesion-induced alteration in 5-HT neurotransmission may, therefore, constitute suitable tools to investigate the role of impulsivity in animal tests of cognitive function. Benzodiazepines also increase impulsive-like behaviour, possibly by decreasing 5-HT neurotransmission. Hence, the effects of modulation of 5-HT systems and of the benzodiazepine-binding site on performance in animals tests of cognitive function will be discussed. It is predicted that the effects of manipulations of serotonergic activity or of benzodiazepine administration depend upon the nature of the response required, and that these effects may be mediated through changes in impulse control.