Opposite effects depending on learning and memory demands in dorsomedial prefrontal cortex lesioned rats performing an olfactory task

Afferents to anterior cingulate areas 24a and 24b and midcingulate areas 24a' and 24b' in the mouse

Areas 24a and 24b of the anterior cingulate cortex (ACC) play a major role in cognition, emotion and pain. While their connectivity has been studied in primate and in rat, a complete mapping was still missing in the mouse. Here, we analyzed the afferents to the mouse ACC by injecting retrograde tracers in the ventral and dorsal areas of the ACC (areas 24a/b) and of the midcingulate cortex (MCC; areas 24a'/b'). Our results reveal inputs from five principal groups of structures: (1) cortical areas, mainly the orbital, medial prefrontal, retrosplenial, parietal associative, primary and secondary sensory areas and the hippocampus, (2) basal forebrain, mainly the basolateral amygdaloid nucleus, the claustrum and the horizontal limb of the diagonal band of Broca, (3) the thalamus, mainly the anteromedial, lateral mediodorsal, ventromedial, centrolateral, central medial and reuniens/rhomboid nuclei, (4) the hypothalamus, mainly the lateral and retromammillary areas, and (5) the brainstem, mainly the monoaminergic centers. The neurochemical nature of inputs from the diagonal band of Broca and brainstem centers was also investigated by double-labeling, showing that only a part of these afferents were cholinergic or monoaminergic. Comparisons between the areas indicate that areas 24a and 24b receive qualitatively similar inputs, but with different densities. These differences are more pronounced when comparing the inputs to ACC's areas 24a/24b to the inputs to MCC's areas 24a'/24b'. These results provide a complete analysis of the afferents to the mouse areas 24a/24b and 24a'/24b', which shows important similarity with the connectivity of homologous areas in rats, and brings the anatomical basis necessary to address the roles of cingulate areas in mice.

Perseveration related to frontal lesion in mice using the olfactory H-maze

The delayed reaction paradigm, consisting to discover two different rules consecutively (delayed alternation and non-alternation task) followed by a delayed reversal task, is a specific marker for the functioning of primate prefrontal cortex. Although several works in rodents report the use of operant delayed alternation tasks, in none of the studies mice with lesion of the prefrontal cortex were used in this paradigm. In the current study, mouse experiments were conducted using a new, totally automated device, the olfactory H-maze. Here, we show that unilateral lesion of the dorsomedial prefrontal cortex in mice induced similar deficits to those observed after frontal lesions in monkeys and humans. These pronounced learning deficits seem to come from difficulty elaborating a new rule and the inability to inhibit the previous rule, characterized by perseveration after prefrontal cortex lesion. The present results demonstrate that this very simple experimental paradigm using the olfactory H-maze presents the advantage to be fast (one training session) and well suited to assess the frontal functions in mice. It should be useful for testing pharmacological or stem cell approaches in order to reduce organic damages or gain insight into the cognitive functions of the frontal cortex using transgenic or gene-targeting mice.

Apparent encoding of sequential context in rat medial prefrontal cortex is accounted for by behavioral variability

Simple sequences can be represented via asymmetrically linked neural assemblies, provided that the elements of the sequence are unique. When elements repeat, however (e.g., A-B-C-B-A), the same element belongs to two separate "sequential contexts," and a more complex encoding mechanism is required. To enable correct sequence performance, some neural structure must provide a disambiguating signal that differentiates the two sequential contexts (i.e., B as an element of "A-B" as opposed to "C-B"). The disambiguating signal may derive from a form of working memory, or, in some cases, a simple timing mechanism may suffice. To investigate the possible role of medial prefrontal cortex in complex sequence encoding, rats were trained on a spatial sequence containing two adjacent repeated segments (e.g., A-B-C-D-B-C-E). The double-repeat procedure minimized behavioral differences in the second leg (C) of the repeat subsequence that arise in the first leg (B) because of differences in the entry point (e.g., A-B vs D-B). Far more cells were context sensitive along the first leg than along the second (36 vs 9%), and most of the differences were accounted for by systematic variations in the rat's trajectory, which were much larger along the first leg. There is thus little evidence for sequential context-discriminative activity in the medial prefrontal cortex that cannot plausibly be accounted for by context-dependent behavior. The finding that the rodent medial prefrontal cortex is highly sensitive to sensory-behavioral variables raises doubts about previous experiments that purport to show working memory-related activity in this region.

A novel experimental paradigm for studying cognitive functions related to delayed response tasks in mice

Rodents are the animals most commonly employed to model human cognitive functions, but serious problems arise from the non-selective use of behavioral paradigms that measure different processes in rodents than those found in humans. To avoid problems stemming from the use of different paradigms on humans and mice, a new experimental paradigm for mice was developed to study the cognitive functions involved in delayed response tasks. The experiments were conducted in an olfactory tubing maze using three successive delayed response tasks: an alternation task, a non-alternation task, and a reversal task. Mice had to discover the rule by themselves by choosing one of two identical odor cues presented simultaneously at the left and right sides of a testing chamber. The success criterion was set at 10, 8, 6, or 4 consecutive correct responses, with a maximum of 80 trials per task, as used in primates. In the delayed alternation task with the criterion of 10 or 8 consecutive successful trials, the rule was discovered but required many more than 80 trials for most of the mice. With a criterion of 6 or 4, the mice were successful but twice as many trials were necessary to reach the criterion of 6 as opposed to 4. In the delayed non-alternation and reversal tasks, more than 80 trials were needed to figure out the new rule with the criterion of 10 or 8. All mice were successful with the criterion of 6 or 4. The results indicated that no matter what criterion was used, mice were able to discover the two rules on the three consecutive delayed response tasks, but they did so with more or less ease. This novel paradigm for mice should be useful in experiments on pharmacological treatments or for testing transgenic or gene-targeting mice to gain insight into the brain structures involved in this type of task.

Modulation of memory processes and cellular excitability in the dentate gyrus of freely moving rats by a 5-HT4 receptors partial agonist, and an antagonist

Firstly, olfactory association learning was used to determine the modulating effect of 5-HT4 receptor involvement in learning and long-term memory. Secondly, the effects of systemic injections of a 5-HT4 partial agonist and an antagonist on long-term potentiation (LTP) and depotentiation in the dentate gyrus (DG) were tested in freely moving rats. The modulating role of the 5-HT4 receptors was studied by using a potent, 5-HT4 partial agonist RS 67333 [1-(4-amino-5-chloro-2-methoxyphenyl)-3-(1-n-butyl-4-piperidinyl)-1-propanone] and a selective 5-HT4 receptor antagonist RS 67532 [1-(4-amino-5-chloro-2-(3,5-dimethoxybenzyloxyphenyl)-5-(1-piperidinyl)-1-propanone]. Agonist or antagonist systemic chronic injections prior to five training sessions yielded a facilitatory effect on procedural memory during the first session only with the antagonist. Systemic injection of the antagonist only before the first training session improved procedural memory during the first session and associative memory during the second session. Similar injection with the 5-HT4 partial agonist had an opposite effect. The systemic injection of the 5-HT4 partial agonist prior to the induction of LTP in the dentate gyrus by high-frequency stimulation was followed by a population spike increase, while the systemic injection of the antagonist accelerated the depotentiation 48 h later. The behavioural and physiological results pointed out the involvement of 5-HT4 receptors in processing related to the long-term hippocampal-dependent memory system, and suggest that specific 5-HT4 agonists could be used to treat amnesic patients with a dysfunction in this particular system.

Olfactory Associative Discrimination: A Model for Studying Modifications of Synaptic Efficacy in Neuronal Networks Supporting Long-term Memory

This review summarizes research that correlates behavioral performance and cellular physiology leading to modifications in the neuronal networks supporting long-term memory in the mammalian brain. Rats were trained in an olfactory associative discrimination task in which natural odors were replaced by mimetic olfactory stimulations. Olfactory learning induced synaptic modifications that affected behavioral performance along the central olfactory pathways. Starting with an early increase in monosynaptic efficacy in the dentate gyrus on the first session, a polysynaptic modification appeared later on in this hippocampal network, when rats began to make associations between cues and rewards. Therefore, only when rats made consistent associations did a long-term potentiation in the synapses of the piriform cortex pyramidal neurons appear. These modifications may correspond to the long-term storage of the meaning of the cue-reward association in a specific cortical area. Based on these cumulative results, a hypothesis is proposed to account for how, when, and where synaptic modifications in neural networks are required to constitute long-term memory.

Prefrontocortical dopamine loss in rats delays long-term extinction of contextual conditioned fear, and reduces social interaction without affecting short-term social interaction memory

Prefrontal dopamine loss delays extinction of cued fear conditioning responses, but its role in contextual fear conditioning has not been explored. Medial prefrontal lesions also enhance social interaction in rats, but the role of prefrontal dopamine loss on social interaction memory is not known. Besides, a role for subcortical accumbal dopamine on mnesic changes after prefrontal dopamine manipulation has been proposed but not explored. The objective was to study the involvement of dopaminergic neurotransmission in the medial prefrontal cortex (mPFC) and nucleus accumbens in two mnesic tasks: contextual fear conditioning and social interaction memory. For contextual fear conditioning, short- and long-term freezing responses after an electric shock were studied, as well as extinction retention. Regarding social interaction memory, the recognition of a juvenile, a very sensitive short-term memory test, was used. Dopamine loss was carried out by injection of 6-hydroxydopamine, and postmortem catecholamine levels were analyzed by high-performance liquid chromatography. Prefrontocortical dopamine loss (>76%) led to a reactive enhancement of accumbal dopamine content (p<0.01), supporting the hypothesis that a hyperdopaminergic tone emerges in the nucleus accumbens after prefrontocortical dopamine loss. In lesioned rats, long-term extinction of contextual fear conditioning was significantly delayed and extinction retention was impaired without changes in acquisition and short-term contextual fear conditioning and, on the other hand, acquisition and short-term social interaction memory were not affected, although time spent on social interaction was significantly reduced. Added dopamine loss in the nucleus accumbens (>76%) did not alter these behavioral changes. In summary, the results of the present study indicate that the dopaminergic network in the mPFC (but not in the nucleus accumbens) coordinates the normal long-term extinction of contextual fear conditioning responses without affecting their acquisition, and it is involved in time spent on social interaction, but not acquisition and short-term social interaction memory.

Prefrontal cortical manipulations alter the effects of intra-ventral striatal dopamine antagonists on fixed-interval performance in the rat

The nature of the functional relationships between areas of prefrontal cortex and ventral striatum remain undefined. This study was designed to examine functional interactions between activity in two areas of prefrontal cortex, the prelimbic (PL) and agranular insular (AI) areas, and ventral striatal (VS) dopamine (DA) function. Interactions were assessed using a Fixed Interval (FI) schedule of reinforcement shown previously in our laboratory to be modulated by VS DA function. The study compared changes in FI performance following intra-VS DA antagonist injections alone (SCH23390 + eticlopride) to those observed when either saline or saline + lidocaine were injected into prefrontal cortex after the intra-VS DA antagonist injections. The intra-VS DA antagonists alone decreased FI response rates and increased postreinforcement pause times at both dose combinations (1/0.1 and 3/0.3 microg of SCH23390/eticlopride per side). Neither saline nor saline + lidocaine injected into the PL area of prefrontal cortex altered the effects of intra-VS DA antagonists on FI performance. Saline administration into the AI area of prefrontal cortex, however, eliminated the FI rate-decreasing effects of intra-VS DA antagonists. The agent or mechanism of this effect, whether it be saline, the act of inserting the cannulae into the cortical tissue, or the act of injecting fluid into this tissue, is not clear. This effect of AI saline was prevented by coadministration of lidocaine with saline into AI. These results, coupled with those from a previous experiment examining lesion effects in PL and AI on FI performance (Evans SB, Cory-Slechta DA. The effects of temporary lesions of the insular and medial prefrontal cortex on fixed-interval schedule-controlled behavior in the rat, Soc Neurosci Abstr 1996;22(1):159) suggest that PL might exert a tonic influence on VS DA function, since FI response rates gradually increase over a 2-week period following lesions of PL. In contrast, AI, although not normally modulating FI performance, can apparently influence VS DA function, possibly when alterations in activity are invoked in AI.

BIMU1 increases associative memory in rats by activating 5-HT4 receptors

Olfactory association learning was used to investigate the involvement of 5-HT4 receptors in learning and long-term memory. The behavioral role of the 5-HT4 receptors was studied by using BIMU1 (3-ethyl-2,3-dihydro-N-[endo-8-methyl-8-azabicyclo(3.2.1)oct-3-yl]-2-oxo -1 H-benzimidazole-1-carboxamide, hydrochloride (Boehringer Ingelheim, Italy); a mixed 5-HT4 agonist/5-HT3 antagonist, and GR125487 (1-[2-[methyl sulphonyl)-amino]ethyl]-4-piperidinyl-methyl 5-fluro-2-methoxy-1H-indole-3- carboxylate; Glaxo Group Research, Hertfordshire, U.K.), a specific 5-HT4 antagonist. The intraperitoneal injections of BIMU1 at 1, 5, and 10 mg/kg were followed by an substantial improvement (> 15% in percentage of correct responses at the dose of 10 mg/kg) in associative memory. Difficulty rapidly reversing behavioral responses to previously learned association, 1 month later indicated that the BIMU1 effect at 10 mg/kg was not transient, but correlated to long-term memory. The effects of BIMU1 are most likely to be mediated by 5-HT4 receptors since they were blocked by GR125487 at 10 mg/kg. These data suggest that activation of 5-HT4 receptors may modulate cognitive processes like learning and memory.