Cortical cholinergic projections from the basal forebrain of the rat, with special reference to the prefrontal cortex innervation

Functionally linked amygdala and prefrontal cortical regions are innervated by both single and double projecting cholinergic neurons

Cholinergic cells have been proposed to innervate simultaneously those cortical areas that are mutually interconnected with each other. To test this hypothesis, we investigated the cholinergic innervation of functionally linked amygdala and prefrontal cortical regions. First, using tracing experiments, we determined that cholinergic cells located in distinct basal forebrain (BF) areas projected to the different nuclei of the basolateral amygdala (BLA). Specifically, cholinergic cells in the ventral pallidum/substantia innominata (VP/SI) innervated the basal nucleus (BA), while the horizontal limb of the diagonal band of Broca (HDB) projected to its basomedial nucleus (BMA). In addition, cholinergic neurons in these two BF areas gave rise to overlapping innervation in the medial prefrontal cortex (mPFC), yet their axons segregated in the dorsal and ventral regions of the PFC. Using retrograde-anterograde viral tracing, we demonstrated that a portion of mPFC-projecting cholinergic neurons also innervated the BLA, especially the BA. By injecting retrograde tracers into the mPFC and BA, we found that 28% of retrogradely labeled cholinergic cells were double labeled, which typically located in the VP/SI. In addition, we found that vesicular glutamate transporter type 3 (VGLUT3)-expressing neurons within the VP/SI were also cholinergic and projected to the mPFC and BA, implicating that a part of the cholinergic afferents may release glutamate. In contrast, we uncovered that GABA is unlikely to be a co-transmitter molecule in HDB and VP/SI cholinergic neurons in adult mice. The dual innervation strategy, i.e., the existence of cholinergic cell populations with single as well as simultaneous projections to the BLA and mPFC, provides the possibility for both synchronous and independent control of the operation in these cortical areas, a structural arrangement that may maximize computational support for functionally linked regions. The presence of VGLUT3 in a portion of cholinergic afferents suggests more complex functional effects of cholinergic system in cortical structures.

Impact of adolescent intermittent ethanol exposure on interneurons and their surrounding perineuronal nets in adulthood

BACKGROUND

Binge alcohol exposure during adolescence results in long-lasting alterations in the brain and behavior. For example, adolescent intermittent ethanol (AIE) exposure in rodents results in long-term loss of functional connectivity among prefrontal cortex (PFC) and striatal regions as well as a variety of neurochemical, molecular, and epigenetic alterations. Interneurons in the PFC and striatum play critical roles in behavioral flexibility and functional connectivity. For example, parvalbumin (PV) interneurons are known to contribute to neural synchrony and cholinergic interneurons contribute to strategy selection. Furthermore, extracellular perineuronal nets (PNNs) that surround some interneurons, particularly PV+ interneurons, further regulate cellular plasticity. The effect of AIE exposure on the expression of these markers within the PFC is not well understood.

METHODS

The present study tested the hypothesis that AIE exposure reduces the expression of PV+ and choline acetyltransferase (ChAT)+ interneurons in the adult PFC and striatum and increases the related expression of PNNs (marked by binding of Wisteria floribunda agglutinin lectin) in adulthood. Male rats were exposed to AIE (5 g/kg/day, 2-days-on/2-days-off, i.e., P25 to P54) or water (CON), and brain tissue was harvested in adulthood (>P80). Immunohistochemistry and co-immunofluorescence were used to assess the expression of ChAT, PV, and PNNs within the adult PFC and striatum following AIE exposure.

RESULTS

ChAT and PV interneuron densities in the striatum and PFC were unchanged after AIE exposure. However, PNN density in the PFC of AIE-exposed rats was greater than in CON rats. Moreover, significantly more PV neurons were surrounded by PNNs in AIE-exposed subjects than controls in both PFC subregions assessed: orbitofrontal cortex (CON = 34%; AIE = 40%) and medial PFC (CON = 10%; AIE = 14%).

CONCLUSIONS

These findings indicate that, following AIE exposure, PV interneuron expression in the adult PFC and striatum is unaltered, while PNNs surrounding these neurons are increased. This increase in PNNs may restrict the plasticity of the ensheathed neurons, thereby contributing to impaired microcircuitry in frontostriatal connectivity and related behavioral impairments.

A Central Amygdala-Substantia Innominata Neural Circuitry Encodes Aversive Reinforcement Signals

Aversive stimuli can impact motivation and support associative learning as reinforcers. However, the neural circuitry underlying the processing of aversive reinforcers has not been elucidated. Here, we report that a subpopulation of central amygdala (CeA) GABAergic neurons expressing protein kinase C-delta (PKC-δ+) displays robust responses to aversive stimuli during negative reinforcement learning. Importantly, projections from PKC-δ+ neurons of the CeA to the substantia innominata (SI) could bi-directionally modulate negative reinforcement learning. Moreover, consistent with the idea that SI-projecting PKC-δ+ neurons of the CeA encode aversive information, optogenetic activation of this pathway produces conditioned place aversion, a behavior prevented by simultaneous ablating of SI glutamatergic neurons. Taken together, our data define a cell-type-specific neural circuitry modulating associative learning by encoding aversive reinforcement signals.

Potential roles of cholinergic modulation in the neural coding of location and movement speed

Behavioral data suggest that cholinergic modulation may play a role in certain aspects of spatial memory, and neurophysiological data demonstrate neurons that fire in response to spatial dimensions, including grid cells and place cells that respond on the basis of location and running speed. These neurons show firing responses that depend upon the visual configuration of the environment, due to coding in visually-responsive regions of the neocortex. This review focuses on the physiological effects of acetylcholine that may influence the sensory coding of spatial dimensions relevant to behavior. In particular, the local circuit effects of acetylcholine within the cortex regulate the influence of sensory input relative to internal memory representations via presynaptic inhibition of excitatory and inhibitory synaptic transmission, and the modulation of intrinsic currents in cortical excitatory and inhibitory neurons. In addition, circuit effects of acetylcholine regulate the dynamics of cortical circuits including oscillations at theta and gamma frequencies. These effects of acetylcholine on local circuits and network dynamics could underlie the role of acetylcholine in coding of spatial information for the performance of spatial memory tasks.

Cholinergic control in developing prefrontal-hippocampal networks

The cholinergic drive enhances input processing in attentional and mnemonic context by interacting with the activity of prefrontal-hippocampal networks. During development, acetylcholine modulates neuronal proliferation, differentiation, and synaptic plasticity, yet its contribution to the maturation of cognitive processing resulting from early entrainment of neuronal networks in oscillatory rhythms remains widely unknown. Here we show that cholinergic projections growing into the rat prefrontal cortex (PFC) toward the end of the first postnatal week boost the generation of nested gamma oscillations superimposed on discontinuous spindle bursts by acting on functional muscarinic but not nicotinic receptors. Although electrical stimulation of cholinergic nuclei increased the occurrence of nested gamma spindle bursts by 41%, diminishment of the cholinergic input by either blockade of the receptors or chronic immunotoxic lesion had the opposite effect. This activation of locally generated gamma episodes by direct cholinergic projections to the PFC was accompanied by indirect modulation of underlying spindle bursts via cholinergic control of hippocampal theta activity. With ongoing maturation and switch of network activity from discontinuous bursts to continuous theta-gamma rhythms, accumulating cholinergic projections acting on both muscarinic and nicotinic receptors mediated the transition from high-amplitude slow to low-amplitude fast rhythms in the PFC. By exerting multiple actions on the oscillatory entrainment of developing prefrontal-hippocampal networks, the cholinergic input may refine them for later gating processing in executive and mnemonic tasks.

Sex difference in the 24-h acetylcholine release profile in the premotor/supplementary motor area of behaving rats

The sex differences in various motor functions suggest a sex-specific neural basis in the nonprimary or primary motor area. To examine the sex difference in the 24-h profile of acetylcholine (ACh) release in the rostral frontal cortex area 2 (rFr2), which is equivalent to the premotor/supplementary motor area in primates, we performed an in vivo microdialysis study in both sexes of rats fed pelleted or powdered diet. The dialysate was automatically collected from the rFr2 for 24 h under freely moving conditions. Moreover, the number of cholinergic neurons in the nucleus basalis magnocellularis (NBM) was examined. Further, to confirm the relation between ACh release in the rFr2 and motor function, the spontaneous locomotor activity was monitored for 24 h. Both sexes showed a distinct 24-h rhythm of ACh release, which was high during the dark phase and low during the light phase. Female rats, however, showed a greater ACh release and more cholinergic neurons in the NBM than male rats. Similarly, spontaneous locomotor activity also showed a 24-h rhythm, which paralleled the changes in ACh release in both sexes, and these changes were again greater in female rats than in male rats. In addition, feeding with powdered diet significantly increased the ACh release and spontaneous locomotor activity. The present study is the first to report the sex difference in the 24-h profile of ACh release in the rFr2 in rats. The sex specific ACh release in the rFr2 may partly contribute to the sex difference in motor function in rats.

Selective lesions of the nucleus basalis magnocellularis impair cognitive flexibility

The authors tested the hypothesis that the cholinergic nucleus basalis magnocellularis (NBM) is involved in solving problems requiring cognitive flexibility. Rats with 192 IgG-saporin lesions of the NBM were assessed for perseveration (i.e., cognitive inflexibility) in the serial reversal of an operant discrimination and during subsequent extinction testing. It was hypothesized that the NBM lesion and control groups would not differ in the acquisition of the initial, simple discrimination, because this task does not demand cognitive flexibility. In contrast, it was hypothesized that the NBM lesion group would show perseveration during serial reversal and extinction testing. Results generally supported these hypotheses, suggesting that the NBM plays an important role in mediating cognitive flexibility.

Songbirds and the revised avian brain nomenclature

It has become increasingly clear that the standard nomenclature for many telencephalic and related brainstem structures of the avian brain is based on flawed once-held assumptions of homology to mammalian brain structures, greatly hindering functional comparisons between avian and mammalian brains. This has become especially problematic for those researchers studying the neurobiology of birdsong, the largest single group within the avian neuroscience community. To deal with the many communication problems this has caused among researchers specializing in different vertebrate classes, the Avian Brain Nomenclature Forum, held at Duke University from July 18-20, 2002, set out to develop a new terminology for the avian telencephalon and some allied brainstem cell groups. In one major step, the erroneous conception that the avian telencephalon consists mainly of a hypertrophied basal ganglia has been purged from the telencephalic terminology, and the actual parts of the basal ganglia and its brainstem afferent cell groups have been given new names to reflect their now-evident homologies. The telencephalic regions that were incorrectly named to reflect presumed homology to mammalian basal ganglia have been renamed as parts of the pallium. The prefixes used for the new names for the pallial subdivisions have retained most established abbreviations, in an effort to maintain continuity with the pre-existing nomenclature. Here we present a brief synopsis of the inaccuracies in the old nomenclature, a summary of the nomenclature changes, and details of changes for specific songbird vocal and auditory nuclei. We believe this new terminology will promote more accurate understanding of the broader neurobiological implications of song control mechanisms and facilitate the productive exchange of information between researchers studying avian and mammalian systems.

Lesions of the rat nucleus basalis magnocellularis disrupt appetitive-to-aversive transfer learning

Rats with selective lesions of the nucleus basalis magnocellularis (NBM) and sham-lesion control animals were tested in an operant appetitive-to-aversive transfer task. We hypothesized that NBM lesions would not affect performance in the appetitive phase, but that performance would be impaired during subsequent transfer to the aversive phase of the task. Additional groups of NBM lesion and control rats were tested in the avoidance condition only, where we hypothesized that NBM lesions would not disrupt performance. These hypotheses were based on the argument that the NBM is not necessary for simple association learning that does not tax attention. Both the appetitive phase of the transfer task and the avoidance only task depend only on simple associative learning and are argued not to tax attention. Consequently, performance in these tasks was predicted to be spared following NBM lesions. Complex, attention-demanding associative learning, however, is argued to depend on the NBM. Performance in the aversive phase of the transfer task is both attentionally demanding and associatively more complex than in either the appetitive or aversive tasks alone; thus, avoidance performance in the NBM lesion group was predicted to be impaired following transfer from prior appetitive conditioning. Results supported our hypotheses, with the NBM lesion group acquiring the appetitive response normally, but showing impaired performance following transfer to the aversive conditioning phase of the transfer task. Impairments were not attributable to disrupted avoidance learning per se, as avoidance behavior was normal in the NBM lesion group tested in the avoidance condition only.

Functional recovery of cholinergic basal forebrain neurons under disease conditions: old problems, new solutions?

Recognition of the involvement of cholinergic neurons in the modulation of cognitive functions and their severe dysfunction in neurodegenerative disorders, such as Alzheimer's disease, initiated immense research efforts aimed at unveiling the anatomical organization and cellular characteristics of the basal forebrain (BFB) cholinergic system. Concomitant with our unfolding knowledge about the structural and functional complexity of the BFB cholinergic projection system, multiple pharmacological strategies were introduced to rescue cholinergic nerve cells from noxious attacks; however, a therapeutic breakthrough is still awaited. In this review, we collected recent findings that significantly contributed to our better understanding of cholinergic functions under disease conditions, and to the design of effective means to restore lost or damaged cholinergic functions. To this end, we first provide a brief survey of the neuroanatomical organization of BFB nuclei with emphasis on major evolutionary differences among mammalian species, in particular rodents and primates, and discuss limitations of the translation of experimental data to human therapeutic applications. Subsequently, we summarize the involvement of cholinergic dysfunction in the pathogenesis of severe neurological conditions, including stroke, traumatic brain injury, virus encephalitis and Alzheimer's disease, and emphasize the critical role of pro-inflammatory cytokines as common mediators of cholinergic neuronal damage. Moreover, we review leading functional concepts on the limited recovery of cholinergic neurons and their impaired plastic re-modeling, as well as on the hampered interplay of the ascending cholinergic and monoaminergic projection systems under neurodegenerative conditions. In addition, recent advances in the dynamic labeling of living cholinergic neurons by fluorochromated antibodies, referred to as in vivo labeling, and novel neuroimaging approaches as potential diagnostic tools of progressive cholinergic decline are surveyed. Finally, the potential of cell replacement strategies using embryonic and adult stem cells, and multipotent neural progenitors, as a means to recover damaged cholinergic functions, is discussed.

Transverse patterning reveals a dissociation of simple and configural association learning abilities in rats with 192 IgG-saporin lesions of the nucleus basalis magnocellularis

This experiment tests the hypothesis that the cholinergic nucleus basalis magnocellularis (NBM) is necessary for complex or configural association learning, but not elemental or simple association learning. Male Long-Evans rats with bilateral 192 IgG-saporin lesions of the NBM (n = 12) and sham-operated controls (n = 8) were tested in the transverse patterning problem, which provides a test of both simple and configural association learning. Rats were trained in phases to concurrently solve first one, then two, and finally three different visual discriminations; Problem 1 (A+ vs B- sign) and Problem 2 (B+ vs C-) could be solved using simple associations, whereas solving Problem 3 (C+ vs A-) required the ability to form configural associations. Consistent with our hypothesis, the NBM lesion group solved the simple discriminations in Problems 1 and 2 but showed impaired configural association learning in Problem 3. Additionally, when Problem 2 was introduced, previously high levels of performance on Problem 1 suffered more in the NBM lesion group than in the control group; this finding suggests an impairment in the ability of animals with NBM lesions to divide attention among multiple stimuli or to shift between strategies for solving different problems. Results support our argument that the NBM is critically involved in the acquisition of associative problems requiring a configural solution but not in problems that can be solved using only simple associations. The observed impairments in configural association learning and the apparent loss of cognitive flexibility or capacity are interpreted as reflecting specific attentional impairments resulting from NBM damage.

Cosmic ray hit frequencies in critical sites in the central nervous system

One outstanding question to be addressed in assessing the risk of exposure to space travelers from galactic cosmic rays (GCR) outside the geomagnetosphere is to ascertain the effects of single heavy-ion hits on cells in critical regions of the central nervous system (CNS). As a first step toward this end, it is important to determine how many "hits" might be received by a neural cell in several critical CNS areas during an extended mission outside the confines of the earth's magnetic field. Critical sites in the CNS: the macula, and an interior brain point (typical of the genu, thalamus, hippocampus and nucleus basalis of Meynert) were chosen for the calculation of hit frequencies from galactic cosmic rays for a mission to Mars during solar minimum (i.e., at maximum cosmic-ray intensity). The shielding at a given position inside the body was obtained using the Computerized Anatomical Man (CAM) model, and a radiation transport code which includes nuclear fragmentation was used to calculate yearly fluences at the point of interest. Since the final Mars spacecraft shielding configuration has not yet been determined, we considered the minimum amount of aluminum required for pressure vessel-wall requirements in the living quarters of a spacecraft, and a typical duty area as a pressure vessel plus necessary equipment. The conclusions are: (1) variation of the position of the "target site" within the head plays only a small role in varying hit frequencies; (2) the average number of hits depends linearly on the cross section of the critical portion of the cell assumed in the calculation; (3) for a three-year mission to Mars at solar minimum (i.e., assuming the 1977 spectrum of galactic cosmic rays), 2% or 13% of the "critical sites" of cells in the CNS would be directly hit at least once by iron ions, depending on whether 60 micrometers2 or 471 micrometers2 is assumed as the critical cross sectional area; and (4) roughly 6 million out of some 43 million hippocampal cells and 55 thousand out of 1.8 million thalamus cell nuclei would be directly hit by iron ions at least once on such a mission for space travelers inside a simple pressure vessel. Also, roughly 20 million out of 43 million hippocampal cells and 230 thousand out of 1.8 million thalamus cell nuclei would be directly hit by one or more particles with z > or = 15 on such a mission.

Lesions of nucleus basalis alter ChAT activity and EEG in rat frontal neocortex

The EEG was recorded from frontal, parietal and visual cortices of sham-operated control rats and rats having ibotenic acid lesions of the nucleus basalis. Recordings were made during a period of rest and during stimulus-evoked desynchronization. Spectral power was determined using a Fast Fourier Transform routine; 3 artifact-free 4 sec epochs of resting activity and two 4 sec epochs of activated EEG were analyzed. Choline acetyltransferase activity (ChAT) was measured in each cortical area and was reduced in lesioned animals an average of 25% in frontal cortex, 19% in the parietal region and 10% in visual cortex. The percent of low frequency activity (1-12 Hz) in the frontal EEG was significantly greater in lesioned animals than in the control group during quiet rest; a significant correlation was found between ChAT activity and power in this band. Desynchronized activity was largely unaffected except for a reduction in 25-31 Hz activity in the frontal cortex of lesioned animals. EEG activity in both the parietal and visual areas was unchanged from control values.

Differential changes in cell size and number in topographic subdivisions of human basal nucleus in normal aging

The age-related cell loss of the nucleus basalis of Meynert is of considerable importance because loss of its neurons may be followed by cognitive decline. Compared to the number found at ages 16-29 years, we found that 50% of the total population of neurons is lost by 90 years of age. This change in number is accompanied by modifications in the morphometric features, including a 17.3% increase in cell size by 60 years of age as compared with values at 16 years, and followed by a gradual decline. Topographic differences were seen both in the neuronal loss and in morphometry: in relation to the youngest group, the posterior subdivision is the most severely affected by 90 years (64.5% decrease in number and 10% reduction in neuronal size), followed by the intermediate subdivision (42% loss of neurons accompanied by 4% increase in cell size). In the anterior subdivision no significant decrease in the number of neurons could be detected, although a 15% increase in cell size occurred.

Dopaminergic modulation of cholinergic responses in rat medial prefrontal cortex: an electrophysiological study

The neuromodulatory action of dopamine (DA) on acetylcholine (ACh)-evoked responses of prefrontal cortex (PFC) neurones were investigated electrophysiologically in rats anaesthetised with a combination of urethane and ketamine. Iontophoretic application of ACh-excited prefrontal cortex neurones. Concurrent application of DA (5-15 nA) resulted in complex changes in the ACh-evoked responses: (1) DA depressed spontaneous background discharges (designated as noise) proportionally more than the ACh-evoked discharges (designated as input signals), thus yielding an enhanced signal/noise ratio. This increase in signal/noise ratio by dopamine was reversed by iontophoretic application of the Da D2 antagonist sulpiride (20-50 nA). Nevertheless, iontophoretic application of D2 agonist quinpirole (5-35 nA) enhanced the ACh-evoked response, but was accompanied by some increase in spontaneous discharge, thus yielding no change in the signal/noise ratio. (2) DA also increased the signal/noise ratio by inducing a net increase of the ACh-evoked response but simultaneously suppressed the spontaneous activity of PFC neurones. This effect was more prominent following blockade of D1 receptors by SCH23390 (6 mg/kg, i.p.), suggesting that D1 receptors may normally inhibit D2 receptor function in the PFC. In addition, endogenous DA in the PFC did not play a significant part in modifying the ACh-evoked responses since the modulation of ACh-evoked response by DA or its D1 and D2 agonists was similar in both saline control and alpha-methyl-p-tyrosine-pretreated rats. (3) When ejected with larger iontophoretic current (16-35 nA), DA suppressed both the ACh-evoked and spontaneous discharge and this effect was mimicked by D1 agonist SKF38393 (5-15 nA). Taken together, these results suggest that complex dopaminergic modulation of the cholinergic responses of prefrontal cortex neurones are mediated by D1 and D2 receptors. This DA action may have a functional role in the cognitive-integrative processes occurring in the prefrontal cortex.

Prefrontal cortex aspiration in pups and juvenile rats: behavioural changes and recovery of function

Male Wistar rats sustaining prefrontal cortex aspiration or sham operation at 6 days or 30 days of age were submitted to the following behavioural tests: open-field, acquisition and retention of two-way active as well as passive avoidance tasks. In the open-field the locomotor activity proved enhanced in all the aspirated animals and this enhancement lasted for 30 days. In the two-day active avoidance task, an acquisition deficit was observed in both aspirated groups; but when retrained one month later, they were able to acquire the avoidance task like sham-operated rats and no difference appeared between the groups aspirated at 6 or at 30 days of age. Concerning the passive avoidance task, no difference could be detected between aspirated and sham-operated animals of both groups except that the rats aspirated at an early age (6 days) seemed to display a better avoidance ability in the retention test. These behavioural alterations (hyperactivity and impairment of the acquisition of the 2-way active avoidance) resulted from the prefrontal cortex aspiration, at whatever age this aspiration was performed (6 days or 30 days). They disappeared after a postoperative recovery period of about one month, as evidenced by this longitudinal study.

Memory disturbances following ibotenic acid injections in the nucleus basalis magnocellularis of the rat

The behavioral effects of lesions of the nucleus basalis magnocellularis (NBM) on two spatial discrimination tasks (place navigation and cross maze) were examined in the rat. These tasks were designed to test reference memory. Lesions by bilateral injection of ibotenic acid into the NBM led to a severe and permanent impairment in the learning of the cross maze task. In the learning of the place navigation task, the rats with lesions showed only a transient deficit. Immediately after the removal of the platform, the rats with lesions explored the quadrant (NE) previously containing the platform as long as controls and above chance levels. The rats with lesions did not extinguish exploration like the controls, seen as a reduction both in time spent in the NE quadrant and in swimming activity. Taken together, the results showed that (1) NBM lesions impair reference memory, but (2) spare other aspects of memory. On the basis of the results in the place navigation task, procedural memory was assumed to remain intact after lesion of the NBM. Biochemical assays of choline acetyltransferase (ChAT) in various brain regions in the lesioned animals demonstrated a reduced ChAT activity in the neocortical projections of the NBM but not in the hippocampus. However, it cannot be decided from this work whether behavioral deficits result from the lesion of cholinergic or of non-cholinergic cells in the NBM.

Distribution and some projections of cholinergic neurons in the brain of the common marmoset, Callithrix jacchus

The distribution of choline acetyltransferase-immunoreactive (ChAT-IR) neurons was studied in the brain of the common marmoset by using immunohistochemistry. ChAT-IR neurons were found in the medial septal nucleus, vertical and horizontal limb nuclei of the diagonal band, the nucleus basalis of Meynert, pedunculopontine nucleus and laterodorsal tegmental nucleus, and also in the striatum, habenula, and brainstem cranial nerve motor nuclei. The organization of ChAT-IR neurons in the basal forebrain, midbrain, and pons is consistent with the Ch1-Ch6 nomenclature introduced by Mesulam et al. ('83). The combination of the retrograde transport of HRP-WGA with ChAT immunohistochemistry revealed the distribution of neurons in the Ch4 cell group projecting to the dorsolateral prefrontal cortex. The activity of ChAT was highest in limbic cortical structures, such as the hippocampus, and lowest in association areas of the neocortex. Lesions at various loci in the basal forebrain resulted in differential patterns of ChAT loss in the cortex, which suggests some degree of topographical organization of Ch4 projections to the cortical mantle.

The effects of excitotoxic lesions of the substantia innominata, ventral and dorsal globus pallidus on the acquisition and retention of a conditional visual discrimination: implications for cholinergic hypotheses of learning and memory

The effects of ibotenic acid-induced lesions of the ventral pallidum/substantia innominata region, the dorsal pallidum or both on the acquisition and retention of a conditional visual discrimination have been studied in the rat. Lesions of the ventral pallidum and large lesions of the dorsal and ventral pallidum severely impaired both the acquisition and retention of the conditional discrimination. Dorsal pallidal lesions had similar, but less marked effects. The same lesions also impaired the retention of a passive avoidance task, but had no effect on a conditioned taste aversion. Neurobiological investigations revealed that the lesions destroyed cholinergic neurons in the magnocellular nucleus basalis and caused reductions in cortical choline acetyltransferase activity of about 30-40%. Tract-tracing experiments indicated that the lesions destroyed, in particular, cholinergic neurons projecting to the frontal dorsolateral cortex and also those projecting to more posterior cortex, but not the occipital lobes. Contingency analysis of the behavioural, neurochemical and neuroanatomical data indicated that those animals with the largest decreases in choline acetyltransferase activity, or the largest areas of neuronal loss in the ventral and dorsal globus pallidus, were most impaired in the retention of the conditional discrimination. The results do not, therefore, indicate a simple relationship between cholinergic neuronal loss and the retention of response rules essential for performance of the task ("reference memory"). The relevance of the results to cholinergic hypotheses of learning and memory is discussed.

Physiological evidence for subpopulations of cortically projecting basal forebrain neurons in the anesthetized rat

Sixty-three cortically projecting basal forebrain neurons were identified in chloral hydrate anesthetized rats by antidromic activation from the cerebral cortex. Two subpopulations were noted: type I neurons exhibited two antidromic action potentials of constant latency and identical waveform in response to double pulse cortical stimulation. In contrast, type II neurons exhibited two antidromic action potentials of constant latency but differing waveforms in response to the double pulse paradigm. The phenomenon exhibited by type II cortically projecting basal forebrain neurons is interpreted as evidence for loss of the somatodendritic portion of the antidromic action potential with high frequency stimulation. The median latency to antidromic activation of type II neurons (13.5 ms) was significantly longer than that of type I neurons (3.9 ms). Spontaneous firing rates varied over a wide range (0-49 Hz), and there was no significant difference between the rates of type I and type II neurons. These data underscore the physiological heterogeneity of this presumptive cholinergic cortical afferent system. Anatomical studies have shown that most, but possibly not all cortically projecting basal forebrain neurons are cholinergic. The relative proportions of type I (87%) and type II (13%) neurons encountered in this study suggest that type I neurons might be cholinergic and type II neurons non-cholinergic. If substantiated, this hypothesis would permit cholinergic and non-cholinergic cortically projecting basal forebrain neurons to be distinguished using a simple test of antidromicity.

Neurotransmitters, pathways and circuits as the neural substrates of self-stimulation of the prefrontal cortex: facts and speculations

Through a multidisciplinary approach considerable progress has been made in understanding the neural substrates of self-stimulation (SS) of the medial prefrontal cortex (MPC). Thus, neuroanatomical studies have revealed that intrinsic neurones in the MPC seem to be the central elements responsible for initiating and maintaining this phenomenon in this area of the brain. Complementary to this central finding are the electrophysiological and neurohistological data reviewed here, showing that neurones in the MPC are directly activated and have monosynaptic feed-back connections with neurones located in areas which also support SS. These findings have given rise to the hypothesis that several single feed-back pathways or single circuits exist between points of SS in the MPC and points of SS in other areas of the brain. This hypothesis implies that SS in a particular area would depend not only on the intrinsic local activity induced by the electrical stimulation but on the functional and specific activity of other nuclei in the brain. The fact that lesions of single circuits, which are apparently involved in SS of the MPC such as the medial prefrontal cortex-ventrotegmental area-medial prefrontal cortex and medial prefrontal cortex-n. dorsomedialis of the thalamus-medial prefrontal cortex, do not produce a permanent decrease of SS, together with the finding that transynaptic connections seem to exist between MPC and other areas of the brain, suggests further that a complex rather than several single independent circuits could be at the neural basis of SS of the MPC. If that were the case, then SS of the MPC would not only depend upon local and single feed-back activity but upon specific functional feed-back activity among the nuclei, which in turn have single feed-back connections with the MPC (see the concept of 'complex circuit' outlined in the section of Behavioural studies). On the basis of this hypothesis no permanent changes should be expected after lesions of single pathways since physiological and even anatomical compensation could be reached through the rest of the undamaged circuit. That terminals containing specific neurotransmitters exist in layers of the PC where electrodes for SS are located has been reviewed in this paper. Some of these neurotransmitters have been suggested to be part of the local substrates activated by SS.(ABSTRACT TRUNCATED AT 400 WORDS)

Lesion of nucleus basalis magnocellularis decreases [3H]hemicholinium-3 binding (as measured by autoradiography) in the amygdala and frontal cortex of the rat

The effects of a unilateral electrolytic lesion of the nucleus basalis magnocellularis on [3H]hemicholinium-3 binding sites in discrete brain regions of the rat were studied through the use of quantitative autoradiography. When compared to the contralateral side this lesion caused a decrease in the density of [3H]hemicholinium-3 binding sites in the medial prefrontal cortex, frontoparietal cortex and basolateral nucleus of the amygdala but not in the caudate-putamen, nucleus accumbens, olfactory tubercle, hippocampus and auditory cortex. These results add further weight to the view that the cholinergic innervation of the rostral cerebral cortex and amygdala originates from the nucleus basalis magnocellularis and suggest that [3H]hemicholinium-3 autoradiography is a suitable means of visualizing cholinergic nerve terminals.

Ultrastructural organization of choline-acetyltransferase-immunoreactive fibres innervating the neocortex from embryonic ventral forebrain grafts

Suspension grafts of foetal tissue rich in cholinergic neurones were transplanted into the frontoparietal cortex of rats that had previously undergone deafferentation of the extrinsic cholinergic innervation of the cortex by injection of ibotenic acid into the nucleus basalis magnocellularis. The cortical tissue containing the graft was processed for electron microscopic immunocytochemistry by using a monoclonal antibody to choline acetyltransferase (ChAT) in order to examine the contacts established between cholinergic fibres from the graft and the host neocortex. The density, distribution, and targets of this graft-host innervation were compared with those seen in the intact and deafferented cortex. ChAT-positive fibres in both grafted and control animals formed extensive synaptic connections with various cortical neural elements--those of graft origin being of similar morphology to those in the intact cortex. However, the distribution of postsynaptic cortical targets of the graft-derived ChAT-immunoreactive boutons was abnormal, such that a greater percentage of such terminals formed synaptic contacts with neuronal perikarya, especially layer V pyramidal neurones, than was seen in control brains. It is possible that the formation of new synaptic contacts between the embryonic graft and host frontoparietal cortex may, in part, be necessary for the restoration of functional activity that has been previously reported in these grafted animals.

Memory related role of the posterior cholinergic system

In order to establish a model for the possible neuropathology of patients with Alzheimer's disease, various behaviors of rats with different chemical lesions in cholinergic regions were studied and compared with those of sham-operated control rats. A battery of neurological tests was used as well as activity measurements and two learning tasks: a positively reinforced place-learning task with delay periods of Os, 1 min, 15 min, and 2h, and a shock-motivated two-way active avoidance task. While in general no intergroup differences were obtained in performance on the neurological test battery or the rats' activity in an open field, there were marked impairments in the three lesioned groups compared to the control group in the two learning tasks. These deficits were less severe in the two groups with lesions of the medial septal/vertical diagonal band of Broca region and the nucleus basalis of Meynert region, but rather marked in the group with lesions of neurons situated in the pontomesencephalic region, although the amount of ibotenic acid injected had been the same for all groups. We conclude from these data that changes in mesencephalic cholinergic regions might play a significant role in the pathology of Alzheimer's disease. The existence of such regions was recently established for primates as well, thus providing a basis for justifying an animal model of this human disease.

Immunohistochemical evidence for degeneration of cholinergic neurons in the forebrain of the rat following injection of AF64A-picrylsulfonate into the dorsal hippocampus

The cholinergic neurotoxin, AF64A-picrylsulfonate, was unilaterally infused into the dorsal hippocampus of Wistar rats (2 nmol/2 microliters/4 min; A 6.2, Ls 1.5, H 6.5, Paxinos and Watson). After 19 days the for immunohistochemical staining of choline acetyltransferase (ChAT). Morphometry and counting of ChAT-immunoreactive profiles revealed shrinkage and disappearance of cholinergic neurons in the medial septum and diagonal band of Broca at the lesioned brain side. These data indicate a retrograde degeneration of cholinergic neurons following injection of AF64A-picrylsulfonate into the dorsal hippocampus of the rat.

Cholinergic limbic projections and behavioral role of basal forebrain nuclei in the rat

The purposes of the present study were to identify cholinergic non-neocortical projections of the basal forebrain and to determine the role of this region in the regulation of estrogen-dependent reproductive behaviors in the rat. Bilateral electrolytic lesions were placed in an area encompassing the horizontal limb of the diagonal band, as well as portions of the substantia innominata and magnocellular preoptic nucleus, and choline acetyltransferase (CAT) activity was assayed in microdissected brain areas seven days after lesion. Compared to sham surgery, lesions of this region significantly reduced CAT activity in the basal amygdala (34%), dorsal hippocampus (14%), cingulate cortex (25%), piriform cortex (36%), and entorhinal cortex (34%). Other limbic and midbrain structures do not appear to receive significant cholinergic innervation from this locus since no reductions in CAT were detected after bilateral lesions. These included the anterior hypothalamus, ventromedial hypothalamus, mammillary nucleus, habenula, subiculum, ventral hippocampus, insular cortex, central gray, and interpeduncular nucleus. Behaviorally, female rats with bilateral lesions of the basal forebrain displayed an unusually high incidence of rejection behavior in response to attempted mounts by stimulus male rats in sexual behavior tests. There was no effect of basal forebrain lesions on the incidence of lordosis exhibited by these females. The dissociation of rejection and lordosis suggests that distinct neural pathways mediate the occurrence of these reproductive behaviors and that rejection behavior may be regulated by basal forebrain pathways.

Effects of agonists and antagonists of cholinergic receptors on self-stimulation of the medial prefrontal cortex of the rat

The effects of agonists and antagonists of muscarinic and nicotinic receptors on self-stimulation (SS) of the medial prefrontal cortex (MPC) were investigated. Rats, implanted chronically with monopolar electrodes in the MPC, received subcutaneous injections of nicotine (0.2, 0.4 and 0.8 mg/kg), mecamylamine (2.0, 4.0 and 8.0 mg/kg), pilocarpine (0.5, 1.0, 2.0 and 4.0 mg/kg), scopolamine (0.05, 0.1, 0.2, 0.4 and 0.8 mg/kg) and physostigmine (0.1, 0.2 and 0.4 mg/kg). In order to assess the possible non-specific effects of drugs such as sedation or motor dysfunction, spontaneous locomotor activity (SLA) was used as control. In those groups of rats in which the drugs produced an effect on SS, an operant behaviour for drinking (DB) on an FR-10 schedule was also used as control. Nicotine and mecamylamine had no effect on SS. Both pilocarpine and physostigmine produced a decrease in SS, SLA and DB. Scopolamine, on the contrary, produced a dose-related decrease on SS rate, which was accompanied by a facilitatory effect on SLA and DB. These results suggest that only muscarinic receptors could play a specific role on SS of the MPC.

Are the neurochemical and behavioral changes induced by lesions of the nucleus basalis in the rat a model of Alzheimer's disease?

A review of the work on the neurochemical, electroencephalographic and behavioral changes induced in the rat by lesions of the nucleus basalis is presented. The similarities and differences between the effects of the lesions and the neurochemical and clinical alterations characterizing senile dementia of Alzheimer type are pointed out. The decrease in choline acetyltransferase (ChAT) activity in the cortex following unilateral or bilateral electrolytic or neurotoxic lesions of the nucleus basalis are described and compared with the decrease in ChAT activity found in the cortex and hippocampus of patients affected by senile dementia. At variance with the latter condition, in rats with lesions of the nucleus basalis a spontaneous recovery in cortical ChAT activity has been observed 3-6 months after the lesion. The lesions of the nucleus basalis decrease high affinity choline uptake activity which, however, undergoes a rapid recovery. Lesions also decrease spontaneous and drug-stimulated ACh release from the cerebral cortex. Transitory changes in the number of muscarinic binding sites have been reported in the cerebral cortex of the lesioned rats while a decrease in the number of muscarinic binding sites has generally been found in the cerebral cortex of patients with senile dementia. [3H] glutamate uptake in the striatum of the lesioned rats was not affected. In both lesioned rats and patients affected by senile dementia, a decrease of low voltage high frequency electrocortical activity has been reported. Unilateral and bilateral lesions of the nucleus basalis bring about an impairment of the acquisition of active and passive avoidance responses and of the rewarded alternation discriminatory tasks involving working memory and spatial memory. On the other hand, memory impairment is a typical symptom of senile dementia. In conclusion, the lesions of the nucleus basalis only partly mimic the complex clinical picture of senile dementia of Alzheimer type. They offer, nevertheless, a useful tool for understanding the critical role of the central cholinergic pathways in some of the cognitive processes and identifying potentially useful pharmacological treatments.

Profound disturbances of spontaneous and learned behaviors following lesions of the nucleus basalis magnocellularis in the rat

It has been shown that a marked decline in the cortical activity of the cholinergic synthesizing enzyme choline-acetyltransferase (ChAT), accompanied by a severe neuronal loss in the nucleus basalis magnocellularis of Meynert occurs in the brains of patients with senile dementia of the Alzheimer type. However, the functional role of these neurons is largely unknown. In fact, very few studies have been done in animals. In this paper we report the behavioral effects of the lesion of the nucleus basalis magnocellularis in the rat either by radiofrequency current or by ibotenic acid injection at the level of the cell bodies. The two kinds of lesion lead to a profound disturbance of spontaneous and learned behaviors. There is a complete disorganization of behavior which is evidenced by an enhanced locomotor activity, an alteration in alimentary and hoarding behavior. In addition, we observed a deterioration of spatial memory and an incapacity to reverse a previously learned response. Biochemical assay showed that radiofrequency and ibotenic acid lesions produced a decrease of ChAT activity in the prefrontal and sensorimotor cortices and in amygdala without affecting the hippocampus or striatum. Ibotenic acid lesions seem to specifically destroy the cell bodies of the nucleus basalis magnocellularis since the dopaminergic and noradrenergic fibers of passage remained intact as measured by the unchanged level of endogenous catecholamine concentration in the terminal region in the prefrontal cortex. Presently, it cannot be said that the behavioral syndrome results solely from the lesion of the cholinergic neurons. Also, it is likely that the lesion of the nucleus basalis magnocellularis in the rat does not exactly reproduce the behavioral syndrome observed in Alzheimer's disease in man. However, this experimental approach in leading to a better knowledge of the functioning of these neurones could improve our understanding of this disease.

The pattern of cortical projections from the intermediate parts of the magnocellular nucleus basalis in the rat demonstrated by tracing with Phaseolus vulgaris-leucoagglutinin

The pattern and distribution of the cortical projections from intermediate parts of the cholinergic basal magnocellular nucleus were studied by anterogradely transported Phaseolus vulgaris-leucoagglutinin. This immunocytochemical tracing technique reveals the detailed morphology and distribution of efferents from this intermediate area in the nucleus basalis to the various areas and layers of cortex and amygdala. Major projections with a relatively high density of terminal boutons were found in layers I, II and VI of the frontal cortex, in layers V and VI of parietal and temporal areas, in the entire perirhinal and entorhinal cortices, and in the basolateral nucleus of the amygdaloid body. From the nucleus basalis area studied, few if any projections could be demonstrated to cingulate and occipital cortical regions.