Regional decreases of cortical choline acetyltransferase after lesions of the septal area and in the area of nucleus basalis magnocellularis

192 IgG-saporin lesions to the nucleus basalis magnocellularis (nBM) disrupt acquisition of learning set formation

Rats with bilateral 192 IgG-saporin lesions to the nucleus basalis magnocellularis (nBM) were tested on olfactory discrimination learning set (ODLS), olfactory discrimination reversal learning set (DRLS), and open field activity. Control animals demonstrated learning set in both the ODLS and DRLS tasks. The nBM-lesioned animals showed initial acquisition impairment in learning set in the ODLS task but eventually demonstrated learning set in both ODLS and DRLS tasks. There were no group differences in open-field activity. Results suggest that removal of the nBM cholinergic system through 192 IgG-saporin lesions impairs early acquisition of learning set compared to control animals, but does not prevent later use of learning set formation. Implications for the non-cholinergic basal forebrain cells in learning set are discussed.

Plastic changes in the central auditory system after hearing loss, restoration of function, and during learning

Traditionally the auditory system was considered a hard-wired sensory system; this view has been challenged in recent years in light of the plasticity of other sensory systems, particularly the visual and somatosensory systems. Practical experience in clinical audiology together with the use of prosthetic devices, such as cochlear implants, contributed significantly to the present view on the plasticity of the central auditory system, which was originally based on data obtained in animal experiments. The loss of auditory receptors, the hair cells, results in profound changes in the structure and function of the central auditory system, typically demonstrated by a reorganization of the projection maps in the auditory cortex. These plastic changes occur not only as a consequence of mechanical lesions of the cochlea or biochemical lesions of the hair cells by ototoxic drugs, but also as a consequence of the loss of hair cells in connection with aging or noise exposure. In light of the aging world population and the increasing amount of noise in the modern world, understanding the plasticity of the central auditory system has its practical consequences and urgency. In most of these situations, a common denominator of central plastic changes is a deterioration of inhibition in the subcortical auditory nuclei and the auditory cortex. In addition to the processes that are elicited by decreased or lost receptor function, the function of nerve cells in the adult central auditory system may dynamically change in the process of learning. A better understanding of the plastic changes in the central auditory system after sensory deafferentation, sensory stimulation, and learning may contribute significantly to improvement in the rehabilitation of damaged or lost auditory function and consequently to improved speech processing and production.

Acoustic frequency tuning of neurons in the basal forebrain of the waking guinea pig

The acoustic responses of cells in the basal forebrain were studied in the adult waking guinea pig. Frequency receptive fields were obtained across wide frequency (0.094-45.0 kHz) and intensity (0-90 dB) ranges. A total of 326 recordings were obtained in 26 electrode penetrations from five subjects; 205 from the globus pallidus (GP), 98 from the caudate-putamen (CPu) and 23 from the central nucleus of the amygdala (ACE). Twenty-nine recordings exhibited acoustic responses (GP=20 (9.8%); CPu=9 (9.2%); ACE=0). Cells in the regions of the GP that project to the primary auditory cortex (ACx) exhibited frequency tuning that was dominantly suppressive. Responses in the CPu were excitatory, but poorly tuned. The spontaneous rate of discharge of GP cells that yielded complete tuning data was positively correlated with power in the beta bands (12-25 and 25-50 Hz) and negatively correlated with power in the delta band (1-4 Hz) of the EEG of the ACx. These findings suggest that acoustically tuned neurons in the GP that are inhibited by tones are involved in the regulation of auditory cortical state, possibly promoting deactivation to unimportant sounds, and may be cholinergic in nature.

Learning-induced physiological memory in adult primary auditory cortex: receptive fields plasticity, model, and mechanisms

It is well established that the functional organization of adult sensory cortices, including the auditory cortex, can be modified by deafferentation, sensory deprivation, or selective sensory stimulation. This paper reviews evidence establishing that the adult primary auditory cortex develops physiological plasticity during learning. Determination of frequency receptive fields before and at various times following aversive classical conditioning and instrumental avoidance learning in the guinea pig reveals increased neuronal responses to the pure tone frequency used as a conditioned stimulus (CS). In contrast, responses to the pretraining best frequency and other non-CS frequencies are decreased. These opposite changes are often sufficient to shift cellular tuning toward or even to the frequency of the CS. Learning-induced receptive field (RF) plasticity (i) is associative (requires pairing tone and shock), (ii) highly specific to the CS frequency (e.g., limited to this frequency +/- a small fraction of an octave), (iii) discriminative (specific increased response to a reinforced CS+ frequency but decreased response to a nonreinforced CS- frequency), (iv) develops extremely rapidly (within 5 trials, the fewest trials tested), and (v) is retained indefinitely (tested to 8 weeks). Moreover, RF plasticity is robust and not due to arousal, but can be expressed in the deeply anesthetized subject. Because learning- induced RF plasticity has the major characteristics of associative memory, it is therefore referred to as "physiological memory". We developed a model of RF plasticity based on convergence in the auditory cortex of nucleus basalis cholinergic effects acting at muscarinic receptors, with lemniscal and nonlemniscal frequency information from the ventral and magnocellular divisions of the medial geniculate nucleus, respectively. In the model, the specificity of RF plasticity is dependent on Hebbian rules of covariance. This aspect was confirmed in vivo using microstimulation techniques. Further, the model predicts that pairing a tone with activation of the nucleus basalis is sufficient to induce RF plasticity similar to that obtained in behavioral learning. This prediction has been confirmed. Additional tests of the model are described. RF plasticity is thought to translate the acquired significance of sound into an increased frequency representation of behaviorally important stimuli.

Changes in the sensitivity of frontal cortical neurones to acetylcholine after unilateral lesion of the nucleus basalis with alpha-amino-3-OH-4-isoxozole propionic acid (AMPA): effects of basal forebrain transplants into neocortex

Unilateral S-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) lesions of the nucleus basalis magnocellularis (nbm), which produced persistent and extensive ChAT-positive cell loss within the nbm and depletion of cortical cholinergic markers in the frontal cortex, increased both the number and sensitivity of individual frontal cortical neurones responding to iontophoretic administration of ACh. The lesion also increased the sensitivity of individual neurones to carbachol but the increase in the number of neurones responding to carbachol was transient and had returned to normal 4 weeks after lesion. The sensitivity of individual neurones to glutamate was unchanged by the lesion. The percentage of cortical neurones responding to ACh, but not the sensitivity of individual neurones was restored to the prelesion level, 6-8 weeks after cholinergic transplants to the lesioned frontal cortex; cholinergic transplants to the more distant parietal cortex were only effective after 6 months whereas noncholinergic transplants were ineffective at both time intervals. Cholinergic transplants placed in the frontal cortex 6-8 weeks or 6 months before nbm lesion offered some protection from the effects of the lesion, particularly at 6 months but were ineffective when placed into the parietal cortex. Lesion of the nbm also reduced basal firing rate of spontaneously active neurones and this was not restored by any of the transplants. The results are discussed in the light of quantitative measurements of acetylcholinesterase-positive fibre outgrowth from the transplant into the recording area, which are described in the preceding manuscript [20].

Plastic changes in the cholinergic innervation of the rat cerebral cortex after unilateral lesion of the nucleus basalis with alpha-amino-3-OH-4-isoxozole propionic acid (AMPA): effects of basal forebrain transplants into neocortex

Unilateral AMPA lesions of the nucleus basalis magnocellularis (nbm) produced a nearly complete loss of cholinergic markers in the ipsilateral frontal and parietal cortices with no recovery at 6 months. The loss was associated with compensatory increases in AChE-positive fibre density in the contralateral cortex, in ipsilateral cortical regions not receiving their cholinergic innervation from the nbm and in the size of cholinergic magnocellular neurones in the contralateral nbm. The hypertrophy and increase in AChE-positive fibre density were apparent at 4-6 weeks after lesion and increased with time. Cholinergic transplants to cholinergically deafferented cortex prevented development of the compensatory increases in AChE-positive fibre density and restored AChE-positive fibre density and ChAT activity to control levels in ipsilateral cholinergically deafferented regions, partially after 6-8 weeks and completely after 6 months. In contrast, when cholinergic grafts were placed into unlesioned cortex, axonal outgrowth was localized to the vicinity of the transplant and did not develop with time. These results support the concept that vacant synapses promote and direct axonal outgrowth from transplanted neurones and that grafted cholinergic neurones integrate into the lesioned forebrain cholinergic projections system and prevent the lesion-induced changes in AChE-positive fibre density and ChAT activity.

Ameliorating effects of SDZ ENA 713 on age-associated decreases in learning performance and brain choline acetyltransferase activity in rats

In the present study, we have investigated the effects of SDZ ENA 713 on spatial learning deficits in aged rats. Using the same animals, the effect of SDZ ENA 713 on choline acetyltransferase was simultaneously studied to obtain a basis for the behavioral study. In the aged rats, the spatial learning and choline acetyltransferase activity in the frontal cortex were significantly deteriorated compared with young adult rats. SDZ ENA 713 (0.2 mg/kg) significantly shortened the time to reach a hidden platform without affecting swim rates in the water maze task. SDZ ENA 713 (0.1 and 0.2 mg/kg) inhibited aging-induced decreases in choline acetyltransferase activity in the frontal cortex. These results suggest that SDZ ENA 713 ameliorates aging-induced learning deficits and cholinergic dysfunction in rats.

Induction of a physiological memory in the cerebral cortex by stimulation of the nucleus basalis

Auditory cortical receptive field plasticity produced during behavioral learning may be considered to constitute "physiological memory" because it has major characteristics of behavioral memory: associativity, specificity, rapid acquisition, and long-term retention. To investigate basal forebrain mechanisms in receptive field plasticity, we paired a tone with stimulation of the nucleus basalis, the main subcortical source of cortical acetylcholine, in the adult guinea pig. Nucleus basalis stimulation produced electroencephalogram desynchronization that was blocked by systemic and cortical atropine. Paired tone/nucleus basalis stimulation, but not unpaired stimulation, induced receptive field plasticity similar to that produced by behavioral learning. Thus paired activation of the nucleus basalis is sufficient to induce receptive field plasticity, possibly via cholinergic actions in the cortex.

Behavioral and adaptive status in an experimental model of Alzheimer's disease in rats

Ten days after bilateral electrolytic lesions of nucleus basalis magnocellularis (NBM) we tested behavioral (spontaneous motor activity, acquisition and performance of two-way active avoidance, fear-response in open field test, foot shock induced aggression, depression-response in learned helplessness test) and adaptive status (body temperature at standard, hot and cold environment as well as cold restraint-induced gastric lesions) in adult male Wistar rats. Compared to intact control and sham-operated rats, the bilateral NBM-lesioned rats showed the significant impairment of learning behavior and reduced fear, aggression and depression as well as altered body temperature at standard and stressed conditions. Namely, it was established that body temperature in NBM-lesioned rats was significantly lower at standard laboratory conditions, but in these rats body temperature significantly was raised after exposing to cold and hot environment. On the other hand, spontaneous motor activity and number and length of cold restraint-induced gastric lesions (erosions and petechiae) in NBM-lesioned rats were similarly to those in both controls. It could be concluded that NBM plays a significant role in cognitive, emotional and adaptive processes in the rats.

Influence of ipsilateral lesions of the nucleus basalis magnocellularis and of choline alphoscerate treatment on histochemically reactive zinc stores and on the ultrastructure of the rat frontal cortex

The influence of ipsilateral lesions of the nucleus basalis magnocellularis (NBM) and of choline alphoscerate treatment on histochemically reactive zinc stores and on the ultrastructure of the neuropil of layer III of the frontal cortex were investigated in the rat. In control animals a dark-brown staining was developed in the neuropil of layers I-III of the frontal cortex. Lesions of the right NBM caused a marked reduction in the density of sulphide-silver staining in the right frontal cortex 4 weeks post lesion. Moreover, changes in the morphology and in the density of synaptic buttons in the neuropil of layer III of the cerebral cortex were also noticeable. Concomitant treatment for 4 weeks with choline alphoscerate restored the density of sulphide-silver staining in the right frontal cortex and countered in part changes of synaptic buttons of the neuropil of layer III of the frontal cortex. These findings suggest that the loss of cerebrocortical histochemically reactive zinc stores occurring in NBM-lesioned rats is due to the alterations of synaptic contacts in the frontal cortex and that treatment with choline alphoscerate may counter these degenerative changes.

Characterization of C6-10A glioma cells highly responsive to beta-adrenergic receptor agonist-induced NGF synthesis/secretion

A subline of rat C6 glioma cells, C6-10A cells, in which epinephrine can induce nerve growth factor (NGF) synthesis/secretion, was isolated. C6-10A cells have retained their sensitivity to epinephrine for more than 2 years in a medium containing 0.5% fetal calf serum (FCS) but easily lose it in 10% FCS. C6-10A cells are S-100- and glial fibrillary acidic protein-positive, and the doubling time is about 60 h in the medium containing 0.5% FCS and about 20 h in 10% FCS. Epinephrine induced NGF synthesis/secretion prominently in serum-free cultures of C6-10A cells and in cultures with a high cell density, but not in serum-containing cultures. The induction did not occur with parent C6 cells under the appropriate conditions in C6-10A cells. NGF secretion was induced by catecholaminergic compounds in the following order isoproterenol > epinephrine = norepinephrine >> dopamine. The induction caused by epinephrine was blocked by propranolol (beta-blocker) but not by phentolamine (alpha-blocker). Various compounds that activate the adenylate cyclase system also induced NGF synthesis/secretion. These results indicate that C6-10A cells are astrocytes that are highly responsive to beta-adrenergic receptor agonists, which stimulate NGF synthesis/secretion via receptors coupled with adenylate cyclase machinery. These cells may be a useful aid in studying the mechanism of NGF synthesis/secretion.

Aging of nucleolar organizer region in rat basal forebrain neurons related to learning and memory

The nucleolar organizer of rat basal forebrain neurons was studied with histochemical and morphometrical techniques, in order to analyze quantitatively the morphological correlate of activity as it changes during learning and aging of the brain. The learning abilities of adult (2 months) and senile (30 months) rats were tested with a conditioned response learning paradigm. Four groups of rats were defined: one group consisted of untrained adults, a second group consisted of trained adults, and the senile rats were subdivided into one group, which was able to learn and another group which was not able to perform the test. Frozen sections through the basal nucleus were silver stained to visualize the nucleolar organizer and the area of its profile was measured. The data showed that the nucleolar organizer in both the trained adult and the senile rats who were able to learn, was much larger than that in the untrained adults and the senile rats who were not able to learn. In the latter groups the nucleolar organizers were of equal size. An additional finding was, that the senile rats who could not be trained, showed a lower neuronal density in the basal nucleus compared with that of the trainable senile and adult rats. The changes in the size of the nucleolar organizer are interpreted as a morphological sign of learning-induced increases in transcription and synthesis of ribosomal RNA.

Reversal of learning and memory impairments following lesion of the nucleus basalis magnocellularis (NBM) by concurrent noradrenergic depletion using DSP4 in the rat

In the following study the behavioural effects of simultaneous lesion of the nucleus basalis magnocellularis (NBM) using ibotenic acid and noradrenergic depletion following a single i.p. administration of DSP4 (50 mg/kg) were examined in the rat. NBM lesion induced a deficit in acquisition of a reinforced T-maze alternation task, a working memory adaptation of a spatial navigation task in a water maze and 24 h retention in a passive avoidance task compared to sham controls. No effect of the lesion on a reference memory version of spatial navigation in a water maze task was found. Animals that received a combination of NBM lesion and DSP4 treatment showed no impairment on any of the tasks that were impaired by NBM lesion alone. This indicates a reversal of the learning and memory deficits consequent to NBM lesion by simultaneous noradrenergic depletion. NBM lesion induced a significant reduction in choline-acetyltransferase activity in the frontal cortex, and DSP4 induced a significant decrease in noradrenaline concentration in occipital cortex and hippocampus, confirming the effects of these treatments. These results suggest an interaction between central noradrenergic and cholinergic systems in learning and memory processes.

High-affinity transport of choline and amino acid neurotransmitters in synaptosomes from brain regions after lesioning the nucleus basalis magnocellularis of young and aged rats

Bilateral lesion of the nucleus basalis with ibotenic acid infusions in young and aged rats results in the degeneration of cholinergic neurons which innervate the cortex. As expected, high-affinity uptake of choline was decreased in the frontal cortex subsequent to the lesion. Twenty one days after surgery there was a significantly decrease of the transport rate of GABA, glutamate and glycine in the frontal cortex of young rats, but those activities showed a recovery six months after lesion. On the contrary, 12-month old rats lesioned with the same experimental protocol showed no recovery of the transport rates in the frontal cortex. Uptake of choline, GABA, glutamate and glycine has also been studied in other areas of the brain, namely, hippocampus, olfactory bulb and cerebellum. The present results suggest that lesioning the nucleus basalis of rats led to a more effective and permanent impairment of some biochemical functions of the brain, when compared to young lesioned animals, and also suggest a functional relationship between the nucleus basalis and other areas of the brain.

The potential for muscarinic receptor subtype-specific pharmacotherapy for Alzheimer's disease

In several neurodegenerative disorders, including Alzheimer's disease, a loss of the cholinergic projections of the basal forebrain to the cerebral cortex and hippocampus occurs. Studies of the anatomic and physiologic characteristics of these ascending cholinergic systems suggest that they are important in processing information and in memory function. Muscarinic receptors are situated at various critical control points in these pathways. Activation of postsynaptic muscarinic receptors often increases the excitability of neurons; thus, the signal-to-noise ratio for sensory processing is enhanced. In addition, muscarinic receptors negatively control cholinergic tone at presynaptic sites. Molecular biologic methods have disclosed the existence of five muscarinic receptors, which are coupled to different second messenger systems. The evidence reviewed suggests that at least four of the five muscarinic receptor genes are expressed as functional receptor proteins in the neocortex and hippocampal formation. On the basis of the current information about their pharmacologic properties and coupling mechanisms in nervous tissue, drugs that selectively affect subtypes of muscarinic receptors could enhance cortical cholinergic function and thereby ameliorate certain cognitive impairments in Alzheimer's disease.

Dissociative effects of ibotenic and quisqualic acid-induced basal forebrain lesions on cortical acetylcholinesterase-positive fiber density and cytochrome oxidase activity

The behavioral effects of excitatory amino acid-induced basal forebrain lesions have been conventionally attributed to the loss of cholinergic neurons innervating cortical areas. However, comparative examinations of quisqualic acid- and ibotenic acid-induced lesions to this region have suggested that the behavioral consequences of ibotenate-induced lesions may not be exclusively related to the loss of cholinergic neurons [Etherington R. et al. (1987) Neurosci. Res. Commun. 1, 135-143; Robbins T. W. et al. (1989) Neuroscience 28, 337-352]. These findings prompted the present investigation of the effects of quisqualic acid- and ibotenic acid-induced basal forebrain lesions on cortical cholinergic fiber density and cytochrome oxidase activity. Parallel brain sections from rats with unilateral lesions produced by each toxin were examined for cytochrome oxidase activity and acetylcholinesterase-positive fiber density, at a period of four, eight and 20 days postlesion. Quisqualic acid-induced lesions resulted in a greater loss of cortical acetylcholinesterase-positive fibers than did ibotenic acid-induced lesions, but the latter lesions produced a greater reduction in cytochrome oxidase activity. These results suggest that the loss of cortical cholinergic afferents does not contribute to the cortical metabolic decrease induced by infusions of ibotenic acid into the basal forebrain. Thus, the behavioral and metabolic consequences of ibotenic acid-induced lesions may be due to the destruction of an additional, noncholinergic pathway.

Neurochemical recovery in the neocortex after colchicine lesions of the nucleus basalis magnocellularis in rats

Neurochemical recovery was investigated in male, Fischer-344 rats up to 3 months after lesions of the nucleus basalis. Bilateral injections of colchicine (1.0 micrograms/site) into the nucleus basalis magnocellularis (NBM) resulted in a 30% decrease in choline acetyltransferase (ChAT) activity in frontal cortex 4 weeks after surgery, compared to unlesioned controls. ChAT activity in the frontal cortex gradually recovered to control levels by 12 weeks. The loss of ChAT-immunoreactive neurons in the NBM observed 4 weeks after surgery was still evident 12 weeks after surgery. These results suggest that surviving cholinergic neurons in the NBM contribute to recovery of ChAT activity in the neocortex.

Tetrahydroaminoacridine improves the spatial acquisition deficit produced by nucleus basalis lesions in rats

We administered tetrahydroaminoacridine (THA), a cholinesterase inhibitor, to rats with bilateral nucleus basalis magnocellularis lesions and measured their performance in a spatial learning task. The subjects, 34 male Fischer-344 rats, received bilateral excitotoxic NBM lesions; 10 other rats served as unlesioned controls. Two weeks later the animals were tested in a circular water maze for time and distance swum to find a submerged platform. We tested three different doses (5.0, 2.5, and 1.25 mg/kg) of daily subcutaneous THA against a lesioned control group receiving saline and a fifth group of untreated unlesioned controls. The saline-treated lesioned group showed a significant impairment of acquisition. The 1.25 mg/kg group performed significantly better than the lesioned controls with respect to latency. Analysis of swim speed data showed slowing in the 2.5 and 5.0 mg/kg groups. Analysis of the distance swum to find the platform, an untimed task that corrects for the difference in swim speeds, showed statistically significant improvement in all three treated groups. Additionally, spatial memory for the platform location was improved by two of the three doses of THA tested. Passive avoidance retention was not impaired by our lesion. All lesioned groups had comparable reductions of cortical choline acetyltransferase. Our data show significantly improved spatial learning with THA. These data provide an additional rationale for further clinical testing of THA and other centrally active cholinergic agents in diseases with cholinergic loss.

Frontal cortex as the site of action of physostigmine in nbM-lesioned rats

The administration of a variety of cholinomimetic agents to nucleus basalis of Meynert-lesioned rats has been shown to alleviate their lesion-induced memory deficits. This experiment attempted to determine whether the frontal cortex was the site of the memory enhancing action of the cholinomimetic physostigmine. Different groups of rats received excitotoxic lesions of the basal forebrain, the frontal cortex or both. Immediately after one trial passive avoidance training, these rats were injected with either saline or a 0.06 mg/kg dose of physostigmine. Physostigmine enhanced the 72-hour retention test performance of sham-operated and basal forebrain-lesioned rats, but failed to affect the performance of rats with cortical lesions. These data were interpreted as consistent with the hypothesis that the memory-enhancing effects of physostigmine are at least partially mediated by the frontal cortex.

Activating the damaged basal forebrain cholinergic system: tonic stimulation versus signal amplification

The hypothesis that the cognitive decline in senile dementia is related to the loss of cortical cholinergic afferent projections predicts that pharmacological manipulations of the remaining cholinergic neurons will have therapeutic effects. However, treatment with cholinesterase inhibitors or muscarinic agonists has been, for the most part, largely unproductive. These drugs seem to disrupt the normal patterning of cholinergic transmission and thus may block proper signal processing. An alternative pharmacological strategy which focuses on the amplification of presynaptic activity without disrupting the normal patterning of cholinergic transmission appears to be more promising. Such a strategy may make use of the normal GABAergic innervation of basal forebrain cholinergic neurons in general, and in particular of the inhibitory hyperinnervation of remaining cholinergic neurons which may develop under pathological conditions. Disinhibition of the GABAergic control of cholinergic activity is assumed to intensify presynaptic cortical cholinergic activity and to enhance cognitive processing. Although the extent to which compounds such as the benzodiazepine receptor antagonist beta-carboline ZK 93,426 act via the basal forebrain GABA-cholinergic link is not yet clear, the available data suggest that the beneficial behavioral effects of this compound established in animals and humans are based on indirect cholinomimetic mechanisms. It is proposed that an activation of residual basal forebrain cholinergic neurons can be achieved most physiologically via inhibitory modulation of afferent GABAergic transmission. This modulation may have a therapeutic value in treating behavioral syndromes associated with cortical cholinergic denervation.

Ontogeny of cholinergic neurons in the mouse forebrain

The development of cholinergic neurons in the mouse forebrain was studied by immunocytochemistry with a monoclonal antibody to choline acetyltransferase (ChAT), the rate-limiting enzyme for acetylcholine synthesis. Since this antibody stained dividing cells in ventricular germinal zones as well as differentiating neurons, likely routes of migration could be inferred on the basis of the location of immunoreactive (IR) cells at different gestational ages. Germinal zones for cholinergic cells were observed in all ventricular zones of the forebrain with the ventral zones generating the earliest cells by gestational day 13.5 (GD13.5). On GD14, ChAT IR cells were visible in the germinal zones of the eye, olfactory ventricle, anterior horn, and dorsolateral aspect of the lateral ventricle, lateral ganglionic eminence, ventro- and dorsolateral third ventricle, and in the pineal anlage (epiphysis). ChAT IR neurons continued to develop in these and additional germinal zones on GD15, including the medial, dorsal, and dorsomedial walls of the lateral ventricle, and the medial and dorsal ganglionic eminence. On GD16, ChAT IR neurons were located in the prelimbic, pyriform, and parietal cortices and the lamina terminalis, and a cluster of IR cells was observed in the ventricular zone of the caudatopallial angle. On GD17-18, neurons in the anterior olfactory nucleus, olfactory tubercle, horizontal and vertical nucleus of the diagonal band, and medial septal nucleus stained more darkly and were multipolar, whereas immature bipolar neurons appeared to continue their migration into the hippocampus and along major fiber tracts, such as the corpus callosum, external capsule, fornix and anterior commissure. This study provides a comprehensive view of the zones of origin, probable routes of migration, and final destination of cholinergic neurons in the mouse forebrain.

Muscarinic receptor plasticity in rats lesioned in the nucleus basalis of Meynert

The present study investigated the effects of chronic treatment with scopolamine (10 mg/kg i.p. for 21 days) on the muscarinic acetylcholine receptors in the frontoparietal cortex of rats, lesioned at the level of the nucleus basalis of Meynert. Ibotenic acid (25 nmol in 0.5 microliters) was injected bilaterally or unilaterally into the area of this nucleus and produced a major impairment of the cortical cholinergic system. These lesions depleted specifically frontoparietal cortical choline acetyltransferase activity. Sham-operated rats were similarly operated but no neurotoxin was injected. The chronic treatment with scopolamine caused a significant increase in the binding of [3H](-)quinuclidinylbenzilate to muscarinic receptors in the frontoparietal cortex of control and sham-operated rats but not in lesioned animals. This increase was due to an up-regulation in the number of muscarinic acetylcholine receptors, without significant change in their affinity. These results suggest that a functional presynaptic cholinergic terminal is necessary for the plasticity of muscarinic receptors in the central nervous system.

Changes in choline acetyltransferase immunoreactivity and the number of immunoreactive fibers remaining after lesions to the magnocellular basal nucleus of rats

Electrolytic and kainic acid lesions of the magnocellular basal nucleus of rats caused a homogeneous reduction in the density of choline acetyltransferase (ChAT)-immunoreactive fibers in the frontal and parietal cortices. ChAT immunoreactivity of the remaining fibers after unilateral lesions was increased ipsilaterally within the deafferentated areas. The number of intact immunoreactive fibers was consistently low through the period from 7 days to 6 months after the lesion. A previous finding that biochemically measured ChAT activity in the lesioned side recovered to the contralateral level should be interpreted as an increase in the content of ChAT in terminal axons rather than fiber sprouting.

Local cerebral glucose utilization following unilateral and bilateral lesions of the nucleus basalis magnocellularis in the rat

To investigate to what extent the loss of cholinergic projections to the neocortex results in functional impairment in the target areas, local rates of cerebral glucose utilization were measured following excitotoxin lesions of the nucleus basalis magnocellularis (NBM) in the rat. Both unilateral and bilateral lesions of NBM resulted in reversible depression of cerebral metabolism. The effects of unilateral lesions were limited to the cortical areas which receive most of the cholinergic projections from NBM. The metabolic defect produced by bilateral lesions was spread to the whole brain. Within 4 months, however, normal metabolic values coexisted with marked changes of the presynaptic cholinergic markers and impairment of conditioned behavior.

Scopolamine induces up-regulation of nicotinic receptors in intact brain but not in nucleus basalis lesioned rats

The effect of chronic scopolamine treatment on muscarinic and nicotinic receptors in frontoparietal cortex in rats was investigated. Administration of the muscarinic antagonist, scopolamine (10 mg/kg i.p./day) for 21 days, produced a significant increase in the density of both muscarinic and nicotinic receptors by 27.7% and 12.1% respectively as measured by the specific binding of (-)-[3H]quinuclidinylbenzilate and (-)-[3H]-nicotine. There was no modification in the affinities for these ligands. Rats, bilaterally lesioned with ibotenic acid at the level of nucleus basalis of Meynert, which innervates the frontoparietal cortex, showed no up-regulation of cortical nicotinic receptors after chronic scopolamine treatment, suggesting the importance of the synaptic integrity in the regulation mechanism.

Distribution of enkephalin-immunoreactive nerve fibres and terminals in the region of the nucleus basalis magnocellularis of the rat: a light and electron microscopic study

This investigation was carried out on the distribution of enkephalin-containing nerve fibres and terminals in the region of the nucleus basalis magnocellularis (NBM) of the rat. At the light microscope (LM) level, enkephalin-immunoreactive sites and endogenous choline acetyltransferase (ChAT) were demonstrated by employing the two-colour immunoperoxidase staining technique, using highly specific monoclonal antibodies against enkephalin and ChAT. A pharmacohistochemical procedure to reveal acetylcholinesterase (AChE)-synthesizing neurons combined with the peroxidase-antiperoxidase (PAP) immunocytochemical technique to detect endogenous enkephalins, provided ultrastructural data on the relationships of neuronal elements containing AChE and enkephalins in the region of the NBM. At the LM level, cholinergic neurons of the NBM were surrounded by a dense network of enkephalin-immunoreactive nerve fibres. Electron microscopic (EM) observations of histochemically characterized structures, that were first identified in the LM, revealed that intensely AChE-stained structures in the region of the NBM received sparse synaptic inputs from enkephalin immunoreactive terminals. Synaptic inputs of immunoreactive terminals onto intensely AChE-stained neuron cell bodies were not detected. Synaptic contacts onto proximal AChE-positive dendrites were sparse, but the density increased on more distal regions of the dendrites. All immunoreactive boutons studied established symmetrical synaptic contacts with AChE-positive post-synaptic structures. The pattern of the synaptic input to these cells differs strikingly from that onto typical globus pallidus neurons. The perikarya and dendrites of the latter neurons were characteristically ensheathed in immunoreactive synaptic boutons. Results are consistent with the view that enkephalin-like substances in the rat might be synaptic transmitters or neuromodulators in the area of the NBM and that cholinergic neurons of the NBM (Ch4) are integrated into the circuitry of the basal ganglia. Enkephalins may play an important role regulating the extrinsic cholinergic innervation of the neocortex.

Change in the distribution of acetylcholinesterase molecular forms in frontoparietal cortex of the rat following nucleus basalis lesions with kainic acid

The unilateral injection of kainic acid into the nucleus basalis magnocellularis (NBM) resulted in an alteration of the distribution of acetylcholinesterase (AChE) molecular forms in frontoparietal cortex ipsilaterally to the lesion. The G4/G1 ratio fell from 5.4 +/- 0.8 in contralateral to 3.0 +/- 0.5 in ipsilateral cortex. The NBM lesion effect thus, mimicks, the loss of tetrameric G4 form reported for various brain cortical areas of Alzheimer's disease (AD) patients. The data support the suggestion that G4 form is enriched in presynaptic nerve terminals.

Characteristics of memory impairment following lesioning of the basal forebrain and medial septal nucleus in rats

Memory impairment in rats with lesions of the basal forebrain (BF) and medial septal nucleus (MS) including cell bodies of the cortical and septohippocampal cholinergic systems, respectively, were compared in order to evaluate the functional contribution of the two cholinergic systems to memory. Biochemical assay revealed that lesioning of the BF and MS resulted in marked and selective decreases in both choline acetyltransferase and acetylcholinesterase activities in the cerebral cortex and hippocampus, respectively. Rats with BF lesions exhibited a severe deficit in a passive avoidance task; acquisition of passive avoidance by repeated training was sluggish, and the acquired response was rapidly eliminated in a subsequent extinction test. However, only slight impairment of passive avoidance was observed in rats with MS lesions. Memory impairment in rats with BF or MS lesions was also investigated using two spatial localization tasks, the Morris water task and the 8-arm radial maze task. Both BF and MS lesions elicited a significant impairment in the Morris water task that required reference memory, as demonstrated by an apparent increase in the latency to escape onto a hidden platform in a large water tank. The impairment was much more obvious in the BF-lesioned rats. In contrast, in the radial maze task primarily requiring working memory, rats with lesions of the MS showed severe disruption, exhibiting a marked increase in total errors, a decrease in the number of initial correct responses, and an apparent change in the strategy pattern. However, corresponding changes in the rats with BF lesions were slight. These results suggest that BF lesions may lead to substantial long-term memory impairment while MS lesions may primarily produce short-term or working memory impairment, indicating a qualitatively different contribution of the two cholinergic systems to memory. It is also suggested that these two experimental animal models may be useful for evaluation of therapeutic drugs for senile dementia of the Alzheimer type.

Correlation of choline acetyltransferase activity between the nucleus basalis of Meynert and the cerebral cortex

Choline acetyltransferase (ChAT) activities were measured in the cerebral cortex and the nucleus basalis of Meynert (nbM) of post-mortem human brains from 8 cases with Alzheimer type dementia (ATD) and 5 age-matched control subjects. The lowest ChAT activity was detected in the temporal cortex (Brodmann's area 22) and the nbM in ATD. A significant correlation was found between ChAT activities in the nbM and those in Brodmann's areas 4, 7, 10, 17 and 22. Present results provide evidence of a cholinergic projection from the nbM to the cerebral cortex observed by retrograde or anterograde degeneration studies in animals.

Changes in cholinergic markers following kainic acid lesion of the ventral globus pallidus in rat

Choline acetyltransferase (CAT) and glutamate decarboxylase (GAD) activities and [3H] quinuclidinyl benzilate ([3H]QNB) binding were determined in the rat frontal cortex following damage to the basal forebrain cholinergic system. Pre- and postsynaptic changes in the cholinergic system with the passage of time were also studied. After a unilateral injection of kainic acid into the right ventral globus pallidus, the GAD levels remained unaffected, but the CAT levels decreased to 63.4% after 7 days. After 12 weeks, the CAT levels had returned to 87% of the control value. The Bmax of [3H]QNB binding for the muscarinic receptor was higher in the ipsilateral cortex up to 4 weeks. On the other hand, the KD value at 12 weeks was higher without a change in the Bmax of the [3H]QNB binding. These findings might indicate an ongoing compensatory receptor mechanism of denervation supersensitivity as a response to early changes in presynaptic cholinergic activity and the production of postsynaptic effect with presynaptic cholinergic damage over a long period of time.

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.

Galanin-like immunoreactivity in hippocampal afferents in the rat, with special reference to cholinergic and noradrenergic inputs

The distribution of galanin-like immunoreactivity in the rat hippocampal formation (hippocampus and dentate gyrus) was studied and its origins were determined using various lesioning techniques. Special reference was made to the known cholinergic and noradrenergic hippocampal inputs from the septum-basal forebrain complex and locus coeruleus, both of which have previously been shown to co-contain galanin-like immunoreactivity at the cell body level. Galanin-immunoreactive fibers in the hippocampal formation were of at least three different morphological types: (1) Fine, slender, faintly immunoreactive fibers were seen throughout the hippocampal formation. (2) A strongly fluorescent varicose fiber population was observed mainly in the strata radiatum and oriens of the ventral CA3 region. (3) A population of fine, faint puncta was seen within the granule and pyramidal cell layers throughout the hippocampal formation. Knife cut lesions of the dorsal afferent pathways resulted in almost complete disappearance of all fiber types, except for the ventral fine fibers. Lesions of the fimbria affected mainly the coarse and punctate fiber types, while lesions of the supracallosal striae depleted mainly the fine fibers. Cuts anterior and ventral to the hippocampal formation caused a decrease in ventral fine fibers. Furthermore, lesions of the dorsal bundle caused an almost complete disappearance of the fine fibers in all regions of the hippocampal formation. Neurotoxin lesions of the diagonal band/septal complex resulted in decreases in faintly immunoreactive puncta within the granule cell layer and adjacent fine fibers. It is concluded that most fine galanin-positive fibers originate in the lower brain stem, presumably the locus coeruleus, and appear to reach the hippocampal formation primarily through the supracallosal striae and the ventral route. The fimbria seems to contain a large proportion of the fibers giving rise to the coarse strongly fluorescent innervation, which appears to originate rostral to the pons. The galanin-immunoreactive fibers originating in cholinergic somata of the diagonal band, medial septal nuclei, previously shown to project to the hippocampal formation, seem to give rise to faintly labeled puncta within the granule and pyramidal cell layers, and to a small proportion of the fine fibers bordering the cell layers, as revealed by immunohistochemistry using our antibody.(ABSTRACT TRUNCATED AT 400 WORDS)

Generation of cortical event-related slow potentials in the rat involves nucleus basalis cholinergic innervation

These experiments were conducted to gather information regarding the role of cholinergic innervation to the cortex in the generation of event-related slow potentials. The effects of unilateral drug treatments or lesions on ipsilateral and contralateral frontal cortex slow potential (SP) responses were examined in rats. The SP responses were recorded with silver-silver chloride electrodes and were generated by a 2 sec light cue which preceded rewarding medial forebrain bundle stimulation. The following approaches were used: microinjection of GABA, procaine or saline into the nucleus basalis magnocellularis; microinjection of atropine or saline subdurally in the SP recording area; electrolytic lesion of the nucleus basalis area; and kainic acid lesion of the nucleus basalis area. The following bilateral measurements were obtained lesion studies: choline acetyltransferase (ChAT) in cortex and hippocampus; serotonin in cortex, hippocampus, striatum and nucleus accumbens; norepinephrine in cortex and hippocampus; dopamine in striatum and nucleus accumbens; and metabolites of serotonin, norepinephrine and dopamine in these areas. The cortical SP responses were reduced on the side ipsilateral to the injections of GABA and procaine into the nucleus basalis, and on the side of the subdural atropine injection. With either type of lesion, the SP responses on the lesioned side were significantly reduced as compared to the non-lesioned side. Reductions in cortical ChAT and other measures were observed ipsilateral to the electrolytic lesion, but only cortical ChAT activity was reduced in the kainic acid-lesioned animals. Thus, pharmacological depression of nucleus basalis neurons, blockade of cholinergic muscarinic receptors in the cortex, and nucleus basalis lesions that reduce cortical choline acetyltransferase activity depress event-related slow potentials in the rat frontal cortex. These results provide evidence that cortical slow potential responses in the rat are dependent upon cholinergic innervation from the nucleus basalis.

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.

The identification of some sources of afferent input to the rat nucleus basalis magnocellularis by retrograde transport of horseradish peroxidase

Neurons of the nucleus basalis magnocellularis (NBm) of the rat are contained within the ventromedial globus pallidus and adjacent internal capsule. Horseradish peroxidase injection limited to the ventromedial globus pallidus result in sparse neuronal labeling in a variety of brainstem, thalamic and hypothalamic nuclei, and the basal nuclei identified after NBm injections. Thus, these contiguous regions have comparable subcortical inputs. By contrast, only NBm injections yielded a large number of labeled neurons in layer V of NBm cholinergic neurons. in addition to the reciprocity observed between NBm and frontal cortex, the ventral tegmental area and NBm likewise appear to be reciprocally connected.

Galanin-like immunoreactivity in cholinergic neurons of the septum-basal forebrain complex projecting to the hippocampus of the rat

It is now well recognized that there are several groups of cholinergic neurons in the basal forebrain with direct projections to various cortical regions. Immunohistochemical investigations of the distribution of the neuropeptide galanin (GAL) have shown that two of these brain areas, the medial septum and diagonal band, contained large numbers of GAL-immunoreactive neurons. In the present study, double staining techniques using antibodies raised against choline acetyltransferase (ChAT) revealed that GAL- and ChAT-like immunoreactivities are colocalized within a subpopulation of the cholinergic neurons within the medial septum and diagonal band. This colocalization of GAL- and ChAT-immunoreactivities was not seen to occur within other groups of forebrain cholinergic neurons. Immunohistochemistry carried out subsequent to injections of fluorescent retrograde tracers into the hippocampal formation revealed that both ChAT/GAL- and ChAT-containing neurons project to the hippocampal formation. The question of GAL as a modulator of cholinergic transmission in this projection is discussed.

Distribution of cholinergic muscarinic binding sites in guinea-pig brain as determined by in vitro autoradiography of [3H]N-methyl scopolamine binding

The distribution of muscarinic binding sites was analyzed in regions of the guinea-pig brain with semi-quantitative densitometry of [3H]N-methyl scopolamine binding, a muscarinic antagonist. In the rostral forebrain, high levels of binding were detected in the caudate putamen, nucleus accumbens and olfactory tubercle while intermediate levels of binding were observed in the medial and lateral septum, bed nucleus, and vertical and horizontal limbs of the diagonal band. The hypothalamus displayed binding that ranged from low levels in the preoptic area to intermediate levels in the mammillary nucleus. In limbic areas such as the thalamus, amygdala and hippocampus, a heterogeneous pattern of binding was evident in various subregions which tended to correspond with known innervation by cholinergic afferents. In the midbrain, binding was high in the superficial layer of the superior colliculus and the medial geniculate while intermediate binding was recorded in the lateral geniculate and the lateral aspect of the central gray. The pattern of muscarinic binding observed in the brain of the guinea-pig is similar to distributions of this binding site previously reported in the rat brain and the human brain.

Quinolinic acid neurotoxicity in the nucleus basalis antagonized by kynurenic acid

Quinolinic acid, a metabolite of tryptophan, behaves as an excitotoxic amino acid. It has been proposed that quinolinic acid might be implicated in neurodegenerative diseases. The related metabolite, kynurenic acid, has been found to be a powerful antagonist of quinolinic acid. The ability of quinolinic acid, alone or in combination with kynurenic acid, to destroy cholinergic neurons projecting to the cortex was examined by morphological and biochemical criteria. The compounds were injected unilaterally into the nbm of the rat. Neuronal destruction of the basal forebrain occurred with quinolinic acid alone; however, no cell loss was observed when kynurenic and quinolinic acid were co-injected. Quinolinic acid lesions of the nucleus basalis caused significant decreases in cortical choline acetyltransferase, acetylcholinesterase, high affinity choline uptake and 3H-acetylcholine release. These reductions in cortical cholinergic markers were prevented by co-injecting kynurenic with quinolinic acid. A significant decrease in cortical choline acetyltransferase activity was observed three months following quinolinic acid lesions of the nucleus basalis. The results indicate that quinolinic acid can be used as an endogenous neurotoxin to produce lesions of the nbm resulting in impaired cortical cholinergic function similar to that seen in Alzheimer's disease.

Transplantation of embryonic ventral forebrain neurons to the neocortex of rats with lesions of nucleus basalis magnocellularis--I. Biochemical and anatomical observations

Unilateral ibotenic acid lesions of the rat nucleus basalis magnocellularis produce approximately 60% depletion of choline acetyltransferase activity in ipsilateral frontal and frontoparietal neocortex. This depletion, which represents the loss of most of the extrinsic neocortical cholinergic input, is stable for at least 6 months. Embryonic ventral forebrain neurons survive transplantation to such cholinergically denervated neocortex. Cholinergic cells abound within these transplants and appear able to reinnervate the cholinergically depleted host cortex, as assessed histochemically and by measurement of choline acetyltransferase activity. Outgrowing fibres may extend beyond 2 mm from the grafts and often appear to be organized in an appropriate laminar pattern within the host cortex. Peptidergic neurons are sparse within the grafts and their fibres frequently appear unable to grow into the host tissue. Control grafts of non-cholinergic embryonic hippocampal cells survive well but have no effect on cortical depletions of acetylcholinesterase or choline acetyltransferase activity. Reconstruction of the extrinsic cholinergic input to the cortex by transplantation provides a useful tool for understanding the functions of this pathway.

Functional and neurochemical cortical cholinergic impairment following neurotoxic lesions of the nucleus basalis magnocellularis in the rat

The effect of kainic and quinolinic acid on cortical cholinergic function was examined following injections of these agents into the nucleus basalis magnocellularis (nbm) or into the frontoparietal cortex. The release of cortical 3H-acetylcholine (3H-ACh), high affinity choline uptake (HACU) and acetylcholinesterase was measured 7 days following injections of saline (control), kainic acid (4.7 nmoles) and quinolinic acid (60, 150 and 300 nmoles) into the nbm. These cortical cholinergic parameters were also examined after injections of saline (control), kainic acid (9.4 nmoles) and quinolinic acid (300 nmoles) into the fronto-parietal cortex. The release of 3H-ACh, HACU and AChE was significantly reduced in animals injected with kainic or quinolinic acid into the nbm. Histological examination of stained sections showed a loss of cell bodies in the region of the nbm and the globus pallidus. The size of the lesion produced by quinolinic acid was proportional to the dose injected into the nbm. In animals injected with kainic acid or quinolinic acid into the cerebral cortex, the release of 3H-ACh, HACU and AChE was not significantly reduced when compared with control animals, although histological examination of stained cortical sections showed a marked loss of cortical neurons. The results show that quinolinic acid, an endogenous neuroexcitant, produces a deficit of cholinergic function similar to that described in the cortical tissue of patients with senile dementia of Alzheimer's type. The toxic effects of quinolinic acid on cortical cholinergic function are due to its action on cholinergic cell bodies in the nbm.(ABSTRACT TRUNCATED AT 250 WORDS)

Memory impairments following basal forebrain lesions

The functional contribution of the nucleus basalis magnocellularis (NBM) and medial septal area (MSA) to memory was evaluated in 4 behavioral tasks. The tasks were postoperative acquisition of a win-stay spatial discrimination in a T-maze, a win-shift spatial discrimination on a radial arm maze, active avoidance in a shuttle box, and passive avoidance in a shuttle box. Bilateral lesions were made by injecting ibotenic acid (IBO) into the NBM or MSA. Control rats received operations in which no neurotoxin was injected. When compared to controls, rats with lesions in either the NBM or MSA had significantly impaired choice accuracy in the T-maze and radial maze tasks, took significantly fewer trials to reach criterion in the acquisition, but not the retention of an active avoidance task, and significantly more trials to reach criterion in the passive avoidance task. The results show that equivalent behavioral changes are obtained from lesions in the NBM and MSA in tasks that vary in their type of motivation, reinforcement, response-reinforcement contingency, and response. These behavioral changes suggest that the NBM and MSA may both be involved in memory.

Basal forebrain lesions produce a dissociation of trial-dependent and trial-independent memory performance

The behavioral effects of lesions in the basal forebrain (BF) of rats were evaluated using two tasks. The BF lesions included both the nucleus basalis magnocellularis (NBM) and the medial septal area (MSA). The first task was a Stone maze, which has 14 consecutive choice points and is a task of complex, trial-independent memory. BF lesions did not impair choice accuracy in this task. The second task was a win-shift spatial discrimination in a radial arm maze, which requires trial-dependent memory. BF lesions produced a significant decrease in choice accuracy in this task. These results demonstrate that BF lesions impair trial-dependent (working) memory but not trial-independent reference memory, and that task difficulty is not the sole factor determining whether BF lesions produce behavioral impairments.

Cholinergic modulation of mediodorsal thalamic input into cingulate cortex

Field potentials in cingulate cortex (area 24) produced by electrical stimulation of the mediodorsal thalamic nucleus were diminished by iontophoretic ejection of the cholinergic agonist, carbachol. The effect was frequency dependent: field potentials produced by 7.0 Hz stimulation were reduced by 34%. Potentials produced by 0.5 Hz stimulation were not significantly changed. This reduction was blocked by muscarinic but not nicotinic antagonists.