Noncollateral projections of basal forebrain neurons to frontal and parietal neocortex in primates

Noradrenergic and cholinergic systems take centre stage in neuropsychiatric diseases of ageing

Noradrenergic and cholinergic systems are among the most vulnerable brain systems in neuropsychiatric diseases of ageing, including Alzheimer's disease, Parkinson's disease, Lewy body dementia, and progressive supranuclear palsy. As these systems fail, they contribute directly to many of the characteristic cognitive and psychiatric symptoms. However, their contribution to symptoms is not sufficiently understood, and pharmacological interventions targeting noradrenergic and cholinergic systems have met with mixed success. Part of the challenge is the complex neurobiology of these systems, operating across multiple timescales, and with non-linear changes across the adult lifespan and disease course. We address these challenges in a detailed review of the noradrenergic and cholinergic systems, outlining their roles in cognition and behaviour, and how they influence neuropsychiatric symptoms in disease. By bridging across levels of analysis, we highlight opportunities for improving drug therapies and for pursuing personalised medicine strategies.

α4β2∗ nicotinic receptors stimulate GABA release onto fast-spiking cells in layer V of mouse prefrontal (Fr2) cortex

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α4β2^∗ nAChRs stimulate IPSCs in FS interneurons, in layer V of the mouse PFC (Fr2).

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In P16–P63 mice, nicotine increased both IPSC and mIPSC frequencies.

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GABAergic terminals adjacent to PV+ cells expressed α4 nAChR.

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The percentage of FS cells with somatic α4β2^∗ currents decreased with age.

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Hence, nAChRs may be able to induce local circuit disinhibition in Fr2 PFC.

Nicotinic acetylcholine receptors (nAChRs) produce widespread and complex effects on neocortex excitability. We studied how heteromeric nAChRs regulate inhibitory post-synaptic currents (IPSCs), in fast-spiking (FS) layer V neurons of the mouse frontal area 2 (Fr2). In the presence of blockers of ionotropic glutamate receptors, tonic application of 10 μM nicotine augmented the spontaneous IPSC frequency, with minor alterations of amplitudes and kinetics. These effects were studied since the 3rd postnatal week, and persisted throughout the first two months of postnatal life. The action of nicotine was blocked by 1 μM dihydro-β-erythroidine (DHβE; specific for α4^∗ nAChRs), but not 10 nM methyllycaconitine (MLA; specific for α7^∗ nAChRs). It was mimicked by 10 nM 5-iodo-3-[2(S)-azetidinylmethoxy]pyridine (5-IA; which activates β2^∗ nAChRs). Similar results were obtained on miniature IPSCs (mIPSCs). Moreover, during the first five postnatal weeks, approximately 50% of FS cells displayed DHβE-sensitive whole-cell nicotinic currents. This percentage decreased to ∼5% in mice older than P45. By confocal microscopy, the α4 nAChR subunit was immunocytochemically identified on interneurons expressing either parvalbumin (PV), which mainly labels FS cells, or somatostatin (SOM), which labels the other major interneuron population in layer V. GABAergic terminals expressing α4 were observed to be juxtaposed to PV-positive (PV+) cells. A fraction of these terminals displayed PV immunoreactivity. We conclude that α4β2^∗ nAChRs can produce sustained regulation of FS cells in Fr2 layer V. The effect presents a presynaptic component, whereas the somatic regulation decreases with age. These mechanisms may contribute to the nAChR-dependent stimulation of excitability during cognitive tasks as well as to the hyperexcitability caused by hyperfunctional heteromeric nAChRs in sleep-related epilepsy.

Spatiotemporal specificity in cholinergic control of neocortical function

Cholinergic actions are critical for normal cortical cognitive functions. The release of acetylcholine (ACh) in neocortex and the impact of this neuromodulator on cortical computations exhibit remarkable spatiotemporal precision, as required for the regulation of behavioral processes underlying attention and learning. We discuss how the organization of the cholinergic projections to the cortex and their release properties might contribute to this specificity. We also review recent studies suggesting that the modulatory influences of ACh on the properties of cortical neurons can have the necessary temporal dynamic range, emphasizing evidence of powerful interneuron subtype-specific effects. We discuss areas that require further investigation and point to technical advances in molecular and genetic manipulations that promise to make headway in understanding the neural bases of cholinergic modulation of cortical cognitive operations.

Learning to ignore: a modeling study of a decremental cholinergic pathway and its influence on attention and learning

Learning to ignore irrelevant stimuli is essential to achieving efficient and fluid attention, and serves as the complement to increasing attention to relevant stimuli. The different cholinergic (ACh) subsystems within the basal forebrain regulate attention in distinct but complementary ways. ACh projections from the substantia innominata/nucleus basalis region (SI/nBM) to the neocortex are necessary to increase attention to relevant stimuli and have been well studied. Lesser known are ACh projections from the medial septum/vertical limb of the diagonal band (MS/VDB) to the hippocampus and the cingulate that are necessary to reduce attention to irrelevant stimuli. We developed a neural simulation to provide insight into how ACh can decrement attention using this distinct pathway from the MS/VDB. We tested the model in behavioral paradigms that require decremental attention. The model exhibits behavioral effects such as associative learning, latent inhibition, and persisting behavior. Lesioning the MS/VDB disrupts latent inhibition, and drastically increases perseverative behavior. Taken together, the model demonstrates that the ACh decremental pathway is necessary for appropriate learning and attention under dynamic circumstances and suggests a canonical neural architecture for decrementing attention.

Phasic acetylcholine release and the volume transmission hypothesis: time to move on

Traditional descriptions of the cortical cholinergic input system focused on the diffuse organization of cholinergic projections and the hypothesis that slowly changing levels of extracellular acetylcholine (ACh) mediate different arousal states. The ability of ACh to reach the extrasynaptic space (volume neurotransmission), as opposed to remaining confined to the synaptic cleft (wired neurotransmission), has been considered an integral component of this conceptualization. Recent studies demonstrated that phasic release of ACh, at the scale of seconds, mediates precisely defined cognitive operations. This characteristic of cholinergic neurotransmission is proposed to be of primary importance for understanding cholinergic function and developing treatments for cognitive disorders that result from abnormal cholinergic neurotransmission.

Modulators in concert for cognition: modulator interactions in the prefrontal cortex

Research on the regulation and function of ascending noradrenergic, dopaminergic, serotonergic, and cholinergic systems has focused on the organization and function of individual systems. In contrast, evidence describing co-activation and interactions between multiple neuromodulatory systems has remained scarce. However, commonalities in the anatomical organization of these systems and overlapping evidence concerning the post-synaptic effects of neuromodulators strongly suggest that these systems are recruited in concert; they influence each other and simultaneously modulate their target circuits. Therefore, evidence on the regulatory and functional interactions between these systems is considered essential for revealing the role of neuromodulators. This postulate extends to contemporary neurobiological hypotheses of major neuropsychiatric disorders. These hypotheses have focused largely on aberrations in the integrity or regulation of individual ascending modulatory systems, with little regard for the likely possibility that dysregulation in multiple ascending neuromodulatory systems and their interactions contribute essentially to the symptoms of these disorders. This review will paradigmatically focus on neuromodulator interactions in the PFC and be further constrained by an additional focus on their role in cognitive functions. Recent evidence indicates that individual neuromodulators, in addition to their general state-setting or gating functions, encode specific cognitive operations, further substantiating the importance of research concerning the parallel recruitment of neuromodulator systems and interactions between these systems.

Effects of cholinergic deafferentation of the rhinal cortex on visual recognition memory in monkeys

Excitotoxic lesion studies have confirmed that the rhinal cortex is essential for visual recognition ability in monkeys. To evaluate the mnemonic role of cholinergic inputs to this cortical region, we compared the visual recognition performance of monkeys given rhinal cortex infusions of a selective cholinergic immunotoxin, ME20.4-SAP, with the performance of monkeys given control infusions into this same tissue. The immunotoxin, which leads to selective cholinergic deafferentation of the infused cortex, yielded recognition deficits of the same magnitude as those produced by excitotoxic lesions of this region, providing the most direct demonstration to date that cholinergic activation of the rhinal cortex is essential for storing the representations of new visual stimuli and thereby enabling their later recognition.

Multiple output pathways of the basal forebrain: organization, chemical heterogeneity, and roles in vigilance

Studies over the last decade have shown that the basal forebrain (BF) consists of more than its cholinergic neurons. The BF also contains non-cholinergic neurons, including gamma-aminobutyric acid-ergic neurons which co-distribute and co-project with the cholinergic neurons. Both types of neuron project, in variable proportions, to the cerebral cortex, hippocampus, thalamus, amygdala, and olfactory bulb, whereas descending projections to the posterior hypothalamus and brainstem nuclei are predominantly non-cholinergic. Some of the cholinergic and non-cholinergic projection neurons contain neuropeptides such as galanin, nitric oxide synthase, and possibly glutamate. To understand better the function of the BF, the organization of the multiple ascending and descending projections of BF neurons is reviewed along with their neurochemical heterogeneity, and possible functions of individual pathways are discussed. It is proposed that BF neurons belong to multiple systems with distinct cognitive, motivational, emotional, motor, and regulatory functions, and that through these pathways, the BF plays a role in controlling both cognitive and non-cognitive aspects of vigilance.

Removal of cholinergic input to rat posterior parietal cortex disrupts incremental processing of conditioned stimuli

Recent research suggests that the basal forebrain cholinergic neurons innervating the cortex play a role in attentional functions in both primates and rodents. Among the cortical targets of these projections in primates is the posterior parietal cortex (PPC), a region shown to be critically involved in the regulation of attention. Recent anatomical studies have defined a cortical region in the rat that may be homologous to the PPC of primates. In the present study, cholinergic innervation of the PPC was depleted by intracortical infusion of the immunotoxin 192 IgG-saporin. Control and lesioned rats were then tested in two associative learning paradigms designed to increase attentional processing of conditioned stimuli (CSs). In one experiment, attention was manipulated by shifting a predictive relation between a light CS and another CS to a less predictive relation. Unlike control rats, lesioned rats failed to increase attention when the predictive relation was modified. In a second experiment, attentional processing of a tone CS was increased when its introduction during training coincided with a change in the value of the unconditioned stimulus, a phenomenon referred to as unblocking. Unlike control rats, lesioned rats failed to exhibit unblocking. In both paradigms, lesioned rats conditioned normally when the training procedures did not encourage increased attentional processing. These findings, across different behavioral paradigms and stimulus modalities, provide converging evidence that intact cholinergic innervation of the PPC is important for changes in attention that can increase the processing of certain cues.

Cognitive functions of cortical acetylcholine: toward a unifying hypothesis

Previous efforts aimed at attributing discrete behavioral functions to cortical cholinergic afferents have not resulted in a generally accepted hypothesis about the behavioral functions mediated by this system. Moreover, attempts to develop such a unifying hypothesis have been presumed to be unproductive considering the widespread innervation of the cortex by basal forebrain cholinergic neurons. In contrast to previous descriptions of the role of cortical acetylcholine (ACh) in specific behavioral phenomena (e.g., mediation of the behavioral effects of reward loss) or mnemonic entities (e.g., working or reference memory), cortical ACh is hypothesized to modulate the general efficacy of the cortical processing of sensory or associational information. Specifically, cortical cholinergic inputs mediate the subjects' abilities to detect and select stimuli and associations for extended processing and to allocate the appropriate processing resources to these functions. In addition to evidence from electrophysiological and behavioral studies on the role of cortical ACh in sensory information processing and attention, this hypothesis is consistent with proposed functions of the limbic and paralimbic networks in regulating the activity of the basal forebrain cholinergic neurons. Finally, while the proposed hypothesis implies that changes in activity in cortical ACh simultaneously occur throughout the cortex, the selectivity and precision of the functions of cholinergic function is due to its coordinated interactions with the activity of converging sensory or associational inputs. Finally, the dynamic, escalating consequences of alterations in the activity of cortical ACh (hypo- and hyperactivity) on cognitive functions are evaluated.

Topographical organization of the sources of discrete cortical projections within the striatum as determined by a retrograde fluorescence tracing technique in the cat

The projections from the neostriatum and the paleostriatum to the cerebral cortex in the cat were examined by means of retrogradely transported fluorescent tracers primuline, Fast Blue, Nuclear Yellow and Evans Blue injected into different neocortical fields. In all cases after dye injections only large labelled cells of sources of striatocortical ipsilateral projections were observed. The main projections from the caudate nucleus and the putamen are directed to the auditory and neighbouring "associative" cortex, and more numerous projections from the globus pallidus are addressed to the motor cortex. No sources of cortical projections within the entopeduncular nucleus were found. Simultaneous injections of Fast Blue and primuline into even closely located and tightly bound functional regions of parietal or temporal cortex failed to reveal double-labelled neurons in the caudate nucleus, internal capsule, putamen and globus pallidus. Thus, our findings on cats are consistent with recent studies on rats and monkeys that suggest that striatal neurons innervate relatively small, restricted fields of the neocortex. Again, the results show evidence for a significant contribution to cholinergic cortical innervation not only of magnocellular neurons of the basal forebrain but also of large neo- and paleostriatal cells.

The organization of the descending ventral pallidal projections in the monkey

This study describes the organization and topography of the descending efferent projections from the monkey ventral pallidum. The main efferent projections from the globus pallidus are to the subthalamus, to the thalamus, and to the substantia nigra. Although these projections have been well established for the dorsal pallidum, the projections of the ventral pallidum have not been explored in primates. The results of this study add an important link in how information from the limbic lobe is channeled through the basal ganglia in monkeys. Anterograde tracers, Phaseolus vulgaris-leucoagglutinin, and tritiated amino acids were injected into various regions of the ventral pallidum. The descending efferent projection from the ventral pallidum in monkeys terminates primarily in the subthalamic nucleus and adjacent lateral hypothalamus, in the substantia nigra, and in the lateral habenular nucleus. Although terminals are also found in the thalamus, these are relatively sparse. The projections to the subthalamic nucleus and the lateral hypothalamus are topographically arranged, while those to the substantia nigra are not. These results suggest that pathways from distinct pallidal regions that receive specific striatal input terminate in distinct regions of the subthalamic/hypothalamic regions, thus maintaining a topographic arrangement. Projections to the substantia nigra, however, overlap extensively, suggesting convergence of terminals from different ventral pallidal regions. The relatively small projection to the thalamus raises the question that without a prominent thalamic projection, is this system parallel to that described for the dorsal globus pallidus?

Pallidocortical projections to the tempolar polar gyrus in the cat

Marked projections from the globus pallidus (GP) to the temporal polar gyrus (TPG) of the cat were found by means of the PHA-L (Phaseolus vulgaris leucoagglutinin) and WGA-HRP (horseradish peroxidase conjugated to wheat germ agglutinin) methods. Pallidocortical fibers to the TPG originate mainly from the middle levels of the GP, and terminate all layers of the TPG cortex, especially in layers I, II and III. The GP neurons projecting to the TPG are large-multipolar or small-bipolar neurons. Almost all of these GP neurons show choline acetyltransferase-like immunoreactivity.

A study of cortical and hippocampal NMDA and PCP receptors following selective cortical and subcortical lesions

The neuronal localization of glutamate and phencyclidine (PCP) receptors was evaluated in the cerebral cortex and hippocampal formation of rat CNS using quantitative autoradiography. Scatchard analysis of [3H]glutamate binding in the cortex (layers I and II and V and VI) showed no difference in the total number of binding sites (Bmax) or apparent affinity (Kd) 1 week, 1 month and 2 months following unilateral ibotenate lesions to nucleus basalis of Meynert (nbM) compared to the non-lesioned side. Quisqualic acid displacement of [3H]glutamate in layers I and II, 1 week following nbM destruction, revealed both high- and low-affinity binding sites (representing the quisqualate (QA) and N-methyl-D-aspartate (NMDA) sites, respectively). Compared to the control side, there was no difference in binding parameters for either of the receptor sites. In similarly lesioned animals, the NMDA receptor was specifically labelled with [3H]glutamate and the associated PCP receptor labelled with [3H]N-(1-[2-thienyl]cyclohexyl)3,4-piperidine ([3H]TCP) in adjacent brain sections. For both receptors, there was no change in the total number of binding sites in the cortex following destruction of nbM. On the other hand, virtually all binding to NMDA and PCP receptors was eliminated following chemical destruction of intrinsic cortical neurons. These results suggest that the NMDA/PCP receptor complex does not exist on the terminals of cortical cholinergic afferents. One week after knife cuts of the glutamatergic entorhinal pathway to the hippocampal formation only an approximate 10% reduction of NMDA and PCP receptors was seen in the dentate gyrus. Conversely, selective destruction of the dentate granule cells using colchicine caused a near identical loss of NMDA and PCP receptors (84% vs 92% respectively). It is concluded from these experiments that glutamate and PCP receptors exist almost exclusively on neurons intrinsic to the hippocampal formation and that no more than 10% of NMDA and PCP receptors exist as autoreceptors on glutamatergic terminals.

Distribution of choline acetyltransferase-immunoreactive axons in monkey frontal cortex

Cholinergic neurons of the nucleus basalis of Meynert project to numerous regions of the cerebral cortex. However, little is known about the regional and laminar distributions of cholinergic axons in monkey frontal cortex. In this study, immunohistochemical techniques were used to identify axons that were immunoreactive for choline acetyltransferase, the enzyme that catalyses the synthesis of acetylcholine, in the frontal cortex of cynomolgus monkeys. Motor cortex contained the greatest density of labeled fibers: the density of labeled fibers was lower in premotor and anterior cingulate cortices and lower still in the association regions of prefrontal cortex. On a laminar basis, choline acetyltransferase-immunoreactive axons were most dense in layer I to superficial layer III. Layer V also contained a distinct band of labeled fibers that was particularly prominent in the agranular regions of frontal cortex. The density of labeled fibers was much lower in the deep portion of layer III to layer IV and in layer VI. These findings demonstrate a specific and regionally distinctive cholinergic innervation of monkey frontal cortex that may reveal the anatomical basis for the influence of acetylcholine on the diverse functions of primate frontal cortex.

Thalamic midline cell populations projecting to the nucleus accumbens, amygdala, and hippocampus in the rat

The organization of the thalamic midline efferents to the amygdaloid complex, hippocampal formation, and nucleus accumbens was investigated in the rat by means of multiple retrograde fluorescent tracing. The present findings indicate that these connections derive from separate cell populations of the thalamic midline, with a low degree of divergent collateralization upon more than one of the targets examined. The neural populations projecting to the amygdala, hippocampus, or accumbens are highly intermingled throughout the thalamic midline, but display some topographical prevalence. Midline thalamo-hippocampal cells are concentrated in the nucleus reuniens; thalamo-accumbens neurons prevail in the ventral portion of the paraventricular nucleus, and in the central medial nucleus. Thalamo-amygdaloid cells display a topographical prevalence in the rostral third of the thalamic midline and are concentrated in the dorsal part of the paraventricular nucleus and in the medial part of the nucleus reuniens. Both dorsally in the paraventricular nucleus and ventrally in the nucleus reuniens, thalamo-amygdaloid cells are located closer to the ependymal lining than the neurons projecting to the hippocampus or nucleus accumbens. Further, thalamo-amygdaloid cells, especially in the paraventricular nucleus, extend their dendritic processes in the vicinity of the ependymal lining, where they arborize profusely. These features indicate a close topographical relationship of neurons projecting to the amygdala with ependymal cells. The fairly discrete origin of midline outputs to the amygdala, hippocampus, and accumbens indicates that the flow of information is conveyed through separate channels from the thalamic midline to limbic and limbic-related targets. Together with the literature on the limbic afferents to the thalamus, these findings emphasize the relationships between the thalamus and the limbic system subserved by parallel input-output routes. However, because of the overlap of the projection cell populations, the thalamic midline may represent a locus of interaction among neurons connected with different parts of the limbic system. The functional implications of these findings are discussed in relation to the "nonspecific" thalamic system, as well as to the circuits involved in memory formation.

Basal forebrain and mesopontine tegmental projections to the reticular thalamic nucleus: an axonal collateralization and immunohistochemical study in the rat

Using a double fluorescence retrograde labeling procedure, the present study sought to determine the degree to which basal forebrain and mesopontine tegmental neurons have axons that innervate both the reticular thalamic nucleus and the cerebral cortex. Immunofluorescence for choline acetyltransferase, somatostatin, and the calcium-binding protein parvalbumin was also performed to elucidate the neurochemical identity of basal forebrain and mesopontine tegmental inputs to the reticular thalamic nucleus. A significant portion (10-15%) of neurons in the basal forebrain and mesopontine tegmentum that were retrogradely labeled from the reticular thalamic nucleus were also found to be retrogradely labeled from the cortex. Many of these neurons stained positively for choline acetyltransferase. Of the basal forebrain neurons retrogradely labeled from the reticular thalamic nucleus, approximately 20% were found to be immunoreactive to choline acetyltransferase, whereas none was stained for somatostatin. A larger portion (up to 50%) of the basal forebrain neurons that were retrogradely labeled from the reticular thalamic nucleus were parvalbumin-immunoreactive, and some of these were also retrogradely labeled from the cortex. These results suggest that a subpopulation of cholinergic and non-cholinergic neurons in the basal forebrain and the mesopontine tegmentum may influence simultaneously the activity of neurons in the reticular thalamic nucleus and the cerebral cortex.

GABAergic neurons in the primate basal forebrain magnocellular complex

Hybridization histochemistry was used to detect messenger ribonucleic acid (mRNA) coding for glutamic acid decarboxylase, the synthesizing enzyme for gamma-aminobutyric acid (GABA), in neurons of the nucleus basalis of Meynert and nucleus of the diagonal band of Broca of one rhesus monkey and 4 baboons. GABAergic neurons were distributed among the unlabeled large, hyperchromic Nissl-stained neurons characteristic of this basal forebrain magnocellular complex, although they were infrequent within the dense islands of large cells. Most GABAergic cells were small to medium in size, but some were large and hyperchromic. These findings demonstrate a heterogeneous population of presumably inhibitory neurons in the basal forebrain magnocellular complex of primates.

Aberrant phosphorylation of neurofilaments accompanies transmitter-related changes in rat septal neurons following transection of the fimbria-fornix

Lesions of the fimbria-fornix (FF) have been reported to cause retrograde changes in neurons of the medial septal nucleus (MSN). To analyze the nature and time course of these events, we investigated changes in cytoskeletal elements (phosphorylated and non-phosphorylated neurofilament (NF) proteins) and transmitter-related enzymes (choline acetyltransferase (ChAT) in MSN neurons following FF transection. During the first week postlesion, ChAT immunoreactivity and size of many perikarya were reduced. Irregular, swollen cholinergic fibers appeared first at postlesion day 2 in caudal septum and soon spread rostrally, reaching rostral septum by day 7. A few perikarya developed abnormal accumulations of phosphorylated NFs. At postlesion days 7-10, many neurons did not stain for ChAT. Phosphorylated NFs were present in many perikarya. At this time, cell loss was apparent in Nissl-stained material. Cholinergic cell loss continued through postlesion weeks 6-8 but at a much slower rate than during the first week. Phosphorylated NF accumulations in MSN perikarya persisted until postlesion week 6, disappearing thereafter. Double-immunostaining procedures showed that MSN neurons expressed both ChAT and phosphorylated NF immunoreactivity at postlesion day 3; however, at days 7 and 14, cells that accumulated phosphorylated NFs did not stain for ChAT. The results of this study indicate that FF transection leads to perikaryal shrinkage with loss of ChAT immunoreactivity, perikaryal phosphorylation of NFs, cholinergic fiber abnormalities, and cell loss. Recent evidence suggests that reduction of transmitter markers and aberrant phosphorylation of NFs may be involved in the pathogenesis of several neurodegenerative disorders, including Alzheimer's disease. Therefore, FF transection provides a useful animal model for further investigations of complex disorders of the central nervous system that involve degeneration of transmitter-specific pathways.

Brainstem projecting neurons in the rat basal forebrain: neurochemical, topographical, and physiological distinctions from cortically projecting cholinergic neurons

Magnocellular regions of the basal forebrain contain cholinergic neurons that project to the cerebral cortex. Neurons in the same basal forebrain regions innervate the brainstem. The present study investigated whether these brainstem projecting neurons are cholinergic, project also to the cortex, and share similar physiological properties as cortically projecting neurons. Data with retrograde tracing from various regions of the pons, medulla, and cortex combined with choline acetyltransferase immunofluorescence indicated that: 1) brainstem projecting neurons are usually segregated from cortically projecting and/or cholinergic neurons in the basal forebrain, 2) virtually no brainstem projecting neurons in the basal forebrain are cholinergic, and 3) only rarely do basal forebrain neurons have axon collaterals that project to both cortex and brainstem. Extracellular recordings from basal forebrain neurons confirmed the paucity of axonal collateralization and the topographic segregation between cortically and brainstem projecting basal forebrain neurons, and, in addition, showed that brainstem projecting neurons have a slower mean conduction velocity than cortically projecting neurons. These observations suggest that basal forebrain neurons projecting to the brainstem (pons, medulla) and the cortex represent separate cell populations in terms of projections, neurotransmitter content, distribution, and physiological properties.

Peptidergic neurons in the basal forebrain magnocellular complex of the rhesus monkey

The basal forebrain magnocellular complex of primates is defined by the presence of large, hyperchromic, usually cholinergic neurons in the nucleus basalis of Meynert and nucleus of the diagonal band of Broca. Because there is growing evidence for noncholinergic neuronal elements in the basal forebrain complex, five neuropeptides and the enzyme choline acetyltransferase were studied immunocytochemically in this region of rhesus monkeys. Galaninlike immunoreactivity coexists with choline-acetyl-transferase-like immunoreactivity in most large neurons and in some smaller neurons of the primate nucleus basalis and nucleus of the diagnonal band. Four other peptides show immunoreactivity in more limited regions of the basal forebrain complex, usually in separate smaller, noncholinergic neurons. Numerous small, somatostatinlike-immunoreactive neurons occupy primarily anterior and intermediate segments of the nucleus basalis, especially laterally and ventrally. Somewhat fewer, small neuropeptide Y-like-immunoreactive somata are found in the same regions. Neurons that show neurotensinlike immunoreactivity are slightly larger than cells that contain immunoreactivity for somatostatin or neuropeptide Y, but these neurons also occur mainly in anterior and intermediate parts of the nucleus basalis. Overall, the usually small, leucine-enkephalin-like-immunoreactive neurons are infrequent in the basal forebrain complex and are most abundant in the rostral intermediate nucleus basalis. Thus, neurons that appear to contain somatostatin, neuropeptide Y, neurotensin, or enkephalin mingle with cholinergic/galaninergic neurons only in some subdivisions of the nucleus basalis/nucleus of the diagonal band, and their distributions suggest that some of these small neurons could be associated with structures that overlap with cholinergic neurons of the labyrinthine basal forebrain magnocellular complex. We also have found light microscopic evidence for innervation of basal forebrain cholinergic neurons by boutons that contain galanin-, somatostatin-, neuropeptide Y-, neurotensin-, or enkephalinlike immunoreactivity. The origins and functions of these putative synapses remain to be determined.

Afferent connections of the forebrain cholinergic projection neurons, with special reference to monoaminergic and peptidergic fibers

Earlier light microscopic data on afferent connections to the cholinergic forebrain neurons are reconsidered in the light of EM cross-identification of neurons and synapses by combinations of tracer and immunocytochemical techniques. Such studies suggest that brainstem monoaminergic afferents terminate on cholinergic forebrain neurons, and may modulate the activity of choline acetyltransferase levels in the postsynaptic neurons. A monosynaptic relationship between cholinergic forebrain neurons and neuropeptide Y and somatostatin containing axons is also supported by studies using double immunolabeling techniques at the EM level. These peptidergic afferents originate in part from locally arborizing neurons. Based upon the new data a circuit model for basal forebrain cholinergic neurons is proposed.

Topographic, non-collateralized basal forebrain projections to amygdala, hippocampus, and anterior cingulate cortex in the rhesus monkey

Projections of the basal forebrain magnocellular complex to the limbic telencephalon of the primate were studied by combining double-retrograde tracing with immunocytochemistry. Tracers were injected into anterior cingulate cortex and hippocampus or into hippocampus and amygdala. Retrogradely labeled populations of neurons were topographically arranged but intermingled peripherally. Double-labeled neurons, found only after amygdala-hippocampus injections, were very rare. Approximately 30% of hippocampopetal, 50-70% of amygdalopetal, and 50-90% of cingulopetal neurons were cholinergic; percentages varied among different regions of basal forebrain. These findings further support the concept of a system with a highly organized efferent circuitry.

Cholinergic divergent projections from rat basal forebrain to the hippocampus and olfactory bulb

Cholinergic neurons in the basal forebrain which project to both the hippocampus and olfactory bulb were identified in the rat by using twin fluorescent retrograde tracers combined with fluorescence immunohistochemistry for choline acetyltransferase (ChAT). The majority of neurons simultaneously labeled with the two tracers were also identified as being ChAT-positive. They were located at the border between the horizontal and vertical limbs of the diagonal band of Broca. Their numbers were very small compared with neurons singly labeled from each of the projection areas.

Fiber pathways of basal forebrain cholinergic neurons in monkeys

In rhesus monkeys, autoradiographic tracing methods, complemented by immunocytochemical and histochemical techniques, were used to delineate pathways by which cholinergic neurons of the nucleus basalis of Meynert (nbM) and nucleus of the diagonal band of Broca (ndbB) project to forebrain targets. Following injections of [3H]amino acids into these nuclei, 5 major fiber pathways were identified: axons of the nbM and ndbB project medially, principally within the cingulum bundle, to dorsomedial portions of the hemispheres; nbM and ndbB fibers exit laterally beneath the pallidum and striatum, enter the external and extreme capsules, and pass within the corona radiata to terminate in lateral and caudal regions of neocortex; axons coursing ventrally from the nbM project to portions of the temporal lobe, including the amygdala; some fibers pass through the fibrae pass orbitofrontales to the orbitofrontal cortex; and, finally axons of the nbM/ndbB project via the fimbria/rornix and a ventral pathway to the hippocampus. The presence of these 5 radiolabeled pathways arising from basal forebrain cholinergic neurons was confirmed by acetylcholinesterase histochemistry and choline acetyltransferase immunocytochemistry.

Morphology of cortically projecting basal forebrain neurons in the rat as revealed by intracellular iontophoresis of horseradish peroxidase

The intracellular horseradish peroxidase technique was employed to study the morphology of basal forebrain neurons that were identified as cortically projecting by antidromic invasion from the cerebral cortex. Four neurons were examined in detail; they were located at different rostrocaudal levels within the basal forebrain. Their somata were large, 30-50 microns in longest dimension, and gave rise to three to eight primary dendrites, which ramified into third- to fifth-order dendrites. The longest observed dendrite in each neuron terminated at a distance of 600-900 microns from the soma. The sizes of soma and dendritic field of the two most rostrally located cells were smaller than those of the other two cells located more caudally. Dendritic spines were seen in all four cortically projecting basal forebrain neurons. Spines had shafts of variable lengths, and usually had spherical or elongated heads. The density of spines varied among the four neurons; one neuron, a type II cortically projecting basal forebrain neurons as defined physiologically by Reiner et al., had a much greater number of dendritic spines than the other three neurons, which were type I neurons. No somatic spines were observed. Presumptive axons were identified in three of the four cortically projecting basal forebrain neurons. These axons originated from either the soma or a primary dendrite, and two of them gave off local collaterals, which displayed occasional bouton-like swellings. The above observations confirm and extend previous findings that cortically projecting neurons in the basal forebrain are large multipolar cells, and provide evidence to support the conclusion that these cells, although somewhat variable in size, generally have extensive dendrites which display frequent spines.

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.

Basal forebrain neurons have axon collaterals that project to widely divergent cortical areas in the cat

Basal forebrain neurons with axon collaterals that project to widely divergent cortical areas were identified using retrograde transport of two labels. A proportion of neurons in the basal forebrain have axon collaterals that project to both anterior (precruciate gyrus) and posterior (marginal and suprasylvian gyri) cortical areas or to medial (precruciate gyrus) and lateral (ectosylvian and anterior suprasylvian gyri) cortical areas. These branched fibers originate from cells located predominantly in the basal nucleus of Meynert. The existence of such neurons suggests that individual basal forebrain cells are capable of influencing widespread neocortical zones in the cat.

Axonal branching of basal forebrain projections to the neocortex: a double-labeling study in the cat

Double-labeling of basal forebrain neurons by retrograde axonal transport demonstrates divergent collateralization among undecussated axonal projections to the neocortex. These branched fibers originate from a considerable complement of large polymorphic cell bodies located mainly in the basal nucleus of Meynert. They terminate in multiple neocortical sites including the precruciate, postcruciate and/or cingulate gyri. This extensive intra- and intergyral axonal branching indicates that neurons in the basal forebrain of the cat have extensive axonal fields innervating adjacent neocortical gyri.

Basal forebrain neurons provide major cholinergic innervation of primate neocortex

In 3 monkeys, lesions were made in the basal forebrain by microinjections of ibotenic acid into the nucleus basalis. Bilateral samples of multiple neocortical gyri were assayed for the activity of choline acetyltransferase. Compared to control hemispheres, enzyme activity was reduced up to 69% in the neocortex ipsilateral to the lesion; in addition, acetylcholinesterase staining was decreased at the lesioned site and in the ipsilateral cortex. These results support the concept that the principal cholinergic innervation of the primate neocortex is derived from axons and nerve terminals of neurons whose perikarya are located in the basal forebrain, particularly the nucleus basalis.