GABAergic basal forebrain neurons project to the neocortex: the localization of glutamic acid decarboxylase and choline acetyltransferase in feline corticopetal neurons

Our objective was to determine whether GABAergic and cholinergic basal forebrain neurons project to the neocortex. The retrograde connectivity marker wheat germ agglutinin lectin-bound horseradish peroxidase was injected into the neocortex of adult cats. Histo- and immunohistochemical methods were combined to label sequentially connectivity and transmitter markers (glutamic acid decarboxylase; choline acetyltransferase) in forebrain neurons. The labels of each marker were identified by correlative light and electron microscopy. Two principal types of doubly labeled neurons were demonstrated. The connectivity marker was colocalized with glutamic acid decarboxylase or choline acetyltransferase. The neurons were located in the basal forebrain. Their ultrastructural, cellular, and regional organization supported 2 conclusions. (1) GABAergic basal forebrain neurons project to the neocortex. This is important new morphological evidence for the origin of inhibitory neocortical afferents from a subcortical brain site. (2) The GABAergic and cholinergic basal forebrain neurons projecting to the neocortex exhibit remarkable structural similarities. The transmitter diversity of these intertwined neocortical afferents may be significant for the pathology and treatment of human neurological disorders such as Alzheimer's disease.     

Distribution of secretagogin-containing neurons in the basal forebrain of mice,with special reference to the cholinergic corticopetal system

Cholinergic and GABAergic corticopetal neurons in the basal forebrain play important roles in cortical activation, sensory processing, and attention. Cholinergic neurons are intermingled with peptidergic, and various calcium binding protein-containing cells, however, the functional role of these neurons is not well understood. In this study we examined the expression pattern of secretagogin (Scgn), a newly described calcium-binding protein, in neurons of the basal forebrain. We also assessed some of the corticopetal projections of Scgn neurons and their co-localization with choline acetyltransferase (ChAT), neuropeptide-Y, and other calcium-binding proteins (i.e., calbindin, calretinin, and parvalbumin). Scgn is expressed in cell bodies of the medial and lateral septum, vertical and horizontal diagonal band nuclei, and of the extension of the amygdala but it is almost absent in the ventral pallidum. Scgn is co-localized with ChAT in neurons of the bed nucleus of the stria terminalis, extension of the amygdala, and interstitial nucleus of the posterior limb of the anterior commissure. Scgn was co-localized with calretinin in the accumbens nucleus, medial division of the bed nucleus of stria terminalis, the extension of the amygdala, and interstitial nucleus of the posterior limb of the anterior commissure. We have not found co-expression of Scgn with parvalbumin, calbindin, or neuropeptide-Y. Retrograde tracing studies using Fluoro Gold in combination with Scgn-specific immunohistochemistry revealed that Scgn neurons situated in the nucleus of the horizontal limb of the diagonal band project to retrosplenial and cingulate cortical areas.     

The neglected constituent of the basal forebrain corticopetal projection system: GABAergic projections

At least half of the basal forebrain neurons which project to the cortex are GABAergic. Whilst hypotheses about the attentional functions mediated by the cholinergic component of this corticopetal projection system have been substantiated in recent years, knowledge about the functional contributions of its GABAergic branch has remained extremely scarce. The possibility that basal forebrain GABAergic neurons that project to the cortex are selectively contacted by corticofugal projections suggests that the functions of the GABAergic branch can be conceptualized in terms of mediating executive aspects of cognitive performance, including the switching between multiple input sources and response rules. Such speculations gain preliminary support from the effects of excitotoxic lesions that preferentially, but not selectively, target the noncholinergic component of the basal forebrain corticopetal system, on performance in tasks involving demands on cognitive flexibility. Progress in understanding the cognitive functions of the basal forebrain system depends on evidence regarding its main noncholinergic components, and the generation of such evidence is contingent on the development of methods to manipulate and monitor selectively the activity of the GABAergic corticopetal projections.     

Local synaptic connections of basal forebrain neurons

Single, biocytin filled neurons in combination with immunocytochemistry and retrograde tracing as well as material with traditional double-immunolabeling were used at the light and electron microscopic levels to study the neural circuitry within the basal forebrain. Cholinergic neurons projecting to the frontal cortex exhibited extensive local collaterals terminating on non-cholinergic, (possible GABAergic) neurons within the basal forebrain. Elaborate axon arbors confined to the basal forebrain region also originated from NPY, somatostatin and other non-cholinergic interneurons. It is proposed that putative interneurons together with local collaterals from projection neurons contribute to regional integrative processing in the basal forebrain that may participate in more selective functions, such as attention and cortical plasticity.     

Neurogenesis of basal forebrain cholinergic neurons in rat

The basal forebrain cholinergic system embodies a heterogeneous group of neurons distributed in the basal telencephalon that project topographically to the cortical mantle. We sought to examine the generation of these neurons to determine whether basal forebrain neurons have unique patterns of neurogenesis or, if, in contrast, they are born along general neurogenic gradients. The techniques of tritiated thymidine autoradiography and choline acetyltransferase (ChAT) immunocytochemistry were combined to determine the birthdays and neurogenic gradients of cholinergic cells in this region of rat brain. Cholinergic neurogenesis throughout the basal forebrain ranged from embryonic days 12 to 17 (E12-17). Neurogenesis in the nucleus basalis magnocellularis occurred over E12-16, with a peak day of generation on E13. The horizontal limb nucleus of the diagonal band which is located rostral to the nucleus basalis was generated over E12-17, with the majority of cells arising on E14-15. The rostral-most nuclei of the basal forebrain cholinergic system, the vertical limb of the diagonal band and the medial septum, were generated between E13-17, with peak days of neurogenesis on E15 and E15-16, respectively. These results were evaluated quantitatively and demonstrated that the basal forebrain cholinergic neurons were generated along the general caudal-to-rostral gradient previously described for all neurons in this brain region. The results of this study, in combination with those of similar investigations, emphasize that position-dependent epigenetic factors appear to be more potent determinants of the time of neuronal origin than factors which influence a cell's transmitter phenotype.     

Specific contributions of the basal forebrain corticopetal cholinergic system to electroencephalographic activity and sleep/waking behaviour

The present study examined the role of the basal forebrain corticopetal cholinergic projection in the regulation of cortical electroencephalographic activity across sleep/wake states in rats. Selective lesions of this projection were effected by local intraparenchymal infusions of the immunotoxin 192 IgG-saporin. Lesions spared the septo-hippocampal cholinergic system, as well as p75-receptor-bearing noncholinergic neurons in the suprachiasmatic nucleus. Relative to sham-lesioned control animals, rats with lesions of basal forebrain cholinergic neurons displayed a significant reduction in high frequency EEG activity, characterized especially by a reduction in gamma EEG power. Lesions did not significantly alter the overall proportion of sleeping and waking states as defined behaviourally, but the attenuation of high frequency EEG activity was apparent across all stages, including REM-like periods. Results are consistent with the view that the basal forebrain corticopetal cholinergic system exerts a general activational effect on the cortical mantle. Although this system may not be essential for sleep/wake stage-switching, it does impact on the cortical states associated with those stages.     

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.     

Morphological and electrophysiological characteristics of noncholinergic basal forebrain neurons

Cholinergic neurons in the basal forebrain are the focus of considerable interest because they are severely affected in Alzheimer's disease. However, both cholinergic and noncholinergic neurons are intermingled in this region. The goal of the present study was to characterize the morphology and in vivo electrophysiology of noncholinergic basal forebrain neurons. Neurons in the ventral pallidum and substantia innominata were recorded extracellularly, labeled juxtacellularly with biocytin and characterized for the presence of choline acetyltransferase immunoreactivity. Two types of ventral pallidal cells were observed. Type I ventral pallidal neurons had axons that rarely branched near the cell body and tended to have smaller somata and lower spontaneous firing rates than did type II ventral pallidal neurons, which displayed extensive local axonal arborizations. Subtypes of substantia innominata neurons could not be distinguished based on axonal morphology. These noncholineregic neurons exhibited local axon arborizations along a continuum that varied from no local collaterals to quite extensive arbors. Substantia innominata neurons had lower spontaneous firing rates, more variable interspike intervals, and different spontaneous firing patterns than did type II ventral pallidal neurons and could be antidromically activated from cortex or substantia nigra, indicating that they were projection neurons. Ventral pallidal neurons resemble, both morphologically and electrophysiologically, previously described neurons in the globus pallidus, whereas the substantia innominata neurons bore similarities to isodendritic neurons of the reticular formation. These results demonstrate the heterogeneous nature of noncholinergic neurons in the basal forebrain. J. Comp. Neurol. 394:186–204, 1998. © 1998 Wiley-Liss, Inc.     

GABAergic control of basal forebrain cholinergic neurons and memory

The involvement of the GABAergic innervation of basal forebrain neurons in the rats' conditional visual discrimination performance was examined. Performance in such a task is based on the subjects's ability to retrieve information about response rules, and previous experiments have demonstrated that basal forebrain lesions interfere with this ability. Following the acquisition of the task, chronic guide cannulae were stereotaxically implanted into the substantia innominata of both hemispheres, and the animals were retrained. Administration of the GABAA-agonist muscimol into the substantia innominata (0, 25, 50 ng/0.5 microliters/hemisphere) dose-dependently decreased the number of correct responses, increased the number of errors of omission, increased response latency, but did not affect side bias. Systemic co-administration of the cholinesterase inhibitor physostigmine (0, 0.1, 0.2 mg/kg; i.p.) exclusively interacted with the effects of muscimol on correct responding. Specifically, physostigmine dose-dependently intensified and attenuated the muscimol-induced reduction in correct responding. Although it cannot be excluded that alternative neuronal mechanisms were involved in the mediation of the effects of muscimol and their interaction with physostigmine, these findings support previous evidence indicating that the activity of basal forebrain cholinergic neurons is controlled by a GABAergic input, and that this neuronal link is involved in mnemonic processing.     

Sleep-waking discharge of basal forebrain projection neurons in cats

We have previously described a population of neurons in the magnocellular basal forebrain which have selectively elevated discharge rates during slow-wave sleep compared to waking; we postulate that these sleep-active neurons are a component of a basal forebrain sleep-promoting system. The purpose of the present experiment was to determine if sleep-active neurons contribute axons to recently described basal forebrain projection pathways. In cats prepared for chronic single unit and EEG-sleep recordings, stimulating electrodes were placed in the mesencephalic reticular formation, and the external capsule and anterior cingulate bundle, fiber bundles known to contain axons of basal forebrain projection neurons. Fifty-nine neurons were antidromically driven; differences in antidromic response latencies were related to sleep-waking discharge profiles. Of the cells with short antidromic latencies (less than 5 msec), the majority (9 of 12) had high discharge rates during waking and low rates during slow-wave sleep. Cells with long antidromic latencies had either very low discharge rates (less than 1 spike/sec) across all states, or had elevated discharge rates in slow-wave sleep. Sleep-active neurons were antidromically driven from external capsule (n = 9), anterior cingulate bundle (n = 9), or mesencephalic reticular formation (n = 5). Projection sleep-active neurons were recorded in the substantia innominata, ventral to the globus pallidus and medial to the central nucleus of the amygdala. Our study found that identified basal forebrain projection neurons in cats exhibit a variety of sleep-waking discharge patterns and conduction velocities. Sleep-active neurons were found to have slowly conducting axons, and to be a source of both ascending and descending projections.     

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.     

Developing cholinergic basal forebrain neurons are sensitive to thyroid hormone

The influence of thyroid hormone on the development of cholinergic neurons in nucleus basalis was assessed in hyperthyroid, hypothyroid, and euthyroid rats by use of CAT immunohistochemistry and single-section Golgi-impregnation histology. Animals were made either hyperthyroid by daily injections of 1.0 micrograms/gm body weight triiodothyronine starting at postnatal day (P) 3 or hypothyroid by providing 0.4% propylthiouracil in the diet of dams from P2. Compared to developing control rats, increased exposure to thyroid hormone resulted in accelerated expression of CAT in nucleus basalis neurons. Overshoot in cell body size, a normal developmental phenomenon of cholinergic neurons in the basal nuclear complex, occurred earlier in hyperthyroid brains and was of a greater magnitude than in controls. Furthermore, increased numbers of primary dendrites and dendritic branchpoints accompanied by dendritic and perisomal filopodia-like structures were observed for nucleus basalis neurons in hyperthyroid rats. These dendritic changes persisted throughout the second postnatal month. After the fifth postnatal week, cell body sizes of these hyperthyroid CAT-positive neurons began to decrease and by P50 were significantly less than controls or similarly treated animals at earlier ages. By P64, the number of cholinergic neurons in nucleus basalis was appreciably less than in age-matched controls. Hypothyroidism resulted in a delay of normal CAT expression that persisted throughout the third postnatal week. After this time, CAT staining increased until normal immunoreactivity was attained in cell bodies, fibers, and terminal regions by P35. A deficit in thyroid hormone during development prevented overshoot in perikaryal size and resulted in diminished cross-sectional areas throughout the cholinergic nucleus basalis at all ages examined. Hypothyroidism also prevented the normal overproduction of dendrites in those cells and produced stunted dendritic trees at all ages examined. These morphological abnormalities persisted throughout the second postnatal month. The effects of thyroid hormone on cholinergic projection neurons in the rat brain appeared relatively selective for cells in the basal nuclear complex because neither hypothyroid nor hyperthyroid treatment produced changes in the cell body areas of the phenotypically similar CAT-positive neurons of the pontomesencephalotegmental complex.(ABSTRACT TRUNCATED AT 400 WORDS)     

Cholinergic neurons in the basal forebrain of aged female mice

Aging is associated with at least down-regulation of several cellular functions and diminished responsiveness to internal and external signals, and possibly with direct cell death. Consequently, pharmacological manipulations may be less effective in aged than in young organisms. In the present study, we investigated whether the basal forebrain cholinergic neurons and the estrogen receptor alpha (ERalpha) which they contain respond to changes in estrogen availability in aged female mice. The mice were sham-operated, ovariectomized, or ovariectomized and treated with 17beta-estradiol at the age of 18 months. Three months later, the mice were perfused and brain sections were double immunostained for choline acetyltransferase (ChAT) and ERalpha. Cell counting with a stereological method revealed that changes in the estrogen level have no effect on the total number of ChAT-immunoreactive (ir) neurons in the basal forebrain. However, the percentage of ChAT-ir neurons containing ERalpha-ir was higher in the ovariectomized mice than in the sham-operated or estrogen-treated mice. This was specific for the medial septum and vertical diagonal band of Broca. The findings indicate that even at old age the ERalphas in cholinergic cells are able to respond to changes in estrogen levels, though in a region-specific manner. This is naturally important for studies aiming to develop therapies for the elderly.     

Tyrosine-hydroxylase-containing neurons in the primate basal forebrain magnocellular complex.

Immunocytochemistry and in situ hybridization for tyrosine hydroxylase (TH) were used to study the distribution of putative catecholaminergic neurons in the basal forebrain magnocellular complex (BFMC) of monkeys and humans. Magnocellular TH-expressing neurons in the primate BFMC are distributed along a rostrocaudal gradient, with the largest proportion of these cells located in the medial septal nucleus and nucleus of the diagonal band of Broca; smaller TH-containing neurons generally follow the same distribution. These findings suggest that, within rostromedial segments of the BFMC, there is a distinct subpopulation of neurons that express catecholamine-synthesizing enzymes. Further research is necessary to establish whether these neurons utilize one or more catecholamines as neurotransmitters.     

Acidic fibroblast growth factor mRNA is expressed by basal forebrain and striatal cholinergic neurons

Evidence for the importance of the basal forebrain cholinergic system in the maintenance of cognitive function has stimulated efforts to identify trophic mechanisms that protect this cell population from atrophy and dysfunction associated with aging and disease. Acidic fibroblast growth factor (aFGF) has been reported to support cholinergic neuronal survival and has been localized in basal forebrain with the use of immunohistochemical techniques. Although these data indicate that aFGF is present in regions containing cholinergic cell bodies, the actual site of synthesis of this factor has yet to be determined. In the present study, in situ hybridization techniques were used to evaluate the distribution and possible colocalization of mRNAs for aFGF and the cholinergic neuron marker choline acetyltransferase (ChAT) in basal forebrain and striatum. In single-labeling preparations, aFGF mRNA-containing neurons were found to be codistributed with ChAT mRNA⁺ cells throughout all fields of basal forebrain, including the medial septum/diagonal band complex and striatum. By using a double-labeling (colormetric and isotopic) technique, high levels of colocalization (over 85%) of aFGF and ChAT mRNAs were observed in the medial septum, the diagonal bands of Broca, the magnocellular preoptic area, and the nucleus basalis of Meynert. The degree of colocalization was lower in the striatum, with 64% of the cholinergic cells in the caudate and 33% in the ventral striatum and olfactory tubercle labeled by the aFGF cRNA. These data demonstrate substantial regionally specific patterns of colocalization and support the hypothesis that, via an autocrine mechanism, aFGF provides local trophic support for cholinergic neurons in the basal forebrain and the striatum. © 1996 Wiley-Liss, Inc.     

NGF mRNA is expressed by GABAergic but not cholinergic neurons in rat basal forebrain

Nerve growth factor (NGF) supports the survival and biosynthetic activities of basal forebrain cholinergic neurons and is expressed by neurons within lateral aspects of this system including the horizontal limb of the diagonal bands and magnocellular preoptic areas. In the present study, colormetric and isotopic in situ hybridization techniques were combined to identify the neurotransmitter phenotype of the NGF-producing cells in these two areas. Adult rat forebrain tissue was processed for the colocalization of mRNA for NGF with mRNA for either choline acetyltransferase, a cholinergic cell marker, or glutamic acid decarboxylase, a GABAergic cell marker. In both regions, many neurons were single-labeled for choline acetyltransferase mRNA, but cells containing both choline acetyltransferase and NGF mRNA were not detected. In these fields, virtually all NGF mRNA-positive neurons contained glutamic acid decarboxylase mRNA. The double-labeled cells comprised a subpopulation of GABAergic neurons; numerous cells labeled with glutamic acid decarboxylase cRNA alone were codistributed with the double-labeled neurons. These data demonstrate that in basal forebrain GABAergic neurons are the principal source of locally produced NGF. © 1995 Wiley-Liss, Inc.     

Inhibition of rostral basal forebrain neurons promotes wakefulness and induces FOS in orexin neurons

The present study examined whether the activities of the rostral basal forebrain neurons alter the activities of the orexin (also known as hypocretin) neurons in the tuberal part of the hypothalamus in rats. We performed microdialysis perfusion of the ventromedial portion of the rostral basal forebrain with the GABAA receptor agonist muscimol to inhibit focally the neuronal activities in the rostral basal forebrain. Then, we monitored sleep/wake behaviour and investigated the pattern of activities of orexin neurons by examining the expression of FOS as an indicator of cellular activation. Bilateral perfusion with muscimol (5, 15, and 50 micro m) produced a dose-dependent decrease in the amount of sleep. This perfusion with muscimol at 50 micro m produced FOS-like immunoreactivity in 37% of the orexin neurons located in the tuberal part of the hypothalamus, whereas the FOS-like immunoreactivity was sparse in orexin neurons of the sleeping control rats (P = 0.001 by Mann-Whitney U-test). Unilateral perfusion with muscimol (50 micro m) also suppressed sleep. In this case, FOS-like immunoreactivity was seen in 40% of the orexin neurons on the side ipsilateral to the perfusion site but only in 10% of orexin neurons on the contralateral side (P = 0.018 by Wilcoxon signed rank test). These functional data suggested that a sleep-generating element in the ventromedial part of the rostral basal forebrain provides an inhibitory influence on the activities of the orexin neurons in the tuberal part of the hypothalamus.     

Galanin-immunoreactive synaptic terminals on basal forebrain cholinergic neurons in the rat

Previous studies have indicated that galanin is one of the most abundant peptides in the basal forebrain and that it has a significant modulatory influence on cholinergic transmission. The aim of the present study was to use a light electron microscopic correlation technique to determine whether galanin-immunoreactive terminals form synaptic contacts with basal forebrain cholinergic cells of the rat. Sections from fixed-perfused brains were stained at the light and electron microscopic levels for galanin and choline acetyltransferase immunoreactivity in the same section by using a dual-colour immunohistochemical method. The results showed that galanin-immunoreactive axonal terminals are unevenly distributed in the medial septal nucleus, the diagonal band, and the nucleus basalis. Galanin-positive synapses were most prominent on choline acetyltransferase-positive neurons in the lateral parts of the nucleus of the diagonal band and in the posterior half of the nucleus basalis, which is where there was the greatest overlap between the distribution of galanin-immunoreactive terminals and choline acetyltransferase-positive neurons. The origins of these galanin-positive terminals are not known, but the results confirm that the basal forebrain galaninergic system has a synaptic influence on basal forebrain cholinergic neurons in the rat. J. Comp. Neurol. 383:82–93, 1997. © 1997 Wiley-Liss, Inc.     

Prefrontal cortical projections to the cholinergic neurons in the basal forebrain

The prefrontal cortex (PFC) projections to the basal forebrain cholinergic cell groups in the medial septum (MS), vertical and horizontal limbs of the diagonal band of Broca (VDB and HDB), and the magnocellular basal nucleus (MBN) in the rat were investigated by anterograde transport of Phaseolus vulgaris leuco-agglutinin (PHA-L) combined with acetylcholinesterase (AChE) histochemistry or choline acetyltransferase (ChAT) immunocytochemistry. The experiments revealed rich PHA-L-labeled projections to discrete parts of the basal forebrain cholinergic system (BFChS) essentially originating from all prefrontal areas investigated. The PFC afferents to the BFChS display a topographic organization, such that medial prefrontal areas project to the MS, VDB, and the medial part of the HDB, whereas the orbital and agranular insular areas predominantly innervate the HDB and MBN, respectively. Since the recurrent BFChS projection to the prefrontal cortex is arranged according to a similar topography, the relationship between the BFChS and the prefrontal cortex is characterized by reciprocal connections. Furthermore, tracer injections in the PFC resulted in anterograde labeling of numerous "en passant" and terminal boutons apposing perikarya and proximal dendrites of neurons in the basal forebrain, which were stained for the cholinergic marker enzymes. These results indicate that prefrontal cortical afferents make direct synaptic contacts upon the cholinergic neurons in the basal forebrain, although further analysis at the electron microscopic level will be needed to provide conclusive evidence.     

Potassium (K+) channel expression in basal forebrain cholinergic neurons

Basal forebrain cholinergic neurons (BFCN) are depleted early in the course of Alzheimer's disease (AD). BFCN voltage-gated K(+) channels regulate acetylcholine release and may play a role in BFCN neurodegeneration. Neuronal voltage-gated K(+) channels are heterotetrameric assemblies of K(v) and accessory subunits. Currently, there is no available information about the K(v) proteins expressed in BFCN. Immunohistochemical techniques were used to investigate the expression of specific K(v) subunits in rat brain BFCN. Our results showed that BFCN express both K(v)3.1 and K(v)2.1 subunits. However, the K(v)2.1 subunit showed a wider distribution in noncholinergic neurons than the K(v)3.1 subunit. K(v)3.1 and K(v)2.1 immunostaining was noticeable not only in neuronal cell bodies but also in the dendritic ramifications of these neurons. Insofar as the K(v)3.1 subunit has been classically associated with "fast-spiking neurons" and BFCN have low firing rates and long-duration action potentials, K(v)3.1 subunits may have functions other than facilitating high-frequency firing in BFCN.