Malocclusion induces chronic stress

We examined the effect of occlusal disharmony in senescence-accelerated (SAMP8) mice on plasma corticosterone levels, spatial learning in the water maze, fos induction, hippocampal neuron number, expression of glucocorticoid receptors (GR) and glucocorticoid receptor messenger ribonucleic acid (GRmRNA) in hippocampus and inhibitor of glucocorticoid (metyrapone). Bite-raised aged mice had significantly greater plasma corticosterone levels than age-matched control mice as well as impaired spatial memory and decreased Fos induction and a number of neurons in hippocampus. GR and GRmRNA expressions were significantly decreased in aged bite-raised mice compared with age-matched control mice. Pretreatment with metyrapone inhibited not only the bite-raised induced increase in plasma corticosterone levels, but also the reduction in the number of hippocampal neurons and impaired spatial learning. These datas suggest that the bite-raised condition may enhance the aging process in hippocampus, thereby leading to impairment of spatial memory by stress.     

Duration-dependent effects of the bite-raised condition on hippocampal function in SAMP8 mice

We evaluated the effect of the duration of occlusal disharmony induced chronic stress on hippocampal function by examining spatial memory in the Morris water maze and on the number of hippocampal neurons in aged senescence-accelerated prone (SAMP8) mice. The bite of SAMP8 mice was raised 0.1 mm using dental adhesive. Groups of mice were tested in the Morris water maze 8, 11, or 22 d after raising the bite. The results indicated that the longer the duration of the bite-raised condition, the greater the impairment in spatial learning ability and the greater the decrease in the number of neurons in the hippocampal CA3 subfield. Thus, behavioral and morphologic deficits induced by the bite-raised condition in aged SAMP8 mice are influenced by the duration of the occlusal disharmony.     

Occlusal disharmony attenuates glucocorticoid negative feedback in aged SAMP8 mice

To evaluate the mechanism underlying impaired cognitive function due to occlusal disharmony, we examined the effect of the bite-raised condition on spatial performance and hippocampal expression of glucocorticoid receptors (GR) and glucocorticoid receptor messenger ribonucleic acid (GRmRNA) using behavioral, immunohistochemical, and in situ hybridization techniques. Learning ability in the water maze test was significantly impaired in aged bite-raised mice compared with age-matched control mice. There was no difference between control and bite-raised young and middle-aged mice. Also, immunohistochemical and in situ hybridization analysis showed that the bite-raised condition enhanced the age-related decrease in GR and GRmRNA expression in the hippocampus. In particular, GR and GRmRNA expressions were significantly decreased in aged bite-raised mice compared to age-matched control mice. These findings suggest that the bite-raised condition in aged SAMP8 mice decreases GR and GRmRNA, which impairs the hypothalamic-pituitary-adrenal feedback inhibition, thereby leading to memory deficits.     

Occlusal disharmony induces spatial memory impairment and hippocampal neuron degeneration via stress in SAMP8 mice

We examined the effect of occlusal disharmony in senescence-accelerated (SAMP8) mice on plasma corticosterone levels, hippocampal neuron number, and spatial performance in the water maze. The bite-raised condition was associated with an accelerated age-related decline in spatial memory, increased plasma corticosterone levels, and a decreased number of neurons in the hippocampal CA3 region. The findings suggest that the bite-raised condition in aged SAMP8 mice induces hippocampal neuron loss, thereby leading to senile memory deficits.     

Intraseptal muscarinic ligands and galanin: influence on hippocampal acetylcholine and cognition

The cholinergic neurons in the septohippocampal projection are implicated in hippocampal functions such as spatial learning and memory. The aim of this study was to examine how septohippocampal cholinergic transmission is modulated by muscarinic inputs and by the neuropeptide galanin, co-localized with acetylcholine (ACh) in septohippocampal cholinergic neurons, and how spatial learning assessed by the Morris water maze test is affected. Muscarinic inputs to the septal area are assumed to be excitatory, whereas galanin is hypothesized to inhibit septohippocampal cholinergic function. To test these hypotheses, compounds were microinjected into the medial septum and hippocampal ACh release was assessed by microdialysis probes in the ventral hippocampus of the rat. Blockade of septal muscarinic transmission by intraseptal scopolamine increased hippocampal ACh release suggesting that septal cholinergic neurons are under tonic inhibition. Stimulation of septal muscarinic receptors by carbachol also increased hippocampal ACh release. Despite this increase, both scopolamine and carbachol tended to impair hippocampus-dependent spatial learning. This finding also suggests a revision of the simplistic notion that an increase in hippocampal ACh may be facilitatory for learning and memory. Galanin infused into the medial septum enhanced hippocampal ACh release and facilitated spatial learning, suggesting that septal galanin, contrary to earlier claims, does not inhibit but excites septohippocampal cholinergic neurons. Galanin receptor stimulation combined with muscarinic blockade in the septal area resulted in an excessive increase of hippocampal ACh release combined with an impairment of spatial learning. This finding suggests that the level of muscarinic activity within the septal area may determine the effects of galanin on hippocampal cognitive functions. In summary, a limited range of cholinergic muscarinic transmission may contribute to optimal hippocampal function, a finding that has important implications for therapeutic approaches in the treatment of disorders of memory function.     

The bite-raised condition enhances the aging process in the dorsal and ventral hippocampus

The bite raised condition decreases the number of neurons and increases the amount of glial fibrillary acidic protein in the hippocampus of aged SAMP8 mice. In the present study, we examined whether these effects differ between the dorsal and ventral hippocampus. In bite-raised SAMP8 mice, the number of neurons was significantly lower in the hippocampal CA1 and dentate gyrus (DG) subfields compared to control mice. In the bite raised condition, the number of neurons was significantly lower in both the dorsal and ventral CA3 subfields, and the number of glial fibrillary acidic protein-labeled astrocytes was increased in the CA1, CA3, and DG subfields, compared to control mice. These data suggest that in aged SAMP8 mice, the bite-raised condition enhanced aging processes in both the dorsal and ventral hippocampus.     

The bite-raised condition in aged SAMP8 mice reduces the expression of glucocorticoid receptors in the dorsal and ventral hippocampus

In the present study, we examined whether the effects induced by the bite-raised condition on glucocorticoid receptor (GR) expression differ between the dorsal and ventral hippocampus in SAMP8 mice. In the bite-raised condition, the number of GR-immunoreactive cells was significantly decreased in both the dorsal and ventral CA1 and dentate gyrus (DG) subfields of the hippocampus compared to control mice, as revealed by immunohistochemical analysis. The decrease in the number of GR-immunoreactive cells tended to be greater in the dorsal hippocampus than in the ventral hippocampus. Only in the DG subfield was there a significant difference in the number of GR-immunoreactive cells between the dorsal and ventral hippocampus. These findings suggest that in aged SAMP8 mice, the bite-raised condition decreases the number of GR-immunoreactive cells in both the dorsal and ventral hippocampus.     

Increased expression of hippocampal cholinergic neurostimulating peptide-related components and their messenger RNAs in the hippocampus of aged senescence-accelerated mice

Hippocampal cholinergic neurostimulating peptide stimulates cholinergic phenotype development by inducing choline acetyltransferase in the rat medial septal nucleus in vitro. Adult senescence-accelerated-prone mice/8, a substrain of the senescence-accelerated-prone mouse, show a remarkable age-accelerated deterioration in learning and memory. We cloned mouse hippocampal cholinergic neurostimulating peptide precursor protein complementary DNA. The deduced amino acid sequence showed that the neurostimulating peptide itself is the same as that found in the rat. In situ hybridization revealed that the highest expression of the precursor protein messenger RNA was in hippocampal pyramidal neurons. Compared with a strain of senescence-accelerated-resistant mouse (control mouse), adult senescence-accelerated-prone mice/8 showed increased expression of both the precursor messenger RNA and the neurostimulating peptide-related immunodeposits in the hippocampal CA1 field. The deposits were intensely and diffusely precipitated in neuropils throughout the strata oriens and radiatum in senescence-accelerated-prone mice/8, but not in control mice. The neurostimulating peptide content in the hippocampus was higher in senescence-accelerated-prone mice/8 than in control mice, while its precursor protein itself was not different between the two strains. Furthermore, our previous and present data show that the medial septal and hippocampal choline acetyltransferase activity was significantly lower in senescence-accelerated-prone mice/8 than in control mice. The data suggest that, in hippocampal neurons in adult senescence-accelerated-prone mice/8, the production of hippocampal cholinergic neurostimulating peptide precursor protein in neuronal somata, which is associated with an increased expression of its messenger RNA in the CA1 field, occurs as a consequence of low activity in their presynaptic cholinergic neurons. This is followed by accelerated processing to generate bioactive peptide and transport to its functional fields. However, certain mechanisms reduce the release of the peptide and lead to its accumulation in the neuropil. These disturbances of the septohippocampal cholinergic system might be the biochemical mechanism underlying the characteristic deterioration of senescence-accelerated-prone mice/8.     

The bite-raised condition in aged SAMP8 mice induces dendritic spine changes in the hippocampal region

The present study examined the effect of the bite-raised condition on the number of dendritic spines on hippocampal pyramidal cells in SAMP8 mice and related the results with learning ability in a water maze test. Aged mice in the bite-raised condition had reduced learning ability and a lower number of CA1 pyramidal cell dendritic spines. The results suggest that the bite-raised condition exacerbates the age-related spatial learning impairment, and that this may be due to the degeneration of hippocampal dendritic spines.     

Histamine innervation and activation of septohippocampal GABAergic neurones: involvement of local ACh release

Recent studies indicate that the histaminergic system, which is critical for wakefulness, also influences learning and memory by interacting with cholinergic systems in the brain. Histamine-containing neurones of the tuberomammillary nucleus densely innervate the cholinergic and GABAergic nucleus of the medial septum/diagonal band of Broca (MSDB) which projects to the hippocampus and sustains hippocampal theta rhythm and associated learning and memory functions. Here we demonstrate that histamine, acting via H₁ and/or H₂ receptor subtypes, utilizes direct and indirect mechanisms to excite septohippocampal GABA-type neurones in a reversible, reproducible and concentration-dependent manner. The indirect mechanism involves local ACh release, is potentiated by acetylcholinesterase inhibitors and blocked by atropine methylbromide and 4-DAMP mustard, an M₃ muscarinic receptor selective antagonist. This indirect effect, presumably, results from a direct histamine-induced activation of septohippocampal cholinergic neurones and a subsequent indirect activation of the septohippocampal GABAergic neurones. In double-immunolabelling studies, histamine fibres were found in the vicinity of both septohippocampal cholinergic and GABAergic cell types. These findings have significance for Alzheimer's disease and other neurodegenerative disorders involving a loss of septohippocampal cholinergic neurones as such a loss would also obtund histamine effects on septohippocampal cholinergic and GABAergic functions and further compromise hippocampal arousal and associated cognitive functions.     

Cholinergic and peptidergic projections from the medial septum and the nucleus of the diagonal band of Broca to dorsal hippocampus, cingulate cortex and olfactory bulb: a combined wheatgerm agglutinin-apohorseradish peroxidase-gold immunohistochemical study

We have examined the distribution pattern and the density of various neuropeptide, neurotransmitter and enzyme containing neurons in the rat medial septum and the nucleus of the diagonal band of Broca to assess their possible involvement in the septohippocampal, septocortical and septobulbar pathways. Immunohistochemical methods were combined with the retrograde transport of a protein-gold complex injected in the hippocampus, the cingulate cortex or the olfactory bulb. Cholinergic neurons were the most numerous. Galanin-positive neurons were about two or three times less numerous than cholinergic cells. Both these cell types had a similar location though the choline acetyl transferase-like immunoreactive cells extended more caudally in the horizontal limb of the nucleus of the diagonal band of Broca. Immunoreactive cells for other neuroactive substances were few (calcitonin gene-related peptide, luteinizing hormone releasing hormone. [Met]enkephalin-arg-gly-leu) or occasional (dynorphin B, vasoactive intestinal polypeptide, somatostatin, neurotensin, cholecystokinin, neuropeptide Y and substance P). No immunoreactive cells for bombesin, alpha atrial natriuretic factor, corticotropin releasing factor, 5-hydroxytryptamine, melanocyte stimulating hormone, oxytocin, prolactin, tyrosine hydroxylase or arg-vasopressin were present. Choline acetyltransferase- and galanin-like immunoreactive cells densely participate to septal efferents. Cholinergic neurons constituted the bulk of septal efferent neurons. Galanin-positive cells were 22% of septohippocampal, 8% of septocortical, and 9% of septobulbar neurons. Galanin containing septohippocampal neurons were found in the medial septum and the nucleus of the diagonal band of Broca; galanin-positive septobulbar and septocortical cells were limited to the nucleus of the diagonal band of Broca. Occasional double-labellings were noticed with some peptides other than galanin. Luteinizing hormone-releasing hormone, calcitonin gene-related peptide and enkephalin were the most often observed; some other projecting cells stained for vasoactive intestinal polypeptide or dynorphin B. Luteinizing hormone-releasing hormone, calcitonin gene-related peptide and enkephalin were observed in septohippocampal neurons; luteinizing hormone-releasing hormone and vasoactive intestinal peptide were observed in septocortical neurons and calcitonin gene-related peptide, luteinizing hormone-releasing hormone and dynorphin B were observed in septo-bulbar cells. These results show that, in addition to acetylcholine, galanin is a major cellular neuroactive substance in septal projections to the hippocampus, the cingulate cortex and the olfactory bulb. The presence of septal projecting neurons immunoreactive for other peptides shows that a variety of distinct peptides may also participate, but in a smaller number, to septal efferent pathways.     

Group I metabotropic glutamate receptor activation produces a direct excitation of identified septohippocampal cholinergic neurons

Septohippocampal cholinergic neurons innervate the hippocampus and provide it with almost its entire acetylcholine. Axon collaterals of these neurons also release acetylcholine within the septum and thereby maintain the firing activity of septohippocampal GABAergic neurons. A loss of septohippocampal cholinergic neurons occurs in various neurodegenerative disorders associated with cognitive dysfunctions. group I metabotropic glutamate receptors have been implicated in septohippocampal-dependent learning and memory tasks. In the present study, we examined the physiological and pharmacological effects of a potent and selective group I metabotropic glutamate receptor (mGluR) agonist S-3,5-dihydroxyphenylglycine (DHPG) on rat septohippocampal cholinergic neurons that were identified in brain slices using a selective fluorescent marker. In whole cell recordings, DHPG produced a reversible, reproducible and a direct postsynaptic and concentration-dependent excitation in 100% of septohippocampal cholinergic neurons tested with an EC(50) of 2.1 microM. Pharmacologically, the effects of DHPG were partially/completely reduced by the mGluR1 antagonists, 7-hydrox-iminocyclopropan[b]chromen-1a-carboxylic acid ethyl ester and (+)-2-methyl-4-carboxyphenylglycine. Addition of the mGluR5 antagonist, 2-methyl-6-(phenylethnyl)pyridine hydrochloride, reduced the remaining response to DHPG, suggesting involvement of both receptor subtypes in a subpopulation of septohippocampal cholinergic neurons. In double-immunolabeling studies, 74% of septohippocampal cholinergic neurons co-localized mGluR1alpha-immunoreactivity and 35% co-localized mGluR5-immunoreactivity. Double-immunolabeling studies at the light and electron-microscopic levels showed that vesicular glutamate transporter 2 terminals make asymmetric synaptic contacts with septohippocampal cholinergic neurons. These findings may be of significance in treatment of cognitive deficits associated with neurodegenerative disorders as a group I mGluR-mediated activation of septohippocampal cholinergic neurons would enhance the release of acetylcholine both in the hippocampus and in the septum.     

Acetylcholine levels increase in rat hippocampus following acute septal lesions: Evidence for interactions between cholinergic and noncholinergic neurons

Acute septal lesions in rat brain resulted in elevation of the amount of particle-bound acetylcholine in the hippocampus irrespective of the extent of damage to the cholinergic septohippocampal projection. Changes in the high affinity choline uptake in the hippocampus were, however, proportional to the degree of destruction of this projection. The results are discussed in terms of possible interactions between the cholinergic and noncholinergic pathways in the system investigated.     

Cholinergic system and memory in the rat: effects of chronic ethanol, embryonic basal forebrain brain transplants and excitotoxic lesions of cholinergic basal forebrain projection system

Oral administration of ethanol (20% v/v) to male Sprague-Dawley rats for different periods of time up to 28 weeks resulted in profound reductions of acetylcholine content, in vitro synthesis and release of acetylcholine, choline uptake, activities of choline acetyltransferase, acetylcholinesterase and pyruvate decarboxylase, content of noradrenaline, serotonin and, to a lesser extent, dopamine throughout the brain. Changes were fully and partially reversible by a 4 weeks' ethanol-free period following a treatment of 8 and 18 weeks, respectively. They remained persistent, however, after 28 weeks of treatment. Performance in an eight arm-radial maze revealed a severe impairment in both spatial and non-spatial reference and working memory. A similar pattern of memory impairment was obtained after ibotenate lesion of the cholinergic basal forebrain projection system. In order to test whether this memory impairment depends on cholinergic deafferentation of the cortex, cholinergic-rich fetal basal forebrain cell suspensions were transplanted into cortex, hippocampus or both these sites in ethanol treated rats. Cholinergic-rich transplants, but not cholinergic-poor transplants, were effective in ameliorating impaired memory function and measures of cholinergic activity in the basal forebrain projection system. The behavioural efficacy of the basal forebrain grafts was well correlated with measures of both transplant volume and the degree to which they restored acetylcholine content at the transplant site; these transplants had no effect, however, on brain monoamine levels. The effects of the cholinergic-rich transplants into cortical and hippocampal sites were additive in their amelioration of performance in the radial maze. Similarly, ibotenate lesions of the sites of origin of the cholinergic projections to neocortex (in the region of the nucleus basalis magnocellularis) and hippocampus (the medial septal areas and nucleus of the diagonal band), respectively, were additive in their deleterious effects on maze performance. There were no qualitative differences in the susceptibility of the four different types of memory performance measured (spatial and non-spatial reference and working memory) to the effects of ethanol, ibotenate lesions of the cholinergic projection system, or cholinergic-rich brain tissue transplants. Thus, overall, the results indicate that the forebrain cholinergic system acts as a whole, without major functional differences between the projections originating in the medial septal area/diagonal band complex and the basal nucleus, and that it discharges a very general function in cognitive processes.(ABSTRACT TRUNCATED AT 400 WORDS)     

Behaviour-dependent changes in acetylcholine release in normal and graft-reinnervated hippocampus: evidence for host regulation of grafted cholinergic neurons.

Grafted neurons obtained from the fetal basal forebrain can provide a functional cholinergic reinnervation of the hippocampal formation in rats with a lesion of the intrinsic septal cholinergic afferents. In the present experiments graft-derived acetylcholine release in the hippocampus was studied by microdialysis in awake rats during different types of behaviours which are known to activate the innate septohippocampal cholinergic system and during different activity periods of the day-night cycle. Two types of basal forebrain grafts were studied: cell suspensions implanted into the hippocampus in rats with an aspirative lesion of the fimbria-fornix, and grafts of solid tissue implanted as a tissue bridge into the fimbria-fornix lesion cavity. Increased acetylcholine overflow was seen in both groups with grafts during sensory stimulation (by handling). The strongest response (50% increase in acetylcholine release) was seen in rats with solid basal forebrain grafts (equivalent to two-thirds of that seen in intact rats). Immobilization stress and motor activity (swimming) also resulted in increased, but more variable, acetylcholine release (+ 30%; about one-third of the normal response). None of these effects was seen in the control rats with fimbria-fornix lesion only. The two-fold difference in hippocampal acetylcholine release in normal animals between day and night was absent in both types of grafted rats. An acute knife-cut, transecting the connections between the solid basal forebrain graft and the host hippocampus, caused an immediate 75% reduction in acetylcholine release (similar to the effect of an acute fimbria-fornix transection in the normal rats) and the response to swimming was no longer evident. The results show that grafted cholinergic neurons can be functionally integrated into the host brain, allowing the grafted neurons to be activated in the correct behavioural contexts, although the changes in acetylcholine overflow were overall smaller and more variable than normal. The ability of the host to influence cholinergic graft activity, most probably mediated via activation of afferent host-graft connections, may contribute to the efficacy of basal forebrain grafts in the amelioration of behavioural impairments in animals with lesions of the forebrain cholinergic system.     

Disturbance of rapid eye movement sleep in senescence-accelerated mouse prone/8 mice is improved by retinoic acid receptor agonist Am80 (tamibarotene)

Senescence-accelerated mouse prone/8 (SAMP8) mice are known to exhibit age-related deterioration in sleep–wake architecture compared with senescence-accelerated mouse resistant/1 (SAMR1) mice. We investigated whether treatment with Am80 (Tamibarotene), a retinoic acid receptor agonist, would improve sleep in 9–10-month-old SAMP8 mice. One week of Am80 administration improved the decrease in rapid eye movement (REM) sleep shown by SAMP8 mice. Real-time RT-PCR analysis demonstrated an impairment in the hippocampal retinoid cascade (retinoic acid receptor alpha and transthyretin) in SAMP8 in comparison to SAMR1 mice. Am80 treatment induced an increase in mRNA expression in the vesicular acetylcholine transporter in the brainstem and transthyretin in the hippocampus. Furthermore, decreased cortical acetylcholine content in SAMP8 was improved by Am80 administration. Decreased non-REM sleep and delta oscillation were also observed in SAMP8 mice; however, this was not improved by Am80 administration. These results partially support the hypothesis that the effects of aging on sleep–wake architecture are improved by the activation of retinoic acid receptors. The improvement may be induced by the activation of the cholinergic pathway.     

Chewing under restraint stress inhibits the stress-induced suppression of cell birth in the dentate gyrus of aged SAMP8 mice

To investigate the mechanisms underlying impaired hippocampal function resulting from masticatory dysfunction, we examined the effects of the molarless condition on cell proliferation and the effect of the administration of metyrapone, which suppresses the stress-induced rise in plasma corticosterone levels, on cell proliferation in the hippocampal dentate gyrus (DG) of aged senescence-accelerated prone (SAMP8) mice. In addition, we examined whether chewing under restraint stress prevents the stress-induced suppression of cell proliferation. In aged mice, the molarless condition suppressed cell proliferation in the hippocampal DG. Vehicle-injected molarless mice had significantly higher plasma corticosterone levels than vehicle-injected control and metyrapone-injected molarless mice, in association with decreased cell proliferation in the hippocampal DG. Pretreatment with metyrapone inhibited the increase in plasma corticosterone levels induced by the bite-raised condition, and also attenuated the reduction in cell proliferation. Immobilization stress suppressed cell proliferation in the hippocampal DG, but chewing under restraint stress blocked the stress-induced suppression of cell proliferation in the DG. These results suggest that the morphologic deficits induced by the molarless condition in aged SAMP8 mice are a result of increased plasma corticosterone levels, and that chewing under restraint stress prevents the stress-induced suppression of cell birth in the DG.     

Age-related loss of galanin-immunoreactive cells in the rat septal area

We have examined the distribution of galanin-like immunoreactive (LI) cell bodies in the medial septal nucleus (MS) and the nucleus of the diagonal band of Broca (nDBB) of young (3 months) and aged (25-30 months) rats, and assessed their respective contribution to the septohippocampal pathway. Immunohistochemical techniques were used alone or combined with the retrograde transport of a protein-gold complex injected into the dorsal hippocampus. In both groups, galanin-LI cells were observed in the MS and the nDBB. In aged rats, a significant decrease in both the staining intensity and the number of galanin-LI perikarya throughout the MS-nDBB complex was observed. Some immunoreactive cells appeared shrunken. The reduction in cell number ranged from 30 to 85%. There was also a decrease in the proportion of septohippocampal neurons containing galanin in aged rats (13% vs 20% in young animals) which however did not reach statistical significance. These results suggest that galanin-positive cells in the medial septal area undergo alterations with aging in the rat.     

An investigation of whether septal gamma-aminobutyrate-containing interneurons are involved in the reduction in the turnover rate of acetylcholine elicited by substance P and beta-endorphin in the hippocampus.

Afferents to the septum that contain substance P or β-endorphin probably have an inhibitory action upon the acetylcholine metabolism of the cholinergic septohippocampal pathway because intraseptal administration of these peptides decreases the turnover rate of acetylcholine in the hippocampus. Bicuculline, an antagonist of γ-aminobutyric acid, blocks the inhibitory action of β-endorphin on acetylcholine turnover suggesting that it is mediated via the septal γ-aminobutyric acid-containing interneurons. Inhibition by substance P of the turnover of acetycholine in the hippocampus is independent of this type of interneuron. Dopamine, like β-endorphin, acts on γ-aminobutyrie acid-containing interneurons of the septum. However, lesions of dopaminergic cell bodies located in area A₁₀ have established that the dopaminergie and β-endorphin-containing inputs to the septal γ-aminobutyric acidcontaining interneurons are independent of each other.     

Electrophysiological characteristics of non-bursting, glutamate decarboxylase messenger RNA-positive neurons of the medial septum/diagonal band nuclei of guinea-pig and rat

Nuclei of the medial septum/diagonal band region of the mammalian forebrain contain neurons that give rise to the septohippocampal pathway, which has separate cholinergic and GABAergic components. This pathway is known to influence hippocampal-dependent memory and learning processes, but the precise role of each component is unclear. In this study, we tested the hypothesis that fast-firing, non-bursting medial septum/diagonal band neurons are GABAergic. We used brain slice preparations from young adult guinea-pigs and rats, or from weanling rats, to perform current-clamp recordings from medial septum/diagonal band neurons. Recorded neurons were injected with biocytin for subsequent visualization with fluorescent avidin, and then hybridized with a 35S-labeled riboprobe for glutamate decarboxylase-67 messenger RNA. As a positive control, guinea-pig cerebellar Purkinje cells were labeled and hybridized with the riboprobe. As expected, labeled Purkinje cells were glutamate decarboxylase-67 messenger RNA positive. Slow-firing, cholinergic (choline acetyltransferase-positive) guinea-pig medial septum/diagonal band neurons were glutamate decarboxylase-67 messenger RNA negative. Contrary to our hypothesis, of the guinea-pig neurons, only three of 11 fast-firing neurons were glutamate decarboxylase-67 positive. Of the rat medial septum/diagonal band neurons, three of four were positive for glutamate decarboxylase-67 messenger RNA.These data suggest that fast-firing, non-bursting neurons of the medial septum/diagonal band, as sampled by sharp-electrode intracellular recordings in brain slices, may be a heterogeneous group of neurons, some of which are GABAergic. Together with recent data demonstrating the presence of another GABAergic marker, parvalbumin, in fast-firing septal neurons, we conclude that GABAergic septohippocampal neurons include a population of fast-firing, non-bursting neurons. The influence of these neurons on the hippocampus is likely to occur on a shorter time-scale and over a wider range of firing frequencies as compared to slowly firing cholinergic septohippocampal neurons.