Amelioration of spatial memory impairment by intrahippocampal grafts of mixed septal and raphe tissue in rats with combined cholinergic and serotonergic denervation of the forebrain

Study of the Potential Endocrine-Disrupting Effects of Phenylurea Compounds on Neurohypophysis Cells In Vitro

Homeostatic disruptor agents, and endocrine disruptor compounds (EDC) specifically, can originate from agricultural and industrial chemicals. If they modify the adaptation of living organisms as direct (e.g., by altering hormone regulation, membrane functions) and/or indirect (e.g., cell transformation mechanisms) factors, they are classified as EDC. We aimed to examine the potential endocrine-disrupting effects of phenylurea herbicides (phenuron, monuron, and diuron) on the oxytocin (OT) and arginine-vasopressin (AVP) release of neurohypophysis cell cultures (NH). In our experiments, monoamine-activated receptor functions of neurohypophyseal cells were used as a model. In vitro NH were prepared by enzymatic (trypsin, collagenase) and mechanical dissociation. In the experimental protocol, the basal levels of OT and AVP were determined as controls. Later, monoamine (epinephrine, norepinephrine, serotonin, histamine, and dopamine) activation (10^-6 M, 30 min) and the effects of phenylurea (10^-6 M, 60 min) alone and in combination (monoamines 10^-6 M, 30 min + phenylureas 10^-6 M, 60 min) with monoamine were studied. OT and AVP hormone contents in the supernatant media were measured by radioimmunoassay. The monoamine-activated receptor functions of neurohypophyseal cells were modified by the applied doses of phenuron, monuron, and diuron. It is concluded that the applied phenylurea herbicides are endocrine disruptor agents, at least in vitro for neurohypophysis function.

Neural Transplantation: Prospects for Clinical use

Neural transplantation has been extensively applied in Parkinson's disease, including numerous clinical studies, studies in animal models, and related basic research on cell biology. There is evidence that the clinical trials of both adrenal medulla transplantation and fetal substantia nigra transplantation have produced a detectable clinical effect, although it is not yet clear whether the clinical benefit is sufficient to justify a more widespread application of these procedures. Studies of long-term outcome and quantitative tests are important in assaying the degree of benefit produced by transplantation procedures in Parkinson's disease and for developing improved and refined procedures. Other disease-related applications of neural transplantation are beginning to be developed. These include Huntington's disease, chronic pain, epilepsy, spinal cord injury, and perhaps even demyelinating diseases and cortical ischemic injury. Although most of these applications lie in the future, it is not too soon to begin to consider the scientific justification that should be required for initiation of human clinical trials.

Intraretrosplenial Cortical Grafts of Fetal Cholinergic Neurons and the Restoration of Spatial Memory Function

The retrosplenial cortex (RSC) receives cholinergic afferent fibers from the medial septal nucleus and diagonal band of Broca (DBB) by way of the cingulate bundle and the fornix. Bilateral lesions of both the cingulate and fornix pathways result in a complete depletion of cholinergic input to the RSC. In the present study we have examined the effects of transplanting cholinergic neurons from fetal rat pups to the RSC of adult rats following lesions of the cingulate bundle and fornix. The animals with lesions exhibited severe spatial memory impairments with a complete loss of extrinsic cholinergic afferents to the RSC. Animals with intraretrosplenial cortical transplants exhibited significant improvements in learning and memory performance as revealed by decreased escape latencies in spatial reference memory tests, increased numbers of platform crossings in spatial navigation tests, and a higher percentage of correct choices in a spatial working memory task. These improvements appeared to be cholinergically mediated because atropine administration significantly disrupted spatial navigation performance. The survival of the transplanted cholinergic neurons and their innervation of the RSC were characterized using a monoclonal antibody to choline acetyltransferase (ChAT). The staining of graft-derived ChAT-positive fibers also revealed a pattern of innervation that mimicked that of the cholinergic input in normal animals. These results indicate that intraretrosplenial cortical transplants of cholinergic neurons can rectify spatial memory deficits produced by the loss of intrinsic cholinergic afferents from the medial septal nucleus. Copyright © 1997 Elsevier Science Inc.

Mechanisms and use of neural transplants for brain repair

Under appropriate conditions, neural tissues transplanted into the adult mammalian brain can survive, integrate, and function so as to influence the behavior of the host, opening the prospect of repairing neuronal damage, and alleviating symptoms associated with neuronal injury or neurodegenerative disease. Alternative mechanisms of action have been postulated: nonspecific effects of surgery; neurotrophic and neuroprotective influences on disease progression and host plasticity; diffuse or locally regulated pharmacological delivery of deficient neurochemicals, neurotransmitters, or neurohormones; restitution of the neuronal and glial environment necessary for proper host neuronal support and processing; promoting local and long-distance host and graft axon growth; formation of reciprocal connections and reconstruction of local circuits within the host brain; and up to full integration and reconstruction of fully functional host neuronal networks. Analysis of neural transplants in a broad range of anatomical systems and disease models, on simple and complex classes of behavioral function and information processing, have indicated that all of these alternative mechanisms are likely to contribute in different circumstances. Thus, there is not a single or typical mode of graft function; rather grafts can and do function in multiple ways, specific to each particular context. Consequently, to develop an effective cell-based therapy, multiple dimensions must be considered: the target disease pathogenesis; the neurodegenerative basis of each type of physiological dysfunction or behavioral symptom; the nature of the repair required to alleviate or remediate the functional impairments of particular clinical relevance; and identification of a suitable cell source or delivery system, along with the site and method of implantation, that can achieve the sought for repair and recovery.

Neural tissue transplantation, repair, and rehabilitation

Transplants of cells and tissues to the central nervous system of adult mammals can, under appropriate conditions, survive, integrate, and function. In particular, the grafted cells can sustain functional recovery in animal models of a range of neurodegenerative conditions including genetic and idiopathic neurodegenerative diseases of adulthood and aging, ischemic stroke, and brain and spinal cord trauma. In a restricted subset of such conditions, cell transplantation has progressed to application in humans in early-stage clinical trials. At the present stage of play, there is clear evidence of clinical efficacy of fetal cell transplants in Parkinson disease (notwithstanding a range of technical difficulties still to be fully resolved), and preliminary claims of promising outcomes in several other severe neurodegenerative conditions, including Huntington disease and stroke. Moreover, the experimental literature is increasingly suggesting that the experience and training of the graft recipient materially affects the functional outcome. For example, environmental enrichment, behavioral activity, and specific training can enhance the recovery process to maximize functional recovery. There are even circumstances where the grafted cells have been demonstrated to restore the neural substrate for new learning. Consequently, it is not sufficient to replace lost cells anatomically; rather, for the grafts to be effective, they need to be integrated functionally into the host circuitry, and the host animal requires training and rehabilitation to maximize function of the reconstructed graft-host circuitry. Such observations require reconsideration of the design of the next generation of clinical trials and subsequent service delivery, to include physiotherapists, cognitive therapists, and rehabilitation experts as core members of the transplant team, along with the neurologists and neurosurgeons that have conventionally led the field.

Thinking about repairing thinking

The work of Sinden et al. suggests that it may be possible to produce improvement in the “highest” areas of brain function by transplanting brain tissue. What appears to be the limiting factor is not the complexity of the mental process under consideration but the discreteness of the lesion which causes the impairment and the appropriateness and accuracy of placement of the grafted tissue.

Therapeutic uses for neural grafts: Progress slowed but not abandoned

In spite of Stein and Glasier's justifiable conclusion that initial optimism concerning the immediate clinical applicability of neural transplantation was premature, there exists much experimental evidence to support the potential for incorporating this procedure into a therapeutic arsenal in the future. To realize this potential will require continued evolution of our knowledge at multiple levels of the clinical and basic neurosciences.

The structure, operation, and functionality of intracerebral grafts

The concept of structure, operation, and functionality, as they may be understood by clinicians or researchers using neural transplantation techniques, are briefly defined. Following Stein & Glasier, we emphasize that the question of whether an intracerebral graft is really functional should be addressed not only in terms of what such a graft does in a given brain structure, but also in terms of what it does at the level of the organism.

The NGF superfamily of neurotrophins: Potential treatment for Alzheimer's and Parkinson's disease

Stein & Glasier suggest embryonic neural tissue grafts as a potential treatment strategy for Alzheimer's and Parkinson's disease. As an alternative, we suggest that the family of nerve growth factor-related neurotrophins and their trk (tyrosine kinase) receptors underlie cholinergic basal forebrain (CBF) and dopaminergic substantia nigra neuron degeneration in these diseases, respectively. Therefore, treatment approaches for these disorders could utilize neurotrophins.

Some practical and theoretical issues concerning fetal brain tissue grafts as therapy for brain dysfunctions

Grafts of embryonic neural tissue into the brains of adult patients are currently being used to treat Parkinson's disease and are under serious consideration as therapy for a variety of other degenerative and traumatic disorders. This target article evaluates the use of transplants to promote recovery from brain injury and highlights the kinds of questions and problems that must be addressed before this form of therapy is routinely applied. It has been argued that neural transplantation can promote functional recovery through the replacement of damaged nerve cells, the reestablishment of specific nerve pathways lost as a result of injury, the release of specific neurotransmitters, or the production of factors that promote neuronal growth. The latter two mechanisms, which need not rely on anatomical connections to the host brain, are open to examination for nonsurgical, less intrusive therapeutic use. Certain subjective judgments used to select patients who will receive grafts and in assessment of the outcome of graft therapy make it difficult to evaluate the procedure. In addition, little long-term assessment of transplant efficacy and effect has been done in nonhuman primates. Carefully controlled human studies, with multiple testing paradigms, are also needed to establish the efficacy of transplant therapy.

Models of neurological defects and defects in neurological models

The transition from research to patient following advances in transplantation research is likely to be disappointing unless it includes a better understanding of critically relevant characteristics of the neurological disorder and improvements in the animal models, particularly the behavioral features. The appropriateness of the model has less to do with the species than with how the species is used.

The neurotoxin 2′-NH2-MPTP degenerates serotonin axons and evokes increases in hippocampal BDNF

1-Methyl-4-(2'-aminophenyl)-1,2,3,6-tetrahydropyridine (2'-NH2-MPTP) causes long-term depletions in cortical and hippocampal serotonin (5-HT) and norepinephrine (NE) that are accompanied by acute elevations in glial fibrillary acidic protein (GFAP) and argyrophilia. To further investigate the hypothesis that these changes are reflective of serotonergic and noradrenergic axonal degeneration, 2'-NH2-MPTP was administered to mice and innervation densities were determined immunocytochemically. Regional responses of the neurotrophin, brain-derived neurotrophic factor (BDNF), to putative damage were also assessed. Three days after 2'-NH2-MPTP, 5-HT axons exhibited a beaded, tortuous appearance indicative of ongoing degeneration. At 21 days, numbers of serotonin axons were significantly decreased, with the greatest axonal losses occurring in cortex and hippocampus. Serotonin axons in the amygdala were contrastingly spared long-term damage, as were 5-HT and NE cell bodies in the brain stem. BDNF protein levels were selectively increased in the hippocampus 3 days post-dose and returned to normal 21 days later. These results, in conjunction with previous findings, demonstrate that 2'-NH2-MPTP causes degeneration of serotonergic axons innervating the cortex and hippocampus on par with depletions in neurotransmitter levels. Moreover, damage to the hippocampus, a brain region important for learning and memory, and the modulation of anxiety and stress responsiveness, results in a transitory increase in BDNF.

Dépression et neuroplasticité : implication des systèmes sérotoninergiques

Neuroplasticity contributes to both normal and pathological brain function. A recent hypothesis links depression to lack of adaptive responses to stress or other aversive stimuli, and effects of antidepressant treatments on adult neurogenesis are more and more extensively studied because of the structural changes involved in the pathophysiology of depression. Indeed, neuronal remodelling in hippocampal formation is associated with chronic stress and is reversed by antidepressant treatments in animals. Decrease in hippocampal volume has also been associated to cognitive deficits in patients with major depression. Interestingly, serotonergic (5-HT) systems play a major role both as antidepressants and by increasing hippocampal neurogenesis through various receptor subtypes. Recently, we have also demonstrated that agomelatine, a new antidepressant drug having serotonergic and melatonergic properties, can increase proliferation and survival of newly formed hippocampal cells. Although the mechanisms underlying such effects are still unknown, these data reinforce the view that changes in hippocampal neurogenesis might belong to the cellular correlates of mood disorders.

Late onset loss of hippocampal 5-HT and NE is accompanied by increases in BDNF protein expression in mice co-expressing mutant APP and PS1

Transgenic mice expressing both mutant amyloid precursor protein (APPswe) and presenilin-1 (PS1DeltaE9) develop amyloid deposits as early as 4 months of age and preliminary evidence suggests that this may be associated with degenerative changes in serotonin axons innervating the dentate gyrus of the hippocampus. In the present investigation, which focused on further delineating the effects of amyloid deposition on hippocampal neurochemistry, decreases in serotonin neurotransmitter levels (-25%) were discovered to be present at 18 months in APP+/PS1+ mice, while norepinephrine was reduced in the hippocampus of 12- (-30%) and 18-month-old (-45%) APP+/PS1+ double mutants. In addition, brain-derived neurotrophic factor (BDNF) protein levels were investigated since changes in BDNF are reported to occur in AD, and BDNF has been shown to have trophic effects on serotonin and norepinephrine neurons. In doubly, but not singly mutant mice, hippocampal BDNF levels were increased at 12 (+70%) and 18 months (+170%). Furthermore, in a different model of serotonergic and noradrenergic degeneration, BDNF protein levels were similarly increased in response to depletions in hippocampal serotonin and norepinephrine caused by the chemical neurotoxin 1-methyl-4-(2'-aminophenyl)-1,2,3,6-tetrahydropyridine (2'-NH2-MPTP). These findings show that early amyloid deposition in mice expressing mutant human APP and PS-1 is associated with a progressive loss of serotonin and norepinephrine neurotransmitter levels in the hippocampus later in life. Furthermore, BDNF protein levels are increased in APP+/PS1+ and 2'-NH2-MPTP-treated mice, possibly as a compensatory response to serotonergic and noradrenergic neurodegeneration in a brain region important for learning and memory.

Serotonin Reinnervation of the Suprachiasmatic Nucleus by Intrahypothalamic Fetal Raphe Transplants, with Special Reference to Possible Influences of the Target

Survival and development of fetal serotonin (5-HT) neurons grafted to various brain areas in adult mammals have been suggested to be under host influences. The aim of this study was to determine whether the suprachiasmatic nucleus of the hypothalamus (SCN), a region receiving a 5-HT input which is one of the densest and the most heavily synaptic in the brain, can actually support the development of transplanted 5-HT neurons. The time course and extent of 5-HT reinnervation were therefore investigated with 5-HT immunocytochemistry in adult rats subjected to intraventricular injection of 5,7-dihydroxytryptamine and subsequent grafting of fetal cell suspension of mesencephalic raphe neurons. The ultrastructural features of the newly formed 5-HT terminal plexa were also examined. Serotonin reinnervation of the SCN remained partial up to 4 months post-transplantation, with no apparent predilection of the reinnervating fibres for any particular portion of the nucleus, thus differing from the normal 5-HT innervation of the SCN both quantitatively and qualitatively. This was in sharp contrast to the 5-HT hyperinnervation observed in neighbouring areas such as the supraoptic nucleus, a structure normally provided with only few 5-HT fibres, and the ventral wall of the third ventricle. The graft-derived 5-HT axons, however, displayed ultrastructural features that did not appear different from those of their normal counterparts; in particular they re-established defined synaptic contacts with the host population. These results may indicate that the mature SCN specifically lacks a trophic factor necessary for the ingrowth of graft-derived 5-HT fibres, or that it represents an inhibitory environment for such an ingrowth. The limited ability of regrowing 5-HT axons to restore a normal density of 5-HT innervation could also be related to the fact that these neurons normally establish a relatively high number of synaptic contacts in the target region.

Serotonergic modulation of hippocampal acetylcholine release after long-term neuronal grafting

Adult female rats sustained aspirative fimbria-fornix lesions and, 2 weeks later, received intrahippocampal grafts of fetal septal or mixed septal-raphe cell suspensions. Twenty-four months later, the extracellular concentration of hippocampal acetylcholine (ACh) was determined by microdialysis. Basal ACh levels (5-65 fmol/5 microl sham-operated rats) were strongly reduced after lesioning (3-7 fmol/5 microl). In septally transplanted and septal-raphe co-transplanted rats, hippocampal ACh concentrations were restored to near-normal levels (15-25 fmol/5 microl), indicating long-term functional survival of hippocampal transplants. After administration of citalopram (100 microM by infusion) and fenfluramine (20 mg/kg i.p.), the hippocampal ACh efflux was increased by 2- to 3-fold in all groups of rats. The relative increase of ACh was highest in co-transplanted rats, an effect which was possibly due to functional interactions between grafted raphe and septal neurons.

Serotonin release from mesencephalic raphe neurons grafted to the 5,7-dihydroxytryptamine-lesioned rat hippocampus: effects of behavioral activation and stress

Transplants of fetal midbrain raphe neurons into the adult brain have been shown to promote recovery of complex behavioral deficits in several experimental models, but the mechanisms underlying these effects are only partially understood. In the present study, we have used a well-characterized model system to ascertain whether midbrain raphe graft can display behaviorally relevant changes in transmitter release and/or metabolism. Fetal mesencephalic raphe neurons were grafted unilaterally into the hippocampus previously deprived of its innate serotonergic innervation by intraventricular injections of 5,7-dihydroxytryptamine. The contralateral hippocampus remained as a nongrafted, lesioned control. Microdialysis probes were implanted in the hippocampus 5-7 months postgrafting. Under baseline conditions, extracellular levels of serotonin were similar to normal in the grafted hippocampi, but undetectable on the contralateral, nongrafted side. Levels of the serotonin metabolite, 5-hydroxyindoleacetic acid (5-HIAA), were markedly higher than normal in the grafted hippocampi, but dramatically reduced on the contralateral nongrafted side. Handling stimulation (gentle stroking of a rat's fur and tail for 15 min) induced a 64% increase in serotonin output in the intact rats and a small but significant 12% increase in the grafted animals. Non-noxious tail-pinch (15 min) enhanced serotonin release by 86% in the intact rats and 28% in the grafted ones. Extracellular 5-HIAA levels remained unchanged during both handling and tail-pinch in both the intact and the grafted rats. Forced immobilization of the rats for 15 min induced a transient 124% increase in extracellular serotonin levels in the intact rats and a significant 19% increase in the grafted animals, whereas swimming in temperate water (25-30 degrees C; 15 min) induced no detectable changes in serotonin output in any of the groups. 5-HIAA levels remained unchanged during forced immobilization, but were significantly reduced during the swimming session in both the intact (-38%) and grafted (-15%) animals. The present results indicate that median raphe grafts can become functionally integrated in the denervated host hippocampus and respond by altered indole output when the animal is exposed to different types of environmental challenges.

Enhancement of hippocampal cholinergic neurotransmission through 5-HT1A receptor-mediated pathways by repeated lithium treatment in rats

Hippocampal cholinergic neuronal activity is reported to be regulated, at least partly, through serotonin1A (5-HT1A) receptors. Chronic lithium treatment has been shown to alter both behavioral and neurochemical responses mediated by postsynaptic 5-HT1A receptors. We investigated whether long-term lithium treatment affects central cholinergic neurotransmission through 5-HT1A receptor-mediated pathways. Changes in acetylcholine (ACh) release induced by 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), a 5-HT1A receptor agonist, in the rat hippocampus were measured using a microdialysis technique and a radioimmunoassay for ACh. Administration of lithium for 21 days resulted in a serum lithium concentration of 1.03 mM and caused little change in density or affinity of [3H]8-OH-DPAT binding sites in the hippocampus. The local application of 8-OH-DPAT into the hippocampus of lithium treated rats increased the ACh efflux in both the absence and the presence of physostigmine, a cholinesterase (ChE) inhibitor, in the perfusion fluid. The basal ACh efflux of lithium treated rats was not different from that of the control rats under normal conditions, but was significantly higher than that of the controls when ChE was inhibited. These results demonstrate that chronic lithium treatment increases spontaneous ACh release in the hippocampus under conditions of ChE inhibition, but not under normal conditions, and enhances cholinergic neurotransmission through 5-HT1A receptor-mediated pathways, and suggest that activation of 5-HT1A receptor function by lithium is related to the enhancement of hippocampal cholinergic neurotransmission.

Key words: Acetylcholine (ACh), hippocampus, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), lithium, serotonin1A (5-HT1A) receptor.

Serotonin may stimulate granule cell proliferation in the adult hippocampus, as observed in rats grafted with foetal raphe neurons

The long-term effects of hippocampal serotonergic denervation and reinnervation by foetal raphe tissue were examined in the dentate gyrus where neurons are continously born in the adult. Complete lesion of serotonin neurons following injections of 5, 7-dihydroxytryptamine in the dorsal and medial raphe nuclei produced long-term decreases in the number of newly generated granule cells identified with 5-Bromo-2'-deoxyuridine (BrdU) and the polysialylated form of neural cell adhesion molecule (PSA-NCAM) immunostaining, as observed in 2-month-survival rats. The raphe grafts, but not the control grafts of embryonic spinal tissue, reversed the postlesion-induced decreases in the density of BrdU- and PSA-NCAM-labelled cells detected in the granule layer. Inhibition of serotonin synthesis in animals with raphe grafts reversed back to lesion-induced changes in granule cell proliferation. Furthermore, extensive serotonergic reinnervation of the dentate gyrus in the area proximal to the raphe graft could be associated with supranormal density of BrdU-labelled cells. These results indicate that serotonin may be considered a positive regulatory factor of adult granule cell proliferation. Finally, the lack of effect of embryonic nonserotonergic tissue grafted to serotonin-deprived rats suggests that neurotrophic factors may not be involved in the effects of serotonin on adult neurogenesis.

Age-related working memory deficits in the allocentric place discrimination task: possible involvement in cholinergic dysfunction

It is well known that learning and memory ability declines with aging. Age-related long-term changes in learning and memory ability in rats were investigated with the place navigation task and the allocentric place discrimination task (APDT) in a water maze using the same animals for each task. In a working memory place navigation task, aged animals could learn the location of the platform as well as when they were young, although strategy shifts were observed. In contrast, accuracy in the APDT significantly declined from 90% to 65% with aging. This impairment was ameliorated by an acetylcholine esterase inhibitor physostigmine at 22-23 months old. No amelioration was, however, detected in the same animals tested when they further aged to 26-27 months old. These results suggest that the APDT performance is sensitive to age-related memory deficits and that this may be due to the cholinergic dysfunction.

Amelioration of fornix lesion induced learning deficits by self-stimulation rewarding experience

Intracranial self-stimulation (ICSS) rewarding experience is known to modulate learning and memory and induce morphological and neurochemical changes in hippocampus. Therefore, we studied the effect of ICSS on the hippocampus-dependent operant and the spatial learning tasks in rats with bilateral electrolytic lesioning of fornix. Bilateral lesioning of fornix induced deficits in acquisition and performance of both the tasks, whereas exposure to 10 days of ICSS experience from ventral tegmental area reversed these behavioural deficits. Hence, we propose that the ICSS experience ameliorates the fornix lesion induced behavioural deficits, by inducing neuronal plasticity in the hippocampus which may act as a compensatory mechanism for the deficits produced by the lesioning of fornix.

Modulation of acetylcholine and 5-hydroxytryptamine release in hippocampal slices of rats with fimbria-fornix lesions and intrahippocampal grafts containing cholinergic and/or serotonergic neurons

Three-month-old Long-Evans female rats sustained aspirative lesions of the dorsal septohippocampal pathways and, 2 weeks later, received intrahippocampal suspension grafts containing fetal cells from the mesencephalic raphe (rich in serotonergic neurons; RAPHE), the medial septum and the diagonal band of Broca (rich in cholinergic neurons; SEPT), or a mixture of both (COTR). Lesion-only (LES) and sham-operated rats (SHAM) were used as controls. Hippocampal slices of these rats (5-9 month after surgery) were preincubated with [3H]choline or [3H]5-HT, superfused continuously (in the presence of hemicholinium-3 or fluvoxamine) and stimulated electrically (360 pulses, 2 ms, 3 Hz, 26-28 mA) in order to study the presynaptic modulation of acetylcholine (ACh) and serotonin (5-HT) release. The accumulation of [3H]choline and the evoked overflow of [3H]ACh were significantly reduced in slices from LES and RAPHE rats, but reached a close-to-normal level in SEPT and COTR rats. As to accumulation and overflow of [3H]5-HT, the lesion-induced reduction was compensated for only in RAPHE and COTR rats. The relative amount of evoked [3H]5-HT release (in % of tissue-3H) was significantly increased in LES and SEPT rats. Only slight differences (group LES) were found in the sensitivity of muscarinic and serotonergic autoreceptors towards oxotremorine and CP 93,129, respectively. Moreover, CP 93,129 induced a significantly weaker inhibition of ACh release in slices of COTR rats than in all other groups. Using the 5-HT1A receptor agonist 8-OH-DPAT and antagonist Way 100,635, no evidence for a modulatory influence of 5-HT1A receptors was found in RAPHE and COTR rats. It is concluded that despite substantial lesion- and graft-induced changes in the amount of ACh and 5-HT released by hippocampal slices of lesion-only or grafted rats, the presynaptic modulation of these transmitters is only slightly affected by changes in the neuronal environment.

Intrahippocampal grafts containing cholinergic and serotonergic fetal neurons ameliorate spatial reference but not working memory in rats with fimbria-fornix/cingular bundle lesions

Three-month-old Long-Evans female rats sustained aspirative lesions of the dorsal septohippocampal pathways and, 2 weeks later, received intrahippocampal suspension grafts containing cells from the mesencephalic raphe, cells from the medial septum and the diagonal band of Broca, or a mixture of both. Lesion-only and sham-operated rats were used as controls. All rats were tested for locomotor activity 1 week, 3 and 5 months after lesion surgery, for spatial working memory in a radial maze from 5 to 9 months, and for reference and working memory in a water tank during the 9th month after lesioning. Determination of hippocampal concentration of acetylcholine, noradrenaline, and serotonin was made after completion of behavioral testing. Compared to sham-operated rats, all rats with lesions, whether grafted or not, exhibited increased levels of locomotor activity and made more errors in the radial maze. The lesioned rats were also impaired in the probe trial (30 first seconds) of the water-tank test made according to a protocol requiring intact reference memory capabilities. While rats with septal or raphe grafts were also impaired, the rats with co-grafts showed performances not significantly different from those of sham-operated rats. With a protocol requiring intact working memory capabilities, all lesioned rats, whether grafted or not, were impaired in the water-tank test. In the dorsal hippocampus of lesion-only rats, the concentration of acetylcholine and serotonin was significantly reduced. In rats with septal grafts or co-grafts, the concentration of acetylcholine was close to normal, as was that of serotonin in rats with raphe grafts or co-grafts. These results confirm previous findings showing that co-grafts enabled the neurochemical properties of single grafts to be combined. Data from the water-tank test suggest that cholinergic and serotonergic hippocampal reinnervations by fetal cell grafts may induce partial recovery of spatial reference, but not working memory capabilities in rats.

Modelling recovery of cognitive function after traumatic brain injury: spatial navigation in the Morris water maze after complete or partial transections of the perforant path in rats

The Morris water maze (MWM) has been used to assess cognitive function in rats after a variety of lesions designed to model brain damage and to assess the effects of drugs, growth factors, and neural transplants on post-operative deficits. The present study examined recovery of spatial navigation in the MWM over time in order to model the spontaneous recovery of cognitive function seen in humans. Diffuse axonal injury, a neuropathology commonly associated with traumatic brain injury (TBI), was modelled by transecting the perforant path (PP) bilaterally, either caudal to the hippocampus or dorsal to it at the decussation of the dorsal hippocampal commissure. Both groups with PP cuts showed substantial deficits initially, but spatial performance recovered with time and training. Recovery of platform finding was nearly complete within 14 days of testing, but recovery of platform searching did not occur for 2 or 3 more weeks. When the platform was moved to a new location, a continuing deficit in learning rate was revealed. When the platform was moved to a new position every day, this deficit was even more evident. These results illustrate the multi-faceted nature of recovery after brain injury and provide a new model for assessing the effects of manipulations designed to modulate recovery.

Hippocampal grafts of acetylcholine-producing cells are sufficient to improve behavioural performance following a unilateral fimbria-fornix lesion

Lesions of the septohippocampal pathway produce cognitive deficits that are partially attenuated by grafts of cholinergic-rich tissue into denervated target regions or by systemic administration of cholinomimetic drugs. In the present study, fibroblasts engineered to produce acetylcholine were used to test the hypothesis that restoration of hippocampal acetylcholine in rats with septohippocampal lesions is sufficient to improve cognitive processing post-damage. Rats received unilateral grafts of acetylcholine-producing or control fibroblasts into the hippocampus immediately prior to an aspirative lesion of the ipsilateral fimbria-fornix. Some rats with fimbria-fornix lesions were implanted with acetylcholine-producing or control fibroblasts into the neocortex, another major target of the basal forebrain cholinergic system, to determine if the site of acetylcholine delivery to the damaged brain is critical for functional recovery. Rats were tested in a hidden platform water maze task, a cued water maze task and activity chambers between one and three weeks post-grafting. Compared to unoperated controls, rats with fimbria fornix lesions only were significantly impaired in hidden platform water maze performance. Hippocampal grafts of acetylcholine-producing cells reduced lesion-induced deficits in the water maze, whereas hippocampal control grafts and cortical grafts of either cell type were without effect. Locomotor activity and cued water maze performance were unaffected by the lesion or the implants. Taken together, these data indicate that water maze deficits produced by fimbria fornix lesions, which disrupt a number of hippocampal neurotransmitter systems, can be attenuated by target specific replacement of acetylcholine in the hippocampus and that this recovery occurs in the absence of circuitry repair.

Embryonic entorhinal transplants partially ameliorate the deficits in spatial memory in adult rats with entorhinal cortex lesions

Our previous studies have demonstrated that axons from grafts of embryonic entorhinal cortex (EC) can reinnervate the deafferented zones in the hippocampus and form synaptic connections with the host dentate gyrus in adult mice and rats deprived of their own entorhinal inputs. Here, we have examined the ability of the EC grafts to ameliorate deficits in spatial memory. Three months after transplantation, the grafted rats and control animals were subjected to Morris water maze testing followed by histological examination. According to the exact position of grafts in the host brain, the rats with lesion and EC transplants were divided into two groups, one with EC grafts contacting both the hippocampus and overlying neocortex (n=7, EC1) and another with EC grafts confined within the hippocampus (n=6, EC2). While EC2 rats were still as impaired as those with lesion and transplants of non-entorhinal cortex (n=10, NEC) or with lesions only (n=7, LES), the EC1 rats performed better than the LES group. In a spatial memory trial, the EC1 group made more crossings over platform site and showed more focused search behavior than EC2, LES, NEC groups. The data suggest that EC grafts could partially ameliorate the deficit in spatial learning behavior in the EC-lesioned adult rats. The requirement for the graft to contact both the neocortex and the hippocampus suggests that the functional effects may be exerted by the formation of new neocortical-EC graft-hippocampal circuits.

Neuropeptide Y and enkephalin immunoreactivity in retinorecipient nuclei of the hamster pretectum and thalamus

This investigation was stimulated by the historical confusion concerning the identity of certain pretectal nuclei and by large differences reported between species with respect to which nuclei receive retinal innervation. Subcortical visual nuclei were studied using immunohistochemistry to identify retinal projections labeled following intraocular injection of cholera toxin, b fragment. In addition, neuropeptide Y (NPY) or enkephalin (ENK) immunoreactive cells and fibers were also evaluated in the retinorecipient pretectal and thalamic areas. The results confirm the established view that the retina directly innervates the nucleus of the optic tract (NOT), posterior (PPT), and olivary pretectal (OPT) nuclei. However, the retina also innervates the hamster medial (MPT) and anterior (APT; dorsal division) pretectal nuclei, results not previously reported in rodents. A commissural pretectal area (CPT) sparsely innervated by retina is also described. The data show for the first time that the posterior limitans nucleus (PLi) receives a moderately dense, direct retinal input. The PLi does not project to the cortex and appears to be a pretectal, rather than thalamic, nucleus. All retinal projections are bilateral, although predominantly contralateral. The PLi contains a moderately dense plexus of NPY- and ENK-IR fibers and terminals. However, peptidergic fibers also traverse the ATP and connect with the dorsomedial pretectium. The OPT contains ENK- and NPY-IR neurons and fibers, but is specifically identifiable by a moderately dense plexus of ENK-IR terminals. Numerous ENK-IR neurons are found in the NOT and PPT. The latter also has moderate numbers of ENK-IR fibers and terminals, but few NPY-IR neurons or fibers. The MPT contains modest numbers of ENK-IR fibers. The APT has no NPY-IR neurons or terminals, but an occasional ENK-IR neuron is seen and there is sparse ENK-IR innervation. Peptidergic innervation of the visual nuclei does not appear to be derived from the retina. The results show a set of retinally innervated, contiguous nuclei extending from the thalamic ventrolateral geniculate nucleus dorsomedially to the midbrain CPT. These nuclei plus the superior colliculus comprise a dorsal "visual shell" embracing a central core of caudal thalamus and rostral midbrain.

The fimbria-fornix/cingular bundle pathways: a review of neurochemical and behavioural approaches using lesions and transplantation techniques

Extensive lesions of the fimbria-fornix pathways and the cingular bundle deprive the hippocampus of a substantial part of its cholinergic, noradrenergic and serotonergic afferents and, among several other behavioural alterations, induce lasting impairment of spatial learning and memory capabilities. After a brief presentation of the neuroanatomical organization of the hippocampus and the connections relevant to the topic of this article, studies which have contributed to characterize the neurochemical and behavioural aspects of the fimbria-fornix lesion "syndrome" with lesion techniques differing by the extent, the location or the specificity of the damage produced, are reviewed. Furthermore, several compensatory changes that may occur as a reaction to hippocampal denervation (sprouting changes in receptor sensitivity and modifications of neurotransmitter turnover in spared fibres) are described and discussed in relation with their capacity (or incapacity) to foster recovery from the lesion-induced deficits. According to this background, experiments using intrahippocampal or "parahippocampal" grafts to substitute for missing cholinergic, noradrenergic or serotonergic afferents are considered according to whether the reported findings concern neurochemical and/or behavioural effects. Taken together, these experiments suggest that appropriately chosen fetal neurons (or other cells such as for instance, genetically-modified fibroblasts) implanted into or close to the denervated hippocampus may substitute, at least partially, for missing hippocampal afferents with a neurochemical specificity that closely depends on the neurochemical identity of the grafted neurons. Thereby, such grafts are able not only to restore some functions as they can be detected locally, namely within the hippocampus, but also to attenuate some of the behavioural (and other types of) disturbances resulting from the lesions. In some respects, also these graft-induced behavioural effects might be considered as occurring with a neurochemically-defined specificity. Nevertheless, if a graft-induced recovery of neurochemical markers in the hippocampus seems to be a prerequisite for also behavioural recovery to be observed, this neurochemical recovery is neither the one and only condition for behavioural effects to be expressed, nor is it the one and only mechanism to account for the latter effects.

The effects of intrahippocampal BDNF and NGF on spatial learning in aged Long Evans rats

Spatial learning rate was compared in cognitively impaired aged rats infused with either brain-derived neurotrophic factor (BDNF) or nerve growth factor (NGF). BDNF or NGF was infused into the dorsal hippocampus/third ventricle while animals were being trained on the Morris water maze. Training continued until all rats met a spatial learning criterion. Seven weeks later, they were tested for retention of the task, and sacrificed for assessment of hippocampal high-affinity choline uptake (HACU) or hypothalamic biogenic amine levels. NGF, but not BDNF, improved spatial learning rate in aged rats and increased hippocampal choline uptake weeks after withdrawal of NGF. Although BDNF did not improve spatial learning, it did induce a partial, long-term normalization of the elevated hypothalamic 5-HT levels observed in our aged rats. These data suggest that (1) intrahippocampal/intraventricular infusion of NGF can improve the learning rate of aged, spatial learning-impaired rats, and that this improvement in acquisition could be associated with increased hippocampal cholinergic activity, and (2) that the BDNF-induced normalization of hypothalamic 5-HT levels in aged rats was not sufficient to improve learning rate in aged, spatial learning-impaired rats.

Hippocampal histamine receptors: possible role on the mechanisms of memory in the rat, II

In this paper it was studied the role of histamine and histamine receptors in the hippocampus of rats on an active avoidance response induced by an ultrasonic tone. The animals had to learn to walk through a swinging door into a safe compartment only after the conditioning ultrasonic tone was on in order to avoid an electric shock to their feet. Trained animals were implanted in the ventral hippocampus with microinjection cannulae and injected twice with 1 microliter of saline solution containing pyrilamine (PYR, H1-HA antagonist), ranitidine (RAN, H2-HA antagonist) or histamine. The histamine antagonists were applied in a dose of 65.5 nmol each while histamine was administered in a dose of 45 nmol. The two variables measured were the time in sec the rats take to present the conditioned avoidance response and the accumulated percentage of conditioned avoidance response (CAR). Results showed that histamine administration significantly increased the latency time to escape and decreased the % CAR. These effects were not blocked by the administration of RAN. However, administration of PYR completely counteracted the HA effects. Present findings confirm our previous findings about the inhibitory effect of histamine on the hippocampal retrieval mechanisms and give further support to the hypothesis that HA acts on the memory processes in the hippocampal formation, by activation of H1-histamine receptors.

Downregulation of muscarinic- and 5-HT1B-mediated modulation of [3H]acetylcholine release in hippocampal slices of rats with fimbria-fornix lesions and intrahippocampal grafts of septal origin

Adult Long-Evans female rats sustained electrolytic fimbria-fornix lesions and, two weeks later, received intrahippocampal suspension grafts of fetal septal tissue. Sham-operated and lesion-only rats served as controls. Between 6.5 and 8 months after grafting, both the [3H]choline accumulation and the electrically evoked [3H]acetylcholine ([3H]ACh) release were assessed in hippocampal slices. The release of [3H]ACh was measured in presence of atropine (muscarinic antagonist, 1 microM), physostigmine (acetylcholinesterase inhibitor, 0.1 microM), oxotremorine (muscarinic agonist, 0.01 microM-10 microM), mecamylamine (nicotinic antagonist, 10 microM), methiothepin (mixed 5-HT1/5-HT2 antagonist, 10 microM), 8-OH-DPAT (5-HT1A agonist, 1 microM), 2-methyl-serotonin (5-HT3 agonist, 1 microM) and CP 93129 (5-HT1B agonist, 0.1 microM-100 microM), or without any drug application as a control. In lesion-only rats, the specific accumulation of [3H]choline was reduced to 46% of normal and the release of [3H]ACh to 32% (nCi) and 43% (% of tissue tritium content). In the grafted rats, these parameters were significantly increased to 63%, 98% and 116% of control, respectively. Physostigmine reduced the evoked [3H]ACh release and was significantly more effective in grafted (-70%) than in sham-operated (-56%) or lesion-only (-54%) rats. When physostigmine was superfused throughout, mecamylamine had no effect. Conversely, atropine induced a significant increase of [3H]ACh release in all groups, but this increase was significantly larger in sham-operated rats (+209%) than in the other groups (lesioned: +80%; grafted: +117%). Oxotremorine dose-dependently decreased the [3H]ACh release, but in lesion-only rats, this effect was significantly lower than in sham-operated rats. Whatever group was considered, 8-OH-DPAT, methiothepin and 2-methyl-serotonin failed to induce any significant effect on [3H]ACh release. In contrast, CP 93129 dose-dependently decreased [3H]ACh release. This effect was significantly weaker in grafted rats than in the rats of the two other groups. Our data confirm that cholinergic terminals in the intact hippocampus possess inhibitory muscarinic autoreceptors and serotonin heteroreceptors of the 5-HT1B subtype. They also show that both types of receptors are still operative in the cholinergic terminals which survived the lesions and in the grafted cholinergic neurons. However, the muscarinic receptors in both lesioned and grafted rats, as well as the 5-HT1B receptors in grafted rats show a sensitivity which seems to be downregulated in comparison to that found in sham-operated rats. In the grafted rats, both types of downregulations might contribute to (or reflect) an increased cholinergic function that results from a reduction of the inhibitory tonus which ACh and serotonin exert at the level of the cholinergic terminal.

Combined treatment with a 5HT1A receptor agonist and a muscarinic acetylcholine receptor antagonist disrupts water maze navigation behavior

The present study was designed to investigate the effects of combined treatment with a serotonin (5-HT)1A receptor agonist, 8-hydroxy-2-(dipropylamino)-tetralin (8-OH-DPAT), and a muscarinic acetylcholine receptor antagonist, scopolamine, on water maze (WM) navigation. Treatment with either 8-OH-DPAT or scopolamine before daily behavioral training disrupted spatial navigation at medium doses and cue navigation at high doses. Pretraining treatment with a combination of subthreshold doses of 8-OH-DPAT and scopolamine impaired WM spatial and cue navigation, but did not impair the WM performance if the drugs were injected post-training. In trained rats, combined injections of subthreshold doses of 8-OH-DPAT and scopolamine given pretraining did not impair the rats' ability to find the platform in a familiar or in a novel position. The combination of 8-OH-DPAT and scopolamine also disrupted WM navigation in rats with central 5-HT depletion. A combination of a peripheral muscarinic acetylcholine receptor antagonist and 8-OH-DPAT had no effect on WM navigation. These data suggest that combined treatment with drugs blocking muscarinic acetylcholine receptors and activating 5-HT1A receptors greatly impairs WM learning/performance, but does not impair spatial memory per se.

Serotonergic modulation of cholinergic function in the central nervous system: cognitive implications

Accumulating evidence suggests that serotonin may modulate cholinergic function in several regions of the mammalian brain and that these serotonergic/cholinergic interactions influence cognition. The first part of this review is an overview of histological, electrophysiological and pharmacological (in vitro, in vivo) data indicating that, in several brain regions (e.g., hippocampus, cortex and striatum), there are neuroanatomical substrates for a serotonergic/cholinergic interaction, and that alterations in serotonergic activity may induce functional changes in cholinergic neurons. In the second part, the review focuses on experimental approaches showing or suggesting that central cholinergic and serotonergic mechanisms are cooperating in the regulation of cognitive functions. These arguments are based on lesion, intracerebral grafting and pharmacological techniques. It is concluded that not all mnesic perturbations induced by concurrent manipulations of the serotonergic and cholinergic systems can be attributed to a serotonergic modification of the cholinergic system. The cognitive faculties of an organism arise from interactions among several neurotransmitter systems within brain structures such as, for instance, the hippocampus or the cortex, but also from influences on memory of other general functions that may involve cerebral substrates different from those classically related to mnesic functions (e.g., attention, arousal, sensory accuracy, etc.).

Synergistic anticholinergic and antiserotonergic effects in humans

Animal research suggests an important interactive role for ascending cholinergic and serotonergic systems in modulation of cerebral function. Employing a randomized, double-blind, crossover design, 11 healthy young adults were tested in each of four conditions: (1) placebo, (2) fenfluramine (a serotonin depleting agent), (3) scopolamine (a muscarinic antagonist), and (4) fenfluramine and scopolamine. P3 latency was slowed by the dual drug treatment to an extent greater than the sum of individual drug effects. EEG mean frequency was decreased by behavioral activation, and this decrease was reversed by the combined drug treatment but not by single drugs. In contrast, verbal memory, EEG alpha power, and P3 amplitude were significantly affected only by scopolamine. No drug effects were found for the N1 and P2 potentials. The results provide the first demonstration of combined anticholinergic and antiserotonergic effects in humans, and offer partial support to the concept of an interactive role of cholinergic and serotonergic systems in cerebral mechanisms.

Effects of tetrahydroaminoacridine and nicotine in nucleus basalis and serotonin-lesioned rats

The present study was designed to investigate the hypothesis that concurrent degeneration of serotonin and acetylcholine cells may decrease the therapeutic effects of cholinergic drugs on cognitive functioning in Alzheimer dementia. Therefore, we compared the effects of pretraining injections of a cholinesterase inhibitor, tetrahydroaminoacridine (1, 3 and 5 mg/kg i.p.), and nicotine (0.03, 0.1 and 0.3 mg/kg i.p.) on spatial navigation (water maze) and passive avoidance in nucleus basalis- and nucleus basalis+p-chlorophenylalanine-lesioned rats. Nicotine (0.1 and 0.3 mg/kg) promoted passive avoidance performance of nucleus basalis-lesioned rats, but nicotine did not improve performance of combined-lesioned rats. Tetrahydroaminoacridine (3 mg/kg) facilitated passive avoidance performance of nucleus basalis- and combined-lesioned rats. However, tetrahydroaminoacridine-treated nucleus basalis+p-chlorophenylalanine-lesioned rats were not performing better than vehicle-treated nucleus basalis-lesioned rats. Spatial navigation of nucleus basalis and nucleus basalis+p-chlorophenylalanine-lesioned rats was slightly impaired during the first training day and tetrahydroaminoacridine 3 mg/kg restored the performance of combined-lesioned rats. Combined-lesioned rats performed as well as the controls during the other training days. The present results suggest that, in Alzheimer's disease, combined degeneration of nucleus basalis cholinergic and brainstem serotonergic cells decreases the therapeutic effect of nicotine, but not that of tetrahydroaminoacridine.

The spinal cord as an alternative model for nerve tissue graft

The spinal cord provides an alternative model for nerve tissue grafting experiments. Anatomo-functional correlations are easier to make here than in any other region of the CNS because of a direct implication of spinal cord neurons in sensorimotor activities. Lesions can be easily performed to isolate spinal cord neurons from descending inputs. The anatomy of descending monoaminergic systems is well defined and these systems offer a favourable paradigm for lesion-graft experiments.

Multiple obstacles to gene therapy in the brain

Neuwelt et al. have proposed gene-transfer experiments utilizing an animal model that offers many important advantages for investigating the feasibility of gene therapy in the human brain. A variety of tissues concerning the viral vector and mode of delivery of the corrective genes need to be resolved, however, before such therapy is scientifically supportable.

Principles of brain tissue engineering

It is often presumed that effects of neural tissue transplants are due to release of neurotransmitter. In many cases, however, effects attributed to transplants may be related to phenomena such as trophic effects mediated by glial cells or even tissue reactions to injury. Any conclusion regarding causation of graft effects must be based on the control groups or other comparisons used. In human clinical studies, for example, comparing the same subject before and after transplantation allows for many interpretations of the causes of clinical changes.

Lessons on transplant survival from a successful model system

Studies on the snailMelampusreveal that connectivity is crucial to the survival of transplanted ganglia. Transplanted CNS ganglia can innervate targets or induce supernumerary structures. Neuron survival is optimized by the neural incorporation that occurs when a transplanted ganglion is substituted for an excised ganglion. Better provision for the trophic requirements of neurons will improve the success of mammalian fetal transplants.

Repairing the brain: Trophic factor or transplant?

Three experiments on neural grafting with adult rat hosts are described. Working memory impairments were produced by lesioning the hippocampus or severing its connections with the septum by ablating the fimbria-fornix. The results suggest that the survival and growth of a neural graft, whether an autograft or a xenograft, is not a necessary condition for functional recovery on a task tapping working memory.

Will brain tissue grafts become an important therapy to restore visual function in cerebrally blind patients?

Grafting embryonic brain tissue into the brain of patients with visual field loss due to cerebral lesions may become a method to restore visual function. This method is not without risk, however, and will only be considered in cases of complete blindness after bilateral occipital lesions, when other, risk-free neuropsychological methods fail.

Difficulties inherent in the restoration of dynamically reactive brain systems

The responses displayed by an injured or diseased nervous system are complex. Some of the responses may effect a functional reorganization of the affected neural circuitry. Strategies aimed at the restoration of function, whether or not these involve transplantation, need to recognize the innate reactive capacity of the nervous system to damage. More successful strategies will probably incorporate, rather than ignore, the adaptive responses of the compromised neural systems.

Elegant studies of transplant-derived repair of cognitive performance

Cholinergic-rich grafts have been shown to be effective in restoring maze-learning deficits in rats with lesions of the forebrain cholinergic projection system. However, the relevance of those studies to developing novel therapies for Alzheimer's disease is questioned.

Neural transplants are grey matters

The lesion and transplantation data cited by Sinden et al., when considered in tandem, seem to harbor an internal inconsistency, raising questions of false localization of function. The extrapolation of such data to cognitive impairment and potential treatment strategies in Alzheimer's disease is problematic. Patients with focal basal forebrain lesions (e.g., anterior communicating artery aneurysm rupture) might be a more appropriate target population.

Immunobiology of neural transplants and functional incorporation of grafted dopamine neurons

In contrast to the views put forth by Stein & Glasier, we support the use of inbred strains of rodents in studies of the immunobiology of neural transplants. Inbred strains demonstrate homology of the major histocompatibility complex (MHC). Virtually all experimental work in transplantation immunology is performed using inbred strains, yet very few published studies of immune rejection in intracerebral grafts have used inbred animals.