Survival of cholinergic forebrain neurons in developing p75NGFR-deficient mice

The functions of the low-affinity p75 nerve growth factor receptor (p75(NGFR)) in the central nervous system were explored in vivo. In normal mice, approximately 25 percent of the cholinergic basal forebrain neurons did not express TrkA and died between postnatal day 6 and 15. This loss did not occur in p75(NGFR)-deficient mice or in normal mice systemically injected with a p75(NGFR)-inhibiting peptide. Control, but not p75(NGFR)-deficient, mice also had fewer cholinergic striatal interneurons. Apparently, p75(NGFR) mediates apoptosis of these developing neurons in the absence of TrkA, and modulation of p75(NGFR) can promote neuronal survival. Cholinergic basal forebrain neurons are involved in learning and memory.

Nerve growth factor promotes regeneration of sensory axons into adult rat spinal cord

Injured adult mammalian axons are unable to regenerate spontaneously in the central nervous tissue. This study investigated in two adult rat models the effects of nerve growth factor (NGF) on the capacity of central primary sensory axons to regenerate back into the spinal cord. Sensory fibers were conditioned by transection of the peripheral nerve 1 week prior to the experiment and identified by anterograde tracing with cholera toxin B subunit injected in the sciatic nerve. In the first model, a predegenerated autologous peripheral nerve graft was implanted as a bridge for the transected sensory fibers into a resection gap in the dorsal columns at the tenth thoracic (T10) spinal cord segment. Vehicle or vehicle with purified mouse or recombinant human NGF was continuously infused for 2 weeks directly into the dorsal column at T9, 3 mm from the rostral border of the nerve graft. With vehicle infusion many ascending sensory axons had grown across the nerve bridge, but essentially none had grown back into the rostral cord. In sharp contrast, NGF promoted the reentry into the denervated dorsal columns of 51% of the sensory axons that had reached the rostral level of the nerve graft. Twenty-six percent had grown 2 mm into the spinal tissue and 10% had reached the NGF-infusion site at 3 mm from the nerve graft. A few fibers were found circling around, but not beyond, the infusion site, perhaps due to the chemoattractant action of NGF. In a second model, the fourth lumbar (L4) dorsal root was crushed 2 mm from its insertion point into the spinal cord and the dorsal roots L2, L3, L5, and L6 were transected. Vehicle or vehicle with purified mouse NGF was infused for 2 weeks directly into the lumbar spinal cord, 2.5 mm rostral to the transition zone of the crushed L4 root. With vehicle, only 6% of the regenerating fibers at the transition zone had crossed the root-spinal cord barrier, but not farther than 0.5 mm into the spinal tissue. With NGF, 18% of the fibers at the transition zone were found at 0.5 mm, 9% at 1.5 mm, and 5% at 2.5 mm (the infusion site) from the transition zone. The present results demonstrate that NGF can promote the regeneration of adult sensory fibers into the otherwise nonpermissive spinal cord white matter.

Regeneration of adult rat sensory axons into intraspinal nerve grafts: promoting effects of conditioning lesion and graft predegeneration

The effect of intraspinally implanted predegenerated peripheral nerve grafts and/or conditioning lesions on the regenerative capacity of central ascending sensory axons was investigated in the adult rat. Regenerating sensory fibers were analyzed after their transganglionic labeling with cholera toxin B subunit, usually 1 month after implantation. A conditioning lesion (transection of the tibial and peroneal nerve) caused a fivefold increase in the number of sensory fibers within the fresh graft when applied on the day of grafting and a sevenfold increase when applied 1 week before. In the latter case, a small portion of the fibers (10%) had reached the rostral end of the nerve graft. In the absence of a conditioning lesion, the number of fibers regenerating into a predegenerated nerve graft (collected from the distal part of the peroneal nerve that had been axotomized 1 week earlier) was similar to that found in a fresh graft. However, predegenerated grafts received three and five times more fibers than a fresh graft when conditioning lesions were applied on or 1 week before the day of grafting. With the combination of a predegenerated graft and a 1-week conditioning, most (> 90%) of the regenerating fibers had reached the rostral graft-host border. In animals with a fresh graft, a portion of the axotomized fibers formed terminal club-like structures. Much fewer fibers displayed such clubs in animals with a predegenerated graft or a conditioning lesion, suggesting a preventive action of either treatment. A time-course study with the combined treatments showed that regenerating sensory fibers had already entered the graft after 3 days. Between 1 and 2 weeks, a maximum number of fibers had reached the rostral end of the nerve graft. However, after 2 months, the number of fibers was decreased, i.e., the initial advantage of predegeneration had diminished. The current results demonstrate the necessity of a conditioning lesion for successful regeneration of central sensory fibers, possibly resulting from an earlier induction of the neuronal growth response which allows the axon to enter the graft before the formation of a graft-host barrier. The predegeneration of the nerve graft augments the growth response of the axotomized central sensory fibers, probably by providing a more supportive terrain and/or enhancement of the neuronal response. The presence of a large number of fibers at the rostral graft-host border now provides the opportunity to investigate the effects of neurotrophic factors on the regenerative capacity of the ascending rat sensory fibers into the denervated spinal cord in vivo.

Distribution of corticospinal motor neurons in the postnatal rat: quantitative evidence for massive collateral elimination and modest cell death

The postnatal development of rat corticospinal motor neurons (CSMN) was studied by retrograde tracing with cholera toxin B subunit (CTB) injected into the upper cervical dorsal spinal cord on the first postnatal day (P0), P3, P10, P20, and at adulthood. CTB-labeled neurons were visualized by immunocytochemistry and extensively quantified throughout the cortex. At P0, CSMN were found to an extent similar to that reported in P3 animals with other neuronal tracers, now permitting in vitro studies of neonatal CSMN. Between P0 and P3, the number of labeled neurons increased by 30% to a total maximum of approximately 185,000 in both cortices. The increase occurred throughout the cortex. At P10, the number of labeled CSMN had decreased to 60% of the number at P3. Fewer CSMN were evident particularly in the perirhinal cortex. Between P10 and P20, the number of CSMN decreased further to 52% of the maximal number at P3. This decrease occurred predominantly in the cingulate and parietal cortex. The number of labeled CSMN in rats injected at P0 and analyzed at P20 was 10% lower than the number in P0-injected littermates that were analyzed at P3, which suggests that only a small portion of the "disappearing" CSMN undergoes developmental neuronal death. Thus, the spinal projection of the remaining 38% is apparently eliminated between P3 and P20. Detailed quantitative analysis of the CSMN distribution demonstrated that neuronal death occurs predominantly in the perirhinal cortex. In contrast, axonal elimination of corticospinal projections occurred throughout the CSMN field, i.e., primarily in the frontal, occipital, and perirhinal cortex between P3-P10 and in the cingulate and parietal cortex between P10-P20.

Ciliary neurotrophic factor (CNTF) promotes low-affinity nerve growth factor receptor and CD4 expression by rat CNS microglia

Ramified parenchymal microglia may provide immune surveillance in the nervous system and become activated in response to injury, showing increases in antigens found on macrophages, e.g. CD4 and MHCs. We investigated in adult rats the effects of a 2-week intraventricular infusion with ciliary neurotrophic factor (CNTF), a nervous system-associated cytokine, on microglia of the normal and injured corpus callosum. CNTF caused morphological changes, induced the expression of low-affinity nerve growth factor receptor and CD4 and increased the expression of complement receptor 3. Such changes were also observed after treatment of pure cultures of neonatal rat microglial cells with highly purified CNTF, suggesting a direct responsiveness to CNTF. Thus, endogenous astroglial and Schwann cell-derived CNTF may be an important component of the immune responses of the nervous system.

Ciliary neurotrophic factor prevents degeneration of adult rat substantia nigra dopaminergic neurons in vivo

We have investigated the neuroprotective effects of recombinant human ciliary neurotrophic factor (CNTF) for injured dopaminergic neurons of the adult rat substantia nigra compacta. Fourteen days after a unilateral transection of the nigrostriatal pathway two-thirds of the neurons (identified by retrograde labeling) had degenerated. In sharp contrast, 73% (a few cases, > 90%) of this cell loss was prevented by continuous infusion of CNTF close to the injured neurons. However, CNTF did not prevent the disappearance of the transmitter-synthesizing enzyme tyrosine hydroxylase. Thus, CNTF has potent neurotrophic effects for injured adult rat dopaminergic substantia nigra neurons, whose degeneration plays a major causative role in Parkinson disease.

Potential regulation by trophic factors of low-affinity NGF receptors in spinal motor neurons

Developing spinal motor neurons (SMN) express low-affinity nerve growth factor receptors (LNGFR) but not high-affinity transducing NGF receptors. Moreover, SMN are not supported by NGF in vitro. In the normal adult rat most SMN are not LNGFR immunoreactive (LNGFR-IR), but they transiently reexpress LNGFR (though not the high-affinity receptor) after peripheral nerve injury. With a cut lesion of the sciatic nerve (when only a neuroma forms), the number of LNGFR-IR SMN at L4-L6 rapidly increases to a maximum between day 1 and 7 and returns to baseline levels by day 30. After a crush lesion (accompanied by regeneration to the muscle), LNGFR-IR SMN appear in about the same numbers, but they start to disappear 1 week later. We speculate that the similar appearance and differential decline of LNGFR-IR seen after the two types of lesions are regulated by the availability of a common signal such as ciliary neurotrophic factor. The adult SMN model provides a good opportunity to investigate the reexpression of LNGFR after peripheral nerve injury, and more generally, the unknown role and regulation of LNGFR.

Neurotropism of nerve growth factor for adult rat septal cholinergic axons in vivo

Nerve growth factor (NGF) can induce sprouting of axotomized adult rat medial septum cholinergic neurons and promote their regeneration into septohippocampal nerve grafts and hippocampal formation. This study investigated the potential neurotropic (chemotactic/attracting) action of NGF in the adult rat cholinergic septohippocampal regeneration model. (i) Some animals received sciatic nerve grafts between the disconnected septum and hippocampal formations on each side. A 4-week infusion with NGF into the rostral portion of the lateral ventricle induced sprouting of cholinergic fibers in the dorsolateral septum with a gradient toward the lateral ventricle. However, the number of cholinergic axons entering the nerve bridge was only one-third that observed in vehicle-infused animals, suggesting that NGF had diverted many of the regrowing axons away from the nerve toward the ventricle. (ii) In animals implanted with nerves for 2 weeks and concurrently infused with NGF into the fornix, proximal to the lesion and grafts, cholinergic sprouting occurred in the mediodorsal septum, i.e., was oriented toward the infused fornix. Essentially no fibers had entered the nerve bridge, suggesting that all regrowing fibers had remained near the NGF source. (iii) When animals with a unilateral fimbria-fornix transection (but no nerve graft) were infused with NGF into the lateral ventricle on the opposite side, cholinergic sprouting was oriented toward the midline of the septum. (iv) Infusion of low doses of NGF directly into the (lesioned) septum induced a sprouting response localized around the infusion site. (v) No sprouting occurred when intraventricular NGF infusion was applied to normal (nonlesioned) animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Laminin immunohistochemistry in brain is dependent on method of tissue fixation

Normal adult and lesioned rat and mouse brains were fixed by formaldehyde perfusion by two methods that differ primarily in the length of the post-fixation period. Sections were subsequently immunostained using monoclonal and polyclonal antibodies to laminin. With relatively short post-fixation periods (up to 4 h), vascular basement membrane (BM)-laminin was immunostained, but intraneuronal laminin-like immunoreactivity was faint. With longer post-fixation periods (18-24 h), intraneuronal laminin-like immunoreactivity was distinct, while vascular BM-laminin immunoreactivity was reduced drastically. These findings are particularly relevant to studies examining laminin immunoreactive blood vessels in response to lesions, especially ischemic stroke. In fact, the present results suggest that the apparent neovascularization or up-regulation of vascular BM-laminin following CNS injury likely relates to differences in regional tissue fixation.

The localization of nerve growth factor-like immunoreactivity in the adult rat basal forebrain and hippocampal formation

The role of nerve growth factor (NGF) as a target derived neurotrophic agent for specific cell populations in the peripheral nervous system has been well documented and much evidence suggests that NGF may serve a similar neurotrophic role in the CNS supporting the cholinergic neurons of the basal forebrain. Previous attempts to localize NGF by immunocytochemical methods, however, have not yielded evidence confirming the regional distribution expected based upon reported levels of extractable NGF. In the present study, affinity purified polyclonal antibodies to beta-NGF and a modified immunohistochemical protocol were used to demonstrate specific NGF-like immunoreactivity in the adult rat hippocampal formation and basal forebrain. In the hippocampal formation, NGF-like immunoreactivity was localized primarily within the hilus of the dentate gyrus and within stratum lucidum of the CA3 and CA2 hippocampal subfields. Staining appeared to be associated with cell processes and was similar to the reported distribution of mossy fibers suggesting that granule cells may either serve as a primary source of hippocampal NGF or that mossy fibers selectively accumulate NGF produced by other cell populations. In the basal forebrain, NGF-like immunoreactivity was localized within neuronal cell bodies of the medial septum, diagonal band, and nucleus basalis of Meynert and was further demonstrated to colocalize exclusively with LNGF-R positive neurons. These findings demonstrate the presence of an NGF-like antigen in association with cholinergic neurons of the basal forebrain and strongly support the hypothesis that NGF may serve as an endogenous trophic factor for this adult neuronal population.

Nerve growth factor receptor immunoreactivity in neurons of the normal adult rat spinal cord and its modulation after peripheral nerve lesions

Motoneurons of the rat spinal cord express low-affinity nerve growth factor receptor (LNGFR) and corresponding mRNA during development, and re-express it after their axotomy by peripheral nerve injury. The present study establishes the anatomical and quantitative baseline of LNGFR immunoreactive (LNGFR-IR) neurons of the entire normal adult female rat and then investigates the temporal course for the re-expression of LNGFR-IR in lumbar motoneurons after either a crush lesion (which is followed by regeneration and reconnection to the muscle) or a cut lesion with removal of the distal stump (where a neuroma but no reconnection is formed). In the normal adult spinal cord, two types of LNGFR-IR neurons were recognized: (1) small populations of large motoneurons located in the ventral horn mainly in correspondence to the regions of the phrenic, cremasteric and dorsolateral nuclei, and (2) a more numerous and more dorsally located population of small neurons. With a sciatic cut lesion, the number of LNGFR-IR motoneurons at spinal levels L4-L6 rapidly and dramatically increased to a maximum between post-lesion days 1 and 7, apparently involving most axotomized motoneurons of the region, and returned to the baseline level by day 30. With a crush lesion, similar numbers and virtually the same time-course of LNGFR-IR appearance were seen, but the onset of progressive disappearance of LNGFR-IR neurons was delayed by one week, so that at 30 days, the most caudal motoneurons (which are last to reach their target) were still LNGFR-IR. Comparison of these two time courses gives clues to the kind of signals that may be involved in initiating and/or maintaining the LNGFR response.

Immunolocalization of ciliary neuronotrophic factor in adult rat sciatic nerve

Two rabbit polyclonal antibodies were raised against synthetic peptides corresponding to residue numbers 45-59 and 181-200 of rat ciliary neuronotrophic factor (CNTF). The resulting antibodies were purified by affinity chromatography and both purified antibodies reacted by enzyme-linked immunoassay (ELISA) and immunoblotting with rat sciatic nerve CNTF. The anti-CNTF peptide antibodies were used to immunostain sections of adult rat sciatic nerve, previously known as the richest tissue source of CNTF. By light microscopy both antibodies appeared to stain exclusively Schwann cells and axons and both did so with the same pattern of specific staining. Immunostaining was eliminated by absorption of the anti-peptide antibodies with either their corresponding peptide or with purified rat nerve CNTF or by using purified nonspecific IgG. Schwann cells were stained and in semi-thin sections this staining appeared to be in the Schwann cell cytoplasm. Axons could be stained in addition to Schwann cells providing higher concentrations of antibodies were used. Epineurial, endoneurial and endothelial cells appeared unstained. Since all Schwann cells and axons appear to contain CNTF and since CNTF is known to act in vitro to support sensory and sympathetic ganglionic and motor neurons, we suggest that Schwann cells may normally provide CNTF to those neurons contributing axons to the peripheral nerve.

Ciliary neurotrophic factor prevents neuronal degeneration and promotes low affinity NGF receptor expression in the adult rat CNS

Recombinant human ciliary neurotrophic factor (CNTF) was infused for 2 weeks into the lateral ventricle of fimbria-fornix transected adult rats, and its effects were compared with those of purified mouse nerve growth factor (NGF). We provide evidence that CNTF can prevent degeneration and atrophy of almost all injured medial septum neurons (whereas NGF protects only the cholinergic ones). CNTF is also involved in up-regulation of immunostainable low affinity NGF receptor (LNGFR) in cholinergic medial septum and neostriatal neurons and in a population of lateral septum neurons. In contrast to NGF, CNTF did not stimulate choline acetyltransferase in the lesioned septum and normal neostriatum (pointing to different mechanisms for the regulation of choline acetyltransferase and LNGFR), cause hypertrophy of septal or neostriatal cholinergic neurons, or cause sprouting of LNGFR-positive (cholinergic) septal fibers.

Dose-dependent responses to nerve growth factor by adult rat cholinergic medial septum and neostriatum neurons

This study describes the relationship between the concentration of intraventricularly infused nerve growth factor (NGF) and several responses by axotomized cholinergic medial septum neurons and normal cholinergic neostriatal neurons of the adult rat. NGF infused for 14 days starting either immediately after a unilateral fimbria-fornix transection or after a 2-week delay period elicited similar dose-response relationships for the maintenance or restoration of ChAT and NGF receptor positivity and cell body size and for intraseptal 'sprouting' of the axotomized medial septum neurons. Thus, in the medial septum it appears that the expression of 'marker' molecules, cell body size and the induction of 'sprouting' are regulated by virtually the same concentrations of NGF in the two treatment strategies. This suggests that NGF has a general regulatory role and injured but untreated neurons remain fully susceptible to NGF at least up to 2 weeks after the lesion. A 14-day infusion with NGF also induced an above-normal cell body size (hypertrophy) both in axotomized medial septum and in intact striatal cholinergic neurons. The hypertrophic response of normal striatal neurons required less NGF than did that of medial septum neurons. Since the striatal response began to be detectable at a similar concentration as that required for the full maintenance or restoration of ChAT and NGF receptor positivity it could be seen as an unwanted side-effect. The definition of a sub-optimal dose with which a significant, but not maximal response can be elicited will allow future evaluations of potentially additive or synergistic actions by other agents.

Nerve growth factor promotes CNS cholinergic axonal regeneration into acellular peripheral nerve grafts

Peripheral nerve grafts promote vigorous regeneration of adult mammalian CNS axons. Elimination of nerve-associated cells by freeze-thawing abolishes this promoting quality, possibly by creating inhibitory cellular debris and/or destroying the production of stimulatory factors by living Schwann or other cells. Here, debris-free acellular peripheral nerve segments placed between the disconnected septum and the hippocampal formation acquired almost no cholinergic axons after 1 month. However, such acellular nerve grafts treated before implantation with purified beta-nerve growth factor (NGF) contained nearly as many longitudinally oriented cholinergic axons as did fresh cellular nerve grafts. These results suggest that (i) NGF is required for the regeneration of adult CNS cholinergic axons into nerve grafts and (ii) an important function of living cells within peripheral nerve may be the production of neuronotrophic factors such as NGF.

Nerve growth factor infusion into the denervated adult rat hippocampal formation promotes its cholinergic reinnervation

The well-documented but little-understood failure of lengthy axonal regeneration after injury of the adult mammalian CNS may be caused by an insufficient availability of local growth-promoting factors. If so, identifying and supplying the missing factors may result in better central axonal regeneration. This hypothesis was tested in an adult rat CNS model in which peripheral nerve grafts were placed into a lesion cavity between the septum and hippocampal formation. Continuous infusion of nerve growth factor (NGF) into the dorsal hippocampal tissue dramatically enhanced and accelerated the regrowth and penetration of cholinergic axons into the hippocampal formation. Thus, NGF can overcome the apparent resistance of the hippocampal CNS tissue to cholinergic reinnervation.

Septohippocampal cholinergic axonal regeneration through peripheral nerve bridges: quantification and temporal development

Axons of the adult mammalian CNS have been shown to regrow vigorously into peripheral nerve grafts. Using a cholinergic septohippocampal model for adult CNS regeneration, involving complete denervation of the hippocampal formation from its basal forebrain cholinergic afferents, this study has established quantitative parameters and a temporal baseline of cholinergic fiber regeneration into the dorsal hippocampal tissue through a peripheral sciatic nerve graft. In nerve-implanted animals (i) the nerve grafts are maximally invaded by AChE-positive fibers between 2 weeks and 1 month postlesion, (ii) the fibers entering the hippocampal formation from the graft show a peak numerical increase and rate of elongation around the first month and/or in the proximal hippocampal region, (iii) an apparently normal innervation pattern and fiber density in the most rostral 1.5 mm of the dorsal hippocampal formation is reached by 6 months postlesion. The present study provides a basis for future quantitative comparisons of manipulations of different components of the system, e.g., the contributing neurons, the bridging material, and the receiving central nervous tissue. The temporal/spatial pattern of fiber regeneration suggests that the hippocampal CNS tissue can be a good axonal growth-promoting environment, albeit with temporal and/or spatial limitations, and is therefore not an immutably restrictive environment for axonal regeneration.

Nerve growth factor (NGF) reverses axotomy-induced decreases in choline acetyltransferase, NGF receptor and size of medial septum cholinergic neurons

Intraventricular nerve growth factor (NGF) infusion in the adult rat can prevent and also, if delayed, reverse the disappearance of most of the axotomized medial septum cholinergic neurons immunostained for choline acetyltransferase (ChAT). We have utilized the delayed NGF treatment protocol to (i) extend to 3 months the delay time between axotomy and NGF treatment, (ii) define the time course of their recovery, (iii) determine that immunostaining for the (lower affinity) NGF receptor (NGFR) parallels loss and reversal of the ChAT marker, and (iv) evaluate changes in cholinergic somal size following axotomy and subsequent NGF treatment. While NGF treatments starting only 7 days after the fimbria-fornix transection (axotomy) almost entirely restored the number of both ChAT- and NGFR-positive medial septum neurons, longer delayed (2-3 weeks) treatment brought about recovery from the baseline of 20-25% to only about 70% of the control numbers. This limited recoverability, however, persisted even after a 95 day delay period. In all cases examined maximal recoveries were achieved within 3-7 days of NGF treatment. Neuronal size analyses provided evidence for an axotomy-induced atrophy. NGF treatments, started with 1 or 2 week delays, not only reversed fully the average somal size loss but also induced an actual hypertrophy of several of those neurons. These results provide additional evidence that at least half of the apparent loss of cholinergic medial septum neurons upon axotomy is due to a loss of markers such as the transmitter-related enzyme ChAT and NGFR rather than to actual neuronal cell death. These results also show that NGF exerts a genuine trophic influence by regulating the size of its target neurons as well as their content of several proteins.

Laminin-like antigen in rat CNS neurons: distribution and changes upon brain injury and nerve growth factor treatment

Using several antibodies against rat or human laminin and an avidin-biotin immunocytochemical protocol, laminin-like immunoreactivity was detectable in the rat nervous system in expected locations, i.e., associated with blood vessels and reactive astrocytes. However, laminin staining was also abundantly present within neuronal cell bodies in most parts of the developing and adult rat CNS. Medial septum neuronal immunoreactivity was lost after septo-hippocampal disconnection, but could be preserved or even restored by intraventricular administration of nerve growth factor. Thus, at least for medial septum neurons, this laminin-like molecule can be accumulated or produced independent of direct hippocampal (target) contact. It remains to be determined whether CNS neuronal "laminin" processes activities similar to those found for laminin in vitro.

Neuronotrophic factors in cellular functional and cognitive repair of adult brain

Neuronotrophic factors (NTFs) are special proteins that control survival, growth and functional competence of selected neuronal populations during their development. A recently articulated CNS neuronotrophic hypothesis proposes that adult central neurons continue to be dependent on endogenous supplies of their NTFs for maintenance, function and repair capabilities. The hypothesis has important implications in two major neurologic areas:i) involutive and degenerative processes and ii) CNS regeneration. Strong evidence favoring this general hypothesis has come from the recently observed effects of Nerve Growth Factor (NGF) on the forebrain cholinergic neurons in adult rats. Medial septum cholinergic (MSC) neuros project to the hippocampal formation largely via the fimbria-fornix tract. Transection of the fimbria-fornix interrupts the retrograde delivery of hippocampal NGF to the MSC neuronal somata, leading to reduction in their cholinergic enzyme and, possibly, progressive cellular atrophy. Intraventricular administration of exogenous NGF compensates for the loss of endogenous NGF supply and can reverse as well as protect against the neuronal damage. In a second model, intraventricular NGF infusions were applied to 2-year old (aged) rats displaying a deficient cognitive behavior. Four-weeks administration resulted in i) a correction of the cognitive deficit and ii) a increased body size of basal forebrain cholinergic neurons Cholinergic deficits are known to accompany cognitive deficits both in the brain aging process and Alzheimer's disease. The NGF effects on aged rats, therefore, strongly encourage the possibility of future clinical application in both such cases of brain dysfunction.

An improved device for continuous intraventricular infusions prevents the introduction of pump-derived toxins and increases the effectiveness of NGF treatments

The recent demonstrations of the ability of nerve growth factor (NGF) to protect and promote the welfare of certain cholinergic neurons in the adult CNS have increased the need for safe, accurate, and reliable procedures for intracerebral administration of protein and other experimental agents. Osmotic minipumps have been used to infuse NGF into the lateral ventricle of adult rats, but a sustained and harmless performance of such infusions has not been fully evaluated. The study reported here has led to (i) the recognition that cytotoxic substances, released from some minipumps into the infusion fluid, may be responsible for various degrees of periventricular tissue damage, and (ii) the redesigning of an infusion device which, among other modifications, uses the osmotic pump to propel infusion fluid into the ventricle but prevents pump-derived materials from entering the infusate itself. Besides several other advantages, the modified infusion device has permitted the demonstration that NGF can fully protect experimentally axotomized medial septum cholinergic neurons and can do so with less variability than previously observed and without creating tissue damage.

Nerve growth factor effects on cholinergic neurons of neostriatum and nucleus accumbens in the adult rat

Following intraventricular nerve growth factor infusion in adult rats, the choline acetyltransferase immunostaining of the neuropil and neuronal cell bodies of the neostriatum (caudate-putamen) and nucleus accumbens was more intense on the side of the infusion. Furthermore, the average cross-sectional size (micron2) of the cholinergic somata was increased by about 40 and 20% in the striatum and accumbens, respectively. This unilateral response could be elicited in intact rats as well as in rats receiving a prior aspirative transection of the fimbria-fornix. The reported lack of (low-affinity) nerve growth factor receptor immunostaining in these neurons suggests that the nerve growth factor effects are most likely transduced by high-affinity receptors. The ability of these apparently undamaged cholinergic interneurons to respond to exogenous nerve growth factor with an increase in choline acetyltransferase content and cell body size suggests that they are benefiting from a less-than-maximal support by endogenous nerve growth factor in the normal young adult rat.

Delayed treatment with nerve growth factor reverses the apparent loss of cholinergic neurons after acute brain damage

Previous studies have shown that the loss after brain injury of adult rat septal cholinergic neurons whose axons are transected can be prevented by immediate intraventricular nerve growth factor (NGF) administration. This loss of axotomized neurons may be due to a reduction in detectability of neurotransmitter-related enzyme rather than to neuronal death. Here we report that NGF treatment, started after most of the neurons were no longer detectable (i.e., 1, 2, and 3 weeks), induced a dramatic reappearance of the apparently lost cholinergic neurons. These results may have important implications for potential trophic factor treatments of CNS trauma and neurodegenerative diseases, such as Alzheimer's dementia, which are characterized by chronic and progressive losses in the function of specific sets of neurons.