Hippocampal tyrosine kinase A receptors are restricted primarily to presynaptic vesicle clusters.

Adult septohippocampal cholinergic neurons are dependent on trophic support for normal functioning and survival; these effects are largely mediated by the tyrosine kinase A receptor (TrkA), which binds its ligand, nerve growth factor (NGF), with high affinity. To determine the subcellular localization of TrkA within septohippocampal terminal fields, two rabbit polyclonal antisera to the extracellular domain of TrkA were localized immunocytochemically in rat dentate gyrus by light and electron microscopy. By light microscopy, TrkA immunoreactivity was found mostly in fine, varicose fibers primarily in the hilus and, to a lesser extent, in the granule cell and molecular layers. By electron microscopy, the central and infragranular regions of the hilus contained the highest densities of TrkA-immunoreactive profiles. Most TrkA-labeled profiles were axons (31% of 3,473), axon terminals (20%), and glia (38%); fewer were dendrites (6%), dendritic spines (5%), and granule cell and interneuron somata (<1%). TrkA immunolabeling in axons and axon terminals was discrete, often concentrated in patches of small synaptic vesicles that were adjacent to somatic and dendritic profiles. TrkA-labeled terminals formed both asymmetric and symmetric synapses, primarily with dendritic shafts and spines. TrkA-immunoreactive glial profiles frequently apposed terminals contacting dendritic spines. The findings that presynaptic profiles contain TrkA immunolabeling in sites of vesicle accumulation suggest that NGF binding to TrkA may influence transmitter release. The presence of TrkA immunoreactivity in somata, dendrites, and glia further suggests that cells within the dentate gyrus may take up NGF.

Brain-derived neurotrophic factor in astrocytes, oligodendrocytes, and microglia/macrophages after spinal cord injury.

Recent studies suggest that the injured adult spinal cord responds to brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT3) with enhanced neuron survival and axon regeneration. Potential neurotrophin sources and cellular localization in spinal cord are largely undefined. We examined glial BDNF localization in normal cord and its temporospatial distribution after injury in vivo. We used dual immunolabeling for BDNF and glial fibrillary acidic protein (GFAP) in astrocytes, adenomatous polyposis coli tumor suppressor protein (APC) for oligodendrocytes or type III CDH receptor (OX42) for microglia/macrophages. In normal cord, small subsets of astrocytes and microglia/macrophages and most oligodendrocytes exhibited BDNF-immunoreactivity. Following injury, the number of BDNF-immunopositive astrocytes and microglia/macrophages increased dramatically at the injury site over time. Most oligodendrocytes contained BDNF 1 day and 1 week following injury, but APC-positive cells were largely absent at the injury site 6 weeks postinjury. Glial BDNF-immunolabeling was also examined 10 and 20 mm from the wound. Ten millimeters from the lesion, astrocyte and microglia/macrophage BDNF-immunolabeling resembled that at the injury at all times examined. Twenty millimeters from injury, BDNF localization in all three glial subtypes resembled controls, regardless of time postlesion. Our findings suggest that in normal adult cord, astrocytes, oligodendrocytes, and microglia/macrophages play roles in local trophin availability and in trophin-mediated injury and healing responses directly within and surrounding the wound site.

Cholinergic septal afferent terminals preferentially contact neuropeptide Y-containing interneurons compared to parvalbumin-containing interneurons in the rat dentate gyrus.

Septal cholinergic neurons may affect hippocampal memory encoding and retrieval by differentially targeting parvalbumin (PARV)-containing basket cells and neuropeptide Y (NPY) interneurons. Thus, the cellular associations of cholinergic efferents, identified by the low-affinity, p75 neurotrophin receptor (p75(NTR)), with interneurons containing either PARV or NPY in the hilus of the rat dentate gyrus were examined in single sections using dual labeling immunoelectron microscopy. Most profiles immunoreactive (IR) for PARV and NPY were perikaryal and dendritic and found within the infragranular and central hilar regions, respectively, whereas most profiles with p75(NTR)-labeling were unmyelinated axons and axon terminals. Although PARV-labeled profiles were more numerous, p75(NTR)-labeled axons and terminals contacted few PARV-IR profiles compared to NPY-labeled profiles (2% of 561 for PARV vs 12% of 433 for NPY). Moreover, structures targeted by p75(NTR)-IR axon terminals varied depending on the presence of PARV or NPY immunoreactivity. p75(NTR)-IR terminals primarily contacted PARV-IR dendrites (87%) compared to somata (13%); however, they contacted more NPY-IR somata (57%) than dendrites (43%). p75(NTR)-labeled terminals formed exclusively symmetric (inhibitory-type) synapses with PARV-IR somata and dendrites; however, they formed mostly symmetric but also asymmetric (excitatory-type) synapses with NPY-IR somata and dendrites. These results suggest that septal cholinergic efferents in the dentate gyrus: (1) preferentially innervate NPY-containing interneurons compared to PARV-containing basket cells; and (2) may provide a more powerful (i.e., somatic contacts), yet functionally diverse (i.e., asymmetric and symmetric synapses), modulation of NPY-containing interneurons. Moreover, they provide evidence that neurochemical subsets of hippocampal interneurons can be distinguished by afferent input.

p75NTR immunoreactivity in the rat dentate gyrus is mostly within presynaptic profiles but is also found in some astrocytic and postsynaptic profiles.

To localize neurotrophin binding sites within the rat dentate gyrus, the distribution of low-affinity p75 neurotrophin receptor (p75NTR) immunoreactivity (IR) was examined by using antiserum raised against the cytoplasmic domain of the receptor. Semiquantitative electron microscopic examination of p75NTR-labeled sections showed that most p75NTR-labeled profiles were axons and axon terminals (72% from a total of 3,975); p75NTR-IR was observed throughout the extent of these structures and was not limited to the plasmalemmal surface. Axons and axon terminals containing p75NTR-IR were distributed in approximately equal proportions across the hilus, infragranular zone, and the inner, middle, and outer molecular layers; significantly fewer p75NTR-labeled profiles were observed in the granule cell layer. Axon terminals containing p75NTR-IR, which made synapses (296 of 552), formed equal proportions of symmetric and asymmetric synapses, primarily with the shafts and spines of dendrites. The remainder of the p75NTR-labeled terminals apposed unlabeled somata and dendrites without forming synapses in the single sections analyzed. In addition, p75NTR-IR was contained within some astrocytes (17.5% of 3,975) and dendritic shafts (3%) and spines (5%). Within dendritic spines, p75NTR-IR was most often associated with the plasmalemmal surface near postsynaptic densities; in dendritic shafts, p75NTR labeling was associated with microfilaments distant from the plasmalemma. Most p75NTR-labeled dendritic profiles were located in the molecular layer, and some originated from granule cells. Moreover, in some granule cell somata (<1% of 3,975), p75NTR-IR was associated with endosomes. The primary localization of p75NTR-IR to presynaptic structures in the dentate gyrus, presumably arising from medial septal/diagonal band neurons, agrees with previous reports. However, p75NTR-IR within some astrocytes, somata, and dendritic structures suggests that this receptor may also be involved in controlling local neurotrophin levels and possibly modulating the viability of local hippocampal cell populations.

Septocingulate and septohippocampal cholinergic pathways: involvement in working/episodic memory.

The contribution of the septohippocampal cholinergic pathway to performance of a working/episodic memory task was compared to that of the septocingulate cholinergic path. The septocingulate and septohippocampal cholinergic pathways were selectively destroyed in male Sprague-Dawley rats using site-specific injections of the anti-neuronal immunotoxin 192-IgG saporin into either the hippocampus or the cingulate cortex. 192-IgG-saporin selectively destroys cholinergic neurons and terminals that express the p75 neurotrophin receptor. Following extensive pre-operative training, working memory was assessed using a delayed nonmatch to sample eight arm radial maze task, with delays of 1, 4 and 8 h. The group with lesions of the septohippocampal cholinergic pathway displayed performance deficits on this task which were not related to length of delay. In contrast, the group with lesions of the septocingulate cholinergic pathway did display delay-dependent deficits which were observed at the 4- and 8-h delays, but not at the 1-h delay. These data suggest that the septocingulate cholinergic pathway is critically involved in working/episodic memory but that the septohippocampal cholinergic pathway is either not contributing to working/episodic memory per se or it is involved only at shorter delays.

Acquisition of a morris water maze task is impaired during early but not late withdrawal from morphine.

Behavioral changes in male Sprague-Dawley rats during early and late withdrawal from morphine were investigated. Morphine-treated subjects (M) were implanted (SC) with osmotic pumps containing 2.0 ml of a 159 mg/ml morphine sulfate solution while control subjects (C) received sham implants. Implants were removed after 7 days. M subjects exhibited a significant decrease in body weight during withdrawal that recovered by 21 days after pump removal. Beginning 1 or 21 days following pump removal, subjects were tested for 8 days in a Morris water maze (MWM) task. M subjects trained in the MWM during early withdrawal exhibited significantly longer escape latencies than C subjects. However, during sequential probe trials, the same subjects exhibited a significant preference for the target quadrant of the maze and executed accurate searches for the escape platform. Though these subjects failed to locate the platform as efficiently as controls during training trials, they learned the location of the escape platform. M rats trained during late withdrawal exhibited no deficits in any measure of MWM performance relative to C subjects. The data suggest that a variety of processes involved in the acquisition and performance of the MWM task are differentially affected during early withdrawal from morphine.

Intraseptal injection of the cholinergic immunotoxin 192-IgG saporin fails to disrupt latent inhibition in a conditioned taste aversion paradigm.

Male Sprague-Dawley rats received injections of 100 or 375 ng 192-IgG saporin (SAP) or artificial CSF (C) into the medial septum/diagonal band complex (MSDB). SAP treated rats exhibited brief increases in daily water consumption which recovered to control levels by twenty one days following surgical treatment. Half of all subjects were preexposed to a 0.2% saccharin solution (CS) for 4 consecutive days prior to the establishment of a conditioned taste aversion (CTA) to saccharin. SAP was not found to alter acquisition of a normal CTA nor CS preexposure effects. All subjects which experienced CS/US pairing displayed aversions to saccharin when given a two bottle choice test. However, those subjects which were preexposed to saccharin prior to CS/US pairing displayed significantly weaker aversions than non-preexposed subjects. These subjects consumed a higher percent saccharin on test day and their CTAs extinguished more rapidly relative to non-preexposed subjects. While these behavioral measures remained undisrupted, intraseptal SAP treatment dose-dependently decreased hippocampal levels of high affinity choline transport as well as significantly decreasing HAChT in cingulate and frontal cortices. No such changes in HAChT were observed in striatum or entorhina cortex. These data suggest that the MSDB cholinergic cell population is not critical to selective attentional processes believed to underlie latent inhibition.

Behavioral and neurobiological alterations induced by the immunotoxin 192-IgG-saporin: cholinergic and non-cholinergic effects following i.c.v. injection.

192-IgG-Saporin is an anti-neuronal immunotoxin that combines the 192 monoclonal antibody to the p75 neurotrophin receptor found on terminals and cell bodies of neurons in the cholinergic basal forebrain with the ribosome-inactivating protein saporin. Bilateral intraventricular injection of the 192-saporin produced a variety of dose-related behavioral, neurochemical, and histological alterations in adult male rats. While both the 2 micrograms and 4 micrograms dose produced comparable cholinergic hypofunction only the high dose produced behavioral changes. Behavioral deficits induced by the 4 micrograms dose of 192-saporin induced alterations in rotorod performance and reactivity on the hot-plate which recovered over 8 weeks. In addition, the 4 micrograms dose produced a persistent impairment in the acquisition and performance of standard Morris water maze task as well as a cued version of the task. The neurobiological alterations induced by 192-saporin involved both cholinergic and non-cholinergic systems. Both doses of 192-saporin produced a 60-80% decrease in high affinity choline transport in the hippocampus and cortex without altering this parameter in the striatum. In addition, there was a significant dose-related decrease of norepinephrine in the hippocampus in the high dose group. 192-saporin did not alter the content of dopamine, serotonin, or their metabolites in any region examined. 192-saporin also produced a loss of Purkinje cells in the cerebellum. This cell type also expresses the p75 receptor and appears to be a target for intraventricular 192-saporin. This complex interplay of factors makes the i.c.v. model of 192-saporin very problematic for studying the functional properties of the cholinergic basal forebrain. However, recent data suggest that injection of 192-saporin directly into components of the cholinergic basal forebrain can be used to further elaborate the function of this brain system and to model disorders of cholinergic hypofunction such as Alzheimer's disease.