To evaluate effective means for delivering exogenous neurotrophins to neuron populations in the brain, we compared the distribution and transport of brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF), and neurotrophin-3 (NT-3) following intracerebral delivery. Rats received an injection of radioiodinated or unlabeled neurotrophin into the lateral ventricle and were killed humanely after 1.5-24 hours. Other rats received continuous infusion of unlabeled neurotrophin into the lateral ventricle, the striatum, or the hippocampus for 3-14 days. The neurotrophins were detected by autoradiography or immunohistochemistry. There were striking differences between BDNF, NGF, and NT-3 in their penetration through brain tissue. These differences occurred regardless of the site or method of delivery, but were most pronounced following a bolus intracerebroventricular (ICV) injection. After ICV injection, NGF was widely distributed in tissues around the ventricles and at the surface of the brain, whereas the penetration of BDNF into brain tissue was distinctly less than that of NGF, and the penetration of NT-3 was intermediate. An ICV injection of NGF produced prominent but transient labeling of cells that contain the low-affinity NGF receptor, whereas an injection of BDNF prominently labeled the ventricular ependyma. During ICV infusion (12 micrograms/day), the distribution of BDNF was no greater than that observed after a 12-micrograms bolus injection. A sixfold increase in the amount of BDNF infused (72 micrograms/day) produced a more widespread distribution in the brain and doubled the depth of penetration into periventricular tissues near the cannula. Corresponding to their differences in penetration, NGF was retrogradely transported by basal forebrain cholinergic neurons after ICV or intrastriatal delivery, whereas NT-3 was transported by a few basal forebrain neurons after ICV delivery, and BDNF was rarely detected in neurons after ICV delivery. Delivery of BDNF directly to the striatum or the hippocampus labeled numerous neurons in nuclei afferent to these structures. In situ hybridization studies confirmed that the high-affinity BDNF receptor (TrkB) was much more widely expressed in neurons than was the high-affinity NGF receptor (TrkA). Moreover, mRNA for truncated forms of TrkB was expressed at high levels in the ependyma, the choroid epithelium, and the gray matter. It is likely that binding of BDNF to TrkB, which appears to be more abundant and ubiquitous than TrkA, restricts the diffusion of BDNF relative to that of NGF.

We examined the effect of intraseptal or intracerebroventricular (i.c. v.) infusions of NT-4/5 or intraseptal infusions of NGF on the level of immunohistochemical staining of choline acetyltransferase (ChAT)and the low-affinity nerve growth factor receptor (LNGFR)in the rat medial septum following unilateral transection of the fimbria. The extent of cell loss in the septum ipsilateral to the lesion, determined by cell counts of ChAT-immunopositive neurons and expressed as a ratio comparing the lesioned to the intact sides, was 0.28 in animals that received an infusion of phosphate-buffered saline (PBS). The ratios were 0.97 and 1.07 in animals that received an infusion of NT-4/5 into the ipsilateral ventricle and septum respectively. Septal infusions of NGF produced a ratio of ChAT-immunopositive cells of 1.03. The ratios of LNGFR-immunopositive neurons increased from 0.50 in PBS-infused animals to 0.79 and 0.83 in animals infused with NT-4/5 via the i.c. v. infusion of NT-4/5 or septal infusion of NT-4/5 or NGF. As determined by immunohistochemical staining, NT-4/5 infused into the lateral ventricle was detected in the periventricular portions of the forebrain ipsilateral to the infusion, while NT-4/5 or NGF infused intraseptally was detected in much of the septum, bilaterally. Furthermore, exogenous NT-4/5 or NGF was detected in numerous neuronal perikarya in the medial septal and diagonal band nuclei. These data demonstrate that, as with NGF, i.c.v. as well as septal infusions of NT-4/5 can maintain the phenotype of basal forebrain cholinergic neurons following axotomy.

Electron microscopic immunohistochemical double-label studies were carried out in pigeons to characterize the ultrastructural organization and postsynaptic targets of enkephalinergic (ENK+) striatonigral projection. ENK+ terminals in the substantia nigra were labeled with antileucine-enkephalin antiserum by using peroxidase-antiperoxidase methods, and dopaminergic neurons were labeled with anti-tyrosine hydroxylase antiserum by using silver-intensified immunogold methods. ENK+ terminals on dopaminergic neurons were equal in abundance to ENK+ terminals on nondopaminergic neurons, although the former were typically somewhat smaller than the latter (mean size: 0.50 vs. 0.75 micron, respectively). ENK+ terminals were evenly distributed on the cell bodies and dendrites of dopaminergic neurons, and they were evenly distributed on dendrites but rare on perikarya of nondopaminergic neurons. Transection of the basal telencephalic output revealed that 75% of the nigral ENK+ terminals were of basal telencephalic origin. These telencephalic ENK+ terminals included over 80% of those smaller than 0.80 micron on dopaminergic neurons and smaller than 1.0 micron on nondopaminergic neurons, and none greater than this in size. Both telencephalic and the nontelencephalic ENK+ nigral terminals made predominantly symmetric synapses on nigral neurons. Although the basal telencephalic ENK+ terminals uniformly targeted dendrites and perikarya, nontelencephalic ENK+ terminals seemed to avoid perikarya. The results indicate that ENK+ striatonigral neurons in birds may directly influence both dopaminergic and nondopaminergic neurons of the substantia nigra. Based on similar data for substance P-containing striatonigral terminals, the roles of enkephalin and substance P in influencing nigral dopaminergic neurons may differ slightly, as they appear to target preferentially different portions of dopaminergic neurons. The overall results in pigeons are similar to those for ENK+ terminals in the ventral tegmental area in rats, suggesting that the synaptic organization of the ENK+ input to the tegmental dopaminergic cell fields is similar in mammals and birds.