Dynamic studies of ototoxicity in mature avian auditory epithelium

Hearing loss induced by ototoxicity is a worldwide problem despite the development of newer antibiotics and chemotherapy agents. The cellular mechanisms responsible for aminoglycoside-induced hearing loss are still poorly understood. We have developed two different methods of studying the dynamic cellular and subcellular changes in the chick auditory sensory epithelium that occur during hair cell death. The first study was performed in mature chicks after a single, high dose injection of gentamicin, which results in the rapid loss of all hair cells in the basal third of the cochlea. Chicks were sacrificed at discrete time points after drug treatment, and transmission electron microscopy was performed to study the ultrastructural changes in basal hair cells during the course of cell death. We noted various changes in the cell morphology including accumulation of cytoplasmic inclusion bodies, dispersion of the cytoplasmic polyribosomes, mitochondrial swelling, and cellular extrusion by 24 h after injection. The next two studies were performed using tissue cultures from mature avian auditory sensory epithelium. Cultured cells were labeled using vital fluorescent markers, and levels of intracellular calcium and reactive oxygen species within hair cells were studied following aminoglycoside exposure. We identified a dose-dependent increase in the levels of intracellular calcium, which was blocked by an inhibitor of voltage-gated calcium channels. We also found that levels of reactive oxygen species in hair cells greatly increased after exposure to gentamicin, and this response was blocked by two different antioxidants. These studies serve to identify key cellular and molecular changes in hair cells in response to ototoxic drugs. Further study of these processes may lead to a better understanding of how ototoxicity is induced and to potential preventative interventions.

Electron microscopic analysis of gamma-aminobutyric acid and glycine colocalization in rat trigeminal subnucleus caudalis

Postembedding immunogold methods were used to examine the distribution of gamma-aminobutyric acid (GABA) and glycine and especially their colocalization in glomerular neuronal profiles adjacent to trigeminal primary afferent profiles in lamina II of rat subnucleus caudalis. We found that 60% of the profiles adjacent to the trigeminal primary afferent terminals exhibited colocalization of GABA and glycine. GABA alone was found to localize in 17% of the adjacent profiles. Glycine alone was found to localize in 18% of the adjacent profiles. Of interest, 10% of the trigeminal primary afferent fibers showed glycine localization. All the profiles with colocalization of GABA and glycine were identified as presynaptic axonal terminals, suggesting a possible cumulative effect by these two inhibitory neurotransmitters in presynaptic inhibition. These findings show that GABA and glycine colocalize in a subpopulation of presynaptic axonal terminals within lamina II of the subnucleus caudalis. The possible origins of these axons are discussed, as well as their potential involvement in presynaptic inhibition of orofacial nociception.

Morphological and functional characterization of an in vitro blood-brain barrier model

Cell culture models have been extensively used for studies of blood-brain barrier (BBB) function. However, several in vitro models fail to reproduce some, if not most, of the physiological and morphological properties of in situ brain microvascular endothelial cells. We have recently developed a dynamic, tridimensional BBB model where endothelial cells exposed to intraluminal flow form a barrier to ions and proteins following prolonged co-culturing with glia. We have further characterized this cell culture model to determine whether these barrier properties were due to expression of a BBB phenotype. Endothelial cells of human, bovine or rodent origin were used. When co-cultured with glia, intraluminally grown endothelial cells developed features similar to in vivo endothelial cells, including tight junctional contacts at interdigitating processes and a high transendothelial resistance. This in vitro BBB was characterized by the expression of an abluminal, ouabain-sensitive Na/K pump, and thus favored passage of potassium ions towards the lumen while preventing K+ extravasation. Similarly, the in vitro BBB prevented the passage of blood-brain barrier-impermeant drugs (such as morphine, sucrose and mannitol) while allowing extraluminal accumulation of lipophylic substances such as theophylline. Finally, expression of stereo-selective transporters for Aspartate was revealed by tracer studies. We conclude that the in vitro dynamic BBB model may become an useful tool for the studies of BBB-function and for the testing of drug passage across the brain endothelial monolayer.

Fluid percussion injury causes disruption of the septohippocampal pathway in the rat

Fluid percussion injury (FPI) causes memory deficits, loss of hippocampal neurons, and basal forebrain cholinergic immunoreactivity in rats. Basal forebrain septohippocampal projections terminate in specific hippocampal regions. The purpose of this study was to examine the effects of FPI on the septohippocampal pathway (SHP). Halothane-anesthetized rats received either a sham injury or a parasagittal FPI. To characterize the anatomical effects of FPI on the SHP, silver stains were performed on brains of animals at 1, 5, and 10 days following FPI and were compared to sham-injured preparations. To characterize the effects of FPI on retrograde transport in the SHP, a separate group of FPI and sham-injured animals with survival times of 2, 5, and 10 days received bilateral WGA-HRP injections into the hippocampal formation 24 h prior to sacrifice. Argyrophilic CA3 neurons were present 1 day following FPI. Five days following FPI, terminal degeneration was present in the inner third of the molecular layer of the dentate gyrus bilaterally that was not present 1 day after injury. Fiber and terminal degeneration was not observed in the basal forebrain until 10 days after FPI. WGA-HRP-labeled septal neurons decreased significantly (P < 0.05) ipsilateral to injury in animals sacrificed 5 and 10 days following FPI but not 2 days after injury. This investigation demonstrated that FPI produces focal injury in the hippocampal formation. In addition, the appearance of terminal degeneration in the dentate molecular layer correlated with the significant reduction in axonal transport 5 days following injury. This correlation illustrates the secondary processes that structurally damage the SHP up to 10 days after injury.

Electron microscopy of colocalization of GABA and tyrosine hydroxylase expression in rat olfactory bulb transplants

Juxtaglomerular (JG) neurons of rat olfactory bulb (OB) are a subset of inhibitory interneurons within the OB, acting via lateral inhibition to modulate the afferent input of the primary olfactory nerve. The JG neurons, composed of periglomerular, external tufted, and short axon cells, have been found to express various neurotransmitters, including gamma-amino butyric acid (GABA) and dopamine. A specific set of neurons within the periglomerular population have also been shown to coexpress these neurotransmitters. Deafferentation or functional odor deprivation of the normal OB causes a loss of tyrosine hydroxylase (TH) (the rate limiting enzyme in the dopamine synthesis pathway) expression within the JG cell population, but appears to have no effect on GABA levels. Our laboratory has developed a transplantation model to further study the effects of deafferentation and subsequent reinnervation within this system. Sections from transplant (TX) OBs were reacted for GABA and TH using immunocytochemical localization protocols and studied by electron microscopy. Numerous neuronal populations were found to be either TH or GABA positive in this study, with a specific subpopulation showing colocalization of both. Although the architecture of the TX OB is somewhat disrupted and the TH- and GABA-positive cells were not as uniform in their arrangement as they are in the normal OB, we found that these cells in the TX OB were morphologically similar to the JG cells of normal OB. Positively labeled profiles were also found to receive and form numerous synaptic contacts with both host olfactory nerve axons as well as with the processes of donor neurons. These synaptic contacts were within areas that resemble the glomeruli of normal OB, suggesting that lateral inhibition may occur within the TX OB as it does in the normal. The coexpression of GABA and TH within specific neurons also indicates that a unique population of JG neurons that occur in normal OB are also found within this transplanted system as well.

Developmental localization of GAP-43 and olfactory marker protein in rat olfactory bulb transplants

In an effort to identify and understand the laminar disorganization that occurs in the transplanted (TX) rat olfactory bulb (OB), we examined the development of fiber systems within these TX OBs. One antibody for olfactory marker protein (OMP) was used to identify axons of mature olfactory receptor neurons (ONs) and a second antibody, for a growth-associated protein (GAP-43), provided a marker for all extending or immature fibers. Donor OBs were taken from fetuses on embryonic days 14 or 15 (sperm-positive day is zero) and TX directly into the cavity produced by removal of an OB in 1-day-old hosts of the same strain. After survival times of 1 and 2 weeks and at maturity, adjacent 8 microns paraffin sections from the TX material were examined for OMP and GAP-43 reactivity. Fiber bundles, reactive for OMP, were found within the TX by 1 week post-TX, indicating rapid re-innervation of the donor OB by ONs. The appearance of OMP reactivity gradually shifted from tightly packed, well-defined fiber bundles at 1 week post-TX to a diffuse reticulated pattern of individual fibers emerging from bundles at maturity. The OMP-reactive fiber bundles of the TX OB also contained GAP-43-reactive fibers, but GAP-43 reactivity also extended to other (OMP-negative) bundles and fields. Reactivity for GAP-43 in the TX OB was nearly ubiquitous at 2 weeks post-TX but, as development progressed (in both the TX and normal OB), such reactivity gradually decreased. Thus, while maturation in sensory afferent fiber systems in the TX OB may be delayed, it eventually follows a pattern similar to that in the normal OB, suggesting that factors other than the timing of fiber extension may be responsible for the laminar disorganization of the TX OB.

Electron microscopic analysis of lesion-induced changes in synaptic structure and immunogold labeling of neurotransmitters within the feline trigeminal nucleus

This report uses lesion and postembedding immunogold protocols to examine the ultrastructural details of lesion-induced synaptic and neurotransmitter changes in the feline trigeminal nucleus. Electron microscopic (EM) analysis concentrated on lamina II (substantia gelatinosa) of the subnucleus pars caudalis (PC) which is one relay site or trigeminal fibers involved in nociception. Special attention was directed to analysis of reoccupation of synaptic sites vacated by primary afferent degeneration. Primary afferents are caused to degenerate by performing unilateral retrogasserian rhizotomy. After survival times of 1, 2, 6, and 7 days, sections of PC were processed for postembedding immunogold labeling with antibodies to the neurotransmitters gamma aminobutyric acid (GABA) and glutamate (Glu). The results show: (1) degenerating primary afferent terminals were easily identified in various stages of degeneration; (2) Glu immunoreactivity was observed in early forms of degenerated primary afferent terminals with clumped vesicles as well as in the highly distorted, electron dense terminals of later degeneration; and (3) some GABA immunoreactive terminals formed atypical synapses which exhibited both asymmetric (excitatory) and symmetric (inhibitory) synaptic densities. A possible model is presented of the progression of events following trigeminal nerve lesion which results in atypical synapse formation. Such altered synaptic relationships seen in PC following trigeminal rhizotomy may be related to hyperactivity that is seen in animals and to the atypical facial pain following nerve lesions in humans.

Post-embedding immunogold labeling of gamma-aminobutyric acid in lamina II of the spinal trigeminal subnucleus pars caudalis: I. A qualitative study

This study examines the normal synaptic organization of the feline spinal trigeminal nucleus pars caudalis (PC). A primary goal of this study is to identify and characterize the synaptic complexes within PC based on their specific neurotransmitter content. Post-embedding immunogold techniques are utilized with electron microscopy to determine the ultrastructural localization of gamma-aminobutyric acid (GABA) immunoreactivity within lamina II of PC. The colloidal gold particles (10 nm) are randomly distributed over immunoreactive (IR) profiles without preference toward membranous or cytoplasmic regions. GABA immunoreactivity occurs on small unmyelinated axons, on terminals which form synaptic contacts, and on some vesicle-containing dendrites. The GABA-IR terminals form symmetric (type II) contracts onto unlabeled somata and dendrites of various sizes, and onto other unlabeled axon terminals. The GABA-IR terminal in axo-axonic complexes is presynaptic to a round vesicle-containing terminal, which itself may form a type I asymmetric contact onto an unlabeled dendrite or soma. A proportion of vesicle-containing dendrites show GABA-immunoreactivity and are post-synaptic to unlabeled terminals with round vesicles. Other, but far fewer, vesicle-containing dendrites are GABA negative and postsynaptic to GABA-IR terminals. In summary, the findings are consistent with the localization of GABA in intrinsic neurons, and may be associated with presynaptic and postsynaptic inhibition within nociceptive related pathways.

Tyrosine hydroxylase expression in rat olfactory bulb transplants: an electron microscope study

Juxtaglomerular (JG) neurons of rat olfactory bulb (OB) have been shown to express tyrosine hydroxylase (TH), the rate-limiting enzyme in the catecholamine synthesis pathway. These JG neurons act as inhibitory dopaminergic interneurons, modulating the incoming signal from the primary olfactory afferents. The JG neurons, comprised of periglomerular, external tufted, and short axon cells, stop expressing TH after lesions of the olfactory nerve or closure of the nares, both of which cause a loss of functional input. Upon reinnervation by a continuously regenerating olfactory nerve, these cells resume their expression of TH. In order to study deafferentation and subsequent reinnervation within this system, our laboratory utilizes a transplantation model. Sections from transplant (TX) OBs are reacted for TH using immunocytochemical localization protocols and studied by light- and especially electron microscopy (EM). Autoradiography of tritiated thymidine-labeled tissue was performed to confirm donor origin of the TX OBs. Although the architecture of the TX OB is somewhat disrupted and the TH-positive cells were not as uniform in their arrangement as they are in the normal OB, we found that the TH cells in the TH OB had a morphology similar to the JG cells observed in normal OB. These TH cells were also found to receive synaptic contacts with host olfactory nerve axons as well as make and receive contacts with the processes of donor neurons. These, synaptic contacts were formed within areas that resemble the glomeruli of normal olfactory bulb, suggesting that the inhibitory synaptic pathway is reestablished within the TX OB. These findings also suggested that host olfactory axons formed a functional contact with the TH cells, possibly inducing them to express this enzyme. This study implies that the TX OB retains a level of plasticity that enables it to recapitulate part of the interneuronal arrangement observed in the normal system.

Recovery of olfactory function in thirteen-day-old rats after olfactory bulb transplantation but not after olfactory bulb ablation

Previous studies have shown recovery of olfactory ability along with reconnectivity of olfactory nerve (ON) following both olfactory bulb (OB) lesions and OB transplants (TX) when performed in newborn rats. The purpose of the present study is to correlate functional recovery with patterns of anatomical reconnectivity in older, postnatal (PN) 13-day-old rats (a possible critical period for plasticity in the system). Reinnervation of olfactory areas was seen in all OB TX animals regardless of the extent of functional recovery. Eight of nineteen animals with OB TXs demonstrated some degree of behavioral recovery. No reinnervation or behavioral recovery of OB lesion animals was observed. At this age, behavioral recovery is dependent upon reconnectivity within the system and transplantation may be required to facilitate this process.

Electron microscopy of immunoreactivity patterns for glutamate and gamma-aminobutyric acid in synaptic glomeruli of the feline spinal trigeminal nucleus (Subnucleus Caudalis)

We studied the ultrastructure of the synaptic organization in the feline spinal trigeminal nucleus, emphasizing specific neurotransmitter patterns within lamina II of the pars caudalis/medullary dorsal horn. Normal adults were perfused, and Vibratome sections from pars caudalis were processed for electron microscopy. Ultrathin sections were reacted with antibodies for the excitatory neurotransmitter glutamate (Glu) and for the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) by using postembedding immunogold techniques. Both single- and double-labeled preparations were examined. Results with single labeling show that Glu-immunoreactive terminals have round synaptic vesicles and form asymmetric synaptic contacts onto dendrites. GABA-immunoreactive axon terminals and vesicle-containing dendrites have pleomorphic vesicles, and the axon terminals form symmetric contacts onto dendrites and other axons. Double labeling on a single section shows glomeruli with central Glu-immunoreactive terminals that are presynaptic to dendrites, including GABA+ vesicle-containing dendrites. These Glu+ terminals are also postsynaptic to GABA+ axon terminals, and these GABA-immunoreactive terminals may also be presynaptic to the GABA+ vesicle-containing dendrites. Quantitative analyses confirm the specificity of the Glu and GABA immunoreactivities seen in the various glomerular profiles. The results suggest that a subpopulation of Glu-immunoreactive primary afferents (excitatory) may be under the direct synaptic influence of a GABA-immunoreactive intrinsic pathway (inhibitory) by both presynaptic and postsynaptic mechanisms.

Light microscopic localization of calcitonin gene-related peptide in the normal feline trigeminal system and following retrogasserian rhizotomy

Calcitonin gene-related peptide (CGRP) is a neuropeptide that has been implicated in the transmission and modulation of primary afferent nociceptive stimuli. In this study, we describe the light microscopic distribution of CGRP immunoreactivity (IR) within the feline trigeminal ganglion and trigeminal nucleus of normal adult subjects and in subjects 10 and 30 days following complete retrogasserian rhizotomy. Within the trigeminal ganglion of normal subjects, cell bodies and fibers showed CGRP-IR, whereas immunoreactive fibers were rare in the central root region. Within the normal spinal trigeminal and main sensory nuclei, CGRP-IR was seen to form a reproducible pattern that varied between the different nuclei. Following rhizotomy, most, but not all, of the CGRP-IR was lost from the spinal trigeminal and main sensory nuclei, except in regions where the upper cervical roots and cranial nerves VII, IX and X project into the trigeminal nucleus. The pattern seen at 10 days contained more CGRP-IR than that seen at 30 days and suggests that degenerating fibers still show CGRP-IR. In contrast to the decrease seen in the nuclei after rhizotomy, examination of the central root that was still attached to the trigeminal ganglion showed an increase in CGRP-IR within fibers, some of which ended in growth conelike enlargements. Rhizotomy induced a dramatic increase in CGRP-IR within trigeminal motoneurons and their fibers, which was strongest 10 days after rhizotomy and weaker at 30 days, which was still stronger than normal. These results indicate that the majority of CGRP-IR found in the trigeminal nucleus originates from trigeminal primary afferents and that an upregulation of CGRP-IR occurs in trigeminal motoneurons and in regenerating fibers in the part of the central root that was still attached to the ganglion. In addition, the persistence of CGRP-IR fibers in the trigeminal nucleus provides one possible explanation for the preservation of pain in humans following trigeminal rhizotomy.

Ligands for EPH-related tyrosine kinase receptors are developmentally regulated in the CNS

Elk is a member of the eph family of receptor-like tyrosine kinases. Although its function is unknown, elk is postulated to play a role in nervous system development. Using Northern analysis, we examined the developmental regulation of RNAs encoding elk, and several ligands for the eph family of RTKs, the LERKs. Expression of elk, LERK-1, and LERK-2 RNAs is high in all regions examined in the embryonic and postnatal rat brain and decreases to low levels with age. One exception is the adult olfactory bulb which continues to express a moderate level of LERK-2. In contrast, moderate LERK-4 expression was limited to the developing hippocampus and cerebral cortex. These data indicate that elk and some of the LERKs may play a role in nervous system development, maintenance, and/or regeneration.

Olfactory bulb transplants establish afferent and efferent connections with host forebrain in rat

We are using wheat germ agglutinin-conjugated horseradish peroxidase (WHRP) to study reconnectivity in the transplanted (TX) olfactory bulb (OB) in Sprague-Dawley rats. Tritium-labeled OBs from fetal rat donors of Embryonic Days 14-15 were immediately grafted into neonatal rats in the site from which the host OB had been removed. Following survival times of 7 weeks and longer, WHRP solution was injected into the TX OB, and subjects were perfused after 24 h. The WHRP transport is seen in fibers from the TX OB into layer I of the host olfactory peduncle (OP) and olfactory cortex (OC) and in cell bodies in layers II and III of the OP and OC, the lateral hypothalamus, and the contralateral anterior olfactory nucleus (AON). These findings reaffirm that the axons from a TX OB make connections with some appropriate areas of the host brain and also indicate that axons from cells in the target areas of the host brain, including contralateral AON, reinnervate the TX OB.

Host primary olfactory axons make synaptic contacts in a transplanted olfactory bulb

Previous light microscopic studies have shown that host olfactory neurons are able to grow into a transplanted fetal olfactory bulb, and behavioral studies have shown that animals with transplanted olfactory bulbs recover functional olfactory abilities. We examined the olfactory bulb transplant at the ultrastructural level to determine whether synaptic contacts are reestablished between host olfactory neurons and donor olfactory bulb. Mature rats that, as neonates, had received embryonic olfactory bulb transplants following olfactory bulb removal were studied. An antibody specific for olfactory marker protein was used to identify the primary olfactory neurons; it was bound by a gold-conjugated secondary antibody for visualization. To preserve the antigenicity of the olfactory marker protein for immunolabeling, Lowicryl K4M hydrophilic resin was used. Synaptic contacts were unmistakable between labeled axons of host olfactory neurons and unlabeled processes within glomerulus-like areas of the transplanted olfactory bulb. The surrounding neuropil contained other elements similar to those found in normal tissue, including synaptic contacts between unlabeled profiles. We clearly show that the transplanted olfactory bulb exhibits sufficient plasticity to form an array of normal synaptic contacts, including the contacts from host primary olfactory neurons.

Recovery of olfactory behavior. I. Recovery after a complete olfactory bulb lesion correlates with patterns of olfactory nerve penetration

The olfactory system is an excellent system in which to study issues related to potential functional recovery after a debilitating brain injury. The olfactory system is well-characterized, easily accessible and there are a vast number of studies available from a variety of perspectives. The experimental aim of this research is to examine the anatomical correlates associated with potential behavioral recovery in rats that receive complete olfactory bulb lesions as neonates or as adults. The results show that behavioral recovery occurs only when olfactory nerve penetration of the central nervous system is observed. Further, both olfactory nerve penetration and behavioral recovery are age-dependent phenomena. The olfactory nerve penetration only occurs when the olfactory bulb lesion is performed in neonates. Behavioral recovery of olfactory ability follows a linear trend and reaches near normal levels during the six weeks behavioral testing period. Histological analysis using an antibody for olfactory marker protein (an olfactory nerve-specific marker) reveals two potential candidates for the anatomical pathway responsible for behavioral recovery: olfactory nerve to orbital frontal cortex and olfactory nerve to olfactory peduncle. This report presents evidence that recovery of olfactory ability can occur in the absence of the olfactory bulb if the lesion is performed when the rat is still a neonate.

Recovery of olfactory behavior. II. Neonatal olfactory bulb transplants enhance the rate of behavioral recovery

Previous experiments in this laboratory have shown that transplants of a fetal olfactory bulb into a neonatal rat are viable and that they establish connections with the olfactory peduncle and olfactory cortex. The focus of this experiment was to investigate the anatomical correlates of any behavioral recovery seen in rats that had one olfactory bulb removed along with an immediate transplant of a fetal olfactory bulb. Anatomical details, such as transplant organization and olfactory nerve repenetration patterns were analyzed using a variety of histological and immunohistochemical techniques. The rats in this experiment showed behavioral recovery of olfactory ability. The recovery rates observed in these animals were compared to two other groups of rats that this laboratory has shown to be behaviorally competent: normal rats and rats with neonatal ablations of the olfactory bulb but no transplant. Although the animals with transplants did not recover to completely normal levels of olfactory ability, they did start behavioral testing in a more behaviorally competent condition than rats with simple neonatal lesions. Anatomical analysis revealed that the transplanted olfactory bulb was heavily penetrated by incoming olfactory nerve fibers but olfactory nerve penetration was not limited to the transplanted olfactory bulb. The extra-bulbar host regions that were penetrated included the orbital frontal cortex and three olfaction-related areas; olfactory cortex, olfactory peduncle and the subependymal cell layer. The olfactory nerve penetration patterns observed beyond the transplant were essentially the same as those observed in rats with only neonatal lesions of the olfactory bulb. Thus, multiple pathways may have contributed to the recovery observed in the rats with olfactory bulb transplants.

Olfactory ensheathing glia and platelet-derived growth factor B-chain reactivity in the transplanted rat olfactory bulb

Using a monoclonal antibody against the B-chain of platelet-derived growth factor as a marker, we have examined the behavior of olfactory ensheathing glia in the normal and transplanted rat olfactory bulb. In the normal postnatal olfactory bulb, these glia are found to ensheath the bundles of incoming primary olfactory nerve fibers as well as those in the olfactory nerve layer. Olfactory marker protein antibody was used to identify the olfactory nerve proper. Within the transplant, the same glia: (1) ensheath bundles of both primary olfactory and non-primary olfactory axons, (2) ensheath axonal bundles deep within the donor tissue, and (3) eventually permit radiation of individual axons from bundles to surrounding neuropil. We believe that ensheathing glia (being rich in growth-related factors and extracellular matrix molecules) may be useful in providing trophic support and guidance for the reconstruction of developmentally or traumatically damaged neuronal pathways not directly related to the olfactory system. The evidence presented here indicates that ensheathing glia are capable of existing in deep brain areas and ensheathing other than primary olfactory axons. The special molecular characteristics of these glia along with the morphological findings presented here provide a foundation for further studies of these unique glia and their potential utility in the restoration of damaged neural pathways.

Electron microscopic localization of nerve growth factor receptor (p75)-immunoreactivity in pars caudalis/medullary dorsal horn of the cat

Previous studies have demonstrated the presence of nerve growth factor receptor [NGFr(p75)]-immunoreactivity (IR) in the spinal trigeminal nucleus of both 8-10 week-old kittens and mature cats. Most of the NGFr(p75)-IR is lost following retrogasserian rhizotomy, indicating that the majority of the NGFr(p75)-IR within the spinal trigeminal nucleus is of trigeminal primary afferent origin. Here, we examined the ultrastructural localization of NGFr(p75)-IR within lamina II outer of pars caudalis/medullary dorsal horn in the mature cat. Lamina II outer represents a location where dense NGFr(p75)-IR is seen with the light microscope. The NGFr(p75)-IR identified with the electron microscope was located within small thinly myelinated and unmyelinated axons and within axon terminals. The terminals with NGFr(p75)-IR typically formed asymmetric synaptic specializations onto dendritic profiles and at times were postsynaptic to other axon terminals at symmetric synaptic specializations. The terminals with NGFr(p75)-IR were either simple (associated with a single profile) or more complex, such as those that typically formed the central element in synaptic glomeruli. The NGFr(p75)-IR in terminals was especially prominent on microtubules and the plasmalemma and these findings are consistent with proposed roles for NGFr(p75) in axoplasmic/neuronal transport and as a membrane protein, respectively. The profiles with NGFr(p75)-IR seen with the electron microscope indicate a primary afferent origin and show some similarities when compared to other markers of primary afferent fibers such as calcitonin gene-related peptide. In addition, a possible role for NGFr(p75) in the transmission of nociceptive stimuli is also discussed.

Nerve growth factor receptor (p75)-immunoreactivity in the normal adult feline trigeminal system and following retrogasserian rhizotomy

The 75 kDa protein nerve growth factor receptor [NGFr(p75)] is a neurotrophin receptor that is able to bind different members of the neurotrophin family of molecules implicated in affecting neuronal survival. Here we describe the light microscopic distribution of NGFr(p75)-immunoreactivity (IR) within the feline trigeminal brainstem sensory nuclear complex and trigeminal ganglion of normal adult subjects and in subjects 10 and 30 days following retrogasserian rhizotomy. Within the trigeminal ganglion of normal subjects, numerous fibers and most of the neuronal cell bodies showed NGFr(p75)-IR that varied in intensity, while cells and fibers with NGFr(p75)-IR were less numerous within the mesencephalic trigeminal nucleus. Within the main sensory and spinal trigeminal nuclei, NGFr(p75)-IR formed a reproducible pattern that varied between the different subnuclei. The NGFr(p75)-IR consisted both of dense pockets and a low level NGFr(p75)-IR that was selective to the trigeminal neuropil. Following rhizotomy, most of the NGFr(p75)-IR was lost from the main sensory and spinal trigeminal nuclei, except in regions where the upper cervical roots and cranial nerves VII, IX, and X project. In contrast, examination of the central root that was still attached to the trigeminal ganglion showed increased NGFr(p75)-IR in fibers and supporting cells, as did the motor root within the peripheral mandibular division. These results indicate that the majority of the NGFr(p75)-IR within the main sensory and spinal trigeminal nuclei originates from primary trigeminal afferents and that retrogasserian rhizotomy leads to an up-regulation of NGFr(p75)-IR in the part of the central root that is contiguous with the ganglion.

Axon hillocks and initial segments in spinal trigeminal nucleus with emphasis on synapses including axo-axo-axonic contacts

As a part of a continuing study of the feline spinal trigeminal nucleus, the fine structure and synaptic arrangements on the axon hillock and axon initial segment of neurons in this region are described here. Transmission electron microscopy has been used to characterize qualitatively the axon hillock and initial segment and associated synapses in pars interpolaris. Axon hillocks and initial segments are easily identified in continuity with somata or as isolated profiles in the neuropil, and they receive synaptic contacts: these we regard as axo-axonic. The presynaptic terminals contain either mainly round or mainly flattened synaptic vesicles and have Type I (asymmetric) or Type II (symmetric) thickenings respectively at their contacts with the axon hillock or initial segment. I report here also the unusual arrangement of three separate axons in a serial synaptic complex. Some of the round vesicle Type I contacts onto the axon hillock-initial segment region also receive Type II contacts from one or more flattened vesicle terminals, thus forming an axo-axo-axonic complex. These flattened vesicle terminals lack the usual features of a presynaptic dendrite. It has been shown that in this nucleus some round vesicle terminals, especially those postsynaptic to flattened vesicle terminals, are primary afferents from the periphery. Therefore the round vesicle terminal presynaptic to the axon hillock-initial segment region, some of which are included in the axo-axo-axonic complex may also be a primary afferent directly contacting the spike generator area of the relay neuron and under presynaptic control of a flattened vesicle synapse. The latter may possibly be an intrinsic contact. This strategic situation of round vesicle terminals and the axo-axo-axonic complex at the axon hillock or initial segment has major implications relevant to the overall output of these neurons.

Light and electron microscopic localization of calcitonin gene-related peptide immunoreactivity in lamina II of the feline trigeminal pars caudalis/medullary dorsal horn: a qualitative study

Calcitonin gene-related peptide (CGRP) is a neuropeptide that is associated with a subset of primary afferent fibers and appears to have a role in nociception. The purpose of the present study was to perform a qualitative light, and especially electron microscopic (LM and EM), examination of CGRP-immunoreactivity (IR) within lamina II (substantia gelatinosa) of the feline pars caudalis/medullary dorsal horn (PC/MDH) of the spinal trigeminal nucleus. The LM investigation revealed massive CGRP-IR within lamina II outer, with fewer fibers that traversed lamina II inner. The EM preparations showed CGRP-IR in small, thinly myelinated and unmyelinated axons, preterminal axons, and in axon terminals that formed asymmetric synaptic contacts onto small dendritic profiles. The terminals with CGRP-IR were often the central element within glomeruli, where the terminal usually formed 2 or more asymmetric synaptic specializations onto 1 or more dendrites. Many of these postsynaptic dendrites contained synaptic vesicles. Other profiles were seen forming presynaptic contacts onto the terminal with CGRP-IR, and these profiles most likely represent presynaptic dendrites and/or other axon terminals of intrinsic origin. The synaptic association of terminals showing CGRP-IR with vesicle-containing dendrites, presynaptic dendrites, and/or other axon terminals suggests that terminals with CGRP-IR are especially susceptible to modulation.

Unilateral retrogasserian rhizotomy causes contralateral degeneration in spinal trigeminal nuclei of cats: an ultrastructural study

We are studying the response to injury within the brainstem trigeminal nucleus following trigeminal nerve lesions. We have previously shown with light microscopy and reduced silver stains that unilateral retrogasserian rhizotomy results not only in massive degeneration throughout the ipsilateral spinal trigeminal nucleus; in addition, degeneration is seen in a ventral position at the periobex region (involving caudal pars interpolaris and rostral pars caudalis) in the contralateral spinal trigeminal nucleus. In the present study, we have used electron microscopy to identify the source of the degenerating elements seen bilaterally after unilateral retrogasserian rhizotomy in eight adult felines with survival times ranging from 3 to 20 days. At short survival times (3-7 days) degenerating terminals with round synaptic vesicles (R terminals) and type 1, asymmetric contacts predominate bilaterally, while fewer degenerating terminals with flattened synaptic vesicles (F terminals) and type 2, symmetric contacts are seen. At longer survival times more F terminal degeneration is seen, especially on the contralateral side. Postsynaptic sites and dendrites show minimal alterations. These findings suggest that the degenerating R terminals seen on the contralateral side originate from primary afferents while the degenerating F terminals seen on the contralateral side originate from intrinsic sources involving a crossed internuclear pathway. In addition, the finding of degenerating F terminals may represent a novel form of selective transynaptic change of intrinsic neurons, associated with minimal dendritic or somatic alterations.

Platelet-derived growth factor B-chain-like immunoreactivity in the developing and adult rat brain

Platelet-derived growth factors (PDGFs) are growth-regulatory molecules that stimulate chemotaxis, proliferation and increased metabolism, primarily of connective tissue cells. In our previous paper, we have demonstrated the ubiquitous localization of PDGF B-chain-containing proteins in neurons and expression of transcripts for PDGF A-chain, B-chain and the two forms of the PDGF receptor in the brains of non-human primates. In the present study, the cellular localization of PDGF B-chain in developing and adult rat brains was analyzed using immunocytochemistry with a PDGF B-chain-specific monoclonal antibody. Intense PDGF B-chain immunoreactivity (PDGFB-I) was distributed around the continuously regenerating primary olfactory neurons at all stages of development from embryo to adult. The major part of PDGFB-I associated with neurons appeared some time after birth and increased with age. PDGFB-I appeared in several nerve fiber systems during earlier stages of development and gradually decreased with age. In conjunction with other data showing the existence of functional PDGF receptor beta-subunits in the neurons, these data suggest a possible role for PDGF B-chain as a neurotrophic or neuroregulatory factor in both developing and mature brains.

Nerve growth factor receptor and the transplanted rat olfactory bulb

Nerve growth factor receptor (NGFR) in the rat olfactory system is developmentally regulated, localizing to the olfactory nerve (ON) in the young rat, and to the olfactory bulb (OB) glomeruli in the adult (Vickland et al. 1991. Brain Res., in press). This pattern of immunoreactivity (IR) may indicate the state of axon growth in the young ON and synaptogenesis in the adult glomeruli. Additional experiments in our laboratory involving lesions to the ON in adult rats have shown a recapitulation of the developmental pattern of expression: NGFR-IR is again found along the ON. Longer survival times after lesioning show the reexpression of the adult pattern of NGFR-IR. This phenomenon was further explored in a transplant (TX) model to determine the changes in NGFR-IR in both the host and TX tissue. A fetal OB labeled with [3H]thymidine is placed into the space created by the removal of the OB of a neonatal rat. After survival times of 1, 2, 8, and 13 weeks, the host animal is sacrificed and the OB TX is processed using monoclonal antibody 192 IgG for NGFR. The host ON shows strong NGFR-IR in TX of 1- and 2-week survival times. In TX survival times of 8 weeks or more, NGFR-IR is observed in glomerulus-like structures. All of these glomerulus-like structures are near groups of neurons in the TX tissue, indicating that they may be functional, with appropriate synapses. Therefore, even though the adult pattern of NGFR-IR takes longer to become established than in normal rats, we have demonstrated that this pattern does so in the TX OB model.

Age-related development of olfactory bulb transplants in rats

We are using the rat olfactory system to study developmental aspects of neurotransplantation (TX). Age-related TX maturation and subsequent establishment of connections are of special concern. Previous studies of deafferentation by olfactory bulb (OB) removal suggested "critical" periods of plasticity in the system. We present here preliminary attempts at relating age of host receiving TX to maturation of the TX and its connections. This investigation used hosts of postnatal age (PN) 13-14 days with fetal donors at Embryonic Day 15; the former having one OB ablated and receiving a fetal donor OB TX immediately placed in the vacated space. The fetal tissue was labeled previously in utero with tritiated thymidine. After 2 months a small coagulation lesion was placed in the OB TX and 2 days later the tissue was taken, serially sectioned, and processed for [3H] autoradiography, degeneration, and olfactory marker protein (OMP). Extensively 3H-labeled OB TXs with localized small lesions were studied. The cellular architecture of the TX is less well organized than in normals but substantial OMP reactivity occurs throughout. Degeneration occurs mainly near the lesion and little if any degeneration is seen beyond the 3H-labeled TX tissue. The results show that OB TX survive and develop in the PN 13-14 age group as they do in the younger animals and that primary olfactory neurons likewise reinnervate the TX but that PN 13-14 TX efferent projections are far more limited than those of younger hosts.

Development of olfactory marker protein and tyrosine hydroxylase immunoreactivity in the transplanted rat olfactory bulb

Evidence suggests that tyrosine hydroxylase (TH) expression by juxtaglomerular (JG) neurons of the olfactory bulb (OB) is dependent upon input from primary olfactory neurons (ONs), which are identifiable using immunocytochemical localization (ICC-L) methods for olfactory marker protein (OMP). When the input from the continuously regenerating ONs is temporarily removed (either surgically or chemically), JG cells cease TH production until ON contact is reestablished. We are studying this transneuronal regulation using the rat OB in a transplantation (TX) model. Fetal OBs, labeled in utero with tritiated thymidine, were transplanted (TX) into a site vacated by removal of a neonatal host OB. Host animals were sacrificed at varying periods after TX. Alternate sets of frozen sections were then processed for autoradiography or using ICC-L for TH and OMP. As early as 1 week post-TX, OMP-positive fibers and glomerulus-like structures were seen throughout the TX OB. Despite this extensive and rapid OMP reinnervation, TH expression returned very slowly and the number of TH expressing cells never approached control levels. The reduced TH activity in TXs may be due to failure of JG cells to survive or to develop the correct phenotype under TX conditions. Alternatively, input from ON fibers may only be necessary, but not sufficient, for the expression of TH.

Nerve growth factor receptor expression in the young and adult rat olfactory system

Nerve growth factor (NGF) and its receptor (NGFR) are proteins that have a role in the normal development and survival of neurons in the peripheral and central nervous systems. During development, NGF is necessary for outgrowth of axons and establishment of synapses, and NGFR is the transmembrane protein that binds NGF and brings it into the cell. NGF and NGFR expression in the rat olfactory system have been studied previously, and age differences in NGFR are explored further in this study, using immunocytochemistry and immunoelectron microscopy to determine the changes in two different ages: postnatal day 5 and the adult. Dramatic differences were found in the distribution of NGFR immunoreactivity in the olfactory system of each of the two ages studied. Electron microscopy revealed that glial cells were responsible for this immunoreactivity.

Localization of nerve growth factor receptor immunoreactivity in the trigeminal nucleus of kittens

A monoclonal antibody raised against the human nerve growth factor receptor (NGFr) was used to map the distribution of NGFr-immunoreactivity (IR) in the trigeminal nuclear complex of 8- to 10-week-old, immature felines. Somata and fibers show NGFr-IR within the trigeminal ganglion and the mesencephalic trigeminal nucleus. NGFr-IR is also found in fibers within the trigeminal root entry zone, the spinal trigeminal tract, and in fibers and terminals within all the central trigeminal sensory nuclei. The NGFr-IR found within the trigeminal sensory nuclei typically occurs in circumscribed zones that vary in position for the different subnuclei. NGFr-IR is found in the dorsomedial and ventrolateral subdivisions of the main sensory nucleus, in the dorsomedial and occasionally in ventral positions within pars oralis, in dorsal and ventral regions within pars interpolaris, and primarily in outer lamina II with fibers that project to lamina V within pars caudalis/medullary dorsal horn. These results show some overlap with the central distribution of trigeminal primary afferent nociceptive fibers such as those found from the tooth pulp and overlap with the central distribution of such peptides as calcitonin gene-related peptide and substance P, but NGFr-IR is more restricted. Thus, it appears that NGFr-IR is associated with the endings of primary afferent fibers in the brain stem, and that these fibers may represent a certain subclass of primary afferent nociceptors. It is speculated that fibers showing NGFr-IR may have the ability to alter their response to peripheral deafferentation when compared to fibers lacking NGFr-IR.

Ultrastructural localization of gold particles within neural grafts labeled with gold-filled Sendai viral envelopes

The ability to discriminate between host and donor cells is required to interpret the organization of neural grafts at the electron microscopic (EM) level. Using light microscopy, Ardizzoni et al. (Ardizzoni, S.C., Michaels A., and Arendash, G.W. [1988] Science 239:635-637) described a method, using gold-filled Sendai viral envelopes, for labeling cell suspensions prior to grafting. As the colloidal gold used in this procedure is especially attractive for use with EM, we have examined the ultrastructural distribution and character of this label with transplanted cells. Cell suspensions taken from the nucleus basalis of fetal rats were labeled using gold-filled Sendai viral envelopes and grafted into the dorsal neocortex of adult host rats with nucleus basalis lesions. After varying survival times ranging from 1 to 14 months, grafts and surrounding host tissue were examined using standard EM techniques. Within the graft site, gold particles ranging from 10-200 nm were found associated with various membranes throughout the cytoplasm of both neurons and glia. Gold particles of similar size were also found within the nuclei of neuronal and non-neuronal cells. Host cells near the graft site contained some small gold particles (10-40 nm). Control injections of non-viable, gold-labeled cells or colloidal gold alone resulted in similar patterns of small gold particles which were readily discriminable from the larger virally inserted gold particles found in viable labeled donor cells. We conclude that this method allows discrimination between closely associated host and donor cells.

Retrogasserian rhizotomy causes expression of nerve growth factor receptor-immunoreactive protein in motoneurons within the adult feline trigeminal motor nucleus

The effect of lesions on nerve growth factor receptor (NGFr) immunoreactivity (IR) in motoneurons within the mature feline trigeminal motor nucleus was investigated. Ten days following complete unilateral retrogasserian trigeminal rhizotomy including transection of the trigeminal motor root, motoneurons within the ipsilateral trigeminal motor nucleus showed NGFr-IR. In contrast, motoneurons within the contralateral trigeminal motor nucleus and within both trigeminal motor nuclei in unoperated control subjects did not show NGFr-IR. It is suggested that the appearance of NGFr-IR in motoneurons within the ipsilateral trigeminal motor nucleus after rhizotomy represents an attempts towards recovery and may be associated with the regrowth of its cut axons.

Age-related remodeling of glutamic-acid decarboxylase-labeled elements in deafferented piriform cortex of rats

Olfactory bulb (OB) removal has been shown to result in plasticity in the piriform cortex (PC) that is age dependent. We are studying this phenomenon using immunoelectron microscopy of glutamic acid decarboxylase immunoreactivity (GAD, the enzymatic precursor for GABA) at selected postnatal ages and in adults with emphasis on short survival times of 4-7 days after OB ablation. Normally GAD-labeled synaptic terminals form type II symmetric contacts onto unlabeled dendrites and GAD-labeled dendrites receive type I, asymmetric contacts from unlabeled terminals (Westenbroek, et al., 1988a). The OB lesion results in degenerating terminals with type I contacts onto unlabeled and onto GAD-labeled dendrites. Type I postsynaptic sites may be seen partially contacted by or entirely devoid of degenerating terminals and occasionally may be apposed to variable degrees by normal unlabeled or by GAD-positive terminals. Subsequently, some GAD-labeled terminals may form asymmetric type I contacts usually with unlabeled dendrites and rarely with GAD-labeled dendrites. The findings are most common in the youngest subjects and essentially absent in the adult subjects. A sequence of reinnervation of deafferented type I sites by GAD-labeled terminals is suggested for the formation of this "atypical" synapse and the sequelae of this reorganization are discussed.

PDGF B-chain in neurons of the central nervous system, posterior pituitary, and in a transgenic model

Platelet-derived growth factors (PDGFs) are growth-regulatory molecules that stimulate chemotaxis, proliferation, and increased metabolism of primarily connective tissue cells. In a survey of normal tissues, we found specific immunostaining for PDGF B-chain in neurons, principal dendrites, some axons, and probable terminals throughout the brain, in the dorsal horn of the spinal cord, and in the posterior pituitary of a nonhuman primate (Macaca nemestrina). PDGF activity was extracted from brain cortex and posterior pituitary, and ubiquitous expression of transcripts for the two chains of PDGF and both PDGF receptors was detected throughout the brain and posterior pituitary. A transgenic model was also evaluated in which the chloramphenicol acetyltransferase gene was placed under transcriptional control of the PDGF B-chain promoter. The transgene was preferentially expressed within neural cell bodies in the cortex, hippocampus, and cerebellum. PDGF may act as a neuronal regulatory agent. Neuronal release of PDGF could contribute to nerve regeneration and to glial proliferation that leads to gliosis and scarring.

Contralateral degeneration in the cat spinal trigeminal nucleus following unilateral retrogasserian trigeminal rhizotomy

Retrogasserian trigeminal rhizotomy was used to study the central projections and patterns of degeneration in the spinal trigeminal nucleus (STN). At survival times of 3-20 days, reduced silver stains show extensive degeneration throughout the ipsilateral STN and in addition, well delineated degeneration was identified in the periobex region of the contralateral STN that varied with survival time. The results suggest that primary afferents may contribute to this contralateral projection.

Is Phaseolus vulgaris leucoagglutinin (PHA-L) a useful marker for labeling neural grafts?

The lectin Phaseolus vulgaris leucoagglutinin (PHA-L) has come into wide use as an anterograde neuroanatomical tracer. The ability of this lectin to fill entire neurons and remain in place over long periods suggested it might be an ideal marker for donor cells to be grafted into hosts for long survival periods. We have used the lectin PHA-L to mark fetal rat olfactory bulb (OB) cells prior to grafting into host rat OBs. Hosts were sacrificed at various times up to 9 weeks after grafting, and tissue was immunohistochemically processed for PHA reactivity. After 2 and 4 weeks survival, sparse patterns of labeled cells were observed within the host OBs. However, after 9 weeks survival, few if any labeled cells were visible within host tissue. We conclude that PHA-L may be a less than satisfactory marker for fetal rat cells (other than astrocytes) which are to be identified in host tissue after a period of several weeks.

Fetal olfactory bulb transplants send projections to host olfactory cortex in the rat

We are using the rat olfactory system to study developmental details of neurotransplantation. Tritiated [3H]thymidine-labeled fetal olfactory bulbs (OBs), were transplanted immediately into sites from which the neonatal host OB was removed. Subsequently, a small lesion was placed in the region of the transplanted OB and the tissue studied, using degeneration methods and autoradiography. Only OB's with extensive [3H]-label and precise lesions confined to the labeled areas were used. Degeneration was found mainly in the ipsilateral piriform cortex with lesser amounts at other nearby sites. The results demonstrate successfully transplanted donor OBs that send axons to specific and appropriate target areas of the host brain.

Central representation of dental structures in the kitten, including projections to the mesencephalic trigeminal nucleus

Horseradish peroxidase (HRP) was injected into either a single maxillary or a single mandibular primary (deciduous) cuspid tooth of 8- to 10-week-old kittens. The large apex of the primary cuspid allowed for some leakage of the HRP from the pulpal chamber to the periodontal ligament (PDL). Thus, the injection procedure resulted in the application of HRP to the PDL as well as to the pulpal tissues. The transganglionic transport of HRP resulted in discrete terminal fields within the spinal trigeminal nucleus (STN) and the main sensory nucleus (MSN). These projections were clearly somatotopically organized within the STN, but less so within MSN. Within pars oralis (PO) and pars interpolaris (PI), mandibular cuspid dental structures (MdCDS) were represented in a dorsal position relative to the maxillary cuspid dental structures (MxCDS), whereas within pars caudalis (PC) and the adjacent reticular formation the somatotopic representation was not dorsoventral, but rather mediolateral, with the MdCDS represented more medially than the MxCDS. Areas of overlap between MxCDS and MdCDS were found within MSN and to a lesser degree within the superficial laminae of PC. In addition, the fiber pathway leading to labeled somata in the mesencephalic trigeminal (Mes V) nucleus was clearly identified. The majority of the fibers traced to the Mes V nucleus exited the spinal trigeminal tract at the level of the transition from PO to the MSN and traversed the nuclear region in a position dorsal to and separate from the trigeminal motor tract. As in STN, fibers within the caudal Mes V tract appeared to be somatotopically organized, with the fibers from the MdCDS generally more dorsal than the ones from the MxCDS. Labeled fibers, some with terminal arbors, were also identified in close association with the trigeminal motor tract. The findings show a complex pattern of central representation in the immature feline central nervous system for deciduous dental structures.

Plasticity-related binding of GABA and muscarinic receptor sites in piriform cortex of rat: an autoradiographic study

This study has used the recently developed in vitro quantitative autoradiographic technique to examine the effects of olfactory bulb (OB) removal on receptor-binding sites in the deafferented piriform cortex (PC) of the rat. The gamma-aminobutyric acid-benzodiazepine receptor (GABA-BZR)- and muscarinic cholinergic receptor (MChR)-binding sites in layer I of PC were localized using [3H]flunitrazepam and [3H]quinuclidinyl benzilate as ligands, respectively. From the resultant autoradiograms the optical densities were measured using a Drexel-DUMAS image analysis system. The densities of BZR and MChR-binding sites were markedly increased in the PC ipsilateral to the lesion as compared to the contralateral side in those subjects that were operated in adulthood (Postnatal Day 100, PN 100). Comparisons between the unoperated and PN 100 operated animals also showed significant increases in the deafferented PC. In the animals operated on the day of birth (PN 0) no significant differences were seen between the operated and the contralateral PC. The difference between the PN 0 deafferented PC and the unoperated controls shows a slight decrease in BZR density in the former group; however, in case of the MChR there is a slight increase on the side of the lesion. These results demonstrate that deafferentation of PC by OB removal appears to modulate both the BZR-binding sites that are coupled with the GABA-A receptor complex and the MChR-binding sites. The results also suggest that possibility of a role for these neurotransmitter receptor-binding sites in plasticity following deafferentation.

Ultrastructural localization of immunoreactivity in the developing piriform cortex

The purpose of this study was to determine the ultrastructural basis for the immunoreactivity patterns in synaptic structures during development in layers I and II of the piriform cortex (PC) of rats. Antisera to cholecystokinin (CCK) and glutamic acid decarboxylase (GAD) were used at several different postnatal days (PN) and in adults to describe the distribution, characteristics, and relative frequency of labeled profiles--especially axons and terminals--with emphasis on details of the synaptic contacts. GAD-positive terminals occur from PN 2 to adulthood but only form contacts in deeper sublayers (Ib and II) initially. Contacts increase in layer I after PN 6 and are reduced in layer II after PN 21 when the GAD-labeled terminals and synapses take on adult features with flattened vesicles and symmetric contacts. CCK-labeled terminals are present in deeper sublayers at PN 2 but are few and rarely form contacts. Both terminals and contacts increase between PN 2 and 9, taking on distinctive shapes and vesicle morphology by PN 13. At PN 21 and older, CCK terminals have mainly flattened vesicles and mostly form symmetric contacts onto dendrites and somata in deeper layers (Ib and II). Superficial sublayer Ia has very few CCK-labeled synapses and axons. Thus immunoreactivity occurs in terminals prior to synapse formation; labeling of the presynaptic specializations precedes subsequent maturation; synaptic vesicle morphology and membrane specializations are similar for the vast majority of both CCK and GAD terminals; inhibitory (GABA) synapses are established sooner than the possibly excitatory CCK synapses; a deep to superficial gradient of synaptogenesis is associated with GAD-positive terminals in the PC; and the labeling patterns may be related to critical developmental or synaptogenic periods.

Ultrastructure of synaptic remodeling in piriform cortex of adult rats after neonatal olfactory bulb removal: an immunocytochemical study

The purpose of this investigation was to study possible remodeling in synaptic structures of the piriform cortex (PC) of adult rats following neonatal deafferentation by removal of the olfactory bulb (OB) at birth. Emphasis was placed on possible qualitative changes in the ultrastructure and immunocytochemical localization of cholecystokinin (CCK, a possible excitatory neurotransmitter or modulator) and glutamic acid decarboxylase (GAD, precursor enzyme to the inhibitory transmitter GABA) in axons, terminals, and synaptic complexes. Light microscopic results in normal adult material show that GAD-positive terminals form a dense band subjacent to the lateral olfactory tract (LOT), become less dense in deeper Ib, and are rare in layer II. Following deafferentation, GAD-positive terminals appear denser and more homogeneously distributed throughout layer I and are also more prevalent in layer II. Ultrastructural results of normals and controls indicate GAD-positive terminals normally contain pleomorphic or flattened vesicles and form symmetric contacts onto dendritic shafts and branches throughout layer I. In deafferented layer I not only do there appear to be greater numbers of symmetric GAD-positive contacts, but in contrast to normals, asymmetric contacts mainly onto spines are now present. Light microscopic results from deafferented material also show an apparent proliferation with spread or sprouting of CCK-positive fibers or axonlike structures mainly into layer Ia, whereas these fibers are normally observed only in the LOT and are generally few in number. Also in normals the few CCK-positive terminals in the area subjacent to the LOT contain flattened or pleomorphic vesicles and form symmetric contacts. Deafferentation results in CCK-positive terminals throughout layer I with a greater frequency of synaptic contacts which now also include a few asymmetric contacts onto spines. The findings clearly show modifications in synaptic patterns of immunocytochemical-labeled terminals that might be compatible with the process of atypical reinnervation of deafferented postsynaptic sites and possible ingrowth of new axons.

Electron microscopy of plasticity in rat olfactory cortex

Electron microscopy (EM) is being used to study the ultrastructural basis for the age-dependent reorganization of afferents in the olfactory cortex (OC) of rat after deafferentation of the area by removal of the ipsilateral olfactory bulb (OB). The double-lesion technique was used with a primary lesion of the OB at various postnatal (PN) ages between PN 0 and 30 and in the adult (PN 100). After appropriate survival times to remove initial lesion-degenerated terminals from the OB lesion, a second lesion was placed in the ipsilateral OC. One to 3 days later the tissue is prepared for EM with emphasis on a study of changes in the superficial and deep dendritic layer (Ia and Ib respectively) rostral to the lesion. In control litter mates with both OBs intact, but with a single OC lesion only, degenerating synaptic terminals occur onto dendritic spines and branches only in deeper Ib. However, in adults with OB lesions at PN 0-9, OC lesions produce degenerating terminals throughout Ia and Ib including immediately subjacent to the pia. In Ia degenerating terminals are greatly reduced in the PN 13 group and rare to absent in experiments with OB lesions at older ages (PN 30-100). Electron-dense debris within glia occurs throughout layer I in each double-lesion group but is greatest in experiments with OB lesions at older ages. Some transsynaptic alterations are seen throughout, especially in the PN 30-100 group even at a distance from the OC lesion. The results support earlier light microscopic (LM) findings, suggesting PN 9-13 as critical ages for developmental plasticity and prove that at least in the younger ages, synapses are involved in the phenomenon. This may be explained by either reinnervation of deafferented sites or persistence of synapses that would otherwise have been eliminated by afferents from the OB. In addition, some of the LM degeneration particles probably are engulfed masses of debris and not synaptic structures, especially in cases which were operated at older ages and survived for 3 days. The various afferent pathways involved in the events as well as factors that limit the phenomenon in older ages are discussed.

Ultrastructural changes in acetylcholinesterase activity in the deafferented spinal trigeminal nucleus

Acetylcholinesterase (AChE) activity was investigated in synaptic areas of the cat spinal trigeminal nucleus (pars interpolaris and pars caudalis) ipsilateral and contralateral to complete retrogasserian rhizotomy. Vibratome sections of tissue taken from animals of 1, 3, 6, 14, and 21 days survival were examined by electron microscopy following a histochemical reaction for AChE activity employing a method based on the Karnovsky-Roots technique for demonstrating reaction product. As degeneration progressed with survival time, enzymatic activity was initially reduced in synaptic clefts of injured afferent terminals and subsequently was enhanced throughout the extracellular space, including within synaptic clefts of possibly reinnervated sites. These changes in enzymatic activity with primary deafferentation are discussed in relation to the process of reinnervation, the development of neuronal hyperactivity, and possible noncholinergic functions of AChE.

Ultrastructure of degenerative changes following ricin application to feline dental pulps

The ultrastructure of degenerative changes within the ipsilateral trigeminal ganglion, and partes caudalis and interpolaris of the spinal trigeminal nucleus in the cat is described following the application of the potent toxin ricin to the tooth pulps of unilateral maxillary and mandibular posterior teeth, including the cuspids. Survival times ranged from 6 to 10 days. Typical changes identified within the ipsilateral trigeminal ganglion included myelin fragmentation and 'compartmentalization' of the axoplasm of medium-sized myelinated axons, while small myelinated and unmyelinated axons underwent a more variable response ranging from electron-lucent to electron-dense changes. The affected cell body was characterized by the presence of swollen, electron-lucent mitochondria, a reduction of cytoplasmic ribosomes and a filamentous hyperplasia. Other changes often included an eccentric nucleus and satellite cell proliferation. Degenerative changes often occurred in isolated elements surrounded by normal profiles, suggesting specificity of ricin within the trigeminal ganglion. Changes within brainstem axons showed both an electron-dense and a lucent, fragmenting type of axonal alteration. Terminal changes ranged from electron-dense to lucent and also included filamentous hyperplasia and 'hyperglycogenesis'. The altered axonal knobs contained round synaptic vesicles that were presynaptic to dendritic profiles and postsynaptic to terminals containing flattened synaptic vesicles. The above brainstem alterations were identified specifically in the following areas: ventrolateral, medial and dorsomedial pars interpolaris; the ventrolateral and mid-dorsal to dorsomedial areas of the marginalis and outer substantia gelatinosa layers of pars caudalis; and in ventral pockets corresponding to lamina V of the medullary dorsal horn. Dense alterations within terminals containing flattened synaptic vesicles that are typically presynaptic to primary afferents in these areas were rare findings, but along with vacuolization of dendritic profiles suggest a trans-synaptic effect possibly due to the exocytosis of ricin. The results are discussed in relation to different reports of dental projections and with regards to patterns of transganglionic degeneration.

Benzodiazepine binding increases in the superficial laminae of the trigeminal subnucleus caudalis following central rhizotomy

Benzodiazepine (BZ) binding is being studied in the spinal trigeminal nucleus of the cat 3 and 11 days following unilateral retrogasserian rhizotomy using in vitro autoradiography and computer-assisted densitometry. At 3 days following rhizotomy there is an increase in number and decrease in affinity of flunitrazepam binding sites in the superficial laminae of subnucleus caudalis of the spinal trigeminal complex. By 11 days, affinity remains below control values and binding site number shows an insignificant but detectable increase. There is no change in binding site characteristics in deeper laminae at either survival time. The results are discussed in relation to the physiological hyperactivity and synaptic changes which occur following such lesions and to other conditions of deafferentation.

Immunocytochemical localization of cholecystokinin and glutamic acid decarboxylase during normal development in the prepyriform cortex of rats

Immunocytochemical localization of specific neurotransmitters in the brain is becoming increasingly important in studies of maturation. We have used the trilaminar prepyriform cortex (PC) of rats to study the distribution, patterns and relative number of cells, fibers and terminals during postnatal development using antisera to cholecystokinin (CCK) and glutamic acid decarboxylase (GAD). Both antisera show distinct patterns of immunoreactivity at birth and subsequent periods of distinct changes in these patterns. CCK immunoreactivity is rare but present at birth mostly in layer II. There is a dramatic increase of CCK-labeled structures between postnatal (PN) days 6 and 9 and between PN 13 and 21. The adult pattern is observed by PN 21 with large numbers of labeled cells in layer II, numerous terminals in layers II and deep I and large immunoreactive fibers in the lateral olfactory tract. At birth GAD-immunoreactive terminals are present mainly in layer I, forming a distinct pattern of superficial and deep bands. Subsequent major changes occur in this pattern between PN 9 and 13 and again between PN 13 and 21. By PN 21 there appears to be a loss in deeper laminae of GAD positive terminals which are possibly replaced by the increasing numbers of CCK terminals in the same sublaminae. The adult pattern of GAD immunoreactivity is established by PN 21 with terminals and a few cells in layer I. Therefore, throughout development of the rat PC, there is a distinct complementary and changing distribution of GAD and CCK. Factors that may influence these changes in immunoreactivity are discussed.

Stereological analysis of synaptogenesis in the molecular layer of piriform cortex in the prenatal rat

The development of synapses in the molecular layer of the rat piriform cortex was studied at embryonic days 15, 17, 19, and 21. The present study has sought to extend past studies of synaptogenesis by identifying not only changes in numbers of synapses, but also changes in numbers of potential precursors of synapses. A stereological method (Cruz-Orive, '80) was used to make volumetric estimations of the numbers of synapses, axonal puncta, vesicle-associated puncta, and unapposed postsynaptic specializations. This stereological method was preferred to other morphometric methods because it is not influenced by changes in the size, shape, or orientation of the structures of interest. This was considered important since such changes might be expected during development. Large numbers of unapposed axonal specializations (axonal puncta and vesicle-associated puncta) were found in all three sublaminae (lateral olfactory tract, Ia, and Ib) at all ages. The numerical density (number per unit volume of neuropil) and relative frequency of these structures changed significantly with time. In all three sublaminae, these changes were associated with changes in the number of synapses, although the numerical density and relative proportions varied between the sublaminae. These results suggested that axonal puncta could accumulate vesicles, thus becoming vesicle-associated puncta, and that vesicle-associated puncta could contact dendrites, thus forming synapses. In contrast, the numerical density of lone postsynaptic specializations remained low and no significant changes in their relative proportion in the population were found. This suggested that although lone postsynaptic sites were observed, they did not appear to play a major role in synaptogenesis in this region of the cortex. In addition to documenting developmental differences between the three sublaminae in the molecular layer, the results support a synaptogenic hypothesis in which the axon can form surface specializations that appear to be involved in synaptogenesis, independent of direct dendritic contact.

Primordial synaptic structures and synaptogenesis in rat olfactory cortex

Mature synaptic contacts and various primordial synaptic elements were studied, counted, and analyzed in rat olfactory cortex from birth to 30 days of age. Primordial structures possess one or a few, but not all, of the features of a true mature synapse and have been grouped into two major classes based upon type of apposition: 1) single or 2) partial and multiple, with vacant postsynaptic sites included in the latter. There is a classical fivefold increase in number of mature synapses between birth and 30 days, but different patterns in the primordial appositions are observed. It is suggested that single apposition contribute to early synapse formation, while partial and multiple appositions participate later on during a time of rapid growth of new afferents to the area. The results suggest a clear role for primordial synaptic structures in synaptogenesis; that the sequence may be more diverse than originally hypothesized, occurring at different stages; and that competition, synapse elimination, and replacement may be more prevalent in normal synaptogenesis than has been previously suspected.

New observations on the substructure of the active zone of brain synapses and motor endplates

This study offers a new concept on the origin and function of the hitherto enigmatic presynaptic dense projections (dps) of neurons and motor endplates. After a deuterium oxide-albumin pretreatment (da), brain tissue and motor endplate of rat and frog reveal an intricate association of smooth endoplasmic reticulum (ser), microtubules (mts) and synaptic vesicles (sv) at the presynaptic grid-active zone of synapses. The ser entwines the mts, which are clothed in svs, and impinges directly onto the presynaptic membrane as sacs or 'tubular-fibrillar' extensions. Since no dps are seen in these sections, whereas they do occur in conventionally processed material (i.e. without da pretreatment), it is suggested that the dps of conventional material may, in part, originate from improperly fixed ser at these points. Thus for the first time we demonstrate an in vivo system of ser which, because its 'finger' processes come into intimate contact with the presynaptic membrane, may be implicated in Ca2+ ion translocation, presumably out of the presynaptic bulb. Since no such tubular ser has been demonstrated in what are claimed to be sophisticated techniques (i.e. high-speed slam-freezing-freeze substitution) the actual sophistication of such methods is questioned.

Mapping of the normal distribution of substance P-like immunoreactivity in the spinal trigeminal nucleus of the cat

Although a recent preliminary report indicated a pattern of substance P-like immunoreactivity within the spinal trigeminal nucleus that is similar to the projection sites for dental afferent fibers, details of this substance P distribution are lacking. Our purpose was to describe in cats the complete normal pattern of this immunoreactivity within each of the spinal trigeminal subnuclei. Special emphasis was given to the distribution of substance P-like immunoreactive axons and terminals located in the rostral subnucleus caudalis and the periobex region of subnucleus interpolaris, as these are regions shown to receive dental afferent fibers. Careful mapping in normal cats showed, within the resolution of the light microscope, a consistent pattern of distribution that included only a portion of the previously identified dental relay sites, but was somewhat broader in certain levels and more restricted in others. The results are compared with those provided by others from regions such as the dorsal horn and subnucleus caudalis of the spinal trigeminal nucleus. The findings also provide an anatomical basis for a recent physiologic report on specific cell types associated with dental nociceptive afferent fibers. This study also provides a baseline control for future investigations of possible changes in substance P-like immunoreactivity that follows various peripheral, including dental and central, lesions.

Plasticity in the rat olfactory cortex

The relationship of age to deafferentation plasticity was studied in the rat olfactory cortex (OC). Ablation of a single olfactory bulb (OB) was performed in each of several rats of selected postnatal (PN) ages: PN2.5, 6, 9, 13, and 21 days and in adults of PN100 days. Following survival times sufficient to remove the resultant degeneration, a cortical lesion was placed in the ipsilateral OC. The patterns of degeneration from the OC lesion were studied and mapped in the adjacent deafferented OC. The results show a spread or sprouting of the usually deep-lying afferents (interrupted by the OC lesion), onto the deafferented superficial dendrites (normally occupied by the OB afferents) in all of the ages. The spread is most striking at PN2.5 to PN9, gradually reduced by PN13 to PN21, and least in the adult (PN100). There is also an apparent increase of afferents to the deeper dendrites nearer the cell bodies in all cases except in the PN 100 group. Shrinkage of layer I is not seen in PN2.5 subjects, is minimal by PN9, but is most marked in the adult PN100 with total OB lesions. Incomplete OB lesions sparing some lateral olfactory tract (LOT) fibers greatly reduce the shrinkage of layer I and the spread of afferents in all ages. Thus, a capacity for reorganization of afferents occurs at least through PN9, with PN13-21 a possible "critical period" after which plasticity is limited and transneuronal effects are more permanent. The association, centrifugal, and olfactory-entorhinal pathways are possible origins for this plasticity. Factors contributing to limitations in this reorganization are discussed.