DISTRIBUTION OF SERINE HYDROXYMETHYLTRANSFERASE AND GLYCINE TRANSAMINASE IN SEVERAL AREAS OF THE CENTRAL NERVOUS SYSTEM OF THE RAT

—The regional distributions of serine hydroxymethyltransferase (SHMT) and glycine transaminase (GT) have been determined in five areas of the CNS of the rat. The SHMT activity per mg protein varied in these areas in the following order: medulia-pons and spinal cord > cerebellum > midbrain > telencephalon. The GT activity per mg protein was essentially the same in the four brain areas, whereas, in the spinal cord it was lower. The activity of GT did not correlate with the glycine content (r=?0.45. P > 0.05). However, SHMT activity per mg protein was correlated with the glycine content in four regions (the telencephalon, midbrain, medulla-pons and spinal cord; r= 0.997, P < 0.05). When the activity of SHMT was expressed per relative number of mitochondria, the enzyme levels were correlated with the glycine content in all five areas (r= 0.952, P < 0.05). The distribution of SHMT was determined in the primary subcellular fractions of the CNS. The SHMT activity in these areas of the CNS appeared to be located predominately in paniculate structures, while only 1 to 4 per cent was found in the soluble fraction. The crude nuclear (P₁) and the crude mitochondrial (P₂) fractions contained 90–97 per cent of the activity. Subfractionation of P₂ pellets obtained from the telencephalon, medulla-pons and spinal cord indicated the SHMT activity was localized in both ‘free’ and occluded mitochondria.     

Scavenger Enzyme Activities in Subcellular Fractions of White Clover (Trifolium repens L.) under PEG-induced Water Stress

Scavenger enzyme activities in subcellular fractions under polyethylene glycol (PEG)-induced water stress in white clover (Trifolium repens L.) were studied. Water stress decreased ascorbic acid (AA) content and catalase (CAT) activity and increased the contents of hydrogen peroxide (H₂O₂), thiobarbituric acid reactive substances (TBARS) (measure of lipid peroxidation), and activities of superoxide dismutase (SOD), its various isozymes, ascorbate peroxidase (APOX), and glutathione reductase (GR) in cellular cytosol, chloroplasts, mitochondria, and peroxisomes of Trifolium repens leaves. In both the PEG-treated plants and the control, chloroplastic fractions showed the highest total SOD, APOX, and GR activities, followed by mitochondrial fractions in the case of total SOD and GR activities, whereas cytosolic fractions had the second greatest APOX activity. However, CAT activity was the highest in peroxisomes, followed by the cytosol, mitochondria, and chloroplasts in decreasing order. Although Mn-SOD activity was highest in mitochondrial fractions, residual activity was also observed in cytosolic fractions. Cu/Zn-SOD and Fe-SOD were observed in all subcellular fractions; however, the activities were the highest in chloroplastic fractions for both isoforms. Total Cu/Zn-SOD activity, the sum of activities observed in all fractions, was higher than other SOD isoforms. These results suggest that cytosolic and chloroplastic APOX, chloroplastic and mitochondrial GR, mitochondrial Mn-SOD, cytosolic and chloroplastic Cu/Zn-SOD, and chloroplastic Fe-SOD are the major scavenger enzymes, whereas cellular CAT may play a minor role in scavenging of O₂⁻ and H₂O₂ produced under PEG-induced water stress in Trifolium repens.     

FRACTIONATION OF OLFACTORY TISSUE HOMOGENATES. ISOLATION OF A CONCENTRATED PLASMA MEMBRANE FRACTION

Abstract— Differential and sucrose-density-gradient centrifugation techniques were used for studies on the separation of subcellular particles from rabbit brain and olfactory tissue. Comparisons were made among various fractions from the two types of tissue. These comparisons included protein concentration and enzyme activities of the individual fractions as well as their distribution in subfractions from density gradient separations. In tissue whole homogenates, the percentage of total ATPase activity as ouabain sensitive Na⁺-K⁺ ATPase activity was about 4 times greater in brain cortex (63 per cent) than in olfactory tissue (17 per cent). Cytochrome oxidase and Na⁺-K⁺ ATPase activities were used to indicate the presence and the concentration of mitochondria and of the plasma membranes. A fraction with properties similar to the mitochondria plus nerve ending fraction from brain homogenates (fraction B) was obtained from olfactory tissue. Nerve ending concentration subfractions (B₂) were prepared from the B primary fractions. Plasma membrane subfractions were obtained by osmotic shock treatment of B₂, In the fraction of plasma membrane from olfactory tissue (E₂), 56 per cent of the total ATPase activity was Na⁺-K⁺ ATPase activity. In E₂ from brain 71 per cent was Na⁺-K⁺ ATPase activity. Deoxycholate (DOC)-treated fractions containing nerve endings from brain preparations showed much greater increase in cytochrome oxidase activity than did similar fractions from olfactory tissue. DOC treatment increased the NADH cytochrome c reductase activity of all fractions and subfractions from brain, while it decreased activity in all but one fraction from olfactory tissue. DOC treatment decreased both the Mg²⁺ and Na⁺-K⁺ ATPase activities in both types of tissue. Electron photomicrographs of olfactory B₂, B₃, E₂ and E₃ show clear morphological differences among these subfractions. The presence of possible cilia and basal bodies on vesicles in B₂ gives morphological evidence for the presence of terminal swellings in this subtraction in agreement with enzyme marker activity results.     

Subcellular distribution and properties of rabbit liver aminoacyl-tRNA synthetases under myocardial ischemia

Subcellular distribution of aminoacyl-tRNA synthetase activities has been studied in normal rabbit liver and under experimental myocardial ischemia (EMI). An increase in the activity of a number of aminoacyl-tRNA synthetases in postmitochondrial and postribosomal supernatants from rabbit liver has been determined 12 hr after EMI. Gel chromatography of the postribosomal supernatant on Sepharose 6B shows that aminoacyl-tRNA synthetase activities are distributed among the fractions with M_r 1.82×10⁶, 0.84×10⁶ (high-M_r aminoacyl-tRNA synthetase complexes) and 0.12–0.35×10⁶. In the case of EMI aminoacyl-tRNA synthetase activities are partly redistributed from the 1.82×10⁶ complex into the 0.84×10⁶ complex. The catalytic properties of both free and complex leucyl-tRNA synthetases have been compared. K_M for all the substrates are the values of the same order in norm and under EMI. A decrease in some aminoacyl-tRNA synthetase activities associated with polyribosomes has been observed 12 hr after EMI. The interaction of aminoacyl-tRNA synthetases with polyribosomes stimulates the catalytic activity of some enzymes and protects them from heat inactivationin vitro. It is assumed that the changes in association of aminoacyl-tRNA synthetases with high-M_r complexes and compartmentalization of these enzymes on polyribosomes may be related to the alteration of protein biosynthesis under myocardial ischemia.     

Expression and Localization of sPLA2-III in the Rat CNS

Phospholipases A₂ (PLA₂) are enzymes that cleave the sn-2 bond of membrane phospholipids to yield free fatty acids and lysophospholipids. Secretory PLA₂-III (sPLA₂-III) has been suggested to be important for neuronal differentiation, growth and survival, and is highly expressed in the spinal cord. The aim of this study is to elucidate its expression and distribution in different regions of the adult rat CNS. Quantitative RT-PCR analyses showed high levels of sPLA₂-III mRNA expression in the brainstem and spinal cord and low expression in the olfactory bulb. Western blot analyses showed high level of expression in the brainstem, spinal cord and cerebral neocortex. A dense band corresponding to the catalytically active, mature/cleaved form, and a faint band corresponding to the full length sPLA₂-III were detected in post-mitochondrial supernatants, from different parts of the CNS. Subcellular fractionation of spinal cord homogenates showed that sPLA₂-III protein is present in the ‘light membrane/cytosol’ fraction, but not the nucleus, synaptosomal membrane or synaptic vesicle-enriched fractions. sPLA₂-III was immunolocalized to neurons in the cerebral neocortex, Purkinje neurons in the cerebellar cortex, periaqueductal gray, red nucleus, spinal trigeminal nucleus and dorsal horn of the spinal cord. Electron microscopy of the spinal cord and cerebral neocortex showed that sPLA₂-III was localized in dendrites or dendritic spines, that formed asymmetrical synapses with unlabeled, putatively glutamatergic, axon terminals. The localization of mature/cleaved form of sPLA₂-III in postsynaptic structures suggest a physiological role of the enzyme in neurotransmission or synaptic plasticity.     

Compartmentation and Activities of Enzymes Involved in the Metabolism of Amino Acids Implicated in Osmoregulatory Mechanisms in Acanthamoeba castellanii1

Glutamate dehydrogenase in Acanthamoeba castellanii is an NAD-dependent cytosolic enzyme. This is similar to glutamate dehydrogenases in Phycomycetes, but very different from the dual coenzyme-specific enzymes located in mitochondria in animals and in mitochondria and chloroplasts in higher plants. Pyrroline-5-carboxylate (P-5-C) reductase occurs also in the cytoplasm in A. castellanii and has very high affinities for L-P-5-C (K_m= 12 μM) and NADH (K_m= 15 _μM). In contrast, ornithine aminotransferase and proline oxidase are mitochondrial enzymes. No proline-inhibited γ-glutamyl kinase was detected while an active glutamine synthetase was found in the cytosolic compartment. Evidence for a mitochondrial transport system for L-proline was obtained. Two possible pathways for proline biosynthesis in A. castellanii are discussed based on information obtained about activities and subcellular compartmentation of enzymes.     

Development of a synaptosomal model to determine drug-induced in vivo changes in GABA-levels of nerve endings in 11 brain regions of the rat

An experimental procedure was developed which allowed the simultaneous measurement of GABA in synaptosomes from 11 regions of one rat brain. Synaptosomal fractions were prepared by conventional subcellular fractionation procedures and characterized by electron microscopy. Post-mortem increases of GABA during removal and dissection of brain tissue, homogenization and fractionation procedures could be sufficiently minimized by rapid processing of the tissue at low temperatures and inclusion of 3-mercaptopropionic acid (1 mM) in the homogenizing medium. Experiments with addition of aminooxyacetic acid (AOOA, 1 mM) to the homogenizing medium indicated that GABA was not being degraded during synaptosome preparation. The presence of exogenous GABA (1 mM) did not alter the GABA levels in the organelles, indicating that no significant redistribution of GABA occurred during subcellular fractionation. On the basis of these findings, it was suggested that synaptosomal fractions could be used as a model to monitor indirectly the drug-induced changes in GABA levels of nerve endings in discrete brain areas of the intact animal. In vivo experiments with AOAA (30 mg/kg i.p.) and valproic acid (VPA, 200 mg/kg i.p.) showed that both drugs caused differential effects on synaptosmal GABA levels in different brain regions. Although AOAA was more potent than VPA in increasing GABA in whole tissue of most brain regions, significant increases of synaptosomal GABA levels after AOAA were only determined in olfactory bulbs and frontal cerebral cortex. In contrast, VPA induced significant synaptosomal GABA increases in olfactory bulbs, hypothalamus, superior and inferior colliculus, substantia nigra, and cerebellum. The data indicate that the synaptosomal model can provide useful information on the in vivo effects of drugs on GABA levels in nerve terminals and their ability to exert this effect in specific brain areas.     

Intracellular distribution of molecular forms of acetylcholinesterase in rat brain and changes after diisopropylfluorophosphate treatment

The subcellular distribution of acetylcholinesterase activities was studied in the striatum and cerebellum of rat brain. The highest percentage of the enzyme activity was found in the crude synaptosomal (P₂) fraction, with striatum much higher than cerebellum. On sucrose density gradient centrifugation analyses all the particulate fractions (P₁, P₂, and P₃) showed a major peak of the 10 S form of acetylcholinesterase activity with very little activity of the 4 S form of the enzyme. The 10 S/4 S ratio was much higher in striatum than in cerebellum. In the soluble fraction (100,000g supernatant) the 10 S form was less than the 4 S form in the adult rat brain, but this was reversed in the 6-day-old rat brain. After diisopropylfluorophosphate administration the recovery of acetylcholinesterase molecular forms in various subcellular fractions differed at different recovery periods. These results indicate that the distribution of molecular forms of acetylcholinesterase in rat brain differs in various subcellular fractions, and also the pattern of distribution differs in different regions of the brain as well as in adult and developing brains.     

PURIFICATION AND CHARACTERIZATION OF CARNOSINE SYNTHETASE FROM MOUSE OLFACTORY BULBS

Carnosine synthetase was purified about 500-fold from mouse olfactory bulb to a specific activity of approx 25 nmol/min/mg. This is an increase of 800-fold over that previously reported for this enzyme from rat brain and 11 times higher than the most highly purified enzyme from chicken pectoral muscle. ATP was essential for activity and could not be replaced by ADP. NAD had no effect on the synthesis of carnosine. Of the β-alanine analogues tested, the purified mouse enzyme incorporated only γ-aminobutyric acid and β-amino-n-butyric acid into peptide linkage with histidine. Synthesis of carnosine by the mouse olfactory bulb enzyme was competitively inhibited by the histidine analogues, 1-methyl histidine and 3-methyl histidine, with K_i values which were at least 40 times the K_m value for histidine (16 μM). Ornithine and lysine were more efficient β-alanine acceptors than 1-methyl histidine for the mouse enzyme. Enzyme from olfactory epithelium and leg skeletal muscle of mice also showed higher K_i values for 1–methyl histidine than the K_m value for histidine. In contrast, carnosine-anserine synthetase from chicken pectoral muscle gave K_m values for histidine, 1-methyl histidine and 3-methyl histidine, which were all in the range of 4–12 μM. The differences in substrate specificity between the enzyme from mouse and chicken implies alternate routes of anserine synthesis in these species and predicts the occurrence of certain novel peptides in mouse brain.     

Localization and developmental expression of GABAB receptors in the rat olfactory bulb

In this study, we investigated the distribution and developmental expression of the GABA_B receptor subunits, GABA_B1 and GABA_B2, in the main and accessory olfactory bulbs of the rat. Antibodies raised against these subunits strongly labelled the glomerular layer, suggesting that olfactory and vomeronasal nerve fibers express functional GABA_B receptors. Using postembedding immunogold cytochemistry, we found that GABA_B receptors can be present at both extrasynaptic and presynaptic sites of olfactory nerve terminals, and in the latter case they are preferentially associated with the peripheral part of the synaptic specialization. Olfactory nerve fibers expressed GABA_B1 and GABA_B2 at early developmental stages, suggesting that GABA_B receptors may play a role in olfactory development. Output and local neurons of the main and accessory olfactory bulbs were also labelled for GABA_B1 and GABA_B2, although the subcellular distribution patterns of the two subunits were not completely overlapping. These results indicate that presynaptically located GABA_B receptors modulate neurotransmitter release from olfactory and vomeronasal nerve fibers and that, in addition to this presynaptic role, GABA_B receptors may regulate neuronal excitability in infraglomerular circuits.     

Postnatal ontogeny of uridine kinase in the cerebellum,hypothalamus, and cerebral cortex of the rat

Postnatal developmental patterns of uridine kinase were determined in crude subcellular fractions of the rat cerebellum, hypothalamus and cerebral cortex at ages 3 through 60 days. The highest specific activity and predominant distribution of enzyme was in the 105,000g supernatant of the 3 brain regions. Enzyme activity in hypothalamus and cerebral cortex was maximum at 3 days and decreased with age; in cerebellum it increased through 13 days and decreased thereafter. Thus, the pattern of activity in hypothalamus and cerebral cortex paralleled changes in DNA and RNA synthesis through age 60 days; in cerebellum, it more closely approximated changes in DNA synthesis during early development. Changes inK _m with aging suggest that the brain regions contain more than one form of enzyme. The highest particulate activity was in the microsomal fraction of the cerebellum and hypothalamus at all ages and in the cortex at 35 and 60 days. Relative specific activity for microsomal fractions of the brain regions at 60 days indicate a concentration of the enzyme which may be relevant in the maintenance of RNA activity in adult brain.     

THE SUBCELLULAR DISTRIBUTION OF β-PHENYLETHYLAMINE, p-TYRAMINE AND TRYPTAMINE IN RAT BRAIN

—The quantitative subcellular distribution of β-phenylethylamine, p-tyramine and tryptamine in rat brain was investigated using the mass spectrometric integrated ion current technique. More of the total cellular tryptamine was found to be associated with paniculate fractions than was the case for phenyiethylamine and p-tyramine but a significant amount of this tryptamine was found to be labile. Analysis of the particulate fractions indicated that each of the amines was localized predominantly in the crude P₂ pellet and that the bulk of this was associated with the synaptosomal (P₂B) fraction. Inhibition of monoamine oxidase systems with pargyline caused an increase in the level of all three amines in all fractions, but the increase was greater in the supernatant than in the combined particulate fractions. This treatment produced changes in the distribution of β-phenylethylamine and p-tyramine between the various particulate subcellular fractions but did not markedly alter the distribution of tryptamine between the same fractions.     

Divalent Cation-dependent ATPase Activities Associated with Cilia and Other Subcellular Fractions of Paramecium: An Electrophoretic Characterization on Triton-polyacrylamide Gels1

Several divalent cation-dependent ATP phosphohydrolases associated with cilia, ciliary axonemes, ciliary membranes, pellicles, trichocysts, nuclei, mitochondria, microsomes, and soluble peripheral cell surface fractions of Paramecium tetraurelia were resolved in this study. Fifteen different activity bands were detected in whole cell sonicates or subcellular fractions by Triton polyacrylamide gel electrophoresis and ATPase activity staining. The ciliary surface membrane contained two major ATPase activities that were distinct from the enzymes associated with all other cell fractions.     

Protein Tyrosine Kinase Activity and Its Endogenous Substrates in Rat Brain: A Subcellular and Regional Survey

The rat CNS contains high levels of tyrosine-specific protein kinases that specifically phosphorylate the tyrosine-containing synthetic peptide poly(Glu80,Tyr20). The phosphorylation of this peptide is rapid and occurs with normal Michaelis-Menten kinetics. Using this peptide to assay for enzyme activity, we have measured the protein tyrosine kinase activity in homogenates from various regions of rat CNS. A marked regional distribution pattern was observed, with high activity present in cerebellum, hippocampus, olfactory bulb, and pyriform cortex, and low activity in the pons/medulla and spinal cord. The distribution of protein tyrosine kinase activity was examined in various subcellular fractions of rat forebrain. The majority of the activity was associated with the particulate fractions, with enrichment in the crude microsomal (P3) and crude synaptic vesicle (LP2) fractions. Moreover, the subcellular distribution of pp60csrc, a well-characterized protein tyrosine kinase, was examined by immunoblot analysis using an affinity-purified antibody specific for pp60csrc. The subcellular distribution of pp60csrc paralleled the overall protein tyrosine kinase activity. In addition, using an antibody specific for phosphotyrosine, endogenous substrates for protein tyrosine kinases were demonstrated on immunoblots of homogenates from the various regions and the subcellular fractions. The immunoblots revealed numerous phosphotyrosine-containing proteins that were present in many of the CNS regions examined and were associated with specific subcellular fractions. The differences in tyrosine-specific protein kinase activity, and in phosphotyrosine-containing proteins, observed in various regional areas and subcellular fractions may reflect specific functional roles for protein tyrosine kinase activity in mammalian brain.     

Subcellular organization of ureide biogenesis from glycolytic intermediates and ammonium in nitrogen-fixing soybean nodules

Subcellular organelle fractionation of nitrogen-fixing nodules of soybean (Glycine max (L.) Merr.) indicates that a number of enzymes involved in the assimilation of ammonia into amino acids and purines are located in the proplastids. These include asparagine synthetase (EC 6.3.1.1), phosphoribosyl amidotransferase (EC 2.4.2.14), phosphoglycerate dehydrogenase (EC 1.1.1.95), serine hydroxymethylase (EC 2.1.2.1), and methylene-tetrahydrofolate dehydrogenase (EC 1.5.1.5). Of the two isoenzymes of asparate aminotransferase (EC 2.6.1.1) in the nodule, only one was located in the proplastid fraction. Both glutamate synthase (EC 1.4.1.14) and triosephosphate isomerase (EC 5.3.1.1) were associated at least in part with the proplastids. Glutamine synthetase (EC 6.3.1.2) and xanthine dehydrogenase (EC 1.2.1.37) were found in significant quantities only in the soluble fraction. Phosphoribosylpyrophosphate synthetase (EC 2.7.6.1) was found mostly in the soluble fraction, although small amounts of it were detected in other organelle fractions. These results together with recent organelle fractionation and electron microscopic studies form the basis for a model of the subcellular distribution of ammonium assimilation, amide synthesis and uredie biogenesis in the nodule.Abbreviations FH₄ tetrahydrofolic acid - PRPP 5-phospho--D-ribose 1-pyrophosphate - PRPP synthetase ribosephosphate pyrophosphokinase (phosphoribosylpyrophosphate synthetase)     

Sex‐ and age‐related differences in c‐fos expression in dog olfactory bulbs

Sex‐ and age‐related differences in cognitive abilities are frequently reported. However, the sex‐ and age‐related differences in dog olfaction due to biological system are still poorly understood. We examined c‐fos expression in dog olfactory bulbs by immunohistochemistry approaches. The c‐fos is mainly expressed in the olfactory glomerular layer (GL), mitral cell layer (ML) and granule cell layer (GRL). We found that a higher density of c‐fos‐positive cells could be detected in the ML of olfactory bulbs of adult female dogs compared with that in males and the c‐fos‐positive cells in females' olfactory bulbs are more distinct. Sex‐related differences in c‐fos expression also appeared in the GL of olfactory bulbs in juvenile dogs. We also discovered that the density of c‐fos‐positive cells in the GRL of adult dogs was much higher than that in the GRL of juvenile dogs. Our results indicate that cells in the olfactory bulbs of female dogs are more active than those in males and female dogs may have much stronger ability for long‐time memory of odours than male dogs. Furthermore, our results also suggest that adult dogs may have much stronger ability for long‐time memory of odours and can deal with more complicated odour information than juvenile dogs.     

REGIONAL AND SUBCELLULAR DISTRIBUTION AND KINETIC PROPERTIES OF RAT BRAIN CHOLINE ACETYLTRANSFERASE–SOME FUNCTIONAL CONSIDERATIONS

The kinetic properties of soluble and membrane-bound choline acetyltransferase (ChAc) were determined as a function of homogenization media and solubilization procedure in various regions of rat brain. Treatment of homogenate and/or subcellular fractions with KCl, Triton X-100, or ether dramatically altered the apparent V_max and the degree of solubilization of the enzyme, but no fraction exhibited K_m values substantially different from 12 μM for acetyl-CoA and 200 μM for choline. On the other hand, increasing the ionic strength of the assay medium for a given fraction from 0-02 M to 0-5 M increased both V_max and K_m values for both substrates. The absolute levels and subcellular distribution of ChAc were determined in 11 brain regions to localize cholinergic cell bodies and nerve endings. Levels of ChAc varied from 139 m-units/g tissue in caudate-putamen to 5-7 m-units/g tissue in cerebellum. The fraction of ChAc activity associated with synaptosomes varied from near 75 per cent in caudate-putamen, hippocampus and cortical regions to near 20 per cent in septum, locus coeruleus area and substantia nigra area. The apparent parallel distribution of cholinergic and catecholaminergic nerve endings is discussed in terms of a hypothetical model for the pathophysiology and treatment of Parkinson's syndrome.     

Subcellular Distribution of Glutamate Decarboxylase in Rat Olfactory Bulb: High Content in Dendrodendritic Synaptosomes

: The olfactory bulbs in the CNS contain reciprocal dendrodendritic synapses between the granule cells and the secondary dendrites of mitral cells. Based on pharmacologic and electrophysiologic evidence, these synapses are believed to utilize GABA as an inhibitory neurotransmitter. A dendrodendritic synaptosomal fraction has been isolated from rat olfactory bulbs. The upper portion (P_B) of the crude nuclear pellet contains 30–40% of the GAD (glutamate decarboxylase) activity of the olfactory bulb homogenate. When P_B is purified on a discontinuous sucrose density gradient, 78–85% of the GAD activity is localized to the region containing the dendrodendritic synaptosomes, which were identified by transmission electron microscopy. The presence of a substantial proportion of GAD, the enzyme that catalyzes synthesis of GABA, in the DDS provides neurochemical support for the hypothesis that GABA functions at the reciprocal dendrodendritic synapses in the olfactory bulb.     

Luteinizing hormone-releasing hormone in olfactory bulbs of primates

This immunohistochemical study of luteinizing hormone-releasing hormone (LHRH) in the olfactory bulbs in primates was undertaken in order to see whether there was an LHRH innervation in these species similar to that found in rodents. One old world (Macaca fascicularis) and two new world (Saimiri sciureus and Aotus trivirgatus) monkeys were studied. Aotus trivirgatus was of particular interest as it is noctural and so presumably more dependent upon olfactory cues. Animals were perfused with fixative, olfactory bulbs removed and sectioned, and tissues reacted immunocytochemically using LR1 (Benoit) antiserum to LHRH. Some LHRH innervation was found in the olfactory bulbs of all three species, comprising a few LHRH neurons and many fibers that ramified within the bulbs. The accessory bulb (not present as a distinct entity in old world primates) had more LHRH innervation than did the main olfactory bulb. Aotus trivirgatus had the greatest representation of LHRH of the three species. The layer of the olfactory bulb with the greatest number of LHRH fibers was the external plexiform layer. This is also true in rodents. There is evidence that LHRH has a role in the mediation of olfactory cues in reproductive behavior in rodents. It is not known how LHRH functions within the olfactory system in primates. However, the fact that it is distributed similarly in the two groups suggests that it may serve a similar function.