Some evidence for the maturity of peripheral adrenergic nerves in new-born guinea-pigs.

The fluorescence histochemical technique of Falck and Hillarp revealed a similar distribution and density of peripheral adrenergic nerves in new-born and adult guinea-pigs. The accumulation of tritiated noradrenaline by tracheae from new-born guinea-pigs, assumed to be uptake into adrenergic nerves, was not less than the accumulation by tracheae from adult animals. There was equal potentiation by cocaine (1 x 10(-5)M) of responses to noradrenaline on tracheal chain preparations taken from new-born and adult guinea-pigs. The evidence supports the hypothesis that the guinea-pig has a functional, well differentiated peripheral adrenergic nervous system at birth. This would account for the apparent inability to produce a long-lasting sympathectomy by administration of 6-hydroxydopamine to new-born guinea-pigs.     

A Comparison of the Distribution of Neurotransmitters in Brain Regions of the Rat and Guinea-Pig Using a Chemiluminescent Method and HPLC with Electrochemical Detection

Six brain areas of rats and guinea-pigs, killed by microwave irradiation, were used for the concomitant measurement of the levels and regional distribution of cholinergic, biogenic amine, and amino acid neurotransmitters and metabolites. Acetylcholine (ACh) and choline (Ch) were quantified by chemiluminescence; noradrenaline (NA), dopamine (DA), 5-hydroxytryptamine (5-HT), and their metabolites by HPLC with electrochemical detection (HPLC-EC); and six putative amino acid neurotransmitters by HPLC-EC following derivatisation. The levels and regional distribution of these transmitters and their metabolites in the rat were similar to those reported in previous studies, except that biogenic amine transmitter levels were higher and metabolite concentrations were lower. The guinea-pig showed a similar regional distribution, but the absolute levels of ACh were lower in striatum and higher in hippocampus, midbrain-hypothalamus, and medulla-pons. In all areas, the levels of Ch were higher and those of NA, 5-HT, and taurine were lower than in the rat. The most marked differences between the rat and guinea-pig were in the relative proportion of DA metabolites and 5-HT turnover, as estimated by metabolite/transmitter ratios. This study can be used as a basis for a comprehensive understanding of the central effects of drugs on the major neurotransmitter systems.     

Capillary-venous differences of free plasma catecholamines at rest and during graded exercise

Levels of free plasma catecholamines were simultaneously determined in 10 cyclists using capillary blood from one ear lobe and venous blood from one cubital vein. Catecholamine concentrations were higher in the ear lobe blood than in the venous blood at rest and during graded exercise. Average differences amounted to 1.7 nmol X 1(-1) (dopamine), 2.1 nmol X 1(-1) (noradrenaline) and 1.9 nmol X 1(-1) (adrenaline) at rest and increased only to 8.8 nmol X 1(-1) for noradrenaline during exercise. We assume that higher concentrations of dopamine and adrenaline in the capillary blood point to a significant neuronal release of these catecholamines, similar to noradrenaline. Catecholamine concentrations in capillary blood may better reflect sympathetic drive and delivery of catecholamines to the circulation than the concentrations in venous blood.     

Comparison of glycogen store in two strains of rat and guinea-pig under fed and fasted conditions

Glycogen content in the liver, skeletal muscle and heart has been determined in Sprague-Dawley (SD) and Wistar (W) rats and in tricoloured (T) and albino Dunkin Hartley (DH) guinea-pigs. The 12-week-old animals were studied under non-fasted or control conditions (N) and after 48 hr of fast (F48). Hepatic glycogen was higher in DH guinea-pigs (95.6 +/- 3.8 mg g-1) than in W (77.2 +/- 5.3 mg g-1) and SD (80.2 +/- 2.3 mg g-1) rats under N conditions. Mean values for the two strains were slightly higher in guinea-pigs than in rats. After fasting, hepatic glycogen was almost exhausted in the two species but was higher in W (1.5 +/- 0.08 mg g-1) and T (1.5 +/- 0.2 mg g-1) than in SD and DH (0.6 +/- 0.1 mg g-1). The content of glycogen in the anterior muscles of the thigh was comparable in the two strains of rat and guinea-pig, but was twice as high in the guinea-pigs (DH:15.1 +/- 0.6; T: 16.4 +/- 0.7 mg g-1) as in the rats (SD: 8.1 +/- 0.2; W: 7.1 +/- 0.5 mg g-1) under N conditions. In F48 animals, muscular glycogen decreased by 41-46% (rats) and 38-39% (guinea-pigs). Hepatic and extra-liver glycogen stores were calculated and found higher in the guinea-pigs than in the rats. The total utilization during fasting was larger in the guinea-pigs (6140 mg/kg body wt) than in the rats (4500 mg/kg body wt).(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of adaptation to intermittent high altitude hypoxia on ischemic ventricular arrhythmias in rats

We compared the effects of adaptation to intermittent high altitude (IHA) hypoxia of various degree and duration on ischemia-induced ventricular arrhythmias in rats. The animals were exposed to either relatively moderate hypoxia of 5000 m (4 or 8 h/day, 2-3 or 5-6 weeks) or severe hypoxia of 7000 m (8 h/day, 5-6 weeks). Ventricular arrhythmias induced by coronary artery occlusion were assessed in isolated buffer-perfused hearts or open-chest animals. In the isolated hearts, both antiarrhythmic and proarrhythmic effects were demonstrated depending on the degree and duration of hypoxic exposure. Whereas the adaptation to 5000 m for 4 h/day decreased the total number of premature ventricular complexes (PVCs), extending the daily exposure to 8 h and/or increasing the altitude to 7000 m led to opposite effects. On the contrary, the open-chest rats adapted to IHA hypoxia exhibited an increased tolerance to arrhythmias that was even more pronounced at the higher altitude. The distribution of PVCs over the ischemic period was not altered by any protocol of adaptation. It may be concluded that adaptation to IHA hypoxia is associated with enhanced tolerance of the rat heart to ischemic arrhythmias unless its severity exceeds a certain upper limit. The opposite effects of moderate and severe hypoxia on the isolated hearts cannot be explained by differences in the occluded zone size, heart rate or degree of myocardial fibrosis. The proarrhythmic effect of severe hypoxia may be related to a moderate left ventricular hypertrophy (27 %), which was present in rats adapted to 7000 m but not in those adapted to 5000 m. This adverse effect can be overcome by an unknown protective mechanism(s) that is absent in the isolated hearts.     

Regional changes in catecholamine content of the pregnant uterus

High-pressure liquid chromatography with electrochemical detection was used to identify and measure catecholamines in rat, rabbit, sheep, guinea-pig and human uteri and follow changes with pregnancy. Noradrenaline was consistently the major catecholamine and pregnancy caused a regionally specific fall in its concentration which, in rat, rabbit and guinea-pig, was associated with a decline in total content. Adrenaline was undetectable (less than 10 pmol/g myometrium) in all species and at all gestational ages studied. Dopamine and its metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) were detected at high concentrations in guinea-pig and particularly sheep uterus. In guinea-pig uterus the dopamine/DOPAC ratio fell dramatically with pregnancy, suggesting that increased quantities of dopamine were released and catabolized. The dopamine/noradrenaline ratios suggested that dopamine is stored with noradrenaline in adrenergic neurones in guinea-pig myometrium and within an additional neuronal or cellular store(s) in sheep uterus.     

Ultrastructure of the gastrointestinal epithelium in the marmoset (Callithrix jacchus)

The epithelium of the stomach and intestine of one and three days old as well as adult marmosets was investigated using transmission electron microscopy and was compared with already existing data of man and the laboratory rodents, rat, mouse and guinea-pig. On postnatal day (PD) 1, the enterocytes possessed an inframicrovillous membrane system and giant lysosomes which were absent in adult marmosets whereas the surface and glandular epithelial cells of the stomach showed all structures which were also typical for adult animals. Enterocytes rich in fat and glycogen were only present on PD 1. In the large intestine the vacuolated cells were more frequently seen on PD 1 and 3 than in adult marmosets. The endocrine cells of newborn animals corresponded to those in the gastric and intestinal epithelium of mature animals, occurred everywhere in the lower digestive tract and could be subdivided at least into EC, ECL, D and L and EG cells respectively; a further subdivision was not possible by conventional transmission electron microscopy. Compared with rats, mice and guinea-pigs mostly used for developmental studies of the digestive tract, marmoset monkeys differed especially from the gastrointestinal epithelium of rats and mice but also from guinea-pigs. By contrast comparisons with the human situation are difficult due to the lack of representative electron microscopic findings on the gastrointestinal epithelium. If one considers the close phylogenic relationship between marmosets and man, the marmoset data should be transferable to the human situation rather than the findings obtained for rats, mice and guinea-pigs. In the epithelium of the adult gastrointestinal tract clear-out ultrastructural differences could not be found between these species.     

Lack of cardiac alpha 1-adrenoceptor involvement in ethanol-induced cardiac hypertrophy

The development of cardiac hypertrophy was examined in rats given ethanol in a nutritionally adequate, liquid diet mixture, by intubation, in severely intoxicating doses at 8-h intervals for up to 96 h, alone or in combination with prazosin. Other groups of rats received isocalorically paired quantities of maltose-dextrin. Adrenal glands of rats receiving ethanol were larger than those from control animals. Prazosin did not affect this measure. In contrast, concurrent treatment with prazosin enhanced the loss of medullary catecholamines and noradrenaline from hearts of rats given ethanol, while it had no such effects in controls. Reflecting these changes, excreted quantities of catecholamines were markedly increased in rats given ethanol and prazosin. Hearts of animals given the combined treatment of ethanol and prazosin showed cardiomegaly at 24 h, when there was an increase of about 20% in proportional heart weight, an increase that persisted through the remaining 3 days of the study. At 48 h, hearts of animals given prazosin and ethanol were heavier than those given ethanol alone. A significant correlation between catecholamine excretion and the development of cardiac hypertrophy was identified. The results of the study show that prazosin can enhance effects of ethanol on the peripheral sympathetic nervous system. Moreover, the results suggest that postsynaptic alpha 1-adrenoceptor stimulation in the heart is not an important contributor to ethanol-induced cardiomegaly.     

Vascular response in a non-uterine site to implantation-stage embryos following interspecies transfers between the rat,mouse, and guinea-pig

Summary Pre-implantation-stage embryos from rats, mice, and guinea-pigs were transferred to a non-uterine site — the anterior chamber of the eye — of female recipients. All 9 combinations of transfers were performed: 3 allogeneic (intraspecies) transfers as controls, and 6 xenogeneic (interspecies) transfers. Implantation, as judged by extravasation from blood vessels of the iris or ciliary body occurred with success rates of 90.4% per transfer in the control rat group, 76.9% in the control mouse group, and 81.8% in the control guinea-pig group. Significantly reduced implantation rates occurred in the rat to guinea-pig (0%), mouse to rat (46.9%), mouse to guinea-pig (6.7%), and guinea-pig to rat (0%) groups compared to controls. Reductions, although not significant, also occurred in the other 2 groups: rat to mouse (77.8%), and guinea-pig to mouse (44.4%). These results together with some ultrastructural and lightmicroscopical observations suggest a degree of species specificity involved in the vascular response to the implanting embryo. We propose that the peri-implantation embryo produces a signal(s) which is to some extent species specific and which in the normal allogeneic situation is responsible for the early vascular effects seen at implantation in most eutherian mammals.     

Catecholamine levels in whole brain of stressed and control domestic and wild rats (Rattus Norvegicus)

Whole brain catecholamine (dopamine, nonadrenaline, adrenaline) levels were measured in control and electric footshocked Wild and domestic (Sprague Dawley and Long Evans) rats. No significant differences were found among the three strains of rats for combined total catecholamine content, or for combined total catecholamines between the control and footshocked groups. Significant differences were found for the total of each catecholamine taken separately, dopamine being present at three times the level of noradrenaline and ten times the level of adrenaline. No significant differences were found for dopamine in both control and footshocked animals among all three rat strains. Noradrenaline was significantly higher in the control domestic rats compared to the control Wild rats, and in the footshocked domestic rats compared to the footshocked Wild rats. No differences in noradrenaline levels were found between Sprague Dawley and Long Evans rats, but noradrenaline increased significantly in the latter following footshock. Adrenaline was significantly highest in the Sprague Dawley controls and lowest in the Wild controls. Footshocking resulted in almost identical levels of adrenaline in the domestic strains and an increase in the F₁ Wild strain.     

Effects of a newly synthesized leukotriene antagonist, NZ-107, on immediate-type hypersensitivity reaction in rats and guinea-pigs.

The effects of 4-bromo-5-(3-ethoxy-4-methoxybenzylamino)-3(2H)-pyridazinone (NZ-107) on immediate type hypersensitivity reactions in rats and guinea-pigs were studied. 1. When NZ-107, at a dose of 50 mg/kg (i.p.) or 100 mg/kg (orally), was administered to rats, 48-h homologous passive cutaneous anaphylaxis (PCA) reaction and histamine-, leukotriene C4 (LTC4)- and leukotriene D4 (LTD4)-induced skin reactions were suppressed by the agent. 2. NZ-107 (10(-6) g/ml) inhibited both LTC4- and LTD4-induced contractions of isolated rat stomach smooth muscle. 3. NZ-107 inhibited antigen-induced histamine release from rat peritoneal mast cells by 26% at a concentration of 10(-4) g/ml. 4. NZ-107, at doses of 25 and 50 mg/kg (orally), significantly inhibited guinea-pig 3-h heterologous PCA reaction. 5. NZ-107 inhibited antigen-induced histamine release from guinea-pig lung tissue by 17% and 48% at concentrations of 5 x 10(-5) and 10(-4) g/ml, respectively. 6. NZ-107, at doses of 25 and 50 mg/kg (i.p.), inhibited antigen-induced bronchoconstriction and eosinophil accumulation in the bronchoalveolar lavage fluid (BALF) of guinea-pigs. These results suggest that NZ-107 has anti-allergic action including inhibition of eosinophil accumulation in an antigen-challenged airway lesion in rats and guinea-pigs. The anti-allergic action of this agent is thought to be due to its action as a histamine and LT antagonist and its consequent inhibition of antigen-induced histamine release.     

Neuropeptide tyrosine (NPY)-induced potentiation of the pressor activity of catecholamines in conscious rats

IV bolus administration of 2.5-50 micrograms NPY (0.6-12.5 nmol) to conscious rats produced a dose- and time-dependent increase in systolic and diastolic blood pressure. Following priming with 2.5 micrograms NPY, or larger doses, the subsequent administrations of noradrenaline produced pressor responses that were potentiated both in magnitude and duration. The NPY-induced potentiation of the pressor response to noradrenaline was dose-dependent and extended to the pressor action of adrenaline and angiotensin II but not to the hypotensions produced by bradykinin or isoproterenol. The potentiation was not related to the fact that multiple doses of catecholamines were repeated. Reserpine did not substantially modify the NPY-induced potentiation of the pressor activity of the catecholamines. Chemical sympathectomy following 6-hydroxydopamine caused a marked supersensitivity to the catecholamines and NPY but obliterated the NPY-induced potentiation of the pressor effect of adrenaline. Nifedipine reduced the pressor action of the catecholamines and NPY but did not attenuate the NPY-induced potentiation of the pressor action of catecholamines. It is concluded that the acute pressor effect of NPY and of the potentiation of the catecholamine pressor effects involve different mechanisms.     

Distribution and function of biogenic amines in the heart of <Emphasis Type="Italic">Nautilus pompilius</Emphasis> L. (Cephalopoda,Tetrabranchiata)

Summary Biogenic amines (serotonin and catecholamines), play an important role in the control of the blood flow not only in vertebrates, but also in invertebrates such as cephalopods. In contrast to the well investigated hearts of the ȁ8modern,ȁ9 coleoid cephalopods, the innervation of the heart of the archaic Nautilus pompilius L. has not been studied in detail. In this study the distribution and effects of biogenic amines in the Nautilus heart were investigated. Serotonin and catecholamines were visualised by the glyxoylic acid induced fluorescence. High performance liquid chromatotography analysis was performed to discriminate between the catecholamines, which showed a high content of noradrenaline in the 4 auricles, the aorta and the ventricle, whereas the ventricle showed a high dopamine content. Adrenaline was found at a very low concentration in the ventricle. Serotonin and dopamine were also immunohistochemically localised to larger nerves and throughout the heart, respectively. In organ bath experiments, the auricles showed little spontaneous activity. After adding serotonin, they displayed rhythmical contractions, which were accelerated dose-dependently by noradrenaline. In summary, these data suggest an important role for biogenic amines in the control of the heart of Nautilus pompilius L., with serotonin possibly stimulating excitatory nerve fibres, whereas noradrenaline is likely to influence the muscle contraction itself.     

Adrenergic innervation of the umbilical vessels

Summary The intra- and extrafetal portions of the umbilical vessels in the guinea-pig and the umbilical cord of man, mouse and rabbit have been investigated by means of the Falck-Hillarp method for the fluorescence microscopical demonstration of catecholamines.The umbilical cord was found to be devoid of nerves in all species investigated. Adrenergic nerves are present only in the immediate vicinity of the umbilicus.The intrafetal portions of the umbilical artery and umbilical vein receive adrenergic nerves, the distribution pattern of which is different for each vessel. In the guinea-pig the ductus venosus is an intrahepatic branch of the vena umbilicalis. No adrenergically innervated sphincter has been detected in the initial segment of the ductus venosus. Regional variations in the pattern of innervation of the intrafetal portion of the umbilical vein are paralleled by regional differences in the construction pattern of the vessel's wall. Regional differences in the noradrenaline concentration (measured by fluorometry) which correspond to the fluorescence microscopical findings have been detected in umbilical vessels: low noradrenaline content of the umbilical cord, high concentrations in the intrafetal sections of the umbilical vessels. The noradrenaline concentration of the guinea-pig umbilical artery is three times that of the umbilical vein.Supported by the Joachim Jungius-Gesellschaft der Wissenschaften, Hamburg.For continuous advice and constructive criticism I am indebted to Prof. Dr. Dr. E. Horstmann.     

Identification of a calcium binding protein highly localized to the cochlear nucleus

A protein, designated as protein 10 (MW = 29 kDa, pI = 5.3), was shown to be highly localized to the cochlear nuclei of guinea-pigs and rats by the use of two-dimensional gel electrophoresis. In gels from guinea-pig brain, this protein appeared greatest in amounts in the ventral cochlear nucleus and within the deep regions of the dorsal nucleus. Intermediate amounts of protein 10 were found in the lateral superior olivary nucleus and dorsal nucleus of the lateral lemniscus with lesser amounts in telencephalic brain regions. Finally, incubation of nitrocellulose blots in ⁴⁵Ca²⁺ revealed that protein 10 is a calcium binding protein.     

Bile formation and hepatic plasma membrane composition in guinea-pigs and rats

We compared bile formation, and biliary and liver plasma membrane composition in guinea-pigs and rats in an attempt to explain the observation that the bile flow rate and the bile acid independent fraction of bile flow (BAIF) in guinea-pigs is about five to seven times higher than in rats. Analysis of electrolytes in bile showed that bicarbonate was significantly [acid] higher in guinea-pigs while Cl⁻, phosphate and Ca²⁺ were markedly lower than in rats. High bile independent secretion in guinea-pigs was associated with a significantly lower concentration of total bile acid, phospholipid and cholesterol than in rats. Bile acid distribution studies showed that glycine conjugated chenodeoxycholate and ketolithocholate were the main bile acids in guinea-pigs, while taurine conjugated cholate and muricholate were the predominant bile acids in rats. Total fatty acid analysis of bile indicated that in rats the major fatty acids were palmitic acid (C16:0) and linoleic acid (C18:2, n-6). In guinea-pigs, the contribution of these fatty acids was lower than in rats and compensated with a significantly higher percentage of oleic acid (C18:1, n-9). Concentrations of anionic polypeptide fraction (APF), an acidic calcium binding apoprotein closely associated with biliary phospholipid and cholesterol secretion was also significantly lower in guinea-pigs. Canalicular plasma membrane analysis showed that as compared with rats, specific activities of Na⁺,K⁺ ATPase, and cholesterol and phospholipid content were markedly lower in guinea-pigs. Total fatty acid analysis of the membrane revealed that palmitic acid (C16:0), stearic acid (C18:0) and linoleic acid (C18:2, n-6) were the predominant fatty acids in guinea-pigs, while palmitic acid (C16:0), stearic acid (C18:0) and arachidonic acid (C20:4, n-6) were the most important in rats. Thus, high bile flow rate and BAIF in the guinea-pig may be attributed to the low bile acid concentration (below the critical micellar concentration), secretion of hypercholeretic bile acids (e.g. ketolithocholate) and high bicarbonate output.     

The cardiac effects of amitraz in the guinea-pig in vivo and in vitro

Intravenous amitraz caused significant hypotension and bradycardia in pentobarbitone anaesthetized guinea-pigs. Depression of blood pressure reached a plateau with a dose of 10 mg/kg but heart rate continued to fall in a dose-dependent manner, up to a fall of 90 beats per minute after a total of 160 mg/kg/min. Amitraz was then tested on spontaneously beating guinea-pig isolated atria. The maximum bath concentration approximated a blood concentration produced by 5 mg/kg amitraz in the guinea-pig (2.3 X 10(-4) M). Amitraz did not significantly shift the dose-response curve to isoprenaline or acetylcholine but antagonized histamine rate responses competitively in the presence of propranolol (2 X 10(-6) M). Propranolol unmasked a dose-dependent depressant effect of amitraz on atrial rate, an effect abolished with atropine (1 X 10(-5) M). Amitraz increased atrial force of contraction, an effect which was not seen when propranolol was present in the bath solution. Amitraz also depressed atrial rate directly, but this effect was minor in comparison to bradycardia seen in the guinea-pig. It is likely that the cardiovascular depression seen in the guinea-pig following amitraz i.v. is caused by an alteration in autonomic drive rather than a significant direct cardiac effect.     

Effect of nitrobenzylthioinosine accumulated by the heart in rats and guinea pigs on the release of adenine nucleotide breakdown products during ischemia and reperfusion

The uptake kinetics of nitrobenzyl thioinosine (NBTI), a nucleoside transport inhibitor, was studied in the isolated Langendorf-perfused guinea pig and rat hearts. In rats the rate constant of NBTI uptake was higher and the extent of NBTI accumulation was less than in guinea pig hearts. Heart-accumulated NBTI inhibited the total release of adenine nucleotide degradation products (ANDP) during reperfusion 25 min after global ischemia. The effect was more pronounced in guinea-pig hearts-in accordance with observed higher myocardial concentration of NBTI. Unlike other ANDP, the release adenosine by guinea-pig hearts was unchanged and that by rat hearts increased. In spite of significant NBTI-induced decrease of ANDP losses recovery of contractile function during reperfusion was not observed to improve.     

Effects of mibefradil, a blocker of T-type Ca2+ channels, in single myocytes and intact muscle of guinea-pig heart.

We investigated the effects of a relatively selective blocker of the T-type Ca2+ channels, mibefradil (MBF), in the isovolumic left ventricles of the isolated, perfused hearts of guinea-pigs and single myocytes isolated from the ventricles of this species. In the myocytes superfused with 0 Na+ solution containing 200 microM lidocaine and pulsed from -90 mV to -40 mV to +5 mV, MBF proved to be about 3 times more potent inhibitor of the T-type than of the L-type Ca2+ current. The effect on the L-type current was strongly voltage and use dependent. In the ventricles and in the myocytes contraction was reduced by 50% by about 1 microM MBF, the concentration 12 times higher than this increasing the coronary flow by 50%. In myocytes the decrease in unloaded shortening paralleled inhibition of the T-type rather, than of the L-type Ca2+ current. Inhibition of electrically stimulated contraction of the myocytes was three times stronger than inhibition of the caffeine contractures regarded as an index of sarcoplasmic reticulum (SR) Ca2+ content. These findings are consistent with the hypothesis that the T-type Ca2+ channels may contribute to release of Ca2+ from the SR. It is concluded that MBF has a definite negative inotropic effect in the ventricular myocardium of guinea-pig heart at the concentrations found in the blood of the patients submitted to the clinical trials.     

Localization of noradrenaline and serotonin in nerves in the pineal gland of rats and guinea-pigs studied by glyoxylic acid histofluorescence and electron microscopy

Summary The nerves in the pineal gland of the rat and guinea-pig contain both noradrenaline and serotonin and fluoresce intensely after histofluorescence procedures. Vesicle-filled terminals in the perivascular space of the pineal body contain numerous clear and dense-cored vesicles. A 5mg/kg dose of reserpine causes disappearance of histofluorescence from the pineal nerves and a virtual elimination of dense-cored vesicles from vesicle-filled terminals. A 1mg/kg dose of reserpine results in loss of fluorescence and virtual depletion of dense cores in nerves in the rat, but the guinea-pig pineal nerves continue to fluoresce lightly and the dense-cored vesicles are still present but reduced to about 1/3 in number. Subsequent treatment of lightly reserpinized guinea-pigs withp-chlorophenylalanine, a specific depletor of serotonin, results in dis ppearance of fluorescence in nerves in the pineal gland and virtual depletion of the remaining dense cores. A dose of 1mg/kg reserpine succeeds in depleting noradrenaline from most peripheral nervous structures of the guinea-pig. Hence, the remaining monoamine in guinea-pig pineal nerves after depletion of noradrenaline appears to be serotonin located in the remaining dense-cored vesicles. Since, in lightly reserpinized guinea-pig pineal nerves, a number of dense-cored vesicles containing serotonin are still present after depletion of noradrenaline, it is suggested that noradrenaline and serotonin are not in the same vesicles at the same time.