Tyrosine hydroxylase containing neurons lacking aromatic amino acid decarboxylase in the hamster brain

Brain neurons partly expressing dopaminergic phenotype: location, development, functional significance, and regulation

In addition to catecholaminergic neurons possessing all the enzymes of catecholamine synthesis and the specific membrane transporters, neurons partly expressing the catecholaminergic phenotype have been found a quarter of a century ago. Most of them express individual enzymes of dopamine (DA) synthesis, tyrosine hydroxylase (TH), or aromatic l-amino acid decarboxylase (AADC), lacking the DA membrane transporter and the vesicular monoamine transporter, type 2. These so-called monoenzymatic neurons are widely distributed throughout the brain in ontogenesis and adulthood being in some brain regions even more numerous than dopaminergic (DA-ergic) neurons. Individual enzymes of DA synthesis are expressed in these neurons continuously or transiently in norm and pathology. It has been proven that monoenzymatic TH neurons and AADC neurons are capable of producing DA in cooperation. It means that l-3,4-dihydroxyphenylalanine (l-DOPA) synthesized from l-tyrosine in monoenzymatic TH neurons is transported to monoenzymatic AADC neurons for DA synthesis. Such cooperative synthesis of DA is considered as a compensatory reaction under a failure of DA-ergic neurons, for example, in neurodegenerative diseases like hyperprolactinemia and Parkinson's disease. Moreover, l-DOPA, produced in monoenzymatic TH neurons, is assumed to play a role of a neurotransmitter or neuromodulator affecting the target neurons via catecholamine receptors. Thus, numerous widespread neurons expressing individual complementary enzymes of DA synthesis serve to produce DA in cooperation that is a compensatory reaction at failure of DA-ergic neurons.

L-amino acid decarboxylase- and tyrosine hydroxylase-immunoreactive cells in the extended olfactory amygdala and elsewhere in the adult prairie vole brain

Neurons synthesizing dopamine (DA) are widely distributed in the brain and implicated in a tremendous number of physiological and behavioral functions, including socioreproductive behaviors in rodents. We have recently been investigating the possible involvement of sex- and species-specific TH-immunoreactive (TH-ir) cells in the male prairie vole (Microtus ochrogaster) principal bed nucleus of the stria terminalis (pBST) and posterodorsal medial amygdala (MeApd) in the chemosensory control of their monogamous pairbonding and parenting behaviors. These TH-ir cells are not immunoreactive for dopamine-beta-hydroxylase (DBH), suggesting they are not noradrenergic but possibly DAergic. A DAergic phenotype would require them to contain aromatic L-amino acid decarboxylase (AADC) and here we examined the existence of cells immunoreactive for both TH and AADC in the pBST and MeApd of adult virgin male and female prairie voles. We also investigated the presence of TH/AADC cells in the anteroventral periventricular nucleus (AVPV), medial preoptic area (MPO), arcuate nucleus (ARH), zona incerta (ZI), substantia nigra (SN) and ventral tegmental area (VTA). Among our findings were: (1) the pBST and MeApd each contained completely non-overlapping distributions of TH-ir and AADC-ir cells, (2) the AVPV contained surprisingly few AADC-ir cells and almost no TH-ir cells contained AADC-ir, (3) approximately 60% of the TH-ir cells in the MPO, ARH, and ZI also contained AADC-ir, (4) unexpectedly, only about half of TH-ir cells in the SN and VTA contained AADC-ir, and (5) notable populations of AADC-ir cells were found outside traditional monoamine-synthesizing regions, including some sites that do not contain AADC-ir cells in adult laboratory rats or cats (medial septum and cerebral cortex). In the absence of the chemical requirements to produce DA, monoenzymatic TH-ir cells in the virgin adult prairie vole pBST, MeApd, and elsewhere in their brain may instead produce L-DOPA as an end product and use it as a neurotransmitter or neuromodulator, similar to what has been observed for monoenzymatic TH-synthesizing cells in the laboratory rat brain.

Non-dopaminergic neurons partly expressing dopaminergic phenotype: distribution in the brain, development and functional significance

Besides the dopaminergic (DA-ergic) neurons possessing the whole set of enzymes of DA synthesis from l-tyrosine and the DA membrane transporter (DAT), the neurons partly expressing the DA-ergic phenotype have been first discovered two decades ago. Most of the neurons express individual enzymes of DA synthesis, tyrosine hydroxylase (TH) or aromatic l-amino acid decarboxylase (AADC) and lack the DAT. A list of the neurons partly expressing the DA-ergic phenotype is not restricted to so-called monoenzymatic neurons, e.g. it includes some neurons co-expressing both enzymes of DA synthesis but lacking the DAT. In contrast to true DA-ergic neurons, monoenzymatic neurons and bienzymatic non-dopaminergic neurons lack the vesicular monoamine transporter 2 (VMAT2) that raises a question about the mechanisms of storing and release of their final synthetic products. Monoenzymatic neurons are widely distributed all through the brain in adulthood being in some brain regions even more numerous than DA-ergic neurons. Individual enzymes of DA synthesis are expressed in these neurons continuously or transiently in norm or under certain physiological conditions. Monoenzymatic neurons, particularly those expressing TH, appear to be even more numerous and more widely distributed in the brain during ontogenesis than in adulthood. Most populations of monoenzymatic TH neurons decrease in number or even disappear by puberty. Functional significance of monoenzymatic neurons remained uncertain for a long time after their discovery. Nevertheless, it has been shown that most monoenzymatic TH neurons and AADC neurons are capable to produce l-3,4-dihydroxyphenylalanine (L-DOPA) from l-tyrosine and DA from L-DOPA, respectively. L-DOPA produced in monoenzymatic TH neurons is assumed to play a role of a neurotransmitter or neuromodulator acting on target neurons via catecholamine receptors. Moreover, according to our hypothesis L-DOPA released from monoenzymatic TH neurons is captured by monoenzymatic AADC neurons for DA synthesis. Such cooperative synthesis of DA is considered as a compensatory reaction under a failure of DA-ergic neurons, e.g. in neurodegenerative diseases like hyperprolactinemia and Parkinson's disease.Thus, a substantial number of the brain neurons express partly the DA-ergic phenotype, mostly individual complementary enzymes of DA synthesis, serving to produce DA in cooperation that is supposed to be a compensatory reaction under the failure of DA-ergic neurons.

Tyrosine hydroxylase positive neurons and their contacts with vasoactive intestinal polypeptide-containing fibers in the hypothalamus of the diurnal murid rodent, Arvicanthis niloticus

Diurnal and nocturnal animals differ with respect to the timing of a host of behavioral and physiological events including those associated with neuroendocrine functions, but the neural bases of these differences are poorly understood. In nocturnal species, rhythms in tyrosine hydroxylase-containing (TH+) neurons in the hypothalamus appear to be responsible for rhythms in prolactin secretion. Here we investigated TH+ cells in a diurnal rodent (Arvicanthis niloticus, the unstriped Nile grass rat), and comparing them with those of a nocturnal rodent (Rattus norvegicus, Sprague-Dawley rat). We also examined relationships between TH+ cells and fibers containing vasoactive intestinal polypeptide (VIP) that are thought to originate from cells in the suprachiasmatic nucleus (SCN), the site of the primary circadian clock in mammals. The distribution of TH+ neurons was very similar in the two species except for a population of cells in the basal forebrain that was only present in grass rats. Fibers containing VIP appeared to contact neuroendocrine TH+ cells in both species. These data suggest that, though there may be subtle species differences, temporal information is likely to be carried along the same direct pathways from the SCN to the TH+ neurons in day- and night-active species.

Prenatal cocaine exposure accelerates morphological changes and transient expression of tyrosine hydroxylase in the cochlea of developing rats

Prenatal cocaine exposure causes alterations in auditory brainstem response in children and experimental animals and has adverse effects on auditory information processing and language skills in children. These effects may result from lesions in the cochlea since this organ is particularly sensitive to chemical insults during the development. We have thus studied here the effect of prenatal cocaine exposure on the maturation of the rat cochlea using the transient non-catecholaminergic expression of tyrosine hydroxylase in spiral ganglion neurons as an index of cochlear maturation and morphometry to evaluate the maturation of primary auditory neurons and the organ of Corti. We showed that prenatal cocaine exposure accelerated the cochlear maturation. In the basal coil of cochleas from PND8 cocaine-treated pups, the Kölliker's organ had disappeared, the tunnel of Corti was opened, and the stria vascularis no longer contained undifferentiated marginal cells. The maximum expression of tyrosine hydroxylase in type I primary auditory neurons occurred at PND8 instead of PND12 in pair-fed controls. On the other hand, the prenatal cocaine exposure had no effect on the width and height of the organ of Corti, spiral ganglion volume and number and size of primary auditory neurons. In conclusion, our data suggest that prenatal cocaine exposure, though not lethal to primary auditory neurons, accelerates aspects of the cochlear sensorineural maturation. This accelerated cochlear maturation in cocaine-treated rat pups could cause auditory dysfunctions by desynchronizing the development of the whole auditory pathway.

Catecholamine-independent transient expression of tyrosine hydroxylase in primary auditory neurons is coincident with the onset of hearing in the rat cochlea

During the last stages of neuronal maturation, tyrosine hydroxylase is transiently expressed in the absence of the other catecholamine-synthesizing enzymes. We show here that it is expressed in rat spiral ganglion neurons between postnatal days 8 and 20, with a peak of expression at postnatal day 12. These tyrosine hydroxylase-immunoreactive neurons did not display aromatic amino acid decarboxylase- or dopamine-beta-hydroxylase-immunoreactivities, ruling out the possibilities of dopamine or noradrenaline synthesis. They also did not display peripherin- or intense neurofilament 200-kDa-immunoreactivities, two indicators of type II primary auditory neurons. Tyrosine hydroxylase-immunoreactive dendrites were seen in synaptic contact with the inner hair cells and expressed the GluR2 subunit of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors, further confirming the type I nature of the neurons transiently expressing the enzyme. The end of the tyrosine hydroxylase expression was not due to cell death because the immunoreactive neurons did not show TUNEL-labelled nuclei. Finally, all the type I neurons expressed the tyrosine hydroxylase mRNA at postnatal day 12, suggesting that the expression of the enzyme is a maturational step common to all these neurons and that the expression of the protein is not synchronized. Because the period of transient expression of tyrosine hydroxylase in type I neurons parallels the periods of maturation of evoked exocytosis in inner hair cells and of appearance and maturation of the cochlear potentials, we propose that the expression of the enzyme indicates the onset of hearing in individual type I primary auditory neurons. This enzyme expression could rely on a Ca2+ activation of its encoding gene subsequent to a sudden and massive Ca2+ entry through voltage-activated Ca2+ channels.

Multiple output pathways of the basal forebrain: organization, chemical heterogeneity, and roles in vigilance

Studies over the last decade have shown that the basal forebrain (BF) consists of more than its cholinergic neurons. The BF also contains non-cholinergic neurons, including gamma-aminobutyric acid-ergic neurons which co-distribute and co-project with the cholinergic neurons. Both types of neuron project, in variable proportions, to the cerebral cortex, hippocampus, thalamus, amygdala, and olfactory bulb, whereas descending projections to the posterior hypothalamus and brainstem nuclei are predominantly non-cholinergic. Some of the cholinergic and non-cholinergic projection neurons contain neuropeptides such as galanin, nitric oxide synthase, and possibly glutamate. To understand better the function of the BF, the organization of the multiple ascending and descending projections of BF neurons is reviewed along with their neurochemical heterogeneity, and possible functions of individual pathways are discussed. It is proposed that BF neurons belong to multiple systems with distinct cognitive, motivational, emotional, motor, and regulatory functions, and that through these pathways, the BF plays a role in controlling both cognitive and non-cognitive aspects of vigilance.

Tyrosine hydroxylase-expressing and/or aromatic L-amino acid decarboxylase-expressing neurons in the mediobasal hypothalamus of perinatal rats: differentiation and sexual dimorphism

In this quantitative and semiquantitative immunocytochemical study, the authors evaluated the differentiation of neurons expressing tyrosine hydroxylase (TH) and/or aromatic L-amino acid decarboxylase (AADC) in the mediobasal hypothalamus (MBH) of male and female rats on embryonic day 18 (E18), E20, and postnatal day 9 (P9). Four neuronal populations were distinguished according to either enzyme expression or neuron location. The earliest and most prominent first population was represented by TH-immunoreactive (IR)/AADC-immunonegative (IN) neurons that were detected initially at E18 and always were located in the ventrolateral region of the MBH. The second population of TH-IN/AADC-IR neurons was observed first at E20 and, after that time, was distributed dorsomedially. The third minor population of TH-IR/AADC-IR neurons initially was detected at E20 and was located dorsomedially. The fourth population was represented by TH-IR/AADC-IN neurons that were distributed in the dorsomedial region at any studied age. The numbers of TH-IR and AADC-IR neurons increased from their initial detection at E18 and E20 until P9. The area of TH-IR and AADC-IR neurons also increased from E18 to E20 and from E20 to P9, respectively. Both TH-IR and AADC-IR neurons showed sex differences in the neuron number, size, and optic density (OD). The numbers of TH-IR neurons in males exceeded those of females at E20 and at P9, although, at P9, sexual dimorphism was a characteristic only of the ventrolateral population. The area and OD of TH-IR neurons from females exceeded those from males in the entire mediobasal hypothalamus (MBH) at E18 and E20 but only in its dorsomedial region at P9. Sexual dimorphism also was an attribute of AADC-IR neurons at E20 and P9. Their number, size, and OD were significantly higher in females than in males. Thus, the MBH of perinatal rats contained two major populations of TH-IR/AADC-IN or TH-IN-AADC-IR neurons and a minor population of TH-IR/AADC-IR neurons. The differentiating neurons expressing either enzyme showed sexual dimorphism.

Tyrosine hydroxylase-like immunoreactive synaptic boutons in the inferior colliculus of the cat

The data on the distribution of catecholaminergic cells and fibers in such a significant subcortical relay auditory center as the inferior colliculus (IC) are both few and controversial, and ultrastructural data are lacking. Young adult mongrel cats of both sexes were used. Following routine preparation procedures, the ultrathin sections were prepared for the ultrastructural examination of tyrosine hydroxylase (TH)-like immunoreactivity. TH-positive neuronal perikarya were not detected in the IC. On the other hand, an appreciable number of TH-immunoreactive unmyelinated axons and synaptic boutons were found in all subdivisions of the IC, most often in the nucleus externus, followed by the nucleus pericentralis, and a few were seen in the dorsomedial part of the central nucleus. The boutons measured 0.5-1.8 microns, contained pleomorphic synaptic vesicles, and established symmetrical synaptic contacts almost exclusively with dendrites of small caliber.

Catecholamine systems in the brain of vertebrates: new perspectives through a comparative approach

A comparative analysis of catecholaminergic systems in the brain and spinal cord of vertebrates forces to reconsider several aspects of the organization of catecholamine systems. Evidence has been provided for the existence of extensive, putatively catecholaminergic cell groups in the spinal cord, the pretectum, the habenular region, and cortical and subcortical telencephalic areas. Moreover, putatively dopamine- and noradrenaline-accumulating cells have been demonstrated in the hypothalamic periventricular organ of almost every non-mammalian vertebrate studied. In contrast with the classical idea that the evolution of catecholamine systems is marked by an increase in complexity going from anamniotes to amniotes, it is now evident that the brains of anamniotes contain catecholaminergic cell groups, of which the counterparts in amniotes have lost the capacity to produce catecholamines. Moreover, a segmental approach in studying the organization of catecholaminergic systems is advocated. Such an approach has recently led to the conclusion that the chemoarchitecture and connections of the basal ganglia of anamniote and amniote tetrapods are largely comparable. This review has also brought together data about the distribution of receptors and catecholaminergic fibers as well as data about developmental aspects. From these data it has become clear that there is a good match between catecholaminergic fibers and receptors, but, at many places, volume transmission seems to play an important role. Finally, although the available data are still limited, striking differences are observed in the spatiotemporal sequence of appearance of catecholaminergic cell groups, in particular those in the retina and olfactory bulb.

Immunocytochemical characterization of catecholaminergic neurons in the rat striatum following dopamine-depleting lesions

It is possible either permanently or transiently to deplete the rat striatum of dopamine. Following such depletions, striatal neurons immunoreactive for tyrosine hydroxylase (TH), aromatic L-amino acid decarboxylase (AADC) or dopamine appear. The presence of dopamine-producing neurons in the striatum has relevance for the treatment of Parkinson's disease, but whether these catecholaminergic phenotypes all produce dopamine is unclear. In the present study we establish that after unilateral 6-hydroxydopamine lesions or methamphetamine administration, striatal TH-immunoreactive neurons differ in size, morphology and location from those that are immunopositive for AADC or dopamine. The TH-positive cells which were localized either to ventral parts of the striatum or to the central and dorsal areas of the caudate-putamen generally have the morphological features of projection neurons, whereas those containing AADC or dopamine were confined to subcallosal positions in the dorsal medial quadrant of the caudate-putamen and resemble small, local-circuit neurons. The fact that AADC-immunoreactive neurons overlap in size, morphology and location with the cells that produce dopamine suggests strongly that this population is dopaminergic. However, the simultaneous appearance of neurons that contain the TH enzyme but clearly do not make dopamine raises questions about the functional role of these cells and the cellular mechanisms responsible for their induction following striatal dopamine loss.

Aromatic L-amino acid decarboxylase- and tyrosine hydroxylase-immunohistochemistry in the adult human hypothalamus

The distribution of cell bodies immunoreactive for tyrosine hydroxylase and aromatic L-amino acid decarboxylase was studied in the adult human hypothalamus. Many neurons in the posterior (A11) and caudal dorsal hypothalamic areas (A13) as well as in the arcuate (A12) and periventricular (A14) zone were immunoreactive for the two enzymes, suggesting that they were dopaminergic. Numerous tyrosine hydroxylase-immunoreactive neurons, which were not immunoreactive for aromatic L-amino acid decarboxylase, could be seen in the paraventricular, supraoptic and accessory nuclei (A15) as well as in the rostral dorsal hypothalamic area. These were considered to be non-dopaminergic. Conversely, large numbers of small neurons immunoreactive for aromatic L-amino acid decarboxylase but not for tyrosine hydroxylase, were identified in the premammillary nucleus (D8), zona incerta (D10), lateral hypothalamic area (D11), anterior portion of the dorsomedial nucleus (D12), suprachiasmatic nucleus (D13), medial preoptic area and bed nucleus of the stria terminalis (D14). In the human hypothalamus, besides dopaminergic cell bodies, there exists a large number of tyrosine hydroxylase-only and aromatic L-amino acid decarboxylase-only neurons, whose physiological roles remain to be determined.

Tyrosine hydroxylase- and/or aromatic L-amino acid decarboxylase-containing cells in the suprachiasmatic nucleus of the Syrian hamster (Mesocricetus auratus)

Catecholamines, including dopamine (DA), affect the activity of cells in the suprachiasmatic nucleus (SCN) of the hypothalamus, the principal circadian clock in mammals. This study examined the distribution of dopaminergic cells in the SCN of the male Syrian hamster, using both single- and double-label immunocytochemistry for tyrosine hydroxylase (TH), the rate-limiting enzyme in DA synthesis and for aromatic L-amino acid decarboxylase (AADC), the second enzyme needed to produce DA. Some neurons immunopositive for TH (TH + ) were found in the SCN, but most of the TH + cells of the region were located just outside the borders of the nucleus, as defined by pyronin Y staining. In the SCN, 91% of these cells were also immunopositive for AADC and thus, likely to be dopaminergic. Cells positive for AADC, many of which were not TH +, were found throughout the SCN, with the highest concentration seen in the ventral aspects of the nucleus. Cells containing AADC, but lacking TH may synthesize products other than DA, such as trace amines. These anatomical observations suggest that local neurons that produce DA and perhaps trace amines, may play a role in SCN function and in the neural control of circadian rhythms.

Distribution of ionotropic glutamate receptor subunit immunoreactivity in the suprachiasmatic nucleus and intergeniculate leaflet of the hamster

Glutamate is thought to mediate the effects of light on the circadian pacemaker contained in the suprachiasmatic nucleus. Glutamate can reset this pacemaker both in vivo and in vitro while glutamate antagonists can reduce photically induced phase shifts in activity rhythms and c-fos expression in the suprachiasmatic nucleus. Most behavioural and gene expression experiments investigating circadian rhythms use hamsters, but the majority of the anatomical data on the presence and distribution of selected glutamate receptor subunits in the suprachiasmatic nucleus has been collected from rat. In the present study, we examined the distribution of ionotropic glutamate receptor subunits in the hamster suprachiasmatic nucleus using mono- and polyclonal antibodies directed against these subunits. In addition, we examined the distribution of immunostaining for these subunits in a second structure of the mammalian circadian system, the intergeniculate leaflet of the thalamus since it also is thought to receive glutamatergic input from the retina and is important in the entrainment of circadian rhythms. The results indicated that all of the subunits investigated (GluR1, GluR2/3, GluR4, GluR5/6/7, and NMDAR1) were present in the suprachiasmatic nucleus and that all but GluR4 were present in the intergeniculate leaflet. Each of the subunits investigated had a unique pattern of distribution and intensity of staining. The distribution of immunoreactivity for these subunits in the hamster suprachiasmatic nucleus and intergeniculate leaflet differed from that reported in the rat. The presence of these subunits in the suprachiasmatic nucleus and intergeniculate leaflet implies the presence of functional NMDA and non-NMDA receptors in these structures that may have a role in photic entrainment of the circadian pacemaker.

Short-photoperiod exposure reduces L-aromatic-amino-acid decarboxylase immunostaining in the arcuate nucleus and median eminence of male Syrian hamsters

In male hamsters, exposure to short photoperiod (SD) results in a decrease in median eminence and posterior pituitary dopamine (DA) concentrations. To determine if the SD-induced decline in DA is due to a decrease in the number of neurons synthesizing DA, immunocytochemistry was used to identify cells containing tyrosine hydroxylase (TH) and L-aromatic amino acid decarboxylase (AADC) in animals housed under long photoperiod (LD) or SD. Immunopositive cells were counted in the arcuate and caudal periventricular nuclei, regions where the DA neurons that project to the median eminence and posterior pituitary are located. AADC immunopositive (AADC +) cells were also counted in the median eminence. Photoperiod did not affect the number of TH immunopositive (TH+) or AADC + cells in the caudal periventricular nuclei. In the arcuate nuclei, SD exposure did not affect the number of TH+ neurons, but produced a significant decline in the number of AADC + cells. The number of AADC + cells was also reduced in the median eminence of SD-exposed animals. This decline in AADC+ cells may play a role in SD-induced changes in hypothalamic regulation of anterior pituitary hormone release, including the decline in median eminence DA concentrations.

Distribution of dopaminergic fibers and neurons in visual and auditory cortices of the harbor porpoise and pilot whale

The distribution of putative dopaminergic fibers in two sensory cortical areas in the brain of the harbor porpoise (Phocoena phocoena) and pilot whale (Globicephala melaena) was analyzed at the light and electron microscopic levels using tyrosine hydroxylase (TH) immunohistochemistry. The quantitative analysis of the distribution of labeled fibers demonstrates that the primary visual cortex located in the lateral gyrus and entolateral sulcus contains a denser dopaminergic innervation than the auditory cortex within the posterior portion of the presylvian gyrus. In both areas, TH-immunoreactive fibers are densest in layer I, while layers IIIab and VI have intermediate densities and layers II and IIIc-V have the lowest fiber counts. Layer I is characterized by the presence of very thick TH-immunoreactive fiber populations, in addition to the thin and varicose fiber plexus observed throughout the cortical layers. Electron microscopic analyses demonstrated that some of these thick fibers represent the dendrites of TH-immunoreactive neurons located in the deep portion of layer I. The patterns observed in the present study suggest that the dopaminergic projections to the neocortex in whales have a different organization than in terrestrial mammals, particularly rodents and primates. These differences may reflect the fact that during evolution, the cetacean neocortex has retained many of the cytoarchitectonic features that are usually observed only in proisocortical regions in progressive terrestrial mammals.

Tyrosine hydroxylase mRNA-containing neurons in the medial amygdaloid nucleus and the reticular nucleus of the thalamus in the Syrian hamster

To confirm previous immunocytochemical findings in colchicine-treated Syrian hamsters, in situ hybridization was used to investigate the distribution of TH mRNA-containing cells in the medial amygdaloid nucleus (Me) and the thalamic reticular nucleus (Rt) of untreated hamsters. TH mRNA-producing neurons were observed in anterior and posterior Me and throughout Rt, similar to the distribution of TH-immunostained cells in these areas of animals receiving colchicine. These data confirm that TH is normally produced in amygdaloid and thalamic cell groups which lie outside the classical catecholamine systems.

Tyrosine hydroxylase neurons in the male hamster chemosensory pathway contain androgen receptors and are influenced by gonadal hormones

Chemosensory and hormonal signals, both of which are essential for mating in the male Syrian hamster, are relayed through a distinct forebrain circuit. Immunocytochemistry for tyrosine hydroxylase, a catecholamine biosynthetic enzyme, previously revealed immunoreactive neurons in the anterior and posterior medial amygdaloid nucleus, one of the nuclei within this pathway. In addition, dopamine-immunoreactive neurons were located in the posterior, but not the anterior, medial amygdala. In the present study, tyrosine hydroxylase-immunostained neurons were also observed in other areas of the chemosensory pathway, including the posteromedial bed nucleus of the stria terminalis and the posterior, lateral part of the medial preoptic area, while dopamine immunostaining was only seen in the posteromedial bed nucleus of the stria terminalis. The colocalization of tyrosine hydroxylase and androgen receptors was examined in these four tyrosine hydroxylase cell groups by a double immunoperoxidase technique. The percentage of tyrosine hydroxylase-immunolabeled neurons that were also androgen receptor-immunoreactive was highest in the posterior medial amygdaloid nucleus (74%) and the bed nucleus of the stria terminalis (79%). Fewer tyrosine hydroxylase-immunostained neurons in the anterior medial amygdala (33%) and the medial preoptic area (4%) contained androgen receptors. Surprisingly, castration resulted in a significant decrease in the number of tyrosine hydroxylase-immunoreactive neurons only in the anterior medial amygdaloid nucleus, and this effect was transient. Six weeks after castration, the anterior medial amygdala contained 61% fewer tyrosine hydroxylase-immunolabeled neurons, but 12 weeks after gonadectomy, immunostaining returned to intact values. The number of immunostained neurons in testosterone-replaced, castrated hamsters was not significantly different from that of intact or castrated animals at any time. The results of this study indicate that a substantial number of tyrosine hydroxylase-immunostained neurons in the chemosensory pathway are influenced by androgens; the majority of these neurons in the posterior medial amygdala and the posteromedial bed nucleus of the stria terminalis produce androgen receptors, and tyrosine hydroxylase immunoreactivity is altered by castration in the anterior medial amygdala.

A novel population of tyrosine hydroxylase immunoreactive neurones in the basal forebrain of the common marmoset (Callithrix jacchus)

We have observed in the basal forebrain of the common marmoset a group of neurones which display tyrosine hydroxylase immunoreactivity (THir) with three different polyclonal antibodies and one monoclonal antibody, and which express TH mRNA as shown by in situ hybridization histochemistry. The population of cells is composed of large multipolar neurones and is located predominantly in the substantia innominata and at the ventral, medial and lateral margins of the external segment of the globus pallidus. The cell morphology and the distribution of THir cells corresponds closely to the caudal portion of the nucleus basalis of Meynert. Adjacent sections demonstrate both THir and choline acetyltransferase immunoreactivity in the cells in this group, as well as strong acetylcholinesterase activity but not dopamine immunoreactivity. These observations indicate that many cholinergic neurones in the posterior nucleus basalis of Meynert of the marmoset contain tyrosine hydroxylase, and suggest that both acetylcholine and catecholamine may be synthesised as co-localised neurotransmitters within the same magnocellular neurones. We observe no THir cells in similar areas of the basal forebrain of either rhesus or talapoin monkeys.

GABA-like immunoreactivity in the gustatory zone of the nucleus of the solitary tract in the hamster: light and electron microscopic studies

The distribution of GABA-like immunoreactive (GABA-LI) somata was studied in the gustatory zone of the nucleus of the solitary tract (NST) in the hamster in order to identify putative inhibitory circuitry in gustatory processing. Immunoreactive somata were located throughout the gustatory NST, in accordance to the distribution of large and small types of neurons as determined in previous morphometric studies. Consequently, GABA-LI somata were mostly found in the dorsal two-thirds of the gustatory zone. Such somata were mostly ovoid in shape and possessed somal areas that averaged 85.5 +/- 2.8 microns 2 (12.7 x 8.4 microns). A narrow range of somal areas (50-125 microns 2) suggested a single functional group. At the electron microscopic level, 18% of the neurons encountered were immunoreactive and their nuclei always possessed deeply invaginated boundaries. This morphological feature indicated that GABA-LI neurons are smaller members of the most common class of neurons within the gustatory NST. Because GABA is often implicated as the neurotransmitter of small inhibitory local circuit neurons, these findings indicate a possible inhibitory aspect to the processing of taste information at the level of the first relay in the brainstem.

Tyrosine hydroxylase-immunoreactive neurons in the nucleus basalis of the common marmoset (Callithrix jacchus)

In the course of characterizing the distribution of putative catecholaminergic neurons in the brain of the common marmoset, we encountered a population of such cells in the basal forebrain. Tyrosine hydroxylase-immunoreactive neurons are abundant within the nucleus basalis magnocellularis throughout its entire rostrocaudal extent, but not in other cholinergic basal forebrain nuclei. Most tyrosine hydroxylase-immunoreactive cells are large and multipolar. Double staining with antibodies to choline acetyltransferase or nerve growth factor receptor confirmed that these tyrosine hydroxylase-immunoreactive neurons are cholinergic, and compose at least 40% of the nucleus basalis cholinergic cells. The presence of a catecholamine-synthesizing enzyme in the neurons that provide the major cholinergic input to the neocortex may have important consequences for cortical function, and may be relevant to the vulnerability of the nucleus basalis in certain neurodegenerative disorders.

Tyrosine-hydroxylase-containing neurons in the primate basal forebrain magnocellular complex

Immunocytochemistry and in situ hybridization for tyrosine hydroxylase (TH) were used to study the distribution of putative catecholaminergic neurons in the basal forebrain magnocellular complex (BFMC) of monkeys and humans. Magnocellular TH-expressing neurons in the primate BFMC are distributed along a rostrocaudal gradient, with the largest proportion of these cells located in the medial septal nucleus and nucleus of the diagonal band of Broca; smaller TH-containing neurons generally follow the same distribution. These findings suggest that, within rostromedial segments of the BFMC, there is a distinct subpopulation of neurons that express catecholamine-synthesizing enzymes. Further research is necessary to establish whether these neurons utilize one or more catecholamines as neurotransmitters.

A species-specific population of tyrosine hydroxylase-immunoreactive neurons in the medial amygdaloid nucleus of the Syrian hamster

The medial amygdaloid nucleus (Me) is part of a neural pathway that regulates sexual behavior in the male Syrian hamster. To characterize the neurochemical content of neurons in this nucleus, brains from colchicine-treated adult male and female hamsters were immunocytochemically labeled using antibodies that recognize the catecholamine-synthesizing enzymes, tyrosine hydroxylase (TH), dopamine-beta-hydroxylase (DBH) and phenylethanolamine-N-methyltransferase (PNMT), as well as dopamine. A large population of TH-immunoreactive (TH-IR) neurons was observed throughout Me of male and female hamsters, primarily concentrated in the midrostral and caudal portions of the nucleus. The somata were generally small to medium in size and bipolar. Brains from animals that did not receive colchicine contained a limited number of TH-IR neurons in Me as reported previously. The DBH and PNMT antisera did not label any cells in Me of colchicine-treated animals, and the dopamine antiserum labeled neurons in the same location as the caudal group of TH-IR cells. Therefore, these caudal TH-IR neurons are interpreted to be dopaminergic. The rostral group of TH-IR neurons, on the other hand, may be producing only the immediate precursor of dopamine, L-3,4-dihydroxyphenylalanine (L-DOPA). The TH-synthesizing neurons in Me of the Syrian hamster appear to be a species-specific group of cells located outside of the previously described catecholaminergic cell groups.

The catecholaminergic innervation of primate prefrontal cortex

This paper reviews recent studies indicating that the marked expansion and differentiation of the prefrontal cortex in primates is associated with an increase in the complexity of both the regional density and laminar distribution of catecholaminergic afferents. The innervation patterns of these systems in monkey prefrontal cortex appear to accurately predict those in human prefrontal cortex, suggesting that studies in non-human primates may be reasonably used to generate hypotheses about the nature of involvement of these systems in disorders such as schizophrenia. In addition, the distinctive developmental pattern of the dopaminergic innervation of primate prefrontal cortex and the possibility of an intrinsic catecholaminergic innervation of primate prefrontal cortex may reveal new avenues of investigation into the roles of prefrontal catecholamines in both normal and pathological states.

Ultrastructural analysis of noradrenergic nerve terminals in the cat lumbosacral spinal dorsal horn: a dopamine-beta-hydroxylase immunocytochemical study

Noradrenaline-containing nerve terminals within the cat spinal dorsal horn were studied by immunocytochemical localization of dopamine-beta-hydroxylase. Immunoreactive terminals formed symmetrical (Gray type II) synaptic specializations with dendrites and somata throughout laminae I-IV, but no junctions were formed with other axons. These findings suggest that noradrenaline regulates sensory transmission through the dorsal horn via a postsynaptic action.

Aminergic innervation of the gall bladder in man and dog

This study has compared the innervations of human and canine gall bladders, using immunohistochemical localization of neuron-specific enolase to visualize all intramural nerves and localization of tyrosine hydroxylase, DOPA decarboxylase and serotonin to visualize different populations of aminergic neurons. The vasculature, the wall smooth musculature and the intramural ganglion cells receive a substantial sympathetic innervation, in both species. By contrast, the mucosa appears to be supplied almost entirely by non-sympathetic fibres. On the basis of DOPA decarboxylase immunoreactivity, a large proportion of the biliary sympathetic nerves in both species may be dopaminergic. Human gall bladder also contains a population of putatively tryptaminergic, intramural neuron-like cells. However, there was no evidence for tryptaminergic innervation of effector structures in the gall bladder wall.

Aromatic L-amino-acid decarboxylase (DOPA decarboxylase) gene expression in dopaminergic and serotoninergic cells of the rat brainstem

In situ hybridization was performed in the rat brain to detect aromatic L-amino acid decarboxylase (AADC) mRNA using 35S-labeled oligonucleotide probes derived from rat kidney AADC cDNA. Results demonstrated AADC mRNA in areas containing dopaminergic and serotoninergic cell bodies. Combined immunohistochemistry for tyrosine- or tryptophan hydroxylase and in situ hybridization for AADC mRNA demonstrated the dopaminergic or serotoninergic nature of cells containing AADC mRNA. Tyrosine hydroxylase-positive mesencephalic neurons containing a very low or no AADC mRNA signal were also observed.

Four groups of tyrosine hydroxylase-immunoreactive neurons in the ventrolateral medulla of rats, guinea-pigs and cats identified on the basis of chemistry, topography and morphology

The data in the preceding paper [Halliday G. M. and McLachlan E. M. (1991) Neuroscience 43, 531-550] suggest that some neurons in the rostral ventrolateral medulla contain some catecholamine-synthesizing enzymes but may not produce catecholamines. The present study addresses this question directly by comparing the anatomical location and morphology of these neurons with those revealed by formaldehyde-induced fluorescence. Catecholamine-containing somata of rats and guinea-pigs have been demonstrated following FAGLU-perfusion in normal untreated animals, in animals pretreated with pargyline (a monoamine oxidase inhibitor), and in animals pretreated with colchicine (to block axoplasmic transport). The number and location of fluorescent somata in the ventrolateral medulla have been determined in serial coronal sections of tissue from the cervical spinal cord to the level of the facial nucleus. Catecholamine-fluorescent neurons at different levels of the ventrolateral medulla varied in their topography and sensitivity to pharmacological manipulation. However, the rostrocaudal distributions in rats and guinea-pigs were quantitatively remarkably similar implying that homologous groups of catecholamine-containing neurons exist. Comparison between these distributions and those of somata stained immunohistochemically for catecholamine-synthesizing enzymes and neuropeptide Y [Halliday G. M. and McLachlan E. M. (1991) Neuroscience 43, 531-550] revealed that the majority of fluorescent neurons in both species probably contain dopamine-beta-hydroxylase and neuropeptide Y as well as tyrosine hydroxylase. Those neurons lying just caudal to the facial nucleus immunoreactive for tyrosine hydroxylase and phenylethanolamine-N-methyltransferase but not dopamine-beta-hydroxylase and neuropeptide Y also lack catecholamine fluorescence. This rostral group of somata can be identified immunohistochemically in cats. The size and morphology of catecholamine-fluorescent neurons have been analysed in detail, and compared with the same features of the immunohistochemically stained neurons. Three morphological types of catecholamine-containing neurons could be distinguished in material prepared by both techniques from rats and guinea-pigs, and in immunohistochemical material from cats. Rostral tyrosine hydroxylase-positive neurons, which differed morphologically from these three types, were present in all three species. On the basis of anatomical location, neuronal morphology and chemical characteristics, four groups of tyrosine hydroxylase-immunoreactive neurons have been identified in the ventrolateral medulla of rats, guinea-pigs and cats. Only the caudal three of these four groups appear to synthesize catecholamine, probably noradrenaline. From published data it seems likely that these four groups of tyrosine hydroxylase-positive neurons have distinct projections and functions related to cardiovascular and respiratory control.