Substantia Nigra Ventral Tegmental Area Research Articles

Dopamine receptor sub-types involvement in nucleus accumbens and ventral tegmentum but not in medial prefrontal cortex: on self-stimulation of lateral hypothalamus and ventral mesencephalon

The involvement of different sub-types of dopamine receptors in the electrical self-stimulation behaviour was investigated using DA receptor subtype specific agonists viz R(+) SKF 38393 and LY 171555 (quinipirole). Rats were chronically implanted with bipolar electrodes in lateral hypothalamus-medial forebrain bundles (LH-MFB) and ventral tegmental area-substantia nigra (VTA-SN) or guide cannula in nucleus accumbens (nACB) or medial prefrontal cortex (mPFRCx) or cannula-cum-electrode in VTA-SN. Combining these, it was possible to inject a given receptor ligand in either nACB or mPFRCx or VTA-SN and assess the changes in intracranial self-stimulation (ICSS) of LH-MFB or VTA-SN. The experimental subjects were trained to press a pedal to obtain 0.25 s trains of cathodal rectangular pulses of 0.1 ms pulse duration (for LH-MFB electrode) or 0.3 ms pulse duration (for VTA-SN electrode). The rate-frequency curve for each subject and each placement were obtained in order to determine M 50 (50% of maximum asymptotic response rate). Microinjection of SKF 38393 (a D 1 agonist) or LY 171555 (a D 2 agonist) into nACB caused facilitatory and suppressive effects on the ICSS of VTA-SN and LH-MFB respectively. On the contrary, intra VTA-SN injection of 10 μg/0.5 μl of SKF 38393 or of LY 171555, reduced its ICSS to half as compared to vehicle injections. Under the effect of these injections, there was no change in the ICSS of LH-MFB. The combined administration of SKF 38393 (5 μg) and LY 171555(10 μg) into nACB produced suppression of ICSS of VTA-SN to greater extent than the LY 171555 (10 μg/0.5 μl). This indictes that coactivation of D 1 and D 2 leads to suppression of ICSS of VTA-SN. The activation of D 1 receptors in nACB facilitates the ICSS of both VTA-SN and LH-MFB by increasing the reinforcement and/locomotor activity, while the activation of D 2 receptors in it has opposite effect. The coactivation of D 1 and D 2 receptors in VTA-SN leads to the suppression of ICSS of both VTA-SN and LH-MFB. Similar injections of these receptor ligands into mPFRCx did not alter the ICSS of either LH-MFB or VTA-SN. The pre- or post-synaptic DA receptors of mPFRCx do not appear to influence the ICSS of either VTA-SN or LH-MFB in any significant manner.

Cholinergic and GABAergic Modulation of Self-Stimulation of Lateral Hypothalamus and Ventral Tegmentum: Effects of Carbachol, Atropine, Bicuculline, and Picrotoxin

Singh, J., T. Desiraju and T. R. Raju: Cholinergic and GABAergic modulation of self-stimulation of lateral hypothalamus and ventral tegmentum: Effects of carbachol, atropine, bicuculline, and picrotoxin. Physiol Behav 61(3) 411–418, 1997.—This study aimed at investigating the effects of modulators of acetylcholine-muscarinic (ACh-M) and GABA A,B-receptors in the lateral hypothalamus and ventral tegmentum on the electrical self-stimulation evoked from these regions. In each Wistar rat, a bipolar electrode and an ipsilateral insulated cannula-cum-electrode were implanted chronically in the right hemisphere. There were two placement combinations: 1. Bipolar electrode in lateral hypothalamus-medial forebrain bundle (LH-MFB) and cannula-cum-electrode in ventral tegmental area-substantia nigra (VTA-SN) or 2. bipolar electrode in VTA-SN and cannula-cum-electrode in LH-MFB. The stimulation parameters were kept at a level to obtain 50% of the maximum possible asymptotic intracranial self-stimulation (ICSS) rates (M 50) from each site. Carbachol injected into VTA-SN increased the ICSS responding rate of VTA-SN as well as, indirectly, that of LH-MFB, and atropine injections had an opposite effect. Similar injections of these ligands into LH-MFB again had facilitatory and inhibitory effects on its ICSS; however, there was no indirect effect on the ICSS of VTA-SN. Bicuculline injection into VTA-SN had no effect on either its ICSS or that of LH-MFB. On the other hand, bicuculline injected into LH-MFB decreased its ICSS and, indirectly, the ICSS of VTA-SN in a dose-dependent manner. In contrast, intraVTA-SN injections of picrotoxin decreased its ICSS and, indirectly, ICSS of LH-MFB. Similar injections into LH-MFB had no effect on the ICSS of VTA-SN. The results suggest that ICSS of VTA-SN is Ach-M receptor mediated and, also, this mediation influences the ICSS of LH-MFB. On the contrary, changes in ICSS of LH-MFB following Ach-M receptor ligands do not influence the ICSS of VTA-SN. The direct effects of GABA-receptor antagonists have shown disassociation because ICSS of LH-MFB was more sensitive to bicuculline, whereas the ICSS of VTA-SN was more sensitive to picrotoxin. Thus, the cholinergic and GABAergic mechanisms of the self-stimulation of lateral hypothalamus and ventral tegmentum are different.

Prenatal alcohol treatment attenuated postnatal cocaine-induced elevation of dopamine concentration in nucleus accumbens: A preliminary study

Prenatal alcohol exposure has been shown to damage the developing central nervous system (CNS) in a variety of ways, including neuroanatomical anomalies, neurochemical imbalance, and neuropharmacological dysfunction. The present study investigated one of the functional aspects of dopaminergic system in neonatal rats exposed prenatally to a binge-like alcohol paradigm by measuring dopamine concentrations following a single postnatal cocaine challenge. Pregnant Sprague-Dawley rats were given daily intragastric intubations of 5.1 g/kg alcohol solution from embryonic day (E) 1 to 20. Pair-fed and ad lib-fed animals served as controls. On E33 (usually postnatal day 10), offspring from all groups were given injections (IP) of either 0, 20, or 40 mg/kg cocaine. Animals were sacrificed and the substantia nigra/ventral tegmental area (SN/VTA) and nucleus accumbens (NAc) were dissected for the determination of dopamine concentrations using HPLC. Basal dopamine levels (0 mg/kg cocaine group) did not alter as a function of prenatal alcohol treatment in either region. However, acute cocaine injection increased the dopamine content in NAc, but not in SN/VTA, in ad lib-fed animals, and this elevation in dopamine level was significantly attenuated by prenatal alcohol treatment in both female and male animals, and by prenatal pair-fed treatment in male animals. Taken together, these results indicate that there appears to be a regional difference in acute cocaine-induced dopamine elevation, and prenatal binge-like alcohol exposure significantly alters the functional responsiveness of dopaminergic system in NAc. Furthermore, these data suggest that male offspring may be more sensitive to stress-associated or nutritional influences during gestation.

Effects of perinatal asphyxia on the mesostriatal/mesolimbic dopamine system of neonatal and 4-week-old male rats.

The present study was undertaken in order to study the effects of perinatal asphyxia on tyrosine hydroxylase (TH) activity, dopamine levels and turnover, and dopamine metabolites (3,4-dihydroxyphenylacetic acid, DOPAC, homovanillic acid, HVA, and 3-methoxytyramine, 3-MT, analyzed by high-performance liquid chromatography, HPLC) measured in the basal ganglia of the 20- to 40-min-old newborn and 4-week-old male rat. Asphyxia was induced in pups by placing the fetuses, still in their uterus horns removed by hysterectomy from pregnant rats at full term, in a 37 degrees C water bath for 15-16 min or 19-20 min. Following asphyxia, the uterus horns were opened, and the pups were removed and stimulated to breathe. A 100% and 50-80% pup survival was obtained following 15-16 min and 19-20 min of asphyxia, respectively. Acute changes were studied in brains from newborn pups 20-40 min after delivery, and long-term changes were studied in brains from 4-week-old rats. No changes in TH-activity could be observed in the substantia nigra/ventral tegmental area (SN/VTA), the striatum, or the accumbens nucleus/olfactory tubercle (ACC/TUB), in the newborn or the 4-week-old rat. In the newborn rat, 19-20 min of asphyxia increased (as compared to controls) dopamine levels in the SN/VTA to 136 +/- 14% and in the ACC/TUB to 160 +/- 10%, indicating an increased synthesis and/or release of dopamine. DO-PAC levels were increased in the SN/VTA to 150 +/- 14% and in the ACC/TUB to 151 +/- 10%, and HVA levels were increased to 152 +/- 16% in the striatum and to 117 +/- 4% in the ACC/TUB. Following 15-16 min of asphyxia, dopamine levels were increased to 130 +/- 12% in the ACC/TUB, and DOPAC levels were increased to 135 +/- 6% and 130 +/- 12% in the SN/VTA and the ACC/TUB, respectively. This suggests that the increased dopamine levels may preferably reflect an increased release of dopamine following perinatal asphyxia. In the 4-week-old rat, dopamine levels were decreased in the SN/VTA to 71 +/- 4%, in the striatum to 52 +/- 8%, and in the ACC/TUB to 53 +/- 7%, following 19-20 min of perinatal asphyxia as compared to controls. No changes were observed in DOPAC, HVA, or 3-MT levels, indicating that the reduced dopamine levels reflect a reduced dopamine synthesis following perinatal asphyxia. A decrease in dopamine utilization was observed in the striatum to 15 +/- 8% and in the ACC/TUB to 9 +/- 13% following 19-20 min of perinatal asphyxia as compared to controls. This indicates that perinatal asphyxia produced long-lasting reductions in activity in the mesostriatal/mesolimbic dopamine systems in the 4-week-old rat.

Comparison of intracranial self-stimulation evoked from lateral hypothalamus and ventral tegmentum: analysis based on stimulation parameters and behavioural response characteristics.

The comparison of intracranial self-stimulation (ICSS) derived across the anteroposterior axis of medial forebrain bundle (MFB) from the anterior border of lateral hypothalamus (LH) to the ventral mesencephalon including ventral tegmental area-substantia nigra (VTA-SN) in Wistar rats was assessed through stimulation parameters and behavioural response characteristics. The interpretation of response rate/charge consumption (muC/min) with respect to rectangular wave and sine wave electrical stimulation parameters suggests that the rectangular wave parameters are better in order to get the maximum responding rates. The most vigorous and robust responding was observed in the VTA or VTA-SN boundary placements, followed by placements in medial sector of LH. The acquisition of ICSS was fastest in the case of VTA-stimulation. The next site with respect to rapidity of ICSS was posterio-ventral LH. The extinction curves indicated that it is faster and exponential in case of VTA-SN, but it is slower with longer duration in case of LH-MFB. ICSS of SN were accompanied by exploratory locomotion and head bobbing. Thirty-one percent subjects with SN/VTA stimulation showed rotational behaviour. Seventy-eight percent of subjects with LH stimulations showed stimulus-bound ejaculations. Thirty-two percent of subjects with posterior LH stimulations showed biting of pedal edges. LH stimulations were accompanied by induced seizures and increased grooming in 18% and 13% of subjects, respectively. There was lateralisation of cerebral hemispheric function as right paw preference was noted in majority of rats, whether sites of stimulation were in the left or right cerebral hemisphere. The various other modes of pedal pressing operants like use of paw and mouth, alternate paw dribbling, use of head electrode assembly to manipulate the pedal were also recorded and analysed.

Phenotypical characterization of neurons expressing the dopamine D3 receptor in the rat brain.

We have established the cellular distribution of the dopamine D3 receptor using tritiated 7-hydroxy-N-N-di-n-propyl-2-aminotetralin and a complementary RNA probe to visualize autoradiographically the protein in binding studies and the gene transcripts by in situ hybridization, respectively. Studies with these two markers confirm the restricted expression of the D3 receptor in few brain areas, i.e. mainly the ventral striatal complex, the substantia nigra-ventral tegmental area and the cerebellum. In nucleus accumbens, the D3 receptor was mainly expressed in medium-sized neurons of the rostral pole and ventromedial shell subdivisions, but not of the core or septal pole, i.e. accumbal subdivisions expressing the D2 receptor. In the ventromedial shell, about 60% of the D3 receptor-expressing neurons were neurotensin neurons, presumably projecting to the ventral pallidum. In the islands of Calleja, both D3 receptor binding and messenger RNA were abundant in the entire population of granule cells. These cells are known to make sparse contacts with dopaminergic axons and also to express the D1 receptor. In the mesencephalon, low levels of D3 messenger RNA were detected in few dopamine neurons of substantia nigra pars lateralis and ventral tegmental area. In addition, some D3 receptor binding but not messenger RNA was detected in medial substantia nigra and lateral ventral tegmental area, where the receptor is presumably located presynaptically on afferents. In the archicerebellum, Purkinje cell perikarya in lobules 9 and 10 expressed the D3 receptor messenger RNA, whereas binding sites were found in the molecular layer, where corresponding dendrites but no known dopaminergic projection from mesencephalon are found. The occurrence of D3 receptor gene expression in some brain areas receiving low dopamine innervation supports the hypothesis that this receptor may mediate non-synaptic actions of dopamine.

Facilitation of self-stimulation of ventral tegmentum by microinjection of opioid receptor subtype agonists

Intracranial self-stimulation (ICSS) evoked from the ventral tegmental area-substantia nigra (VTA-SN) and lateral hypothalamus-medial forebrain bundle (LH-MFB) was assessed following microinjections of mu (Tyr-D-Ala 2-N-Me-Phe 4-Gly 5ol: DAGO), delta-(D-Ala 2, D-Met 5−)-enkephalin: DADME) or kappa (Dynorphin-B or Rimorphin) opioid receptor subtype agonists or saline into either VTA-SN or LH-MFB. The current intensity was fixed at an optimum level to obtain 60–70% of the maximum asymptotic response rate. DAGO (5 μg/0.5 μl), DADME (2 μg/0.5 μl) or Dynorphin B (0.5 μg/0.5 μl) injected into VTA-SN facilitated the self-stimulation rates of VTA-SN by 27%, 32%, and 59%, respectively. These microinjections did not alter the self-stimulation of LH-MFB when effects of these injections were still persisting in VTA-SN. Similar doses of these opioid receptor agonists injected into LH-MFB had no significant effect on the self-stimulation rates of either LH-MFB or VTA-SN. The facilitatory effects of DADME were completely abolished by naloxone (30 mg/kg IP). Taken together, these results suggest that all three opioid receptor subtypes of ventral tegmentum and not of lateral hypothalamus are involved in the electrically evoked self-stimulation of VTA-SN.

Decrease in mesencephalic dopamine autoreceptors in experimental herpes simplex encephalitis.

Brain dopamine receptors were determined in experimental herpes encephalitis using an animal model, where herpes simplex virus type 1 was inoculated onto the cornea of rabbits. The animals exhibit an asymmetric posture and circling to the side of inoculation, which appears to be connected to the altered dopamine transmission in the mesostriatal system. In this study striatal and mesencephalic D-1 and D-2 dopamine receptors were measured by radioligand techniques using 3H-SCH 23390 and 3H-spiroperidol as ligands. In the striatal D-1 and D-2 receptors there were no significant differences between HSV-inoculated and control rabbits. In the substantia nigra-ventral tegmental area there was a significant decrease in the D-2 receptors (Bmax) on the side contralateral to the primary virus inoculation and the direction of the rotational behaviour, without any changes in the D-1 receptors. Thus experimental herpes simplex virus infection seems to affect the mesencephalic dopamine autoreceptors, leading to unilateral activation of the mesostriatal dopamine system and rotational behaviour.

Increase of dendritic branching of CA3 neurons of hippocampus and self-stimulation areas in subjects experiencing self-stimulation of lateral hypothalamus and substantia nigra-ventral tegmental area

Golgi examination of neurons of self-stimulation areas of the lateral hypothalamus and substantia nigra-ventral tegmental areas of adult Wistar rats that had experienced self-stimulation for 10 days revealed a significantly higher number of dendritic branching points in the two self-stimulation areas, and also in the hippocampus (CA3 pyramidal neurons) than in inexperienced rats.

Fluorescent histochemical localization of neutral endopeptidase‐24.11 (enkephalinase) in the rat brainstem

Characterization of the distribution of the peptide-degrading enzyme neutral endopeptidase-24.11 (E.C. 3.4.24.11; NEP; enkephalinase) in the rat brainstem was examined by means of a unique fluorescent histochemical method. Enzyme staining was completely blocked by three potent NEP inhibitors (thiorphan, phosphoramidon, and JHF-26) at a concentration of 50 nM, supporting the specificity of this method to visualize sites of NEP activity selectively. At all levels of the brainstem, NEP was localized to cell bodies, cell processes or terminal-like fields and was localized to more than 90 distinct nuclei or subnuclei. In the mesencephalon these included the central gray, cuneiform n., dorsal and lateral tegmental n., inferior colliculus, interpeduncular n., lateral and medial geniculate n., central linear raphe n., mesencephalic n. of the trigeminal nerve, mammillary nuclei, occulomotor n., red n., superior colliculus, ventral n. of the lateral lemniscus, substantia nigra-ventral tegmental area, and the zona incerta. In the pons, NEP staining was restricted to fewer regions or nuclei, including the dorsal and ventral cochlear n., facial n., motor trigeminal n., principal sensory trigeminal n., parabrachial nuclei, pontine n., the oral and caudal pontine reticular n., pontine olivary nuclei, several pontine tegmental nuclei, pontine raphe nuclei, and the trapezoid n. In the cerebellum, staining was localized largely to the granule cell layer of the cerebellar cortex. Scattered staining was observed in the molecular cell layer. The medulla contained extensive NEP staining localized to nuclei that included the ambiguous n., dorsal motor n. of the vagus, hypoglossal n., inferior olivary n., prepositus hypoglossus n., solitary tract n., nuclei of the spinal tract of the trigeminal n., and the lateral, medial, and superior vestibular nuclei. Nuclei of the medullary reticular formation that were also richly stained for NEP included the raphe magnus n., raphe obscurus n., raphe pallidus n., dorsal, lateral, and ventral reticular nuclei of the medulla, and the gigantocellular, lateral paragigantocellular, linear, paramedian and parvicellular reticular nuclei. The widespread distribution of NEP in the brainstem suggests the existence of a number of functional systems, including the pathways involved in the mechanisms of pain and analgesia, which are potential targets of NEP inhibitors. In most regions, the distribution of NEP closely overlapped with that reported for the enkephalins, and showed a more restricted overlap with the reported distribution of substance P.

6-Hydroxydopamine lesions of the ventral tegmentum abolish d-amphetamine and morphine analgesia in the formalin test but not in the tail flick test

The effect of bilateral 6-hydroxdopamine (with desipramine) lesions of the ventral tegmental area-substantia nigra region on analgesia produced by morphine and d-amphetamine was examined with the formalin and tail flick tests in rats. These lesions depleted dopamine, but not noradrenaline, in the ventral and anterior striatum. Morphine and damphetamine produced analgesia in the formalin test in sham lesioned rats but not in lesioned rats. In contrast, morphine produced the same degree of analgesia in the tail flick test in sham lesioned and lesioned rats while d-amphetamine did not produce analgesia in this test. These data suggest that morphine and d-amphetamine analgesia in the formalin test involves dopamine, whereas morphine analgesia in the tail flick test does not.

Evidence for a distinct nigropallidal dopaminergic projection in the squirrel monkey

Injections of the retrograde fluorescent tracer fast blue in the striatum (STR) and nuclear yellow in the internal segment of the globus pallidus (GPi) in the squirrel monkey ( Saimiri sciureus) revealed a nigropallidal projection whose cellular origin was largely distinct from that of the nigrostriatal pathway. Neurons containing the tracer injected in GPi were scattered throughout the substantia nigra-ventral tegmental area complex where they formed approximately 20–25% of the total number of retrogradely labeled cells. Only about 5–10% of all positive neurons were double-labeled after STR-GPi injections. In experiments combining the use of the fluorescent tracer propidium iodide with immunofluorescence, the majority of neurons projecting to GPi displayed tyrosine hydroxylase immunoreactivity. Hence, in addition to their important role at striatal level, midbrain dopaminergic neurons may influence directly the output neurons of the basal ganglia at pallidal level in primates.

Brainstem dopaminergic neurons project to monkey parietal cortex

In this study, retrograde transport of Fast blue was combined with tyrosine hydroxylase immunohistochemistry to reveal the presence of projections from both dopaminergic and non-dopaminergic neurons of the substantia nigra-ventral tegmental area to the parietal cortex of cynomolgus monkeys ( Macaca fascicularis). These findings confirm the results of previous studies demonstrating the existence of a dopaminergic innervation of monkey parietal cortex.

Dopamine and opiate receptors: Localization in the striatum and evidence for their axoplasmic transport in the nigrostriatal and striatonigral pathways

The axoplasmic transport of dopamine and opiate receptors in the striatonigral and nigrostriatal pathways was investigated by placing coronal knife cuts through these pathways and examining autoradiographically the accumulation of receptors at the site of the cut. In otherwise normal animals build-up of both receptors was found both rostral and caudal to the cut after a survival time of 24 h. Build-up of both receptors was reduced caudal to the cut by prior 6-hydroxydopamine lesions of the substantia nigra-ventral tegmental area, and reduced rostral to the cut by prior kainic acid lesions of the striatum. In addition, it was found by both autoradiographic and membrane binding methods that kainic acid lesions of the striatum produced a larger reduction of striatal opiate compared with dopamine receptors. The results suggest that opiate and dopamine receptors are axonally transported in fibers of both the nigrostriatal and striatonigral pathways possibly to their respective presynaptic terminals. The differential sensitivity of opiate and dopamine receptors in the striatum to local kainic acid lesions suggests the preferential localization of postsynaptic opiate receptors on dendritic trunks and neuronal somata, whereas the major localization of postsynaptic dopamine receptors may be on striatal dendritic spines, which to some extent appear to survive the lesions.

Organization of the striatum: Collateralization of its Efferent Axons

The anatomical structure of the basal ganglia indicates that the input from the cerebral cortex is funnelled through the striatum to the globus pallidus and substantia nigra. This structure implies integration of the information as it is transferred through the basal ganglia. In order to investigate this integration, we studied the collateralization of striatal efferents to the globus pallidus and the substantia nigra-ventral tegmental area. Retrogradely transported fluorescent tracers were injected into the target areas of striatal efferents. Nuclear yellow or propidium iodide was injected into the substantia nigra-ventral tegmental area (SN-VTA) and 4-acetamido, 4′-isothiocyanostilbene-2,2′-disulfonic acid (SITS) into the globus pallidus (GP) of adult albino rats. SITS was chosen for the pallidal injections because it is not taken up by fibers-of-passage. The pressure injections resulted in large injection sites which covered the majority of each efferent target area, and as a result retrogradely labeled cell bodies were found throughout the entire extent of the striatum. Cell bodies double-labeled with both dyes were found intermingled with single-labeled cell bodies. In rats injected with propidium iodide in the SN-VTA and SITS in the GP, 70% of all neurons (as revealed by Nissl staining) were labeled. Of these labeled cells, 40% were double labeled, 20% contained only SITS and 40% contained only propidium iodide. Thus a substantial number of the striatal neurons that project to the SN-VTA also possess collateral axons to the GP. Some striatal neurons appear to project to only the SN-VTA or only to the GP. The cells projecting to only one of these striatal target regions tend to cluster together in patches. The organizationl pattern of these patches does not seem to coincide in any simple way with the mosaic pattern of striatal opiate receptors, nor with the previously described mosaic pattern of striatal afferents and various neurotransmitter substances.

Projection neurons of the nucleus accumbens: an intracellular labeling study

Projection neurons of nucleus accumbens (NAC) of the rat were identified by either antidromic activation from stimulation of midbrain ventral tegmental area-substantia nigra (VTA-SN) regions, or by tracing axons of intracellularly labeled NAC neurons into the ventral pallidum. The morphology of these NAC projection neurons were determined to be medium spiny neurons similar to those identified in the caudate-putamen.

Dynorphin-containing pathways in the substantia nigra and ventral tegmentum: A double labeling study using combined immunofluorescence and retrograde tracing

A rich plexus of dynorphin (DYN)-containing fibers and terminals is present throughout the substantia nigra-ventral tegmental area (SN-VTA). Lesion studies have indicated that these terminals originate, in part, from DYN-containing cell bodies in the caudate-putamen. We have utilized combined immunofluorescence and retrograde tracing techniques to confirm this finding and to demonstrate the presence of other potential DYN pathways in the region of SN-VTA. The location of DYN-containing cell bodies and fibers was determined by immunohistochemistry in untreated and colchicine-treated adult albino rats. In further double-labeling experiments, animals received injections of fluorescent retrograde tracers into SN-VTA, brainstem or amygdala, followed one to fourteen days later by colchicine treatment. Brains were subsequently processed for combined FITC immunofluorescence and fluorescent dye localization. These experiments provided neuroanatomical evidence for heterogeneous DYN-containing pathways in the SN-VTA. Topographically organized afferents arose from the caudate putamen, amygdala and hypothalamus, while efferents arose from only the most lateral sectors of the SN to the amygdala. While most of the observed descending projections terminated in the SN-VTA, hypothalamic and amygdala projections also continued en passage to the brainstem. These findings implicate DYN systems in striatal and limbic, as well as neuroendocrine, functions.

Dopamine- and cholecystokinin-containing neurons of the crossed mesostriatal projection

The projection of the substantia nigra-ventral tegmental area (SN-VTA) to the ipsilateral and contralateral caudate-putamen was studied with double-labeling fluorescence techniques in the albino rat. By combining glyoxylic acid histofluorescence with retrograde fluorescence tracers, we were able to determine that 93% of the minor (1–2%) contralateral mesostriatal pathwayis catecholaminergic. In addition, combined immunofluorescence and retrograde fluorescence tracer studies showed that 50% of the crossed mesostriatal pathway also arises from cholecystokinin-containing neurons. These results provide evidence for crossed catecholamine- and peptide-containing pathways in the mesostriatal system which is part of the ‘prefrontal system’ that links the prefrontal cortex through the caudate-putamen, with the thalamus and tectum of each hemisphere.

The origin of cholecystokinin terminals in the basal forebrain of the rat: Evidence from immunofluorencence and retrograde tracing

The origins of cholecystokinin (CCK) fibers in the olfactory tubercle, nucleus accumbens and amygdala of the basal forebrain of the albino rat were studied with combined immunofluorescence and fluorescent retrograde tracing techniques. In each case, the majority of the CCK innervation arises topograpically from subpopulations of neurons in the substantia nigra-ventral tegmental area of the midbrain. This ascending CCK input to the forebrain appears to exceed the amount of descending CCK input from the cortex. In this regard, the CCK innervation of limbic structures is quite different from that of the neostriatum. It has been reported that the CCK innervation of the neostriatum is derived primarily from piriform cortex as a descending corticostriatal projection. Limbic structures, on the other hand, are primarily innervated by ascending CCK, as well as local circuit, projections.

Monoamine innervation of the forebrain: Collateralization

The forebrain is characterized by a dense, localized dopamine (DA) innervation pattern, a diffuse, widespread norepinephrine (NE) innervation pattern, and a serotonin (5-HT) innervation intermediate between the DA and NE patterns. These innervation patterns have implied that basic differences exist in the way DA, NE and 5-HT axons collateralize to different brain structures; that is, DA axons are thought to be poorly collateralized and NE and 5-HT axons are presumed to be more highly collateralized. In the present study, we used injections of retrograde labeling fluorescent dyes into various forebrain regions in order to determine axonal branching patterns from nuclei that contain DA, NE and 5-HT neurons, namely the substantia nigra-ventral tegmental area (SN-VTA), locus coeruleus (LC) and raphe nuclei (DR-MR). The results suggest that at least two subpopulations of neurons can be defined in each monoamine nucleus with respect to the way their axons collateralize. Each area contains a centrally located nuclear area with highly collateralized neurons, and more peripherally situated areas with less highly collateralized neurons. Thus, previous suppositions of the branching of monoamine axons must be revised to account for the existence of cells exhibiting totally different Collateralization patterns within each monoamine nucleus.