12
Neuroscience Letters, 134 (1991) 12 16 @) 1991 Elsevier Scientific Publishers Ireland Ltd. All rights reserved 0304-3940/91/$ 03.50
NSL 08261
The development of ventral tegmental area (VTA) projections to the visual cortex of the rat A t h a n a s i o s D i n o p o u l o s I a n d J o h n G. P a r n a v e l a s 2 1Department of Anatomy, School of Veterinary Medicine, University of Thessaloniki, Thessaloniki (Greece) and 2Department of Anatomy and Developmental Biology, University College London, London (U. K. )
(Received 31 July 1991; Revised version received 20 September 1991;Accepted 20 September 1991) Key words: WGA-HRP tracing; Development; Ventral tegmental area; Visual cortex; Dopaminergic projection
The development of the ventral tegmental area (VTA) projections to the rat visual cortex was studied with the wheat germ agglutinin-horseradish peroxidase (WGA-HRP) retrograde tracing technique. Large injections of WGA-HRP in the visual cortex of newborn, early postnatal, and adult rats resulted in a substantial number of retrogradely labelled neurons in the VTA showing the same distribution pattern at all ages examined. Contrary to other reports, labelled cells were never found in the pars compacta of the substantia nigra but occasionally were seen in the contralateral VTA near the midline. These neurons showed a continuous growth from the day of birth to the end of the second postnatal week, when they acquired morphological features comparable to the adult; they subsequently showed a substantial decrease in soma size. The present results clearly demonstrate that there exists a substantial mesocortical projection to the rat visual cortex which arises exclusively from the VTA. This projection is already established at birth, but the neurons which give rise to it appear morphologically immature during the first two postnatal weeks.
The ventral tegmental area (VTA) o f the midbrain has been considered to be the main site of origin o f the mesolimbocortical dopaminergic system, which sends projection fibers through the medial forebrain bundle to the nucleus accumbens, amygdaloid complex, lateral septal nucleus, bed nucleus o f the stria terminalis, olfactory tubercle, anterior olfactory nucleus, olfactory bulb and the cerebral cortex [6]. The latter projection was initially thought to be limited to prefrontal, cingulate and entorhinal areas [7], but recent evidence indicates that it is more widespread and includes m o t o r and visual cortical areas [3, 4, 15, 16]. The aim o f the present study is to describe the V T A projections into the rat visual cortex, following large wheat germ agglutinin horseradish peroxidase ( W G A - H R P ) injections into the cortex, and to examine their development during postnatal life. In 23 Sprague Dawley albino rats a 4% W G A - H R P solution in sterile water was injected into the visual cortex, using a glass micropipette attached to a l ¢tl Hamilton syringe. Animals o f the following ages were used: postnatal day (P) 0 (P0, day o f birth; n = 6), P3 (n = 4), P6 (n--5), Pl 3 (n = 3), P20 (n = 2) and adult (P90; n = 3). Correspondence." A. Dinopoulos, Department of Anatomy, School of Veterinary Medicine, University of Thessaloniki, 54006 Thessaloniki, Greece.
Rat pups between P0 and P6 were anaesthetized with ether, whereas animals at P I3 and older were anaesthetized with pentobarbital. Animals between P0 and P20 received a single injection o f approximately 20 nl W G A H R P , while adult rats received a total volume o f 100 nl placed in two different sites within the primary visual cortex. Injection coordinates in adults were determined from the atlas o f the rat brain by Paxinos and W a t s o n [10]. Animals were allowed to survive for approximately 48 h (P20 and adults) or 24 h (P13 and younger), after which they were anaesthetized with ether and perfused through the heart with saline followed by a mixture o f 1% paraformaldehyde and 1.25% glutaraldehyde in 0.1 M phosphate buffer (pH 7.4). The brains were removed and immersed in fixative for approximately 5 h, and then placed in 30% sucrose in phosphate buffer overnight. Frozen sections were cut serially in the coronal plane at 40 itm. Every third section was collected in cold phosphate buffer and was processed with tetramethyl benzidine histochemistry according to the procedure o f Mesulain [8]. These sections were subsequently counterstained with neutral red, whilst adjacent sections were stained with toluidine blue. W G A - H R P - l a b e l l e d cells were plotted, with the aid o f a drawing tube attached to a microscope, to establish their distributions within the VTA. Their soma size was estimated as the projected surface
13 area (Sp) of a prolate spheroid using the equation Sp = (n/4) AB in which A and B were the lengths of the major and minor axes, respectively [17]. The minor axis of the soma was considered as the longest axis perpendicular to the major axis. Altogether 907 cells were measured from 8 rats, 2 rats each at P1 (206 cells), P7 (221 cells), P14 (276 cells) and adult (204 cells). The one-way analysis of variance (Duncan procedure) was used for statistical analysis o f this sample. In all cases analyzed, the W G A - H R P injections were
restricted to the visual cortex. In these cases, labelling appeared in all cortical layers throughout most of areas 17 and 18 as well as the medial portion of area 18a. Examination of sections through the thalamus showed retrogradely labelled neurons in the dorsal lateral geniculate nucleus, the lateral posterior nucleus and in the intralaminar nuclei, but not in the ventral posterior nucleus or the medial geniculate body, suggesting that the injections were indeed confined to the visual cortex. Retrogradely labelled cells in the adult were found in
..-..
? g
Q
.i'..;:--..
y--
Y "." Fig. I. A-D: drawings of coronal sections through the ventral tegrnental area of the adult (A) and at postnatal days 14 (B), 7 (C), and 1 (D) showing the pattern of retrograde labelling within the ventral tegmental area following WGA-HRP injections in the visual cortex. Each dot represents one labelled cell. Bar = 1 mm. Abbreviations used in this and the followingfigures: Aq, aqueduct; CG, central gray; cp, cerebral peduncle; fr, fasciculus retroflexus; IP, interpeduncular nucleus; MB, mammillarybody; MG, medial geniculatenucleus; ml, medial lemniscus; RN, red nucleus; SC, superior colliculus; SN, substantia nigra.
14 the VTA, mainly within the parabrachial pigmented nucleus (Fig. IA). Rostrally they were in continuity with labelled cells in the lateral hypothalamic area. Labelled cells were more numerous in rostral parts of the VTA and located dorsolateral to the mammillary body and medial to the substantia nigra. Fewer cells were located near the midline, within the linear nucleus. Caudally, they were fewer in number and were found at all levels until the interpeduncular nucleus disappeared. Labelled neurons were never observed in the pars compacta of the substantia nigra but occasionally were found in the contralateral VTA near the midline. They were generally medium to large in size (Fig. 2c) with fusiform, round and, less often, triangular perikarya. Two of four dendrites were commonly seen to emanate from the somata showing no preferential orientation. Their mean projected surface area was approximately 107 Ftm2 (Fig. 3d). In the newborn and all other postnatal ages examined,
retrogradely labelled cells showed the same distribution pattern as that seen in the adult (Fig. 1B-D). In newborn animals, however, labelled cells were not as densely stained as their adult counterparts and were significantly smaller with a mean projected surface area of about 66 /zm2 (Fig. 3a); some showed rudimentary processes (Fig. 2f). By the end of the first postnatal week they showed a substantial growth in soma size (mean projected surface area 98 gm2; Fig. 3b), but still appeared to give rise to short processes (Fig. 2e). However, during the second postnatal week they showed a sudden growth and at P14 they had a mean projected surface area of about 133/~m 2 (Fig. 3c). At this stage, labelled neurons showed morphological features comparable to the adult, i.e., they were densely stained and showed fairly extensive dendritic processes (Fig. 2a,b,d). Thereafter, they showed a significant decrease in soma size to adult values. The present results show a substantial mesocortical
~A tP It
~
t f
0
,
•
,?
p
4
Fig. 2. Retrogradelylabelledneurons in the ventral tegmentalarea in the adult (c) and at postnatal days 14 (a,b,d), 7 (e) and 1 (f), followingWGAHRP injectionsin the visual cortex, a,b, x 45; c f, x 130.
15
projection to the rat visual cortex. Such a projection was for long underestimated, although it has been described on the basis of retrograde tracing in the cat [12, 16] and rat [11]. In the latter study, retrogradely labelled cells were found in the VTA and a few cells in the ipsilateral pars compacta of the substantia nigra. However, the present results, in agreement with recent findings [15], show that the mesocortical projection to the visual cortex in the rat strictly arises within the ipsilateral VTA with a minor contralateral component. In fact, not a single labelled neuron was found in the pars compacta of the substantia nigra in all animals and ages examined. This is in accordance with the findings in the cat [16] and the suggestion that in this species only A10 cells show cortical projections [12]. There is no direct evidence that the retrogradely
30.
~w
10- l
labelled cells in the VTA described here are dopaminergic. Previous studies have shown that dopamine-conraining neurons in the VTA are intermixed with nondopaminergic perikarya, and that both dopaminergic and non-dopaminergic neurons project to the same target areas [6]. However, the fact that many cells in the VTA contain dopamine and that non-dopaminergic neurons represent a minor part (10-15%) of the VTA neurons projecting to the frontal cortex in the rat [1], suggest that the VTA projection to the rat visual cortex may be to a large extent dopaminergic. Dopaminergic fibers, identified with immunocytochemistry using an antibody against dopamine, were found to be distributed throughout the cortical thickness but chiefly in the deeper layer, of the rat visual cortex [9, 11]. Developmental studies have shown that the biochemical differentiation of dopaminergic neurons within the mesencephalon occurs soon after their mitotic activity has ceased, but before they fully achieve morphological maturity [13, 14, 18]. The present results suggest that neurons in the VTA undergo extensive alterations in their morphological features and soma size during the first two postnatal weeks. It has also been shown, that dopaminergic fibers enter cortical areas very early in life, but at least for the prefrontal and cingular cortex adult patterns of innervation are only achieved by the end of postnatal week 2 and 3 respectively [2, however see also ref. 5] which coincides or follows the time when cells in the VTA were found here to exhibit mature morphological characteristics. We wish to thank Dr. C. van Eden for his helpful comments, Dr. C. Batzios for the statistical analysis, and Mrs. A. Tsipinia for secretarial help. This work was supported by a grant from the Greek Ministry of Research and Technology.
o ..a 50_a 4O
:t 3O gO 150 210 27O 33O PROJECTED SURFACEAREA (IJm~)
Fig. 3. a-d: projected surface area distributions of neurons retrogradely labelled in the ventral tegmental area following WGA-HRP injections in the visual cortex at postnatal days 1 (a), 7 (b), 14 (c) and in the adult (d).
I Albanese, A. and Bentivoglio, M., The organization of dopaminergic and non-dopaminergic mesencephalo-cortical neurons in the rat, Brain Res., 238 (1982) 421-425. 2 Berger, B. and Verney, C., Development of the catecholamine innervation in rat neocortex: morphological features. In L. Descarties, T.R. Reader and H.H. Jasper (Eds.), Monoamine Innervation of Cerebral Cortex, Liss, New York, 1984, pp. 95-121. 3 Berger, B., Verney, C., Alvarez, C., Vigny, A. and Helle, K.B., New dopaminergic terminal fields in the motor, visual (area 18b) and retrosplenial cortex in the young and adult rat. Immunocytochemical and catecholamine histochemical analyses, Neuroscience, 15 (1985) 983-998. 4 Descarries, L., Lemay, B., Doucet, G. and Berger, B., Regional and laminar density of the dopamine innervation in adult rat cerebral cortex, Neuroscience, 21 (1987) 807-824. 5 Kalsbeek, A., Voorn, P., Buijs, R.M., Pool, C.W. and Uylings, H.B.M., Development of the dopaminergic innervation in the prefrontal cortex of the rat, J. Comp. Neurol., 269 (1988) 58-72.
16 6 Lindvall, O. and Bj6rklund, A., Dopamine- and norepinephrinecontaining neuron systems: their anatomy in the rat brain. In P.C. Emson (Ed.), Chemical Neuroanatomy, Raven, New York, 1983, pp. 229-255. 7 Loughlin, S.E. and Fallon, J.H., Substantia nigra and ventral tegmental area projections to cortex: topography and collateralization, Neuroscience, 11 (1984) 425-435. 8 Mesulam, M.-M., Tetramethyl benzidine for horseradish peroxidase neurohistochemistry: a non-carcinogenic blue reaction product with superior sensitivity for visualizing neural afferents and efferents, J. Histochem. Cytochem., 26 (1978) 106-I 17. 9 Papadopoulos, G.C., Parnavelas, J.G. and Buijs, R.M., Light and electron microscopic immunocytochemical analysis of the dopamine innervation of the rat visual cortex, J. Neurocytol., 18 (1989) 303-310. 10 Paxinos, G. and Watson, C., The Rat Brain in Stereotaxic Coordinates, Academic Press, New York, 1982. 11 Phillipson, O.T., Kilpatrick, I.C. and Jones, M.W., Dopaminergic innervation of the primary visual cortex in the rat, and some correlations with human cortex, Brain Res. Bull., 18 (1987) 621~533. 12 Scheibner, T. and T6rk, I., Ventromedial mesencephalic tegmental (VMT) projections to ten functionally different cortical areas in the cat: topography and quantitative analysis, J. Comp. Neurol., 259 (1987) 247-265.
13 Specht, L.A., Pickel, V.M., Joh, T.H. and Reis, D.J., Light-microscopic immunocytochemical localization of tyrosine hydroxylase in prenatal rat brain. I. Early ontogeny, J. Comp. Neurol., 199 (1981) 233-253. 14 Specht, L.A., Pickel, V.M., Joh, T.H. and Reis, D.J., Light-microscopic immunocytochemical localization of tyrosine hydroxylase in prenatal rat brain. II. Late ontogeny, J. Comp. Neurol., 199 (1981) 255-276. 15 Takada, M. and Hattori, T., Organization of ventral tegmental area cells projecting to the occipital cortex and forebrain in the rat, Brain Res., 418 (1987) 27 33. 16 T6rk, I. and Turner, S., Histochemical evidence for a catecholaminergic (presumably dopaminergic) projection from the ventral mesencephalic tegmentum to visual cortex in the cat, Neurosci. Lett., 24 (1981) 215-219. 17 Uylings, H.B.M., Van Pelt, J., Verwer, R.W.H. and McConnell, P., Statistical analysis of neuronal populations. In J.J. Capowski (Ed.), Computer Techniques in Neuroanatomy, Plenum, New York, 1989, pp. 241 264. 18 Voorn, P., Kalsbeek, A., Jorritsma-Byham, B. and Groenewegen, H.J., The pre-and postnatal development of the dopaminergic cell groups in the ventral mesencephalon and the dopaminergic innervation of the striatum of the rat, Neuroscience, 25 (1988) 857-887.
Neuroscience Letters, 134 (1991) 12 16 @) 1991 Elsevier Scientific Publishers Ireland Ltd. All rights reserved 0304-3940/91/$ 03.50
NSL 08261
The development of ventral tegmental area (VTA) projections to the visual cortex of the rat A t h a n a s i o s D i n o p o u l o s I a n d J o h n G. P a r n a v e l a s 2 1Department of Anatomy, School of Veterinary Medicine, University of Thessaloniki, Thessaloniki (Greece) and 2Department of Anatomy and Developmental Biology, University College London, London (U. K. )
(Received 31 July 1991; Revised version received 20 September 1991;Accepted 20 September 1991) Key words: WGA-HRP tracing; Development; Ventral tegmental area; Visual cortex; Dopaminergic projection
The development of the ventral tegmental area (VTA) projections to the rat visual cortex was studied with the wheat germ agglutinin-horseradish peroxidase (WGA-HRP) retrograde tracing technique. Large injections of WGA-HRP in the visual cortex of newborn, early postnatal, and adult rats resulted in a substantial number of retrogradely labelled neurons in the VTA showing the same distribution pattern at all ages examined. Contrary to other reports, labelled cells were never found in the pars compacta of the substantia nigra but occasionally were seen in the contralateral VTA near the midline. These neurons showed a continuous growth from the day of birth to the end of the second postnatal week, when they acquired morphological features comparable to the adult; they subsequently showed a substantial decrease in soma size. The present results clearly demonstrate that there exists a substantial mesocortical projection to the rat visual cortex which arises exclusively from the VTA. This projection is already established at birth, but the neurons which give rise to it appear morphologically immature during the first two postnatal weeks.
The ventral tegmental area (VTA) o f the midbrain has been considered to be the main site of origin o f the mesolimbocortical dopaminergic system, which sends projection fibers through the medial forebrain bundle to the nucleus accumbens, amygdaloid complex, lateral septal nucleus, bed nucleus o f the stria terminalis, olfactory tubercle, anterior olfactory nucleus, olfactory bulb and the cerebral cortex [6]. The latter projection was initially thought to be limited to prefrontal, cingulate and entorhinal areas [7], but recent evidence indicates that it is more widespread and includes m o t o r and visual cortical areas [3, 4, 15, 16]. The aim o f the present study is to describe the V T A projections into the rat visual cortex, following large wheat germ agglutinin horseradish peroxidase ( W G A - H R P ) injections into the cortex, and to examine their development during postnatal life. In 23 Sprague Dawley albino rats a 4% W G A - H R P solution in sterile water was injected into the visual cortex, using a glass micropipette attached to a l ¢tl Hamilton syringe. Animals o f the following ages were used: postnatal day (P) 0 (P0, day o f birth; n = 6), P3 (n = 4), P6 (n--5), Pl 3 (n = 3), P20 (n = 2) and adult (P90; n = 3). Correspondence." A. Dinopoulos, Department of Anatomy, School of Veterinary Medicine, University of Thessaloniki, 54006 Thessaloniki, Greece.
Rat pups between P0 and P6 were anaesthetized with ether, whereas animals at P I3 and older were anaesthetized with pentobarbital. Animals between P0 and P20 received a single injection o f approximately 20 nl W G A H R P , while adult rats received a total volume o f 100 nl placed in two different sites within the primary visual cortex. Injection coordinates in adults were determined from the atlas o f the rat brain by Paxinos and W a t s o n [10]. Animals were allowed to survive for approximately 48 h (P20 and adults) or 24 h (P13 and younger), after which they were anaesthetized with ether and perfused through the heart with saline followed by a mixture o f 1% paraformaldehyde and 1.25% glutaraldehyde in 0.1 M phosphate buffer (pH 7.4). The brains were removed and immersed in fixative for approximately 5 h, and then placed in 30% sucrose in phosphate buffer overnight. Frozen sections were cut serially in the coronal plane at 40 itm. Every third section was collected in cold phosphate buffer and was processed with tetramethyl benzidine histochemistry according to the procedure o f Mesulain [8]. These sections were subsequently counterstained with neutral red, whilst adjacent sections were stained with toluidine blue. W G A - H R P - l a b e l l e d cells were plotted, with the aid o f a drawing tube attached to a microscope, to establish their distributions within the VTA. Their soma size was estimated as the projected surface
13 area (Sp) of a prolate spheroid using the equation Sp = (n/4) AB in which A and B were the lengths of the major and minor axes, respectively [17]. The minor axis of the soma was considered as the longest axis perpendicular to the major axis. Altogether 907 cells were measured from 8 rats, 2 rats each at P1 (206 cells), P7 (221 cells), P14 (276 cells) and adult (204 cells). The one-way analysis of variance (Duncan procedure) was used for statistical analysis o f this sample. In all cases analyzed, the W G A - H R P injections were
restricted to the visual cortex. In these cases, labelling appeared in all cortical layers throughout most of areas 17 and 18 as well as the medial portion of area 18a. Examination of sections through the thalamus showed retrogradely labelled neurons in the dorsal lateral geniculate nucleus, the lateral posterior nucleus and in the intralaminar nuclei, but not in the ventral posterior nucleus or the medial geniculate body, suggesting that the injections were indeed confined to the visual cortex. Retrogradely labelled cells in the adult were found in
..-..
? g
Q
.i'..;:--..
y--
Y "." Fig. I. A-D: drawings of coronal sections through the ventral tegrnental area of the adult (A) and at postnatal days 14 (B), 7 (C), and 1 (D) showing the pattern of retrograde labelling within the ventral tegmental area following WGA-HRP injections in the visual cortex. Each dot represents one labelled cell. Bar = 1 mm. Abbreviations used in this and the followingfigures: Aq, aqueduct; CG, central gray; cp, cerebral peduncle; fr, fasciculus retroflexus; IP, interpeduncular nucleus; MB, mammillarybody; MG, medial geniculatenucleus; ml, medial lemniscus; RN, red nucleus; SC, superior colliculus; SN, substantia nigra.
14 the VTA, mainly within the parabrachial pigmented nucleus (Fig. IA). Rostrally they were in continuity with labelled cells in the lateral hypothalamic area. Labelled cells were more numerous in rostral parts of the VTA and located dorsolateral to the mammillary body and medial to the substantia nigra. Fewer cells were located near the midline, within the linear nucleus. Caudally, they were fewer in number and were found at all levels until the interpeduncular nucleus disappeared. Labelled neurons were never observed in the pars compacta of the substantia nigra but occasionally were found in the contralateral VTA near the midline. They were generally medium to large in size (Fig. 2c) with fusiform, round and, less often, triangular perikarya. Two of four dendrites were commonly seen to emanate from the somata showing no preferential orientation. Their mean projected surface area was approximately 107 Ftm2 (Fig. 3d). In the newborn and all other postnatal ages examined,
retrogradely labelled cells showed the same distribution pattern as that seen in the adult (Fig. 1B-D). In newborn animals, however, labelled cells were not as densely stained as their adult counterparts and were significantly smaller with a mean projected surface area of about 66 /zm2 (Fig. 3a); some showed rudimentary processes (Fig. 2f). By the end of the first postnatal week they showed a substantial growth in soma size (mean projected surface area 98 gm2; Fig. 3b), but still appeared to give rise to short processes (Fig. 2e). However, during the second postnatal week they showed a sudden growth and at P14 they had a mean projected surface area of about 133/~m 2 (Fig. 3c). At this stage, labelled neurons showed morphological features comparable to the adult, i.e., they were densely stained and showed fairly extensive dendritic processes (Fig. 2a,b,d). Thereafter, they showed a significant decrease in soma size to adult values. The present results show a substantial mesocortical
~A tP It
~
t f
0
,
•
,?
p
4
Fig. 2. Retrogradelylabelledneurons in the ventral tegmentalarea in the adult (c) and at postnatal days 14 (a,b,d), 7 (e) and 1 (f), followingWGAHRP injectionsin the visual cortex, a,b, x 45; c f, x 130.
15
projection to the rat visual cortex. Such a projection was for long underestimated, although it has been described on the basis of retrograde tracing in the cat [12, 16] and rat [11]. In the latter study, retrogradely labelled cells were found in the VTA and a few cells in the ipsilateral pars compacta of the substantia nigra. However, the present results, in agreement with recent findings [15], show that the mesocortical projection to the visual cortex in the rat strictly arises within the ipsilateral VTA with a minor contralateral component. In fact, not a single labelled neuron was found in the pars compacta of the substantia nigra in all animals and ages examined. This is in accordance with the findings in the cat [16] and the suggestion that in this species only A10 cells show cortical projections [12]. There is no direct evidence that the retrogradely
30.
~w
10- l
labelled cells in the VTA described here are dopaminergic. Previous studies have shown that dopamine-conraining neurons in the VTA are intermixed with nondopaminergic perikarya, and that both dopaminergic and non-dopaminergic neurons project to the same target areas [6]. However, the fact that many cells in the VTA contain dopamine and that non-dopaminergic neurons represent a minor part (10-15%) of the VTA neurons projecting to the frontal cortex in the rat [1], suggest that the VTA projection to the rat visual cortex may be to a large extent dopaminergic. Dopaminergic fibers, identified with immunocytochemistry using an antibody against dopamine, were found to be distributed throughout the cortical thickness but chiefly in the deeper layer, of the rat visual cortex [9, 11]. Developmental studies have shown that the biochemical differentiation of dopaminergic neurons within the mesencephalon occurs soon after their mitotic activity has ceased, but before they fully achieve morphological maturity [13, 14, 18]. The present results suggest that neurons in the VTA undergo extensive alterations in their morphological features and soma size during the first two postnatal weeks. It has also been shown, that dopaminergic fibers enter cortical areas very early in life, but at least for the prefrontal and cingular cortex adult patterns of innervation are only achieved by the end of postnatal week 2 and 3 respectively [2, however see also ref. 5] which coincides or follows the time when cells in the VTA were found here to exhibit mature morphological characteristics. We wish to thank Dr. C. van Eden for his helpful comments, Dr. C. Batzios for the statistical analysis, and Mrs. A. Tsipinia for secretarial help. This work was supported by a grant from the Greek Ministry of Research and Technology.
o ..a 50_a 4O
:t 3O gO 150 210 27O 33O PROJECTED SURFACEAREA (IJm~)
Fig. 3. a-d: projected surface area distributions of neurons retrogradely labelled in the ventral tegmental area following WGA-HRP injections in the visual cortex at postnatal days 1 (a), 7 (b), 14 (c) and in the adult (d).
I Albanese, A. and Bentivoglio, M., The organization of dopaminergic and non-dopaminergic mesencephalo-cortical neurons in the rat, Brain Res., 238 (1982) 421-425. 2 Berger, B. and Verney, C., Development of the catecholamine innervation in rat neocortex: morphological features. In L. Descarties, T.R. Reader and H.H. Jasper (Eds.), Monoamine Innervation of Cerebral Cortex, Liss, New York, 1984, pp. 95-121. 3 Berger, B., Verney, C., Alvarez, C., Vigny, A. and Helle, K.B., New dopaminergic terminal fields in the motor, visual (area 18b) and retrosplenial cortex in the young and adult rat. Immunocytochemical and catecholamine histochemical analyses, Neuroscience, 15 (1985) 983-998. 4 Descarries, L., Lemay, B., Doucet, G. and Berger, B., Regional and laminar density of the dopamine innervation in adult rat cerebral cortex, Neuroscience, 21 (1987) 807-824. 5 Kalsbeek, A., Voorn, P., Buijs, R.M., Pool, C.W. and Uylings, H.B.M., Development of the dopaminergic innervation in the prefrontal cortex of the rat, J. Comp. Neurol., 269 (1988) 58-72.
16 6 Lindvall, O. and Bj6rklund, A., Dopamine- and norepinephrinecontaining neuron systems: their anatomy in the rat brain. In P.C. Emson (Ed.), Chemical Neuroanatomy, Raven, New York, 1983, pp. 229-255. 7 Loughlin, S.E. and Fallon, J.H., Substantia nigra and ventral tegmental area projections to cortex: topography and collateralization, Neuroscience, 11 (1984) 425-435. 8 Mesulam, M.-M., Tetramethyl benzidine for horseradish peroxidase neurohistochemistry: a non-carcinogenic blue reaction product with superior sensitivity for visualizing neural afferents and efferents, J. Histochem. Cytochem., 26 (1978) 106-I 17. 9 Papadopoulos, G.C., Parnavelas, J.G. and Buijs, R.M., Light and electron microscopic immunocytochemical analysis of the dopamine innervation of the rat visual cortex, J. Neurocytol., 18 (1989) 303-310. 10 Paxinos, G. and Watson, C., The Rat Brain in Stereotaxic Coordinates, Academic Press, New York, 1982. 11 Phillipson, O.T., Kilpatrick, I.C. and Jones, M.W., Dopaminergic innervation of the primary visual cortex in the rat, and some correlations with human cortex, Brain Res. Bull., 18 (1987) 621~533. 12 Scheibner, T. and T6rk, I., Ventromedial mesencephalic tegmental (VMT) projections to ten functionally different cortical areas in the cat: topography and quantitative analysis, J. Comp. Neurol., 259 (1987) 247-265.
13 Specht, L.A., Pickel, V.M., Joh, T.H. and Reis, D.J., Light-microscopic immunocytochemical localization of tyrosine hydroxylase in prenatal rat brain. I. Early ontogeny, J. Comp. Neurol., 199 (1981) 233-253. 14 Specht, L.A., Pickel, V.M., Joh, T.H. and Reis, D.J., Light-microscopic immunocytochemical localization of tyrosine hydroxylase in prenatal rat brain. II. Late ontogeny, J. Comp. Neurol., 199 (1981) 255-276. 15 Takada, M. and Hattori, T., Organization of ventral tegmental area cells projecting to the occipital cortex and forebrain in the rat, Brain Res., 418 (1987) 27 33. 16 T6rk, I. and Turner, S., Histochemical evidence for a catecholaminergic (presumably dopaminergic) projection from the ventral mesencephalic tegmentum to visual cortex in the cat, Neurosci. Lett., 24 (1981) 215-219. 17 Uylings, H.B.M., Van Pelt, J., Verwer, R.W.H. and McConnell, P., Statistical analysis of neuronal populations. In J.J. Capowski (Ed.), Computer Techniques in Neuroanatomy, Plenum, New York, 1989, pp. 241 264. 18 Voorn, P., Kalsbeek, A., Jorritsma-Byham, B. and Groenewegen, H.J., The pre-and postnatal development of the dopaminergic cell groups in the ventral mesencephalon and the dopaminergic innervation of the striatum of the rat, Neuroscience, 25 (1988) 857-887.
