Literally How to use a word that literally drives some pe Is Singular 'They' a Better Choice? The awkward case of 'his or her'. Take the quiz. Our Favorite New Words How many do you know? How Strong Is Your Vocabulary? Note the different fiber density depending on the target structure. One of the BDA microdeposits was located at 0. The other four BDA deposits were placed more medially, approximately at 0.
One of these deposits very slightly invaded the PN but, since the distribution of the axonal terminal fields was similar to that of the other deposits located exclusively within PBP, we did not discard it.
Because the animals received bilateral injections into PBP the putative contralateral projections could not be assessed in this analysis see Table 1 in Supplementary Material. The terminal axonal fields were composed of thin collaterals bearing varicosities or clusters of axon terminals.
A single BDA deposit resulted in terminal axonal fields of different sizes and fiber density depending on the target structure Figures 3B—D. For this reason, based on visual observation, we have qualitatively distinguished high, medium and low innervations in the various target structures of each BDA deposit, as shown in Table 2. Table 2. Brain structures innervated by each PBP deposit and intensity of innervation.
The most intensely innervated regions were related to the olfactory system, such as the anterior olfactory area AO , island of Calleja ICj , Tu Figure 3B , ventral tenia tecta VTt and the piriform cortex Pir. PBP also projected to multiple other cortical areas, either with high intensity in the case of the lateral orbital cortex LO , which is a major source of inputs to DAergic VTA neurons Watabe-Uchida et al.
Within the diencephalon and brainstem, the more densely innervated structures were the lateral hypothalamic area LH , the magnocellular nucleus of the lateral hypothalamus MCLH , and the oral part of the pontine reticular nucleus PnO. Other diencephalic and brainstem structures shared light labeling with low intensity Table 2.
The neural structures innervated by each BDA microdeposit and the degree of innervation observed in each of the targets, estimated qualitatively, are summarized in Table 3. The morphological features of the terminal axonal fields labeled from PN neurons were very similar to those observed with PBP neurons and mostly composed of thin varicose axons. Table 3. Brain structures innervated by each PN deposit and intensity of innervation.
As observed in PBP, the PN neurons labeled from the BDA deposits also project with different intensities to multiple cortical and subcortical structures, but the projections to the cerebral cortex from PN were more markedly restricted than those of PBP neurons. The results gathered from the analysis of the BDA microdeposits placed in PBP and PN indicate that these subdivisions possess neurons with widespread cortical and subcortical prosencephalic and brainstem projections.
Since the BDA microdeposits invariably label small groups of neurons, these findings suggest that, at least within these two main VTA subdivisions, there are either distinct types of closely intermingled projection neurons or single neurons whose axons are extensively ramified to innervate widely scattered brain regions. To ascertain whether the patterns of labeling observed with the BDA microdeposits reflect the axon morphology of individual cells or a mixture of different cell phenotypes, we undertook the task of labeling single neurons located in different VTA subdivisions and tracing their entire axonal arborization.
A total of 30 neurons infected with the GFP-producing Sindbis viral vector have been analyzed here. For each neuron we first examined whether it expressed TH which is a reliable marker for the DAergic neurons in the ventral mesencephalon Fu et al.
Although the other 25 neurons were not demonstrated to express TH, we cannot be sure that at least some of them did not use DA as a neurotransmitter see Methodological Considerations in the Discussion.
Two neurons out of 30 6. The two somata were found in the medial aspect of PBP 0. The two neurons had round-shaped perikarya from which emerged 4 to 6 poorly ramified dendrites. Figure 4.
Mesocorticolimbic neurons. A Sagittal reconstruction of a single PBP mesocorticolimbic DAergic neuron superimposed over a calbindin CB -stained composition of two different mediolateral levels 0. The main axon thick and red reaches the cerebral cortex after providing three main collaterals that are indicated from caudal to rostral with light blue, green and dark blue arrowheads.
Terminal axonal fields placed at mediolateral levels that do not correspond to the level of the CB-section are indicated between parentheses. D Sagittal drawing of a PBP mesocorticolimbic neuron superimposed over an acetylcholinesterase AChE -stained composition in which the dashed line separates a medial section containing PBP 0. The main axon thick and red reached the cerebral cortex after providing three main collaterals indicated with light blue, green, and dark blue arrowheads.
Terminal axonal fields provided by the light blue and green collaterals that reach the AStr, CPu, Cl, and S2 at very lateral levels are indicated in gray color. The axons of both mesocorticolimbic neurons had the following common features: the main axonal branch traversed LH and the substantia innominata SI emitting some thin and short collaterals at these areas, and, as it traversed the accumbens Acb , it gave off collaterals toward the basal forebrain.
The specific features of these two axons can be observed in Figure 4. A second collateral green color was emitted as the main axon traversed the AcbSh and this collateral targeted VP, Pir and more laterally BL and the perirhinal cortex PRh. It also innervated the Cl and the FrA. As the main axon traversed the Cl it provided abundant varicosities or boutons en passant , and a third collateral dark blue color that headed ventrally through the posterior part of the anterior olfactory area AOP toward VP.
The axon of the second mesocorticolimbic neuron Figures 4D—H provided a short collateral at the subthalamic nucleus STh and, before entering Acb, it gave off its first long collateral light blue , which ran laterally to profusely innervate the AStr before targeting the most lateral aspect of the CPu, the Cl, and S2. As the main axon traversed the VP it emitted a second collateral green that extensively targeted the Tu and the anterior amygdaloid area AAV , then continued toward the capsular part of the Ce CeC and the AStr where it joined the first collateral and innervated the same lateral aspect of CPu, Cl, and S2.
As the main axon approached the cortex it emitted some thin branches at Cl as well as a third collateral dark blue that innervated the AI and FrA. Finally, it branched into numerous varicose fibers that spread in all layers of the M2 as well as the deep layers of the S2 and Cg, giving a last fiber that headed ventrally and laterally toward the DEn.
Their cell bodies were located in the caudal sector of PBP at 0. The neurons had ovoid or fusiform perikarya from which emerged 2 to 3 poorly branched primary dendrites.
Figure 5. Mesocortical and mesolimbic neurons. A Sagittal reconstruction of two mesocortical neurons superimposed over a calbindin-stained composition of two different mediolateral levels 0. B Terminal arborizations at cortical S1 deep layers from the two neurons.
C Sagittal reconstruction of a PBP mesolimbic neuron superimposed over an acetylcholinesterase-stained composition in which the dashed line separates the more medial caudal part 0. F Photomicrograph of the somatodendritic domain of the neuron.
The axon of the neuron located in PBP Figure 5A , after emitting a local collateral that innervated the contralateral PBP, ran rostrally, giving two short collaterals that lightly targeted the SI and the lateral globus pallidus LGP and then traversed the CPu, emitting only one branch toward Cl.
Finally, the main axon entered the S1 cortical area, branching into numerous thin processes scattered in all its layers Figure 5B. The axon of the second neuron from rVTA was similar to the first except that it also innervated the S2 moderately Figure 5A. Both reconstructed mesocortical neurons were located in the same hemisphere of the same animal, so both axons merged at S1, making it impossible to separate the terminal fields provided by each neuron.
Therefore, the cortical highly dense field at S1 depicted in Figure 5A actually corresponds to two mesocortical neurons. It is noteworthy that all the PN neurons reconstructed in the present study were mesolimbic and that major targets of this neuronal type, namely Acb and Tu, were not innervated by the same neurons.
The mesolimbic neurons had ovoid, fusiform or polygonal perikarya with 3—4 poorly branched dendrites. The length of the terminal axonal arbor at Tu and VP was The other three cells were located at PN: one DAergic neuron also collateralized at VP and arborized profusely in Tu and ICj; another one targeted the deepest Tu layers as well as VP, the superficial layer of Pir and the amygdala, and was the only neuron from this group that provided local collaterals at PN and PBP; the third axon sparsely innervated the Tu as well as other structures see Table 4 for further details on each neuronal arborization pattern.
The Acb was targeted by another six neurons, whose axons, after traveling through LH and SI without giving any collaterals, provided different arborization patterns. Two of these neurons arose from the lateral aspect of PBP mediolateral level 0. One of them was located at medial PBP 0. Another two neurons from rVTA 0.
There were two other neurons that exclusively targeted the LS PN neuron; mediolateral level 0. Figure 6. Mesolimbic neuron. A Sagittal reconstruction of a single mesolimbic neuron located in PN superimposed over a calbindin-stained section corresponding to 0.
The neurons were located at 0. The axon from the PBP neuron branched profusely within the central sector of the CPu, producing a dense terminal arbor with a length of The other neuron targeted the dorsal part of the CPu and the subcallosal stripe providing a heavy terminal field with numerous varicose branches. Figure 7. A Sagittal reconstruction of a single mesostriatal neuron located at PBP superimposed over an acetylcholinesterase-stained section corresponding to 1.
B Photomicrograph of the neuron whose axon is shown in A. C Terminal fibers at the central aspect of CPu. E Cell body of the neuron whose axon is shown in D. Figure 8. VTA projection neuron phenotypes. Note that for each neuron type, the drawing represents the main structures innervated by the sum of all the neurons of that type reconstructed in the present study. The relative proportion of each VTA projection neuron phenotype is indicated.
The black lines at the bottom are thicker at the levels at which the neurons of a given type are especially abundant. These neurons had ovoid or fusiform perikarya from which emerged 3 to 5 dendrites. In contrast to other highly ramified VTA neurons such as the mesocorticolimbic ones, whose main axons exited the VTA region and could be traced upstream to their final targets, the axons of this type of neuron broke out into numerous collaterals of similar thicknesses within VTA, thus precluding the identification of a putative main axonal branch, and the terminal fields provided by these collaterals at the target structures were very limited.
For a detailed description of each neuronal arborization pattern, see Table 4. The hypothalamus was frequently innervated by these neurons 9 out of 12 scattered through the four VTA subdivisions; the LH and posterior hypothalamic nucleus PH were the main targets of three PBP cells. In contrast, the rest of the terminals at the thalamus were seen at medial thalamic nuclei [reuniens Re , rhomboid Rh , submedius Sub , and paraventricular PV ] and were provided by rVTA or VTT neurons.
The amount of innervation that mesocorticolimbic, mesolimbic and mesostriatal VTA neurons provide at their main target sites is depicted in Table 5. The longest length of terminal axonal arbor was found in the dorsal CPu Terminal arbors at Tu also tended to be dense, though the amount of fibers was much lower than in CPu, and varied considerably depending on the neuronal type that provided the field.
Thus, the length of axonal arbor at Tu provided by a mesolimbic neuron In the mesocorticolimbic neuronal type the terminals at the cerebral cortex were double those at Tu. In terms of the length of the total terminal axonal arbor provided by single neurons the highest value corresponded to the mesostriatal neuron, with It is also worth noting that the length of the total terminal axonal arbor provided by two mesolimbic neurons was very different, one was The present study was designed to elucidate the cellular diversity of the mouse VTA projection neurons and whether the various cytoarchitectonic subdivisions of VTA innervate different or equivalent sets of targets.
To explore these questions, we first examined the brain structures containing terminal axonal fields labeled from small deposits of the anterograde BDA tracer placed in a single VTA subdivision. This analysis demonstrated that small numbers of neurons located in PBP and PN project to widely distributed cortical and subcortical prosencephalic and brainstem structures, indicating either that these VTA subdivisions are populated by distinct and closely intermingled projection neuron types or that their projection neurons have extensively ramified axons.
To ascertain whether the patterns of BDA labeling reflected the axonal morphology of individual cells or a mixture of different cell phenotypes, we undertook the task of labeling single neurons located at different VTA subdivisions and tracing their entire axonal arborization. Moreover, the projection targets of the forebrain-projecting neurons distinguished four different types: neurons that project to the neocortex and basal forebrain mesocorticolimbic , other neurons that almost exclusively innervate the neocortex mesocortical , others whose axons project to the basal forebrain AcbC and CPu mesolimbic , and mesostriatal neurons that extensively innervate the CPu.
We also used immunohistochemistry against TH to examine the DA phenotype of the neurons whose axons were reconstructed and confirmed that DA was present in the forebrain-projecting neurons.
Takahiro Furuta and Dr. This vector is highly efficient for visualizing the whole arborization of single axons in their entirety because of the massive synthesis of GFP protein once a codifying RNA, in principle a single one, has entered a cell Kuramoto et al. Next, we performed immunofluorescence staining for TH to determine the DAergic phenotype of the infected neurons.
The suspicion that something could be hampering the expression of TH in many Sindbis-pal-eGFP infected VTA neurons was strongly supported when we performed a large injection of the fluorescent tetramethylrodamine anterograde tracer in VTA, and found that of the 24 neurons that were labeled by the tracer, 12 expressed TH immunostaining. We consequently suspect that many of the reconstructed TH-negative neurons might have actually been DAergic. The half-life of the TH protein is 30 h Tank et al.
It should be noted that in the study of the nigrostriatal pathway by Matsuda et al. This finding is still more striking when the survival time used in that study was even shorter than the one used here, ranging from 36 to 42 h. In this section we will review the projections of these VTA subdivisions in mice. Numerous studies have reported an extensive cortical innervation with origin in VTA Beckstead et al.
In the present study we have visualized wide cortical innervation from PBP neurons, whose axons targeted sensory somatosensory , motor, limbic retrosplenial, cingular, entorhinal , prefrontal prelimbic , and association frontal, orbital territories.
Within the motor and somatosensory cortices, the axon terminals labeled from PBP injections innervated primary and secondary territories.
According to the results shown here, the projections from PBP to the M1 or S1 cortices could have origin in neurons that are different from those that target M2 or S2 territories; though larger number of such neurons needs to be traced in the future so as to confirm these preliminary observations. This finding indicates that PBP holds neurons focused on exclusively innervating either primary or secondary cortices, and that at least some of those targeting M1 and S1 are DAergic.
The selectivity that some PBP neurons show toward innervating primary or secondary sensorimotor territories strengthens the functional specificity of the DA innervation of M1 and M2, which in M1 is necessary for motor skill learning Hosp et al.
Therefore, the nigrostriatal pathway is involved in voluntary movement, in contrast, the mesolimbic and mesocortical pathways are involved in cognition and emotion Phillipson, ; Swanson, ; Oades and Halliday, ; van Domburg and ten Donkelaar, These results suggest that there may be differences in neural connectivity between the SN and the VTA.
Diffusion tensor imaging DTI has a unique advantage in evaluation of white matter by virtue of its ability to visualize water diffusion characteristics Basser et al.
Recently developed multi-tensor model DTI allows to estimate more than one fiber population in the each imaging voxel and suggests that probability corresponds to multiple fiber populations whereas the single tensor model DTI analyzes only a dominant fiber bundle Smith et al.
Many multi-tensor model DTI studies have reported on neural connectivity in normal subjects Behrens et al. We recruited 63 healthy subjects males: 31, females: 32, mean age: All subjects understood the purpose of the study and provided written, informed consent prior to participation.
DTI data were acquired using a 6-channel head coil on a 1. For each of the 32 non-collinear diffusion sensitizing gradients, we acquired 67 contiguous slices parallel to the anterior commissure-posterior commissure line.
Affine multi-scale two-dimensional registration was used for correction of head motion effect and image distortion due to the eddy current. Mean translation and rotation was observed the sub-one pixel 0. Fiber tracking was performed using a probabilistic tractography method based on a multi-fiber model, and applied in the present study utilizing tractography routines implemented in FMRIB Diffusion streamline samples, 0.
This fiber tracking method by multi-fiber model calculated and generated streamline samples from seed region of interest ROI with consideration of the both dominant and non-dominant orientation of diffusion in each voxel and showed how connects the brain regions. Therefore, it has advantage to solve the problem of the crossing fiber. Especially, cross points of the corpus callosum and corona radiata, corticospinal tract fibers and pontocerebellar fibers at pons, and superior and medial frontal gyri are known to be the crossing fiber point Wiegell et al.
For the connectivity of the SN, a seed ROI was placed on the isolated SN of the upper midbrain on the color-coded map dorsomedially next to the cerebral peduncle of the upper midbrain Mori et al. We identified the VTA by reconstructing the adjacent structures: interpeduncular nucleus anterior boundary , central tegmental tract posterior www. Out of samples generated from the seed voxel, results for contact were visualized with the threshold at a minimum of five streamline through each voxel for analysis.
Connectivity represented the percentage as all hemispheres of 63 subjects. Figure 1. Neural connectivity of substantia nigra and ventral tegmental area.
A The region of interest ROI : a seed ROI for substantia nigra SN, orange , is placed on the isolated SN of the upper midbrain on the B0 and color-coded map dorsomedially next to the cerebral peduncle of the upper midbrain.
We use other structures to isolate the VTA such as interpeduncular nucleus anterior boundary, red , central tegmental tract posterior boundary, white-lined rectangular , midline medial boundary , red nucleus blue and SN lateral boundary. SPSS software v. The Chi-square test was used for determination of the difference in connectivity between the right and left hemispheres, and between the SN and VTA. In addition, we performed an independent t -test for determination of differences in size of ROI between the SN and VTA, and between the right and left hemispheres.
The significant level of the p value was set at 0. Table 1. Comparison of connectivity between substantia nigra and ventral tegmental area with brain regions. These data indicate that the neural connectivity of the SN could be much higher than that of the VTA.
Therefore, our results that the neural connectivity of the SN was higher than that of the VTA are in accordance with the results of previous studies Hirsch et al. As for the connection areas, many animal studies have described the difference in the working areas of the SN and VTA Phillipson, ; Swanson, ; Oades and Halliday, ; van Domburg and ten Donkelaar, The classical concept was that the SNc mainly works for the nigrostriatal pathway, whereas the VTA works for both the mesolimbic and mesocortical pathways Phillipson, ; Swanson, ; Oades and Halliday, ; van Domburg and ten Donkelaar, Therefore, it has been known that SNc is mainly connected with the striatum and VTA is mainly with nucleus accumbens and frontal cortex.
In , Menke et al. These maps showed that the SNc was connected with the posterior striatum, the pallidum, the anterior limb of the internal capsule, anterior thalamic nuclei, and anterior thalamic radiation leading to the prefrontal cortex. By contrast, the SNr was connected to the posterior striatum, ventral thalamus, posterior limb of internal capsule, and tracts leading to premotor and primary sensori-motor cortices.
Thus, the VTA is also activated when one experiences something rewarding, and this integral role in the mediation of rewarding experiences has caused some to propose that activity in the VTA may be necessary to the development of addiction.
Dopamine is also important to normal cognition and so it is not surprising that the VTA has been implicated in the pathophysiology of disorders other than addiction. For example, dopaminergic neurons in the VTA have been proposed to play a role in schizophrenia, a disorder that is thought by some to be associated with high levels of dopamine.
In truth, the dopaminergic projections from the VTA are so extensive that they are likely involved to some degree in a wide variety of normal and pathological behavior, but it is still not very clear exactly what role they play in most cognitive processesboth normal and disordered.
It does seem clear, however, that due to the importance of dopamine signaling throughout the brain and the widespread dopaminergic projections of the VTA, the integrity of the VTA is crucial to proper brain function. Kalivas, P. Neurotransmitter regulation of dopamine neurons in the ventral tegmental area Brain Research Reviews, 18 1 , DOI:
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