This is the first rigorous analysis of cortical DAergic innervation between human and nonhuman primate species and the first characterization of cortical DAergic innervation in the chimpanzee. Direct comparative studies of the DAergic system have heretofore been restricted to analyses of rats versus humans and macaque monkeys. The differences between these two groups in both the organization of the frontal cortex and in DAergic innervation of this area are striking and suggest evolutionary changes that paralleled increases in both the size and functional differentiation of the cerebral cortex (e.g., van Eden et al., 1987
; Berger et al., 1991
; Preuss, 1995
; Sesack et al., 1995
; Williams and Goldman-Rakic, 1998
). For example, the primary motor cortex is thought to be the latest development of a primary cortical field in mammalian neocortical evolution (Sanides, 1970
; Kaas, 2004
), and is the most densely DA-innervated cortical area in primates (Lewis et al., 1987
; Gaspar et al., 1989
; Berger et al., 1991
). However, homologous motor areas in rodents are sparsely innervated (Berger et al., 1991
). Also, the molecular layer is a widespread target for dense DAergic innervation in all areas of the macaque and human neocortex, but DAergic innervation of layer I is restricted to only a few areas in rats. Indeed, the motor, parietal, and temporal cortical areas of rats do not have DAergic afferents in the molecular layer. Finally, primates share neurochemical properties of DAergic afferents that are not found in rats, suggesting that the DAergic neurons innervating the frontal cortex of humans and nonhuman primates are fundamentally distinct from the DAergic neurons that innervate the infragranular layers in rodents (Studler et al., 1988
; Gaspar et al., 1990
; Berger et al., 1991
; Berger et al., 1992
). Such significant differences between rodents and primates prevent direct comparisons of cortical DAergic function within the frontal cortex. The importance of these differences in the presynaptic component of the cortical DAergic system between these taxonomic groups is further supported by the finding that primates have an accelerated rate of protein evolution for the DA receptor gene, DRD2
, relative to rodents (Dorus et al., 2004
A broader view of phylogenetic differences has been provided by Hof and collaborators in an analysis of cortical TH-ir axon distribution in the harbor porpoise and pilot whale (Hof et al., 1995
). Their findings revealed a different pattern of innervation of auditory and visual cortices of cetaceans compared to that of other mammals. Most other mammals share in common a sparser DAergic innervation in primary sensory cortices relative to all other cortical areas. This is particularly true of the primary visual cortex, where humans and other primates exhibit TH-ir axons only in layer I and DAergic axons are rarely found in rodent visual cortex (Gaspar et al., 1989
; Berger et al., 1991
). In contrast, the cetacean primary visual cortex is innervated throughout all layers, and it is more densely innervated than the auditory cortex, whereas the reverse is true for the other mammals (Hof et al., 1995
). Such phylogenetic differences strongly suggest a potential role for this neurotransmitter in brain evolution.
Additional lines of evidence suggest that DAergic systems may have been subtly altered in human evolution. For example, there is considerable evidence indicating that DA dysfunction plays an important role in a number of neuropsychiatric disorders presenting with cognitive deficits that preferentially afflict humans, including Alzheimer’s disease, Parkinson’s disease, and schizophrenia (Akil et al., 1999
; Ciliax et al., 1999
; Venator et al., 1999
; Sutoo et al., 2001
; Winterer and Weinberger, 2004
). Further, a substantial decrease in TH-ir axons in layer VI of area 9 occurs in schizophrenic subjects (Akil et al., 1999
), potentially underlying working memory deficits in this disease (Akil et al., 1999
; Abi-Dargham, 2004
). Humans appear to be uniquely susceptible to these disease states that are associated with devastating cognitive deficits. This susceptibility may be due to an increased reliance on DAergic systems to support intellectual capabilities. Based on this evidence, we hypothesized that humans would have significantly more cortical TH-ir fibers, particularly in prefrontal cortical areas relative to other primate species. Previc (1999)
proposed that DAergic systems increased and expanded within the human neocortex, and that this increase was responsible for the origins of human intelligence. However, the results of the present study do not fully support this hypothesis and indicate that humans do not exhibit an overt increase of DAergic innervation within the cerebral cortex in comparison to their close phylogenetic relatives, chimpanzees.
Nonetheless, the evaluation of TH-ir axon length density within each species demonstrated interesting differences among humans, chimpanzees, and macaques, which might have functional implications. Although these differences cannot be used in direct between-species comparisons because they do not take into account differential tissue shrinkage, these analyses illustrate that the species examined vary in regional pattern of cortical DAergic innervation within the frontal cortex (see Tables and ). Briefly, macaques display a denser innervation in layers I and II relative to layers III and V/VI in all three cortical regions examined. In contrast, humans display a much higher innervation in layers V/VI only in areas 9 and 32. The pattern of innervation for chimpanzees is altogether different, with layer I being the most densely innervated layer only in area 4.
Direct comparisons of ALv/Nv in each layer and area among humans, chimpanzees, and macaques found general similarities in laminar patterns, however, comparisons made between the cortical areas involved in cognition versus the primary motor cortex revealed interesting species differences. Of the different cortical regions, the primary motor cortex has the densest DAergic innervation in primates, including humans (Lewis et al., 1987
; Gaspar et al., 1989
). Our analyses detected significant differences in layers III and V/VI of macaques. Density of TH-ir axon length relative to neuron density was significantly less in these layers in areas 9 and 32 when compared to area 4. No such differences were found in either humans or chimpanzees, with areas 9 and 32 being as densely innervated as area 4. These results suggest an evolutionary shift towards relatively denser DAergic innervation of layers III and V/VI of these prefrontal areas in humans and chimpanzees.
The morphologic appearance of TH-ir fibers in the molecular layer of areas 9 and 32 in humans was considerably different from the distribution observed in macaques and chimpanzees. Innervation of layer I in humans is mostly restricted to the lower portion of the layer, whereas the entire layer is equally innervated in the other species. This observation was previously made in humans, with the suggestion that this pattern could have evolutionary implications (Gaspar et al., 1989
). This pattern of sublaminar innervation in the molecular layer has been reported in agranular cortices of long-tailed macaque monkeys (Berger et al., 1988
), but may have been extended in humans to include both agranular and granular cortices (Gaspar et al., 1989
Another species difference concerns the coil-like accumulations of TH-ir fibers within the cortical mantle of only humans and chimpanzees, most commonly observed in layer III. Although coils of TH-ir axons were previously described in humans, the functional significance of these structures is unknown (Gaspar et al., 1989
; Benavides-Piccione and DeFelipe, 2003
). We did not detect the presence of coils in any of the areas examined in macaques, consistent with earlier reports. A detailed analysis that included long-tailed macaques (Macaca fascicularis
) and squirrel monkeys (Saimiri sciureus
) did not report the presence of TH-ir coils within the cortical mantle (Lewis et al., 1987
). Another study of Macaca fascicularis
reported “clusters” of DAergic fibers in the upper portion of layer III of the motor areas 4 and 6, but not in other cortical areas (Berger et al., 1988
). Similarly, we did not find any coil-like TH-stained structures in the frontal cortex of any species of Old World monkeys that we examined, whereas coils were consistently present in great apes. These results suggest that TH-ir axon coils represent an important morphological variant that is present only in apes and humans.
Interestingly, analogous morphological features (i.e., coils/clusters of axons) have been reported for cholinergic and serotonergic axons in human and chimpanzee cortex, and have been interpreted to represent local events of cortical plasticity or circuit changes (Mesulam et al., 1992
; Raghanti et al., 2007; Raghanti et al., 2008
). Neuromodulatory transmitters have well described functions in modifying cortical neuron response properties as mediated by numerous receptor subtypes (Gu, 2002
; von Bohlen und Halbach and Dermietzel, 2006
). Specific effects include long-term potentiation and long-term inhibition, depending on the properties of the post-synaptic element. The finding that coils of TH-ir axons are most numerous in layer III is important because of its putative role as the terminal input layer in corticocortical connections (e.g., Fuster, 1997
). If coils are indicative of cortical plasticity, these findings suggest increased synaptic reorganization that may be manifested in increased cognitive and behavioral flexibility. Specifically, capacities such as an awareness of “self” (Gallup, 1982
), transmission of social traditions (Whiten and van Schaik, 2001; van Schaik and Pradhan, 2003
), and symbolic language acquisition (Gardner and Gardner, 1985
) that are unique to the great ape and human clade may require a greater capacity for cortical plasticity.
There exist limitations for interpreting results from comparative studies that are noteworthy. First, there is little functional evidence indicating that other species possess strict homologues for human cortical areas. Although this limits interpretation of comparative results to some extent, it does allow for an evaluation of potential human-specific attributes. This is particularly relevant for our analysis of area 32. The integrity of this cortical area is necessary for the putatively unique human capacity of TOM (e.g., Gallagher and Frith, 2003
). Because macaques do not possess a structural homologue to human area 32, it was necessary to analyze the closest anatomical region to evaluate the emergence of potential neuroanatomical substrates that support TOM in humans. In addition, while the current study detected differences between the prefrontal areas versus primary motor cortex, further cortical regions need be examined to determine if this is a functional adaptation, or a general feature of all prefrontal cortical areas on a phylogenetic level.
Currently, very little is known regarding differences in cortical histology between humans other species (Preuss, 2000
; Preuss, 2006
; Sherwood and Hof, 2007
). This report represents a first step towards a broader understanding of human-specific cortical DAergic specializations that may support the evolution of cognition. Future studies that include a wider variety of primate species, cortical areas, and incorporate additional measures of innervation (such as varicosity densities) will further elucidate the functional roles of DA within the cortical mantle.
Several differences in cortical DAergic innervation were observed among species which may have functional implications. Specifically, humans exhibited a sublaminar pattern of innervation in layer I of areas 9 and 32 that differed from macaques and chimpanzees. In addition, in statistical analyses of axon length density within species, humans displayed a greater density of innervation to infragranular layers exclusively in cortical areas involved in high-level cognitive processing (areas 9 and 32), but not in primary motor cortex. The other species displayed different regional and laminar variation in DAergic innervation. Macaques consistently displayed denser innervation of layers I and II relative to layers III and V/VI for all cortical areas examined. In contrast, chimpanzees did not demonstrate a consistent direction of denser innervation. Further, the among-species analysis of ALv/Nv revealed that humans and chimpanzees together deviated from macaques in having increased DAergic afferents in layers III and V/VI of areas 9 and 32. Finally, morphological specializations that may be indicative of cortical plasticity events were observed in humans and chimpanzees, but not macaques.
Taken together, these findings suggest that significant modifications of DA’s role in cortical organization occurred in the evolution of the apes, with further changes in the descent of humans. However, we did not find an overt quantitative increase in cortical DAergic innervation in humans relative to chimpanzees. In this regard, our results highlight the importance of including chimpanzees in comparative neuroanatomical studies to determine human brain specializations (e.g., Preuss, 2000
). The addition of chimpanzees in this analysis has enhanced our ability to understand the role of cortical DAergic innervation in humans in a phylogenetic perspective. Finally, the distinctive innervation patterns and morphological specializations shared by humans and chimpanzees imply functional specializations of cortical DAergic innervation that may provide new insight into normal and pathological functioning of the human brain.