Axon terminals immunolabeled for dopamine or tyrosine hydroxylase synapse on GABA-immunoreactive dendrites in rat and monkey cortex

Synaptic targets of pyramidal neurons providing intrinsic horizontal connections in monkey prefrontal cortex

xũ I sLxxJ In monkey prefrontal cortex, the intrinsic axon collaterals of supragranular pyramidal neurons extend horizontally for considerable distances through the gray matter and give rise to stripe-like clusters of axon terminals (Levitt et al. [1993] J. Comp. Neurol. 338:360-376). Because understanding the functional role of these connections requires knowledge of their synaptic targets, we made injections of biotinylated dextran amine (BDA) into layer 3 of macaque prefrontal area 9 and examined the labeled intrinsic axon collaterals by electron microscopy. Labeled axon terminals formed exclusively asymmetric synapses, and 95.6% of the postsynaptic structures were dendritic spines, presumably belonging to other pyramidal neurons. The remaining postsynaptic structures were dendritic shafts, many of which had the morphological characteristics of local circuit neurons. The prefrontal injections also labeled associational projections that traveled through the white matter to terminate in other areas of prefrontal cortex. All of the synapses formed by these associational axons were asymmetric, and 91.9% were onto dendritic spines. The similarities in synaptic targets of the prefrontal intrinsic and associational axon terminals suggested that these projections might arise from the same neurons, an interpretation confirmed in dual label, retrograde tracing studies. To determine the specificity of the synaptic targets of these prefrontal connections, two additional comparisons were made. In the posterior parietal cortex (area 7a), 94.2% of the synapses furnished by BDA-labeled intrinsic collaterals of supragranular pyramidal neurons were also with dendritic spines. In contrast, only 75.6% of unlabeled asymmetric synapses in the prefrontal cortex were onto dendritic spines. These comparisons suggest that the axons of supragranular pyramidal neurons in primate association cortices are preferentially directed to specific targets. Finally, after injections of BDA, a small number of retrogradely labeled pyramidal neurons were observed within the anterogradely labeled clusters of intrinsic axon terminals. At the ultrastructural level, synapses between anterogradely labeled axon terminals and retrogradely labeled dendritic spines were identified. These findings suggest that reciprocal, monosynaptic connections may exist between pyramidal neurons located in different stripe-like clusters, providing a potential anatomical substrate for reverberating excitatory circuits within the primate association cortices.

Quantitative three-dimensional analysis of the catecholaminergic innervation of identified neurons in the macaque prefrontal cortex

The present study provides a complete quantitative three-dimensional analysis of neurons in primate prefrontal cortex targeted by catecholaminergic axons. Individual pyramidal and nonpyramidal cells in fixed slices were filled with Lucifer yellow (LY) and recovered with anti-LY antibody combined with anti-tyrosine hydroxylase (TH) antisera to reveal catecholaminergic axons. The total number of TH contacts and TH apposition density (THAD) was obtained for pyramidal and nonpyramidal cells in different layers. Four TH contacts (two on spines and two on shafts) were selected for correlated electron microscopic examination and serially sectioned; all four were confirmed as membrane appositions. Quantitative analysis revealed 90 TH contacts per pyramidal neuron in layer III, with a density of 0.8 per 100 microm of dendritic length (i.e., averaging one contact per basal dendrite). Remarkably, pyramids of layers III, V, and VI had the same THAD values, with a highly regular distribution of TH terminals on their spiny dendritic trees. In contrast, TH contacts on nonpyramidal neurons in layer III were half as dense and, moreover, were distributed irregularly and showed large variation from cell to cell. Neurons in layers II and superficial III had the highest THAD, as compared with deeper layers (1.4 vs 0.7 per 100 micron of dendritic length for pyramids; 0.53 vs 0.4 for interneurons). The highly organized TH innervation of pyramidal neurons, with at least one contact on virtually every dendrite, indicates that catecholaminergic, presumably dopaminergic, terminals are placed strategically along the entire dendritic tree to modulate most, if not all, of the excitatory input of a neuron. At the same time, the sparsity of contacts per dendrite may explain cortical vulnerability in diseases involving dopamine.

Calretinin neurons in human medial prefrontal cortex (areas 24a,b,c, 32', and 25)

The calcium-binding protein calretinin (CR) is present in a subpopulation of local-circuit neurons in the mammalian cerebral cortex containing gamma-aminobutyric acid. This light microscopic investigation provides a detailed qualitative and quantitative morphological analysis of CR-immunoreactive (CR+) neurons in the medial prefrontal cortex (mPFC; areas 24a,b,c, 32', and 25) of the normal adult human. The morphology of CR+ neurons and their areal and laminar distributions were consistent across human mPFC. The principal organisational features of CR+ labelling were the marked laminar distribution of immunoreactive somata and the predominantly vertical orientation of labelled axon-like and dendritic processes. Several types of CR- neurons were present in layer 1, including horizontally aligned Cajal-Retzius cells. In layers 2-6, CR+ neurons displayed a variety of morphologies: bipolar cells (49% of CR+ population), vertically bitufted cells (35%), and horizontally bitufted cells (3.5%). These neuron types were mainly located in layer 2/upper layer 3, and their dendritic processes were commonly aspiny and sometimes highly beaded. Aspiny (8%) and sparsely spiny multipolar (5%) CR+ neurons were also found. The mean somatic profile diameter of CR+ cells was 11.6 +/- 0.3 microm (mean +/- S.D). CA+ puncta formed pericellular baskets around unlabelled circular somatic profiles in layers 2/3 and around unlabelled pyramidal-shaped somata in layers 5/6. The somatic sizes of these unlabelled cell populations were significantly different. Immunolabelled puncta were also found in close contact with CR+ somata. Cortical depth distribution histograms and laminar thickness measurements defined the proportions of the overall CR- cell population in each layer: 7% in layer 1, 78% in layers 2/3, 14% in layers 5/6, and 1% in the white matter. In the tangential plane, CR+ neurons were distributed uniformly at all levels of the cortex. By using stereological counting procedures on immunoreacted Nissl-stained sections, CR+ neurons were estimated to constitute a mean 8.0% (7.2-8.7%) of the total neuron population in each cortical area. These data are compared with similar information obtained for the mPFC in monkey and rat (Gabbott and Bacon [1996b] J. Comp. Neurol. 364:657-608; Gabbott et al., [1997] J. Comp. Neurol. 377:465-499). This study provides important morphological insights into a neurochemically distinct subclass of local-circuit inhibitory neurons in the human mPFC.

Local-circuit neurones in the medial prefrontal cortex (areas 25, 32 and 24b) in the rat: morphology and quantitative distribution

This paper is a light microscopical study describing the detailed morphology and quantitative distribution of local circuit neurones in areas 25, 32, and 24b of the medial prefrontal cortex (mPFC) in the rat. Cortical interneurones were identified immunocytochemically by their expression of calretinin (CR), parvalbumin (PV), and calbindin D-28k (CB) immunoreactivity. Neurones immunoreactive for gamma-aminobutyric acid (GABA) were also investigated, as were interneurones containing reduced nicotinamide adenine dinucleotide phosphate (NADPH) diaphorase activity. Several distinct classes of CR+, PV+, and CB+ neurones were identified; the most frequent were: bipolar/bitufted CR+ cells in upper layer 3; multipolar PV+ neurones in layers 3 and 5; and bitufted/multipolar CB+ neurones in lower layer 3. CB+ neurones resembling Martinotti and neurogliaform cells were also present in layers 5/6. The morphologies and depth distributions of each cell type were consistent across the three areas of mPFC studied. Seven classes of diaphorase-reactive mPFC neurone are described; these cells were composed about 0.8% of the total neurone population and had a peak distribution located in mid- to lower layer 5 in each area. In areas 32 and 25, three defined bands of diffuse NADPH diaphorase staining were located in layer 2 and in upper and deep layer 5. Diaphorase reactivity was very infrequently colocalised with either CR, PV, or CB immunoreactivities. The numerical densities of neurones (N(V), number of cells per mm3) in each layer were calculated stereologically. The mean total neuronal N(V) estimate for areas 25, 32, and 24b was 51,603 +/- 3,324 (mean +/- S.D.; n = 8). Significant interareal differences were detected. From cortical thickness data and neuronal N(V) estimates, the absolute number of neurones under 1 mm2 of cortical surface (N(C)) have been derived. The mean N(C) value for areas 25, 32, and 24b was 57,328 +/- 7,505 neurones. In immunolabelled Nissl-stained sections, CR+ neurones constituted an overall 4.0%, PV+ cells 5.6%, and CB+ 3.4% of the total neurone populations in mPFC. GABA+ cells represented a mean of 16.2% (14.8-17.2%) of neurones in areas 25, 32 and 24b. The absolute numbers of CR+, PV+, CB+, and GABA+ neurones within individual layers in a column of cortex under 1 mm2 of cortical surface (N(L)) have also been derived, with significant interareal differences in N(L) values being detected. The data provide the structural basis for a qualitative and quantitative definition of local cortical circuits in the rat mPFC.

Ultrastructural immunocytochemical localization of the dopamine D2 receptor within GABAergic neurons of the rat striatum

Classical antipsychotics, which block dopamine (DA) D2 receptors, showing intrastriatal variation in their effectiveness in modulating GABAergic function. To determine the cellular basis for such differences, we examined the electron microscopic immunocytochemical labeling of D2 receptors and GABA in the dorsolateral caudate-putamen (CPn) and the nucleus accumbens (Acb) shell. In both regions, peroxidase reaction product and gold-silver deposits representing D2 receptor immunoreactivity (D2-IR) and GABA immunoreactivity (GABA-IR), respectively, were detected in dendrites and perikarya having characteristics of either spiny projection neurons or aspiny interneurons. Some perikarya in both regions are dually labeled with D2-IR and GABA-IR. Neurons axon terminals in each region also contained one or both markers. However, there were notable regional differences in the immunolabeling patterns. In the CPn, D2-IR was more commonly seen in dendrites/spines than in axon terminals, and proportionally more dendrites were dually labeled than in the Acb. In the Acb shell, D2-IR was detected with similar frequency in terminals and dendrites/spines, but more terminals co-localized D2-IR and GABA-IR in this region compared with the CPn. These results provide the first ultrastructural evidence for direct D2-mediated effects of DA on striatal GABAergic neurons. They further suggest that modulation of GABAergic neurons by DA acting at D2 receptors may be relatively more postsynaptic in the CPn, but more presynaptic in the Acb shell.

Regional and cellular fractionation of working memory

This chapter recounts efforts to dissect the cellular and circuit basis of a memory system in the primate cortex with the goal of extending the insights gained from the study of normal brain organization in animal models to an understanding of human cognition and related memory disorders. Primates and humans have developed an extraordinary capacity to process information "on line," a capacity that is widely considered to underlay comprehension, thinking, and so-called executive functions. Understanding the interactions between the major cellular constituents of cortical circuits-pyramidal and nonpyramidal cells-is considered a necessary step in unraveling the cellular mechanisms subserving working memory mechanisms and, ultimately, cognitive processes. Evidence from a variety of sources is accumulating to indicate that dopamine has a major role in regulating the excitability of the cortical circuitry upon which the working memory function of prefrontal cortex depends. Here, I describe several direct and indirect intercellular mechanisms for modulating working memory function in prefrontal cortex based on the localization of dopamine receptors on the distal dendrites and spines of pyramidal cells and on interneurons in the prefrontal cortex. Interactions between monoamines and a compromised cortical circuitry may hold the key to understanding the variety of memory disorders associated with aging and disease.

Hippocampal afferents to the rat prefrontal cortex: synaptic targets and relation to dopamine terminals

Afferents to the prefrontal cortex (PFC) from the hippocampal formation and from midbrain dopamine (DA) neurons have been implicated in the cognitive and adaptive functions of this cortical region. In the present study, we investigated the ultrastructure and synaptic targets of hippocampal terminals, as well as their relation to DA terminals within the PFC of adult rats. Hippocampal afferents were labeled either by anterograde transport of wheat germ agglutinin-horseradish peroxidase (WGA-HRP) from the ventral hippocampal formation or by anterograde degeneration following fimbria lesion. Hippocampal terminals in the PFC, identified by either method, formed primarily asymmetric axospinous synapses, with a small percentage forming asymmetric axodendritic synapses. Dopamine terminals in the PFC were identified by peroxidase immunocytochemistry for either tyrosine hydroxylase or DA and formed primarily symmetric synapses onto dendritic spines and small caliber dendritic shafts. Spines that received symmetric synaptic contact from DA terminals invariably also received an asymmetric synapse from an unlabeled terminal, forming a triadic complex. Hippocampal and DA terminals in the PFC were not often observed in the same area of the neuropil, and no examples of convergence of hippocampal and DA terminals onto common postsynaptic targets were observed. Further analysis revealed that spines receiving synaptic contact from hippocampal terminals did not receive additional synaptic contact from any other source. However, when localized to the same area of the neuropil, hippocampal and DA terminals were often in direct apposition to one another, without forming axo-axonic synapses. These results suggest that 1) hippocampal terminals primarily form excitatory synapses onto spiny pyramidal neurons, 2) hippocampal afferents are unlikely to be synaptically modulated by DA or non-DA terminals at the level of the dendritic spine, and 3) appositions between hippocampal and DA terminals may facilitate presynaptic interactions between these afferents to the PFC.

Local circuit neurons of the prefrontal cortex in schizophrenia: selective increase in the density of calbindin-immunoreactive neurons

Schizophrenia has been reported to be associated with alterations in GABAergic local circuit neurons of the prefrontal cortex. In this study, immunocytochemical techniques and antibodies against the calcium-binding proteins calbindin (CB) and calretinin (CR) were used to determine the laminar distribution and relative density of separate subpopulations of local circuit neurons in prefrontal cortical areas 9 and 46 from five pairs of schizophrenic and control subjects, matched for age, sex, and post-mortem interval. The laminar distribution pattern of CB-immunoreactive local circuit neurons was similar in both schizophrenic and control subjects. In both prefrontal regions, however, the density of CB-labeled neurons was 50-70% greater in schizophrenic subjects compared with control subjects, with cortical layers III and V/VI being preferentially affected. In contrast, the density of CR-IR neurons did not differ significantly between schizophrenic and control subjects. These findings reveal a selective increase in the density of a subpopulation of GABAergic local circuit neurons in the prefrontal cortex. Although other explanations for these observations must be considered, they may be consistent with the hypothesis that gene expression in GABAergic neurons is altered in schizophrenia.

Ultrastructural associations between dopamine terminals and local circuit neurons in the monkey prefrontal cortex: a study of calretinin-immunoreactive cells

Dopamine terminals in the monkey prefrontal cortex (PFC) synaptically target the distal dendrites of both pyramidal cells and GABA interneurons. We sought to determine whether the latter input includes the innervation of interneurons that utilize calretinin (CalR) as a calcium-binding protein. Sections through prefrontal area 9 of cynomolgus monkeys were processed by immunoperoxidase for tyrosine hydroxylase (TH) to label dopamine varicosities and by pre-embedding immunogold for CalR. Electron microscopic examination of layers 1-3 revealed numerous TH-immunoreactive (TH-ir) terminals, but few were located in the vicinity of CalR-ir dendrites. Although close appositions were sometimes detected between these labeled processes, no synaptic inputs from TH-ir terminals to CalR-ir dendrites were observed. However, in adjacent sections from the same animals, TH-ir terminals were observed to synapse on GABA-ir dendrites. These findings suggest that dopamine afferents to the monkey PFC target the subclasses of GABA interneurons that do not contain CalR.

Synchronous development of pyramidal neuron dendritic spines and parvalbumin-immunoreactive chandelier neuron axon terminals in layer III of monkey prefrontal cortex

Postnatal development of the primate cerebral cortex involves an initial proliferation and the subsequent attrition of cortical synapses. Although these maturational changes in synaptic density have been observed across the cortical mantle, little is known about the precise time course of developmental refinements in synaptic inputs to specific populations of cortical neurons. We examined the postnatal development of two markers of excitatory and inhibitory inputs to a subpopulation of layer III pyramidal neurons in area 9 and 46 of rhesus monkey prefrontal cortex. These neurons are of particular interest because they play a major role in the flow of information both within and between cortical regions. Quantitative reconstructions of Golgi-impregnated mid-layer III pyramidal neurons revealed substantial developmental changes in the relative density of dendritic spines, the major site of excitatory inputs to these neurons. Relative spine density on both the apical and basilar dendritic trees increased by 50% during the first two postnatal months, remained at a plateau through 1.5 years of age, and then decreased over the peripubertal age range until stable adult levels were achieved. As a measure of the postnatal changes in inhibitory input to the axon initial segment of these pyramidal neurons, we determined the density of parvalbumin-immunoreactive axon terminals belonging to the chandelier class of local circuit neurons. The density of these distinctive axon terminals (cartridges) exhibited a temporal pattern of change that exactly paralleled the changes in dendritic spine density. These results suggest that subpopulations of cortical neurons may be regulated by dynamic interactions between excitatory and inhibitory inputs during development and, in concert with other data, they emphasize the cellular specificity of postnatal refinements in cortical circuitry.