Axonal arborization of corticostriatal and corticothalamic fibers arising from prelimbic cortex in the rat

Recurrent connection patterns of corticostriatal pyramidal cells in frontal cortex

Corticostriatal pyramidal cells are heterogeneous in the frontal cortex. Here, we show that subpopulations of corticostriatal neurons in the rat frontal cortex are selectively connected with each other based on their subcortical targets. Using paired recordings of retrogradely labeled cells, we investigated the synaptic connectivity between two projection cell types: those projecting to the pons [corticopontine (CPn) cell], often with collaterals to the striatum, and those projecting to both sides of the striatum but not to the pons [crossed corticostriatal (CCS) cell]. The two types were morphologically differentiated in regard to their apical tufts. The dendritic morphologies of CCS cells were correlated with their somatic depth within the cortex. CCS cells had reciprocal synaptic connections with each other and also provided synaptic input to CPn cells. However, connections from CPn to CCS cells were rarely found, even in pairs showing CCS to CPn connectivity. Additionally, CCS cells preferentially innervated the basal dendrites of other CCS cells but made contacts onto both the basal and apical dendrites of CPn cells. The amplitude of synaptic responses was to some extent correlated with the contact site number. Ratios of the EPSC amplitude to the contact number tended to be larger in the CCS to CCS connection. Therefore, our data demonstrate that these two types of corticostriatal cells distinct in their dendritic morphologies show directional and domain-dependent preferences in their synaptic connectivity.

Cellular and subcellular localization of neurokinin-1 and neurokinin-3 receptors in primate globus pallidus

The primate globus pallidus receives massive innervations from GABAergic striatal neurons that co-release the neuropeptide substance P (SP). To expand our knowledge regarding SP interaction at pallidal level, we used single and double antigen retrieval methods to study the cellular and subcellular localization of SP and its high-affinity receptors neurokinin-1 (NK-1R) and neurokinin-3 (NK-3R) in the globus pallidus of the squirrel monkey (Saimiri sciureus). At the light microscopic level, a large number of neurons and fibers located in both the external (GPe) and internal (GPi) segments of the globus pallidus expressed NK-1R or NK-3R immunoreactivity. At the electron microscopic level, both NK-1R and NK-3R were mainly associated with intracellular sites or located at extrasynaptic positions on the plasma membrane. Presynaptic axon terminals forming symmetric and asymmetric synapses occasionally contained NK-1R and NK-3R. Neurokinin receptors were also observed in a proportion of SP-immunoreactive axon terminals, but these terminals preferentially expressed NK-3R. The pattern of distribution of NK-1R and NK-3R in GPe and GPi indicates that SP effects at pallidal level are mediated through postsynaptic receptor as well as presynaptic autoreceptors and heteroreceptors. These morphological data suggest that, either alone or in conjunction with GABA, SP could have a wide range of effects at pallidal level. This neuroactive peptide may influence in a significant manner the integration and treatment of neural information that flows through the basal ganglia.

Prefrontal cortex in the rat: projections to subcortical autonomic, motor, and limbic centers

This paper describes the quantitative areal and laminar distribution of identified neuron populations projecting from areas of prefrontal cortex (PFC) to subcortical autonomic, motor, and limbic sites in the rat. Injections of the retrograde pathway tracer wheat germ agglutinin conjugated with horseradish peroxidase (WGA-HRP) were made into dorsal/ventral striatum (DS/VS), basolateral amygdala (BLA), mediodorsal thalamus (MD), lateral hypothalamus (LH), mediolateral septum, dorsolateral periaqueductal gray, dorsal raphe, ventral tegmental area, parabrachial nucleus, nucleus tractus solitarius, rostral/caudal ventrolateral medulla, or thoracic spinal cord (SC). High-resolution flat-map density distributions of retrogradely labelled neurons indicated that specific PFC regions were differentially involved in the projections studied, with medial (m)PFC divided into dorsal and ventral sectors. The percentages that WGA-HRP retrogradely labelled neurons composed of the projection neurons in individual layers of infralimbic (IL; area 25) prelimbic (PL; area 32), and dorsal anterior cingulate (ACd; area 24b) cortices were calculated. Among layer 5 pyramidal cells, approximately 27.4% in IL/PL/ACd cortices projected to LH, 22.9% in IL/ventral PL to VS, 18.3% in ACd/dorsal PL to DS, and 8.1% in areas IL/PL to BLA; and 37% of layer 6 pyramidal cells in IL/PL/ACd projected to MD. Data for other projection pathways are given. Multiple dual retrograde fluorescent tracing studies indicated that moderate populations (<9%) of layer 5 mPFC neurons projected to LH/VS, LH/SC, or VS/BLA. The data provide new quantitative information concerning the density and distribution of neurons involved in identified projection pathways from defined areas of the rat PFC to specific subcortical targets involved in dynamic goal-directed behavior.

Synaptic differences in the patch matrix compartments of subjects with schizophrenia: a postmortem ultrastructural study of the striatum

The striatum processes motor, cognitive, and limbic circuitry. Striatal patch and matrix compartments are organized differently in many aspects including connectivity. Abnormalities in either compartment could have different functional consequences. The present study compares the synaptic organization in the patches and matrix in subjects with schizophrenia (SZ, n = 14) versus normal controls (NC, n = 8). Postmortem striatal tissue was processed for calbindin immunocytochemistry to identify the patch versus matrix compartments, prepared for electron microscopy, and analyzed using stereology. Several synaptic changes were observed in the SZ subjects vs. NCs including a higher density of cortical-type synapses in the putamen patch (44% higher) and in the caudate matrix (36% higher) in SZ cases on typical antipsychotic drugs. These changes appeared to be normalized rather than caused by treatment. The abnormal connectivity may represent a failure of normal synaptic pruning and may play a role in limbic or cognitive dysfunction in schizophrenia.

Bidirectional activity-dependent plasticity at corticostriatal synapses

Corticostriatal projections originate from the entire cerebral cortex and provide the major source of glutamatergic inputs to the basal ganglia. Despite the importance of corticostriatal connections in sensorimotor learning and cognitive functions, plasticity forms at these synapses remain strongly debated. Using a corticostriatal slice preserving the connections between the somatosensory cortex and the target striatal cells, we report the induction of both non-Hebbian and Hebbian forms of long-term potentiation (LTP) and long-term depression (LTD) on striatal output neurons (SONs). LTP and LTD can be induced selectively by different stimulation patterns (high-frequency trains vs low-frequency pulses) and were evoked with similar efficiency in non-Hebbian and Hebbian modes. Combination of LTP-LTD and LTD-LTP sequences revealed that bidirectional plasticity occurs at the same SONs and provides efficient homeostatic mechanisms leading to a resetting of corticostriatal synapses avoiding synaptic saturation. The effect of temporal relationship between cortical stimulation and SON activity was assessed using spike-timing-dependent plasticity (STDP) protocols. An LTP was observed when an action potential was triggered in the striatal neuron before the cortical stimulus, and, conversely, an LTD was induced when the striatal neuron discharge was triggered after the cortical stimulation. Such STDP was reversed when compared with those described so far in other mammalian brain structures. This mechanism may be essential for the role of the striatum in learning of motor sequences in which sensory and motor events are associated in a precise time sequence.

The striatofugal fiber system in primates: a reevaluation of its organization based on single-axon tracing studies

The current model of basal ganglia rests on the idea that the striatofugal system is composed of two separate (direct and indirect) pathways originating from distinct cell populations in the striatum. The striatum itself is divided into two major compartments, the striosomes and the matrix, which differ by their neurochemical makeup and input/output connections. Here, neurons located in either striosomes or the extrastriosomal matrix in squirrel monkeys were injected with biotin dextran amine, and their labeled axons were entirely reconstructed with a camera lucida. Twenty-four of 27 reconstructed axons arborized into the three main striatal targets (external pallidum, globus pallidus, and substantia nigra pars reticulata), a finding that is at odds with the concept of a dual striatofugal system. Axons of striosomal neurons formed several columnar terminal fields in the substantia nigra pars reticulata. These data indicate that the substantia nigra pars compacta is neither the only nor the main target of striosomal neurons, a finding that calls for a reevaluation of the organization of the striatonigral projection system.

Synaptic differences in the postmortem striatum of subjects with schizophrenia: a stereological ultrastructural analysis

The striatum processes motor, cognitive, and limbic function, all of which are perturbed in schizophrenia. The present study examined the synaptic organization of the caudate and putamen in schizophrenia. Postmortem striatum was obtained from 10 normal controls (NC) and 17 subjects with schizophrenia (SZ), prepared for electron microscopy, and analyzed using stereological principles. The densities of total synapses, asymmetric synapses (characteristic of excitatory inputs), and asymmetric axospinous synapses (characteristic of cortical input) were higher in the caudate of the SZs vs. NCs. These changes were most profound in the off-drug SZ cases and were also elevated in subjects on antipsychotic drugs (APDs). In comparison to NCs, there were no significant differences in the putamen of the SZ cohort as a whole group; however, there were more asymmetric axospinous synapses in the off-drug subgroup. The increase in density of synapses in the SZs does not appear to be caused by antipsychotic medication and may represent failure of normal synaptic pruning or abnormal sprouting. Higher density of cortical-type synapses in SZs vs. NCs may reflect adaptation of corticostriatal circuitry or hyperstimulation of striatal projection neurons. The abnormal synaptic organization could have several important and different downstream effects depending on the precise circuitry involved and may be related to limbic or cognitive dysfunction in schizophrenia.

Intermittent cortical stimulation evokes sensitization to cocaine and enduring changes in matrix and striosome neuron responsiveness

Both the behavioral sensitization syndrome and the changes in the responsiveness of striatal neurons evoked by chronic cocaine exposure may be linked to enhanced neocortical activity, yet a direct demonstration of the effects of cortical stimulation on these parameters is lacking. We have found that repeated stimulation of the rat prelimbic cortex with picrotoxin (0.25 microg/0.25 microl, five injections on alternate days followed by 7-day withdrawal) contributed to increase c-Fos protein expression in the striosomes of the dorsolateral striatum, while producing the opposite effect in the matrix compartment, after a single exposure to cocaine (25 mg/kg). Moreover, rats exposed to cortical stimulation showed decreased locomotor activation but enhanced stereotypy following acute cocaine treatment. Thus, pulsatile stimulation of the prelimbic cortex facilitated modifications in striatal activity typically produced by chronic cocaine treatment and sensitized drug-naive animals to acute cocaine challenge. These results suggest that enhanced activation of the prelimbic cortex may contribute to the long-term adaptations induced by cocaine on neural activity and behavior.

Evidence for differential cortical input to direct pathway versus indirect pathway striatal projection neurons in rats

The two main types of corticostriatal neurons are those that project only intratelencephalically (IT-type), the intrastriatal terminals of which are 0.41 microm in mean diameter, and those that send their main axon into pyramidal tract and have a collateral projection to striatum (PT-type), the intrastriatal terminals of which are 0.82 microm in mean diameter. We used three approaches to examine whether the two striatal projection neuron types (striatonigral direct pathway vs striatopallidal indirect pathway) differ in their input from IT-type and PT-type neurons. First, we retrogradely labeled one striatal projection neuron type or the other with biotinylated dextran amine (BDA)-3000 molecular weight. We found that terminals making asymmetric axospinous contact with striatonigral neurons were 0.43 microm in mean diameter, whereas those making asymmetric axospinous contact with striatopallidal neurons were 0.69 microm. Second, we preferentially immunolabeled striatonigral neurons for D1 dopamine receptors or striatopallidal neurons for D2 dopamine receptors and found that axospinous terminals had a smaller mean size (0.45 microm) on D1+ spines than on D2+ spines (0.61 microm). Finally, we combined selective BDA labeling of IT-type or PT-type terminals with immunolabeling for D1 or D2, and found that IT-type terminals were twice as common as PT-type on D1+ spines, whereas PT-type terminals were four times as common as IT-type on D2+ spines. These various results suggest that striatonigral neurons preferentially receive input from IT-type cortical neurons, whereas striatopallidal neurons receive greater input from PT-type cortical neurons. This differential cortical connectivity may further the roles of the direct and indirect pathways in promoting desired movements and suppressing unwanted movements, respectively.

The striatum in the hedgehog tenrec: histochemical organization and cortical afferents

In order to get insight into the striopallidal organization in mammals with little differentiated brain the striatum of the lesser hedgehog tenrec (Afrotheria) was characterized histochemically and analysed with regard to its cortical afferents using axonal tracer substances. The majority of neocortical cells projecting to the striatum were found bilaterally in the layers 2 and 3 of the frontal hemisphere; caudalwards the relative number of cells increased somewhat in the upper layer 5. There was a topographical organization as far as the allocortical projections appeared confined to the ventral striatum, and the efferents from hippocampal, posterior paleocortical, somatosensory and audiovisual areas were distributed in largely different striatal territories. Projections from the anterior frontal cortex, on the other hand, terminated extensively upon the caudate-putamen and also involved the nucleus accumbens and the olfactory tubercle. In the latter region the molecular layer was especially involved. The entorhinal cortex also projected heavily to the olfactory tubercle but unlike other species it scarcely involved the nucleus accumbens. The cortical fibers were distributed in a relatively homogenous fashion within their striatal territory and there was little evidence for patches of high density terminations. Islands of low density labeling, however, were noted occasionally in the caudate-putamen. These islands were partly similar in size as the patches of neuropil staining obtained with anti-calretinin and anti-substance P. There were also hints for the presence of a shell-like region in the nucleus accumbens stained with anti-dopamine transporter and NADPh-diaphorase. The classical striosome-matrix markers such as calbindin, acetylcholinesterase and enkephalin, however, failed to reveal any compartmental organization.

Striosomes are enriched in glutamic acid decarboxylase in primates

The compartmental distribution of glutamic acid decarboxylase (GAD) in the striatum was investigated in squirrel monkeys and rats with antibodies raised against the two isoforms of this enzyme (GAD65 and GAD67) and with calbindin D-28k (CB) and/or micro-opiate receptor (MOR) as striosomal markers. In primates, immunostaining for both GAD65 and GAD67 was much more intense in striosomes than in the surrounding matrix. A thin immunoreactive strip of GAD labeling was also present in the dorsolateral part of both caudate nucleus and putamen. This narrow band appears to correspond to the so-called subcallosal streak (SS) found in rodent striatum. Although the immunostaining intensity for the two enzymes was similar at pallidal level, that for GAD65 was more intense than that for GAD67 at the striatal level. The GAD immunostaining was more uniformly distributed in the rat striatum, which did not display GAD-rich patches that corresponded to MOR-positive striosomes. Moreover, in contrast to the findings obtained in monkeys, the subcallosal streak in rats was less intensely stained for GAD than for the remaining regions of the striatum. These results reveal that GAD65 and GAD67 are faithful markers of striosomes in primates but not in rodents. They suggest the existence of a significant species difference between rodents and primates in respect to the chemical organization of the striatum, a difference that should be taken into account when using rodents as animal models to study the functional organization of the basal ganglia and the pathogenesis of neurodegenerative diseases that affect the striatum.

Differential projections of the infralimbic and prelimbic cortex in the rat

The medial prefrontal cortex has been associated with diverse functions including attentional processes, visceromotor activity, decision-making, goal-directed behavior, and working memory. The present report compares and contrasts projections from the infralimbic (IL) and prelimbic (PL) cortices in the rat by using the anterograde anatomical tracer, Phaseolus vulgaris-leucoagglutinin. With the exception of common projections to parts of the orbitomedial prefrontal cortex, olfactory forebrain, and midline thalamus, PL and IL distribute very differently throughout the brain. Main projection sites of IL are: 1) the lateral septum, bed nucleus of stria terminalis, medial and lateral preoptic nuclei, substantia innominata, and endopiriform nuclei of the basal forebrain; 2) the medial, basomedial, central, and cortical nuclei of amygdala; 3) the dorsomedial, lateral, perifornical, posterior, and supramammillary nuclei of hypothalamus; and 4) the parabrachial and solitary nuclei of the brainstem. By contrast, PL projects at best sparingly to each of these structures. Main projection sites of PL are: the agranular insular cortex, claustrum, nucleus accumbens, olfactory tubercle, the paraventricular, mediodorsal, and reuniens nuclei of thalamus, the capsular part of the central nucleus and the basolateral nucleus of amygdala, and the dorsal and median raphe nuclei of the brainstem. As discussed herein, the pattern of IL projections is consistent with a role for IL in the control of visceral/autonomic activity homologous to the orbitomedial prefrontal cortex of primates, whereas those of PL are consistent with a role for PL in limbic-cognitive functions homologous to the dorsolateral prefrontal cortex of primates.

Novel aspects of the chemical anatomy of the striatum and its efferents projections

This paper summarizes the results of some of our previous neuroanatomical and immunohistochemical studies on the organization of the striatum and its efferent projections in rodents, monkeys and humans. It also reports recent functional calcium-imaging data obtained in rat brain slices, as well as developmental results gathered with bromodeoxyuridine (BrdU) in monkeys. On one hand, single-axon tracing studies in rats and monkeys have revealed that the majority of striatofugal axon arborizes within most striatal target structures. In humans, SP-positive fibers were found to arborize in the two segments of the globus pallidus, where they were closely apposed to pallidal neurons that expressed the neurokinin-1 receptor (NK-1r). In agreement with such findings, calcium-imaging studies in rats have revealed that pallidal and nigral neurons are both responsive to SP. These findings suggest that the striatofugal projection system is much more widely distributed than previously thought and exerted a multifaceted effect upon its target sites. On the other hand, immunostaining studies in humans have shown the presence of several types of putative dopaminergic neurons intrinsic to the striatum. Furthermore, BrdU labeling experiments in monkeys have demonstrated that new neurons are generated throughout adult life in the striatum of normal monkeys and that their number can be markedly increased by the administration of neuronal growth factors. These findings open new therapeutic avenues for the treatment of neurodegenerative disorders that specifically affect the striatum.

Maturation of layer V pyramidal neurons in the rat prefrontal cortex: intrinsic properties and synaptic function

Layer V pyramidal neurons in the rat medial prefrontal cortex (PFC) were examined with whole cell patch-clamp recording in acute slices from postnatal day 1 (P1) to P36. In the first few days after birth, layer V pyramidal neurons had low resting potentials, high-input resistance, and long membrane time constant. During the next 2 wk, the resting potential shifted by -14 mV, while the input resistance and time constant decreased by 15- and 4-fold, respectively. Between P3 and P21, the surface area of the cell body doubled, while the total lengths of apical and basal dendrites increased by 5- and 13-fold, respectively. Action potentials (APs) were observed at all aged tested. The peak amplitude of APs increased by 30 mV during the first 3 wk, while AP rise time and half-maximum duration shortened significantly. Compared with neurons at P21 or older, neurons in the first week required much smaller currents to reach their maximum firing frequencies, but the maximum frequencies were lower than those at older ages. Stimulation of layer II/III induced monosynaptic responses in neurons older than P5. Paired-pulse responses showed a short-term depression at P7, which shifted progressive to facilitation at older ages. These results demonstrate that, similar to other neurons in the brain, layer V pyramidal neurons in the PFC undergo a period of rapid development during the first 3 wk after birth. These findings suggest that the intrinsic properties of neurons and the properties of synaptic inputs develop concomitantly during early life.

Serotonin induces tonic firing in layer V pyramidal neurons of rat prefrontal cortex during postnatal development

The effects of serotonin (5-HT) on neuronal activity were examined during postnatal development in layer V pyramidal neurons of the rat prefrontal cortex (PFC) in vitro. Whole-cell patch-clamp recordings were made in slices obtained from rats aged between postnatal day (P) 6 and P31. In P14 or younger neurons, bath application of 5-HT (10 microM) induced a large depolarization followed by tonic firing at 2-5 Hz. The excitatory effects of 5-HT decreased rapidly after P14, so that by P21, 5-HT produced a small depolarization or hyperpolarization without cell firing. The excitatory effects of 5-HT at younger ages were attributed to 5-HT2A receptors because the effects were mimicked by the 5-HT2 agonist alpha-methyl-5-HT but not by the 5-HT3 agonist 1-(m-chlorophenyl)-biguanide, nor by the 5-HT2B/2C agonist 1-(3-chlorophenyl)piperazine, and were blocked by the 5-HT2A antagonists ketanserin and alpha-phenyl-1-(2-phenylethyl)-4-piperidinemethanol. The excitatory responses persisted in 0 [Ca2+]o and high [Mg2+]o in the presence of TTX or blockers of ionotropic glutamate receptors, suggesting that the effects were mediated essentially by postsynaptic mechanisms. The responses to 5-HT involve a reduction of K+ conductance and an enhancement of the hyperpolarization-activated Na+/K+ current. The developmental decline of 5-HT-induced excitatory effects was associated with a downregulation of 5-HT2A receptor function and a decrease in the input resistance during early life. These results suggest that 5-HT is an important regulator of neuronal activity in the neonatal PFC and may play a role in activity-dependent developmental processes.

Differential morphology of pyramidal tract-type and intratelencephalically projecting-type corticostriatal neurons and their intrastriatal terminals in rats

Two types of corticostriatal projection neurons have been identified: 1) one whose intrastriatal arborization arises as a collateral of a projection to the ipsilateral brainstem via the pyramidal tract (PT-type); and 2) one that projects intratelencephalically to the cortex and striatum, in many cases bilaterally, but not extratelencephalically (IT-type). To assess possible functional differences between these two neuron types, we characterized their laminar location in the cortex, their perikaryal size, and the morphology of their intrastriatal terminals. IT-type neurons were retrogradely labeled by tetramethylrhodamine-dextran amine (RDA)3k injection into the contralateral striatum, whereas their intrastriatal terminals were labeled anterogradely by biotinylated dextran amine (BDA)10k injection into the contralateral motor or primary somatosensory cortex. To label PT-type neurons and their ipsilateral intrastriatal terminals retrogradely, BDA3k was injected into the pontine pyramidal tract. We found that IT-type neuronal perikarya are medium-sized (12-13 microm) and located in layer III and upper layer V, whereas PT-type perikarya are larger (18-19 microm) and most commonly located in lower layer V. At the electron microscopic level, the intrastriatal terminals of both corticostriatal neuron types made asymmetric synaptic contact with spine heads and less frequently with dendrites. IT-type axospinous terminals were characteristically small (0.4-0.5 microm) and regular in shape, whereas PT-type terminals were typically large (0.8-0.9 microm) and often irregular in shape. Perforated postsynaptic densities were common for PT-type terminals, but not IT-type. The clear differences between these two corticostriatal neuron types in perikaryal size and laminar location in the cortex, and in the size and shape of their intrastriatal terminals, suggest that they may differ in the nature of their influence on the striatum.

Projections from the amygdaloid complex to the claustrum and the endopiriform nucleus: a Phaseolus vulgaris leucoagglutinin study in the rat

The claustrum and the endopiriform nucleus contribute to the spread of epileptiform activity from the amygdala to other brain areas. Data of the distribution of pathways underlying the information flow between these regions are, however, incomplete and controversial. To investigate the projections from the amygdala to the claustrum and the endopiriform nucleus, we injected the anterograde tracer Phaseolus vulgaris leucoagglutinin into various divisions of the amygdaloid complex, including the lateral, basal, accessory basal, central, anterior cortical and posterior cortical nuclei, the periamygdaloid cortex, and the amygdalohippocampal area in the rat. Analysis of immunohistochemically processed sections reveal that the heaviest projections to the claustrum originate in the magnocellular division of the basal nucleus. The projection is moderate in density and mainly terminates in the dorsal aspect of the anterior part of the claustrum. Light projections from the parvicellular and intermediate divisions of the basal nucleus terminate in the same region, whereas light projections from the accessory basal nucleus and the lateral division of the amygdalohippocampal area innervate the caudal part of the claustrum. The most substantial projections from the amygdala to the endopiriform nucleus originate in the lateral division of the amygdalohippocampal area. These projections terminate in the central and caudal parts of the endopiriform nucleus. Lighter projections originate in the anterior and posterior cortical nuclei, the periamygdaloid cortex, the medial division of the amygdalohippocampal area, and the accessory basal nucleus. These data provide an anatomic basis for recent functional studies demonstrating that the claustrum and the endopiriform nucleus are strategically located to synchronize and spread epileptiform activity from the amygdala to the other brain regions. These topographically organized pathways also provide a route by means of which the claustrum and the endopiriform nucleus have access to inputs from the amygdaloid networks that process emotionally significant information.

Corticostriatal combinatorics: the implications of corticostriatal axonal arborizations

The complete striatal axonal arborizations of 16 juxtacellularly stained cortical pyramidal cells were analyzed. Corticostriatal neurons were located in the medial agranular or anterior cingulate cortex of rats. All axons were of the extended type and formed synaptic contacts in both the striosomal and matrix compartments as determined by counterstaining for the mu-opiate receptor. Six axonal arborizations were from collaterals of brain stem-projecting cells and the other 10 from bilaterally projecting cells with no brain stem projections. The distribution of synaptic boutons along the axons were convolved with the average dendritic tree volume of spiny projection neurons to obtain an axonal innervation volume and innervation density map for each axon. Innervation volumes varied widely, with single axons occupying between 0.4 and 14.2% of the striatum (average = 4%). The total number of boutons formed by individual axons ranged from 25 to 2,900 (average = 879). Within the innervation volume, the density of innervation was extremely sparse but inhomogeneous. The pattern of innervation resembled matrisomes, as defined by bulk labeling and functional mapping experiments, superimposed on a low background innervation. Using this sample as representative of all corticostriatal axons, the total number of corticostriatal neurons was estimated to be 17 million, about 10 times the number of striatal projection neurons.

Susceptibility to kindling and neuronal connections of the anterior claustrum

The claustrum has been implicated in the kindling of generalized seizures from limbic sites. We examined the susceptibility of the anterior claustrum itself to kindling and correlated this with an anatomical investigation of its afferent and efferent connections. Electrical stimulation of the anterior claustrum resulted in a pattern of rapid kindling with two distinct phases. Early kindling involved extremely rapid progression to bilaterally generalized seizures of short duration. With repeated daily kindling stimulations, early-phase generalized seizures abruptly became more elaborate and prolonged, resembling limbic-type seizures as triggered from the amygdala. We suggest that the rapid rate of kindling from the anterior claustrum is an indication that the claustrum is functionally close to the mechanisms of seizure generalization. In support of our hypothesis, we found significant afferent, efferent, and often reciprocal connections between the anterior claustrum and areas that have been implicated in the generation of generalized seizures, including frontal and motor cortex, limbic cortex, amygdala, and endopiriform nucleus. Additional connections were found with various other structures, including olfactory areas, nucleus accumbens, midline thalamus, and brainstem nuclei including the substantia nigra and the dorsal raphe nucleus. The anatomical connections of the anterior claustrum are consistent with its very high susceptibility to kindling and support the view that the claustrum is part of a forebrain network of structures participating in the generalization of seizures.

Characterization of spontaneous excitatory synaptic currents in pyramidal cells of rat prelimbic cortex

Spontaneous excitatory postsynaptic currents (sEPSCs) were recorded with the whole-cell patch-clamp technique from 41 pyramidal cells in the layers V-VI of the prelimbic (PL) cortex. The sEPSCs occurred randomly and the averaged frequency in 41 cells was 1.81+/-0.27 Hz. The amplitude distribution was skewed toward larger events and could be adequately fitted by a sum of two or three Gaussian distributions, but they could not be fitted by a sum of Gaussian distributions with equidistant separation in all cells studied (n=24). In eight of 24 cells, after the transformation of the amplitudes into logarithms, the skewed histogram became bell-shaped and could be adequately fitted by a single Gaussian distribution, whereas in the other 16 cells, after the transformation the histograms were still skewed. However, for those latter cells, when the logarithms were transformed into difference, the distribution of the differences in 15 of 16 cells became bell-shaped and could be adequately fitted by a single Gaussian distribution. The pie distribution of different rise times within one cell in 1 ms bin showed that there were four different patterns of the rise time distribution. The amplitude distribution of the sEPSCs was unchanged in 10 of 22 cells after TTX, but in the other 12 cells, it was changed significantly. However, for these cells although TTX had a marked effect, it could not change the skewed distribution into a single Gaussian distribution in case of both original and transformed data.

Double retrograde tracer study of the corticostriatal projections to the cat caudate nucleus

The distribution of corticostriatal neurons projecting to the caudate nucleus was examined in the cat by retrograde fluorescent tracers. Thus, Fast Blue and Diamidino Yellow were concomitantly injected in different rostrocaudal, dorsoventral, or mediolateral sectors of the caudate nucleus. The main findings of this study are: 1) few double-labeled cells were found after two injections in different sectors of the caudate nucleus; 2) double-labeled neurons were more abundant after adjacent injections and they were mainly located in 6 alpha beta, dorsolateral prefrontal, dorsomedial prefrontal, prelimbic, anterior limbic, sylvian anterior, and rostral part of cingulate cortical areas; and 3) there were variations in the spatial organization of the corticostriatal neurons in different cortical areas projecting to various parts of the caudate nucleus.

Limited collateralization of neurons in the rat prefrontal cortex that project to the nucleus accumbens

The specificity and selectiveness of a neuronal message depends in part on the number of recipient neurons that simultaneously receive this message. Hence, projections involved in higher order cognitive processes might be expected to exhibit a lower degree of collateralization than projections that mediate more basic brain functions. This study sought to determine the degree to which neurons projecting from the prefrontal cortex to the nucleus accumbens collateralize to major cortical and subcortical regions: the contralateral prefrontal cortex, the basolateral amygdala or the ventral tegmental area. Fluoro-Gold and cholera toxin-b were used to label prefrontal cortex neurons that project to these targets, and the proportion of neurons singly and dually labeled by immunofluorescence for these tracers was determined. The prefrontal cortex neurons projecting to these regions exhibited a partially complementary laminar distribution. Furthermore, of the neurons projecting to the nucleus accumbens, 13% sent a collateralized projection to the contralateral prefrontal cortex, 7% collateralized to the basolateral amygdala, and 3% sent a branched projection to the ventral tegmental area. No differences were observed in the degree of collateralization of neurons in superficial versus deep layers.Thus, the degree of collateralization of corticoaccumbens neurons was overall limited, but significantly greater to a cortical target than to subcortical regions. These branching patterns provide anatomical substrates for temporal and spatial coordination of activity in limbic circuits.

Amphetamine withdrawal alters bistable states and cellular coupling in rat prefrontal cortex and nucleus accumbens neurons recorded in vivo

Repeated amphetamine administration is known to produce changes in corticoaccumbens function that persist beyond termination of drug administration. We have found previously that long-term alteration in dopamine systems leads to changes in gap junction communication, expressed as dye coupling, between striatal neurons. In this study, the cellular bases of amphetamine-induced changes were examined using in vivo intracellular recordings and dye injection in ventral prefrontal-accumbens system neurons of control and amphetamine-treated rats. Rats that had been withdrawn from repeated amphetamine displayed a significant increase in the incidence of dye coupling in the prefrontal cortex and nucleus accumbens, which persisted for up to 28 d after withdrawal. The increased coupling was limited to projection neurons in both prefrontal cortical and accumbens brain regions, as identified by their axonal trajectory or the absence of interneuron-selective immunocytochemical markers. These changes occurred with no substantial loss of tyrosine hydroxylase-immunoreactive terminals in these cortical and accumbens regions, ruling out dopamine degeneration as a precipitating factor. Previous studies showed that nitric oxide plays a role in the regulation of coupling; however, amphetamine-withdrawn rats had fewer numbers of neurons and processes that stained for nitric oxide synthase immunoreactivity. In amphetamine-treated rats, a higher proportion of cortical cells fired in bursts, and a larger proportion of accumbens and prefrontal cortical neurons exhibited bistable membrane oscillations. By increasing corticoaccumbens transmission, amphetamine withdrawal may lead to neuronal synchronization via gap junctions. Furthermore, this adaptation to amphetamine treatment persists long after the drug is withdrawn.

The axonal arborization of single nigrostriatal neurons in rats

Neurons of the substantia nigra pars compacta (SNc) were iontophoretically injected with biotin dextran and their anterogradely labeled axons individually reconstructed from serial sagittal sections. Most nigrostriatal axons travelled directly to the striatum, where they branched abundantly. Other axons arborized profusely in various extrastriatal structures, including the globus pallidus, the entopeduncular and subthalamic nuclei, and branched only sparsely in the striatum. This heterogeneous organization of the nigrostriatal projection allows single SNc neurons to influence differently striatal neurons and to act directly upon extrastriatal components of the basal ganglia via a highly patterned set of collaterals.