A neurophysiological analysis of the development of cortico-caudate connections in the cat

Translating striatal activity from brain slice to whole animal neurophysiology: A guide for neuroscience research integrating diverse levels of analysis

An important goal of this review is highlighting research in neuroscience as examples of multilevel functional and anatomical analyses addressing basic science issues and applying results to the understanding of diverse disorders. The research of Dr. Michael Levine, a leader in neuroscience, exemplifies this approach by uncovering fundamental properties of basal ganglia function and translating these findings to clinical applications. The review focuses on neurophysiological research connecting results from in vitro and in vivo recordings. A second goal is to utilize these research connections to produce novel, accurate descriptions for corticostriatal processing involved in varied, complex functions. Medium spiny neurons in striatum act as integrators combining input with baseline activity creating motivational "events." Basic research on corticostriatal synapses is described and links developed to issues with clinical relevance such as inhibitory gating, self-injurious behavior, and relative reward valuation. Work is highlighted on dopamine-glutamate interactions. Individual medium spiny neurons express both D₁ and D₂ receptors and encode information in a bivalent manner depending upon the mix of receptors involved. Current work on neurophysiology of reward processing has taken advantage of these basic approaches at the cellular and molecular levels. Future directions in studying physiology of reward processing and action sequencing could profit by incorporating the divergent ways dopamine modulates incoming neurochemical signals. Primary investigators leading research teams should mirror Mike Levine's efforts in "climbing the mountain" of scientific inquiry by performing analyses at different levels of inquiry, integrating the findings, and building comprehensive answers to problems unsolvable without this bold approach.

Neurophysiological, pharmacological and morphological properties of human caudate neurons recorded in vitro

Tissue samples from the caudate nucleus were obtained from eight children (eight to 172 months of age) who underwent hemispherectomies for the relief of intractable seizures. Neurophysiological, pharmacological and morphological properties of caudate neurons were characterized by intracellular recordings in an in vitro slice preparation. These properties were compared with those of tissue obtained from animal studies. Electrophysiological properties of human caudate neurons that were similar to those of cat caudate and rat neostriatal cells included resting membrane potential, input resistance, action potential rise time, fall time, duration and action potential afterhyperpolarization amplitude, as well as the general characteristics of locally evoked synaptic responses. Properties that were different included action potential amplitudes and time-constants. Human caudate neurons also displayed responses similar to those of cat caudate or rat neostriatal cells to manipulation of excitatory amino acid receptor systems and to dopamine application. Kynurenic acid, a broad-spectrum excitatory amino acid receptor antagonist, decreased the amplitude of evoked synaptic responses, indicating that they were partially mediated by excitatory amino acids. In Mg2+ free Ringer's solution, the amplitudes and durations of postsynaptic responses were increased and bursts of action potentials were induced. These effects were mediated by activation of N-methyl-D-aspartate receptors since they were blocked by 2-amino-5-phosphonovalerate, a specific N-methyl-D-aspartate-receptor antagonist. Iontophoretic application of N-methyl-D-aspartate also induced membrane oscillations and bursts in almost all caudate neurons. Dopamine decreased the amplitude of postsynaptic responses, an effect antagonized by domperidone, a selective D2 dopamine receptor antagonist. Developmentally, the greatest change was an increase in action potential amplitude, although input resistance decreased and action potential afterhyperpolarization amplitude increased. Postsynaptic responses were similar across age. All but one of the caudate neurons identified by intracellular injection of biocytin or Lucifer Yellow were medium-sized spiny cells. These experiments show that human caudate neurons display a number of electrophysiological properties similar to rat neostriatal or cat caudate neurons recorded in brain slices. Furthermore, few electrophysiological parameters changed significantly over the age period examined suggesting that the human caudate at eight months displays many of the neuronal functions of the more mature caudate nucleus.

AMPA glutamate receptor subunits are differentially distributed in rat brain

To demonstrate the regional, cellular and subcellular distributions of non-N-methyl-D-aspartate glutamate receptors in rat brain, we generated antipeptide antibodies that recognize the C-terminal domains of individual subunits of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-preferring glutamate receptors (i.e. GluR1, GluR4, and a region highly conserved in GluR2, GluR3 and GluR4c). On immunoblots, antibodies detect distinct proteins with mol. wts ranging from 102,000 to 108,000 in homogenates of rat brain. Immunocytochemistry shows that glutamate receptor subunits are distributed abundantly and differentially within neuronal cell bodies and processes in cerebral cortex, basal ganglia, limbic system, thalamus, cerebellum and brainstem. The precise patterns and cellular localizations of glutamate receptor subunit immunoreactivities are unique for each antibody. In neocortex and hippocampus, pyramidal neurons express GluR1 and GluR2/3/4c immunoreactivities; many non-pyramidal, calcium-binding, protein-enriched neurons in cerebral cortex are selectively immunoreactive for GluR1. In striatum, the cellular localizations of GluR1, GluR2/3/4c and GluR4 immunoreactivities are different; in this region, GluR1 co-localizes with many cholinergic neurons but is only present in a minor proportion of nicotinamide adenine dinucleotide phosphate diaphorase-positive striatal neurons. GluR1 co-localizes with most dopaminergic neurons within the substantia nigra. In several brain regions, astrocytes show GluR4 immunoreactivity. Within the cerebellar cortex, cell bodies and processes of Bergmann glia express intense GluR4 and GluR1 immunoreactivities; perikarya and dendrites of Purkinje cells show GluR2/3/4c immunoreactivity but no evidence of GluR1 or GluR4. Ultrastructurally, GluR subunit immunoreactivities are localized within cell bodies, dendrites and dendritic spines of specific subsets of neurons and, in the case of GluR1 and GluR4, in some populations of astrocytes. This investigation demonstrates that individual AMPA-preferring glutamate receptor subunits are distributed differentially in the brain and suggests that specific neurons and glial cells selectively express glutamate receptors composed of different subunit combinations. Thus, the co-expression of all AMPA receptor subunits within individual cells may not be obligatory for the functions of this glutamate receptor in vivo.

Neurophysiological maturation of cat caudate neurons: evidence from in vitro studies

The membrane properties and synaptic physiology of developing cat caudate (Cd) nucleus neurons were studied in in vitro slice preparations. Recordings were obtained from 98 cells in kittens from fetal day (F) 56 to postnatal day (P) 90. With increasing age, the following maturational changes occurred; resting membrane potentials became more negative, action potential rise times decreased, action potential amplitudes increased, and action potential durations and input resistances decreased. The frequency of occurrence of afterhyperpolarizations and of anomalous rectification increased with age. The primary response to local extracellular stimulation was a depolarization usually accompanied by an action potential. Evoked hyperpolarizing responses were seen after P28 but only occurred if the membrane was depolarized by intracellular current injection. Cells identified by intracellular injection of Lucifer yellow were primarily medium-sized spiny neurons although it was not always possible to determine the cell type in slices from animals less than P5. Somatic diameter, dendritic length, and spine density increased with age. Dye-coupling occurred in slices less than P20. Its frequency decreased with age. These results show that Cd neurons undergo significant maturation during late prenatal and early postnatal periods. In contrast, substantia nigra neurons mature more rapidly and should be capable of influencing the less mature Cd neurons during development.(ABSTRACT TRUNCATED AT 250 WORDS)

Postnatal development of striatal connections in the rat: a transport study with wheat germ agglutinin-horseradish peroxidase

This paper deals with the postnatal development of afferent and efferent connections of the rat striatum as revealed by the transport of horseradish peroxidase conjugated with wheat germ agglutinin (WGA-HRP). Tracer was injected weekly from birth to the end of the first postnatal month in the head of the caudate nucleus. To control for transport from cortical areas contaminated by the micropipette, injections in newborn rats were made by either vertical or lateral penetrations. In addition some newborn and 14-day-old animals were injected only in the cortex. The results showed that at birth there was retrograde transport to the thalamus, substantia nigra and raphe nuclei. Labelling in the cortex was seen at birth but was probably due to cortical contamination. Transport from the striatum was clearly established on day 7, when a few labelled neurons were observed on both the ipsi and contralateral sides. These neurons increased in number and were distributed through layers III to VI by day 14. At this time labelled cell bodies were observed in the claustrum and lateral amygdaloid nucleus as well as in the globus pallidus and entopeduncular nucleus. On day 21 the contralateral labelling of the lateral amygdaloid nucleus was apparent. The anterograde transport from the striatum to globus pallidus, entopeduncular nucleus and substantia nigra was already visible at birth although its intensity increased during the first postnatal month.

The early development of major projections to the dorsal striatum in the North American opossum

We have employed immunocytochemical and axonal transport techniques to study the development of major projections to the dorsal striatum of the North American opossum. The opossum is born in a very immature state, 12-13 days after conception, and climbs into an external pouch where it remains attached to a nipple for several months. Its immaturity at birth and its protracted postnatal development make the opossum a good model for developmental studies. Although tyrosine hydroxylase-like immunoreactive (TH-LI), presumably dopaminergic, neurons were present in the ventral mesencephalon at birth (the presumptive substantia nigra and ventral tegmental area), there was no evidence for TH-LI axons in the striatal anlage. By postnatal day (PD)6, a few immunostained axons were found within the putamen. The subsequent growth of TH-LI axons into the striatum followed general caudal to rostral and ventrolateral to dorsomedial gradients and, at any age, they were most numerous in the areas exhibiting the greatest cytodifferentiation. By estimated (E)PD45, TH-LI axons were present in most, if not all, areas of the striatum. Serotoninergic (5-HT)-LI axons were found lateral to the presumptive striatum at birth but not within it. By PD7, however, a few 5-HT-LI axons could be identified in the putamen. The growth of 5-HT-LI axons into the striatum generally followed the same gradients described for TH-LI axons although at all ages their density was much less. Using the orthograde transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP), evidence was obtained for the existence of thalamostriatal projections by PD5 and for corticostriatal projections by PD10. Crossed corticostriatal projections were present by EPD23. Our results suggest that the development of major projections to the striatum occurs postnatally in the opossum, rather than prenatally as in placental animals. The timetable for striatal innervation is discussed in light of the developmental sequences established for other motor circuits.

The morphogenesis of glutamic acid decarboxylase in the neostriatum of the cat: neuronal and ultrastructural localization

Correlative light and electron microscopic immunohistochemical methods were adapted for a descriptive analysis of the normal time course and pattern of expression and intraneuronal localization of the enzyme glutamic acid decarboxylase (GAD) in the neostriatum (Ns) of fetal, postnatal and adult cats. The differentiation of this synthesizing enzyme demonstrated the establishment of gamma-aminobutyric acid (GABA) transmitter identity in these neurons and their connections. The structural modifications of these GABAergic profiles revealed the morphogenesis of important inhibitory synaptic inputs in the Ns. The expression of GAD began during late fetal development and proceeded in a diagonal gradient from the first-formed ventrolateral putamen to the last-formed dorsomedial caudate nucleus. The frequency of GAD-positive elements increased with age particularly during the early postnatal period. After the initial expression of GAD, 3 interrelated processes contributed to its differentiation: (1) enzyme accumulation; (2) enzyme association with membranous organelles and (3) progressive elaboration of neuronal infrastructure. Synaptogenesis was both coincident and subsequent to GAD differentiation. Two principal types of GABAergic structures, cell bodies and axonal 'terminals', were evident from the initiation of GAD expression. The GABAergic cell bodies were polymorphic by and after the day of birth and consisted of ubiquitous medium sized cells (often having somatic and/or dendritic spines) and rare large sized cells (apparently aspiny and confined to a limited region of the Ns). The GABAergic axonal terminals changed from growth cone and prototerminal forms to mature bouton en passage and bouton terminaux forms establishing axosomatic and axodendritic contracts, having symmetric synaptic specializations and providing inputs to both medium- and large-sized GABAergic target neurons.

Neurophysiological alterations in caudate neurons in aged cats

These neurophysiological studies provide information on the alterations in functional capacity of neurons in the aging caudate nucleus (Cd) of the cat. The major finding is that there is a marked loss of excitation in the Cd during the aging process. This loss is most apparent in animals 11-14 years of age but is demonstrable in animals 6-7 years of age. Extracellular recording techniques were used to test the ability of Cd neurons to respond to activation of two of their major inputs, the precruciate cortex (CX) and the substantia nigra (SN). Types of responses that were evoked in both 1-3- and 11-14-year groups were similar and consisted of excitation, excitation followed by inhibition of action potentials or inhibition alone without preceding excitation. The frequency of occurrence of these responses was altered in the aged animals when either input was stimulated. In 1-3-year-old cats CX stimulation evoked initially excitatory responses in 75% of the cells tested while in 11-14-year-old cats excitatory responses occurred in 62% of the cells. When the SN was stimulated the decrease in initial excitation was greater (69% in 1-3- vs 35% in 11-14-year groups). In all aged animals but not in 1-3-year-old cats stimulation thresholds were higher (39-79%) for evoking excitatory responses than for evoking inhibitory responses. In order to assess synaptic security, the ability of Cd neurons to respond to iterative stimulation was determined. Distributions of the minimum interval necessary to evoke two excitatory responses were constructed. There was a marked increase in the proportion of longer intervals in the aged animals indicating that the synaptic response was less secure. There was a tendency for more of the responses in aged animals to have shorter latencies. This result was probably due to loss of less secure longer latency responses that are mediated via multisynaptic pathways. These findings indicate that there are functional changes in a population of Cd neurons in aged cats that impair their ability to process information.

Quantitative morphology of medium-sized caudate spiny neurons in aged cats

These studies were designed to assess some of the morphological alterations that occur in medium-sized spiny neurons of the caudate nucleus in aged cats. Computer assistance was used to quantify in three dimensions the extent of the dendritic trees of 164 neurons from 11 cats (5 1-3 years and 6 over 10 years of age) stained by the rapid Golgi technique. In all animals beyond 10 years of age there was a decrease in the density of spines on distal dendritic segments. This decrease was moderate (16%) in 13 year old cats and reached about 50% in 15 and 18 year old animals. In addition, there was an increase in the frequency of occurrence of spines with enlarged heads in all aged cats. In cats over 13 years there was a marked loss of portions of distal dendritic segments. All measures of dendrite length displayed statistically significant decreases of 30-40% in cats 15 and 18 years of age. There were no significant age-related alterations in numbers of dendrites, number of branches per dendrite or soma diameter. These morphological results indicate that there is a sequence of age-related changes that occurs in caudate medium-sized spiny neurons and provides a basis from which to assess functional alterations.

Late development of intrinsic excitation in the rat neostriatum: an in vitro study

Functional maturation of intrinsic circuitry in the neostriatum was studied by intracellular recording and intracellular staining with Lucifer yellow in slices obtained from rat pups at postnatal days (P)1-20 and from adult rats. The most striking observation was that intrastriatal stimulation elicited predominantly inhibitory responses in slices obtained from animals of P1-6. In contrast, intrastriatally evoked responses in slices obtained after P10 were predominantly excitatory. The inhibitory postsynaptic potentials (IPSPs) recorded in slices obtained from pups were blocked by bicuculline (50 microM) and exhibited a reversal potential at about -60 mV which shifted in depolarizing direction when intracellular Cl- activity increased. Thus, these IPSPs correspond to IPSPs observed in adult animals. It is concluded that maturation of excitatory synapses is the main change during postnatal development. The changes of postsynaptic potentials were paralleled by the appearance of spines on dendrites around P7 as revealed by intracellular staining. The apparent input resistances and time constants of young neurons were very high and responses to large current injections were often distorted by humps which could not be observed in adult neurons. Only young neurons responded with bursts to synaptic activation in the presence of bicuculline (50 microM). It appears that dendritic conductances have a stronger influence on somatic discharge in the electrotonically compact young neurons than in adult neurons.

Postnatal development of identified medium-sized caudate spiny neurons in the cat

The morphology of intracellularly recorded neurons in the cat caudate nucleus (Cd) was studied during postnatal development. After intracellular recording of evoked responses in these neurons, horseradish peroxidase (HRP) was injected iontophoretically through the recording micropipette. Fifty-eight Cd neurons in cats ranging from 6 days of age through adulthood were identified morphologically. All of the recovered Cd cells were medium-sized spiny neurons. The basic somatodendritic morphology of these neurons was evident in the youngest kittens. The most striking morphological change was the postnatal formation of an extensive local axonal collateral plexus. The development of these local axonal collaterals was also quantified with computer assistance in medium-sized Cd spiny neurons selected from silver-impregnated material. This analysis showed that the major development of the branches of this local plexus occurred between birth and 3-4 months of postnatal age. Data from both the HRP-filled and silver-stained axons indicated that the postnatal growth of the local axonal collaterals of the medium spiny cells was associated with the elaboration and increasing prevalence of evoked inhibitory postsynaptic potentials in Cd neurons.

Ontogeny of striatal unit activity and effects of single or repeated haloperidol administration in rats

Development of striatal unit activity recorded from chloral hydrate anesthetized, neonatal rats was characterized electrophysiologically following acute or repeated haloperidol administration. No spontaneously active single units were detected in 8 day old pups. Spontaneous activity was recorded by 17 days of age, although the number of active cells, firing frequency and the variety of firing patterns were less diverse than those observed in 28 day olds. There were also age related differences in striatal unit responses to haloperidol. A significant increase in activity was induced by acute haloperidol administration only in 28 day old animals. No tolerance to the acute effects was demonstrated. Both 17 and 28 day olds responded to repeated haloperidol injections, followed by a 24 h recess, with an increase in striatal activity. These results may assist our understanding of the effects of human fetal, neonatal and/or adolescent exposure to neuroleptics.

Postnatal development of caudate input neurons in the cat

Lectin-bound horseradish peroxidase (WG-HRP) was pressure-injected into the caudate nucleus (Cd) of neonatal (less than 24 hours of age) and adult cats in order to assess the postnatal development of monosynaptic Cd input neurons. Tissue was processed for peroxidase activity with a benzidine dihydrochloride chromagen. The injection of WG-HRP produced relatively similar labelled zones of marker uptake in the caudate nuclei of both neonates and adults. Similar axonal projections were also labelled in both age groups. While many characteristics of retrogradely labelled CD input neurons were apparently constant throughout postnatal life, each of these features had a particular developmental modification. (1) Regardless of age, neuronal somata that projected to the CD were located in the neocortex, thalamus, substantia nigra, mesencephalic raphe nuclei, and globus pallidus. In each of these brain sites, labelled CD input neurons appeared to migrate postnatally. (2) The Cd afferent axons originated from the same neuronal lines in neonates and adults--small-to-medium-sized cortical neurons and medium-sized-to-large fusiform cells in all other brain sites. In each of the brain sites, labelled neurons displayed marked postnatal somatic growth. (3) In both age groups, there was a characteristic intrasomatic reaction product density in the labelled neurons located in each brain site (substantia nigra greater than thalamus = raphe = globus pallidus greater than cortex). In each of these brain sites, the intrasomatic reaction product density was less in neonates than in adults.

Effects of caudate nuclear or frontal cortical ablation in neonatal kittens or adult cats on the spontaneous firing of forebrain neurons

In this paper we have determined the long-lasting consequences of caudate and frontal cortical lesions on spontaneous neuronal firing. Lesions were made both in neonatal and adult cats. All recordings were made in adults. Qualitatively, the effects of the caudate ablations were similar whether they had been carried out in kittens or in adult cats. Caudate lesions produced long-lasting (greater than or equal to 1 year) decreases in the spontaneous firing of cortical neurons. These changes were more pronounced when made in neonates than in adults. The distributions of mean interspike intervals were also altered by these caudate lesions in the pallidum and in the ventral lateral nucleus of the thalamus. Again these effects were more marked if the animals were lesioned as neonates than as adults. Frontal cortical lesions inflicted upon adult cats produced more widespread changes in spontaneous firing rates than similar lesions made in neonates. In both groups frontal lesions slowed spontaneous firing and changed the distributions of mean interspike intervals of caudate neurons. These effects were long-lasting (greater than or equal to 1 year in neonatally-ablated animals). Cortical lesions made in adult cats markedly altered thalamic and pallidal spontaneous activity. Similar lesions made in neonates produced relatively small changes in thalamic and pallidal activity.

Quantitative developmental studies of feline neostriatal spiny neurons

This research documents aspects of the quantitative development of the 'medium' spiny neuron in the kitten from 2 to 143 days of age. Using material derived from 244 Golgi-impregnated neurons in 15 kittens and with computer assistance the changes in somatic, dendritic and spine development were quantified. Although mean somatic diameter increased only slightly from 2--3 to 8--10 days of age, the proportion of neurons with large diameters increased significantly during the developmental period. In addition, the radius of the dendritic field of caudate spiny neurons increased significantly over the age period examined. An unexpected finding was that the number of dendrites per neuron decreased with age, probably due to a decrease in the proportion of neurons with 6 or more dendrites in animals 90--143 days of age. Growth of dendritic segments occurred throughout the age period studied. This growth was apparently caused by lengthening of all dendritic segments and this resulting increase was proportional to the initial length of the individual branches. Number of branches per dendrite and frequency of dendritic branches with different orders remained constant across age indicating that the basic dendritic branching pattern is probably set for the cat before birth. With maturation the density of spines on distal dendritic branches increased while on proximal dendritic branches spine density decreased. The time course of these quantitative changes was related to alterations in synaptogenesis and physiological changes in caudate neurons.

Development of spiny and aspiny neurons in the caudate nucleus of the dog during the first postnatal month

The adult and developmental morphology of spiny and aspiny neurons in the dog caudate nucleus was examined using the Golgi-Kopsch technique. In the adult, three types each of spiny and aspiny neurons were identified based upon dendritic morphology and cell soma size. They corresponded in large part to those neurons described previously in the caudate nuclei of the rat, cat, and monkey. At birth, dendrites of spiny neurons possessed varicosities, filopodia, and thick proximal dendritic stumps--all characteristic of immaturity. Maturation of these processes involved the thinning of proximal dendrites, lengthening of dendritic shafts, and growth of dendritic spines. Although most of the dendritic maturation occurred during the first postnatal month, spine densities and dendritic lengths of spiny I neurons at 30 days were still less than those seen in the adult. Aspiny I neurons were also immature at birth but lacked the filopodia and thicker proximal dendrites that characterized immature spiny neurons. Aspiny dendritic development involved primarily the lengthening of dendritic processes; by 30 days the aspiny I neurons were indistinguishable from those seen in the adult. These results suggest that dendritic development of spiny I neurons may extend well past the end of the first postnatal month and that studies investigating functional development in the caudate nucleus should consider the relatively extended time period required for maturation of these primary synaptic sites.

Development of motor activity in kittens

Developmental alterations in motor activity were quantified for the 1st 7 postnatal months in the kitten. Motor activity measured with a stabilimeter was low and constant for the 1st 9 postnatal days, increased markedly until Day 14, and remained stable until Day 21. Locomotor activity measured in an open field decreased slightly during the 1st 3 weeks, increased markedly during the next 2 weeks, and then remained relatively constant until the 9th week. Activity increased again during the 3rd and 4th month and then declined until 7 months of age. We suggest that the marked increase in motor activity during the 2nd week reflects development of visual and auditory systems whereas the increase in locomotor activity during the 4th and 5th weeks represents maturation of neural systems concerned with motor control. The underlying causes of the final activity changes probably represent the maturation of many neural systems and concomitant development of adult behavior in the cat.

Intracellular analysis of the development of responses of caudate neurons to stimulation of cortex, thalamus and substantia nigra in the kitten

Intracellular recordings were made from caudate neurons in anesthetized kittens of 2-72 days of age. In adult cats, results of intracellular recordings indicate that caudate neurons respond most frequently to stimulation of their major afferents from cortex, thalamus and substantia nigra with a sequence of excitation followed by inhibition (EPSP-IPSP sequence). The results of the present study show that the prominent IPSP of this sequence is not well developed in young kittens and does not reach adult values in terms of frequency of occurrence until beyond 40 days of age. Amplitude and duration of the IPSP evoked by cortical stimulation also did not reach adult values until beyond 40 days of age. In contrast, EPSPs can be evoked in the youngest kittens by stimulation of afferents to the caudate. These findings suggest that the caudate nucleus may alter its role during development. In early postnatal periods it functions as a simple relay system transmitting incoming information to its outputs in a relatively unaltered fashion. Later in development it becomes a system capable of complex modulation and filtering of neural information.

The postnatal development of the caudate nucleus: a Golgi and electron microscopic study of kittens

This study used light and electron microscopy to describe changes in the synaptic organization of developing caudate nucleus and to quantitate postnatal synaptogenesis in this region. Observation at the light microscopic level focus on the perinatal period and suggest an early maturation of spiny interneurons of the caudate nucleus. Golgi impregnation of these cells at early postnatal ages (birth to 7 days) reveals 3-5 primary dendrites which radiate from the cell body and extend for distances of 8-16 micron before branching. Secondary dendritic branches contain spines and extend, with further branching, for additional distances of 60-160 micron. The dendritic fields of neighboring caudate neurons overlap and the axons which arise from these cells course and branch within the dendritic fields. Examination of perinatal caudate neuropil (birth to 5 days) by electron microscopy reveals an extensive and well-developed axodendritic connectivity. Axonal profiles form multiple synapses en passant along single dendrites and dendritic spines or on several adjacent dendritic branches. At these ages, terminals contain few synaptic vesicles and synaptic junctions are slightly asymmetrical. By the fifteenth postnatal day, boutons are filled with vesicles, junctional complexes are distinctly asymmetrical, and axondendritic connectivity has been modified by the increase of dendritic spines and branchlets. Thus, the basic pattern underlying the organization of synapses in the mature caudate nucleus, is established within the first week postpartum and subsequent changes are primarily quantitative.