DNA image cytometry on sections as compared with image cytometry on smears and flow cytometry in melanoma

The distribution and morphology of neurocalcin-immunopositive neurons have been studied in the rat accessory olfactory bulb. Different subsets of neurons displaying neurocalcin immunoreactivity were found in the glomerular layer, the external plexiform layer and the internal plexiform layer. The most abundant staining was detected in the glomerular layer where neurocalcin-immunoreactive periglomerular cells and external tufted cells were observed in the lateral glomeruli, whereas the central region of this layer was practically devoid of immunopositive neurons. In the external plexiform layer, medial tufted cells and Van Gehuchten cells displayed neurocalcin immunoreactivity. In the internal plexiform layer, interneurons classified as horizontal cells and vertical cells of Cajal were neurocalcin-immunoreactivity. In the internal plexiform layer, interneurons classified as horizontal cells and vertical cells of Cajal were neurocalcin-immunostained. The staining pattern for neurocalcin in the accessory olfactory bulb showed similarities with the immunostaining described in this brain region for another EF-hand calcium binding protein, calbindin D-28k. However, after double immunohistochemical labeling, colocalization of both proteins in the same neuron was not observed, reflecting a biochemical heterogeneity within morphologically homogeneous neuronal groups.     

Calretinin- and parvalbumin-immunoreactive neurons in the rat main olfactory bulb do not express NADPH-diaphorase activity

The presence of nitric oxide synthase (NOS) in neuronal elements expressing the calcium-binding proteins calretinin (CR) and parvalbumin (PV) was studied in the rat main olfactory bulb. CR and PV were detected by using immunocytochemistry and the nitric oxide (NO) -synthesizing cells were identified by means of the reduced nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-diaphorase) direct histochemical method. The possible coexistence of NADPH-diaphorase and each calcium-binding protein marker was determined by sequential histochemical-immunohistochemical double-labeling of the same sections. Specific neuronal populations were positive for these three markers. A subpopulation of olfactory fibers and olfactory glomeruli were positive for either NADPH-diaphorase or CR. In the most superficial layers, groups of juxtaglomerular cells, superficial short-axon cells and Van Gehuchten cells demonstrated staining for all three markers. In the deep regions, abundant granule cells were NADPH-diaphorase- and CR-positive and a few were PV-immunoreactive. Scarce deep short-axon cells demonstrated either CR-, PV-, or NADPH-diaphorase staining. Among all these labeled elements, no neuron expressing CR or PV colocalized NADPH-diaphorase staining. The present data contribute to a more detailed classification of the chemically- and morphologically-defined neuronal types in the rodent olfactory bulb. The neurochemical differences support the existence of physiologically distinct groups within morphologically homogeneous populations. Each of these groups would be involved in different modulatory mechanisms of the olfactory information. In addition, the absence of CR and PV in neuronal groups displaying NADPH-diaphorase, which moreover are calmodulin-negative, indicate that the regulation of NOS activity in calmodulin-negative neurons of the rat olfactory bulb is not mediated by CR or PV.     

Organization of inhibition in the rat olfactory bulb external plexiform layer

1. Intracellular recordings were made from the output neurons (mitral and tufted cells) of the rat olfactory bulb during electrical orthodromic stimulation of the olfactory nerve layer (ONL) and antidromic stimulation of the lateral olfactory tract and posterior piriform cortex (pPC) to test for physiological differences among the neuron types. Many of these neurons were identified by intracellular injections of biocytin, and others were identified by their pattern of antidromic activation. 2. Both marked and unmarked mitral cells showed large inhibitory postsynaptic potentials (IPSPs) in response to antidromic stimulation of the pPC, whereas tufted cells exhibited small IPSPs in response to pPC stimulation. Tufted cells, however, showed large IPSPs in response to ONL stimulation. In many cases, these tufted cell responses to ONL stimulation were larger than the mitral cell responses. The marked superficial tufted cells, those with basal dendrites in the superficial sublayer of the external plexiform layer (EPL), had the smallest IPSPs in response to pPC stimulation. These data support anatomic observations suggesting that the granule cell populations responsible for the IPSPs may be different for mitral and for superficial tufted cells. 3. The different types of output cells also showed differences in their responses to orthodromic stimulation. Type I mitral cells, which have basal dendrites confined to the deep sublayer of the EPL, were significantly less excitable by ONL stimulation than were the type II mitral cells, which have basal dendrites distributed within the intermediate sublayer of the EPL. Half of the type I mitral cells could not be excited at all by ONL stimulation. Superficial tufted cells showed even greater orthodromic excitability than type II mitral cells, usually responding to ONL stimulation with two or more spikes. 4. The ionic basis of the IPSPs in the superficial tufted cells appeared similar to those described for mitral cells. These IPSPs could be reversed by chloride injection and were associated with increased membrane conductance. 5. For both mitral and tufted cells, the number of ONL electrodes evoking IPSPs was greater than the number evoking spikes. These data suggest a kind of center-surround organization of inputs to these cells from the ONL, although this does not yet imply that the sensory receptive field of these output cells has a center-surround organization. 6. In conclusion, the properties of rat olfactory bulb output cells correlate with the sublayers of the EPL in which their basal dendrites lie.(ABSTRACT TRUNCATED AT 400 WORDS)     

Beta-galactosidase-labelled relay neurons of homotopic olfactory bulb transplants establish proper afferent and efferent synaptic connections with host neurons

The vertebrate olfactory system has long been an attractive model for studying neuronal regeneration and adaptive plasticity due to the continuous neurogenesis and synaptic remodelling throughout adult life in primary and secondary olfactory centres, its precisely ordered synaptic network and accessibility for manipulation. After homotopic transplantation of fetal olfactory bulbs in bulbectomized neonatal rodents, newly regenerated olfactory neurons form glomeruli within the graft, and the efferent mitral/tufted cells of the transplant innervate the host brain, terminating in higher olfactory centres. However, the synaptic connections of the transplanted relay neurons within the graft and/or host's olfactory centres could not be characterized mainly because of lack of suitable cell-specific markers for these neurons. In this study, we have used olfactory bulbs from transgenic fetuses, in which the majority of the mitral/tufted cells express the bacterial enzyme beta-galactosidase, for homotopic olfactory bulb transplantation following complete unilateral bulbectomy. In the transplants, the cell bodies and terminals of the donor mitral/tufted cells were identified by beta-galactosidase histochemistry and immunocytochemistry at both light and electron microscope levels. We demonstrate that transplanted relay neurons re-establish specific synaptic connections with host neurons of the periphery, source of the primary signal and central nervous system, thereby providing the basis for a functional recovery in the lesioned olfactory system.     

Basic fibroblast growth factor promotes the generation and differentiation of calretinin neurons in the rat cerebral cortex in vitro

Calretinin-expressing neurons are some of the earliest postmitotic cells to appear in the developing cerebral cortex. Lineage studies have shown that the expression of this calcium-binding protein in cortical neurons is not genetically programmed and is likely to be induced by external factors. A number of studies have clearly shown that basic fibroblast growth factor (bFGF) and a number of neurotrophins promote the proliferation and differentiation of cortical progenitor cells to a particular lineage. Here, using a culture system of dissociated rat cortical cells, we found that brain-derived neurotrophic factor and neurotrophin-3 promoted the morphological differentiation of one of the calretinin-containing neuronal subpopulations, the Cajal-Retzius cells. Another subpopulation of calretinin-expressing cells of smaller size and bipolar form was generated when cultures were treated with bFGF. The progenitors of these neurons were stimulated by bFGF to divide a number of times before initiating their differentiation programme. The number of calretinin-expressing neurons increased further when cultures were treated with a combination of bFGF and retinoic acid.     

Pharmacokinetics and plasma protein binding of sulfamethoxypyridazine in camels (Camelus dromedarius)

Transplant-to-host neuron migration and neurite projection were demonstrated using the mouse allelic Thy-1 system, namely, BALB/c (Thy-1.2) embryonic olfactory bulb (OB) as the graft and 5- to 6-week-old AKR (Thy-1.1) OB as the host. From OB transplants inserted into the host OB, small neurons were often extensively moved mainly in the internal granular layer and showed almost the same morphology as the normal granule neurons. Some large neurons also migrated. Furthermore, inside OB the transplants sent axons mainly into the internal granular layer and dendrites into the external plexiform layer. Outside OB the axons arrived at the anterior olfactory nucleus, primary olfactory cortex, olfactory tubercle, and cortical nucleus of the amygdaloid complex. These fibers appeared to terminate in normal target areas. These findings show that the olfactory system at 5-6 weeks of age still has the capacity to integrate newly migrated neurons and to receive newly growing fibers from the transplant.     

Odor-induced, activity-dependent transneuronal gene induction in vitro: mediation by NMDA receptors

Expression of tyrosine hydroxylase (TH) by juxtaglomerular (JG) neurons of the olfactory bulb (OB) requires innervation of the bulb by olfactory receptor neurons (ORNs). ORN lesion selectively downregulates TH in JG neurons. In reversible odor deprivation, TH expression is downregulated as the naris is closed and then upregulated upon naris reopening. The mechanism or mechanisms regulating this dependence are unknown. TH expression could be regulated by trophic factor release and/or synaptic activity from ORN terminals. We investigated TH expression in cocultures of dissociated postnatal rat OB cells and embryonic olfactory neuroepithelium (OE) slice explants. TH-positive neurons in control dissociated OB cell cultures alone comprise only a small fraction of the total population of cells present in the culture. However, when OE slice explants are cocultured with dispersed OB cells, there is a mean 2.4-fold increase in the number of TH-positive neurons. ORNs in vivo use glutamate as a neurotransmitter. Broad spectrum excitatory amino acid antagonists (kyurenic acid) or selective antagonists of the NMDA receptor (APV) both prevent induction of TH expression in OE-OB cocultures. Furthermore, pulse application of NMDA stimulates TH expression in OB neurons in the absence of OE. In vitro, OB TH neurons express NMDA receptors, suggesting that NMDA stimulation is acting directly on TH neurons. Exposure of OE explants to natural odorants results in upregulation of TH, presumably through increased ORN activity, which could be blocked by APV. These findings indicate that odorant-stimulated glutamate release by ORN terminals regulates TH expression via NMDA receptors on JG dopaminergic neurons.     

Cell-specific differential expression of Na(+)-channel beta 1-subunit mRNA in the olfactory system during postnatal development and after denervation

Activity-dependent mechanisms have been implicated in olfactory system development but, although such activity requires ion channels, few reports have described their expression in the olfactory system. We investigated the developmental and denervation-induced regulation of the Na(+)-channel beta 1 subunit (Na beta 1) in rat olfactory bulb (OB) and piriform cortex (PC). In situ hybridization shows that Na beta 1 mRNA expression is upregulated developmentally, but with different time courses in mitral, tufted, and pyramidal cells. In mitral cells, label was detected at postnatal day 4 (P4) and gradually increased to P14. Tufted cells were devoid of Na beta 1 mRNA before P14, when most cells expressed adult levels. In pyramidal cells of PC, Na beta 1 expression was not detectable clearly until P14, with maximal expression at P28. To examine the regulation of Na beta 1 mRNA, we surgically deafferented the OB at P30 and compared the effects on Na beta 1 with those for Na(+)-channel alpha-subunit (Na alpha) mRNAs. Within 5 d of surgery, the Na beta 1 and Na alpha II signals within tufted cells disappeared almost completely. Na beta 1 and Na alpha II expression was decreased in mitral cells to low-to-moderate levels. In pyramidal cells, Na beta 1 mRNA expression was decreased moderately without significant changes in Na alpha II mRNA. Deafferentation had no detectable effects on Na alpha I or III mRNAs in either OB or PC. These data indicate that Na beta 1 mRNA is expressed differentially in subpopulations of cells in the olfactory system during development and after deafferentation and suggest that the expression of Na beta 1 is regulated independently of Na alpha mRNAs via cell-specific and pathway-specific mechanisms.     

Excitatory synapses in the glomerular triad of frog olfactory bulb in vitro

Whole-cell patch clamp recording techniques were applied to periglomerular (PG) cells in slices of the frog olfactory bulb (OB) to study the properties of the excitatory synapses in the triad formed by the olfactory nerve (ON) and the dendrites of mitral/tufted (MT) cells and PG cells. The postsynaptic response evoked by ON stimulation was glutamatergic and could be dissected into NMDA and non-NMDA components of equivalent amplitudes. The dendro-dendritic synapse between MT and PG cells could be activated following antidromic stimulation of the lateral and medial olfactory tract (LOT and MOT). In this case the postsynaptic potentials had amplitudes and durations comparable to those obtained by ON stimulation, the neurotransmitter was glutamate, but the synapse was largely dominated by the slow NMDA component.     

Identification of olfactory receptor mRNA sequences from the rat olfactory bulb glomerular layer

In situ hybridization has demonstrated mRNA for olfactory receptors (OR) in the axon terminals of olfactory receptor neurons. Neurons that express the same OR appear to send their axons to two stereotyped glomeruli in the olfactory bulb (OB). Based on these observations, we tested the feasibility of using RT-PCR to isolate and sequence OR mRNA from small samples of the rat OB glomerular layer. Biomagnetic mRNA isolation followed by RT-PCR yielded partial sequences for 21 novel members of the OR family. The results suggest that the topography of OR mRNA can be mapped across the OB, to study synaptic specificity and odor representation in the olfactory system.     

Two novel monoclonal antibodies (1.9.E and 4.11.C) against olfactory bulb ensheathing glia

We produced and characterized two monoclonal antibodies, termed 1.9.E and 4.11.C, that specifically recognize olfactory bulb ensheathing glia. Both antibodies were generated using the olfactory nerve layer (ONL) of newborn rat olfactory bulbs (P0, P1) as immunogens. The specificity of these antibodies was tested by immunofluorescence techniques on tissue sections and cultures of adult and neonatal rat olfactory bulbs, and by Western blot analysis. 1.9.E labeled the ONL and glomerular layer of the olfactory bulb (OB) of adult rats. In newborn rats, 1.9.E immunostained ensheathing cells from the ONL and peripheral olfactory fascicles. Furthermore, 1.9.E reacted with some processes of the radial glia in the periventricular germinal layer of the newborn rat. Although 4.11.C also specifically labeled ensheathing cells in the adult OB, it did not stain any cell type in the ONL of newborn rats. The lack of double labeling with either 1.9.E or 4.11.C and anti-olfactory marker protein (OMP) antibody, a specific marker for olfactory axons, indicated that none of the monoclonals recognized olfactory axons. Double immunostaining of adult OB cultures with 1.9.E or 4.11.C and anti-p75-nerve growth factor receptor revealed that both antibodies specifically recognized ensheathing glia in those cultures. Filaments were strongly labeled throughout the entire cytoplasm of ensheathing cells, suggesting that 1.9.E and 4.11.C immunoreacted with ensheathing glia cytoskeleton. 4.11.C stained a few Schwann cells in adult sciatic nerve sections. Moreover, 4.11.C immunostained cortical astrocyte cultures from newborn rats (P1). In Western blot analysis both antibodies recognized a major component, migrating with an apparent molecular weight of 60 kDa, from olfactory nerve and glomerular layer (ONGL) extracts of adult and neonatal rats. The pattern of immunoreactivity of 1.9.E and 4.11.C antibodies suggest that both antibodies are specific markers for olfactory ensheathing glia in the adult rat central nervous system (CNS).     

Development and further characterization of a small subclass of rat olfactory receptor neurons that shows immunoreactivity for the HSP70 heat shock protein

We previously described a rat olfactory receptor neuron (ORN) subpopulation [the 2A4(+) ORNs] that shows uniquely strong reactivity with antibodies to the 70-kD heat shock protein (HSP70) family of molecular chaperones (Carr et al. [1994] J. Comp. Neurol. 348:150-160). The 2A4(+)ORNs are dispersed through zones II-IV of the olfactory epithelium (OE), and their axons project to only two or three glomeruli that are located consistently in each olfactory bulb (OB). To date, the 2A4(+)ORN subpopulation is the only cell population to show such distinct HSP70 immunoreactivity as well as the most discrete ORN subpopulation to be so labeled. The present report shows that 2A4(+)ORN neurons first appear between postnatal days 7 (P7) and P10. Initially, low cell numbers rise to a density of 0.1 2A4(+)ORNs/mm OE length by P14, plateau at 0.9 2A4(+)ORNs/mm by P49, then fall to adult values of 0.4 cells/mm. Autoradiographic birthdating indicates that almost all of these early appearing 2A4(+)ORNs are generated postnatally, in contrast to the prenatal generation of all ORN subpopulations characterized to date by their expression of olfactory receptor protein mRNAs. A developmentally related increase in the mean depth of 2A4(+)ORNs within the OE also occurs. In the OB, initial 2A4(+)axonal projections are to only two or three glomeruli, as in adults. Slight but significant rostral shifts in (+)glomerular location occur with development. The 2A4(+)ORN immunoreactivity was found to be due to expression of HSP70, the dominant stress-inducible member of the HSP70 family, rather than constitutively expressed HSC70. In addition, despite their presence in rat OE, no 2A4(+)ORNs were found in mice, gerbils, guinea pigs, or hamsters.     

Rearrest among mentally ill offenders

The accessory olfactory bulb (AOB) is a primary center of the vomeronasal system. In the dog, the position and morphology of the AOB remained vague for a long time. Recently, the morphological characteristics of the dog AOB were demonstrated by means of lectin-histochemical, histological, and immunohistochemical staining, although the distribution of each kind of neuron, especially granule cells, remains controversial in the dog AOB. In the present study, we examined the distribution of neuronal elements in the dog AOB by means of immunohistochemical and enzyme-histochemical staining. Horizontal paraffin or frozen sections of the dog AOB were immunostained with antisera against protein gene product 9.5 (PGP 9.5), brain nitric oxide synthase (NOS), glutamic acid decarboxylase (GAD), tyrosine hydroxylase (TH), substance P (SP), and vasoactive intestinal polypeptide (VIP) by avidin-biotin peroxidase complex method. In addition, frozen sections were stained enzyme-histochemically for NADPH-diaphorase. In the dog AOB, vomeronasal nerve fibers, glomeruli, and mitral/tufted cells were PGP 9.5-immunopositive. Mitral/tufted cells were observed in the glomerular layer (GL) and the neuronal cell layer (NCL). In the NCL, a small number of NOS-, GAD-, and SP-immunopositive and NADPH-diaphorase positive granule cells were observed. In the GL, GAD-, TH-, and VIP-immunopositive periglomerular cells were observed. In the GL and the NCL, TH-, and VIP-immunopositive short axon cells were also observed. In addition to these neurons, TH- and SP-immunopositive afferent fibers were observed in the GL and the NCL. We could distinctly demonstrate the distribution of neuronal elements in the dog AOB. Since only a small number of granule cells were present in the dog AOB, the dog AOB did not display such a well-developed GCL as observed in the other mammals.     

Waiting lists for psychogeriatric nursing home admissions: insight into the problematic waiting times

The distribution, morphological features, and postnatal development of parvalbumin (PV) immunoreactive neurons in the main olfactory bulb (MOB) of the house musk shrew, Suncus murinus, were studied to report for the first time on PV positive bulbar interneurons in the order Insectivora. In adult animals, PV neurons are distributed in the glomerular layer (GL), the external plexiform layer (EPL), the internal plexiform layer (IPL) and the granule cell layer (GCL) of the MOB. These neurons were identified as a subpopulation of periglomerular cells and perinidal cells [Alonso et al., 1995] in the GL and at the GL-EPL border, respectively, and as bipolar and multipolar neurons in the EPL and four types of the interneurons (horizontal cells, Cajal cells, Golgi cells, and bitufted cells) in the layers deeper than the mitral cell layer. During development of PV neurons, neurons exhibiting extremely faint PV immunoreactivity first appeared in the GCL at postnatal day 14 and increased markedly in number and intensity of their PV immunoreactivity from postnatal days 14 to 28. At postnatal day 21, PV neurons were identified as periglomerular cells in the GL, perinidal cells at the GL-EPL border, and morphologically unidentifiable neurons in the EPL, IPL and GCL. At postnatal day 28, PV neurons exhibited a nearly adult pattern with respect to distribution and structural features. The present results strongly suggest that a wide variety of PV positive neurons in the MOB of the house musk shrew may develop postnatally.     

PCTAIRE 2, a Cdc2-related serine/threonine kinase, is predominantly expressed in terminally differentiated neurons

PCTAIRE are members of a subfamily of Cdc2-related kinases that have been shown to be preferentially expressed in post-mitotic cells. To examine the neural functions of PCTAIRE, rat cDNA clones encoding PCTAIRE 1, 2, and 3 were isolated, and their expression patterns in the brain were analyzed. Among the three rat PCTAIREs, only PCTAIRE 2 was found to be specifically expressed in the brain. Furthermore, its expression was transiently increased during brain development, peaking 7-15 days after birth. Within the brain, PCTAIRE 2 was concentrated in the neuronal layers of the hippocampus and olfactory bulb, which mostly consist of post-mitotic neurons. In an immunocytochemical experiment, immunoreactivity for PCTAIRE 2 was detected in the cell bodies and extended neurites of neurons, but not in astrocytes. The PCTAIRE 2 protein was recovered in the particulate fraction and resistant to solubilization with non-ionic detergent, suggesting that PCTAIRE 2 might be present as a component of a large protein complex. An immunoprecipitation assay revealed that the PCTAIRE 2 was associated with Ser/Thr-phosphorylating activity for histone H1, and that its activity depended on association with a regulatory partner that can be released under high-salt conditions. These findings suggest that PCTAIRE 2 is a Ser/Thr kinase that might play a unique role in terminally differentiated neurons.     

FMRF amide-like-immunoreactive primary sensory neurons in the olfactory system of the terrestrial mollusc, Limax marginatus

The distribution of FMRF amide-like immunoreactivity was investigated in the olfactory organs in the tentacle tip of the terrestrial slug, Limax marginatus. Approximately 0.7% of the neurons in the lobules of the tentacle ganglia demonstrated FMRF amide-like immunoreactivity. Most of the FMRF amide-like-immunoreactive somata lay at superficial positions within the lobules, and dendritic processes extended to the outer surface of the sensory epithelium, whereas the axons traveled toward the cerebral ganglion through the ventral part of the tentacle nerve. From their morphological features, FMRF amide-like-immunoreactive cells were considered to be primary sensory neurons.     

Alpha isoform of calcium-calmodulin dependent protein kinase II (CAM II kinase-alpha) restricted to excitatory synapses in the CA1 region of rat hippocampus

Pre-embedding immunoperoxidase staining for CAM II kinase-alpha and post-embedding immunogold staining for glutamate and GABA, were used to reveal the subcellular distribution of CAM II kinase-alpha at transmitter-characterized synapses in the CA1 region of rat hippocampus. Immunoelectron microscopy showed that the majority of CAM II kinase-alpha-immunostained neuronal profiles were dendritic spines presumably derived from pyramidal cells. CAM II kinase-alpha immunoreactivity was mainly localized in postsynatic densities associated with glutamatergic axon terminals. No CAM II kinase-alpha immunoreactivity was detected in GABA-immunoreactive profiles or at GABAergic synapses. This study provides morphological evidence that CAM II kinase-alpha is involved only in excitatory neuronal transmission in the CA1 region. The enzyme is unlikely to be involved in plasticity at GABA synapses.     

Synaptic input and output of parvalbumin-immunoreactive neurons in the neostriatum of the rat

Previous studies have demonstrated that the calcium-binding protein parvalbumin, is located within a population of GABAergic interneurons in the neostriatum of the rat. Anatomical studies have revealed that these cells receive asymmetrical synaptic input from terminals that are similar to identified cortical terminals and that they innervate neurons with the ultrastructural features of medium spiny cells. Furthermore, electrophysiological studies suggest that some GABAergic interneurons in the neostriatum receive direct excitatory input from the cortex and inhibit medium spiny cells following cortical stimulation. The main objectives of the present study were (i) to determine whether parvalbumin-immunoreactive neurons in the rat receive direct synaptic input from the cortex, (ii) to determine whether parvalbumin-immunopositive axon terminals innervate identified striatal projection neurons and (iii) to chemically characterize this anatomical circuit at the fine structural level. Rats received stereotaxic injections of biocytin in the frontal cortex or injections of neurobiotin in the substantia nigra. Following an appropriate survival time, the animals were perfused and the brains were sectioned and treated to reveal the transported tracers. Sections containing the neostriatum were treated for simultaneous localization of the transported tracer and parvalbumin immunoreactivity. Tracer deposits in the cortex gave rise to massive terminal and fibre labelling in the neostriatum. Parvalbumin-immunoreactive elements located within fields of anterogradely labelled terminals were examined in the electron microscope and corticostriatal terminals were found to form asymmetrical synaptic specializations with all parts of parvalbumin-immunoreactive neurons that were examined. Tracer deposits in the substantia nigra produced retrograde labelling of a subpopulation of striatonigral neurons. Areas of the neostriatum and nucleus accumbens containing retrogradely labelled neurons and parvalbumin-immunoreactive structures were selected for electron microscopy. Parvalbumin-immunopositive axon terminals formed symmetrical synaptic specializations with the perikarya of retrogradely labelled medium spiny projection neurons. Postembedding immunocytochemistry for GABA revealed that parvalbumin-immunoreactive boutons in synaptic contact with medium spiny neurons were GABA-positive. These data demonstrate directly a neural circuit whereby cortical information may be passed to medium spiny cells, via GABAergic interneurons, in the form of inhibition and provide an anatomical substrate for the feed-forward inhibition that has been detected in spiny neurons in electrophysiological experiments.