Core-and-belt organisation of the mesencephalic and forebrain auditory centres in turtles: expression of calcium-binding proteins and metabolic activity.

The distribution of immunoreactivity to the calcium-binding proteins parvalbumin, calbindin and calretinin and of cytochrome oxidase activity was studied in the mesencephalic (torus semicircularis), thalamic (nucleus reuniens) and telencephalic (ventromedial part of the anterior dorsal ventricular ridge) auditory centres of two chelonian species Emys orbicularis and Testudo horsfieldi. In the torus semicircularis, the central nucleus (core) showed intense parvalbumin immunoreactivity and high cytochrome oxidase activity, whereas the laminar nucleus (belt) showed low cytochrome oxidase activity and dense calbindin/calretinin immunoreactivity. Within the central nucleus, the central and peripheral areas could be distinguished by a higher density of parvalbumin immunoreactivity and cytochrome oxidase activity in the core than in the peripheral area. In the nucleus reuniens, the dorsal and ventromedial (core) regions showed high cytochrome oxidase activity and immunoreactivity to all three calcium-binding proteins, while its ventrolateral part (belt) was weakly immunoreactive and showed lower cytochrome oxidase activity. In the telencephalic auditory centre, on the other hand, no particular region differed in either immunoreactivity or cytochrome oxidase activity. Our findings provide additional arguments in favour of the hypothesis of a core-and-belt organisation of the auditory sensory centres in non-mammalian amniotes though this organisation is less evident in higher order centres. The data are discussed in terms of the evolution of the auditory system in amniotes.

Preoptic FMRF-amide-like immunoreactive projections to the retina in the lamprey (Lampetra fluviatilis).

A centrifugal visual system showing FMRF-amide-like immunoreactivity has been demonstrated in Lampetra fluviatilis by using immunocytochemical and hodological techniques. From 50 to 60 immunoreactive neurons, labelled after contralateral intraocular injection of rhodamine beta-isothiocyanate, form a small, clearly defined, nucleus in the lateral neural plate of the magnocellular preoptic nucleus. These cells give rise to immunoreactive axons which have been observed at the base of the nucleus, in the optic chiasma and in the optic nerve, to project into the intermediate plexiform layer of the retina, which separates the layer of internal horizontal cells from the layer of external horizontal cells. This FMRF-amide-like immunoreactive centrifugal visual system is compared to that described in Gnathostomes.

Synaptic circuitry in the retinorecipient layers of the optic tectum of the lamprey (Lampetra fluviatilis). A combined hodological, GABA and glutamate immunocytochemical study.

The ultrastructure of the retinorecipient layers of the lamprey optic tectum was analysed using tract tracing techniques combined with GABA and glutamate immunocytochemistry. Two types of neurons were identified; a population of large GABA-immunonegative cells, and a population of smaller, highly GABA-immunoreactive interneurons, some of whose dendrites contain synaptic vesicles (DCSV). Five types of axon terminals were identified and divided into two major categories. The first of these are GABA-immunonegative, highly glutamate-immunoreactive, contain round synaptic vesicles, make asymmetrical synaptic contacts, and can in turn be divided into AT1 and AT2 terminals. The AT1 terminals are those of the retinotectal projection. The origin of the nonretinal AT2 terminals could not be determined. AT1 and AT2 terminals establish synaptic contacts with DCSV, with dendrites of the retinopetal neurons (DRN), and with conventional dendritic (D) profiles. The terminals of the second category are GABA-immunoreactive and can similarly be divided into AT3 and AT4 terminals. The AT3 terminals contain pleiomorphic synaptic vesicles and make symmetrical synaptic contacts for the most part with glutamate-immunoreactive D profiles. The AT4 terminals contain rounded synaptic vesicles and make asymmetrical synaptic contacts with DRN, with DCSV, and with D profiles. A fifth, rarely observed category of terminals (AT5) contain both clear synaptic vesicles and a large number of dense-core vesicles. Synaptic triads involving AT1, AT2 or AT4 terminals are rare. Our findings are compared to these of previous studies of the fine structure and immunochemical properties of the retinorecipient layers of the optic tectum or superior colliculus of Gnathostomes.

Fine structure of the visual dorsolateral anterior thalamic nucleus of the pigeon (Columba livia): a hodological and GABA-immunocytochemical study.

The ultrastructure of the lateroventral subcomponent of the visual dorsolateral anterior thalamic nucleus of the pigeon (DLLv) was analyzed using hodological techniques and GABA-immunocytochemistry. Two types of GABA-immunonegative hyperpalliopetal neurons and a single type of strongly GABA-immunoreactive (-ir) interneuron were identified, the latter displaying long dendrites with some containing synaptic vesicles (DCSV). Ten types of axon terminal were identified and divided into two categories. The first, GABA-immunonegative and making asymmetrical synaptic contact, contain round (RT1, RT2, RT3) or pleiomorphic synaptic and many dense-core vesicles (DCT). RT1 terminals are retinothalamic and RT2 terminals hyperpalliothalamic; both mainly contact dendrites of projection neurons (72% and 78% respectively), less frequently dendrites of interneurons and sometimes DCSV; RT1 terminals are rarely involved in synaptic triads. The second category are consistently GABA-immunopositive. Four types (PT1-4), distinguished by their pleiomorphic synaptic vesicles, make symmetrical synaptic contact essentially with dendrites of projection neurons, more rarely on dendrites of interneurons (PT2). PT1 terminals are very probably those of interneurons, whereas the rare PT4 terminals are of retinal origin. A fifth type (RgT) contains round synaptic vesicles and makes asymmetrical synaptic contact with dendrites of projection neurons and interneurons. PT2 and RgT terminals occasionally contact DCSV of interneurons, which are sometimes involved in synaptic triads. Two final subcategories (DCgT1-2) contain many dense-core vesicles. Our findings are compared with those of previous studies concerning the fine structure and neurochemical properties of the GLd of reptiles and mammals, with special reference to the origin of the extraretinal and extracortical projections to this structure.

Tectorotundal connections in turtles: an electron microscopic tracing and GABA-immunocytochemical study.

The nucleus rotundus of the turtles Emys orbicularis and Testudo horsfieldi was analysed by axonal tracing methods and post-embedding GABA immunocytochemistry. After injections of horseradish peroxidase or biotinylated dextran amine into the optic tectum, electron microscopic observations showed that the vast majority of ipsilateral tectorotundal axon terminals were small in size, had smooth contours and contained small, round, densely packed synaptic vesicles. These terminals were GABA-immunonegative, often gathered in clusters, and established asymmetrical synaptic contacts with either small- or medium-sized GABA-negative dendritic profiles and with GABA-immunoreactive (GABA-ir) dendrites, which did not contain synaptic vesicles. Occasional GABA-ir-labelled axon terminals were observed; these may arise from the rare GABAergic neurons in the central tectal layer, or from neurons in the ventral pretectal nucleus, which projects both to the optic tectum and nucleus rotundus. In addition to tracer-labelled axon terminals, we observed both GABA-negative and GABA-ir cell bodies and dendrites also labelled by the tracer. No GABA-ir presynaptic dendritic profiles containing synaptic vesicles were observed. The existence in reptiles of reciprocal connections between the nucleus rotundus and the optic tectum as a phylogenetically ancient feedback system is discussed.

Distribution of ganglion cells in the pigeon retina labeled via retrograde transneuronal transport of the fluorescent dye rhodamine beta-isothiocyanate from the telencephalic visual Wulst.

The distribution of retinal ganglion cells (RGCs) providing input to the thalamofugal visual system in the pigeon was studied with an anatomical transneuronal transport technique using the fluorescent dye rhodamine beta-isothiocyanate (RITC). Unilateral injections of RITC made into the telencephalic visual Wulst resulted in the retrograde (1) first-order labeling (FOL) of dorsal thalamic (n. dorsolateralis anterior and n. superficialis parvocellularis: SPC) and brainstem somata as well as (2) second-order labeling of other cell populations within the brain and of retinal ganglion cells in both eyes obtained after transneuronal transfer of the tracer from neurons labeled directly via FOL. The mapping and counting of labeled RGCs in retinal flat-mounts showed that they were mainly distributed within the nasal portion of the retinal yellow field (YF) and that their total numbers were consistently higher (averaging 57%) in the eye contralateral to the tracer injection. Labeled RGCs in the retinal red field (RF) represented 13.4% and 12.0% of total labeled cells in the ipsilateral and contralateral eye, respectively. Moreover, the average densities of labeled cells/mm(2) in the RF and YF were respectively 8.4 and 42.8 (ipsilateral) and 17.9 and 54.0 (contralateral). The preferential distribution of labeled RGCs within the nasal YF supports the notion that the thalamofugal visual system in the lateral-eyed pigeon is mainly concerned with viewing in the lateral visual field. Conversely, the relatively low numbers of labeled RGCs observed within the specialized RF indicate that, unlike the case in frontal-eyed bird species and mammals, this system does not appear to be involved in binocular visual processing.

Immunocytochemical localization of the GABAB2 receptor subunit in the glomeruli of the mouse main olfactory bulb.

The olfactory input to the brain is carried out by olfactory nerve axons that terminate in the olfactory bulb glomeruli and make synapses onto dendrites of glutamatergic projection neurons, mitral and tufted cells, and GABAergic interneurons, periglomerular cells. The dendrites are reciprocally connected through asymmetric synapses of mitral/tufted cells with periglomerular cells and symmetric synapses of the opposite direction. Transmission at the first synapse in the olfactory pathway is regulated presynaptically, and this regulation is mediated, in part, by metabotropic GABAB receptors that, when activated, inhibit transmitter release from the olfactory nerve. Functional GABAB receptors are heterodimers composed of the GABAB1 and GABAB2 subunits. Studies using double immunofluorescence have shown colocalization of both subunits in the glomerular neuropil, and ultrastructural studies have localized GABAB1 to extrasynaptic, synaptic, and perisynaptic sites on the plasma membrane of olfactory nerve terminals. We studied the subcellular localization of GABAB2 in the mouse olfactory glomeruli using a subunit-specific antibody and preembedding immunogold labeling. Immunoreactivity for GABAB2 was associated with symmetric dendrodendritic synapses of periglomerular cells with mitral/tufted cells and was localized to the extrasynaptic plasma membrane of presynaptic dendrites, and extrasynaptic, synaptic, and perisynaptic sites on the plasma membrane of postsynaptic dendrites. The results suggest that postsynaptic, and perhaps presynaptic, GABAB receptors may be expressed at GABAergic synapses between dendrites of periglomerular interneurons and projection neurons. Immunolabeling was observed at junctions of the olfactory nerve with mitral/tufted cell dendrites, providing ultrastructural evidence for the expression of the GABAB2 subunit at the primary olfactory synapse.

Role of the calcium modulated cyclases in the development of the retinal projections.

Transmembrane isoforms of adenylate cyclases (AC) integrate a wide variety of extracellular signals from neurotransmitters to morphogens and can also regulate cAMP production in response to calcium entry. Based on observations in the barrelless mouse strain, the Adcy1 gene (AC1) was involved in the segregation of binocular retinal inputs. To determine the potential role of other AC isoforms we localized the Adcy genes in the visual centres during development, using in situ hybridization. Six different AC subtypes were found in the developing retinal ganglion cell layer (RGC; AC1, AC2, AC3, AC5, AC8, and AC9), and three AC subtypes were expressed in the central brain targets, the dorsal lateral geniculate nucleus (AC1 and AC8), the ventral lateral geniculate nucleus (AC2 and AC8) and the superior colliculus (AC1, AC2, AC8). Using a genetic approach we tested the role of the calcium modulated cyclases AC1, AC5 and AC8 for the segregation retinal fibres. Ipsilateral retinal axons remained exuberant in the AC1(-/-) mice, with overlapping retinal projections from both eyes in the superior colliculus and the visual thalamus. These abnormalities were similar to those of barrelless mouse mutants. No abnormalities were detectable in the AC5(-/-) or the AC8(-/-) mice. Similar abnormalities were noted in the single AC1(-/-) and the AC1/AC8 double-knockout mice (DKO). Thus, only AC1 is required for the maturation of the retinal axon terminals whereas AC5 and AC8 are not needed. The specificity of AC1's action is linked to its cellular localization in the RGCs and to its distinctive functional profile, compared with the other cyclases expressed in the same cells.

The turtle thalamic anterior entopeduncular nucleus shares connectional and neurochemical characteristics with the mammalian thalamic reticular nucleus.

Neurochemical and key connectional characteristics of the anterior entopeduncular nucleus (Enta) of the turtle (Testudo horsfieldi) were studied by axonal tracing techniques and immunohistochemistry of parvalbumin, gamma-aminobutyric acid (GABA) and glutamic acid decarboxylase (GAD). We showed that the Enta, which is located within the dorsal peduncle of the lateral forebrain bundle (Pedd), has roughly topographically organized reciprocal connections with the dorsal thalamic visual nuclei, the nucleus rotundus (Rot) and dorsal lateral geniculate nucleus (GLd). The Enta receives projections from visual telencephalic areas, the anterior dorsal ventricular ridge and dorsolateral cortex/pallial thickening. Most Enta neurons contained GABA and parvalbumin, and some of them were retrogradely labeled when the tracer was injected into the visual dorsal thalamic nuclei. Further experiments using double immunofluorescence revealed colocalization of GAD and parvalbumin in the vast majority of Enta neurons, and many of these cells showed retrograde labeling with Fluoro-gold injected into the Rot and/or GLd. According to these data, the Enta may be considered as a structural substrate for recurrent inhibition of the visual thalamic nuclei. Based on morphological and neurochemical similarity of the turtle Enta, caiman Pedd nucleus, the superior reticular nucleus in birds, and the thalamic reticular nucleus in mammals, we suggest that these structures represent a characteristic component which is common to the thalamic organization in amniotes.

GnRH-immunoreactive centrifugal visual fibers in the Nile crocodile (Crocodylus niloticus).

Thin varicose centrifugal visual fibers, between 30-45 in number and displaying cGnRH-I immunoreactivity, were identified in Crocodylus niloticus. Approximately 80% of these fibers were also FMRF-amide-like immunoreactive. The cGnRH-I fibers extended from the preoptic region to the retina where they appeared to terminate in the external portion of the inner plexiform layer. The location of their neurons of origin could not be determined precisely following the intraocular injection of the retrograde axonal tracer RITC. Nevertheless, the presence of cGnRH-I-immunoreactive neurons exclusively within the complex comprising the terminal nerve and the septo-preoptic region, and of several retinopetal fibers labelled retrogradely with the axonal tracer at the septo-preoptic junction, indicates that the cGnRH-immunoreactive centrifugal visual system originates from within this complex.

Rhodopsin-like immunoreactivity in the 'all cone' retina of the chameleon (Chameleo chameleo).

Chameleons (Order, Reptilia: Family, Lacertilia) are unique among vertebrates in being able to make independent eye movements. The organisation of their retina, however, closely ressembles that of other diurnal lizards; based on morphological studies, it is typically described as containing only cone photoreceptors. We show here that a subpopulation of the photoreceptors are immunolabelled by an antibody directed against rhodopsin, suggesting the presence of rods. We conclude that in the nonmammalian retina, rods and cones cannot be exclusively distinguished on purely morphological grounds.

Presumptive FMRF-amide-like immunoreactive retinopetal fibres in Crocodylus niloticus.

A small contingent of 30-50 of centrifugal visual fibres, showing FMRF-amide-like immunoreactivity, has been identified in C. niloticus; these fibres extend from the chiasmatic region into the retina. They do not take the marginal optic tract, but pass medially to the chiasmatic fascicles, from the preoptic region. The cells of origin of these fibres have not been identified. However, none of the retinopetal neurons of the brainstem [M. Medina, J. Reperant, R. Ward, D. Miceli, Centrifugal visual system of Crocodylus niloticus : a hodological, histochemical and immunocytochemical study, J. Comp. Neurol. 468 (2004) 65-85], labelled by retrograde transport of rhodamine beta-isothiocyanate after intraocular injection of this tracer, show FMRF-amide-like immunoreactivity; neither are any of the FMRF-amide-like immunopositive neurons in the crocodile brain, particularly those of the complex involving the terminal nerve and the septo-preoptic region, labelled by rhodamine after its intraocular injection.

Pretectal and tectal afferents to the dorsal lateral geniculate nucleus of the turtle: an electron microscopic axon tracing and gamma-aminobutyric acid immunocytochemical study.

The pretectal and tectal projections to the dorsal lateral geniculate nucleus (GLd) of two species of turtle (Emys orbicularis and Testudo horsfieldi) were examined under the electron microscope by using axonal tracing techniques (horseradish peroxidase or biotinylated dextran amine) and postembedding gamma-aminobutyric acid (GABA) immunocytochemistry. After injection of tracer into the pretectum, two types of axon terminals were identified as those of pretectogeniculate pathways. Both contained pleomorphic synaptic vesicles and were more numerous in the inner part of the nucleus. They could be distinguished on the bases of size and shape of their synaptic vesicles, type of synaptic contact, and level of GABA immunoreactivity. One type had a higher density of immunolabeling and established symmetric synaptic contacts, whereas the other, less densely immunolabeled, made asymmetric synaptic contacts. In both cases, synaptic contacts were mainly with relay cells and occasionally with interneurons. We suggest that these two types of pretectogeniculate terminals originate in two separate pretectal nuclei. After injection of tracer into the optic tectum, a single population of GABA-immunonegative tracer-labeled terminals was identified as belonging to the tectogeniculate pathway. These were small, had smooth contours, contained very small round synaptic vesicles, and established asymmetric synaptic contacts with long active zones, predominantly with relay cells and less frequently with interneurons, in the inner part of the nucleus. In addition, a population of GABA-negative and occasionally GABA-positive terminals, labeled by tracer injected into either the pretectum or the tectum, was identified as retinal terminals; these were presumably labeled by the retrograde transport of tracer in collateral branches of visual fibers innervating both the GLd and the pretectum or tectum. Comparison of the present ultrastructural findings in turtles with those previously reported in mammals shows that the cytological features, synaptic morphology, and immunochemical properties of the pretectogeniculate and tectogeniculate terminals of both groups share many similarities. Nevertheless, the postsynaptic targets of these two categories of terminals display some pronounced differences between the two groups, which are discussed in terms of their possible functional significance.

Centrifugal visual system of Crocodylus niloticus: a hodological, histochemical, and immunocytochemical study.

The retinopetal neurons of Crocodylus niloticus were visualized by retrograde transport of rhodamine beta-isothiocyanate or Fast Blue administered by intraocular injection. Approximately 6,000 in number, these neurons are distributed in seven regions extending from the mesencephalic tegmentum to the rostral rhombencephalon, approximately 70% being located contralaterally to the injected eye. None of the centrifugal neurons projects to both retinae. The retinopetal neurons are located in rostrocaudal sequence in seven regions: the formatio reticularis lateralis mesencephali, the substantia nigra, the griseum centralis tectalis, the nucleus subcoeruleus dorsalis, the nucleus isthmi parvocellularis, the locus coeruleus, and the commissura nervi trochlearis. The greatest number of cells (approximately 93%) is found in the nucleus subcoeruleus dorsalis. The majority are multipolar or bipolar in shape and resemble the ectopic centrifugal visual neurons of birds, although a small number of monopolar neurons resembling those of the avian isthmo-optic nucleus may also be observed. A few retinopetal neurons in the griseum centralis tectalis were tyrosine hydroxylase (TH) immunoreactive. Moreover, in the nuclei subcoeruleus dorsalis and isthmi parvocellularis, both ipsilaterally and contralaterally, approximately one retinopetal neuron in three (35%) was immunoreactive to nitric oxide synthase (NOS), and a slightly higher proportion (38%) of retinopetal neurons were immunoreactive for choline acetyltransferase (ChAT). Some of them contained colocalized ChAT and NOS/reduced nicotinamide adenine dinucleotide phosphate-diaphorase. Fibers immunoreactive to TH, serotonin (5-HT), neuropeptide Y (NPY), or Phe-Met-Arg-Phe-amide (FMRF-amide) were frequently observed to make intimate contact with rhodamine-labeled retinopetal neurons. These findings are discussed in relation to previous results obtained in other reptilian species and in birds.

Tectothalamic visual projections in turtles: their cells of origin revealed by tracing methods.

In two species of turtle (Emys orbicularis and Testudo horsfieldi), retrograde and anterograde tracer techniques were used to study projections from the optic tectum to the nucleus rotundus (Rot) and to the dorsal lateral geniculate nucleus (GLd). The ipsilateral Rot received the most massive tectal projections, stemming from numerous neurons located in the stratum griseum centrale (SGC). These neurons varied in size and shape, many of them having a wide zone of dendritic arborization within both the (SGC) and the stratum griseum et fibrosum superficiale (SGFS). Projections from the tectum to the GLd were ipsilateral, were extremely scarce, and arose from a small number of neurons of various shapes situated in the SGFS; these cells were, as a rule, smaller than those projecting to the Rot. For the most part, these neurons were radially oriented, with rather restricted dendritic arborizations in the most superficial sublayers of the SGFS; smaller numbers of projection neurons were horizontally oriented, with long dendrites branching throughout the layer. Some neurons located in the stratum griseum periventriculare (SGP) projected to both the Rot and the GLd. Most of these neurons had dendritic arborizations within the retinorecipient zone of the SGFS. We were unable to rule out the possibility that some cells projecting to the GLd were situated in the SGC. Both the GLd and the main body of the Rot did not contain neurons projecting to the optic tectum. Thalamic neurons projecting to the tectum were observed in the ventral lateral geniculate nucleus, the intergeniculate leaflet and the interstitial nuclei of the tectothalamic tract, and the nucleus of the decussatio supraoptica ventralis. The question of whether variation in the laminar organization of the tectorotundal and tectogeniculate projection neurons in reptiles, birds, and mammals may be related to different degrees of differentiation of the tectal layers is discussed.

Serotonin immunoreactivity in the retinal projecting isthmo-optic nucleus and evidence of brainstem raphe connections in the pigeon.

Serotonin (5-HT) immunoreactive (-ir) profiles within the isthmo-optic nucleus (ION) of the centrifugal visual system (CVS) were studied in the pigeon using light microscopic immunohistofluorescent and electron microscopic immunocytochemical pre-embedding techniques. The brainstem origin of the 5-HT input upon the ION was determined by combining 5-HT immunohistofluorescence (FITC) and retrograde transneuronal tracing after intraocular injection of Rhodamine beta-isothiocyanate. The light microscopic results showed that 5-HT endings were mainly localised within the neuropillar zones of the ventral ION. The 5-HT-ir cell bodies, belonging to a lateral extension of the dorsal raphe system, were observed within the same region as the centrifugal ectopic neurons (EN) underlying the ION and some displayed dendritic processes which penetrated the nucleus. Double-labeled neurons, representing 5-HT-ir afferents to the ION, were identified only within the n. linearis caudalis region of the ventral raphe. The electron microscopic results confirmed the presence of 5-HT-ir dendritic processes within the ventral part of the nucleus and showed that they were contacted by axon terminals belonging to intrinsic interneurons. The functional organisation of the ION and the possible contribution of serotonergic raphe afferents and efferents are discussed in relation to present hypotheses linking the avian CVS to mechanisms of visual attention.

Development of tyrosine hydroxylase-immunoreactive systems in the brain of the larval lamprey Lampetra fluviatilis.

The development of the catecholaminergic system of the brain of the lamprey (Lampetra fluviatilis) was studied with immunocytochemistry in a series of larvae of different sizes by using two different antibodies directed against tyrosine hydroxylase (TH), the rate-limiting enzyme of catecholamine synthesis. In group 1 larvae (length: 29-54 mm, ages: 8 months to 1.5 years), the only TH-immunoreactive somata observed were located in the caudal wall of the recessus praeopticus (RP) and in the nucleus tuberculi posterioris (NTP). In group 2 larvae (length: 55-80 mm, ages: 1.5-2.5 years), the somata of immunolabeled cells of the NTP give rise to fibers, most of which are ascending and terminate in the corpus striatum. Additional immunoreactive cells are observed in the nucleus praeopticus (NP), which has differentiated, and in the spinal cord. In group 3 larvae (length: 81-110 mm, ages: 2.5-4 years), the spatial distribution of TH-immunoreactive elements (somata, fibers, and terminals) bears many resemblances to that seen in the adult. Immunolabeled cells may be observed in the olfactory bulb, in the nucleus commissurae postopticae (NCP), and in the nucleus dorsalis hypothalami (NDH). Nevertheless, some groups of TH-immunoreactive cells found in the adult are not observed in group 3 larvae; these may appear during the metamorphic phase. By comparative analysis, we show that, in spite of several differences, the spatiotemporal sequence of appearance of TH-immunoreactive cell bodies and fibers in the lamprey presents many similarities to that described in gnathostomes.

Expression of Kin, a nuclear protein binding to curved DNA, in the brain of the frog (Rana esculenta), turtle (Trachemys scripta), quail (Coturnix coturnix) and mouse (Mus musculus).

The distribution of Kin protein, the vertebrate homologue of the bacterial recA nuclear protein involved in illegitimate recombinant DNA repair and gene regulation, was analysed in the brain of the mouse, quail, turtle and frog by immunocytochemical methods. The protein was expressed in all brains, but not in a uniform manner. Immunoreactivity was absent from major fibre tracts. In the cerebral nuclei, immunolabelling in the various species showed an important variation. A comparative analysis, based on the homologies between different brain structures in these species, showed that this variation was not due to interspecific variation but that of an ancestral pattern of distribution of Kin protein. It is also shown that whatever the species examined, Kin protein is consistently more highly expressed in those regions of the brain with a conservative evolutionary history (e.g. the olfactory and limbic systems, the hypothalamus, the monoaminergic system, the cerebellum, and the nuclei of sensory and motor cranial nerves). The protein is markedly less heavily expressed in the dorsal striatum and the sensory nuclei of the thalamus.