Journal of Comparative Neurology

Receptive field characteristics of single cells in primary visual cortex of rabbit were studied. Seventy‐two percent of cells were found to be orientation selective, and the remainder had concentric, uniform, movement selective or pure direction selective receptive fields. Single cells were also recorded from primary visual cortex of cat to permit a comparison of visual cortical organization in cats and rabbits. Laminar organization of receptive field types was observed in rabbits which was similar in most respects to that described in the cat. Although the major categories of orientation selective cells (simple, complex, hypercomplex) were similar for both cat and rabbit, many differences emerged: (I) tuning of orientation selectivity was narrower in cats than in rabbits; (II) units which preferred oblique orientations were less frequently represented in rabbits than in cats; (III) orientation preferences appeared to be arranged in cluster in rabbit cortex; in rabbits we found no evidence of the columnar organization of orientation selectivity which characterizes cat visual cortex.

A comparison of our data with those previously reported for mouse, rat, hamster and opossum visual cortex suggest that mammals in which a significant proportion of visual cortical cells are not orientation selective have in common certain patterns of cortical organization involving a less precise and less specialized representation of stimulus orientation.

Stereological methods were used to examine the consequences of prenatal exposure to ethanol on the structure of area 3, primary somatosensory cortex, of the mature hooded rat. Pregnant rats were fed a liquid diet containing 6.7% (v/v) ethanol (Et), pairfed an isocaloric liquid control diet (Ct), or fed a diet of chow and water (Ch). Cresyl violet‐stained sections of 3‐month‐old pups were examined. The corrected mean size of the cell bodies of neurons in layers other than layer V was significantly smaller in the Ettreated rats; conversely, the mean somatic size of glia in each layer was significantly larger in the Ettreated rats. The laminar cell packing density for neurons and glia, however, was similar in rats from both treatment groups. The overall volume of area 3 and the volume of individual layers were about 33% smaller in Et‐treated rats than in the pair‐fed controls. Thus, the estimated total number of neurons in Et‐treated rats (1.79*10⁶) was significantly fewer than in Chtreated rats (2.77*10⁶) and in Ct‐treated rats (2.66*10⁶). The total number of glia also was about 30% fewer in Et‐treated rats than in the controls. Not all layers were affected equivalently. The space occupied by the neuropil was significantly greater in Et‐treated rats, but only in layers II/III, IV, and VI; hence, the cell body/neuropil ratio in these layers was less in Et‐treated rats than in the controls. Therefore, microcephaly caused by prenatal exposure to ethanol results not only from a miniaturization of the brain, but also from a permanent abnormal organization of cerebral cortex.

The generation, migration, and morphogenesis of atypically oriented pyramidal neurons in the rat visual cortex were examined. In the mature cortex, these neurons were distributed through layers II–VI. Moreover, the atypically oriented pyramidal neurons in a particular layer tended to be oriented in a specific way; atypically oriented pyramidal neurons in layer II, layers III–VIa, and layer VIb were obliquely, radially, and obliquely oriented, respectively. Ultrastructurally, the somata of atypically oriented pyramidal neurons contained large euchromatic ovoid nuclei and cytoplasm that was replete with rough endoplasmic reticulum and Golgi apparatus. These somata formed only symmetric axosomatic synapses. Many atypically oriented pyramidal neurons projected axons into the white matter as demonstrated by a Golgi method and by a retrograde tract‐tracing technique; however, some of these pyramidal neurons in layers III–V had axons that ascended to layer I.

By using a technique which combined retrograde tract tracing with [³H]thymidine autoradiography, it was determined that most atypically oriented pyramidal neurons in layers V and VIa, layer IV, and layer II were generated on gestational days (GD) 15–17, GD 17–19, and GD 20–21, respectively. Atypically oriented pyramidal neurons were identified during the period from postnatal day 0 (day of birth) to day 30. On day 0, obliquely oriented pyramidal neurons were distributed in the deep cortical plate, i.e., immature layer VI. On day 3, the youngest atypically oriented pyramidal neurons were radially oriented and were located in layer IV. Some obliquely oriented pyramidal neurons were present in layer II on day 6, but the greatest number and the most severely canted pyramidal neurons in layer II were evident on day 9. The orientations of the cell body and the apical dendrite did not change appreciably after migration was complete, except for those in layers V and VI with obliquely oriented cell bodies and radially oriented apical dendrites. The second and third postnatal weeks were marked by substantial morphological differentiation of all pyramidal neurons as noted by the lengthening and branching of dendrites and by the appearance of dendritic spines. By the fourth postnatal week, atypically oriented pyramidal neurons achieved their mature morphology.

The generation, migration, and morphogenesis of atypically oriented pyramidal neurons proceed by an inside‐to‐outside sequence. This development is similar and concurrent with that of typically oriented pyramidal neurons.

Postmitotic neurons migrate from a zone(s) near the ventricles to the neocortex. During this migration, neurons associate with radial glia. After serving their role as guides for neuronal migration, the radial glia transform into astrocytes. Prenatal exposure to ethanol causes abnormal neuronal migration. We examined the effects of gestational exposure to ethanol on radial glia and astrocytes. Radial glia were stained immunohistochemically with the antibody RAT‐401, and astrocytes were labeled with an antibody directed against glial‐fibrillary acidic protein (GFAP). The subjects were the offspring of rats fed an ethanol‐containing liquid. diet (Et), pair‐fed a liquid control diet (Ct), or fed chow and water (Ch). During the first postnatal week, radial glial fibers (in Et‐treated rats and controls) stretched from the ventricular surface through the developing. cerebral wall to the pial surface. In the Et‐treated rats, the radial processes were less dense and more poorly fasciculated than they were in the Ch‐and Ct‐treated rats. Moreover, by postnatal day (P) 5, there was a significant reduction in RAT‐401 immunostaining in the Et‐treated rats, particularly in the superficial cortex. A similar reduction in control rats did not begin until P10. In all three treatment groups, GFAP‐immunoreactive astrocytes were in the cortex throughout the period from P1 to P45. In neonates, GFAP‐positive cells were distributed in the marginal zone (layer I) and the intermediate zone (the white matter). The number of GFAP‐positive cells in the cortical plate increased steadily with time so that, by P26, GFAP‐immunoreactive astrocytes were distributed evenly through all cortical laminae. Interestingly, between P5 and P12, the number of astrocytes was significantly greater in Et‐treated rats than in controls.

Thus prenatal exposure to ethanol induces the premature loss of RAT‐401‐positive processes and the precocious increase in GFAP immunostaining. These ethanol‐induced changes in glial development indicate that ethanol accelerates the transformation of radial glia into astrocytes. Moreover, the ethanol‐induced premature degradation of the network of radial glial fibers may underlie the migration of late‐generated neurons to ectopic sites. © 1993 Wiley‐Liss, Inc.

This study demonstrates that exposure to alcohol during a period of rapid brain growth can lead to severe and permanent deficits in the number of granule cells and mitral cells in the main olfactory bulb. Sprague‐Dawley rat pups were reared artificially and were administered alcohol over postnatal days (PD) 4 through 9, a period of brain development comparable to part of the human third trimester. The daily alcohol dose of 6.6 g/kg was concentrated into two of the twelve daily feedings, producing high peak blood alcohol concentrations followed by near total clearance. Pups were either sacrificed on PD10 or were allowed to grow to adulthood and sacrificed on PD115. The total number of granule cells and mitral cells in the main olfactory bulb were estimated with the aid of unbiased stereological principles and systematic sampling techniques. Exposure to alcohol resulted in significant reductions in the number of both granule cells and mitral cells on PD10. Significant deficits in both neuronal populations remained on PD115. The results support the hypothesis that alcohol exposure can kill developing neurons and lead to permanent neuronal deficits.

Substantial developmental changes also occurred in the total number of mitral cells and granule cells between PD10 and PD115 in the control groups. In untreated rats, the number of granule cells increased from 2.20 · 10⁶on PD10 to 5.06 · 10⁶on PD115, while the number of mitral cells decreased from 5.30 · 10⁴to 4.33 · 10⁴over the same time period. These results demonstrate that there is a natural loss of mitral cells during postnatal development at the same time that granule cell number is increasing.

Clinical and experimental evidence shows that prenatal exposure to ethanol causes craniofacial malformations, microcephaly, and abnormal development of the central nervous system. This study describes the effects of ethanol on the development of the principal sensory nucleus of the trigeminal nerve (PSN). The offspring of two groups of rats were examined. Pregnant females in one group were fed a liquid diet containing 6.7% (v/v) ethanol (Et) and rats in the other group were fed an isocaloric liquid control diet (Ct). Each pregnant rat was administered [³H]thymidine on one day during the period from gestational day (G) 10 to G22. After pups grew to 30 days of age, they were killed and their brains were processed by an autoradiographic procedure.

Qualitatively, the PSN of Ct‐ and Et‐treated rats appeared similar; they were composed chiefly of small neurons and a few scattered large neurons. On the other hand, quantitative analyses revealed significant differences between both groups. Although the volume of the PSN of Et‐treated rats was not significantly different (‐3.2%) than that for Ct‐treated rats, the PSN of Et‐treated rats had significantly (P <0.01) fewer (30.0%) neurons than did the PSN of Ct‐treated rats. The number of the small neurons, but not of the large neurons, was affected most by the ethanol exposure. Prenatal exposure to ethanol also altered the generation of PSN neurons. Most neurons in the PSN of Ct‐treated rats were born between G12 and G15, the small neurons being generated before the large neurons. In Et‐treated rats, too, small neurons were born before the large neurons; however, the time frame of neuronogenesis was delayed as it occurred between G13 and G16.

Thus, prenatal exposure to ethanol produces profound developmental abnormalities that lead to permanent alterations in the structure of the mature central nervous system.

The afferent connections of the main and accessory olfactory bulbs in the rat were examined by injecting horseradish peroxidase (HRP) into one or the other of these structures either by microelectrophoresis or by hydraulic pressure. Alternate sections were stained with newly developed HRP‐procedures using either benzidine dihydrochloride (de Olmos and Heimer, '77) or tetramethyl‐benzidine.

Eighteen to twenty‐four hours after unilateral HRP injections confined to the main olfactory bulb, a large number of HRP‐labeled perikaria appeared in the following telencephalic structures on the ipsilateral side: All portions of the anterior olfactory nucleus (AON) except its external part, the lateral transitional field (LT) between AON and the paleocortex, the whole extent of the primary olfactory cortex (POC); The medial forebrain bundle area deep to the olfactory tubercle, the nucleus of the horizontal limb of the diagonal band (NHDB) and the nucleus of the lateral olfactory tract (NLOT). A moderate to small number of labeled cells, furthermore, were seen in the dorsal (DT) and medial (MT) transition fields, the ventral praecommissural hippocampus (tt₂), the ventral superficial part of the nucleus of the vertical limb of the diagonal band (NVDB), the sublenticular part of the substantia innominata (SI), the anterior amygdaloid area, the posterolateral cortical amygdaloid nucleus (C₂) and the transition region (28 L') between the olfactory cortex and the lateral entorhinal area proper. On the contralateral side a large number of labeled cells were found in all parts of the AON, with especially heavy labeling in its external part. A moderate number of labeled cells could also be detected in the lateral transition field (LT) and the NLOT.

In the diencephalon and the brain stem a moderate number of HRP‐labeled perikaria were observed in the dorsal, perifornical, and lateral hypothalamus, as well as in locus coeruleus and the dorsal and medial raphae nuclei.

Following large HRP injections in the main olfactory bulb a moderate to small number of labeled cells were seen also in the posterior and premammillary hypothalamus and in field CA₁ of the retrocommissural hippocampus on the ipsilateral side, as well as in POC on the contralateral side. It is possible, however, that the uptake of label took place in an undetected pool of HRP in the very rostral part of AON rather than in the olfactory bulb.

HRP injections in the accessory olfactory bulb resulted in labeled neurons in the posterior ventro‐lateral part of the bed nucleus of the stria terminalis, the nucleus of the accessory olfactory tract, the rostrodorsal portions of the medial amygdaloid nucleus, and whole extent of the posteromedial cortical amygdaloid nucleus (C₃) on the ipsilateral side. A few lightly labeled cells were seen also in the contralateral C₃.

The axoplasmic retrograde transport of horseradish peroxidase (HRP) from axon terminals to their parent cell bodies and histochemical fluorescence microscopy have been used to study the ipsilateral centrifugal fibers to the olfactory bulbs and anterior olfactory nucleus in the rabbit. Focal injections of peroxidase were placed unilaterally into the main or accessory olfactory bulb or into the anterior olfactory nucleus. In animals with injected HRP confined within the main bulb, perikarya retrogradely labeled with the protein in the ipsilateral forebrain were observed in the anterior prepyriform cortex, horizontal limb of the nucleus of the diagonal band, and far lateral preoptic and rostral lateral hypothalamic areas. Brain stem cell groups that contained HRP‐positive somata include the locus coeruleus and midbrain dorsal raphe nucleus. Except for the prepyriform cortex, the basal forebrain structures with labeled perikarya correlate well with locations of cell bodies containing acetylcholinesterase and choline acetyltransferase. These somata may represent a cholinergic afferent system to the main olfactory bulb. Peroxidase‐labeled cell bodies in the locus coeruleus and midbrain raphe are indicative of noradrenergic and serotonergic innervations respectively of the olfactory bulb. In rabbits in which peroxidase was injected or diffused into the accessory olfactory bulb and anterior olfactory nucleus, HRP‐positive somata were identified in the prepyriform cortex bilaterally, the horizontal limb of the diagonal band nucleus, lateral hypothalamic region, nucleus of the lateral olfactory tract, corticomedial complex of the amygdala, mitral and tufted cell layers of the ipsilateral main olfactory bulb, locus coeruleus, and the midbrain raphe. Evidence for centrifugal fibers to the accessory olfactory bulb from the corticomedial complex of the amygdala, locus coeruleus, and possibly the nucleus of the lateral olfactory tract and midbrain raphe is discussed. A similar distribution of labeled perikarya in the forebrain and brain stem was seen in rats in which peroxidase injected into the main olfactory bulb had spread into the accessory bulb and anterior olfactory nucleus.

Histochemical fluorescence microscopy of the main and accessory olfactory bulbs in the rabbit and rat revealed fine caliber, green fluorescent fibers and varicosities predominantly in the granule cell layer and less so among cells in the glomerular layer. In sections through the root of the main olfactory bulb, a similar fluorescence was seen in the deep half of the plexiform layer of the pars externa of the anterior olfactory nucleus. These fluorescent fibers likely represent the noradrenergic innervation of the olfactory bulbar and retrobulbar formations. A fluorescent yellow hue was observed in the glomerular layer of the main bulb and may signify a serotonergic innervation of this lamina.

The results of this study provide continued support for the existence of main and accessory olfactory systems and offer greater insight into the organization of the olfactory brain and the interrelationship of the hypothalamus and olfaction.

Müller glial cells are the major type of glia in the mammalian retina. To identify the molecular machinery that defines Müller glial cell identity and function, single cell gene expression profiling was performed on Affymetrix microarrays. Identification of a cluster of genes expressed at high levels suggests a Müller glia core transcriptome, which likely underlies many of the functions of Müller glia. Expression of components of the cell cycle machinery and the Notch pathway, as well as of growth factors, chemokines, and lipoproteins might allow communication between Müller glial cells and the neurons that they support, including modulation of neuronal activity. This approach revealed a set of transcripts that were not previously characterized in (Müller) glia; validation of the expression of some of these genes was performed by in situ hybridization. Genes expressed exclusively by Müller glia were identified as novel markers. In addition, a novel BAC transgenic mouse that expresses Cre in Müller glia cells was generated. The molecular fingerprint of Müller glia provides a foundation for further studies of Müller glia development and function in normal and diseased states. J. Comp. Neurol. 509:225–238, 2008. © 2008 Wiley‐Liss, Inc.

Dopamine is the putative transmitter of eight neurons in the hermaphrodite form of the nematode Caenorhabditis elegans. These include the cephalic and deirid neurons, which are believed to be mechanosensory. The male has an additional six dopaminergic neurons in the tail. Mutants have been selected which have defects in the formaldehyde induced fluorescence and lack dopamine to varying degrees, but they are not insensitive to touch. The dopaminergic neurons of C. elegans are compared with the homologous neurons in Ascaris lumbricoides.