Roberto Alesci

Changing distribution of GABA-like immunoreactivity in pigeon visual areas during the early posthatching period and effects of retinal removal on tectal GABAergic systems

The distribution of GABA-like immunoreactivity in the pigeon visual system was studied during the first 9 days after hatching using a mouse monoclonal antibody, mAb 3A12, to glutaraldehyde linked GABA (Matute & Streit, 1986). GABA-like immunoreactivity was seen in cell bodies as well as in neuropil at the level of both the retina and central visual regions at any posthatching age. However, the distribution of putative GABAergic cells and processes varied with age reaching the adult pattern at around 9 days. As a general observation, almost no cell bodies in the retina (except for some perikarya in the ganglion cell layer) were labeled at hatching but densely packed immunostained processes were present in the inner plexiform layer. During the next few days, GABA-immunoreactive amacrine and horizontal cells appeared and the adult distribution of GABA-like immunoreactivity was reached at around 9 days. In the other visual regions examined, the general trend in the variation of GABA-like immunoreactivity included: (1) a progressive decrease in the density of immunostained cell bodies and (2) an almost parallel increase in the concentration of stained neuropil. Since in pigeons the adult organization of visual pathways and the final distribution of putative GABAergic systems are reached at around the same age, we suggest the possibility that incoming ganglion cell axons play a role in regulating the distribution of GABA-like immunoreactivity in visual areas. This hypothesis is supported by the fact that the distribution of GABA-like immunoreactivity in the superficial layers of the optic tectum was altered following ablation of the contralateral retina immediately after hatching.

Pharmacological dissociation of the b-wave and pattern electroretinogram

Electroretinographic responses to modulation of either luminance (focal ERG) or spatial contrast (pattern ERG) were recorded from the pigeon eye before and after intravitreal injection of glutamate analogues DLα amino adipic acid (DLα AA) and 2-amino-4 phosphonobutyric acid (APB). Both toxins reversibly abolished the b-wave. The pattern ERG was still present, however, when the b-wave had been abolished by the toxins. This result demonstrates that the b-wave and the pattern ERG can be pharmacologically dissociated and suggests the possibility that in pigeons the b-wave and pattern ERG reflect the activity of different generators.

Norepinephrine levels in developing pigeon brain: effect of monocular deprivation on the Wulst noradrenergic system.

The endogenous level of norepinephrine (NE) was measured in discrete brain areas of the pigeon during post-hatching development. The pontine tegmentum showed the highest NE content, which remained constant during the post-hatching period. On the contrary, the NE content in the Wulst and cerebellum gradually decreased from hatching to 6 days. After this period, the Wulst NE level did not change significantly. In fact, there was no significant difference between NE values at 6 days and those at 6 months of age. In contrast, the difference between the cerebellar NE level at 6 days and that at the adult stage was highly significant. The NE content in the Wulst could be related to noradrenergic afferents originating in the ipsilateral locus coeruleus and substantia grisea centralis, since an electrolytic lesion of the pontine tegmentum caused a 60% reduction in the NE level in the ipsilateral Wulst. In line with the hypothesis that NE plays an important role in cortical plasticity, effects of early monocular deprivation on the Wulst NE content were also observed. After monocular deprivation during the first 6 months of life, the NE level increased by 40% in the Wulst ipsilateral to the deprived eye in comparison to the other side, where the NE level was normal. Monocular deprivation performed in adult animals did not affect the NE content in the Wulst. These results indicate that noradrenergic systems in the Wulst are affected by early, but not late visual deprivation.

Endogenous levels of serotonin and 5-hydroxyindoleacetic acid in specific areas of the pigeon CNS: effects of serotonin neurotoxins

Endogenous levels of serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) were determined by high-performance liquid chromatography (HPLC) in specific regions of the pigeon central nervous system (CNS). High endogenous 5-HT levels in the visual wulst and brainstem and medium 5-HT content in the optic lobes were found. The cerebellum and retina showed low endogenous 5-HT levels. Similar endogenous 5-HIAA levels were measured in the visual wulst, optic lobes and brainstem, whereas the 5-HIAA content of the cerebellum and retina was significantly lower. The effects of para-chloroamphetamine (p-CA) and 5,7-dihydroxytryptamine (5,7-DHT) on the 5-HT and 5-HIAA content of the same regions were studied. Six days after p-CA treatment, the 5-HT content of the visual wulst, optic lobes, brainstem and the 5-HIAA content of the optic lobes and cerebellum markedly decreased. Nine days after 5,7-DHT administration, the 5-HT and 5-HIAA content of the visual wulst and optic lobes was significantly reduced. At longer survival times, serotonergic systems were differentially affected depending on both the neurotoxin treatment and the specific brain regions examined. The 5-HT content of the pigeon retina was not modified by p-CA treatment, whereas 5,7-DHT intravitreally injected caused a pronounced 5-HT depletion. Our results demonstrate that selective neurotoxins for serotonergic systems can provide a useful denervation tool for the study of serotonergic function in the pigeon CNS.

Serotonin depletion modifies the pigeon electroretinogram

Significant amounts of endogenous serotonin have been detected in the retina of many nonmammalian vertebrates. In the pigeon retina, serotonin-like immunoreactivity has been localized within a subpopulation of bipolar and amacrine cells, and serotonin-containing terminals have been found to be segregated in different laminae of the inner plexiform layer. In the current experiments 5,7-dihydroxytryptamine was injected intravitreally in the pigeon eye in order to examine the effect of serotonin depletion on the functional activity of the retina. The physiological modifications induced by the serotonin depletion were examined by recording electroretinographic responses to light flashes of different intensity under conditions of light and dark adaptation. Our results show that 5,7-dihydroxytryptamine treatment selectively increases b-wave amplitude and modifies the function relating b-wave amplitude to Log flash intensity without affecting the peak latency and the amplitude of oscillatory potentials. These results can be interpreted in terms of a possible inhibitory role of serotonin on b-wave generators.

Evoked responses to sinusoidal gratings in the pigeon optic tectum.

Tectal evoked potentials (TEPs) in response to sinusoidal gratings of different contrast, spatial and temporal frequency have been recorded from the tectal surface of the pigeon. Responses have been digitally filtered in order to isolate transient oscillatory (fast) potentials (50-150 Hz), transient slow potentials (1-50 Hz), and the steady-state second-harmonic component (16.6 Hz). Transient slow potentials, as well as the steady-state second-harmonic component, are band-pass spatially tuned with a maximum at 0.5 cycles/deg and attenuation at higher and lower spatial frequencies. The high spatial frequency cutoff is 4-5 cycles/deg. Both transient slow potentials and the steady-state second-harmonic component increase in amplitude as a function of log contrast and saturate at about 20% contrast. The contrast sensitivity, as determined by extrapolating TEP amplitude to 0 V is 0.1-0.2%. These characteristics of spatial-frequency selectivity and contrast sensitivity are similar to those reported for single tectal cells. Unlike slow potentials, oscillatory potentials are not band-pass spatially tuned. In addition, their contrast response function does not saturate at moderate contrast. These differences suggest that tectal evoked slow and fast potentials reflect the activity of different neuronal mechanisms.

Developmental changes of serotonin and 5-hydroxyindoleacetic acid levels in specific regions of the pigeon central nervous system

Serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) levels were determined in the visual Wulst, optic lobes, retina, cerebellum and brainstem of the pigeon during embryonic and posthatching periods. 5-HT content increased during development in almost all regions. 5-HIAA content generally showed the highest values within the second posthatching week and then decreased to reach adult values. The high 5-HT turnover (as indicated by high (5-HIAA/5-HT ratio) observed over the first posthatching week suggests a possible role of 5-HT on developmental processes which occur in pigeon visual areas over the same time.

Dark-rearing modifies serotonin and 5-hydroxyindoleacetic acid contents in specific regions of rabbit brain.

The contents of serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) in specific brain regions of rabbits dark-reared from birth until 3 months of age are higher than those of controls. After light exposure, 5-HT and 5-HIAA levels become similar to normal values. In adult rabbits kept in darkness for 3 months, 5-HT and 5-HIAA contents increase only in the superior colliculi. These results are discussed in relation to the early development of the vestibulo-ocular reflex.

p -Chloroamphetamine treatment modifies evoked responses to sinusoidal gratings in the pigeon optic tectum

This study was performed in order to establish whether selective depletion of serotonin (5-HT) and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) in the pigeon optic tectum (TeO) induced by p-chloroamphetamine (p-CA) modified tectal evoked potentials (TEPs). TEPs in response to sinusoidal gratings of different contrast, spatial and temporal frequency were recorded in control pigeons and in pigeons intraperitoneally injected with p-CA (10 mg/kg; two administrations in consecutive days). TEPs of p-CA treated pigeons, as compared to those of control pigeons, were reduced in amplitude as a function of contrast, spatial and temporal frequency. In addition, TEPs of p-CA treated pigeons differed from those recorded in controls in their transfer characteristics of contrast and spatial frequency. In particular, TEPs of p-CA treated pigeons did not saturate at moderate contrast, unlike those of controls. Furthermore, the TEP spatial tuning in p-CA treated pigeons is broader than that in controls; it thus suggests a reduction of spatial-frequency selectivity. These findings indicate that a selective neurotoxin for serotonergic systems, such as p-CA, can serve as a useful denervation tool for the study of the serotonergic function in the pigeon TeO. In addition, selective changes of TEP properties suggest the possibility that serotonergic afferents play a modulatory role on the receptive-field characteristics of tectal neurons.

Changes in the Avian Visual Wulst Following Early Monocular Deprivation

Since the work of Wiesel and Hubel (1963), the mammalian visual system has been extensively used as a model for studying developmental plasticity in the brain. Recently, the avian visual system has also been used for research examining the neuronal correlates of experience-dependent changes. In particular the visual Wulst was shown to be involved in plastic changes occurring as a result of early monocular deprivation (Burkhalter et al, 1982).