Peter W. Land

Metabolic and structural correlates of the vibrissae representation in the thalamus of the adult rat

Cytochrome oxidase (CO) histochemistry was used to examine patterns of metabolic activity in the ventral posteromedial nucleus of the adult rat thalamus. In sections cut in an oblique horizontal plane, CO staining reveals distinct patches of heightened activity arranged in a fashion remniscent of the pattern of vibrissae on the contralateral face and which corresponds to the known somatotopic organization of the nucleus. The CO-reactive zones coincide with oval cylinders of thalamic neurons that appear to be anatomically linked with corresponding barrels in the contralateral somatosensory cortex.

The development of callosal projections in normal and one-eyed rats

The course of callosal development in area 17 of rats suggests that, unlike immediately adjacent regions, axons of callosal origin do not normally gain access to upper cortical layers, and this results in the loss of an early exuberant callosal pathway. Removal of optic input, however, permits invasion of these layers of area 17 by callosal axons and results in survival of callosally projecting neurons in area 17.

A reliable technique for marking the location of extracellular recording sites using glass micropipettes

A simple and highly reliable technique is described for marking the locations of extracellularly recorded neurons using double-barreled glass micropipettes. One barrel contains 3 M NaCl for recording; the other contains horseradish peroxidase (HRP) that is iontophoretically ejected using low currents. With appropriate processing of the tissue, small well-localized spots are produced that contain a small number of HRP-filled neurons. The technique is effective in marking sequential recording sites within individual electrode tracks and in multiple penetrations using the same microelectrode.

Metabolic activity in SmI cortical barrels of adult rats is dependent on patterned sensory stimulation of the mystacial vibrissae.

Cytochrome oxidase (CO) histochemistry was used to examine the effect of sensory deprivation on metabolic activity in the somatosensory cortex (SmI) of adult rats. Chronic trimming of one or several rows of mystacial vibrissae resulted in a decrease in CO reactivity in the corresponding barrels in layer IV. Reduced CO staining also was observed in cortical laminae superficial and deep to the affected layer IV barrels, suggesting that patterned deflections of the whiskers are important for maintaining the metabolic activity of neurons at least 3 and perhaps 4 synapses removed from the periphery.

Immunocytochemical Localization of GABAA Receptors in Rat Somatosensory Cortex and Effects of Tactile Deprivation

Immunocytochemical techniques were used to investigate the distribution of gamma-aminobutyric acidA (GABAA) receptors in the rat primary somatosensory cortex (SI). Monoclonal antibody 62-3G1 (de Blas et al., 1988; Victorica et al., 1988), which recognizes an epitope common to the beta 2 and beta 3 subunits of the GABAA receptor, produces staining of small punctate structures throughout the neuropil, and around somata and linear processes in all laminae of SI. Receptor immunostaining is relatively intense in upper lamina I and in lamina IV, where patches of intense receptor staining are interleaved with narrow zones of moderate immunoreactivity. Staining is lightest in lamina Vb, where stained puncta appear to be aligned with radially oriented processes, and moderate in the remaining laminae. Tangential sections through lamina IV reveal that each large cortical barrel encompasses several patches of intense receptor staining that are aligned with the corners or edges of individual barrels; interbarrel septa are moderately of intense cytochrome oxidase (CO) histochemical staining. A similar correspondence is apparent between a complex lattice of dense receptor clustering and a plexus of dark CO staining in the cortical trunk representation. Six to eight weeks of tactile deprivation produced by simple whisker trimming have no visible effect on GABAA receptor distribution. This is the case for rats whose whiskers were trimmed only during adulthood and for rats deprived from the day of birth until examination 6-8 weeks later. However, electrocautery ablation of whisker follicles leads to a marked decline in GABAA receptor immunoreactivity in cortical barrels associated with the ablated follicles. Our findings indicate that there is reasonable, though not perfect, correspondence between the distribution of GABAA receptors and the distribution of GABA-containing neurons and terminals in rat SI. These elements are associated with regions of intense oxidative metabolic activity revealed by CO staining. The density of GABAA receptors is reduced in lamina IV following complete loss of peripheral afferent input. However, less severe tactile deprivation, which is known to affect cortical neuron responsiveness, produces little or no change in receptor distribution.

Experience-dependent alteration of synaptic zinc in rat somatosensory barrel cortex.

Chelatable zinc is co-localized with glutamate in the synaptic vesicles of a distinct population of telencephalic neurons. The present study used a histochemical technique to localize zinc-containing terminals within the somatosensory barrel cortex (S1) of normal adult rats and rats that had been subjected to 4-6 weeks of tactile deprivation produced by simple whisker trimming beginning either at birth or during adulthood. In normal adult rats intense staining for synaptic zinc was observed in laminae I, II/III and V. Lamina VI was moderately stained, particularly within sublamina VIa where thalamic axons terminate. Thalamorecipient barrel centers in lamina IV were distinguished by the relative absence of synaptic zinc and were bounded by darkly stained inter-barrel septa. This pattern was unchanged in rats whose whiskers were trimmed only during adulthood, suggesting that synaptic zinc constitutes a relatively stable pool that is resistant to subtle changes in sensory input. By contrast, neonatal whisker trimming resulted in a marked increase in density of synaptic zinc specifically within the centers of deprived barrels in lamina IV. This redistribution of synaptic zinc appears to be permanent since altered staining of deprived barrels persists after extended periods of tactile experience with regrown whiskers. The results in normal rats indicate that zinc-containing circuits are distributed heterogeneously within S1 where they most likely subserve intracortical vs thalamocortical processing. The altered distribution of zinc-ergic circuits following neonatal whisker trimming suggests that zinc-sequestering neurons in developing S1 are particularly sensitive to early tactile experience.

Vesicular glutamate transporters in the lateral geniculate nucleus: expression of VGLUT2 by retinal terminals.

We used immunohistochemistry to localize vesicular glutamate transporters VGLUT1 and VGLUT2 in the rat lateral geniculate nucleus. The lateral geniculate nucleus is intensely immunoreactive for both transporters. Monocular eye removal abolished staining for VGLUT2 in a pattern corresponding to the distribution of terminals from the missing eye, without affecting distribution of VGLUT1 immunoreactivity. These data indicate retinal ganglion cells are the source of VGLUT2-containing synapses in the lateral geniculate nucleus.

Serotonin immunoreactivity in the circumesophageal nervous system of Hermissenda crassicornis

Immunohistochemical techniques were used to examine the distribution of serotonin-immunoreactive (5-HT-IR) neurons and processes in the circumesophageal nervous system of Hermissenda. Both the pedal and the cerebropleural ganglia contained immunoreactive neuronal somata, with the majority occurring in the pedal ganglia. Immunoreactive fibers and varicosities were identified in portions of the central neuropil, where we noted a consistent and specific relationship between 5-HT-IR axons, the optic nerve and the synaptic region in the neuropil near the photoreceptor terminals.

Zinc accumulation after target loss: an early event in retrograde degeneration of thalamic neurons

Accumulation of cytoplasmic zinc is linked with a cascade of events leading to neuronal death. In many in vivo models of zinc‐induced cell death, toxic concentrations of synaptically released zinc enter vulnerable neurons via neurotransmitter‐ or voltage‐gated ion channels. In vitro studies demonstrate, in addition, that zinc can be liberated from intracellular stores following oxidative stress and contribute to cell death processes, including apoptosis. Here we describe accumulation of intracellular zinc in an in vivo model of cell death in the absence of presynaptic zinc release. We focused on the lateral geniculate nucleus (LGN) because LGN neurons undergo apoptosis when separated from their target, the primary visual cortex (V1), and the LGN is mostly devoid of zinc‐containing presynaptic terminals. Infant and adult rats and adult mice received unilateral ablation of V1, either by aspiration or kainate injection. One to 14 days later, brain sections were stained with selenium autometallography or fluorescently labeled to localize zinc, or stained immunochemically for activated caspase‐3. V1 lesions led to zinc accumulation in LGN neurons in infant and adult subjects. Zinc‐containing neurons were evident 1–3 days after aspiration lesions, depending on age, but not until 14 days after kainate injection. Zinc accumulation was followed rapidly by immunostaining for activated caspase‐3. Our data indicate that like neurotrauma and excitotoxicity, target deprivation leads to accumulation of zinc in apoptotic neurons. Moreover, zinc accumulation in vivo can occur in the absence of presynaptic zinc release. Together these findings suggest that accumulation of intracellular zinc is a ubiquitous component of the cell death cascade in neurons.

Chronic sensory deprivation affects cytochrome oxidase staining and glutamic acid decar☐ylase immunoreactivity in adult rat ventrobasal thalamus

The rodent ventrobasal thalamus (VB) contains groups of vibrissa-related neurons (barreloids) that are highly reactive for the enzyme cytochrome oxidase. The present experiments show that each barreloid also contains a dense accumulation of glutamic acid decarboxylase (GAD) immunoreactive terminals. Chronic vibrissa trimming results in parallel declines in staining for both cytochrome oxidase (CO) and GAD in barreloids associated with the trimmed hairs. Thus, thalamic metabolism like that in the cortex is dependent upon normal sensory input. This includes projection neurons as well as neurons in the reticular nucleus, which are the major source of gamma-aminobutyric acid (GABA)ergic input to the rat VB.