Complexity of sensory environment drives the expression of candidate-plasticity gene, nerve growth factor induced-A.

Exposure of animals to an enriched environment triggers widespread modifications in brain circuitry and function. While this paradigm leads to marked plasticity in animals chronically or acutely exposed to the enriched environment, the molecular mechanisms that enable or regulate such modifications require further characterization. To this end, we have investigated the expression profiles of both mRNA and protein products of a candidate-plasticity gene, nerve growth factor induced-A (NGFI-A), in the brains of rats exposed to increased environmental complexity. We found that NGFI-A mRNA is markedly up-regulated throughout the brains of animals exposed to the enriched environment, but not in the brains of either handled-only or undisturbed control groups. The most pronounced effects were observed in the somatosensory and visual cortices, in layers III and V, while more modest increases were observed in all other cortical layers, with the exception of layer I. A striking NGFI-A mRNA up-regulation was also observed in the striatum and hippocampal formation, notably in the CA1 subfield, of animals exposed to the enriched environment paradigm. Immunocytochemistry was also used to investigate the distribution of NGFI-A protein in response to the environmental enrichment protocol. A marked increase in the number of NGFI-A positive nuclei was identified in the enriched environment condition, as compared to undisturbed and handled-only controls, throughout the rat brain. While the greatest number of NGFI-A immunolabeled neurons was found in cortical layers III and V, up-regulation of NGFI-A protein was also detectable in layers II, IV and VI, in both the somatosensory and visual cortices. NGFI-A immunopositive neurons were also more numerous in the CA1 subfield of the hippocampal formation of animals exposed to the enriched environment, but remained at basal levels in both control groups. Our results implicate NGFI-A as one of the possible early genetic signals that ultimately lead to plastic changes in the CNS.

Role of inhibition in cortical reorganization of the adult raccoon revealed by microiontophoretic blockade of GABA(A) receptors.

Cortical reorganization was induced by amputation of the 4th digit in 11 adult raccoons. Animals were studied at various intervals, ranging from 2 to 37 wk, after amputation. Recordings were made from a total of 129 neurons in the deafferented cortical region using multibarrel micropipettes. Several types of receptive fields were described in reorganized cortex: restricted fields were similar in size to the normal receptive fields in nonamputated animals; multi-regional fields included sensitive regions on both adjacent digits and/or the underlying palm and were either continuous over the entire field or consisted of split fields. The proportion of neurons with restricted fields increased with time after amputation and was greater than previously found in subcortical regions. A GABA(A) receptor antagonist (bicuculline methiodide), glutamate, and GABA were administered iontophoretically to these neurons while determining their receptive fields and thresholds. Bicuculline administration resulted in expansion of the receptive field in 60% of the 93 neurons with cutaneous fields. In most cases (33 neurons) this consisted of a simple expansion around the borders of the predrug receptive field, and the average expansion (426%) was not different from that seen in nonamputated animals. In some neurons (n = 4), bicuculline produced an expansion from one digit onto the adjacent palm or another digit, an effect never seen in control animals. Bicuculline also changed the split fields of seven neurons into continuous fields by exposing a responsive region between the split fields. Finally, bicuculline changed the internal receptive field organization of 10 neurons by revealing subfields with reduced thresholds. In contrast to the situation in nonamputated animals, iontophoretic administration of glutamate also produced receptive field expansion in some neurons (n = 6), but the size and/or shape of the change was different from that produced by bicuculline, indicating that the effects of bicuculline were not due to an overall facilitation of neuronal activity. These results are consistent with the hypotheses that an important component of long-term cortical reorganization is the gradual reduction in effective receptive field size and that intracortical inhibitory networks are partially responsible for these changes.

Expansion of receptive fields in raccoon somatosensory cortex in vivo by GABA(A) receptor antagonism: implications for cortical reorganization.

The effect of antagonism of GABA(A) receptors on the receptive fields of raccoon primary somatosensory cortical neurons was tested using microiontophoretic administration of bicuculline methiodide (BMI). The size of cutaneous receptive fields was examined using minimal suprathreshold mechanical stimulation before, during, and after BMI administration. In 65 of 102 rapidly adapting neurons, BMI produced a clear expansion of the receptive field. The mean increase in receptive-field size was 286%. The receptive fields on the distal digit, which were initially smaller, showed smaller increases in absolute area than more proximal receptive fields, but the percentage increase did not vary with location. Greater expansion was seen in superficially located neurons than in those below 800 microm. Of particular significance was the finding that the expansion of receptive fields produced by BMI never extended from one digit onto an adjacent digit or onto the palm, even when the original receptive field was at the base of a digit. This finding indicates that intracortical GABAergic inhibition is insufficient to explain cortical reorganization following digit amputation.

Light-induced zif268 expression is dependent on noradrenergic input in rat visual cortex.

In the present paper we investigated the role of the noradrenergic projection from the locus coeruleus on the expression of the immediate early gene zif268 in the visual cortex of rats exposed to ambient light stimulation. Local administrations of 6-hydroxydopamine (6-OHDA), a specific toxin directed against the catecholaminergic system, were performed in the locus coeruleus prior to visual stimulation. Animals were stimulated for 2 h by ambient light, after a 2-week dark adaptation period. Sham-operated controls displayed a massive increase in the number of zif268 positive cells after light stimulation. To the contrary, lesioned animals demonstrated a dramatic reduction in the number of zif268 positive nuclei across all cortical layers. A few scattered immunopositive nuclei were identified in cortical layer IV, however, this region also underwent a significant reduction in the number of zif268 immunopositive nuclei. Our results indicate that the noradrenergic system plays an important role in the expression of zif268 in the visual cortex of rats exposed to ambient light after dark isolation.

Antibody for human p75 LNTR identifies cholinergic basal forebrain of non-primate species.

192-IgG is an antibody directed against the p75 low affinity nerve growth factor receptor in rats, whereas ME 20.4 was raised against the analogous protein in humans. Coupled to saporin, 192-IgG and ME 20.4 have been used to lesion basal forebrain neurons in rats and primates, respectively. We compared the cross-reactivity of 192-IgG and ME 20.4 in the basal forebrain of rat, human, dog, cat, raccoon, pig, and rabbit. We found excellent species cross-reactivity of ME 20.4 in dog, raccoon, cat, pig and rabbit. In contrast, 192-IgG did not label neurons in any species other than rat. Our findings suggest that ME 20.4-saporin could be used to produce cholinergic basal forebrain lesions in several non-primate species.

Two distinct populations of cholinergic neurons in the septum of raccoon (Procyon lotor): evidence for a separate subset in the lateral septum.

The present study focused on cholinergic neurons in the lateral septal region of the raccoon detected by choline acetyltransferase (ChAT)-immunostaining. For comparison of the cholinergic neurons of the medial and lateral septal nuclei, soma sizes were measured, and several antibodies were applied that differentially characterize these cells in several species: low-affinity neurotrophin receptor p75 (p75(NTR)), calbindin-D(28k) (CALB), and constitutive nitric oxide synthase (cNOS). To compare the basic organization of the raccoon septum with that in other mammals, parvalbumin (PARV) immunocytochemistry and Wisteria floribunda-agglutinin (WFA) lectin histochemistry also were used in double-staining experiments. The ChAT-immunoreactive neurons of the rostral lateral septum are arranged in laminae. Accumulations of cholinergic varicosities, often clearly ensheathing noncholinergic neurons, occupy small territories of the rostral septum. Such regions become larger in the caudal septum. They are assumed to correspond to the septohippocampal and septofimbrial nuclei of the rat. In contrast to the large medial septal cholinergic neurons of the raccoon that contain p75(NTR), CALB, and cNOS, the cholinergic neurons of the lateral septum are smaller and do not express these markers. A further peculiarity is that the region of the lateral septum that contains cholinergic neurons corresponds to WFA-labelled extracellular matrix zones that contain chondroitin sulfate proteoglycans. In addition to clustered thread- or ring-like accumulations of the WFA, sparsely labelled perineuronal nets surround the lateral septal cholinergic neurons. Similar to other species that have been investigated, perineuronal nets are completely absent around cholinergic cells of the medial septum. The PARV-containing neurons of this region, however, are enwrapped by perineuronal nets as they are in the rat. Within the medial septum, the PARV-containing neurons are restricted to ventral bilateral territories that are devoid of cholinergic cells. In this respect, they differ from the more vertically arranged PARV-containing medial septal cells in rodents and primates. Apart from striking differences in numbers and distribution patterns, the raccoon lateral septal cholinergic neurons resemble those detected by Kimura et al. (Brain Res [1990] 533:165-170) in the ventrolateral septal region of rat and monkey. Their participation in the functions of the lateral septum remains to be elucidated.

Co-expression of p75NTR- and calbindin-immunoreactivity in cholinergic neurons of the raccoon basal forebrain.

The cholinergic system of the basal forebrain is involved in the modulation of sensory information. This has previously been investigated in the raccoon, an animal especially interesting because of its highly developed somatosensory cortex. The present study focused on the co-expression of the low-affinity neurotrophin receptor p75NTR and calbindin in cholinergic neurons of the raccoon basal forebrain and neostriatum. Carbocyanine immunofluorescence double labelling revealed the co-localization of choline acetyltransferase and p75NTR as well as calbindin in a large portion of basal forebrain neurons, but not in the neostriatum. In contrast, immunolabelling of two other calcium-binding proteins, parvalbumin and calretinin, was found exclusively in non-cholinergic neurons.

Calcitonin gene-related peptide and neuropeptide Y in the raccoon cuneate nucleus.

Immunocytochemical methods were used to determine the distribution of calcitonin gene-related peptide (CGRP) and neuropeptide Y (NPY) in the middle region of the adult raccoon cuneate nucleus. Extensive CGRP-immunoreactive fibers and darkly stained punctate structures, thought to be terminals, were concentrated in the dorsal cap and basal region of the middle cuneate nucleus. These regions receive input from the claws and the hairy skin of the paw and forearm. The NPY-immunoreactive fibers and terminals were also found within the dorsal cap of the cuneate nucleus, but were less abundant than CGRP. However, most of the NPY-immunostained fibers and terminals were found in the cluster region of the cuneate nucleus, which receives input from glabrous skin. No CGRP- or NPY-immunoreactive cell bodies were found in the raccoon middle cuneate nucleus. This description of the distribution of CGRP and NPY in the normal animal provides a baseline for future investigations into injury-induced neuropeptide plasticity in the raccoon middle cuneate nucleus.