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.

Amino acids modify thalamo-cortical response transformation expressed by neurons of the ventrobasal complex.

The hypothesis has been tested that inhibitory mechanisms, active spatially and temporally between the input and the output of thalamic neurons, determine the nature of the information transmitted to the cerebral cortex. To enable this assessment, in barbiturate-anesthetized cats and urethane-anesthetized rats juxtacellular recordings were performed together with microiontophoretic ejection of transmitter agonists and antagonists. The effects of these drugs were studied on responses evoked by mechanical stimulation of cutaneous receptive fields (RFs) of neurons in the thalamic ventrobasal complex (VB). Neurons from different parts of the VB were investigated: 29 units were located medially, in the ventral posteromedial nucleus (VPM; facial RFs), and 11 units were located laterally, in the ventral posterolateral nucleus (VPL; forepaw and body RFs). A further eleven VB units had no detectable RF. Twenty-six neurons were tested with electrical stimulation of the somatosensory cortex (SI), 17 of these being identified as thalamo-cortical relay neurons and 5 being classified as presumed interneurons; the remaining 4 could not be activated. Four additional recordings were from trigemino-thalamic or thalamo-cortical fibers. For the quantitative assessment of the neurons' input and output, neuronal activity was induced by feedback-controlled, mechanical trapezoidal and/or sinusoidal stimuli applied to sinus hairs, fur or skin and the numbers of prepotentials and soma spikes were compared in peristimulus time histograms (PSTHs) generated simultaneously for both types of signal from 'DC' recordings. Iontophoretic administration of excitatory amino acids (EAAs) or bicuculline methiodide (BMI) increased output-input ratios in 87% of the cases tested, due to a higher rate of conversion of prepotentials into soma spikes taking place. In cases of neurons exhibiting a sustained-to-transient response pattern, changes to sustained-to-sustained patterns were demonstrated. Tests with gamma-aminobutyric acid (GABA) produced decreased output-input ratios in 90% of the neurons, due to a lower conversion rate of prepotentials into soma spikes taking place. In cases of neurons exhibiting high output-input ratios (sustained-to-sustained type), the responses changed to the sustained-to-transient pattern. For cortically evoked antidromic spikes of VB neurons, GABA produced a failure of the initial segment (IS-) spike to invade the soma, whereas BMI and glutamate (Glu) facilitated soma depolarization. When ejected with relatively higher currents than those needed to alter output-input ratios, EAAs decreased prepotential amplitudes while GABA produced increases in 16 of 18 neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Disposition of amino acid synaptic transmitters, acetylcholine and substance P in the LM-suprageniculate nuclear complex of the cat's thalamus.

Immunocytochemical analysis with antibodies raised against aspartate, glutamate, gamma-aminobutyrate (GABA), choline acetyltransferase (ChAT), and substance P (SP) have allowed the transmitter characterisation and distribution of cells of the lateralis medialis-nucleus suprageniculatus (LM-SG) complex to be made at the level of the light microscope. We have found that the intranuclear distributions of aspartate and glutamate differed substantially from that of GABA, as well as there being specific and, in some cases, major differences in the respective populations of cells labelled with all three amino-acid-sensitive antibodies. ChAT-labelled elements were disposed very similarly to acetylcholinesterase (AChE)-positive subregions of the nuclear complex, while SP labelling was comparatively weak, albeit present, throughout the region. These data provide an important first step towards the further understanding of the details of the neurochemical and functional identity of the LM-SG complex.

Amino acids as transmitters of synaptic excitation in neocortical sensory processes.

Few synaptic transmitters are known to exist that are not represented in some region or another, or at some layer or other, in the cerebral cortex of mammalian brain. The more difficult job than mere identification of which substances are present, is that of the assignment of particular functional role(s) of such substances, and as well, of determining upon exactly which element(s) of the known synaptic circuitry of neocortex, such transmitters operate. Current wisdom subscribes to the view that the excitatory amino acids, most likely L-glutamate, and L-aspartate but perhaps also L-cysteate, L-homocysteate, L-cysteine sulfinate or even (although much less likely) the endogenous dipeptide substance, N-acetyl-L-aspartyl-L-glutamate, are the major excitatory synaptic transmitters of intracortical (associational) fibres, of corticofugal projections, and, as this article will attest, of thalamocortical inputs, as well. What particular limits, or restrictions, are imposed upon these generalizations, such as whether the data pertain only to primary sensory areas or follow some other yet to be determined rule, remains to be discovered in future experiments. This paper first presents an overview of the advances in understanding that have come about during the past few decades concerning the synaptic roles of amino acid transmitters. Next, an experimental section presents new evidence based on release studies and the microiontophoretic approach, which supports the view that the amino acids, glutamate and aspartate, interact with specific, pharmacologically identified subtypes of receptors in neocortex as transmitters of synaptic excitation released from thalamic afferent terminals.

Organization of cortical and subcortical projections to the feline insular visual area, IVA.

Extracellular recordings with carbon fiber-filled microelectrodes were used to identify the visually responsive area within the insular cortex (referred to hereafter as the insular visual area, IVA) of anaesthetized cats. Broadly speaking, IVA comprises the cortex surrounding the anterior ectosylvian sulcus (AEs) along its ventral bank and the major portion of the anterior sylvian gyrus. Visually sensitive cells were recorded along the whole length of the AEs. In the same animals, the afferent connections of IVA were studied through the use of the retrograde tracers wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) and fluorescent Diamidino yellow (DY), in combination with standard electrophysiological stimulation and recording techniques. The results indicate that: (1) the IVA receives a wide variety of telencephalic inputs, not only from visual, sensorimotor, auditory, limbic and association cortical areas, and from the claustrum, amygdala and basal nucleus of Meynert, as well, but also from the diencephalic projections arising mainly from the lateralis medialis-suprage niculate nuclear complex (LM-Sg) and the ventral medial nucleus (VM). (2) The gyral part of IVA (gIVA) receives afferents mainly from the lateral part of the lateral suprasylvian visual area (LS) throughout almost its entire length, as well as from area 20, the posterior suprasylvian sulcal area (PS), the frontal eye fields, areas 6 and 36, and almost the whole length of the cortical area lying along the anterior ectosylvian sulcus (AEs). (3) By contrast with (2), the sulcal part of IVA (sIVA) which corresponds to the anterior part of the anterior ectosylvian visual area (AEV) of Norita et al. ('86), receives cortical projections mainly from the lateral and medial parts of the anterior half of LS, area 20, PS, the frontal eye fields, area 36, and most parts of the cortical area extending along the AEs. (4) Subcortically, IVA receives thalamic afferents mainly from VM and LM-Sg. The connections between IVA and LM-Sg are organized topographically, with the more anterior part of IVA being related to the more ventral portion of LM-Sg, and with sIVA being related chiefly to the mid-portions of LM-Sg. These results thus suggest that IVA may function as an integrative centre among structures belonging to the extrageniculostriate system, the sensorimotor system, as well as to the limbic system. Furthermore, our electrophysiological and anatomical findings, together with previous reports concerning AEV, suggest that the posterior part of AEV (AEV proper) is distinctive from gIVA, and that the sIVA apparently serves as a transitional region between AEV and gIVA.

Bicuculline-induced alterations of response properties in functionally identified ventroposterior thalamic neurones.

Extracellular recordings of 105 neurones in the cat's somatosensory thalamus were obtained with carbon fibre-containing multibarrel micropipettes. The responses of cells to natural stimulation of cutaneous or deep structures were characterized and the responses to electrical stimulation of primary somatosensory cortex were determined. Receptive fields were mapped and the functional properties were examined before and during the microiontophoretic administration of glutamate, gamma-aminobutyric acid (GABA) and bicuculline methiodide (BMI). Modality and submodality properties of all cells tested apparently remained unchanged qualitatively, despite all pharmacological interventions. BMI lowered the response threshold of a majority of the 48 cells tested for this variable, although almost 25% responded with elevated thresholds. BMI changed the temporal properties of the responses of both thalamocortical relay neurones and of presumed interneurones. Discharges evoked by natural stimuli and by electrical stimulation of the cortex were prolonged and their pattern was altered. Decreases in the frequency of bursts of discharges were often observed with BMI, and these bursts were invariably prolonged and the interspike interval profiles were altered. Receptive field size changes were observed only in 8 of 48 neurones. For two of these, the field size decreased, while for the others there were small increases.

On the configuration of the receptors for excitatory amino acids.

Separate receptors are recognized for the excitation of mammalian neurones by (a) L-glutamic and quisqualic acids and (b) N-methyl-D-aspartic (NMDA), and other amino acids which have conformationally restricted molecules. Several other compounds, both agonists and antagonists, have been examined, and it is concluded that (i) the NMDA receptor reacts preferentially with substances in a relatively extended configuration, (ii) the glutamate/quisqualate receptor prefers folded molecules and (iii) the distance separating the amino group from the distal anionic function is the critical one determining receptor preference.

The excitation of mammalian central neurones by amino acids.

1. The relative potencies of a number of analogues of L-glutamate as excitants of thalamic neurones in the rat have been compared. The most powerful compounds were kainate, ibotenate and (+/-)cis-1-amino-1,3-dicarboxycyclopentane. The D- and L-isomers of glutamate and aspartate were also compared. Whereas D-glutamate is approximately one-half as active as the L-form, D-aspartate is more potent than L-aspartate. 2. Computer analysis has indicated that ibotenate and cis-1-amino-1,3-dicarboxy-cyclopentane have relatively fixed and similar C alpha-N, Comega-N and C alpha-Comega interatomic distances which can also be achieved by glutamate in certain conformations of the molecule, but not by aspartate. 3. Parallel examination of the antagonists glutamate diethylester and D-alpha-aminoadipate has shown that the former preferentially reduces L-glutamate effects while the latter blocks the actions of other amino acid excitants more readily than those of L-glutamate. 4. The evidence is consistent with the hypothesis that at least two populations of neuronal receptors for the excitatory amino acids exist.

Ranking of excitatory amino acids by the antagonists glutamic acid diethylester and D-alpha-aminoadipic acid.

A separation of the excitatory actions of the amino acids upon thalamic neurones of rats anaesthetized with urethane has been accomplished through the use of two antagonists. It has been possible to rank the excitatory compounds in their order of susceptibility to D-alpha-aminoadipic acid (DalphaAA) and L-glutamic acid diethylester (GDEE). The observation that the ranking orders of the excitants differ for these two antagonists permits an analysis of the types of receptors with which the amino acid excitants react. The results support the proposition that more than one neuronal receptor sensitive to the amino acids exists.