Adaptive responses of monkey somatosensory cortex to peripheral and central deafferentation.

This study deals with two kinds of activity-dependent phenomena in the somatosensory cortex of adult monkeys, both of which may be related: (1) mutability of representational maps, as defined electrophysiologically; (2) alterations in expression of genes important in the inhibitory and excitatory neurotransmitter systems. Area 3b of the cerebral cortex was mapped physiologically and mRNA levels or numbers of immunocytochemically stained neurons quantified after disrupting afferent input peripherally by section of peripheral nerves, or centrally by making lesions of increasing size in the somatosensory thalamus. Survival times ranged from a few weeks to many months. Mapping studies after peripheral nerve lesions replicated results of previous studies in showing the contraction of representations deprived of sensory input and expansion of adjacent representations. However, these changes in representational maps were in most cases unaccompanied by significant alterations in gene expression for calcium calmodulin-dependent protein kinase isoforms, for glutamic acid decarboxylase, GABA(A) receptor subunits, GABA(B) receptors, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) or N-methyl-D-aspartate (NMDA) receptor subunits. Mapping studies after lesions in the ventral posterior lateral nucleus (VPL) of the thalamus revealed no changes in cortical representations of the hand or fingers until >15% of the thalamic representation was destroyed, and only slight changes until approximately 45% of the representation was destroyed, at which point the cortical representation of the finger at the center of a lesion began to shrink. Lesions destroying >60% of VPL resulted in silencing of the hand representation. Although all lesions were associated with a loss of parvalbumin-immunoreactive thalamocortical fiber terminations, and of cytochrome oxidase staining in a focal zone of area 3b, no changes in gene expression could be detected in the affected zone until >40-50% of VPL was destroyed, and even after that changes in mRNA levels or in numbers of GABA-immunoreactive neurons in the affected zone were remarkably small. The results of these studies differ markedly from the robust changes in gene expression detectable in the visual cortex of monkeys deprived of vision in one eye. The results confirm the view that divergence of the afferent somatosensory pathways from periphery to cerebral cortex is sufficiently great that many fibers can be lost before neuronal activity is totally silenced in area 3b. This divergence is capable of maintaining a high degree of cortical function in the face of diminishing inputs from the periphery and is probably an important element in promoting representational plasticity in response to altered patterns of afferent input.

Progressive transneuronal changes in the brainstem and thalamus after long-term dorsal rhizotomies in adult macaque monkeys.

This study deals with a potential brainstem and thalamic substrate for the extensive reorganization of somatosensory cortical maps that occurs after chronic, large-scale loss of peripheral input. Transneuronal atrophy occurred in neurons of the dorsal column (DCN) and ventral posterior lateral thalamic (VPL) nuclei in monkeys subjected to cervical and upper thoracic dorsal rhizotomies for 13-21 years and that had shown extensive representational plasticity in somatosensory cortex and thalamus in other experiments. Volumes of DCN and VPL, number and sizes of neurons, and neuronal packing density were measured by unbiased stereological techniques. When compared with the opposite, unaffected, side, the ipsilateral cuneate nucleus (CN), external cuneate nucleus (ECN), and contralateral VPL showed reductions in volume: 44-51% in CN, 37-48% in ECN, and 32-38% in VPL. In the affected nuclei, neurons were progressively shrunken with increasing survival time, and their packing density increased, but there was relatively little loss of neurons (10-16%). There was evidence for loss of axons of atrophic CN cells in the medial lemniscus and in the thalamus, with accompanying severe disorganization of the parts of the ventral posterior nuclei representing the normally innervated face and the deafferented upper limb. Secondary transneuronal atrophy in VPL, associated with retraction of axons of CN neurons undergoing primary transneuronal atrophy, is likely to be associated with similar withdrawal of axons from the cerebral cortex and should be a powerful influence on reorganization of somatotopic maps in the somatosensory cortex.

Laminar and cellular distribution of AMPA, kainate, and NMDA receptor subunits in monkey sensory-motor cortex.

In situ hybridization histochemistry and immunocytochemistry were used to examine lamina- and cell-specific expression of glutamate receptor (GluR) mRNAs and polypeptide subunits in motor and somatosensory cortex of macaque monkeys. Radioactive complementary RNA (cRNA) probes were prepared from cDNAs specific for alpha-amino-3-hydroxy-5-methylisoxozolepropionate (AMPA)/kainate (GluR1-GluR4), kainate (GluR5-GluR7), and N-methyl-D-aspartate (NMDA; NR1, NR2A-NR2D) receptor subunits. AMPA/kainate and NR1, NR2A, and NR2B receptor transcripts show higher expression than other transcripts. All transcripts show lamina-specific patterns of distribution. GluR2 and GluR4 mRNAs show higher expression than do GluR1 and GluR3 mRNAs. GluR6 transcript expression is higher than that of GluR5 and GluR7. NR1 mRNA expression is much higher than that of NR2 mRNAs. NR2C subunit expression is very low except for a very distinct band of high expression in layer IV of area 3b. Immunocytochemistry, using subunit-specific antisera and double labeling for calbindin, parvalbumin, or alpha type II Ca2+/calmodulin-dependent protein kinase (CaMKII-alpha), allowed identification of cell types expressing different subunit genes. GluR1 and GluR5/6/7 immunoreactivity is found in both pyramidal cells and gamma-amino butyric acid (GABA) cells; GluR2/3 immunoreactivity is preferentially found in pyramidal cells, whereas GluR4 immunoreactivity is largely restricted to GABA cells; NMDA receptor subunit immunoreactivity is far greater in excitatory cells than in GABA cells. The density of expression of AMPA/kainate, kainate, and NMDA receptor subunit mRNAs differed within and across the architectonic fields of sensory-motor cortex. This finding and the lamina- and cell-specific patterns of expression suggest assembly of functional receptors from different arrangements of available subunits in specific neuronal populations.

Extensive divergence and convergence in the thalamocortical projection to monkey somatosensory cortex.

This study examined the extent of thalamocortical divergence as a potential determinant of activity-dependent representational plasticity in area 3b of adult monkey somatosensory cortex. Single or paired injections of anterogradely transported tracers, of varying anteroposterior extent, were made horizontally from behind in defined parts of the body representation in the ventral posterior lateral (VPL) and/or ventral posterior medial (VPM) thalamic nuclei, and the distribution and density of labeled thalamocortical terminations were mapped in cortex. Injections of increasing size in any dimension of VPL or VPM resulted in increasing accumulation of labeled terminals within the region of projection, implying extensive convergence of individual axons. Anteroposteriorly elongated injections labeled mediolaterally extended but anteroposteriorly restricted zones in cortex. Dorsoventral placement of an injection in VPL or VPM determined anteroposterior location of labeling in cortex. Dual injections separated mediolaterally or dorsoventrally by approximately 1 mm, and in different parts of the thalamic body or head-face representation gave rise to labeled thalamocortical terminations that overlapped extensively. For injection sites at different anteroposterior levels in VPL or VPM, the area of cortical convergence was related to their extent of anteroposterior coincidence. Labeled terminations arising from injections in immediately adjacent parts of VPL and VPM did not overlap in cortex. The extent of thalamocortical divergence and convergence revealed by these experiments is greater than that predictable from labeling of single axons and is sufficiently great to account for representational plasticity that exceeds the 1.5 mm cortical "distance limit."

Maintenance of a somatotopic cortical map in the face of diminishing thalamocortical inputs.

This study addresses the extent of divergence in the ascending somatosensory pathways of primates. Divergence of inputs from a particular body part at each successive synaptic step in these pathways results in a potential magnification of the representation of that body part in the somatosensory cortex, so that the representation can be expanded when peripheral input from other parts is lost, as in nerve lesions or amputations. Lesions of increasing size were placed in the representation of a finger in the ventral posterior thalamic nucleus (VPL) of macaque monkeys. After a survival period of 1-5 weeks, area 3b of the somatosensory cortex ipsilateral to the lesion was mapped physiologically, and the extent of the representation of the affected and adjacent fingers was determined. Lesions affecting less than 30% of the thalamic VPL nucleus were without effect upon the cortical representation of the finger whose thalamic representation was at the center of the lesion. Lesions affecting about 35% of the VPL nucleus resulted in a shrinkage of the cortical representation of the finger whose thalamic representation was lesioned, with concomitant expansion of the representations of adjacent fingers. Beyond 35-40%, the whole cortical representation of the hand became silent. These results suggest that divergence of brainstem and thalamocortical projections, although normally not expressed, are sufficiently great to maintain a representation after a major loss of inputs from the periphery. This is likely to be one mechanism of representational plasticity in the cerebral cortex.

Hand/face border as a limiting boundary in the body representation in monkey somatosensory cortex.

Horizontal intracortical connections may form one substrate for representational plasticity in somatosensory cortex. Electrophysiological mapping demonstrated the finer details of the representations of the hand, lower jaw, neck, and face in area 3b of normal macaque monkeys. Injections of two fluorescent tracers then defined the extent to which horizontal connections crossed from the face into the hand representations and vice versa in area 3b. Connections are widely distributed within cortical representations of skin areas innervated by cervical nerves or by the trigeminal nerve but do not cross a border defined by the anterior limit of the representation of skin innervated by the second cervical nerve. This border separates the representation of the muzzle, innervated only by the mandibular nerve, and the representation of the lower jaw and neck region, innervated by the second and third cervical nerves but overlapped by the mandibular nerve. Thus, the muzzle representation lacks connections with the hand and with the lower jaw and neck representations, but the representations of the hand and of the lower jaw and neck are strongly interconnected. Overlap of the hand and of the lower jaw and neck representations and of their horizontal intracortical connections may form one basis for expansions of the lower jaw representation into that of the hand when peripheral input from the hand is lost. Lack of connections with the rest of the face representation may limit this spread.

Representation of face and intra-oral structures in area 3b of macaque monkey somatosensory cortex.

Details of the representation of body regions innervated by the trigeminal nerve were elucidated in monkey cerebral cortex. Microelectrode recording was used to generate somatosensory maps in the posterior bank of the central sulcus and on the exposed cortical surface lateral to the lateral tip of the central sulcus in Macaca nemestrina. The area innervated by the contralateral trigeminal nerve is represented in an 8-mm mediolateral extent of area 3b lateral to the representation of the hand. Lateral to this, still within area 3b, there is an expanded representation of ipsilateral intra-oral structures measuring 6 mm in mediolateral extent. Both representations fill area 3b anteroposteriorly. The ipsilateral representation forms approximately 40% of the trigeminal representation, consistent with the amount of the ventroposterior medial (VPM) thalamic nucleus devoted to representation of ipsilateral intra-oral structures. Comparison of the present results with maps of the face representation in other species of monkey shows a consistent somatotopy of the face between species; size variations are mainly related to the enlarged ipsi- and contralateral representations of the cheek pouches in macaques. The general somatotopy of the trigeminal representation in monkeys is consistent with that in other mammalian species.

Plasticity of the somatosensory cortical map in macaque monkeys after chronic partial amputation of a digit.

Activity-dependent changes in cortical representational maps have been reported in many studies of adult mammals. Limits in extent of change have been attributed to limited divergence in the thalamocortical projection. However, studies have commonly been restricted to animals surviving less than a year following relatively modest peripheral sensory perturbations. After extensive deafferentation and long-term survival, more extensive changes, seemingly beyond the limits of thalamocortical divergence, have been reported. We report changes in the somatotopic map in area 3b of an adult macaque monkey, in which part of the index finger of one hand had been amputated two years previously. The representation of the remaining stump occupied the whole region of area 3b normally devoted to the representation of the entire digit. The skin surrounding the stump appeared to have been hyperinnervated by axons severed during the amputation. The hyperinnervation of remaining skin may have reactivated neurons of the somatosensory system silenced by the amputation, leading to the recovery of a cortical map but with a modified organization.

Laminar patterns of expression of GABA-A receptor subunit mRNAs in monkey sensory motor cortex.

Radioactive complementary RNA probes, made from monkey-specific cDNAs specific for the alpha 1, alpha 2, alpha 4, alpha 5, beta 1, beta 2, and gamma 2 subunits of the gamma-aminobutyric acid A (GABAA) receptor were used for in situ hybridization histochemistry of the primary motor, somatosensory, and anterior parietal areas of the cerebral cortex in macaque monkeys. mRNAs for the alpha 1, beta 2, and gamma 2 subunit polypeptides, which form receptors with the full range of classical properties, are expressed at much higher levels in all areas and show laminar- and sublaminar-specific concentrations. alpha 2, alpha 4, alpha 5, and beta 1 subunit transcripts are expressed at much lower levels but also display individual, laminar-specific concentrations; alpha 5 expression, in particular, is highly expressed in layer IV in the somatosensory and parietal areas and in a layer IV-like band in the motor cortex. In layers in which expression of a particular transcript is high, all neurons may express the gene, but in layers in which expression is moderate, it is possible to detect differences in the degree of labeling of individual neurons for a particular mRNA, and some neurons may not express certain subunit transcripts in detectable amounts. These findings indicate the variability in expression of different GABAA receptor subunits in the cerebral cortex. Laminar differences may indicate the assembly of functional receptors from different arrangements of available subunits in different classes of cells.

Representation of the face and intraoral structures in area 3b of the squirrel monkey (Saimiri sciureus) somatosensory cortex, with special reference to the ipsilateral representation.

Studies of the representation of the trigeminal nerve in the thalamus and cerebral cortex of mammals have revealed representations of both contra- and ipsilateral intraoral structures. However, the relative extent of both representations is subject to considerable species variation. The present study employed microelectrode mapping and anatomical tracing to investigate the location and extent of the ipsilateral representation in area 3b of the somatosensory cortex of squirrel monkeys. A small region, approximately 2 mm2, was found to be responsive to stimulation of ipsilateral intraoral structures. This region was located on the anteromedial border of area 3b, surrounded by the representation of the contralateral roof of the mouth. This region corresponded to areas of intense anterograde labeling following injections placed in the ventromedial portion of the ventral posterior medial nucleus of the thalamus at the only sites where neural responses could be elicited by stimulation of ipsilateral intraoral structures. The amount of thalamus and cortex given over to the ipsilateral representation in the squirrel monkey is small compared with that of the macaque monkey. This difference may be related to the lack of cheek pouches in the squirrel monkey, and therefore a different strategy for eating. The representation of the contralateral lower lip in area 3b was split by the representation of the contralateral upper lip. This split representation is in agreement with previous studies of the trigeminal representation in area 3b of the macaque monkey and may be a general feature of the representation of the trigeminal nerve in area 3b of primate cerebral cortex.

Pharmacokinetics of cefoperazone and tobramycin alone and in combination.

Certain beta-lactams have been shown to increase the clearance of tobramycin. We evaluated the pharmacokinetics of cefoperazone and tobramycin, alone and in combination, in healthy volunteers. No significant alteration in pharmacokinetic behavior was noted for cefoperazone or tobramycin alone or in combination.

Cerebroventricular and intravascular metabolism of [125I]angiotensins in rat.

This study compared the metabolism of [125I]angiotensin II (AII), [125I]angiotensin III (AIII), and [125I]Sar1,Ile8-AII (SI-AII) in the vascular and cerebroventricular compartments. Using HPLC methods to monitor degradation the following t1/2 values were established in the vascular compartment: AII, 12.7 +/- 1.4 s; AIII, 16.3 +/- 0.7 s; and SI-AII, 100.7 +/- 7.3 s. HPLC analysis also revealed that [125I]AII is converted in an obligatory manner to [125I]AIII during its degradation sequence. Cerebrospinal fluid contained no degradative capacity for [125I]AII but exhibited a significant capacity to degrade [125I]AIII. A technique that combined the intra-cerebroventricular injection of [125I]angiotensins followed by focused microwave fixation to stop all peptidase activity was used to determine the half-life of [125I]angiotensins in the ventricular space. Results indicated very rapid metabolism of angiotensins with the following t1/2 values: AII, 23.0 s; and AIII, 7.7 s. This extremely rapid, differential, and sequential metabolism of AII and AIII in two relevant body fluid compartments underscores the need for caution when interpreting data derived from intravascular and intracerebroventricular application of angiotensins. In addition the faster metabolism of AIII than AII in the ventricular space indicates that the actual potency of AIII at central angiotensin receptors is being underestimated.