Topiramate promotes neurite outgrowth and recovery of function after nerve injury.

Topiramate is a structurally novel neurotherapeutic agent with a unique combination of pharmacological properties and currently is available in most world markets for treating several seizure disorders. Because its pharmacological profile was suggestive of possible activity as a neuroprotectant, topiramate was evaluated and found to be active in several animal models of stroke or neuropathic pain. This prompted an evaluation of topiramate as a possible neurotrophic agent. In this study, topiramate enhanced the recovery of facial nerve function after injury when administered orally at therapeutically relevant doses, and significantly increased neurite outgrowth in cell cultures derived from fetal rat cortical and hippocampal tissues.

Protease-activated receptor-2 (PAR-2) is present in the rat hippocampus and is associated with neurodegeneration.

Protease-activated receptor-2 (PAR-2) is a seven-transmembrane G protein-coupled receptor that possesses a structure and activation mechanism similar to those of the thrombin receptor. It is activated by low concentrations of trypsin (300 pM) and a synthetic hexapeptide [sequence of serine, leucine, isoleucine, glycine, arginine, leucine (SLIGRL), the rodent PAR-2 "tethered ligand"] representing the first six amino acids following the putative PAR-2 cleavage site. Previous studies have indicated that alpha-thrombin and SFLLRN (synthetic hexapeptide sequence of serine, phenylalanine, leucine, leucine, arginine, asparagine; the human thrombin receptor "tethered ligand") induce neurite retraction and neurotoxicity. Because of the strong similarities between thrombin receptor and PAR-2, we have proposed that PAR-2 may also participate in neurodegeneration. In the present study, we used reverse transcriptase polymerase chain reaction and immunocytochemistry to provide the first evidence that PAR-2 is present in the rat hippocampus. Moreover, we found SLIGRL to be toxic to hippocampal neurons in a concentration-dependent manner (> or = 100 microM). Calcium signaling studies were performed to aid in determining the mechanism by which PAR-2 activation is neurotoxic.

Bacterial alkaloids mitigate seizure-induced hippocampal damage and spatial memory deficits.

Studies of human patients with temporal lobe epilepsy and animal models of epilepsy have established relationships between seizures, excitotoxic hippocampal damage, and memory impairment. We report that bacterial alkaloids, recently shown to mimic actions of neurotrophic factors in cell culture, attenuate seizure-induced damage to hippocampal neurons and memory impairment in adult rats when administered subcutaneously. Intrahippocampal administration of convulsant doses of kainic acid (KA) to adult rats resulted in degeneration of neurons in CA3, CA1, and hilus. Rats administered KA exhibited (24 h later) deficits in performance on both goal latency and probe trial tasks in Morris water maze (MWM) tests of visuospatial memory. Seizure-induced damage to hippocampal neurons was significantly reduced, to varying extents, in rats administered the bacterial alkaloids K252a, K252b, or staurosporine (daily injections of 4 micrograms/kg body weight) prior to KA administration. The KA-induced deficits in MWM goal latency performance were abrogated in rats administered K252a or K252b, and K252a and staurosporine completely prevented seizure-induced impairment on the MWM probe trial. The alkaloids did not suppress electroencephalographic seizure activity, suggesting a dissociation between synchronization of activity and synaptically mediated excitotoxic injury to hippocampal neurons. Each alkaloid caused an increase in levels of protein tyrosine phosphorylation as determined by Western blot analysis of hippocampal tissue. Our data indicate that these bacterial alkaloids have potent antiexcitotoxic activities which may have clinical utility in epilepsy and other disorders that involve excitotoxic damage.

Metyrapone, an inhibitor of glucocorticoid production, reduces brain injury induced by focal and global ischemia and seizures.

Increasing evidence indicates that glucocorticoids (GCs), produced in response to physical/emotional stressors, can exacerbate brain damage resulting from cerebral ischemia and severe seizure activity. However, much of the supporting evidence has come from studies employing nonphysiological paradigms in which adrenalectomized rats were compared with those exposed to constant GC concentrations in the upper physiological range. Cerebral ischemia and seizures can induce considerable GC secretion. We now present data from experiments using metyrapone (an 11-beta-hydroxylase inhibitor of GC production), which demonstrate that the GC stress-response worsens subsequent brain damage induced by ischemia and seizures in rats. Three different paradigms of brain injury were employed: middle cerebral artery occlusion (MCAO) model of focal cerebral ischemia; four-vessel occlusion (4VO) model of transient global forebrain ischemia; and kainic acid (KA)-induced (seizure-mediated) excitotoxic damage to hippocampal CA3 and CA1 neurons. Metyrapone (200 mg/kg body wt) was administered systemically in a single i.p. bolus 30 min prior to each insult. In the MCAO model, metyrapone treatment significantly reduced infarct volume and also preserved cells within the infarct. In the 4VO model, neuronal loss in region CA1 of the hippocampus was significantly reduced in rats administered metyrapone. Seizure-induced damage to hippocampal pyramidal neurons (assessed by cell counts and immunochemical analyses of cytoskeletal alterations) was significantly reduced in rats administered metyrapone. Measurement of plasma levels of corticosterone (the species-typical GC of rats) after each insult showed that metyrapone significantly suppressed the injury-induced rise in levels of circulating corticosterone. These findings indicate that endogenous corticosterone contributes to the basal level of brain injury resulting from cerebral ischemia and excitotoxic seizure activity and suggest that drugs that suppress glucocorticoid production may be effective in reducing brain damage in stroke and epilepsy patients.

Gustatory neural coding in the monkey cortex: mixtures.

1. Psychophysicists have shown that the intensity and quality of a taste stimulus, as perceived by humans, is modified by including that stimulus in a mixture. Gustatory neurons in the primary taste cortex (anterior insula and frontal operculum) of the cynomolgus macaque are involved with the coding of stimulus intensity and quality, and so should reflect the impact of these stimulus interactions. 2. We recorded the activity of 48 neurons in primary taste cortex in response to the oral application of each of the four basic stimuli, their six possible dyads, the four triads, and the tetrad of all four. Stimuli were maintained at a constant intensity in all mixtures by increasing their concentrations as the number of components rose. 3. Glucose was the most effective basic stimulus, followed by quinine HCl, NaCl, and HCl. The mean response to dyads was suppressed by 50% from the sum of responses to the two unmixed components. The response to triads was 62% lower than the sum of responses to their three components, and activity evoked by the tetrad was suppressed by 74% from the sum of all four individual responses. Therefore there was nearly total suppression in the sense that the responses to the mixtures were approximately 1/2, 1/3, and 1/4 the sums of responses to two, three, and four components, respectively. 4. Neurons could be divided into four subtypes: those that responded best to each of the basic stimuli. All subtypes except HCl cells were about equally suppressed when their preferred stimulus was included in a mixture. HCl was a particularly ineffective stimulus, such that this subtype responded poorly and so was less susceptible to mixture suppression. 5. Taste quality, as indexed by correlation coefficients among profiles of activity, was quite predictable for dyads. If the mixture included HCl, the profile it generated correlated poorly (about +0.20) with that of HCl and rather well (about +0.60) with that of the other component. If HCl was not included, the mixture's profile correlated about +0.40 with that of each component. 6. The profile generated by the mixture of three stimuli was predictable only if one of the components was HCl. In that case, the triad elicited a profile midway between those of the other two components, i.e., the contribution of HCl was largely ignored. When HCl was not involved, or when all four basic stimuli were combined, the resulting profiles were poorly correlated with those of all basic stimuli. 7. The contribution made by each basic taste to human perception and to the macaque's neurophysiological response was compared for all mixtures. The contribution was often quite similar for human and macaque, but when differences occurred, they were typically due to lower activity from HCl cells in the macaque, a loss that was replaced mainly by larger responses from glucose neurons. 8. The magnitude of responses to mixtures in the macaque taste cortex matches well with expectations from human psychophysical studies. The presumed quality of the response to mixtures is also similar, except that HCl is less effective in monkeys and sugars more so.

Brain injury and tumor necrosis factors induce calbindin D-28k in astrocytes: evidence for a cytoprotective response.

Calbindin is a 28 kDa calcium-binding protein expressed in restricted neuronal populations in the mammalian brain where it may play a role in protecting neurons against excitotoxic insults. Recent findings indicate that electrical activity and some neurotrophic factors can induce the expression of calbindin in neurons. We now report that brain injury, effected by systemic administration of the excitotoxin kainate or mechanical trauma, induces expression of calbindin in cells of the corpus callosum and subcortical white matter. Immunohistochemical analysis using antibodies to the astrocyte-specific proteins (glial fibrillary acidic protein and S-100 beta) established the identity of calbindin immunoreactive cells as astrocytes. Because brain injury is known to induce the expression of several neurotrophic factors and cytokines, we employed cultures of hippocampal and neocortical astrocytes to test the hypothesis that such factors can induce expression of calbindin in astrocytes. Tumor necrosis factors (TNF alpha and TNF beta), cytokines that are expressed in response to brain injury, induced the expression of calbindin in cultured rat hippocampal and neocortical astrocytes. Two neurotrophic factors, basic fibroblast growth factor and nerve growth factor, did not induce calbindin in astrocytes. TNF-treated, calbindin-expressing astrocytes were resistant to acidosis and calcium ionophore toxicity, suggesting that TNFs and calbindin may serve a cytoprotective role in astrocytes in the injured brain.

Opposing actions of thrombin and protease nexin-1 on amyloid beta-peptide toxicity and on accumulation of peroxides and calcium in hippocampal neurons.

Amyloid beta-peptide (A beta) is the principal component of neuritic plaques in the brain in Alzheimer's disease (AD). Recent studies revealed that A beta can be neurotoxic by a mechanism involving free radical production and loss of cellular ion homeostasis, thus implicating A beta as a key factor in the pathogenesis of AD. However, other proteins are present in plaques in AD, including the protease thrombin and protease nexin-1 (PN1), a thrombin inhibitor. We therefore tested the hypothesis that thrombin and PN1 modify neuronal vulnerability to A beta toxicity. In dissociated rat hippocampal cell cultures the toxicity of A beta was significantly enhanced by coincubation with thrombin, whereas PN1 protected neurons against A beta toxicity. A beta induced an increase in levels of intracellular peroxides and calcium. Thrombin enhanced, and PN1 attenuated, the accumulation of peroxides and calcium induced by A beta. Taken together, these data demonstrate that thrombin and PN1 have opposing effects on neuronal vulnerability to A beta and suggest that thrombin and PN1 play roles in the pathogenesis of neuronal injury in AD.

Protease nexin-1 and thrombin modulate neuronal Ca2+ homeostasis and sensitivity to glucose deprivation-induced injury.

Protease nexin-I (PN-1) is a 44 kDa serine proteinase inhibitor that rapidly inhibits thrombin by forming SDS stable complexes with serine at the catalytic site of the protease. Levels of both PN-1 and thrombin are increased in the brain in response to insults such as ischemia, suggesting roles in neural injury and repair processes. We now report that PN-1-protected cultured rat hippocampal neurons against glucose deprivation- induced damage (GDID), and the protection was abolished by equimolar thrombin. PN-1 reduced resting intracellular free calcium levels ([Ca2+]i) and attenuated the elevation of [Ca2+]i normally associated with GDID. Thrombin reduced neuronal survival and caused a significant increase in [Ca2+]i. Submaximally toxic levels of thrombin exacerbated GDID. Calcium responses to thrombin were attenuated in neurons contacting PN-1 immunoreactive astrocytes. These findings suggest that PN-1 and thrombin play important roles in modulating neuronal calcium responses, and vulnerability, to metabolic/excitotoxic insults.

Gustatory neural coding in the monkey cortex: acid stimuli.

1. We sought to define the gustatory neural code for acidic stimuli. Therefore we analyzed the responses of 44 single neurons in the insular cortex of four alert cynomolgus macaques in response to the oral application of four basic taste stimuli (glucose, NaCl, HCl, and quinine HCl) and fruit juice, and to a series of 20 additional acids. 2. Neurons responsive to gustatory stimulation were encountered within a volume of 38.2 mm3 (3.5 mm anteroposterior x 2.1 mm mediolateral x 5.2 mm dorsoventral). Taste cells constituted 81 (5.2%) of the 1,552 neurons whose sensitivities were tested. Of these, the activity of 44 was followed through at least one complete application of the stimulus series, and those responses compose the data of this study. Nongustatory cells included those responsive to mouth movements (36.3%), tactile stimulation within the mouth (2.1%), visual approach of the taste stimulus (1.4%), and extension of the tongue (0.1%). The functions of the remaining 54.8% were not determined. 3. The mean spontaneous discharge rate of these cortical taste cells was 3.0 spikes/s (range 0.0-14.4 spikes/s). The mean breadth of tuning coefficient was a moderate 0.72 (range 0.26-0.98). Most evoked activity was excitatory, although inhibition was a prominent response option for four (9%) taste cells. 4. There was no evidence that taste cells with similar functional characteristics were clustered within the cortex, i.e., there was no apparent topographic organization of taste quality. 5. Thirty-four of the 44 cells were divisible into three functional types on the basis of their response profiles to the four basic stimuli used here.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence that actin depolymerization protects hippocampal neurons against excitotoxicity by stabilizing [Ca2+]i.

Calcium influx through glutamate receptors and voltage-dependent channels mediates an array of functional and structural responses in neurons. However, unrestrained Ca2+ influx can injure and kill neurons; a mechanism implicated in both acute and chronic neurodegenerative disorders. Data reported here indicate that depolymerization of actin filaments can stabilize intracellular free calcium levels ([Ca2+]i) and protect hippocampal neurons against excitotoxic injury. Studies with fluorescein-labeled phalloidin showed that cytochalasin D and glutamate each induced actin filament depolymerization. The microfilament-disrupting agent cytochalasin D protected cultured rat hippocampal neurons against glutamate toxicity, whereas the actin filament-stabilizing agent jasplakinolide potentiated glutamate toxicity. The microtubule-disrupting agent colchicine was ineffective in protecting neurons against glutamate toxicity. Cytochalasin D did not protect neurons against calcium ionophore toxicity or iron toxicity, indicating that its actions were not due to nonspecific effects on Ca2+ or free radical metabolism. Cytochalasin D markedly attenuated kainate-induced damage to hippocampus of adult rats, suggesting an excitoprotective role for actin depolymerization in vivo. Elevations of [Ca2+]i induced by glutamate were attenuated in cultured hippocampal neurons pretreated with cytochalasin D and potentiated in neurons pretreated with jasplakinolide. The [Ca2+]i response to a Ca2+ ionophore was unaffected by cytochalasin D, suggesting that actin depolymerization reduced Ca2+ influx through membrane channels. Taken together with previous patch clamp data, our findings suggest that depolymerization of actin in response to Ca2+ influx may serve as a feedback mechanism to attenuate potentially toxic levels of Ca2+ influx.

Secreted forms of beta-amyloid precursor protein protect against ischemic brain injury.

The beta-amyloid precursor protein (beta APP) is the source of the amyloid beta-peptide that accumulates in the brain in Alzheimer's disease. A major processing pathway for beta APP involves an enzymatic cleavage within the amyloid beta-peptide sequence that liberates secreted forms of beta APP (APPss) into the extracellular milieu. We now report that postischemic administration of these APPss intracerebroventricularly protects neurons in the CA1 region of rat hippocampus against ischemic injury. Treatment with APPs695 or APPs751 resulted in increased neuronal survival, and the surviving cells were functional as demonstrated by their ability to synthesize protein. These data provide direct evidence for a neuroprotective action of APPss in vivo.

Gustatory neural coding in the monkey cortex: the quality of saltiness.

1. We analyzed the activity of single neurons in the insularopercular cortex of four alert cynomolgus monkeys in response to the oral application of four basic taste stimuli (glucose, NaCl, HCl, and quinine HCl) and fruit juice and to a range of 17 sodium and lithium salts with a variety of anions. 2. Neurons responsive to gustatory stimulation were encountered in an area of 34.5 mm3 (2.0 mm A-P x 2.5 mm M-L x 6.9 mm D-V). Taste cells composed 46 (4.5%) of the 1,028 neurons whose sensitivities were tested. Nongustatory cells included those responsive to mouth movements (20.4%), tactile stimulation in the mouth (3.4%), and visual approach of the stimulus (0.7%). The functions of the remaining 71.0% could not be determined. 3. The mean spontaneous discharge rate of these cortical taste cells was 4.4 spikes/s (range, 0.1-23.8 spikes/s). The mean breadth-of-tuning coefficient was a moderate 0.72 (range, 0.15-1.00). Inhibitory responses were nearly nonexistent. 4. There was no evidence that taste cells with similar functional characteristics were clustered within the cortex, i.e., there was no apparent topographic organization of taste qualities. 5. The 46 taste cells were divisible into three functional types, based on their response profiles to the four basic stimuli used here. The types could be characterized as sweet-, salt-, and quinine-oriented. 6. A taste space was generated from correlations among the response profiles evoked by the stimulus array. The 17 salts formed a coherent group from which the other basic stimuli were separated. Glucose was closest to the salt group, followed by quinine and HCl. 7. Within the salt group, the four halides (NaCl, LiCl, NaBr, LiBr) formed a tight cluster; the 11 stimuli with acetate, citrate, phosphate, sulfate, and tartrate anions joined with monosodium glutamate and Na bicarbonate to form two closely related clusters; Na succinate was somewhat distinct from the others, and Na carbonate was most separate. 8. The relative qualities of the salts did not relate systematically to anionic size, promotion of sodium transport, or molar conductivity. 9. The configuration of stimuli in this taste space was compared with that in a space derived from human descriptions of the relative similarities of many of these same stimuli. Using the position of NaCl as a reference, the distances to all other stimuli common to the two studies was measured.(ABSTRACT TRUNCATED AT 400 WORDS)

Glutamate, beta-amyloid precursor proteins, and calcium mediated neurofibrillary degeneration.

In this article we present evidence supporting the interaction between excitotoxicity, beta APP mismetabolism, metabolic compromise and intracellular calcium destabilization in the process of neurodegeneration associated with Alzheimer's disease (AD). AD is characterized by the presence of neurofibrillary tangles and amyloid-containing plaques in specific regions of the brain. There appear to be several processes which contribute to the neurodegeneration associated with AD. Although AD has been linked to genetic mutations on chromosomes 21, 19 and 14, there are sporadic forms of AD that have no known genetic mutation involved. Aging is the major risk factor for AD. During the course of normal aging several metabolic compromises may occur in the brain. Both decreased glucose transport and utilization, and increased glucocorticoid levels are known to occur with aging and may lead to decreased energy supplies, ATP depletion, failure of Ca2+ buffering systems, excess glutamate release and activation of glutamate receptors. In addition, a reduction in antioxidant enzymes and consequently an increase in free radicals has also been associated with aging. Each of the preceeding alterations would lead to an increase in neuronal [Ca2+]i. Elevated calcium could then activate calcium-dependent proteases which degrade particular cytoskeletal proteins, and lipases which generate free radicals resulting in membrane damage and possible cell death. In this article we provide evidence that amyloid beta-peptide (A beta), the substance which accumulates in AD plaques, exacerbates excitotoxic and metabolic compromises to neurons resulting in changes in the cytoskeleton which resemble those seen in the neurofibrillary tangles of AD. We also provide evidence that secreted forms of beta-amyloid precursor protein (beta APP) are neuroprotective against excitotoxic insults. Recent findings concerning the normal function of beta APP and the mechanism of A beta toxicity place beta APP at the center of changes leading to neuronal degeneration in AD.

beta-Amyloid precursor protein metabolites and loss of neuronal Ca2+ homeostasis in Alzheimer's disease.

Recent findings link altered processing of beta-amyloid precursor protein (beta APP) to disruption of neuronal Ca2+ homeostasis and an excitotoxic mechanism of cell death in Alzheimer's disease. A major pathway of beta APP metabolism results in the release of secreted forms of beta APP, APPss. These secreted forms are released in response to electrical activity and can modulate neuronal responses to glutamate, suggesting roles in developmental and synaptic plasticity. beta APP is upregulated in response to neural injury and APPss can protect neurons against excitotoxic or ischemic insults by stabilizing the intracellular Ca2+ concentration [Ca2+]i. An alternative beta APP processing pathway liberates intact beta-amyloid peptide, which can form aggregates that disrupt Ca2+ homeostasis and render neurons vulnerable to metabolic or excitotoxic insults. Genetic abnormalities (e.g. certain beta APP mutations or Down syndrome) and age-related changes in brain metabolism (e.g. reduced energy availability or increased oxidative stress) may favor accumulation of [Ca2+]i-destabilizing beta-amyloid peptide and diminish the release of [Ca2+]i-stabilizing, neuroprotective APPss.

beta-Amyloid precursor protein mismetabolism and loss of calcium homeostasis in Alzheimer's disease.

The suspected involvement of the beta-amyloid precursor protein (beta APP) in the etiology of Alzheimer's disease (AD) has been strengthened by recent genetic evidence, but pursuit of the mechanisms involved will initially require basic cell biology approaches. Several studies have concentrated on toxic activities of beta-amyloid peptide (beta AP) itself, illuminating its contributions to excitotoxicity and calcium-mediated degeneration in general. We now know that generation of beta AP from beta APP also compromises the production of an important set of trophic factors: the secreted forms of beta APP (APPS), which may act--ironically--by conferring protection from calcium-mediated insults. Therefore, conditions which contribute to the formation of beta AP (possibly including ischemia) not only produce an agent which exacerbates calcium-mediated cell death, but also reduce the levels of one of the few factors able to rescue calcium homeostasis. The implications of these postulates and their relationship to the process of aging are discussed.

Altered calcium signaling and neuronal injury: stroke and Alzheimer's disease as examples.

Several cellular signaling systems have been implicated in the neuronal death that occurs both in development ("natural" cell death) or in pathological conditions such as stroke and Alzheimer's disease (AD). Here we consider the possibility that neuronal degeneration in an array of disorders including stroke and AD arises from one or more alterations in calcium-regulating systems that result in a loss of cellular calcium homeostasis. A long-standing hypothesis of neuronal injury, the excitatory amino acid (EAA) hypothesis, is revisited in light of new supportive data concerning the roles of EAAs in stroke and the neurofibrillary degeneration in AD. Two quite new concepts concerning mechanisms of neuronal injury and death are presented, namely: 1) growth factors normally "stabilize" intracellular free calcium levels ([Ca2+]i) and protect neurons against ischemic/excitotoxic injury, and 2) aberrant processing of beta-amyloid precursor protein (APP) can cause neurodegeneration by impairing a neuroprotective function of secreted forms of APP (APPs) which normally regulate [Ca2+]i. Altered APP processing also results in the accumulation of beta-amyloid peptide which contributes to neuronal damage by destabilizing calcium homeostasis; in AD beta-amyloid peptide may render neurons vulnerable to excitotoxic conditions that accrue with increasing age (e.g., altered glucose metabolism, ischemia). Growth factors may normally protect neurons against the potentially damaging effects of calcium influx resulting from energy deprivation and overexcitation. For example, bFGF, NGF and IGFs can protect neurons from several brain regions against excitotoxic/ischemic insults. Growth factors apparently stabilize [Ca2+]i by several means including: a reduction in calcium influx; enhanced calcium extrusion or buffering; and maintenance or improvement of mitochondrial function. For example, bFGF can suppress the expression of a N-methyl-D-aspartate (NMDA) receptor protein that mediates excitotoxic damage in hippocampal neurons. Growth factors may also prevent the loss of neuronal calcium homeostasis and the increased vulnerability to neuronal injury caused by beta-amyloid peptide. Since elevated [Ca2+]i can elicit cytoskeletal alterations similar to those seen in AD neurofibrillary tangles, we propose that neuronal damage in AD results from a loss of calcium homeostasis. The data indicate that a variety of alterations in [Ca2+]i regulation may contribute to the neuronal damage in stroke and AD, and suggest possible means of preventing neuronal damage in these disorders.

Gustatory neural coding in the monkey cortex: the quality of sweetness.

1. We analyzed the activity of single neurons in gustatory cortex of alert cynomolgus monkeys in response to the four basic taste stimuli and to a range of chemicals, all of which are predominantly sweet to humans. 2. We recorded taste-evoked responses from a cortical area that measured 4.0 mm in its anteroposterior extent, 5.6 mm dorsoventrally and 2.2 mm mediolaterally. Taste-responsive neurons constituted 4.7% of the 3,066 neurons tested in the course of 66 recording tracks. Nongustatory cells included those responsive to mouth movement (34.1%), tongue touch (1.9%), stimulus approach (0.7%), and tongue extension (0.5%). The functions of 58.2% of the cells we isolated could not be determined. 3. The mean breadth of tuning of these cortical taste neurons was a moderate 0.59 (range 0.00-0.93). 4. There was no evidence that taste cells with similar functional attributes were clustered in the cortex, i.e., there was no apparent topographic organization of taste qualities. 5. A taste space was generated from the correlations among patterns of neural activity evoked by the stimulus array. Within the space, NaCl was most isolated from other stimuli; the profiles elicited by HCl, quinine HCl, and water were all moderately intercorrelated and were clearly distinct from the cluster of sweet stimuli. 6. The 19 sweet chemicals formed a coherent cluster centered on the simple carbohydrates (glucose, fructose, sucrose, maltose) and sorbitol. Nearest this core were calcium cyclamate, aspartame, and cran-raspberry juice. In the next concentric ring were acesulfame potassium, xylose, xylitol, sorbose, polycose, and myoinositol. Increasingly distant from the sugars were sodium saccharin, stevioside, neohesperidin DHC, L-tryptophan and monellin. 7. We compared these results with those of a human psychophysical study of sweet stimuli. Using the position of glucose as a reference, we measured the distances to all other stimuli that were common to the two studies (n = 15). The correlation between the human psychophysical data and those derived from evoked activity in the macaque cortex was +0.82. 8. The high correlation between human psychophysical and macaque electrophysiological data implies that the subtle distinctions among stimuli that are predominantly sweet are quite similar for these two species and reinforces the value of this neural model for human taste perception.

Gustatory neural coding in the monkey cortex: L-amino acids.

1. Single-neuron activity in the primary gustatory cortex of the alert cynomolgus monkey (Macaca fascicularis) was analyzed in response to a range of taste stimuli. Tastants included the four prototypical stimuli (glucose, NaCl, HCl, and quinine), fruit juice, and 12 amino acids selected for their chemical characteristics, nutritional significance, and biological importance, as well as for the availability of human psychophysical data on their perceived qualities. 2. Taste-evoked responses could be recorded from a cortical area that measured 3.5 mm in its anteroposterior extent, 2.0 mm mediolaterally, and 6.0 mm dorsoventrally. Gustatory cells constituted 4.8% of the 1,129 neurons tested. Nongustatory cells gave responses associated with mouth movements (11.1%), somatosensory stimulation (3.8%), approach or anticipation of the taste stimulus (2.2%), and tongue extension (0.4%). 3. The most effective taste stimuli were those with qualities that humans describe as salty or sweet: NaCl, monosodium glutamate, glucose, proline, glycine, and fruit juice. The least effective tastants were those rated bitter or insipid: tyrosine, tryptophan, phenylalanine, and leucine. Accordingly, 79% of the gustatory neurons responded best to glucose (46%) or NaCl (33%) among the basic stimuli; only 19% responded best to quinine (13%) or HCl (6%). One cell (2%) responded exclusively to fruit juice. 4. Cortical gustatory neurons showed a moderate breadth of sensitivity, with a mean breadth of tuning coefficient of 0.71 across 54 cells. There was no evidence of chemotopic organization in the taste cortex. 5. The taste quality of each stimulus was inferred from the relative similarity of the profiles they evoked. The clearest distinction among stimuli was between those that humans characterize as sweet versus those with other qualities. Several amino acids that have dominant sweet (glycine and proline), salty (arginine and monosodium glutamate), sour (tryptophan), or bitter (phenylalanine) components to humans evoked activity profiles that were associated with those of the appropriate prototypical stimuli. Others (cysteine and lysine) were not closely related to any single prototype. 6. Conclusions based on the responses of cortical cells in the monkey are in close agreement with those that derive from human psychophysical studies of L-amino acids, reinforcing the value of this neural model for human taste perception.

Gustatory neural coding in the monkey cortex: stimulus quality.

1. Extracellular action potentials were recorded from 50 single neurons in the insular-opercular cortex of two alert cynomolgus monkeys during gustatory stimulation of the tongue and palate. 2. Sixteen stimuli, including salts, sugars, acids, alkaloids, monosodium glutamate, and aspartame, were chosen to represent a wide range of taste qualities. Concentrations were selected to elicit a moderate gustatory response, as determined by reference to previous electrophysiological data or to the human psychophysical literature. 3. The cortical region over which taste-evoked activity could be recorded included the frontal operculum and anterior insula, an area of approximately 75 mm3. Taste-responsive cells constituted 50 (2.7%) of the 1,863 neurons tested. Nongustatory cells responded to mouth movement (20.7%), somatosensory stimulation of the tongue (9.6%), stimulus approach or anticipation (1.7%), and tongue extension (0.6%). The sensitivities of 64.6% of these cortical neurons could not be identified by our stimulation techniques. 4. Taste cells had low spontaneous activity levels (3.7 +/- 3.0 spikes/s, mean +/- SD) and showed little inhibition. They were moderately broadly tuned, with a mean entropy coefficient of 0.76 +/- 0.17. Excitatory responses were typically not robust. 5. Hierarchical cluster analysis was used to determine whether neurons could be divided into discrete types, as defined by their response profiles to the entire stimulus array. There was an apparent division of response profiles into four general categories, with primary sensitivities to sodium (n = 18), glucose (n = 15), quinine (n = 12), and acid (n = 5). However, these categories were not statistically independent. Therefore the notion of functionally distinct neuron types was not supported by an analysis of the distribution of response profiles. It was the case, however, that neurons in the sodium category could be distinguished from other neurons by their relative specificity. 6. The similarity among the taste qualities represented by this stimulus array was assessed by calculating correlations between the activity profiles they elicited from these 50 neurons. The results generally confirmed expectations derived from human psychophysical studies. In a multidimensional representation of stimulus similarity, there were groups that contained acids, sodium salts, and chemicals that humans label bitter and sweet. 7. The small proportion of insular-opercular neurons that are taste sensitive and the low discharge rates that taste stimuli are able to evoke from them suggest a wider role for this cortical area than just gustatory coding.(ABSTRACT TRUNCATED AT 400 WORDS)