The effects of phenytoin on the performance of rats in a delayed match-to-place task.

We have shown previously that phenytoin impairs learning in rats in several different behavioral paradigms (Churchill et al., 1998, 2003; Banks et al., 1999). The present study has examined this drug's effects on performance in a delayed match-to-place water maze paradigm developed by Steele and Morris (1999). We find that phenytoin retards performance, but only when the inter-trial interval (ITI) is short (i.e., 15-sec). With longer ITIs (i.e., 20-min, 2-hr), the performance of the phenytoin-treated rats was quite comparable to the controls. We suggest that this pattern of results stems from a disruption of spatial working memory, perhaps due to the effects of the drug on hippocampal function (cf., Churchill et al., 1998, 2003). This disruption is, however, not so profound that consolidation is prevented.

The effects of estradiol on avoidance learning in ovariectomized adult rats.

The present study examined the effects of ovariectomy and subsequent estradiol replacement on learning in young adult rats using a set of instrumental avoidance paradigms differing in the nature and extent of prior experience in the learning context. Thus, one group of animals was placed directly into avoidance learning (AV). A second group was trained on an appetitive task first, and then transferred into the aversive context (AP-AV). The third group was exposed to the training context without any specific appetitive response requirement, and then required to learn an active avoidance response (Context-AV). We found that estradiol (OVX+E) impaired avoidance acquisition in all cases relative ovariectomized controls (OVX). In contrast, while avoidance learning is improved following appetitive training or context exposure in both OVX+E and OVX animals, the OVX+E animals profit to a greater extent from the appetitive or context experience than do the OVX controls. We suggest that this difference may be due to enhanced attentional processes or improved hippocampal processing of contextual factors. Thus, estradiol negatively influences simple associative avoidance learning in ovariectomized rats, but appears to promote positive transfer.

Discrete and contextual cue alterations eliminate the instrumental appetitive-to-aversive transfer impairment in phenytoin-treated rats.

We have shown previously that the antiepileptic phenytoin impairs transfer in an instrumental learning task (Banks et al., 1999). The present study examined the effects of contextual alterations on appetitive-to-aversive transfer performance of rats treated with either phenytoin or tang. Adult rats were tested in tone-signaled appetitive and aversive instrumental tasks, where the animal bar-pressed to obtain a food reward (sugar pellet) or to avoid shock. Rats were trained on the appetitive task for 31 days. Beginning on the twenty-first day, rats were gavaged with either phenytoin or tang twice daily. Animals were then transferred to aversive training, with the phenytoin or tang treatment continuing throughout the 25 testing days. For some animals, contextual changes were introduced as they shifted from appetitive to aversive training, while for other animals these changes were not made. Phenytoin-treated rats that were presented with changes in context as they transferred from the appetitive to the aversive task learned the avoidance response to levels substantially higher than drug-treated rats not presented with the contextual changes. These results indicate that phenytoin impairs avoidance learning following transfer from the appetitive task, and that this impairment can be eliminated by introducing changes in context at the point of transfer. In the tang-treated control subjects, on the other hand, there was no improvement in transfer learning performance associated with the changes in contextual cues. This pattern of results suggests that contextual encoding processes in rats being trained in an instrumental appetitive-to-aversive paradigm are dramatically affected by phenytoin.

Lesions of the rat nucleus basalis magnocellularis disrupt appetitive-to-aversive transfer learning.

Rats with selective lesions of the nucleus basalis magnocellularis (NBM) and sham-lesion control animals were tested in an operant appetitive-to-aversive transfer task. We hypothesized that NBM lesions would not affect performance in the appetitive phase, but that performance would be impaired during subsequent transfer to the aversive phase of the task. Additional groups of NBM lesion and control rats were tested in the avoidance condition only, where we hypothesized that NBM lesions would not disrupt performance. These hypotheses were based on the argument that the NBM is not necessary for simple association learning that does not tax attention. Both the appetitive phase of the transfer task and the avoidance only task depend only on simple associative learning and are argued not to tax attention. Consequently, performance in these tasks was predicted to be spared following NBM lesions. Complex, attention-demanding associative learning, however, is argued to depend on the NBM. Performance in the aversive phase of the transfer task is both attentionally demanding and associatively more complex than in either the appetitive or aversive tasks alone; thus, avoidance performance in the NBM lesion group was predicted to be impaired following transfer from prior appetitive conditioning. Results supported our hypotheses, with the NBM lesion group acquiring the appetitive response normally, but showing impaired performance following transfer to the aversive conditioning phase of the transfer task. Impairments were not attributable to disrupted avoidance learning per se, as avoidance behavior was normal in the NBM lesion group tested in the avoidance condition only.

Acute reductions in GABAA receptor binding in layer IV of adult primate somatosensory cortex after peripheral nerve injury.

Following peripheral nerve transection, reorganizational plasticity has been reported to occur in two phases, one immediate and one more protracted. GABA (gamma-aminobutyric acid) has been implicated in the immediate "unmasking" phase of reorganization. We have used quantitative autoradiography to assess potential changes in GABA(A) and GABA(B) receptor binding in primate somatosensory cortex following peripheral nerve injury. Here we report reductions in GABA(A) receptor binding in layer IV of primate somatosensory cortex deprived of its normal activating inputs for 2-5 h by peripheral nerve transection.

Discrimination reversal conditioning of an eyeblink response is impaired by NMDA receptor blockade.

In the present study we examined the effects of the specific NMDA receptor antagonist CPP on discrimination reversal learning in rabbits. We report two primary findings. First, the institution of NMDA receptor blockade had no effect on a learned discrimination. Second, after stimulus reversal, CPP treatment impaired acquisition of the discrimination reversal. This impairment manifested itself early in training as a retardation in acquisition of a CR to the new CS+ and late in training as an inability to suppress responsiveness to the new CS-. Given the comparability of the present results with previously published results for phenytoin-treated rabbits, we suggest that the effects of phenytoin on learning in this paradigm is at least in part mediated by its effects on NMDA receptors. We further suggest that these findings emphasize the need to better define the role of NMDA receptor activation and hippocampally-mediated circuits in a variety of associative learning paradigms.

Somatotopic reorganization in the brainstem and thalamus following peripheral nerve injury in adult primates.

Injury-induced reorganization of central somatotopic maps is a phenomenon that has proven to be useful for elucidating the mechanisms and time course of neural plasticity. To date, the overwhelming majority of this line of research has focused on such plastic events in cortical areas, at the expense of subcortical structures. In this study, we used multi-unit electrophysiological recording techniques to assess the somatotopic organization of brainstem and thalamic areas following chronic survival from paired median and ulnar nerve section in adult squirrel monkeys. We report that the extent of cutaneously-driven reorganization in both the cuneate nucleus of the brainstem and the ventroposterior lateral nucleus of the thalamus is comparable to that previously documented for area 3b of cortex. These observations are consistent with those previously reported in thalamus, and are unique for brainstem.

The effects of carbamazepine on an appetitive-to-aversive transfer task: comparison to untreated and phenytoin.

1. Concerns over negative consequences resulting from chronic maintenance with antiepileptic medications have led to increased research regarding such impairments, often with disparate results. The authors have previously reported that phenytoin profoundly impairs the ability of adult rats, in comparison to controls. To learn a tone-signaled active avoidance response after learning a tone-signaled appetitive response (Banks et al., 1995; Banks et al., 1999). Such results lend further support to the suggestion that pharmacological treatment itself can produce cognitive difficulties that are comparable to those experienced by epileptic patients (Meador, 1994; Smith et al., 1987). 2. In the present experiments, the authors have continued their investigation of antiepileptic compounds by treating rats with carbamazepine, another commonly prescribed "first-line defense" antiepileptic medication. In comparison to intact animals, carbamazepine-treated rats demonstrate variable deficiencies in the acquisition of the secondarily acquired avoidance response. 3. This result is in agreement with the finding for phenytoin-treated animals, albeit to a lesser degree. Continuing experiments are needed to investigate the relative nature of the deficits produced by such antiepileptic medications, as well as the underlying neurobiological mechanism(s).

Role of NMDA receptors in adult primate cortical somatosensory plasticity.

We have previously shown that most of the reorganization that typically follows median nerve transection in adult squirrel monkeys is dependent on normally functioning N-methyl-D-aspartate (NMDA) receptors. Here, we have evaluated two additional hypotheses: (1) is the immediate "unmasking" found after median nerve transection NMDA receptor-dependent? and (2) are NMDA receptors necessary for both the initiation and maintenance of the second phase of reorganizational changes, or only the former? To address these issues, we implanted osmotic minipumps subcutaneously to deliver an NMDA receptor antagonist (3-((+/-)-2- carboxypiperazin-4-yl)propyl-1-phosphonic acid, CPP) systemically either before examining the immediate effects of median nerve transection, or after reorganization had presumably occurred. For the first set of experiments, NMDA receptor blockade was initiated either 1 or 4 weeks prior to multi-unit mapping in area 3b followed by transection of the median nerve and remapping of the cortex. In the second set of experiments, median nerve transection was followed 4 weeks later by either 1 or 4 weeks of NMDA receptor blockade prior to terminal mapping. We report that the immediate unmasking of new receptive fields after acute nerve injury is not prevented by NMDA receptor blockade; nor are completely reorganized cortical maps dependent upon NMDA receptors for their maintenance. We conclude that the immediate changes in cortical topography are not due to an NMDA receptor-dependent mechanism, but more likely due to release from tonic inhibition. Furthermore, the later phase of reorganization, as for some forms of hippocampal long-term potentiation (LTP), is dependent on normally functioning NMDA receptors for its initiation, but not for its maintenance.

Asymmetric connections, duplicate layers, and a vertically inverted map in the primary visual system.

The achiasmatic mutation is a remarkable and rare visual system mutation carried in a line of black sheepdogs. In affected animals, the optic chiasm is missing, and each retina projects entirely to the ipsilateral hemisphere. As a result of this navigational error, maps of visual space in the lateral geniculate nucleus (LGN) have a unique structure with mirror reversals of field position across the A-A1 border. Animals also have a persistent and severe congenital nystagmus. In this report we analyze a novel variant of the achiasmatic mutation, one in which retinal axons from only one eye successfully cross midline and in which the great majority of fibers from both eyes terminate in a single lateral geniculate nucleus. The dominant optic tract contains four times as many axons as the other tract. The hyperinnervated LGN has a lamination pattern consisting of duplicate and partly interwoven layers. A multiunit mapping study of visual cortex (primarily area 17 along the marginal gyrus) shows that receptive field topography and orientation selectivity are normal. The size of central binocular visual space is nearly normal and is flanked by monocular domains in the periphery. However, there is an inexplicable vertical inversion in the orientation of the cortical representation: superior fields are located rostrally, and inferior fields are located caudally. Despite a host of drastic abnormalities at all level of the visual system, from retina to cortex, this animal was behaviorally indistinguishable from normal dogs and did not have any detectable oculomotor abnormalities.

The effects of phenytoin on instrumental appetitive-to-aversive transfer in rats.

Antiepileptic medications are the primary treatment for seizure conditions. Over the past several years, it has become clear that the medications themselves may contribute to the negative cognitive side effects that people with epilepsy often report. In the experiments reported here, the effects of phenytoin treatment have been evaluated in rats performing an instrumental appetitive-to-aversive transfer task. We find that rats treated with phenytoin fail to acquire the avoidance response when transferred from an appetitive to an aversive context. This deficit is not due to any sensory or motor slowing resulting from the drug, nor is it a deficit that is specific to learning in an aversive context. Rather, we suggest that the deficits shown by phenytoin-treated rats in the appetitive-to-aversive transfer reflect a fundamental inability in altering the associations that were formed during the initial appetitive training.

Phenytoin blocks the reversal of a classically conditioned discriminative eyeblink response in rabbits.

PURPOSE: Cognitive deficits associated with chronic treatment with phenytoin (PHT) have been reported. PHT blocks transfer from a signaled appetitive bar press to an active avoidance response in rats. We investigated the effects of PHT and the prodrug fosphenytoin in rabbits required to learn a discrimination and reversal of a classical eyeblink conditioning paradigm. METHODS: Before drug treatment was started, rabbits were trained to produce a discriminated eyeblink response. PHT (n = 7) was administered centrally or the prodrug fosphenytoin (n = 2) was given systemically. Control animals were similarly treated centrally with either saline (n = 3) or no drug treatment (n = 13). Rabbits were then challenged with a stimulus reversal while being maintained on the respective drug. RESULTS: On the first day of reversal training, control animals typically displayed high response rates to both tones, followed by a reduction in responsiveness to the new conditioned stimulus (CS-) in the ensuing days. In contrast, PHT-treated animals failed to suppress responsiveness to the new CS-. CONCLUSIONS: The response patterns observed are similar to those observed in rabbits with hippocampal ablations, leading us to suggest that the adverse effects of phenytoin may be due to actions in the hippocampus.

Specification of retinogeniculate X and Y axon arbors in cats: fundamental differences in developmental programs.

Monocular enucleation at E36, followed by intracellular labeling of single, physiologically identified X and Y axons, demonstrates fundamental differences in their termination patterns within the lateral geniculate nucleus (LGN). X axons have arbors that appear normal in their dorsoventral extent, though some are located in inappropriate regions of the LGN. Y axons have arbors that are either abnormally tall, spanning the entire extent of the LGN, or of normal height but located in inappropriate regions of the LGN. These termination patterns resemble patterns seen after monocular enucleation at E44, and reinforce the conclusion that X and Y axons differ fundamentally in the cues that constrain the dorsoventral extents of their arbors.

Somatotopic consolidation: a third phase of reorganization after peripheral nerve injury in adult squirrel monkeys.

It has previously been demonstrated that the central somatosensory topographic reorganization within deprived cortex that follows peripheral nerve injury in adult monkeys occurs in at least two stages: an immediate unmasking period; and a more prolonged period where deprived areas of cortex come to express new receptive fields in a topographically arranged manner. In the present experiments, we have compared cortical topography many months after combined median and ulnar nerve transection with "complete" reorganization evident at relatively short (i.e., 2-5 months) survival times. We find further reorganizational changes in cortical topography with longer survival times. That is, the roughly somatotopic, generally multiple-digit receptive fields frequently observed at the shorter survival times are generally sharpened to more distinct, single-digit receptive fields at longer survival times. We hypothesize that the early crudely topographic maps reflect all available inputs while the refined map is the outcome of an extraction process where only the most useful subset of available inputs is expressed. It is further suggested that this distillation process is a use-dependent phenomenon.

Denervation-induced sprouting of intact peripheral afferents into the cuneate nucleus of adult rats.

In adult monkeys with dorsal rhizotomies extending from the second cervical (C2) to the fifth thoracic (T5) vertebrae, cortex deprived of its normal inputs regained responsiveness to inputs conveyed by intact peripheral afferents from the face [T.P. Pons, P.E. Garraghty, A.K. Ommaya, J.H. Kaas, E. Taub, M. Mishkin, Massive reorganization of the primary somatosensory cortex after peripheral sensory deafferentation, Science 252 (1991) 1857-1860]. It has been suggested that the extent of this massive topographic reorganization may be due to the establishment of novel connections between intact afferents and neurons denervated after dorsal rhizotomy [P.E. Garraghty, D.P. Hanes, S.L. Florence, J.H. Kaas, Pattern of peripheral deafferentation predicts reorganizational limits in adult primate somatosensory cortex, Somatosens. Motor Res. 11 (1994) 109-117]. Using adult rats with comparably extensive dorsal rhizotomies, we employed anatomical tracing techniques to address this possibility. Subcutaneous hindpaw injections of horseradish peroxidase conjugated to either wheat germ agglutinin or cholera toxin subunit B revealed aberrant expansions of gracile projections into the cuneate and, in one case, external cuneate nucleus within three months of the deafferentation. It seems plausible that such modest sprouting of ascending projections at the level of the brainstem may form functional connections which, through divergence, ultimately drive a larger population of neurons in cortex. This new growth may well account for both the substantial cortical reorganization observed in the 'Silver Spring monkeys' [T.P. Pons, P.E. Garraghty, A.K. Ommaya, J.H. Kaas, E. Taub, M. Mishkin, Massive reorganization of the primary somatosensory cortex after peripheral sensory deafferentation, Science 252 (1991) 1857-1860] and the 'referred sensation' phenomena (see J.P. Donoghue, Plasticity of adult sensorimotor representations, Curr. Opin. Neurobiol., 5 (1995) 749-754 for review) reported to follow proximal limb amputations in humans.

Cell size in the lateral geniculate nucleus of cats reared with esotropia and sagittal transection of the optic chiasm.

We have attempted to investigate the role of extraretinal influences in controlling visual system development by rearing cats with esotropia in combination with sagittal transection of the optic chiasm. This combination leave two intact A1 laminae, one innervated by the deviated eye and one innervated by the unoperated eye and thus minimizes the contributions of retinally mediated influences. Cell size measurements in the dorsal lateral geniculate nucleus revealed that the neurons in the A1 lamina innervated by the deviated eye were, on average, 10-15% smaller than their counterparts in the contralateral A1 lamina.

Emergence of radial nerve dominance in median nerve cortex after median nerve transection in an adult squirrel monkey.

Throughout the glabrous representation in Area 3b, electrical stimulation of the dominant (median or ulnar) input produces robust, short-latency excitation, evident as a net extracellular "sink" in the Lamina 4 current source density (CSD) accompanied by action potentials. Stimulation of the collocated nondominant (radial nerve) input produces a subtle short-latency response in the Lamina 4 CSD unaccompanied by action potentials and followed by a clear excitatory response 12-15 ms later. Laminar response profiles for both inputs have a "feedforward" pattern, with initial activation in Lamina 4, followed by extragranular laminae. Such corepresentation of nondominant radial nerve inputs with the dominant (median or ulnar nerve) inputs in the glabrous hand surface representation provides a likely mechanism for reorganization after median nerve section in adult primates. To investigate this, we conducted repeated recordings using an implanted linear multi-electrode array straddling the cortical laminae at a site in "median nerve cortex" (i.e., at a site with a cutaneous receptive field on the volar surface of D2 and thus with its dominant afferent input conveyed by the median nerve) in an adult squirrel monkey. We characterized the baseline responses to median, radial, and ulnar nerve stimulation. We then cut the median nerve and semichronically monitored radial nerve, ulnar nerve and median nerve (proximal stump) evoked responses. The radial nerve response in median nerve cortex changed progressively during the weeks after median nerve transection, ultimately assuming the characteristics of the dominant nerve profile. During this time, median, and ulnar nerve profiles displayed little or no change.

NMDA receptors and plasticity in adult primate somatosensory cortex.

Topographic maps in adult primate somatosensory cortex are capable of dramatic reorganizations after peripheral nerve injuries. In the present experiments, we have deprived a circumscribed portion of the hand map in somatosensory cortex of our adult squirrel monkeys by transecting the median nerve to one hand, and evaluated the hypothesis that N-methyl-d-aspartate (NMDA) glutamatergic receptors are necessary for the reorganization that follows within four weeks. In one monkey, we confirm previous results demonstrating that the deprived cortex has regained responsiveness in its expanse four weeks after median nerve transection. However, in three monkeys in which NMDA receptors were concurrently blocked, most of the deprived cortex remained unresponsive. Thus, much of the cortical "recovery" that typically follows peripheral nerve injury in adult monkeys is apparently dependent on NMDA receptors and may well be due to Hebbian-like changes in synaptic strength. Perhaps the elimination of the normally dominant inputs to "median nerve cortex" permits the gradual strengthening of correlations between the activity of the formally impotent presynaptic and deprived postsynaptic elements. These enhanced correlations may also have been made possible by reductions in intracortical inhibition as a necessary but not sufficient condition.

Electrophysiological evidence for overlapping dominant and latent inputs to somatosensory cortex in squirrel monkeys.

1. The pattern of reorganization in area 3b of adult primates after median or ulnar nerve section suggests that somatic afferents from the dorsum of the hand, carried by the radial nerve, have preferential access to the cortical territories normally expressing glabrous inputs carried by the median and ulnar nerves. A likely mechanism underlying preferential access is preexisting, but silent, radial nerve inputs to the glabrous region of cortex. 2. We tested this by comparing the effects of electrical stimulation of median or ulnar versus radial nerves, on responses in the hand representation of area 3b. Laminar current source density and multiunit activity profiles were sampled with the use of linear array multicontact electrodes spanning the laminae of area 3b. Data were obtained from three squirrel monkeys anesthetized during recording. 3. Compared with colocated median or ulnar nerve responses, the radial nerve response had 1) an initial short-latency response in the middle laminae that was subtle; there was a small transmembrane current flow component without a discernable multiunit activity correlate; and 2) a laminar sequence and distribution of activity that was similar to those of the median or ulnar nerve responses (i.e., initial activation of the middle, followed by upper and lower laminae), but the significant current flow and multiunit response to radial nerve stimulation occurs 12-15 ms later. 4. Normal corepresentation of nondominant dorsum hand (radial) inputs with the dominant (median or ulnar) inputs in the glabrous hand surface representation provides a clear vehicle for the biased patterns of reorganization occurring after peripheral nerve section. The initial, "subtle" activity phase in the nondominant response is believed to reflect intracortical inhibition, and the later "significant" response phase, a rebound excitation, possibly compounded by an indirect or extralemniscal input. The spatiotemporal pattern of nondominant input is proposed to play a role in normal somatosensory perception.

Possible use-dependent changes in adult primate somatosensory cortex.

Topographic changes in adult primate somatosensory cortical maps have been reported to follow well-regulated manipulations in the animals' experience. We report briefly here cortical mapping data from one monkey which arrived at our laboratory with a chronic paralytic condition in one hand that resulted in a unique pattern of skin surface stimulation. Isolated receptive fields across the mediolateral extent of cortical area 3b were highly unusual with respect to normal topography, but they were completely consistent with the hypothesis that the correlated activation of peripheral afferents acts to shape expressed cortical receptive fields.