Tuning out the noise: limbic-auditory interactions in tinnitus

Tinnitus, the most common auditory disorder, affects about 40 million people in the United States alone, and its incidence is rising due to an aging population and increasing noise exposure. Although several approaches for the alleviation of tinnitus exist, there is as of yet no cure. The present article proposes a testable model for tinnitus that is grounded in recent findings from human imaging and focuses on brain areas in cortex, thalamus, and ventral striatum. Limbic and auditory brain areas are thought to interact at the thalamic level. While a tinnitus signal originates from lesion-induced plasticity of the auditory pathways, it can be tuned out by feedback connections from limbic regions, which block the tinnitus signal from reaching auditory cortex. If the limbic regions are compromised, this "noise-cancellation" mechanism breaks down, and chronic tinnitus results. Hopefully, this model will ultimately enable the development of effective treatment.

Neuroanatomical localization of an internal clock: A functional link between mesolimbic, nigrostriatal, and mesocortical dopaminergic systems

The effects of selective dopamine (DA) depleting lesions with 6-hydroxydopamine microinjection into the SN, CPu, and NAS, as well as radiofrequency lesions of the CPu on the performance characteristics of rats trained on a single-valued 20-s peak-interval (PI) timing procedure or a double-valued 10-s and 60-s PI procedure were evaluated. A double dissociation in the performance of duration discriminations was found. Rats with CPu lesions were unable to exhibit temporal control of their behavior suggesting complete insensitivity to signal duration but were able to show discrimination of the relative reward value of a signal by differentially modifying their response rates appropriately. In contrast, rats with NAS lesions were able to exhibit temporal control of their behavior by differentially modifying their response rates as a function of signal duration(s), suggesting no impairment of sensitivity to signal duration, but were unable to show discrimination of the relative reward value of a signal.

Cholesterol synthesis and lipoprotein reuptake during synaptic remodelling in hippocampus in adult rats

Apolipoprotein E is synthesized and secreted by astrocytes in the hippocampus following lesions of the entorhinal cortex. It was proposed that apolipoprotein E, by analogy to its role in cholesterol transport in circulation, could be involved in the salvage and reutilization of non-esterified cholesterol released during terminal breakdown. The salvaged cholesterol could then be transported to neurons by apolipoprotein E-complexes and taken up via the apolipoprotein E/apolipoprotein B (low-density lipoprotein) receptor. To test this hypothesis, we have examined low-density lipoprotein receptor binding in brain sections of rats undergoing hippocampal reinnervation. The number of neuronal cells labelled by fluorescent Dil-low-density lipoprotein as well as the density of [125I]low-density lipoprotein binding sites in the dentate gyrus were found to increase in parallel with the extent of cholinergic reinnervation occurring in the deafferented hippocampus. In contrast, hippocampal cholesterol synthesis fell by more than 60% at eight days post-lesion, but eventually returned to control levels at 30 days post-lesion. The transient loss of cholesterol synthesis coincided with a peak in hippocampal apolipoprotein E expression. A concomitant accumulation of sudanophilic lipids (cholesterol esters and phospholipids) was detected in the outer molecular layer of the dentate gyrus and in the hilar region. The present findings suggest that non-esterified cholesterol released during terminal breakdown is esterified, transported via the apolipoprotein E transport system to neurons undergoing reinnervation, and take-up through the low-density lipoprotein receptor pathway where it is presumably used as a precursor molecule for the synthesis of new synapses and terminals.

Proliferative and migratory activity of glial cells in the partially deafferented hippocampus

The proliferative response of the glial cell population of the adult rat hippocampus deafferented by unilateral lesion of the entorhinal cortex was studied using 3H-thymidine autoradiography. Two experimental paradigms were used, involving: (1) intraventricular 3H-thymidine injection at a number of post-lesion intervals with sacrifice six hours later and (2) intraventricular injection at 30 hours post-lesion with sacrifice at 6, 96, or 192 hours later. The first increase in the number of labeled glial cells was obtained at 20 hours post-lesion and was confined to areas of degenerating axons. By 30 hours a large and uniformly dense proliferative response was observed throughout the ipsilateral, and medial aspects of the contralateral, hippocampus encompassing both deafferented and intact regions. Cell division continued through 50 and 65 hours post-lesion particularly in directly deafferented regions, but diminished to control levels by 80 hours. Although oligodendroglia and astrocyte-like cells were sometimes found to have incorporated the label the most common proliferative element within the hippocampus corresponded to previous light microscopic descriptions of "microglial" cells. The experiments using thymidine injection given at the peak proliferative period followed by survival periods of varying lengths indicated that a progressive redistribution of labeled nuclei occurred resulting in an accumulation of labeled cells in the zones of deafferentation. Multiple division of cells within these areas as well as the migration of nuclei from non-deafferented regions was found to contribute to this effect. The possible involvement of glial proliferation with other morphological effects of deafferentation, including the sprouting response of intact afferents, is discussed.

Impaired object recognition with increasing levels of feature ambiguity in rats with perirhinal cortex lesions

It has been proposed that the perirhinal cortex is involved in the representation of the characteristics of objects. In particular it has been proposed that it is critical for discriminating between stimuli which have some features in common and thus it has been described as being involved in resolving feature ambiguity. The present experiments demonstrate that lesions of perirhinal cortex in the rat cause impairments in object recognition which increase with the level of feature ambiguity present in the discrimination. Although increasing feature ambiguity increases the overall difficulty of discriminations, lesions of the perirhinal cortex resulted in a disproportionate impairment when feature ambiguity was increased and not when the difficulty of the discrimination was increased through enlargement of the stimulus set. The present experiments therefore support the view that perirhinal cortex in the rat is critical to resolution of feature ambiguity in stimulus specification.

Comparison of mania and depression after brain injury: causal factors

Patients who developed secondary mania after brain injury (N = 17) had a significantly greater frequency of injury to right hemisphere areas connected with the limbic system than poststroke patients with major depression (N = 31), who had injury primarily in the left frontal cortex and basal ganglia. For patients without mood disturbance after brain injury (N = 28), the location of the lesion was not significant. Secondary mania patients also had a significantly greater frequency of family history of affective disorder than did the other two groups. These results suggest that an interaction between injury to certain areas of the right hemisphere and genetic factors or other neuropathological conditions produces secondary mania.

Selective impairment of reasoning about social exchange in a patient with bilateral limbic system damage

Social exchange is a pervasive feature of human social life. Models in evolutionary biology predict that for social exchange to evolve in a species, individuals must be able to detect cheaters (nonreciprocators). Previous research suggests that humans have a cognitive mechanism specialized for detecting cheaters. Here we provide neurological evidence indicating that social exchange reasoning can be selectively impaired while reasoning about other domains is left intact. The patient, R.M., had extensive bilateral limbic system damage, affecting orbitofrontal cortex, temporal pole, and amygdala. We compared his performance on two types of reasoning problem that were closely matched in form and equally difficult for control subjects: social contract rules (of the form, "If you take the benefit, then you must satisfy the requirement") and precaution rules (of the form, "If you engage in hazardous activity X, then you must take precaution Y"). R.M. performed significantly worse in social contract reasoning than in precaution reasoning, when compared both with normal controls and with other brain-damaged subjects. This dissociation in reasoning performance provides evidence that reasoning about social exchange is a specialized and separable component of human social intelligence, and is consistent with other research indicating that the brain processes information about the social world differently from other types of information.

Dissociable effects of anterior and posterior cingulate cortex lesions on the acquisition of a conditional visual discrimination: facilitation of early learning vs. impairment of late learning

Two experiments investigated the effects of quinolinic acid induced lesions of the anterior and posterior cingulate cortices on the acquisition and performance of a conditional visual discrimination (CVD) task, in which rats were required to learn a rule of the type: "If lights are flashing FAST, press the right lever; if SLOW press left". In Experiment 1, animals with lesions of the anterior cingulate cortex (ANT group) demonstrated a significant enhancement in learning during the early stages of task acquisition. Conversely, animals with lesions of the posterior cingulate cortex (POS group) were impaired in learning during the later stages of acquisition. There were no significant differences between the ANT and POS groups on the performance of the task when either variable inter-trial intervals or reduced stimulus durations were imposed. In Experiment 2, the specificity of the lesion effects for processes operative during the early and late stages of learning was tested. Animals were trained to a criterion of 70% correct choices on two consecutive sessions prior to lesioning, and subsequently allowed to continue to acquire the task to the mean asymptotic performance level of 85% correct choices on two consecutive sessions. Animals of the POS group were impaired in learning during this later stage of task acquisition, thus replicating the pattern of results obtained in Experiment 1. The animals in Experiment 2 were then tested following a 30-day retention interval and during extinction (removal of sucrose from the magazine). The extinction test revealed an impairment in the ability of animals in the ANT group to omit lever responses in the absence of reinforcement. These results indicate that the anterior and posterior cingulate cortices are functionally dissociable, and suggest that they may form part of complementary, but competing, learning and memory systems.

The role of the rat prelimbic/infralimbic cortex in working memory: Not involved in the short-term maintenance but in monitoring and processing functions

Contrary to human and primate, working memory in the rodent is usually considered as a simple short term memory buffer and mainly investigated using delayed response paradigms. The aim of the present study was to further investigate the role of the rat prelimbic/infralimbic cortex in different spatial delayed tasks in order to dissociate its involvement in temporary storage from other information processes, such as behavioral flexibility and attention. In experiment 1 rats were trained in a standard elimination win-shift task in a radial-arm maze after which a 1-min delay was inserted mid trial. Prelimbic/infralimbic lesions induced only a transient disruption of performance following introduction of the delay. In experiment 2, rats were trained directly in a win-shift task with a 5-min delay that was subsequently extended to 30 min. Prelimbic/infralimbic lesions did not significantly affect behavior. Nevertheless, transient disruptions of performance (correlated with lesion extent) were noted repeatedly in lesioned rats when sets of interfering events were presented. The present findings indicate that prelimbic/infralimbic cortex is not directly involved in the short term maintenance of specific information but is implicated when changes, such as sudden introduction of a delay or exposure to unexpected interfering events, alter the initial situation. It appears that working memory in rodents should be considered, as in humans and primates, to encompass both storage and monitoring functions. The present results along with previous ones strongly suggest that prelimbic/infralimbic cortex is not involved in the temporary on-line storage but rather in the control of information required to prospectively organize the ongoing action.

Effects of ganglioside treatments on lesion-induced behavioral impairments and sprouting in the CNS

Recent findings suggest that exogenous gangliosides improve recovery of a learned behavior (alternation in a T-maze) which is thought to be related to sprouting after lesions of the entorhinal cortex. In the present investigation, we studied an unlearned behavior (the open-field hyperactivity resulting from bilateral entorhinal lesions) to evaluate whether ganglioside treatments reduce the severity of initial postlesion impairments or improve recovery. We also examined whether the treatments enhance the sprouting of septodenate fibers which parallels the recovery of open-field activity. The typical behavioral changes induced by bilateral entorhinal lesions include hyperactivity, reduced habituation of activity, and a gradual time-dependent return toward control levels. We found that rats treated with total brain gangliosides (30 mg/kg) showed a smaller lesion-induced increase, consistently lower levels, and greater within-session habituation of activity than did saline-treated counterparts. Control rats treated with gangliosides did not exhibit a reduction in activity, suggesting that the effect was on lesion-induced hyperactivity rather than on activity, per se. Ganglioside-treated rats showed a slight, but consistently smaller lesion-induced sprouting response by the septodentate pathway than did untreated counterparts at all postlesion intervals examined (3, 5, 7, and 10 days). The present findings indicate that ganglioside treatments reduce the severity of the initial behavioral effects after entorhinal lesions without enhancing the sprouting by septodentate fibers.

Limbic-cortical neuronal damage and the pathophysiology of schizophrenia

Neurobiological studies of patients with schizophrenia suggest that abnormalities of both anatomy and function occur in limbic-cortical structures. An anatomical circuit links the functioning of the ventral striatum (i.e., nucleus accumbens) with the hippocampus and other limbic-cortical structures where neurobiological abnormalities have been found. In animals, lesions of limbic-cortical neurons cause decreases in glutamatergic input to the nucleus accumbens and are also associated with decreases in presynaptic dopamine release, increases in the density of D2-like dopamine receptors, and insensitivity to the actions of dopamine antagonists such as haloperidol. These experiments suggest a plausible pathophysiology of schizophrenia, in that schizophrenic symptoms may be caused by an abnormal dopaminergic state brought about by a primary limbic-cortical lesion and deficits in glutamatergic inputs to the ventral striatum.

Synthesis and transport of GAP-43 in entorhinal cortex neurons and perforant pathway during lesion-induced sprouting and reactive synaptogenesis

Metabolic labeling and quantitative 2D gel autoradiography were used to assess changes in the synthesis and transport of GAP-43 in entorhinal cortex (EC) neurons and perforant pathway during lesion-induced sprouting and reactive synaptogenesis. In normal adult rats, there is a high constitutive level of GAP-43 synthesis and transport in EC neurons projecting to the hippocampus. Following unilateral EC lesions, there is a 2-fold (100%) increase in the transport of newly synthesized GAP-43 to the contralateral or 'sprouting' hippocampus. The timing of this upregulation (between 6 and 15 days) suggests that changes in GAP-43 expression occur in response to the growth of presynaptic terminals during sprouting.

Entorhinal cortex lesion induces differential responses in [125I]insulin-like growth factor I, [125I]insulin-like growth factor II and [125I]insulin receptor binding sites in the rat hippocampal formation

The hippocampus can be induced by deafferentation to selectively reorganize its neuronal input. Entorhinal cortex lesion, which causes degeneration of the perforant pathway, evokes sprouting of septal afferents as well as glutamatergic commissural/associational fibers in the deafferentated zone of the molecular layer of the dentate gyrus. Although the process of reactive synaptogenesis that follows deafferentation has been extensively studied, at present little is known about its molecular basis and the mechanism of initiation. In this study, following unilateral lesion of the entorhinal cortex, the time-course of possible alterations of insulin-like growth factors I and II, and insulin binding sites were evaluated by in vitro quantitative receptor autoradiography. [125I]Insulin-like growth factor I receptor binding sites did not exhibit any significant variation between the contralateral and ipsilateral hippocampal formation at any time periods following lesion except in the molecular layer of the dentate gyrus (P < 0.05) at day 8. However, when compared with the unlesioned animals, a differential time-dependent response of [125I]insulin-like growth factor I binding sites was noted in selective layers of the hippocampus. [125I]Insulin-like growth factor II receptor binding sites showed a significant decrease (P < 0.05) in the ipsilateral granular cell layer of the dentate gyrus only at day 14 post lesion. Interestingly, compared to controls, a dramatic bilateral increase (P < 0.05) in [125I]insulin-like growth factor II binding was evident between days 1 and 8 in most layers of the hippocampal formation. A lesion-induced bilateral increase (P < 0.05) in [125I]insulin binding sites was evident in all layers of the hippocampus between two to eight days and at 30 days post lesion. In selective layers, however, a significant increase (P < 0.05) in [125I]insulin binding sites was also observed at days 1 and 14 after lesion. These results, which are compatible with the process of degeneration and/or sprouting of the terminal fibers, suggest possible involvement of insulin-like growth factors and insulin in the sequence of molecular events that occur to facilitate neuronal repair and to promote neuronal survival following entorhinal cortex lesion.

A reaffirmation of the retrosplenial contribution to rodent navigation: reviewing the influences of lesion, strain, and task

Retrosplenial cortex (RS) is situated both anatomically and functionally between neocortical and limbic structures involved in spatial navigation. Initial anatomical, electrophysiological and behavioural evidence in both humans and rodents strongly suggested a role for RS in spatial navigation as well. Later studies using more selective cytotoxic lesions in rodents, however, cast doubt on earlier RS studies by failing to find spatial deficits following RS lesions. Contrasting reports from behavioural results on spatial tasks following RS damage have continued to be reported during the past decade. That RS does indeed contribute spatial behaviour even in rodents has been recently reaffirmed. The ambiguity surrounding RS is shown to result from differences in the choice of spatial tasks and rat strains between studies that find RS deficits and those that do not. The reconciliation of behavioural results following RS lesions strengthens the view that RS forms a part of the neural circuitry that underlies spatial navigation.

Sparing and recovery of spatial alternation performance after entorhinal cortex lesions in rats

Groups of adult rats were first trained on a spatial alternation task and then subjected to unilateral entorhinal cortex lesions, unilateral entorhinal cortex lesions followed by dorsal psalterium transections, or bilateral entorhinal cortex lesions. After this surgery, the rats were then tested for retention of spatial alternation. Neither unilateral lesions alone nor unilateral lesions followed by dorsal psalterium transections resulted in long-term spatial performance deficits; however, animals with bilateral lesions exhibited severe impairments from which they eventually recovered. The results from animals with bilateral entorhinal damage indicate that extensive postoperative training may facilitate the recovery of spatial alternation performance. Histological analyses indicated that the crossed entorhinal projection proliferated in the dentate gyrus after unilateral entorhinal lesions and such anomalous growth occurred independently of any changes in alternation performance.

Effects of lesions to the glutamatergic afferents to the nucleus accumbens in the modulation of reactivity to spatial and non-spatial novelty in mice

The purpose of this study was to compare the effects of selective lesions of the three main sources of limbic afferents to the nucleus accumbens-fornix, prelimbic cortex and amygdala-with those induced by N-methyl-D-aspartate receptor blockage in this structure, in a non-associative task designed to estimate the ability of rodents to encode spatial and non-spatial relationships between discrete stimuli. The task consists of placing mice in an open field containing five objects and, after three sessions of habituation, examining their reactivity to object displacement (spatial novelty) and object substitution (object novelty). Focal administrations of the competitive N-methyl-D-aspartate antagonist DL-2-amino-5-phosphonopentanoic acid (0.1 microg/side) induced a selective impairment in the ability of mice to react to the spatial change. Lesions to the different structures affect the response of mice to spatial and non-spatial novelty in different ways. In particular, while fornix lesions induced a decrease in re-exploration of the displaced objects, prelimbic cortex lesions enhanced the exploration of both displaced and non-displaced objects. Finally, the basolateral amygdala lesions did not induce any impairment in the detection of the displaced objects but decreased the latencies to approach novel objects. It is concluded that N-methyl-D-aspartate receptor blockage in the nucleus accumbens subsumes the effects of the three lesions. Some hypotheses on the role of glutamatergic transmission in the accumbens on information processing are briefly discussed.

Testing the importance of the retrosplenial navigation system: lesion size but not strain matters: a reply to Harker and Whishaw

In their review on the retrosplenial cortex Harker and Whishaw [Neurosci Biobehav Rev, 2004] claim that there is continued disagreement over the importance of this region for navigation. They argue that discrepancies in the published effects of retrosplenial lesions reflect two principal variables, choice of rat strain and choice of spatial task. In this reply, evidence is provided showing that Harker and Whishaw [Neurosci Biobehav Rev, 2004] have created a misleading impression and, in fact, there is a clear consensus that the rat retrosplenial cortex is necessary for navigation. Likewise, there is no dispute that the effects of retrosplenial lesions will differ for different tests of spatial learning. While Harker and Whishaw [Neurosci Biobehav Rev, 2004] also conclude that choice of rat strain has a critical impact on whether a lesion-induced deficit is found, a comprehensive review of the published data shows no systematic strain difference. There is, however, growing evidence that when interpreting the effects of retrosplenial lesions, account should be given of the lesion method and its interaction with lesion size.

Striatal dopamine receptors and transporters in monkeys with neonatal temporal limbic damage

Developmental cortical damage has been implicated in the basic neurobiology of schizophrenia. Adult rhesus monkeys with neonatal temporal limbic damage show a stimulus-dependent disinhibition of subcortical dopamine (DA) release. We measured dopamine D2 receptors and transporters in vivo in rhesus monkeys with neonatal and adult mesial temporal limbic lesions and control monkeys to explore further the effects of this developmental lesion on striatal DA function. All monkeys were studied with [I-123]IBZM SPECT to assess the availability of striatal dopamine D2 receptors and with [I-123]beta-CIT SPECT to measure the availability of dopamine transporters in the striatum. IBZM binding was significantly reduced in monkeys with neonatal limbic lesions. No group difference in beta-CIT binding was found. The reduction in IBZM binding was significantly correlated with subcortical dopamine release after monoaminergic prefrontal stimulation as determined with in vivo microdialysis. Our findings imply specific interactions between age at lesion and the availability of DA transporter and receptors in non-human primates, and suggest that stimulus-dependent DA activity affects the expression of DA receptors.

Klüver-Bucy syndrome in man: experiences with posttraumatic cases

After the original description of characteristic behavioral changes in rhesus monkeys after bilateral resection of major portions of the temporal lobes by Klüver and Bucy in 1937 [11], similar syndromes have repeatedly been reported in human pathology. The present paper is based on clinical analysis of 40 cases of traumatic apallic syndrome (TAS), 30 of whom developed a complete or partial Klüver-Bucy syndrome during recovery. The dynamics in the development of a Klüver-Bucy syndrome during remission of a TAS are analyzed. The delineation between a pre-stage, full-stage and remission-stage of the Klüver-Bucy syndrome is suggested.

Transneuronal changes in dendrites of GABAergic parvalbumin-containing neurons of the rat fascia dentata following entorhinal lesion

The perforant path fibers from the entorhinal cortex form synapses with both granule cells and GABAergic, parvalbumin-containing (PARV) nongranule cells. The authors recently reported a persistent reduction of PARV-positive dendrites in the termination zones of entorhinal fibers in the hippocampus proper and fascia dentata after lesion of the entorhinal cortex. In the present study the authors analyzed the effects of de-entorhination on the ultrastructure of postsynaptic PARV-positive dendrites in the molecular layer of the fascia dentata. PARV immunocytochemistry was performed 2, 8, 55, and 360 days after an ipsilateral entorhinal lesion and, for comparison, 10 days after an ipsilateral fimbria-fornix transection that disconnects the hippocampus from its septal and commissural afferents. Two days after entorhinal lesion, the authors observed swelling of the tissue close to the hippocampal fissure. Adjacent distal dendritic tips of PARV-positive dentate neurons appeared bloated and reduced in number. Reduction of PARV-positive dendrites in the former perforant path termination zone persisted 55 days after entorhinal lesion and could still observed after postlesional survival times for 1 year. Degenerating axon terminals were still present 55 days following lesion and PARV-positive dendrites exhibited abnormal invaginations. Fimbria transection did not result in similar dendritic changes in PARV-positive neurons. The results indicate a long-lasting process of reorganization in the molecular layer of the fascia dentata following entorhinal lesion and persisting changes in the morphology of PARV-immunoreactive dendrites. Entorhinal fibers seem to play a specific role for the maintenance of these dendrites, since similar changes did not occur following removal of septal and commissural fibers.

Histochemical evidence for a post-lesion reorganization of cholinergic afferents in the hippocampal formation of the mature cat

We have utilized acetylcholinesterase (AChE) histochemistry to analyze possible post-lesion changes in the distribution of AChE containing afferents to the hippocampal formation of the cat following unilateral destruction of the entorhinal cortex. In the cat, the entorhinal area gives rise to a massive projection to the ipsilateral fascia dentata, and to regio inferior and regio superior of the hippocampus proper. Sixty days following unilateral entorhinal lesions, histochemical preparations for AChE indicate a dramatic increase in the density of the reaction product in the zones normally occupied by entorhinal afferents in the fascia dentata and regio inferior of the hippocampus proper, whereas little if any increase in the density of the reaction product was observed in the entorhinal terminal zone in regio superior. In addition to these increases in the density of the AChE reaction product, there was also evidence for a widening of an AChE free zone in the inner stratum moleculare of the fascia dentata denervated by the lesion. The time course of these changes in the pattern of AChE staining was analyzed by sacrificing animals 7, 10, 13, 14, 16, 17,, 19, and 20 days following entorhinal cortical lesions. The increase in the density of the AChE reaction product in the denervated zones was not apparent at seven days post-lesion, while at ten days post-lesion, a slight increase in the density of the AChE reaction product could be observed. By 13 days post-lesion, the differences between the denervated and normally innervated (contralateral) hippocampal formation were prominent, and by 16 days post-lesion, the pattern of staining appeared comparable to that which was observed at longer post-lesion intervals. The present experiments indicate that following entorhinal cortical lesions in mature cats the final post-lesion pattern of altered AChE staining is quite comparable to that which is observed following similar lesions in rats. In the rat, such changes in AChE staining have been interpreted as a reflection of a proliferation of cholinergic septal afferents within the denervated zones. If this interpretation is correct, the present results suggest a similar proliferation of cholinergic afferents following entorhinal lesions in cats. The time course of this apparent proliferation is considerably slower in the cat then in the rat, however, since the earliest changes are observed at approximately five days post-lesion in the rat, and ten days post-lesion in the cat.

Mossy fiber sprouting in the fascia dentata after unilateral entorhinal lesions: quantitative analysis using computer-assisted image processing

Axon sprouting typically occurs in a brain region that has been partially denervated. The present study demonstrates, quantitatively, evidence for sprouting outside the region of deafferentation. A modification of the Timm sulfide silver histochemical method was used to monitor an increase in the mossy fiber terminal field in the fascia dentata of adult rats following severe deafferentation of the outer three-fourths of stratum moleculare by unilateral entorhinal lesions. Computer-assisted image processing techniques were used to quantify mossy fiber sprouting. In stratum granulosum, and to a lesser extent in the deep (supragranular) portion of stratum moleculare (areas separated from the zone of deafferentation), there was a three-fold increase in the area of mossy fiber staining on the side of the lesion compared to the non-operated side (and unoperated animals). Much of the increased staining was located near the tip of the infrapyramidal (ventral) blade of the fascia dentata. Since mossy fiber sprouting apparently occurs in the absence of degeneration-produced synaptic dilution in that region, it may represent an example of post-lesion growth initiated by conditions fundamentally different from those normally believed to induce sprouting.