Physiological and anatomical correlates of the human dentate gyrus: consequences or causes of epilepsy.

The presence of paroxysmal discharges in the epileptic human dentate gyrus provides a physiologic basis for hyperexcitability that may initiate seizure discharges during the development of epilepsy. Although these responses can occur with single orthodromic stimulation, data obtained under conditions that weaken synaptic inhibition (e.g., 1 Hz stimulation or bicuculline disinhibition) suggest that paroxysmal discharges may be a more common feature of tissue from temporal lobe epileptic patients than has been reported previously. Hilar cell loss and weakened synaptic inhibition may provide conditions favorable for the activation of N-methyl-D-aspartate acid (NMDA) receptors that would allow triggering of paroxysmal discharges that normally never are evoked in dentate granule cells in nonepileptic humans. As the dentate gyrus in normal animal tissue is not susceptible to intrinsic bursting behavior and is characterized by a relatively short duration excitatory postsynaptic potential even under pharmacologic disinhibition, paroxysmal discharges in the epileptic human dentate gyrus may provide an important clue to understanding the prerequisite conditions for seizure discharge.

Paroxysmal discharges in the EL mouse, a genetic model of epilepsy.

The EL/Suz (EL) mouse is a strain that is highly susceptible to convulsive seizures after repeated sensory stimulation. Its control strain, DDY/Jc1 (DDY), is less susceptible under similar conditions. The seizure prone phenotype is the result of differences at several genetic loci. In vivo electrical recordings from the seizure prone EL mouse brain have shown that the appearance of abnormal discharges in the hippocampus are critical to the onset of generalized seizures, indicating that the hippocampus plays an important role in EL mouse seizure activity. In the present study, electrophysiological differences between EL and DDY mice (9-15 weeks of age) were examined by comparing field potentials recorded from the dentate granule cell layer of hippocampal brain slices from mice that had not been stimulated to induce seizures. In control physiological solution, no significant differences were observed in characteristics of perforant path evoked field potentials or in paired pulse depression of evoked field potentials using 20 to 300 ms interstimulus intervals. After 60 min of disinhibition following bicuculline (10 microM) exposure, however, prolonged large amplitude potentials, paroxysmal discharges, were evoked by perforant path stimulation in the dentate gyrus of EL mice but were absent in the DDY strain. Paroxysmal discharges were curtailed by APV and were similar to responses recorded from the dentate gyrus in hippocampal brain slices from temporal lobe epileptic patients. The field response to hilar stimulation was identical in both strains and was composed of a single population spike before and after bicuculline exposure. Mossy fiber terminals were not present in the molecular layer of either strain. We propose that the mechanisms leading to a greater likelihood of paroxysmal discharge generation in EL mouse may be important in the development and/or generation of epileptic seizures in this mouse strain and may be a significant phenotypic difference between the EL mouse and its parent strain.

Mossy fiber reorganization and its possible physiological consequences in the dentate gyrus of epileptic humans.

It is unlikely that MF reorganization is the cause of epilepsy, but it may affect the progression of the disease, i.e., the frequency or severity of seizures. We propose that early events, yet undiscovered, lead to an increased likelihood of excitability. This hyperexcitability, which initially may not be manifested in overt seizures, may erode vulnerable hilar neurons that serve an important inhibitory function, as illustrated in Fig. 6-15. As inhibition is lost, hyperexcitability reaches the level of clinically manifested seizures that are severe enough to lead to substantial loss of hilar neurons. When the loss of these cells is sufficiently high, MF reorganization occurs, first to neighboring hilar neurons and later to dendrites of granule cells (Fig. 6-15). Thus, the functional consequence of MF reorganization may provide a compensatory form of inhibition, as well as a circuit for feedback excitation. Although definitive evidence indicating that MF reorganization contributes to the acceleration or progression of epilepsy is missing, the findings to date are consistent with this hypothesis. In the event that reorganization contributes to the epileptic condition, treatments that reduce indicators of neuropathology may lead to a reduction of seizure frequency and severity. Evidence suggests that reorganization in the dentate gyrus may follow the pathways of neuronal processes of hilar neurons that have died. Thus, further study of the events that guide MF reorganization may hold important clues for developing methods for targeting regenerating axons following central nervous system injury.

Prolonged field potentials evoked by 1 Hz stimulation in the dentate gyrus of temporal lobe epileptic human brain slices.

An abnormal electrophysiological response in brain slices of the dentate gyrus from biopsy material from patients surgically treated for intractable epilepsy (46/57), exhibited characteristics similar to the physiological hallmark of epilepsy, the paroxysmal discharge, a prolonged (30-600 ms) and often large amplitude field potential. The most striking feature of the prolonged response to a single perforant path stimulus was a predominantly biphasic field potential (23/46 cases). The biphasic response was characterized by a negative field potential of substantial duration exceeding 180 ms which followed an initial shorter duration positive field potential. Multiple population spikes occurred during both phases of the response. During a 1 Hz stimulus train applied to the perforant path, the magnitude and duration of the negative component of the field response was significantly increased. Approximately half of the cases (Group 1; 30/57) exhibited potentiation of the biphasic response, while the remaining cases (Group 2; 27/57) exhibited no negative field component during 1 Hz stimulation trains. This repetitive stimulation, in general, increased the area of the field response in a large majority of cases (44/57) regardless of the sign of the field potential. The number of population spikes following 1 Hz stimulation increased significantly for cases in both groups, although the increase was greater for those in Group 1 than in Group 2. Paired pulse depression (20 ms ISI) was reduced in cases that exhibited potentiated biphasic responses during 1 Hz stimulation (Group 1) in comparison to cases that exhibited no negative field potentials (Group 2). Paired pulse depression at a 200 ms ISI was not significantly different between the groups. During a single stimulus, bicuculline disinhibition (20 microM) resulted in either a prolonged positive or biphasic field potential. Intracellularly recorded responses to single perforant path stimuli also exhibited prolonged and large depolarizations that were comparable in time course to the duration of field potentials recorded in the same area whether generated in the absence or presence of bicuculline. The prolonged field potential after bicuculline was reduced by APV (20 microM). We suggest that the prolonged field response, whether biphasic or monophasic when generated by either 1 Hz stimulation or bicuculline disinhibition, may be due directly or indirectly to an increase in membrane depolarization mediated by activation of the NMDA receptor.

Changes in dentate granule cell field potentials during afterdischarge initiation triggered by 5 Hz perforant path stimulation.

A failure of early paired pulse depression often precedes the onset of intermittent spontaneous seizures in animal models of status epilepticus. In the present study, changes in the strength of early and late paired pulse depression of dentate granule cell field potentials were compared in the unanesthetized rat during the initiation of a single afterdischarge (AD) evoked by perforant path stimulation (0.1 ms pulse duration, 5 Hz, 12-18 s duration, 50-1000 microA). Late paired pulse depression was measured by sequential changes in the population spike (PS) amplitude during 5 Hz stimulation (200 ms interpulse interpulse interval, IPI). When 5 Hz stimulation triggered an AD, the population spike (PS) was initially depressed and then increased to above pre-train values, indicating a loss of late paired pulse depression by the middle of the train. Early paired pulse depression was measured by inserting paired pulses (20 ms IPI) at spaced intervals throughout the 5 Hz train. In contrast to late paired pulse depression, early paired pulse depression remained at maximum strength until an abrupt failure was detected coincident with AD initiation. Two experimental treatments shown to increase the strength of late paired pulse depression, administration of the N-methyl-D-aspartate antagonist, MK-801 (0.25 mg/kg, i.p.), and the development of kindled seizures, produced an increase in AD thresholds and in the initial depression in the PS amplitude during 5 Hz stimulation. Together, these results suggest that a failure of late paired pulse depression may be a precipitating event in AD initiation triggered by 5 Hz stimulation in the unanesthetized rat.

Effects of bicuculline and baclofen on paired-pulse depression in the dentate gyrus of epileptic patients.

Paired-pulse field responses were recorded from the granule cell layer of the dentate gyrus in brain slices from temporal lobe epileptic patients. Paired-pulse depression (PPD) was examined using perforant path stimulation of low to moderate intensity at an inter-stimulus interval (ISI) of 20 ms. The paired-pulse ratio (PS2/PS1) was expressed as the population spike amplitude of the second response (PS2) relative to that of the first response (PS1). Representative tissue response from each patient biopsy were divided into two groups that were significantly different based on the magnitude of the highest paired-pulse ratio recorded for each biopsy specimen: the strong paired-pulse depression group (PS2/PS1 = 0.12 +/- 0.03; n = 15) and the weak paired-pulse depression group (PS2/PS1 = 0.68 +/- 0.06; n = 13). Paired-pulse ratios from the strong PPD group were relatively independent of stimulus intensity, whereas, PPD was dependent on stimulus intensity in the weak PPD group; i.e., PPD was greatest at the lowest intensity and reached a plateau at higher intensities. Bicuculline (20 microM) and low concentrations of baclofen (0.1-0.2 microM) reduced paired-pulse depression in the strong PPD group, but did not significantly change the paired-pulse ratio in the weak PPD group. Paired-pulse facilitation was observed in some cases after inhibition was blocked pharmacologically. The number of population spikes was increased in the presence of bicuculline but was unchanged by baclofen. In the strong PPD group, baclofen significantly altered the EPSP-population spike (E-S) relationship by increasing the slope of the relationship for the second response, without having an effect on the slope of the first response. Baclofen had no effect on the E-S relationship of either response in the weak PPD group. The data are consistent with (1) less inhibition in the weak PPD group compared to the strong PPD group, (2) reduction of feedback inhibition in the strong PPD group by bicuculline and by low concentrations of baclofen, and (3) the occurrence of paired-pulse facilitation when inhibition was pharmacologically reduced in the dentate gyrus of temporal lobe epileptic patients. The results are also consistent with the presence of GABAB receptors on human inhibitory interneurons that, when activated by baclofen, result in disinhibition of granule cells through feedback circuits. Although inhibition may be compromised in some epileptic human biopsy specimens, the presence of strong inhibition in other patients' biopsy material suggest the re-evaluation of the role of inhibition in epilepsy.

Stimulus parameters affecting paired-pulse depression of dentate granule cell field potentials. II. Low-frequency stimulation.

Low frequency (1 Hz) stimulation of the perforant path produces a depression in the population spike (PS) of dentate granule cell field potentials and also may affect the strength of paired pulse depression. The effects of 1 Hz stimulation (30 s train) on paired pulse depression (20 and 200 ms interpulse intervals, IPI) were evaluated in the unanesthetized rat under two conditions: (i) when the stimulus intensity of both pulses was increased simultaneously (5-100%); and (ii) when the stimulus intensity of the first (conditioning) pulse was increased (5-100%), while the stimulus intensity of the second (test) pulse was held constant (50%). The test PS amplitude was predicted based upon either the conditioning PS amplitude at the end of the 1 Hz train or upon the additive effects of paired pulse depression and 1 Hz stimulation. These predicted values then were assessed for the best fit to observed values following 1 Hz trains. Under both stimulus conditions, the 1 Hz depression in the conditioning PS amplitude exhibited characteristics that were identical to late paired pulse depression recorded before the train. A decrease in the test PS amplitude also was observed following 1 Hz stimulation at the 20 and 200 ms IPIs. The best fit to observed values of the test PS at the end of 1 Hz trains was provided by estimates based upon the additive effects of 1 Hz stimulation and paired pulse depression. These results indicate that the strength of paired pulse depression in the unanesthetized rat is unchanged following 1 Hz stimulation, and further, that the 1 Hz depression in dentate granule cell field potentials most likely reflects the cumulative influence of late paired pulse depression.

Longitudinal variation in cell density and mossy fiber reorganization in the dentate gyrus from temporal lobe epileptic patients.

Variation in cell loss and mossy fiber reorganization was examined along the longitudinal axis of the dentate gyrus from temporal lobe epileptic (TLE) patients. Previous evidence has indicated that the anterior hippocampus is prone to seizure activity. We compared granule and hilar cell number in addition to Timm stain density of the molecular layer and hilus in more anterior and more posterior specimens of hippocampus obtained from patients surgically treated for intractable epilepsy by the removal of the anterior half of the hippocampus. Granule cells/mm in the more anterior specimen were less than or equal to those in the more posterior specimen locations in 77% of the patients, while there was no significant difference in hilar neuron density between the two blocks. These results demonstrate a significantly greater pathology in the granule cell layer in more anterior specimens and no difference in pathology for hilar neurons. Molecular layer Timm stain density was significantly greater in the more anterior specimen of 71% of the patients. The molecular layer Timm stain density ratio was inversely related to hilar cell density in more anterior specimens, whereas in more posterior specimens there was no significant relationship with hilar cell density. Our observations show that although differences exist among TLE patients for these neuroanatomic measures, pathology was greater in more anterior specimens. The latter result is consistent with the conclusion that seizure activity may originate in the anterior region of the hippocampus in a majority of patients.

Polyamine effects on the NMDA receptor in human brain.

Polyamines are thought to modulate the activation of NMDA receptors through a unique allosteric regulatory site. The effects of polyamines on the binding of [3H]MK-801 were measured in cortical and hippocampal tissue surgically removed from patients with temporal lobe epilepsy (TLE). The polyamine agonist spermidine increased the binding of [3H]MK-801 in the cortex in a dose-dependent manner and this effect could be blocked by the weak partial agonist diethylenetriamine (DET). Spermidine decreased the Kd of [3H]MK-801 for the NMDA receptor but did not alter the density of receptors. Spermidine had essentially the same effect on Kd and Bmax measured in the dentate gyrus of TLE subjects and the cortex and dentate gyrus of postmortem controls. Moreover, there was no difference in the density of binding sites between postmortem and TLE subjects in either region. The binding of [3H]MK-801 in human cortex was decreased by 30% by incubation with DET or by prewashing the tissue sections. In contrast, DET did not alter the binding of [3H]MK-801 in rat cortex and prewashing sections produced an increase rather than a decrease in binding. These results suggest that there are different endogenous modulators for the polyamine site in rat and human tissue. The inverse agonist 1,10-diaminodecane decreased the binding of [3H]MK-801 in a dose-dependent manner. These results suggest that the fundamental modulatory properties of polyamines in rat and human tissues are essentially the same and that endogenous polyamines may regulate human NMDA receptors.

Alterations of inhibitory synaptic responses in the dentate gyrus of temporal lobe epileptic patients.

The number of orthodromically evoked population spikes was used to classify brain slice tissue from the dentate gyrus of temporal lobe epileptic patients as "more excitable" (multiple population spikes) or "less excitable" (a single population spike). During orthodromic stimulation, "more excitable" tissue exhibited less paired-pulse depression in comparison to "less excitable" tissue. During antidromic stimulation, both multiple population spikes and paired-pulse depression were observed in "more excitable" tissue. "Less excitable" tissue exhibited a single antidromic spike and often no antidromically evoked paired-pulse depression. The strength of antidromic paired-pulse depression was correlated positively with the number of antidromic spikes and was correlated negatively with orthodromic paired-pulse depression. Although orthodromic and antidromic paired-pulse depression were correlated to the number of orthodromically evoked population spikes, this correlation was not as strong as that between orthodromic paired-pulse depression, antidromic paired-pulse depression, and number of antidromically evoked population spikes. The antidromic paired-pulse depression observed in tissue exhibiting antidromically evoked multiple population spikes was enhanced rather than blocked by bicuculline. In addition, the blockade of the antidromic paired-pulse depression by CNQX indicated that this inhibition is mediated by an AMPA-type glutamatergic synapse. We suggest that alterations in circuitry occur in the dentate gyrus of some temporal lobe epileptic patients and were manifested by both a loss of inhibitory input as well as an increase of inhibition, which was dependent on the pathway of stimulation. The results of pairing antidromic and orthodromic stimuli were consistent with these conclusions.

Hyperexcitability associated with localizable lesions in epileptic patients.

Intracellular recordings from neurons were carried out in cortical slices obtained from tissue removed from patients suffering from intractable seizures. The patients were divided into two groups based on the presence or absence of an anatomical abnormality that could be imaged preoperatively. The lesion or its surround was the presumptive epileptogenic area. The tissue removed from the patients without lesions was removed either for biopsy purposes or for access to epileptic tissue and was not considered epileptogenic. All neurons from patients without an imageable lesion, and some (19%) from patients with an imageable lesion, responded to orthodromic stimuli with a sequence of synaptic excitation followed by inhibition; these properties resembled those of normal rodent cortical slices. Different responses, classified as abnormal, were observed in 81% of the neurons in tissue specimens obtained near lesions. The most common was prolonged synaptic excitation with no noticeable inhibition, even at high stimulus strengths. In three resections, long latency all-or-none depolarization shifts were observed that resemble the classic paradoxical depolarization shift seen in in vivo extracellular recordings. Loss of specific inhibitory systems within the cortex may contribute in part to these abnormal responses.

The functional relationship between antidromically evoked field responses of the dentate gyrus and mossy fiber reorganization in temporal lobe epileptic patients.

Field recordings from the dentate granule cell layer of in vitro brain slices of temporal lobe epileptic patients were evoked by antidromic stimulation. Tissue from the same specimen was stained by the Timm-sulfide method to assess the pattern and degree of mossy fiber reorganization into the supragranular layer. A wide range of physiological responses and Timm staining patterns was present across patients. A significant correlation was observed between the abnormality of antidromic responses, reflected by multiple secondary population spikes, and the degree of Timm staining of the supragranular layer. This relationship lends support to the hypothesis that mossy fiber synapses located in the supragranular layer may have functional implications for granule cell excitability in human epileptic tissue.

NMDA receptor activation during epileptiform responses in the dentate gyrus of epileptic patients.

We previously showed that a low frequency (1 Hz) train of perforant path stimulation evokes burst discharges in the dentate gyrus of hippocampal slices obtained from patients surgically treated for intractable temporal lobe epilepsy. We report here that multiple population spikes that characterize the burst discharge are blocked reversibly by the specific NMDA receptor antagonist, D-(-)-2-amino-5-phosphonovaleric acid (D-APV). The epileptiform discharge evoked in human dentate gyrus by stimulation trains of 1 Hz could be reproduced in the rat dentate gyrus in vitro by the same stimulation protocol but required the presence of low concentrations (0.2-0.6 mM) of extracellular magnesium. We suggest that low frequency orthodromic stimulation of dentate granule cells through the perforant path progressively evokes an increase in the activation of NMDA receptors resulting in burst discharges in tissue from epileptic patients.

Distribution of single-channel conductances in cultured rat hippocampal neurons.

1. The nonhomogeneous spatial distribution of ionic channels in neurons has been implied from intracellular recordings at somatic and dendritic locations. These reports indicate that Na- and Ca-dependent regenerative currents are distributed differently throughout the neuron. Although a variety of K conductances and a noninactivating Na conductance have been described in intracellular studies, little is known about the spatial distribution of inward and outward currents throughout different regions of the neuron. 2. We recorded from cell-attached patches from cultured hippocampal cells from 1-day-old rats. The cells were cultured for 3-21 days. The spatial distribution of a variety of ionic channels was determined by comparing the conductances from somatic and dendritic membranes. Single-channel currents obtained from cell-attached patches were identified by the time course of ensemble (averaged) responses, voltage dependence, and the effect of channel blocking agents. 3. We consistently observed that only the rapidly inactivating inward current was localized to the soma. The other channel types that we studied, including an inward noninactivating, delayed rectifier and transient A-type currents, were observed in both the somatic and dendritic regions. 4. We suggest that the distribution of ionic conductances that we have observed may be functional in limiting excitability during development of neurons.

Hippocampal neuronal density in temporal lobe epilepsy with and without gliomas.

The majority of patients with temporal lobe epilepsy show hippocampal sclerosis, which pathologically represents neuronal loss and gliosis. We studied volumetric neuronal density on a representative mid to mid-posterior level slice of hippocampi surgically removed from intractable temporal lobe epilepsy cases, and compared the results between 25 non-tumor epilepsy (NTE) cases and 5 tumor-associated epilepsy (TAE) cases. Eleven age-matched non-epileptic autopsy cases were studied as controls. Cells were counted in the CA1 through CA4 fields and the stratum granulosum of the dentate fascia. In NTE every hippocampal field showed statistically significant loss of neurons, the neuronal density in each field ranging from 35% to 50% of that of control. The mean neuronal density between the TAE and NTE groups also showed statistically significant differences in all hippocampal fields. The neuronal density of hippocampal fields of NTE ranged from 43% to 58% of that of TAE. Tumor-associated epilepsy cases, however, failed to show any statistically significant deviation from the control in their neuronal density. The etiology of the difference in neuronal density between the TAE and NTE groups is discussed.

Epileptiform discharges evoked in hippocampal brain slices from epileptic patients.

We have recorded from diseased hippocampal tissue which was surgically removed from epileptic patients for therapeutic purposes. When the perforant path was stimulated at a low frequency (1 Hz), the number of population spikes evoked in the dentate gyrus increased by a factor of as great as 8 during a 15 s train. This effect was transient. A similar epileptiform discharge could be generated in normal rat hippocampal brain slices by the same stimulus paradigm, but only in the presence of a low concentration (0.2 microM) of bicuculline. These results suggest that this frequency-dependent epileptiform discharge, evoked in the dentate gyrus of epileptic patients, may be due to a small reduction in GABAA-mediated inhibition and may involve factors that lead to the initiation of seizures.

Excitable properties of olfactory receptor neurons.

Action potential-generating properties of olfactory receptor neurons in the olfactory epithelium of the salamander, Ambystoma tigrinum, were studied in control animals, and 2 and 4 weeks after olfactory nerve transection. The threshold for impulse generation in response to injected current was extremely low (74 +/- 46 pA). In addition, the discharge frequencies of the receptor neurons were exquisitely sensitive to small increments of injected current. These high sensitivities may be characteristic of small neurons and stand in contrast to the much lower sensitivities reported for large neurons. The high sensitivity has important implications for the input-output functions of this cell. After nerve transection, both the threshold and the frequency sensitivity decreased. These changes appear to be associated with increased potassium conductance, suggested by prominent membrane rectification and reduced amplitudes of later membrane action potentials in the spike trains. The olfactory receptor neuron appears to be a favorable model for exploring these properties.

Temperature dependence of intrinsic membrane properties and synaptic potentials in hippocampal CA1 neurons in vitro.

The temperature dependence of intrinsic membrane conductances and synaptic potentials in guinea pig hippocampal CA1 pyramidal neurons were examined in vitro as they were cooled from 37 degrees C to between 33 and 27 degrees C. Cooling reversibly increased resting input resistance in a voltage-independent manner (Q10 = 0.58 to 0.75). The amplitude and duration of orthodromically evoked action potentials were increased by cooling (Q10 = 0.87 and 0.52 to 0.53, respectively), whereas the maximum rates of rise and fall were reduced (Q10 = 1.27 to 1.49 and 2.19 to 2.44, respectively). The amplitude and duration of the afterhyperpolarization which follows a directly evoked train of action potentials were substantially increased at low temperatures. It is possible to attribute this increase to an augmentation of Ca2+ influx during the train and also to a slowing of Ca2+ removal from the cytoplasm. Spike frequency adaptation during prolonged depolarizing pulses was enhanced at low temperatures. In addition, there was a decrement in spike amplitude during the train of action potentials. These observations all suggest an increase in Ca2+-activated K+ conductance at low temperature. A late, slow, hyperpolarizing synaptic potential in response to orthodromic stimulation became apparent at low temperature. This potential had an apparent reversal potential more negative than the early inhibitory postsynaptic potential, suggesting that it was mediated by a K+ conductance, possibly activated by Ca2+ influx. We conclude that reductions in temperature of as little as 5 to 10 degrees C from normal can significantly alter the intrinsic and synaptic physiology of hippocampal neurons and should, therefore, be considered an important variable in in vitro brain slice experiments.

Electrophysiological properties of identified cells in the in vitro olfactory epithelium of the tiger salamander.

An in vitro preparation of the salamander olfactory epithelium has been developed for electrophysiological analysis. Intracellular measurements of membrane properties of the main epithelial cell types have been carried out, combined with Lucifer Yellow injections. The most prevalent type of cell had a high resting membrane potential and relatively low input resistance. This cell never discharged impulses, either spontaneously or to injected current. Lucifer Yellow injections identified this cell type as a supporting cell. A less frequent type had a medium resting potential and a very high input resistance. This type always discharged impulses in response to injected depolarizing current. Lucifer Yellow injections identified this cell type as an olfactory receptor neuron. The least frequent type had a medium resting potential and a high input resistance. It never generated action potentials. This nonspiking type was tentatively identified as an immature receptor neuron in the process of differentiating from basal stem cells in the epithelium. These are the first results to document physiological properties for the main cell types and morphological identification of two of the types in the same preparation of the olfactory epithelium. Our results support previous suggestions regarding the glial-like properties of the supporting cells. The membrane properties of the receptor neurons appear to be well suited for mediating the olfactory sensory response of these cells.

Changes in the electrical properties of olfactory epithelial cells in the tiger salamander after olfactory nerve transection.

Transection of olfactory nerves causes degeneration of receptor neurons in the olfactory epithelium, followed by generation of new receptor neurons. We have carried out intracellular recordings to document changes in epithelial cell populations during receptor neuron degeneration and regrowth at 1, 2, and 4 weeks following olfactory nerve transection in the salamander. Receptor neurons were greatly reduced in numbers at 1 week, and gradually returned to the normal percentage of intracellular penetrations by 4 weeks. They had a resting membrane potential between -30 and -50 mV and high input resistance, 100 to 600 megohms, characteristically seen in normal epithelium. However, at 1 week, the receptor neurons were able to generate only a single spike in response to injected current, and did not re-acquire their ability to respond repetitively until 4 weeks. Cells with the properties of immature receptor neurons (resting membrane potential between -30 and -50 mV and high input resistance, 100 to 600 megohms, but unable to generate spikes) increased significantly in number in the post-transection period. This correlates with the burst of mitotic activity giving rise to new receptor neurons after nerve transection. Supporting cells changed their properties in the aftermath of transection. One type (A) showed a decrease in resting membrane potential and a small increase in input resistance. A second type (B) showed a very large increase in input resistance. These results imply that the degenerating receptor neurons transmit a signal that leads to changes in the functional properties of the glial-like supporting cells. These may involve changes in the membrane properties or in electrical coupling between cells.