Characterization and distribution of basic fibroblast growth factor-containing cells in the rat hippocampus

Basic fibroblast growth factor (bFGF), a member of the heparin-binding growth factor family, is present in relatively high levels in the brain where it may play an important role in the maintenance, repair, and reorganization of the tissue. Although bFGF is associated mainly with astrocytes throughout most of the central nervous system (CNS), a narrow but prominent band of pyramidal neurons, which coincides with the CA2 subregion of Ammon's horn in the hippocampus, stains intensely for bFGF. In order to gain an understanding of which cells express bFGF and whether or not bFGF is a good marker for CA2 neurons, we have used a mouse monoclonal antibody directed against recombinant human bFGF to characterize the distribution and localization of bFGF expression in the hippocampus. We find that about one-quarter of the neurons in CA2 are bFGF positive, and they appear smaller and have more irregular-shaped nuclei than their unstained counterparts. In addition, all glial fibrilary acidic protein (GFAP)-positive astrocytes in the hippocampus stain for bFGF, and the distribution of these astrocytes is heterogeneous in the hippocampus. Finally, in both astrocytes and CA2 pyramidal neurons, bFGF immunoreactivity is localized primarily in the nucleus and to a lesser extent in the cytoplasm and processes of stained cells. © 1996 Wiley-Liss, Inc.     

Action of tachykinins in the rat hippocampus: modulation of inhibitory synaptic transmission

Substance P and other neuropeptides of the tachykinin family can powerfully excite CA1 hippocampal interneurons present in the CA1 region. In the present work we show that, by exciting hippocampal interneurons, tachykinins can indirectly inhibit pyramidal neurons. We found that tachykinins caused a decrease in the inhibitory synaptic current interval and an increase in the inhibitory synaptic current amplitude in almost all pyramidal neurons tested. This effect was tetrodotoxin sensitive. Tachykinins did not alter the frequency or amplitude of miniature inhibitory synaptic currents and were without effect on evoked inhibitory synaptic currents. Thus, these neuropeptides acted at the somatodendritic membrane of GABAergic interneurons, rather than at their axon terminals. The effect of substance P on spontaneous inhibitory synaptic currents could be mimicked by a selective agonist of NK1 receptors, but not by selective agonists of NK2 and NK3 receptors. It was suppressed by an NK1 receptor antagonist. In CA1 interneurons located in stratum radiatum, substance P generated a sustained tetrodotoxin-insensitive inward current or induced membrane depolarization and action potential firing. This direct excitatory action was mediated by NK1 receptors. Current-voltage relationships indicate that the net tachykinin-evoked current reversed in polarity at or near the K+ equilibrium potential, suggesting that a suppression of a resting K+ conductance was involved. By increasing the excitability of CA1 GABAergic interneurons, tachykinins can powerfully facilitate the inhibitory synaptic input to pyramidal neurons. This indirect inhibition could play a role in regulating short-term and/or long-term synaptic plasticity, promoting neuronal circuit synchronization or, in some physiopathological situations, influencing epileptogenesis.     

Cellular changes in the postmortem hippocampus in major depression.

BACKGROUND: Imaging studies report that hippocampal volume is decreased in major depressive disorder (MDD). A cellular basis for reduced hippocampal volume in MDD has not been identified. METHODS: Sections of right hippocampus were collected in 19 subjects with MDD and 21 normal control subjects. The density of pyramidal neurons, dentate granule cell neurons, glia, and the size of the neuronal somal area were measured in systematic, randomly placed three-dimensional optical disector counting boxes. RESULTS: In MDD, cryostat-cut hippocampal sections shrink in depth a significant 18% greater amount than in control subjects. The density of granule cells and glia in the dentate gyrus and pyramidal neurons and glia in all cornv ammonis (CA)/hippocampal subfields is significantly increased by 30%-35% in MDD. The average soma size of pyramidal neurons is significantly decreased in MDD. CONCLUSION: In MDD, the packing density of glia, pyramidal neurons, and granule cell neurons is significantly increased in all hippocampal subfields and the dentate gyrus, and pyramidal neuron soma size is significantly decreased as well. It is suggested that a significant reduction in neuropil in MDD may account for decreased hippocampal volume detected by neuroimaging. In addition, differential shrinkage of frozen sections of the hippocampus suggests differential water content in hippocampus in MDD.     

Shape of frequency-current curves in CAI pyramidal cells in the hippocampus

Frequency-current curves for CA1 hippocampal pyramidal neurones are shown to have basically the same shape as those for spinal motoneurones, with a region of shallow slope at low frequencies, preceding a steep linear or upward convex region at higher frequencies. The frequency range is, however, displaced towards lower frequencies. The results suggest, in a qualitative sense, that the firing behaviour of CA1 pyramidal cells is regulated by the afterhyperpolarization, at least in the low frequency range.     

Differential developmental refinement of the intrinsic electrophysiological properties of CA1 pyramidal neurons from the rat dorsal and ventral hippocampus

The dorsal and ventral regions of the rat longitudinal hippocampal axis are functionally distinct. That is, each region is associated with different behavioral tasks and disease susceptibilities due to underlying anatomical, and physiological differences. These differences are especially pronounced in area CA1, where significant differences in morphology, synaptic physiology, intrinsic excitability, and gene expression have been reported between CA1 pyramidal neurons from the dorsal (DHC) and ventral hippocampus (VHC). However, despite a significant amount of recent attention, a cogent picture of the intrinsic electrophysiological profile of DHC and VHC neurons has remained elusive, due, in part, to experiments performed on rats at different developmental time points. Moreover, the resulting intrinsic electrophysiological profiles are sufficiently different as to warrant a thorough investigation of the spatial and temporal changes in the intrinsic excitability of CA1 pyramidal neurons across developmental time. Accordingly, in this study, I have characterized the intrinsic electrophysiological properties of CA1 pyramidal neurons from acute hippocampal slices prepared from the DHC and VHC throughout an approximately 3‐week developmental period (P14–P37). DHC and VHC neurons exhibited distinct intra‐region changes (DHC or VHC) and inter‐region differences (DHC versus VHC) in their intrinsic electrophysiological properties, which yielded two developmental timelines: (a) a common developmental timeline describing changes observed in both DHC and VHC neurons, and (b) a differential developmental timeline highlighting unique features observed in DHC neurons. Specifically, DHC neurons exhibited significant inter‐region differences in RMP, input resistance, threshold, and spike frequency adaptation relative to VHC neurons, as well as an intra‐region change in the rebound slope (a proxy for I_h). These observations both integrate and reconcile previous work performed with rats at different developmental stages and suggest a distinct developmental trajectory for DHC neurons that might shed light on the normal physiological functions and disease susceptibility of the DHC.     

Enkephalin inhibition of inhibitory input to CA1 and CA3 pyramidal neurons in the hippocampus

Enkephalin-induced excitation in the hippocampus has been attributed to the attenuation of inhibitory input as well as to augmentation of excitatory input to pyramidal neurons. We have further examined these possible mechanisms of enkephalin action, as well as the possibility that enkephalins may be affecting intrinsic membrane properties, by recording intracellularly from CA1 and CA3 pyramidal cells in the guinea pig hippocampal brain slice preparation. It was observed that the inhibitory synaptic potential was significantly decreased in the presence of leucine enkephalin and D-alanine, D-leucine-enkephalin (DADL), whereas the excitatory synaptic potential, revealed by block of the inhibitory postsynaptic potential (IPSP) by bicuculline, was unaltered. In addition, the response of pyramidal cells to pressure-applied GABA was unaffected by enkephalin, as were the voltage-dependent membrane conductances. The increase in excitability which was observed in both field potential and intracellular recordings to drop application of DADL must, then, be due to a purely presynaptic block of inhibitory interneurons in both the CA1 and CA3 areas of the hippocampus.     

Effects of age and alcohol exposure during early life on pyramidal cell numbers in the CA1-CA3 region of the rat hippocampus

We have previously shown that exposing rats to a relatively high dose of ethanol during early postnatal life can result in an alteration in spatial learning ability. The hippocampal formation is known to be involved in the control of this ability. The purpose of the present study was to determine whether exposure of rats to ethanol during early postnatal life had either immediate or delayed effects on the numbers of pyramidal cells in the CA1-CA3 subregion of the hippocampus. Wistar rats were exposed to a relatively high daily dose of ethanol at postnatal day 10-15 by placing them for 3 h/day in a chamber containing ethanol vapor. Groups of ethanol-treated (ET), separation control (SC), and mother-reared control (MRC) rats were anesthetized and killed at 16 and 30 days of age by perfusion with phosphate-buffered 2.5% glutaraldehyde. The Cavalieri principle was used to determine the volumes of the CA1 and CA2+CA3 regions. The physical disector method was used to estimate the numerical density of neurons in each of the subdivisions. The total number of pyramidal cells was calculated by multiplying the appropriate estimates of the numerical density by the volume. There were significant age-related reductions in the total numbers of pyramidal cells at 16-30 days of age irrespective of the groups examined. Ethanol treated rats were found to have slightly but significantly fewer pyramidal cell neurons than either the MRC or SC groups. These observations indicate that pyramidal cells in the hippocampus may be vulnerable to a relatively high dose of ethanol exposure during this short period of early postnatal life.     

Properties of the spike afterhyperpolarization in pyramidal tract neurons

Features of the spike afterhyperpolarization (AHP) recorded intracellularly have been analyzed in fast pyramidal tract neurons of cats. Cell input conductance increases during the AHP, possibly because of a change in potassium conductance, as suggested by an AHP equilibrium potential 10–15 mV negative to the resting membrane potential. When more spikes are evoked in succession, AHPs following the first one are strongly reduced in amplitude. The effect is virtually maximal (30–50% of the control) after a single spike and fades out by 200–400 ms after the last spike. At short interspike intervals the initial time course of the depression is hidden by summation occurring between consecutive AHPs.     

Distribution of glutamate decarboxylase65 immunoreactive puncta on pyramidal and nonpyramidal neurons in hippocampus of schizophrenic brain

Recent studies have reported an increase in GABA_A receptor binding activity in several key corticolimbic regions, including the hippocampal formation, of postmortem schizophrenic brain. Because this change has been postulated to represent a compensatory upregulation of this receptor, the current report has sought to determine whether a decrease of glutamate decarboxylase (GAD), the enzyme responsible for the synthesis of GABA, may also be present in the hippocampus of schizophrenic subjects. A standard immunoperoxidase technique, together with a computer-assisted microscopic analysis, has been employed to evaluate the distribution of the 65 kDalton isoform of GAD (GAD₆₅) in 12 normal controls and 13 schizophrenic subjects matched for age and postmortem interval (PMI). The results show no significant difference in the density of GAD₆₅-immunoreactive (-IR) puncta in contact with pyramidal neurons (PN), nonpyramidal neurons (NP), or neuropil (NPL) in sectors CA_1–4 and their various sub-laminae. When the data were considered in relation to neuroleptic exposure, a significant positive correlation between the density of GAD₆₅-IR puncta and drug dose was found on both PNs (r = 0.814, P = 0.002; r = 0.777, P = 0.005, respectively) and NPs (r = 0.673, P = 0.023; r = 0.672, P = 0.024, respectively) in sectors CA₄ and CA₃. A similar result was found in the stratum oriens of CA₃ (r = 0.704, P = 0.016) and CA₂ (r = 0.774, P = 0.009). In each instance, two neuroleptic free schizophrenics showed the lowest density of GAD₆₅-IR puncta. There was no significant relationship between the density of GAD₆₅-IR puncta with either age or PMI. Taken together with previous data showing an upregulation of GABA_A receptor activity in sectors CA₃ and CA₂, particularly the stratum oriens, this study provides further evidence in support of the hypothesis that an intrinsic defect of GABAergic activity may occur in the hippocampal formation of schizophrenic patients and show dose-related increases in relation to neuroleptic exposure. Synapse 29:323–332, 1998. © 1998 Wiley-Liss, Inc.     

Lack of experience‐dependent intrinsic plasticity in the adolescent infralimbic medial prefrontal cortex

Fear extinction, an inhibitory learning that suppresses a previously learned fear memory, is diminished during adolescence. Earlier studies have shown that this suppressed fear extinction during adolescence involves an altered glutamatergic plasticity in infralimbic medial prefrontal cortical (IL‐mPFC) pyramidal neurons. However, it is unclear whether the excitability of IL‐mPFC pyramidal neurons plays a role in this development‐dependent suppression of fear extinction. Therefore, we examined whether fear conditioning and extinction affect the active and passive membrane properties of IL‐mPFC layer 5 pyramidal neurons in preadolescent, adolescent and adult mice. Both preadolescent and adult mice exhibited a bidirectional modulation of the excitability of IL‐mPFC layer 5 pyramidal neurons following fear conditioning and extinction, i.e., fear conditioning reduced membrane excitability, whereas fear extinction reversed this effect. However, the fear conditioning‐induced suppression of excitability was not reversed in adolescent mice following fear extinction training. Neither fear conditioning nor extinction affected GABAergic transmission in IL‐mPFC layer 5 pyramidal neurons, suggesting that GABAergic transmission did not play a role in experience‐dependent modulation of neuronal excitability. Our results suggest that the extinction‐specific modulation of excitability is impaired during adolescence.     

Electrical fields directly contribute to action potential synchronization during convulsant-induced epileptiform bursts

Synchronous field-potential bursts were induced in hippocampal slices with picrotoxin. Differential recording between intracellular and adjacent extracellular electrodes during paroxysmal depolarization shifts revealed rapid transmembrane depolarizations (TMDs), which were spike prepotentials generated by electrical field effects. These experiments demonstrate that endogenous electrical fields contribute to spike synchronization in the presence of convulsant drugs when excitatory chemical synapses are functional.     

Learning-induced reduction in post-burst after-hyperpolarization (AHP) is mediated by activation of PKC

We studied the role of protein kinase C (PKC) and protein kinase A (PKA) in mediating learning-related long lasting reduction of the post-burst after-hyperpolarization (AHP) in cortical pyramidal neurons. We have shown previously that pyramidal neurons in the rat piriform (olfactory) cortex from trained (TR) rats have reduced post-burst AHP for 3 days after odour-discrimination learning, and that this reduction is due to decreased conductance of calcium-dependent potassium current. In the present study, we examined whether this long-lasting reduction in AHP is mediated by second messenger systems. The broad-spectrum kinase inhibitor, H7, increased the AHP in neurons from TR rats, but not in neurons from pseudo-trained (pseudo-TR) and naive rats. Consequently, the difference in AHP amplitude between neurons from TR and control animals was diminished. This effect was also obtained by application of the specific PKC inhibitor, GF-109203x. The PKC activator, 1-Oleoyl-2-acetyl-sn-glycerol (OAG), significantly reduced the AHP in neurons from naive and pseudo-TR rats, but not in neurons from TR rats, so that the difference between the groups was abolished. The PKA-specific inhibitor, H-89, increased the AHP in neurons from all groups to a similar extent, and the difference in AHP amplitude between neurons from TR rats and neurons from controls was maintained. We suggest that while the post-burst AHP in piriform cortex pyramidal neurons is modulated by both PKC and PKA, a PKC-dependent process maintains the learning-related reduction of the AHP in these cells.     

Differential distribution of SK channel subtypes in the brain of the weakly electric fish Apteronotus leptorhynchus

Calcium signals in vertebrate neurons can induce hyperpolarizing membrane responses through the activation of Ca(2+)-activated potassium channels. Of these, small conductance (SK) channels regulate neuronal responses through the generation of the medium after-hyperpolarization (mAHP). We have previously shown that an SK channel (AptSK2) contributes to signal processing in the electrosensory system of Apteronotus leptorhynchus. It was shown that for pyramidal neurons in the electrosensory lateral line lobe (ELL), AptSK2 expression selectively decreases responses to low-frequency signals. The localization of all the SK subunits throughout the brain of Apteronotus then became of substantial interest. We have now cloned two additional SK channel subunits from Apteronotus and determined the expression patterns of all three AptSK subunits throughout the brain. In situ hybridization experiments have revealed that, as in mammalian systems, the AptSK1 and 2 channels showed a partially overlapping expression pattern, whereas the AptSK3 channel was expressed in different brain areas. The AptSK1 and 2 channels were the primary subunits found in the major electrosensory processing areas. Immunohistochemistry further revealed distinct compartmentalization of the AptSK1 and 2 channels in the ELL. AptSK1 was localized to the apical dendrites of pyramidal neurons, whereas AptSK2 channels are primarily somatic. The distinct expression patterns of all three AptSK channels may reflect subtype-specific contributions to neuronal function, and the high homology between subtypes from a number of species suggests that the functional roles for each channel subtype are conserved from early vertebrate evolution.     

Ontogeny of altered dendritic morphology in the rat prefrontal cortex, hippocampus, and nucleus accumbens following Cesarean delivery and birth anoxia

We used a delayed Cesarean birth model and the Golgi-Cox staining method to investigate the effects of perinatal anoxia on prefrontal cortex (PFC) and hippocampal (CA1) pyramidal neurons as well as nucleus accumbens (NAcc) medium spiny neurons. Dendritic morphology in these regions was studied on postnatal days (P) 2, 7, 14, 21, 35, and 70 in male Sprague-Dawley rats born either vaginally (VAG) or by Cesarean section either with (C + anoxia) or without (C-only) anoxia. The most striking birth group differences seen were at the level of dendritic spine densities on P35. During this postnatal period the dendritic spine density of PFC neurons was significantly lower in C + anoxia and C-only animals than in VAG controls; however, by P70 PFC spine densities in all birth groups were comparable. In contrast, hippocampal spine densities on P35 were comparably greater in C + anoxia animals than in VAG controls, whereas in C-only animals spine densities were lower than controls; here again, by P70 all groups had comparable hippocampal spine densities. In NAcc greater spine densities were seen on medium spiny neurons of C + anoxia animals on P35. These findings provide evidence that perinatal insult in the form of Cesarean birth with or without anoxia alters the dendritic development of PFC and hippocampal pyramidal neurons and to some extent also of NAcc medium spiny neurons. They also suggest that perinatal anoxia can alter the neuronal development of key structures thought to be affected in such late-onset dopamine-related disorders as schizophrenia and Attention Deficit Hyperactivity Disorder (ADHD).     

Juvenile Taiep rats have shorter dendritic trees in the dorsal field of the hippocampus without spatial learning disabilities

Myelin mutant taiep rats show a progressive demyelination in the central nervous system due to an abnormal accumulation of microtubules in the cytoplasm and the processes on their oligodendrocytes. Demyelination is associated with electrophysiological alterations and the mutant had a progressive astrocytosis. The illness is associated with change in cytokine levels and in the expression of different nitric oxide synthase and concomitantly lipoperoxidation in several areas of the brain. However, until now there has been no detailed anatomical analysis of neurons in this mutant. The aim of this study was to analyze the dendritic morphology in the hippocampus using Golgi‐Cox staining and spatial memory through Morris water maze test in young adult (3 months old) taiep rats and compare them with normal Sprague‐Dawley. Our results showed that taiep rats have altered dendritic tree morphology in pyramidal neurons in the CA1 field of the hippocampus, but not in the CA3 region. These morphological changes did not produce a concomitant deficit in spatial memory acquisition or recall at this early stage of the disease. Our results suggest that impairment of dendritic morphology in the CA1 field of the hippocampus is a landmark of the pathology of this progressive multiple sclerosis model.     

Synchronization of GABAergic interneuronal networks during seizure-like activity in the rat horizontal hippocampal slice

We studied the contribution of GABAergic (gamma-aminobutyric acid) neurotransmission to epileptiform activity using the horizontal hippocampal rat brain slice. Seizure-like (ictal) activity was evoked in the CA1 area by applying high-frequency trains (80 Hz for 2 s) to the Schaffer collaterals. Whole-cell recordings from stratum oriens-alveus interneurons revealed burst firing with superimposed high-frequency spiking which was synchronous with field events and pyramidal cell firing during ictal activity. On the other hand, interictal interneuronal bursts were synchronous with large-amplitude inhibitory postsynaptic potentials (IPSPs) in pyramidal cells. Excitatory and inhibitory postsynaptic potentials were simultaneously received by pyramidal neurons during the ictal afterdischarge, and were synchronous with interneuronal bursting and field potential ictal events. The GABAA receptor antagonist bicuculline greatly reduced the duration of the ictal activity in the CA1 layer, and evoked rhythmic interictal synchronous bursting of interneurons and pyramidal cells. With intact GABAergic transmission, interictal field potential events were synchronous with large amplitude IPSPs (9.8 +/- 2.4 mV) in CA1 pyramidal cells, and with interneuronal bursting. Simultaneous dual recordings revealed synchronous IPSPs received by widely separated pyramidal neurons during ictal and interictal periods, indicative of widespread interneuronal firing synchrony throughout the hippocampus. CA3 pyramidal neurons fired in synchrony with interictal field potential events recorded in the CA1 layer, and glutamate receptor antagonists abolished interictal interneuronal firing and synchronous large amplitude IPSPs received by CA1 pyramidal cells. These observations provide evidence that the interneuronal network may be entrained in hyperexcitable states by GABAergic and glutamatergic mechanisms.     

Synchronized clusters of action potentials can increase or decrease the excitability of the axons of magnocellular hypothalamic neurosecretory cells

Extracellular recordings were made from supraoptic nucleus (SON) cells in urethane anaesthetized male rats in vivo. Two stimulating electrodes were positioned to activate the cells antidromically, one in the mid axon region of the cells and the other at the axon terminals. Trains of 5-20 just-subthreshold stimuli at 5 s intervals decreased the threshold for antidromic activation from both sites. Whereas neither single stimuli, nor the stimuli at the beginning of a train of 20 stimuli evoked antidromic action potentials, later action potentials did so. Paradoxically, trains of 20 just-suprathreshold stimuli increased the threshold for activation of both axons and terminals. In recordings from the same cells, stimuli were applied singly at 5 s intervals at an intensity which almost invariably evoked an antidromic action potential. Identical stimuli were then applied in trains of 20 stimuli at 50 Hz. After the first train, the initial stimulus pulses of the trains frequently fell below threshold. Following a conditioning train of five stimuli applied to one electrode, the period of decreased threshold (increased excitability) at the other electrode lasted less than 100 ms and the period of increased threshold (decreased excitability) after 12 trains of 20 stimuli lasted between 5 and 10 s. Both decreased and increased excitability were seen at axons and terminals of both putative oxytocin and vasopressin cells. Since the excitability changes were shown in vivo at frequencies encountered during recordings, it is likely that they influence the probability of spike propagation and hormone secretion under physiological conditions.     

Action of vasopressin on CA1 pyramidal neurons in rat hippocampal slices

The effects of arginine⁸-vasopressin (AVP) on the excitability of 47 pyramidal cells of the CA1 region of the hippocampus were determined by using intracellular recording techniques in a submerged slice preparation. Addition of 10⁻⁶ M AVP to the bathing medium evoked an increase in spike discharge which was slow in onset and only gradually reversible. The discharge was accompanied by an increase in excitatory postsynaptic potentials without significant change of the resting input resistance. AVP-induced excitation was found in 81% of ventral and 29% of dorsal hippocampal CA1 pyramidal cells. In low Ca²⁺, high Mg²⁺ solution this excitatory action by AVP was blocked. Microiontophoretic application of AVP onto apical or basal dendrites or the cell body did not result in excitation. These observations suggest that the action of AVP on CA1 pyramidal cells is transsynaptic and is more pronounced in ventral than dorsal CA1.