The expression pattern of somatostatin and calretinin by postnatal hippocampal interneurons is regulated by activity-dependent and -independent determinants

Nrg1 haploinsufficiency alters inhibitory cortical circuits

Neuregulin 1 (NRG1) and its receptor ERBB4 are schizophrenia (SZ) risk genes that control the development of both excitatory and inhibitory cortical circuits. Most studies focused on the characterization ErbB4 deficient mice. However, ErbB4 deletion concurrently perturbs the signaling of Nrg1 and Neuregulin 3 (Nrg3), another ligand expressed in the cortex. In addition, NRG1 polymorphisms linked to SZ locate mainly in non-coding regions and they may partially reduce Nrg1 expression. Here, to study the relevance of Nrg1 partial loss-of-function in cortical circuits we characterized a recently developed haploinsufficient mouse model of Nrg1 (Nrg1^tm1Lex). These mice display SZ-like behavioral deficits. The cellular and molecular underpinnings of the behavioral deficits in Nrg1^tm1Lex mice remain to be established. With multiple approaches including Magnetic Resonance Spectroscopy (MRS), electrophysiology, quantitative imaging and molecular analysis we found that Nrg1 haploinsufficiency impairs the inhibitory cortical circuits. We observed changes in the expression of molecules involved in GABAergic neurotransmission, decreased density of Vglut1 excitatory buttons onto Parvalbumin interneurons and decreased frequency of spontaneous inhibitory postsynaptic currents. Moreover, we found a decreased number of Parvalbumin positive interneurons in the cortex and altered expression of Calretinin. Interestingly, we failed to detect other alterations in excitatory neurons that were previously reported in ErbB4 null mice suggesting that the Nrg1 haploinsufficiency does not entirely phenocopies ErbB4 deletions. Altogether, this study suggests that Nrg1 haploinsufficiency primarily affects the cortical inhibitory circuits in the cortex and provides new insights into the structural and molecular synaptic impairment caused by NRG1 hypofunction in a preclinical model of SZ.

Expression of Transcripts Selective for GABA Neuron Subpopulations across the Cortical Visuospatial Working Memory Network in the Healthy State and Schizophrenia

Visuospatial working memory (WM), which is impaired in schizophrenia, depends on a distributed network including visual, posterior parietal, and dorsolateral prefrontal cortical regions. Within each region, information processing is differentially regulated by subsets of γ-aminobutyric acid (GABA) neurons that express parvalbumin (PV), somatostatin (SST), or vasoactive intestinal peptide (VIP). In schizophrenia, WM impairments have been associated with alterations of PV and SST neurons in the dorsolateral prefrontal cortex. Here, we quantified transcripts selectively expressed in GABA neuron subsets across four cortical regions in the WM network from comparison and schizophrenia subjects. In comparison subjects, PV mRNA levels declined and SST mRNA levels increased from posterior to anterior regions, whereas VIP mRNA levels were comparable across regions except for the primary visual cortex (V1). In schizophrenia subjects, each transcript in PV and SST neurons exhibited similar alterations across all regions, whereas transcripts in VIP neurons were unaltered in any region except for V1. These findings suggest that the contribution of each GABA neuron subset to inhibitory regulation of local circuitry normally differs across cortical regions of the visuospatial WM network and that in schizophrenia alterations of PV and SST neurons are a shared feature across these regions, whereas VIP neurons are affected only in V1.

Prolonged withdrawal from cocaine self-administration affects prefrontal cortex- and basolateral amygdala-nucleus accumbens core circuits but not accumbens GABAergic local interneurons

Withdrawal from extended-access cocaine self-administration leads to progressive intensification ('incubation') of cocaine craving. After prolonged withdrawal (1-2 months), when craving is high, expression of incubation depends on strengthening of excitatory inputs to medium spiny neurons (MSN) of the nucleus accumbens (NAc). These excitatory inputs interact with the intra-NAc GABAergic 'microcircuit', composed of MSN axon collaterals and GABAergic interneurons. Here, we investigated whether the increased glutamatergic neurotransmission observed after prolonged withdrawal is accompanied by altered GABAergic neurotransmission, focusing on NAc core. Rats self-administered cocaine or saline (6 hours/day) and then underwent >40 days of withdrawal. First, we investigated parvalbumin positive (PV+) interneurons, GABAergic fast-spiking interneurons that regulate MSN activity. Immunohistochemical studies revealed no significant change in PV signal intensity or the number of PV+ cells in cocaine rats versus saline controls. We then screened PV and other interneuron markers using immunoblotting. We detected no changes in levels of PV, calretinin, calbindin or neuronal nitric oxide synthase. Because expression of these markers is activity dependent, our results suggest no marked changes in interneuron activity. Finally, we utilized local field potential recording, which can detect GABA-mediated alterations at the circuit level, to investigate potential changes in two circuits implicated in cocaine craving: prelimbic prefrontal cortex to NAc core and basolateral amygdala to NAc core. We detected differential adaptations in these circuits, some of which may involve GABA. Overall, our results suggest that alterations in GABA transmission may accompany incubation of cocaine craving, but they are circuit specific and less pronounced than alterations in glutamate transmission.

Cortical Gene Expression After a Conditional Knockout of 67 kDa Glutamic Acid Decarboxylase in Parvalbumin Neurons

In the cortex of subjects with schizophrenia, expression of glutamic acid decarboxylase 67 (GAD67), the enzyme primarily responsible for cortical GABA synthesis, is reduced in the subset of GABA neurons that express parvalbumin (PV). This GAD67 deficit is accompanied by lower cortical levels of other GABA-associated transcripts, including GABA transporter-1, PV, brain-derived neurotrophic factor (BDNF), tropomyosin receptor kinase B, somatostatin, GABAA receptor α1 subunit, and KCNS3 potassium channel subunit mRNAs. In contrast, messenger RNA (mRNA) levels for glutamic acid decarboxylase 65 (GAD65), another enzyme for GABA synthesis, are not altered. We tested the hypothesis that this pattern of GABA-associated transcript levels is secondary to the GAD67 deficit in PV neurons by analyzing cortical levels of these GABA-associated mRNAs in mice with a PV neuron-specific GAD67 knockout. Using in situ hybridization, we found that none of the examined GABA-associated transcripts had lower cortical expression in the knockout mice. In contrast, PV, BDNF, KCNS3, and GAD65 mRNA levels were higher in the homozygous mice. In addition, our behavioral test battery failed to detect a change in sensorimotor gating or working memory, although the homozygous mice exhibited increased spontaneous activities. These findings suggest that reduced GAD67 expression in PV neurons is not an upstream cause of the lower levels of GABA-associated transcripts, or of the characteristic behaviors, in schizophrenia. In PV neuron-specific GAD67 knockout mice, increased levels of PV, BDNF, and KCNS3 mRNAs might be the consequence of increased neuronal activity secondary to lower GABA synthesis, whereas increased GAD65 mRNA might represent a compensatory response to increase GABA synthesis.

Amelioration of improper differentiation of somatostatin-positive interneurons by triiodothyronine in a growth-retarded hypothyroid mouse strain

Thyroid hormone (TH) plays an important role in brain development, and TH deficiency during pregnancy or early postnatal periods leads to neurological disorders such as cretinism. Hypothyroidism reduces the number of parvalbumin (PV)-positive interneurons in the neocortex and hippocampus. Here we used a mouse strain (growth-retarded; grt) that shows growth retardation and hypothyroidism to examine whether somatostatin (Sst)-positive interneurons that are generated from the same pool of neural progenitor cells as PV-positive cells are also altered by TH deficiency. The number of PV-positive interneurons was significantly decreased in the neocortex and hippocampus of grt mice as compared with normal control mice. In contrast to the decrease in the number of PV neurons, the number of Sst-positive interneurons in grt mice was increased in the stratum oriens of the hippocampus and the hilus of the dentate gyrus, although their number was unchanged in the neocortex. These changes were reversed by triiodothyronine administration from postnatal day (PD) 0 to 20. TH supplementation that was initiated after PD21 did not, however, affect the number of PV- or Sst-positive cells. These results suggest that during the first three postnatal weeks, TH may be critical for the generation of subpopulations of interneurons.

The development of hippocampal interneurons in rodents

Interneurons are GABAergic neurons responsible for inhibitory activity in the adult hippocampus, thereby controlling the activity of principal excitatory cells through the activation of postsynaptic GABAA receptors. Subgroups of GABAergic neurons innervate specific parts of excitatory neurons. This specificity indicates that particular interneuron subgroups are able to recognize molecules segregated on the membrane of the pyramidal neuron. Once these specific connections are established, a quantitative regulation of their strength must be performed to achieve the proper balance of excitation and inhibition. We will review when and where interneurons are generated. We will then detail their migration toward and within the hippocampus, and the maturation of their morphological and neurochemical characteristics. We will finally review potential mechanisms underlying the development of GABAergic interneurons.

Loss of Embryonic MET Signaling Alters Profiles of Hippocampal Interneurons

Hippocampal interneurons arise in the ventral forebrain and migrate dorsally in response to cues, including hepatocyte growth factor/scatter factor which signals via its receptor MET. Examination of the hippocampus in adult mice in which MET had been inactivated in the embryonic proliferative zones showed an increase in parvalbumin-expressing cells in the dentate gyrus, but a loss of these cells in the CA3 region. An overall loss of calretinin-expressing cells was seen throughout the hippocampus. A similar CA3 deficit of parvalbumin and calretinin cells was observed when MET was eliminated only in postmitotic cells. These data suggest that MET is required for the proper hippocampal development, and embryonic perturbations lead to long-term anatomical defects with possible learning and memory dysfunction.

Calcium-binding protein calretinin immunoreactivity in the dog superior colliculus

We studied calretinin-immunoreactive (IR) fibers and cells in the canine superior colliculus (SC) and studied the distribution and effect of enucleation on the distribution of this protein. Localization of calretinin was immunocytochemically observed. A dense plexus of anti-­calretinin-IR fibers was found within the upper part of the superficial gray layer (SGL). Almost all of the labeled fibers were small in diameter with few varicosities. The intermediate and deep layers contained many calretinin-IR neurons. Labeled neurons within the intermediate gray layer (IGL) formed clusters in many sections. By contrast, labeled neurons in the deep gray layer (DGL) did not form clusters. Calretinin-IR neurons in the IGL and DGL varied in morphology and included round/oval, vertical fusiform, stellate, and horizontal neurons. Neurons with varicose dendrites were also labeled in the IGL. Most of the labeled neurons were small to medium in size. Monocular enucleation produced an almost complete reduction of calretinin-IR fibers in the SC contralateral to the enucleation. However, many calretinin-IR cells appeared in the contralateral superficial SC. Enucleation appeared to have no effect on the distribution of calretinin-IR neurons in the contralateral intermediate and deep layers of the SC. The calretinin-IR neurons in the superficial dog SC were heterogeneous small- to medium-sized neurons including round/oval, vertical fusiform, stellate, pyriform, and ­horizontal in shape. Two-color immunofluorescence revealed that no cells in the dog SC ­expressed both calretinin and GABA. Many horseradish peroxidase (HRP)-labeled retinal ganglion cells were seen after injections into the superficial layers. The vast majority of the double-labeled cells (HRP and calretinin) were small cells. The present results indicate that antibody to calretinin labels subpopulations of neurons in the dog SC, which do not express GABA. The results also suggest that the calretinin-IR afferents in the superficial layers of the dog SC originate from small class retinal ganglion cells. The expression of calretinin might be changed by the cellular activity of selective superficial collicular neurons. These results are valuable in delineating the basic neurochemical architecture of the dog visual system.

A role for L-type calcium channels in the maturation of parvalbumin-containing hippocampal interneurons

While inhibitory interneurons are well recognized to play critical roles in the brain, relatively little is know about the molecular events that regulate their growth and differentiation. Calcium ions are thought to be important in neuronal development and L-type voltage gated Ca(+2) channels have been implicated in activity-dependent mechanisms of early-life. However, few studies have examined the role of these channels in the maturation of interneurons. The studies reported here were conducted in hippocampal slice cultures and indicate that the L-type Ca(+2) channel agonists and antagonists accelerate and suppress respectively the growth of parvalbumin-containing interneurons. The effects of channel blockade were reversible suggesting they are not the result of interneuronal cell death. Results from immunoblotting showed that these drugs have similar effects on the expression of the GABA synthetic enzymes, glutamic acid decarboxylase65, glutamic acid decarboxylase67 and the vesicular GABA transporter. This suggests that L-type Ca(+2) channels regulate not only parvalbumin expression but also interneuron development. These effects are likely mediated by actions on the interneurons themselves since the alpha subunits of L-type channels, voltage-gated calcium channel subunit 1.2 and voltage-gated calcium channel subunit 1.3 were found to be highly expressed in neonatal mouse hippocampus and co-localized with parvalbumin in interneurons. Results also showed that while these interneurons can contain either subunit, voltage-gated calcium channel subunit 1.3 was more widely expressed. Taken together results suggest that an important subset of developing interneurons expresses L-type Ca(+2) channels alpha subunits, voltage-gated calcium channel subunit 1.2 and especially voltage-gated calcium channel subunit 1.3 and that these channels likely regulate the development of these interneurons in an activity-dependent manner.

Homeostatic activity-dependent paradigm for neurotransmitter specification

Calcium-signaling plays a central role in specification of the chemical transmitters neurons express, adjusting the numbers of cells that express excitatory and inhibitory transmitters as if to achieve homeostatic regulation of excitability. Here we review the extent to which this activity-dependent regulation is observed for a range of different transmitters. Strikingly the homeostatic paradigm is observed both for classical and for peptide transmitters and in mature as well as in embryonic nervous systems. Transmitter homeostasis adds another dimension to homeostatic regulation of function in the nervous system that includes regulation of levels of voltage-gated ion channels, densities of neurotransmitter receptors, and synapse numbers and strength.

Mice deficient for the extracellular matrix glycoprotein tenascin-r show physiological and structural hallmarks of increased hippocampal excitability, but no increased susceptibility to seizures in the pilocarpine model of epilepsy

Recognition molecules provide important cues for neuronal survival, axonal fasciculation, axonal pathfinding, synaptogenesis, synaptic plasticity, and regeneration. Our previous studies revealed a link between perisomatic inhibition and the extracellular matrix glycoprotein tenascin-R (TN-R). Therefore, we here studied neuronal excitability and epileptic susceptibility in mice constitutively deficient in TN-R. In vitro analysis of populational spikes in hippocampal slices of TN-R-deficient mice revealed a significant increase in multiple spikes in the CA1 region, as compared with wild-type mice. This difference between genotypes was only partially reduced after blockade of GABA(A) receptors with picrotoxin, indicating a deficit in GABAergic inhibition and an increase in intrinsic excitability of CA1 pyramidal cells in TN-R-deficient mice. Using a battery of immunohistochemical markers and histological stainings, we were able to identify two abnormalities in the hippocampus of TN-R-deficient mice possibly related to increased excitability: the high number of glial fibrillary acidic protein-positive astrocytes and low number of calretinin-positive interneurons in the CA1 and CA3 regions. In order to test whether the revealed abnormalities give rise to increased susceptibility to seizures in TN-R-deficient mice, we used the pilocarpine model of epilepsy. No genotype-specific differences were found with regard to the time-course of pilocarpine-induced and spontaneous seizures, neuronal cell loss, aberrant sprouting and distribution of synaptic and inhibitory interneuron markers. However, pilocarpine-induced astrogliosis and reduction in calretinin-positive interneurons were less pronounced in TN-R mutants, thereby resulting in an occlusion of effects induced by TN-R deficiency and pilocarpine. Thus, TN-R-deficient mutants show several electrophysiological and morphological hallmarks of increased neuronal excitability, which, however, do not give rise to more accelerated or severe epileptogenesis in the pilocarpine model of epilepsy.

Immunocytochemical localization of calretinin in the superficial layers of the cat superior colliculus

We localized calretinin-immunoreactive (IR) fibers and cells in the superior colliculus (SC) of the cat and studied the distribution and effect of enucleation on the distribution of this protein. Calretinin was localized with antibody immunocytochemistry. A dense plexus of anti-calretinin-IR fibers was found within the upper part of the superficial gray layer. Almost all of the labeled fibers were small diameter fibers with few varicosities. Monocular enucleation produced an almost complete reduction of calretinin-IR fibers in the SC contralateral to the enucleation. Furthermore, many calretinin-IR cells appeared in the contralateral SC. The newly appeared cells had small- to medium-sized vertical fusiform, oval or round, or stellate cell bodies. Two-color immunofluorescence revealed that no cells in the superficial layers expressed both calretinin and GABA. Many retinal ganglion cells were labeled after injections of retrograde axonal transport horseradish peroxidase (HRP) in the superficial layers. However, no large cells were double-labeled with calretinin and HRP. More than 95% of the double-labeled cells were small cells (<15 microm). Based on the retinal ganglion cell size, we believe that the vast majority of calretinin-IR retinocollicular fibers in cat SC are small gamma type cells that have W type physiologies.

Physiological effects of sustained blockade of excitatory synaptic transmission on spontaneously active developing neuronal networks--an inquiry into the reciprocal linkage between intrinsic biorhythms and neuroplasticity in early ontogeny

Spontaneous bioelectric activity (SBA) taking the form of extracellularly recorded spike trains (SBA) has been quantitatively analyzed in organotypic neonatal rat visual cortex explants at different ages in vitro, and the effects investigated of both short- and long-term pharmacological suppression of glutamatergic synaptic transmission. In the presence of APV, a selective NMDA receptor blocker, 1-2- (but not 3-)week-old cultures recovered their previous SBA levels in a matter of hours, although in imitation of the acute effect of the GABAergic inhibitor picrotoxin (PTX), bursts of action potentials were abnormally short and intense. Cultures treated either overnight or chronically for 1-3 weeks with APV, the AMPA/kainate receptor blocker DNQX, or a combination of the two were found to display very different abnormalities in their firing patterns. NMDA receptor blockade for 3 weeks produced the most severe deviations from control SBA, consisting of greatly prolonged and intensified burst firing with a strong tendency to be broken up into trains of shorter spike clusters. This pattern was most closely approximated by acute GABAergic disinhibition in cultures of the same age, but this latter treatment also differed in several respects from the chronic-APV effect. In 2-week-old explants, in contrast, it was the APV+DNQX treated group which showed the most exaggerated spike bursts. Functional maturation of neocortical networks, therefore, may specifically require NMDA receptor activation (not merely a high level of neuronal firing) which initially is driven by endogenous rather than afferent evoked bioelectric activity. Putative cellular mechanisms are discussed in the context of a thorough review of the extensive but scattered literature relating activity-dependent brain development to spontaneous neuronal firing patterns.

Requirement of neural activity for the maintenance of dopaminergic neurons in rat midbrain slice cultures

Chronic treatment of organotypic midbrain slice cultures with L-type Ca(2+) channel blocker nicardipine (3-10 microM) or verapamil (10 microM) for 18 days resulted in a drastic decrease in the number of dopaminergic neurons. A voltage-dependent Na(+) channel blocker tetrodotoxin (1 microM) was also effective in decreasing the number of dopaminergic neurons. Concurrent application of forskolin (20 microM) or dibutyryl cyclic AMP (1 mM) counteracted the effects of nicardipine and tetrodotoxin. These results suggest that spontaneous neuronal activity within midbrain slice cultures, causing Ca(2+) influx through L-type Ca(2+) channels that maintains intracellular cyclic AMP levels, is required for the maintenance of dopaminergic neurons.

Brain-derived neurotrophic factor differentially regulates excitatory and inhibitory synaptic transmission in hippocampal cultures

Brain-derived neurotrophic factor (BDNF) has been postulated to be a key signaling molecule in regulating synaptic strength and overall circuit activity. In this context, we have found that BDNF dramatically increases the frequency of spontaneously initiated action potentials in hippocampal neurons in dissociated culture. Using analysis of unitary synaptic transmission and immunocytochemical methods, we determined that chronic treatment with BDNF potentiates both excitatory and inhibitory transmission, but that it does so via different mechanisms. BDNF strengthens excitation primarily by augmenting the amplitude of AMPA receptor-mediated miniature EPSCs (mEPSCs) but enhances inhibition by increasing the frequency of mIPSC and increasing the size of GABAergic synaptic terminals. In contrast to observations in other systems, BDNF-mediated increases in AMPA-receptor mediated mEPSC amplitudes did not require activity, because blocking action potentials with tetrodotoxin for the entire duration of BDNF treatment had no effect on the magnitude of this enhancement. These forms of synaptic regulations appear to be a selective action of BDNF because intrinsic excitability, synapse number, and neuronal survival are not affected in these cultures. Thus, although BDNF induces a net increase in overall circuit activity, this results from potentiation of both excitatory and inhibitory synaptic drive through distinct and selective physiological mechanisms.

Anterograde tracing and immunohistochemical characterization of potentially mechanosensitive vagal afferents in the esophagus

Vagal mechanosensitive afferents with an important functional role in esophageal peristalsis are well known from physiological studies. It is not known whether these fibers represent a separate subpopulation among all vagal afferents projecting to the esophageal wall. A morphological and immunohistochemical description of vagal afferents was undertaken to define their possible homo- or heterogeneity. The peripheral projections of vagal afferents were anterogradely labeled by injection of wheatgerm agglutinin conjugated to horseradish peroxidase into the nodose ganglion of rats. The anterogradely transported tracer was detected by tyramide amplification in conjunction with immunohistochemistry for Ca(2+)-binding proteins recently identified in different types of mechanosensory endings. It was found that vagal afferents represented a morphologically and structurally homogeneous population projecting to the myenteric ganglia of the esophagus, where they terminated as highly branched endings. Vagal afferent terminals, however, were different in their staining intensity for calretinin and calbindin, which ranged from intense to no detectable immunofluorescence. The fluorescence intensity of Ca(2+)-binding proteins within the vagal terminating branches was graded and the average staining intensity determined of all terminating branches in the upper, middle, and lower thirds of the esophagus. The average staining intensity was highest in the upper third of the esophagus and then declined in a statistically significant manner in the middle and lower thirds. This result suggests different requirements for intracellular Ca(2+)-buffering capacities in vagal afferents depending on their position along the esophageal axis and corroborates studies reporting a segmental organization of esophageal motility. Immunohistochemical evidence of substance P (SP) in a subset of vagal terminals was demonstrated. Hence, an effector role of vagal afferents on esophageal peristalsis by the release of SP, as has been proposed by physiological studies, is also supported by immunohistochemical data.

Consequences of cortical dysplasia during development in rats

PURPOSE

To determine whether focal cortical dysplasia alters excitability in regions distant to the region of the dysplasia.

METHODS

We studied the physiological consequences of cortical dysplasia induced by either one or three freeze lesions at birth. Seizure susceptibility was assessed at age 35 days by amygdala kindling. c-fos immunostaining was performed after kainic acid-induced seizures at 10, 20, or 30 days to evaluate the patterns of neuronal activation.

RESULTS

Freeze lesions consistently produced uniform regions of dysplasia. No significant differences in seizure susceptibility, as measured by afterdischarge threshold and kindling rate, were seen between controls and rats receiving either one or three freeze lesions. c-fos activation after kainic acid injection was not observed in the region of the dysplasia. However, rats with freeze lesions at age 30 days demonstrated asymmetric c-fos staining with greater staining in CA1 ipsilateral, than contralateral, to the lesion.

CONCLUSIONS

Focal cortical dysplasia results in enhancement of c-fos activation in regions outside the borders of the dysplasia. However, as indicated by kindling rate, the functional consequences of these alterations do not appear to be robust.

BDNF and NT-4 differentiate two pathways in the modulation of neuropeptide protein levels in postnatal hippocampal interneurons

Neuropeptide protein levels in hippocampal interneurons exhibit a considerable maturation in postnatal animals. This study characterizes the role of neuronal activity in determining neuropeptide protein levels in postnatal hippocampal interneurons, and the involvement of neurotrophins. In hippocampal slices from 7-day-old rats cultured for 2 weeks, treatment with the gamma-aminobutyric acidA (GABAA) receptor antagonist bicuculline increased the staining intensity and the number of neurons immunoreactive for neuropeptide Y (NPY). An opposite effect was observed when non-N-methyl-d-aspartate (non-NMDA) excitatory transmission was blocked. The effects of either treatment were reversed after return to control medium. These findings were similar to those previously obtained on the effects of activity on somatostatin immunostaining. Blockade of endogenous tyrosine kinase neurotrophin receptors using K252a prevented the effects of bicuculline on NPY- and somatostatin-immunoreactive neurons. Application of exogenous neurotrophin-3 (NT-3) increased NPY and somatostatin protein levels in long-term but not short-term cultures, while nerve growth factor (NGF) had no effect. In contrast, brain-derived neurotrophic factor (BDNF) or neurotrophin-4 (NT-4) did not affect equally NPY and somatostatin immunoreactivity: they mimicked the effects of bicuculline treatment on NPY-immunoreactive neurons, but exerted no conspicuous effect on somatostatin immunostaining. These results indicate that although neuronal activity plays a major role in determining neuropeptide protein levels in postnatal hippocampal interneurons, its effects on different neuropeptides might be exerted through different mechanisms, with or without the mediation of BDNF or NT-4.

How many subtypes of inhibitory cells in the hippocampus?

Hippocampal inhibitory cells are diverse. It is supposed that they fall into functionally distinct subsets defined by a similar morphology and physiology. Switching between functions could be accomplished by activating receptors for modulating transmitters expressed selectively by different subsets of interneurons. We tested this hypothesis by comparing morphology, physiology, and neurotransmitter receptor expression for CA1 hippocampal interneurons. We distinguished 16 distinct morphological phenotypes and 3 different modes of discharge. Subsets of inhibitory cells were excited or inhibited by agonists at receptors for noradrenaline, muscarine, serotonin, and mGluRs. Most cells responded to 2 or 3 agonists, and 25 different response combinations were detected. Subsets defined by morphology, physiology, and receptor expression did not coincide, suggesting that hippocampal interneurons cannot easily be segregated into a few well-defined groups.