Layer selective presynaptic modulation of excitatory inputs to hippocampal cornu Ammon 1 by mu-opioid receptor activation

Chronic and acute activation of mu-opioid receptors (MOR) in hippocampal cornu Ammon 1 (CA1) disrupts rhythmic activity, alters activity-dependent synaptic plasticity and impairs spatial memory formation. In CA1, MORs act by hyperpolarizing inhibitory interneurons and suppressing inhibitory synaptic transmission. MOR modulation of inhibitory synaptic function translates into an increase in excitatory activity in all layers of CA1. However, the exact anatomical sites for MOR actions are not completely known. Therefore, we used voltage-sensitive dye imaging, whole cell patch clamping, photolysis of alpha-carboxy-2-nitrobenzyl ester, trifluoroacetic acid salt (CNB) -caged GABA, and micro-sectioned slices of rat hippocampus to investigate the effect of MOR activation in CA1. First, we investigated the effect of MOR activation using a MOR agonist [d-Ala2, NMe-Phe4, Gly-ol5]-enkephalin (DAMGO) on the direct activation of GABA receptors by photolysis of CNB-caged GABA in all layers of CA1. MOR activation did not affect hyperpolarizations due to direct GABA receptor activation in any layer of CA1, but MOR activation did suppress GABAergic inhibitory postsynaptic potentials suggesting that MOR activation acts by presynaptically inhibiting interneuron function. We next examined whether MOR activation was equivalently effective in all anatomical layers of CA1. To do this, cuts were made between anatomical layers of CA1 and isolated layers were stimulated electrically (five pulses at 20 Hz) to produce excitatory postsynaptic potentials (EPSPs). Under these conditions, MOR activation significantly increased EPSP areas in stratum radiatum (SR), stratum pyramidale (SP) and stratum oriens (SO) relative to stratum lacunosum-moleculare (SLM). When compared with the effect of GABA(A) and GABA(B) receptor antagonists on EPSP areas, the effect of DAMGO was proportionately larger in SR, SP and SO than in SLM. We conclude that MOR activation is more effective at directly modulating activity in SR, SP and SO, and the smaller effect in SLM is likely due to a smaller MOR inhibition of GABA release in SLM.     

Adenosine A2a receptor antagonists attenuate striatal adaptations following dopamine depletion

The motor symptoms of Parkinson's disease (PD) are widely thought to arise from an imbalance in the activity of the two major striatal efferent pathways following the loss of dopamine (DA) signaling. In striatopallidal, indirect pathway spiny projection neurons (iSPNs), intrinsic excitability rises following the loss of inhibitory D2 receptor signaling. Because these receptors are normally counterbalanced by adenosine A2a adenosine receptors, antagonists of these receptors are being examined as an adjunct to conventional pharmacological therapies. However, little is known about the effects of sustained A2a receptor antagonism on striatal adaptations in PD models. To address this issue, the A2a receptor antagonist SCH58261 was systemically administered to DA-depleted mice. After 5 days of treatment, the effects of SCH58261 on iSPNs were examined in brain slices using electrophysiological and optical approaches. SCH58261 treatment did not prevent spine loss in iSPNs following depletion, but did significantly attenuate alterations in synaptic currents, spine morphology and dendritic excitability. In part, these effects were attributable to the ability of SCH58261 to blunt the effects of DA depletion on cholinergic interneurons, another striatal cell type that co-expresses A2a and D₂ receptors. Collectively, these results suggest that A2a receptor antagonism improves striatal function in PD models by attenuating iSPN adaptations to DA depletion.     

Effects of brain-derived neurotrophic factor on the development of NADPH-diaphorase/nitric oxide synthase-positive amacrine cells in the rodent retina.

Amacrine neurons expressing nitric oxide synthase (NOS) contain brain-derived neurotrophic factor (BDNF) receptors and respond to exogenous BDNF [Klöcker, N., Cellerino, A. & Bähr, M. (1998) J. Neurosci., 18, 1038–1046]. We analysed the effects of BDNF on the development of neurons which express NOS in the mouse and rat retina. Rat pups received a total of three intraocular injections of BDNF at intervals of 48 h, starting at postnatal day 16 (P16), and were killed at P22. The retinas were stained for NADPH-diaphorase, a histological marker of NOS. NOS-expressing neurons were found in both the inner nuclear layer (INL) and the ganglion cell layer (GCL). Two classes of NOS-expressing neurons, type I and type II, had already been distinguished in the INL [Koistinaho, J. & Sagar, S.M. (1995) In Osborne, N.N. & Chader, G.J. (eds), Progress in Retinal and Eye Research, Vol. 15. Oxford University Press, pp. 69–87] and a third one in the GCL. Up-regulation of NADPH-diaphorase activity was observed after BDNF treatment. The number of type I neurons remained stable, whereas the number of type II neurons and NOS-positive neurons in the GCL increased significantly (P < 0.001). Type I and type II neurons were significantly larger in BDNF-treated retinas. Double-labelling experiments revealed that BDNF induces NADPH-diaphorase in dopaminergic neurons and amacrine cells displaced to the GCL, but not in retinal ganglion cells. In mice homozygous for a null mutation of the bdnf gene, the intensity of NADPH-diaphorase labelling in both somata and processes was reduced, but the number of labelled neurons was not dramatically reduced. These findings indicate that BDNF regulates the neurotransmitter phenotype of NOS-expressing amacrine neurons under physiological conditions, but is not required for their survival.     

On the effect of chemically activated fine muscle afferents on interneurones mediating group I non-reciprocal inhibition of extensor ankle and knee muscles in humans

In a previous paper it was shown that muscle nociceptive discharge depressed the activity of interneurones mediating group I non-reciprocal inhibition (or Ib interneurones) in humans [A. Rossi, B. Decchi, Changes in Ib heteronymous inhibition to soleus motoneurons during cutaneous and muscle nociceptive stimulation in humans, Brain Res. 774 (1997) 55–61.]. However, since nociceptive discharge depressed the size of the soleus H-reflex (by which Ib inhibition was tested) the question arises as to whether modification of motoneurone membrane conductance per se could depress the size of Ib inhibitory post-synaptic potentials. The results of the present study suggest that the contribution of motoneurone hyperpolarization to Ib disinhibition is negligible and that muscle nociceptive discharge actually depresses the activity of these pathways.     

Distribution of prepro-nociceptin/ orphanin FQ mRNA and its receptor mRNA in developing and adult mouse central nervous systems

Nociceptin/orphanin FQ (N/OFQ) and its receptor share similarities to opioids and their receptors in terms of the molecular structure and signaling pathway, but the two systems exhibit different actions in vivo. To understand the mechanism of N/OFQ-system actions, we examined, by in situ hybridization analysis, the distribution of preproN/OFQ and N/OFQ receptor mRNAs in the developing and adult mouse central nervous systems (CNS). In most neural regions, preproN/OFQ mRNA was mainly expressed in a small population of middle-sized neurons. These neurons were scattered between large projection-type neurons or within the neuropil, suggestive of interneurons. In some other nuclei (lateral septum, bed nucleus of the stria terminalis, reticular thalamic nucleus, inferior colliculus, and rostral periolivery nucleus), preproN/OFQ mRNA was expressed in a number of large projection-type neurons. By contrast, N/OFQ receptor mRNA was evenly expressed in most neurons of the adult CNS. Considering the inhibitory actions of N/OFQ, the distinct cellular expression pattern of the N/OFQ system suggests that the release of N/OFQ from interneurons may lower neuronal and synaptic activities of neighboring neurons, leading to integration or modulation of local circuits. Furthermore, the cellular expression pattern, distinct from that of the opioid system, may provide a possible molecular/cellular basis for the different in vivo actions of N/OFQ and opioids. In embryonic stages, both preproN/OFQ and N/OFQ receptor mRNAs were highly and widely expressed in the mantle zone, suggesting the possible importance of N/OFQ signaling in CNS development. J. Comp. Neurol. 399:139–151, 1998. © 1998 Wiley-Liss, Inc.     

Abnormalities in cortical interneuron subtypes in ephrin‐B mutant mice

To explore roles for ephrin‐B/EphB signaling in cortical interneurons, we previously generated ephrin‐B (Efnb1/b2/b3) conditional triple mutant (TM^lz) mice using a Dlx1/2.Cre inhibitory neuron driver and green fluorescent protein (GFP) reporters for the two main inhibitory interneuron groups distinguished by expression of either glutamic acid decarboxylase 1 (GAD1; GAD67‐GFP) or 2 (GAD2; GAD65‐GFP). This work showed a general involvement of ephrin‐B in migration and population of interneurons into the embryonic neocortex. We now determined whether specific interneurons are selectively affected in the adult brains of TM^lz.Cre mice by immunostaining with antibodies that identify the different subtypes. The results indicate that GAD67‐GFP‐expressing interneurons that also express parvalbumin (PV), calretinin (CR) and, to a lesser extent, somatostatin (SST) and Reelin (Rln) were significantly reduced in the cortex and hippocampal CA1 region in TM^lz.Cre mutant mice. Neuropeptide Y (NPY) interneurons that also express GAD67‐GFP were reduced in the hippocampal CA1 region, but much less so in the cortex, although these cells exhibited abnormal cortical layering. In GAD65‐GFP‐expressing interneurons, CR subtypes were reduced in both cortex and hippocampal CA1 region, whereas Rln interneurons were reduced exclusively in hippocampus, and the numbers of NPY and vasoactive intestinal polypeptide (VIP) subtypes appeared normal. PV and CR subtype interneurons in TM^lz.Cre mice also exhibited reductions in their perisomatic area, suggesting abnormalities in dendritic/axonal complexity. Altogether, our data indicate that ephrin‐B expression within forebrain interneurons is required in specific subtypes for their normal population, cortical layering and elaboration of cell processes.     

Disruption of perineuronal nets increases the frequency of sharp wave ripple events

Hippocampal sharp wave ripples (SWRs) represent irregularly occurring synchronous neuronal population events that are observed during phases of rest and slow wave sleep. SWR activity that follows learning involves sequential replay of training‐associated neuronal assemblies and is critical for systems level memory consolidation. SWRs are initiated by CA2 or CA3 pyramidal cells (PCs) and require initial excitation of CA1 PCs as well as participation of parvalbumin (PV) expressing fast spiking (FS) inhibitory interneurons. These interneurons are relatively unique in that they represent the major neuronal cell type known to be surrounded by perineuronal nets (PNNs), lattice like structures composed of a hyaluronin backbone that surround the cell soma and proximal dendrites. Though the function of the PNN is not completely understood, previous studies suggest it may serve to localize glutamatergic input to synaptic contacts and thus influence the activity of ensheathed cells. Noting that FS PV interneurons impact the activity of PCs thought to initiate SWRs, and that their activity is critical to ripple expression, we examine the effects of PNN integrity on SWR activity in the hippocampus. Extracellular recordings from the stratum radiatum of horizontal murine hippocampal hemisections demonstrate SWRs that occur spontaneously in CA1. As compared with vehicle, pre‐treatment (120 min) of paired hemislices with hyaluronidase, which cleaves the hyaluronin backbone of the PNN, decreases PNN integrity and increases SWR frequency. Pre‐treatment with chondroitinase, which cleaves PNN side chains, also increases SWR frequency. Together, these data contribute to an emerging appreciation of extracellular matrix as a regulator of neuronal plasticity and suggest that one function of mature perineuronal nets could be to modulate the frequency of SWR events.     

Cholinergic control of striatal neurons to modulate L‐dopa‐induced dyskinesias

L‐dopa induced dyskinesias (LIDs) are a disabling motor complication of L‐dopa therapy for Parkinson's disease (PD) management. Treatment options remain limited and the underlying network mechanisms remain unclear due to a complex pathophysiology. What is well‐known, however, is that aberrant striatal signaling plays a key role in LIDs development. Here, we discuss the specific contribution of striatal cholinergic interneurons (ChIs) and GABAergic medium spiny projection neurons (MSNs) with a particular focus on how cholinergic signaling may integrate multiple striatal systems to modulate LIDs expression. Enhanced ChI transmission, altered MSN activity and the associated abnormal downstream signaling responses that arise with nigrostriatal damage are well known to contribute to LIDs development. In fact, enhancing M4 muscarinic receptor activity, a receptor favorably expressed on D1 dopamine receptor‐expressing MSNs dampens their activity to attenuate LIDs. Likewise, ChI activation via thalamostriatal neurons is shown to interrupt cortical signaling to enhance D2 dopamine receptor‐expressing MSN activity via M1 muscarinic receptors, which may interrupt ongoing motor activity. Notably, numerous preclinical studies also show that reducing nicotinic cholinergic receptor activity decreases LIDs. Taken together, these studies indicate the importance of cholinergic control of striatal neuronal activity and point to muscarinic and nicotinic receptors as significant pharmacological targets for alleviating LIDs in PD patients.     

Integrated anatomical and physiological mapping of striatal afferent projections

The dorsomedial striatum, a key site of reward‐sensitive motor output, receives extensive afferent input from cortex, thalamus and midbrain. These projections are integrated by striatal microcircuits containing both spiny projection neurons and local circuit interneurons. To explore target cell specificity of these projections, we compared inputs onto D1‐dopamine receptor‐positive spiny neurons, parvalbumin‐positive fast‐spiking interneurons and somatostatin‐positive low‐threshold‐spiking interneurons, using cell type‐specific rabies virus tracing and optogenetic‐mediated projection neuron recruitment in mice. While the relative proportion of retrogradely labelled projection neurons was similar between target cell types, the convergence of inputs was systematically higher for projections onto fast‐spiking interneurons. Rabies virus is frequently used to assess cell‐specific anatomical connectivity but it is unclear how this correlates to synaptic connectivity and efficacy. To test this, we compared tracing data with target cell‐specific measures of synaptic efficacy for anterior cingulate cortex and parafascicular thalamic projections using novel quantitative optogenetic measures. We found that target‐specific patterns of convergence were extensively modified according to region of projection neuron origin and postsynaptic cell type. Furthermore, we observed significant divergence between cell type‐specific anatomical connectivity and measures of excitatory synaptic strength, particularly for low‐threshold‐spiking interneurons. Taken together, this suggests a basic uniform connectivity map for striatal afferent inputs upon which presynaptic–postsynaptic interactions impose substantial diversity of physiological connectivity.     

The evolution of motor cortical dysfunction in amyotrophic lateral sclerosis

Objective

The present study aimed to investigate alterations in cortical function in amyotrophic lateral sclerosis (ALS) related to disease progression.