Biomedical subjects

Matthew Ennis

Publications and source records attributed to Matthew Ennis.

11 recordsLinked to original sources

PubMed2007

Group I metabotropic glutamate receptors are differentially expressed by two populations of olfactory bulb granule cells.

In the main olfactory bulb, several populations of granule cells (GCs) can be distinguished based on the soma location either superficially, interspersed with mitral cells within the mitral cell layer (MCL), or deeper, within the GC layer (GCL). Little is known about the physiological properties of superficial GCs (sGCs) versus deep GCs (dGCs). Here, we used patch-clamp recording methods to explore the role of Group I metabotropic glutamate receptors (mGluRs) in regulating the activity of GCs in slices from wildtype and mGluR-/- mutant mice. In wildtype mice, bath application of the selective Group I mGluR agonist DHPG depolarized and increased the firing rate of both GC subtypes. In the presence of blockers of fast synaptic transmission (APV, CNQX, gabazine), DHPG directly depolarized both GC subtypes, although the two GC subtypes responded differentially to DHPG in mGluR1-/- and mGluR5-/- mice. DHPG depolarized sGCs in slices from mGluR5-/- mice, although it had no effect on sGCs in slices from mGluR1-/- mice. By contrast, DHPG depolarized dGCs in slices from mGluR1-/- mice but had no effect on dGCs in slices from mGluR5-/- mice. Previous studies showed that mitral cells express mGluR1 but not mGluR5. The present results therefore suggest that sGCs are more similar to mitral cells than dGCs in terms of mGluR expression.

Animals↗

PubMed2006

Metabotropic glutamate receptors in the main olfactory bulb drive granule cell-mediated inhibition.

Main olfactory bulb (MOB) granule cells (GCs) express high levels of the group I metabotropic glutamate receptor (mGluR), mGluR5. We investigated the role of mGluRs in regulating GC activity in rodent MOB slices using whole cell patch-clamp electrophysiology. The group I/II mGluR agonist (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (ACPD) or the selective group I agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) depolarized ( approximately 20 mV) and increased the firing rate of GCs. In the presence of ionotropic glutamate and GABA receptor antagonists, DHPG evoked a more modest depolarization ( approximately 8 mV). In voltage clamp, DHPG, but not group II [(2S,2'R,3)-2-(2',3'-dicarboxycyclopropyl)glycine, DCG-IV] or group III [L(+)-2-amino-4-phosphonobutyric acid, L-AP4] mGluR agonists, induced an inward current. The inward current reversed polarity near the potassium equilibrium potential, suggesting mediation by closure of potassium channels. The DHPG-evoked inward current was unaffected by the mGluR1 antagonist (S)-(+)-alpha-amino-4-carboxy-2-methylbenzeneacetic acid (LY367385), was blocked by the group I/II mGluR antagonist (alphaS)-alpha-amino-alpha-[(1S,2S)-2-carboxycyclopropyl]-9H-xanthine-9-propanoic acid (LY341495), and was absent in GCs from mGluR5 knockout mice. LY341495 also attenuated mitral cell-evoked voltage-sensitive dye signals in the external plexiform layer and mitral cell-evoked spikes in GCs. These results suggest that activation of mGluR5 increases GC excitability, an effect that should increase GC-mediated GABAergic inhibition of mitral cells. In support of this: DHPG increased the frequency of spontaneous GABAergic inhibitory postsynaptic currents in mitral cells and LY341495 attenuated the feedback GABAergic postsynaptic potential elicited by intracellular depolarization of mitral cells. Our results suggest that activation of mGluR5 participates in feedforward and/or feedback inhibition at mitral cell to GC dendrodendritic synapses, possibly to modulate lateral inhibition and contrast in the MOB.

Action Potentials↗

PubMed2006

Complementary postsynaptic activity patterns elicited in olfactory bulb by stimulation of mitral/tufted and centrifugal fiber inputs to granule cells.

Main olfactory bulb (MOB) granule cells receive spatially segregated glutamatergic synaptic inputs from the dendrites of mitral/tufted cells as well as from the axons of centrifugal fibers (CFFs) originating in olfactory cortical areas. Dendrodendritic synapses from mitral/tufted cells occur on granule cell distal dendrites in the external plexiform layer (EPL), whereas CFFs preferentially target the somata/proximal dendrites of granule cells in the granule cell layer (GCL). In the present study, tract tracing, and recordings of field potentials and voltage-sensitive dye optical signals were used to map activity patterns elicited by activation of these two inputs to granule cells in mouse olfactory bulb slices. Stimulation of the lateral olfactory tract (LOT) produced a negative field potential in the EPL and a positivity in the GCL. CFF stimulation produced field potentials of opposite polarity in the EPL and GCL to those elicited by LOT. LOT-evoked optical signals appeared in the EPL and spread subsequently to deeper layers, whereas CFF-evoked responses appeared in the GCL and then spread superficially. Evoked responses were reduced by N-methyl-d-aspartate (NMDA) receptor antagonists and completely suppressed by AMPA receptor antagonists. Reduction of extracellular Mg(2+) enhanced the strength and spatiotemporal extent of the evoked responses. These and additional findings indicate that LOT- and CFF-evoked field potentials and optical signals reflect postsynaptic activity in granule cells, with moderate NMDA and dominant AMPA receptor components. Taken together, these results demonstrate that LOT and CFF stimulation in MOB slices selectively activate glutamatergic inputs to the distal dendrites versus somata/proximal dendrites of granule cells.

Action Potentials↗

PubMed2006

Olfactory nerve-evoked, metabotropic glutamate receptor-mediated synaptic responses in rat olfactory bulb mitral cells.

The group I metabotropic glutamate receptor (mGluR) subtype, mGluR1, is highly expressed on the apical dendrites of olfactory bulb mitral cells and thus may be activated by glutamate released from olfactory nerve (ON) terminals. Previous studies have shown that mGluR1 agonists directly excite mitral cells. In the present study, we investigated the involvement of mGluR1 in ON-evoked responses in mitral cells in rat olfactory bulb slices using patch-clamp electrophysiology. In voltage-clamp recordings, the average EPSC evoked by single ON shocks or brief trains of ON stimulation (six pulses at 50 Hz) in normal physiological conditions were not significantly affected by the nonselective mGluR antagonist LY341495 (50-100 microM) or the mGluR1-specific antagonist LY367385 (100 microM); ON-evoked responses were attenuated, however, in a subset (36%) of cells. In the presence of blockers of ionotropic glutamate and GABA receptors, application of the glutamate uptake inhibitors THA (300 microM) and TBOA (100 microM) revealed large-amplitude, long-duration responses to ON stimulation, whereas responses elicited by antidromic activation of mitral/tufted cells were unaffected. Magnitudes of the ON-evoked responses elicited in the presence of THA-TBOA were dependent on stimulation intensity and frequency, and were maximal during high-frequency (50-Hz) bursts of ON spikes, which occur during odor stimulation. ON-evoked responses elicited in the presence of THA-TBOA were significantly reduced or completely blocked by LY341495 or LY367385 (100 microM). These results demonstrate that glutamate transporters tightly regulate access of synaptically evoked glutamate from ON terminals to postsynaptic mGluR1s on mitral cell apical dendrites. Taken together with other findings, the present results suggest that mGluR1s may not play a major role in phasic responses to ON input, but instead may play an important role in shaping slow oscillatory activity in mitral cells and/or activity-dependent regulation of plasticity at ON-mitral cell synapses.

Amino Acids↗

PubMed2005

Olfactory bulb external tufted cells are synchronized by multiple intraglomerular mechanisms.

In rat olfactory bulb slices, external tufted (ET) cells spontaneously generate spike bursts. Only ET cells affiliated with the same glomerulus exhibit significant synchronous activity, suggesting that synchrony results mainly from intraglomerular interactions. The intraglomerular mechanisms underlying their synchrony are unknown. Using dual extracellular and patch-clamp recordings from ET cell pairs of the same glomerulus, we found that the bursting of ET cells is synchronized by several mechanisms. First, ET cell pairs of the same glomerulus receive spontaneous synchronous fast excitatory synaptic input that can also be evoked by olfactory nerve stimulation. Second, they exhibit correlated spontaneous slow excitatory synaptic currents that can also be evoked by stimulation of the external plexiform layer. These slow currents may reflect the repetitive release of glutamate via spillover from the dendritic tufts of other ET or mitral/tufted cells affiliated with the same glomerulus. Third, ET cells exhibit correlated bursts of inhibitory synaptic activity immediately after the synchronous fast excitatory input. These bursts of IPSCs were eliminated by CNQX and may therefore reflect correlated feedback inhibition from periglomerular cells that are driven by ET cell spike bursts. Fourth, in the presence of fast synaptic blockers, ET cell pairs exhibit synchronous slow membrane current oscillations associated with rhythmic spikelets, which were sensitive to the gap junction blocker carbenoxolone. These findings suggest that coordinated synaptic transmission and gap junction coupling synchronize the spontaneous bursting of ET cells of the same glomerulus.

Animals↗

PubMed2004

External tufted cells: a major excitatory element that coordinates glomerular activity.

The glomeruli of the olfactory bulb are the first site of synaptic processing in the olfactory system. The glomeruli contain three types of neurons that are referred to collectively as juxtaglomerular (JG) cells: external tufted (ET), periglomerular (PG), and short axon (SA) cells. JG cells are thought to interact synaptically, but little is known about the circuitry linking these neurons or their functional roles in olfactory processing. Single and paired whole-cell recordings were performed to investigate these questions. ET cells spontaneously fired rhythmic spike bursts in the theta frequency range and received monosynaptic olfactory nerve (ON) input. In contrast, all SA and most PG cells lacked monosynaptic ON input. PG and SA cells exhibited spontaneous, intermittent bursts of EPSCs that were highly correlated with spike bursts of ET cells in the same but not in different glomeruli. Paired recording experiments demonstrated that ET cells provide monosynaptic excitatory input to PG/SA cells; the ET to PG/SA cell synapse is mediated by glutamate. ET cells thus are a major excitatory linkage between ON input and other JG cells. Spontaneous bursting is highly correlated among ET cells of the same glomerulus, and ET cell activity remains correlated when all fast synaptic activity is blocked. The findings suggest that multiple, synchronously active ET cells synaptically converge onto single PG/SA cells. Synchronous ET cell bursting may function to amplify transient sensory input and coordinate glomerular output.

Action Potentials↗

PubMed2004

Regulation of main olfactory bulb mitral cell excitability by metabotropic glutamate receptor mGluR1.

In the rodent main olfactory bulb (MOB), mitral cells (MCs) express high levels of the group I metabotropic glutamate receptor (mGluR) subtype, mGluR1. The significance of this receptor in modulating MC excitability is unknown. We investigated the physiological role of mGluR1 in regulating MC activity in rat and mouse MOB slices. The selective group I agonist (RS)-3,5-dihydroxyphenylglycine (DHPG), but not group II or III agonists, induced potent, dose-dependent, and reversible depolarization and increased firing of MCs. These effects persisted in the presence of blockers of fast synaptic transmission, indicating that they are due to direct activation of mGluRs on MCs. Voltage-clamp recordings showed that DHPG elicited a voltage-dependent inward current consisting of multiple components sensitive to potassium and calcium channel blockade and intracellular calcium chelation. MC excitatory responses to DHPG were absent in mGluR1 knockout mice but persisted in mGluR5 knockout mice. Broad-spectrum LY341495, MCPG, as well as preferential mGluR1 LY367385 antagonists blocked the excitatory effects of DHPG and also potently modulated MC spontaneous and olfactory nerve-evoked excitability. mGluR antagonists altered spontaneous membrane potential bistability, increasing the duration of the up and down states. mGluR antagonists also substantially attenuated MC responses to sensory input, decreasing the probability and increasing the latency of olfactory nerve-evoked spikes. These findings suggest that endogenous glutamate tonically modulates MC excitability and responsiveness to olfactory nerve input, and hence the operation of the MOB circuitry, via activation of mGluR1.

Animals↗

PubMed2004

Olfactory bulb glomeruli: external tufted cells intrinsically burst at theta frequency and are entrained by patterned olfactory input.

Glomeruli, the initial sites of synaptic processing in the olfactory system, contain at least three types of neurons collectively referred to as juxtaglomerular (JG) neurons. The role of JG neurons in odor processing is poorly understood. We investigated the morphology, spontaneous, and sensory-evoked activity of one class of JG neurons, external tufted (ET) cells, using whole-cell patch-clamp and extracellular recordings in rat olfactory bulb slices. ET cells have extensive dendrites that ramify within a single glomerulus or, rarely, in two adjacent glomeruli. All ET neurons exhibit spontaneous rhythmic bursts of action potentials (approximately 1-8 bursts/sec). Bursting is intrinsically generated; bursting persisted and became more regular in the presence of ionotropic glutamate and GABA receptor antagonists. Burst frequency is voltage dependent; frequency increased at membrane potentials depolarized relative to rest and decreased during membrane potential hyperpolarization. Spontaneous bursting persisted in blockers of calcium channels that eliminated low-threshold calcium spikes (LTS) in ET cells. ET cells have a persistent sodium current available at membrane potentials that generate spontaneous bursting. Internal perfusion with a fast sodium channel blocker eliminated spontaneous bursting but did not block the LTS. These results suggest that persistent sodium channels are essential for spontaneous burst generation in ET cells. ET cell bursts were entrained to ON stimuli delivered over the range of theta frequencies. Thus, ET cells appear to be tuned to the frequency of sniffing.

Action Potentials↗

PubMed2003

Optimization of the mechanical nociceptive threshold testing with the Randall-Selitto assay.

The Randall-Selitto (RS) assay is widely used for quantification of thresholds of the rat hindpaw withdrawal reflex to nociceptive pressure stimulation. Despite a report by Taiwo et al. [Brain Research 487 (1989)] that the sensitivity of the RS assay can be significantly improved by pre-training, many researchers still conduct this test in untrained rats. In part, this is because the study of Taiwo et al. employed heavily-restrained and thus very stressed animals. That study also examined bradykinin-induced hyperalgesia rather than hyperalgesia associated with persistent inflammatory models used in many other studies. Therefore, it is conceivable that pre-training may be unnecessary with a less restraining RS testing paradigm and the use of inflammation-producing agents. To resolve these issues, we re-examined the need for pre-training with the RS assay with minimal animal restraint and inflammation produced by Complete Freund's Adjuvant (CFA) injection. We also examined the sensitivity of this assay to detect analgesia induced by swim stress. We found that without training the differences between untreated, CFA-injected, swim stress-exposed, and CFA + swim stress-treated animal groups did not reach statistical significance. Four days of training, however, enlarged these differences to statistically significant levels. Furthermore, we found that the use of only the last measurement within a testing session, rather than the average of all collected measurements, may further enhance the sensitivity of the RS assay.

Analysis of Variance↗

PubMed2002

Age-dependency of analgesia elicited by intraoral sucrose in acute and persistent pain models.

Treatment of pain in newborns is associated with problematic drug side effects. Previous studies demonstrate that an intraoral infusion of sucrose and other sweet components of mother's milk are effective in alleviating pain in infant rats and humans. These findings are of considerable significance, as sweet tastants are used in pain and stress management in a number of clinical procedures performed in human infants. The ability of sweet stimuli to induce analgesia is absent in adult rats, suggesting that this is a developmentally transient phenomenon. However, the age range over which intraoral sucrose is capable of producing analgesia is not known. We investigated the effects of intraoral sucrose (7.5%) on nocifensive withdrawal responses to thermal and mechanical stimuli in naive and inflamed rats at postnatal days (P) P0-21. In some rats, Complete Freund's adjuvant (CFA) was injected in a fore- or hindpaw to produce inflammation. In non-inflamed animals, for noxious thermal stimuli, sucrose-induced analgesia emerged at P3, peaked at P7-10, then progressively declined and was absent at P17. For mechanical forepaw stimuli, sucrose-induced analgesia emerged, and was maximal at approximately P10, then declined and was absent at P17. By contrast, maximal sucrose-induced analgesia for mechanical hindpaw stimuli was delayed (P13) compared to that for the forepaw, although it was also absent at P17. In inflamed animals, sucrose reduced hyperesthesia and hyperalgesia assessed with mechanical stimuli. Sucrose-induced analgesia in inflamed animals was initially present at P3 for the forepaw and P13 for the hindpaw, and was absent by P17 for both limbs. Intraoral sucrose produced significantly greater effects on responses in fore- and hindpaws in inflamed rats than in naive rats indicating that it reduces hyperalgesia and allodynia beyond its effects on responses in naive animals. These findings support the hypothesis that sucrose has a selective influence on analgesic mechanisms and that an enhanced sucrose effect takes place in hyperalgesic, inflamed animals as compared to naive animals. Taken together, these results indicate that intraoral sucrose alleviates transient pain in response to thermal and mechanical stimuli, and also effectively reduces inflammatory hyperalgesia and allodynia. Sucrose-induced analgesia is age-dependent and limited to the pre-weaning period in rats. The age-dependency of sucrose-induced analgesia and its differential maturation for the fore- and hindpaw may be due to developmental changes in endogenous analgesic mechanisms and developmental modulation of the interaction between gustatory and pain modulatory pathways.

Acute Disease↗

PubMed1990

The effect of GABA and its antagonists on midbrain periaqueductal gray neurons in the rat.

Injection of GABA into the midbrain periaqueductal gray (PAG) activates medullary neurons that are involved in pain inhibition and potentiates morphine-induced analgesia. These observations suggest that GABAergic mechanisms in the PAG may modulate the descending pain inhibitory system that arises from this structure. In the present study, the effects of GABA and GABA antagonists on membrane properties and baseline activity of PAG neurons were examined using both in vitro and in vivo preparations. Application of bicuculline methiodide (BICM), at a dose that blocked the response to GABA, potently increased the baseline firing rate in 53% of cells recorded in vitro and 74% of cells recorded in the intact preparation. Application of BICM often yielded multiple or burst spiking episodes in both preparations. In 69% of cells the effect of BICM was diminished or totally abolished when the slice was perfused with high-magnesium, calcium-free, physiological saline solution. Intracellular recordings revealed that bicuculline caused depolarization of the membrane (70% of cells), increased the firing frequency (94% of cells) and increased the frequency of excitatory postsynaptic potentials (18% of cells). The effect of bicuculline on membrane resistance was not pronounced and in 64% of neurons it did not cause any measurable change in the resting membrane resistance. PAG neurons responsive to GABA and its antagonists were observed in all regions of the PAG. However, the highest number of neurons that responded to GABA and its antgonists was found in the medial and medioventral parts of the PAG. These results indicate that PAG may contain a tonically active GABAergic network that operates, at least in part, through GABAA receptors. This GABAergic system may modulate activity in descending pain inhibitory pathways emanating from PAG.

Action Potentials↗