Effects of Mild Closed-Head Injury and Subanesthetic Ketamine Infusion on Microglia, Axonal Injury, and Synaptic Density in Sprague-Dawley Rats

Mild traumatic brain injury (mTBI) affects millions of people in the U.S. Approximately 20-30% of those individuals develop adverse symptoms lasting at least 3 months. In a rat mTBI study, the closed-head impact model of engineered rotational acceleration (CHIMERA) produced significant axonal injury in the optic tract (OT), indicating white-matter damage. Because retinal ganglion cells project to the lateral geniculate nucleus (LGN) in the thalamus through the OT, we hypothesized that synaptic density may be reduced in the LGN of rats following CHIMERA injury. A modified SEQUIN (synaptic evaluation and quantification by imaging nanostructure) method, combined with immunofluorescent double-labeling of pre-synaptic (synapsin) and post-synaptic (PSD-95) markers, was used to quantify synaptic density in the LGN. Microglial activation at the CHIMERA injury site was determined using Iba-1 immunohistochemistry. Additionally, the effects of ketamine, a potential neuroprotective drug, were evaluated in CHIMERA-induced mTBI. A single-session repetitive (ssr-) CHIMERA (3 impacts, 1.5 joule/impact) produced mild effects on microglial activation at the injury site, which was significantly enhanced by post-injury intravenous ketamine (10 mg/kg) infusion. However, ssr-CHIMERA did not alter synaptic density in the LGN, although ketamine produced a trend of reduction in synaptic density at post-injury day 4. Further research is necessary to characterize the effects of ssr-CHIMERA and subanesthetic doses of intravenous ketamine on different brain regions and multiple time points post-injury. The current study demonstrates the utility of the ssr-CHIMERA as a rodent model of mTBI, which researchers can use to identify biological mechanisms of mTBI and to develop improved treatment strategies for individuals suffering from head trauma.

Adenosine A2A receptor modulates microglia-mediated synaptic pruning of the retinogeniculate pathway during postnatal development

Synapse pruning is essential not only for the developmental establishment of synaptic connections in the brain but also for the pathogenesis of neurodevelopmental and neurodegenerative disorders. However, there are no effective pharmacological means to regulate synaptic pruning during early development. Using the eye-specific segregation of the dorsal lateral geniculate nucleus (dLGN) as a model of synaptic pruning coupled with adenosine A2A receptor (A2AR) antagonism and knockout, we demonstrated while genetic deletion of the A2AR throughout the development attenuated eye-specific segregation with the attenuated microglial phagocytosis at postnatal day 5 (P5), selective treatment with the A2AR antagonist KW6002 at P2-P4 facilitated synaptic pruning of visual pathway with microglial activation, increased lysosomal activity in microglia and increased microglial engulfment of retinal ganglion cell (RGC) inputs in the dLGN at P5 (but not P10). Furthermore, KW6002-mediated facilitation of synaptic pruning was activity-dependent since tetrodotoxin (TTX) treatment abolished the KW6002 facilitation. Moreover, the A2AR antagonist also modulated postsynaptic proteins and synaptic density at early postnatal stages as revealed by the reduced immunoreactivity of postsynaptic proteins (Homer1 and metabotropic glutamate receptor 5) and colocalization of presynaptic VGlut2 and postsynaptic Homer1 puncta in the dLGN. These findings suggest that A2AR can control pruning by multiple actions involving the retinal wave, microglia engulfment, and postsynaptic stability. Thus, A2AR antagonists may represent a novel pharmacological strategy to modulate microglia-mediated synaptic pruning and treatment of neurodevelopmental disorders associated with dysfunctional pruning.

Bisphenol A exposure perturbs visual function of adult cats by remodeling the neuronal activity in the primary visual pathway

Bisphenol A (BPA), a common environmental xenoestrogen, has been implicated in physiological and behavioral impairment, but the neuronal basis remains elusive. Although various synaptic mechanisms have been shown to mediate BPA-induced brain deficits, there are almost no reports addressing its underlying physiological mechanisms at the individual neuron level, particularly in the primary visual system. In the present study, using multiple-channel recording technique, we recorded the responses of single neurons in the primary visual system of cats to various direction stimuli both before and after BPA exposure. The results showed that the orientation selectivity of neurons in the primary visual cortex (area 17, A17) was obviously decreased after 2 h of intravenous BPA administration (0.2 mg/kg). Moreover, there were worse performances of information transmission of A17 neurons, presenting markedly decreased signal-to-noise ratio (SNR). To some extent, these functional decreases were attributable to the altered information inputs from lateral geniculate nucleus (LGN), which showed an increased spontaneous activity. Additionally, local injection of BPA (3.3 μg/ml) in A17 resulted in an obvious increase in orientation selectivity and a decrease in neuronal activity, involving enhanced activity of fast-spiking inhibitory interneurons. In conclusion, our results first demonstrate that acute BPA exposure can restrict the visual perception of cats, mainly depending on the alteration of the LGN projection, not the intercortical interaction. Importantly, BPA-induced-brain deficits might not only be confined to the cortical level but also occur as early as at the subcortical level.

Auditory cue absence as a conditioned stimulus for delay eyeblink conditioning

The present experiment was designed to determine if the absence of an auditory cue (i.e., a "tone-off" cue) would be an effective conditioned stimulus (CS) for delay eyeblink conditioning and to test if the medial geniculate nucleus (MGN) is part of the sensory pathway for tone-off conditioning. Rats were given paired or unpaired delay eyeblink conditioning to examine if responding to a tone-off CS was due to an associative process. An inactivation technique was performed on a separate group of rats to determine if the MGN is needed to express tone-off conditioning. The results showed that rats given paired conditioning acquired robust conditioned eyeblink responses (CRs) compared with rats given unpaired conditioning and that expression of tone-off elicited CRs was impaired when the MGN was inactivated. The findings suggest that tone-on and tone-off eyeblink conditioning may share a common neural pathway. (PsycINFO Database Record

GABAergic inhibition shapes SAM responses in rat auditory thalamus

Auditory thalamus (medial geniculate body [MGB]) receives ascending inhibitory GABAergic inputs from inferior colliculus (IC) and descending GABAergic projections from the thalamic reticular nucleus (TRN) with both inputs postulated to play a role in shaping temporal responses. Previous studies suggested that enhanced processing of temporally rich stimuli occurs at the level of MGB, with our recent study demonstrating enhanced GABA sensitivity in MGB compared to IC. The present study used sinusoidal amplitude-modulated (SAM) stimuli to generate modulation transfer functions (MTFs), to examine the role of GABAergic inhibition in shaping the response properties of MGB single units in anesthetized rats. Rate MTFs (rMTFs) were parsed into "bandpass (BP)", "mixed (Mixed)", "highpass (HP)" or "atypical" response types, with most units showing the Mixed response type. GABAA receptor blockade with iontophoretic application of the GABAA receptor (GABAAR) antagonist gabazine (GBZ) selectively altered the response properties of most MGB neurons examined. Mixed and HP units showed significant GABAAR-mediated SAM-evoked rate response changes at higher modulation frequencies (fms), which were also altered by N-methyl-d-aspartic acid (NMDA) receptor blockade (2R)-amino-5-phosphonopentanoate (AP5). BP units, and the lower arm of Mixed units responded to GABAAR blockade with increased responses to SAM stimuli at or near the rate best modulation frequency (rBMF). The ability of GABA circuits to shape responses at higher modulation frequencies is an emergent property of MGB units, not observed at lower levels of the auditory pathway and may reflect activation of MGB NMDA receptors (Rabang and Bartlett, 2011; Rabang et al., 2012). Together, GABAARs exert selective rate control over selected fms, generally without changing the units' response type. These results showed that coding of modulated stimuli at the level of auditory thalamus is at least, in part, strongly controlled by GABA neurotransmission, in delicate balance with glutamatergic neurotransmission.

Epibatidine blocks eye-specific segregation in ferret dorsal lateral geniculate nucleus during stage III retinal waves

The segregation and maintenance of eye-specific inputs in the dorsal lateral geniculate nucleus (dLGN) during early postnatal development requires the patterned spontaneous activity of retinal waves. In contrast to the development of the mouse, ferret eye-specific segregation is not complete at the start of stage III glutamatergic retinal waves, and the remaining overlap is limited to the C/C1 lamina of the dLGN. To investigate the role of patterned spontaneous activity in this late segregation, we disrupted retinal waves pharmacologically for 5 day windows from postnatal day (P) 10 to P25. Multi-electrode array recordings of the retina in vitro reveal that the cholinergic agonist epibatidine disrupts correlated retinal activity during stage III waves. Epibatidine also prevents the segregation of eye-specific inputs in vivo during that period. Our results reveal a novel role for cholinergic influence on stage III retinal waves as an instructive signal for the continued segregation of eye-specific inputs in the ferret dLGN.

A mechanism study on propofol's action on middle latency auditory evoked potential by neurons in ventral partition of medial geniculate body in rats

OBJECTIVES

To investigate the effect of propofol on the middle latency auditory evoked potentials (MLAEP) by neurons in the ventral partition of medial geniculate body (MGBv) in rats and study their mechanism.

MATERIALS AND METHODS

Sprague-Dawley (SD) rats were randomly divided into 7 groups (n = 6): group normal saline (NS), group intralipid (I), and groups of different concentrations of propofol (5.6, 16.8, 56, 168, 560 µmol/L) (P1-P5). These animals were anesthetized with ether, tracheostomized, and mechanically ventilated. After anesthesia, rats were paralyzed with vecuronium and fixed. A recording needle electrode with drugs was inserted into MGBv by means of stereotaxis. After injection of 0.2 µl propofol, normal saline or intralipid, correct insertion was verified by MLAEP response to standard sound. MLAEP including amplitudes and latencies of N0, P0, Na, Pa, and Nb waves were recorded. To identify which ion channel could be impacted by propofol, SD rats were divided into Ni2+ plus propofol (A1 group), Cd2+ plus propofol (A2 group), 4-AP plus propofol (A3 grouop), and TTX plus propofol (A4 group). The changes of MLAEP were recorded between injecting 4 ion channel blockers and propofol. Whole-cell patch clamp technique was used to confirm these variations.

RESULTS

There was no significant changes in all waves of MLAEP in MGBv after drug administration as compared with the baselins before injection in group NS, I, P1, and P2. The latency of Na, Pa, and Nb wave was significantly prolonged and the amplitude of Pa wave was decreased after injection as compared with the baseline in group P3, P4, and P5. The latency of Na, Pa, and Nb waves was significantly longer and the amplitude of Pa wave was significantly lower in group P3, P4, and P5 than in group NS and I. With TTX, amplitudes were decreased in wave Na-Pa. After given propofol, waves were unchanged in Na-Pa and latency was increased in Na, Pa, and Nb. With Cd2+, latency was unchanged in Na, Pa, and Nb as well as amplitudes decreasing in Na-Pa. Nevertheless, after given Ni2+ and 4-AP, every wave of MLAEP had no changes. After injected propofol, amplitudes were decreased in wave Na-Pa and latency was increased in Na, Pa, and Nb. The results of patch clamp showed 56 µmol/L, 168 µmol/L and 560 µmol/L propofol inhibited the persistent sodium currents and high voltaged activated calcium currents in the brain slices of rats.

CONCLUSIONS

Propofol in 56 µmol/L, 168 µmol/L and 560 µmol/L can inhibit MLAEP in MGBv of rats in a dose-dependent manner and these changes may be caused by blocking the ion channel of persistent sodium currents and high voltaged activated calcium currents.

Methylphenidate and atomoxetine enhance sensory-evoked neuronal activity in the visual thalamus of male rats

Attention deficits and inappropriate regulation of sensory signal processing are hallmarks of many neuropsychiatric conditions, including attention deficit hyperactivity, for which methylphenidate (MPH) and atomoxetine (ATX) are commonly prescribed therapeutic treatments. Despite their widespread use and known mechanism of blocking reuptake of catecholamine transmitters in the brain, the resultant actions on individual neuron and neural circuit function that lead to therapeutic efficacy are poorly understood. Given the ability of MPH and ATX to improve cognitive performance in humans and rodent assays of attention, we were interested in their influence on early sensory processing in the dorsal lateral geniculate nucleus (dLGN), the primary thalamic relay for visual information from the retina to the visual cortex. In male rats, dLGN neuronal responses to light stimuli were altered in multiple ways after doses of MPH or ATX observed to enhance performance in visually guided assays of attention (MPH = 2 mg/kg; ATX = 0.5 mg/kg). Latencies to response onset and to the peak of the primary response were decreased, while the peak intensity and area of the primary response were increased. In addition, some cells that were unresponsive to light stimuli prior to drug treatment displayed a "gating effect," wherein prominent responses to light stimuli were evident after drug administration. Our results begin to reveal unique effects of MPH and ATX in enhancing sensory signal transmission through visual circuitry, and may yield new insights for understanding the pathophysiology of certain cognitive disorders and inform development of improved therapeutic treatments for these conditions.

Activation of both Group I and Group II metabotropic glutamatergic receptors suppress retinogeniculate transmission

Relay cells of dorsal lateral geniculate nucleus (LGN) receive a Class 1 glutamatergic input from the retina and a Class 2 input from cortical layer 6. Among the properties of Class 2 synapses is the ability to activate metabotropic glutamate receptors (mGluRs), and mGluR activation is known to affect thalamocortical transmission via regulating retinogeniculate and thalamocortical synapses. Using brain slices, we studied the effects of Group I (dihydroxyphenylglycine) and Group II ((2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine) mGluR agonists on retinogeniculate synapses. We showed that both agonists inhibit retinogeniculate excitatory postsynaptic currents (EPSCs) through presynaptic mechanisms, and their effects are additive and independent. We also found high-frequency stimulation of the layer 6 corticothalamic input produced a similar suppression of retinogeniculate EPSCs, suggesting layer 6 projection to LGN as a plausible source of activating these presynaptic mGluRs.

The impact of hemodynamic stress on sensory signal processing in the rodent lateral geniculate nucleus

Hemodynamic stress via hypotensive challenge has been shown previously to cause a corticotropin-releasing factor (CRF)-mediated increase in tonic locus coeruleus (LC) activity and consequent release of norepinephrine (NE) in noradrenergic terminal fields. Although alterations in LC-NE can modulate the responsiveness of signal processing neurons along sensory pathways, little is understood regarding how continuous CRF-mediated activation of LC-NE output due to physiologically relevant stressor affects downstream target cell physiology. The goal of the present study was to investigate the effects of a physiological stressor [hemodynamic stress via sodium nitroprusside (SNP) i.v.] on stimulus evoked responses of sensory processing neurons that receive LC inputs. In rat, the dorsal lateral geniculate nucleus (dLGN) of the thalamus is the primary relay for visual information and is a major target of the LC-NE system. We used extracellular recording techniques in the anesthetized rat monitor single dLGN neuron activity during repeated presentation of light stimuli before and during hemodynamic stress. A significant decrease in magnitude occurred, as well as an increase in latency of dLGN stimulus-evoked responses were observed during hemodynamic stress. In another group of animals the CRF antagonist DpheCRF12-41 was infused onto the ipsilateral LC prior to SNP administration. This infusion blocked the hypotension-induced changes in dLGN stimulus-evoked discharge. These results show that CRF-mediated increases in LC-NE due to hemodynamic stress disrupts the transmission of information along thalamic-sensory pathways by: (1) initially reducing signal transmission during onset of the stressor and (2) decreasing the speed of stimulus evoked sensory transmission.

Altered neuronal intrinsic properties and reduced synaptic transmission of the rat's medial geniculate body in salicylate-induced tinnitus

Sodium salicylate (NaSal), an aspirin metabolite, can cause tinnitus in animals and human subjects. To explore neural mechanisms underlying salicylate-induced tinnitus, we examined effects of NaSal on neural activities of the medial geniculate body (MGB), an auditory thalamic nucleus that provides the primary and immediate inputs to the auditory cortex, by using the whole-cell patch-clamp recording technique in MGB slices. Rats treated with NaSal (350 mg/kg) showed tinnitus-like behavior as revealed by the gap prepulse inhibition of acoustic startle (GPIAS) paradigm. NaSal (1.4 mM) decreased the membrane input resistance, hyperpolarized the resting membrane potential, suppressed current-evoked firing, changed the action potential, and depressed rebound depolarization in MGB neurons. NaSal also reduced the excitatory and inhibitory postsynaptic response in the MGB evoked by stimulating the brachium of the inferior colliculus. Our results demonstrate that NaSal alters neuronal intrinsic properties and reduces the synaptic transmission of the MGB, which may cause abnormal thalamic outputs to the auditory cortex and contribute to NaSal-induced tinnitus.

Effects of early postnatal exposure to ethanol on retinal ganglion cell morphology and numbers of neurons in the dorsolateral geniculate in mice

BACKGROUND

The adverse effects of fetal and early postnatal ethanol intoxication on peripheral organs and the central nervous system are well documented. Ocular defects have also been reported in about 90% of children with fetal alcohol syndrome, including microphthalmia, loss of neurons in the retinal ganglion cell (RGC) layer, optic nerve hypoplasia, and dysmyelination. However, little is known about perinatal ethanol effects on retinal cell morphology. Examination of the potential toxic effects of alcohol on the neuron architecture is important because the changes in dendritic geometry and synapse distribution directly affect the organization and functions of neural circuits. Thus, in the present study, estimations of the numbers of neurons in the ganglion cell layer and dorsolateral geniculate nucleus (dLGN), and a detailed analysis of RGC morphology were carried out in transgenic mice exposed to ethanol during the early postnatal period.

METHODS

The study was carried out in male and female transgenic mice expressing yellow fluorescent protein (YFP) controlled by a Thy-1 (thymus cell antigen 1) regulator on a C57 background. Ethanol (3 g/kg/d) was administered to mouse pups by intragastric intubation throughout postnatal days (PDs) 3 to 20. Intubation control (IC) and untreated control (C) groups were included. Blood alcohol concentration was measured in separate groups of pups on PDs 3, 10, and 20 at 4 different time points, 1, 1.5, 2, and 3 hours after the second intubation. Numbers of neurons in the ganglion cell layer and in the dLGN were quantified on PD20 using unbiased stereological procedures. RGC morphology was imaged by confocal microscopy and analyzed using Neurolucida software.

RESULTS

Binge-like ethanol exposure in mice during the early postnatal period from PDs 3 to 20 altered RGC morphology and resulted in a significant decrease in the numbers of neurons in the ganglion cell layer and in the dLGN. In the alcohol exposure group, out of 13 morphological parameters examined in RGCs, soma area was significantly reduced and dendritic tortuosity significantly increased. After neonatal exposure to ethanol, a decrease in total dendritic field area and an increase in the mean branch angle were also observed. Interestingly, RGC dendrite elongation and a decrease in the spine density were observed in the IC group, as compared to both ethanol-exposed and pure control subjects. There were no significant effects of alcohol exposure on total retinal area.

CONCLUSIONS

Early postnatal ethanol exposure affects development of the visual system, reducing the numbers of neurons in the ganglion cell layer and in the dLGN, and altering RGCs' morphology.

Differential region-specific regulation of α4β2* nAChRs by self-administered and non-contingent nicotine in C57BL/6J mice

Neuronal nAChR upregulation is the hallmark of chronic nicotine exposure. Neuroplasticity to abused drugs, however, depends on whether their administration is forced by the experimenter or is under the control of the experimental animal. Neuroadaptation to chronic nicotine self-administration was examined with a yoked-control paradigm, using nose-poking as the operating procedure. Freely moving C57BL/6J mice that responded for 0.03 mg/kg/infusion of intravenous nicotine under a continuous schedule of reinforcement (FR-1), had control over the rate and amount of drug intake that a yoked littermate passively received (n = 11). The impact of response dependency on neurobiological changes in nicotinic and dopaminergic systems was subsequently assessed using quantitative autoradiography. Cytisine-sensitive [(125)I]epibatidine binding, [³H]SCH23390, [³H]raclopride and [³H]mazindol were used to label nAChRs with α4β2* subtype properties, D1 and D2 dopaminergic receptors, and dopamine transporters, respectively. During a period of 12 days, self-administration was reliably initiated and maintained in animals receiving response-contingent nicotine. Region specific changes in the density of α4β2* nAChRs were found to be dependent on the contingency of nicotine treatment. Higher levels of α4β2* receptor binding were observed in the dorsal lateral geniculate nucleus and the ventral tegmental area of self-administering mice, compared to non-contingent animals. Moreover, response-independent increases in D2 binding were observed following chronic nicotine administration. No change in D1 and DAT binding was observed among groups. These findings indicate regional specific alterations in the regulation of the nicotinic cholinergic system following contingent and non-contingent nicotine exposure, and underline the importance of response dependency on the development of nicotine addiction.

Retinogeniculate EPSPs recorded intracellularly in the ferret lateral geniculate nucleus in vitro: Role of NMDA receptors

We used an in vitro preparation of the ferret lateral geniculate nucleus (LGN) to examine the role of the NMDA class of excitatory amino acid (EAA) receptors in retinogeniculate transmission. Intracellular recordings revealed that blockade of NMDA receptors both shortened the time course and reduced the amplitude of fast and slow components of excitatory postsynaptic potentials (EPSPs) evoked by optic tract stimulation. The amplitude and width of the EPSPs mediated by NMDA receptors increased as membrane potential was depolarized towards spike threshold. Individual LGN cells were influenced to varying extents by blockade of NMDA receptors; NMDA and non-NMDA receptor blockade together attenuated severely the entire retinogeniculate EPSP. The dependence of all components of retinogeniculate EPSPs (and action potentials) on NMDA receptor activation supports the hypothesis that the NMDA receptor participates in fast (<10 ms) synaptic events underlying conventional retinogeniculate transmission. The voltage dependence of the NMDA receptor-gated conductance suggests strongly that the transmission of retinal information through the LGN is subject to modulation by extraretinal inputs that affect the membrane potential of LGN neurons.

Deleterious effects of minocycline after in vivo target deprivation of thalamocortical neurons in the immature, metallothionein-deficient mouse brain

Compared with adults, immature metallothionein I and II knockout (MT(-/-)) mice incur greater neuronal loss and a more rapid rate of microglia accumulation after target deprivation-induced injury. Because minocycline has been proposed to inhibit microglial activation and associated production of neuroinflammatory factors, we investigated its ability to promote neuronal survival in the immature, metallothionein-deficient brain. After ablation of the visual cortex, 10-day-old MT(-/-) mice were treated with minocycline or saline and killed 24 or 48 hr after injury. By means of stereological methods, the number of microglia and neurons were estimated in the ipsilateral dorsal lateral geniculate nucleus (dLGN) by an investigator blinded to the treatment. No effect on neuronal survival was observed at 24 hr, but 48 hr after injury, an unanticipated but significant minocycline-mediated increase in neuronal loss was detected. Further, while failing to inhibit microglial accumulation, minocycline treatment increased the proportion of amoeboid microglia in the ipsilateral dLGN. To understand the molecular mechanisms underlying this neurotoxic response, we identified minocycline-mediated changes in the expression of three potentially proapoptotic/inflammatory genes: growth arrest- and DNA damage-inducible gene 45gamma (GADD45gamma); interferon-inducible protein 1 (IFI1), and cytokine-induced growth factor. We also observed increased mitogen-activated protein kinase p38 phosphorylation with minocycline treatment. Although minocycline inhibited calpain activity at 12 hr after injury, this effect was not sustained at 24 hr. Together, these results help to explain how minocycline has a deleterious effect on neuronal survival in this injury model.

Electrophysiology and pharmacology of the optic input to the rat intergeniculate leaflet in vitro

The mammalian intergeniculate leaflet (IGL) of the thalamus is a neuronal element of the circadian timing system, which receives direct photic input from the retina. The purpose of this study was to analyze responses of rat IGL neurons in vitro to optic tract stimulation and to identify neurotransmitters released from the terminals of retinal ganglion cells in this structure. Following optic tract stimulation, most of the responding IGL cells were excited and only a minority of them were inhibited. Neurons showing the excitatory response were tested in the presence of AP-5, a selective antagonist of NMDA receptors. In most cases the responses were only partially inhibited by the presence of AP-5. Complete disappearance of excitatory responses was achieved by adding CNQX, an AMPA/kainate receptor-selective antagonist, to the standard incubation fluid. Inhibitory responses were blocked or considerably attenuated in the presence of bicuculline, a GABA(A) receptor antagonist, in the ACSF. This study demonstrated that glutamate is the main neurotransmitter mediating optic tract input to the IGL, acting mainly via non-NMDA ionotropic receptors. It was also shown that NMDA and GABA(A) receptors are involved in passing photic input to the IGL, albeit to a much lesser extent.

Time course of cortically induced fos expression in auditory thalamus and midbrain after bilateral cochlear ablation

Expression of c-fos in the medial geniculate body (MGB) and the inferior colliculus (IC) in response to bicuculline-induced corticofugal activation was examined in rats at different time points after bilateral cochlear ablation (4 h-30 days). Corticofugal activation was crucial in eliciting Fos expression in the MGB after cochlear ablation. The pars ovoidea (OV) of the medial geniculate body ventral division (MGv) showed dense Fos expression 4 h after cochlear ablation; the expression declined to very low levels at 24 h and thereafter. In turn, strong Fos expression was found in the pars lateralis (LV) of the MGv 24 h after cochlear ablation and dropped dramatically at 14 days. The dorsal division of the MGB (MGd) showed high Fos expression 7 days after cochlear ablation, which persisted for a period of time. Using multi-electrode recordings, neuronal activity of different MGB subnuclei was found to correlate well with Fos expressions. The temporal changes in cortically activated Fos expression in different MGB subnuclei after bilateral cochlear ablation indicate differential denervation hypersensitivities of these MGB neurons and likely point to differential dependence of these nuclei on both auditory ascending and corticofugal descending inputs. After bilateral cochlear ablation, significant increases in Fos-positive neurons were detected unilaterally in all IC subnuclei, ipsilateral to the bicuculline injection.

Medial auditory thalamus inactivation prevents acquisition and retention of eyeblink conditioning

The auditory conditioned stimulus (CS) pathway that is necessary for delay eyeblink conditioning was investigated using reversible inactivation of the medial auditory thalamic nuclei (MATN) consisting of the medial division of the medial geniculate (MGm), suprageniculate (SG), and posterior intralaminar nucleus (PIN). Rats were given saline or muscimol infusions into the MATN contralateral to the trained eye before each of four conditioning sessions with an auditory CS. Rats were then given four additional sessions without infusions to assess savings from the initial training. All rats were then given a retention test with a muscimol infusion followed by a recovery session. Muscimol infusions through cannula placements within 0.5 mm of the MGm prevented acquisition of eyeblink conditioned responses (CRs) and also blocked CR retention. Cannula placements more than 0.5 mm from the MATN did not completely block CR acquisition and had a partial effect on CR retention. The primary and secondary effects of MATN inactivation were examined with 2-deoxy-glucose (2-DG) autoradiography. Differences in 2-DG uptake in the auditory thalamus were consistent with the cannula placements and behavioral results. Differences in 2-DG uptake were found between groups in the ipsilateral auditory cortex, basilar pontine nuclei, and inferior colliculus. Results from this experiment indicate that the MATN contralateral to the trained eye and its projection to the pontine nuclei are necessary for acquisition and retention of eyeblink CRs to an auditory CS.

The responsiveness of the rat intergeniculate leaflet neurons to glutamatergic agonists

The intergeniculate leaflet (IGL) has been shown to be a functional constituent of the circadian timing system. The IGL receives a monosynaptic input from the retina and is known to mediate some of the effects of light on the circadian clock. In the majority of retinal ganglion cells, glutamate functions as an excitatory neurotransmitter. The effect of monosodium glutamate and N-methyl-D-aspartate (NMDA), on the extracellularly recorded discharge activity of IGL neurons was studied in vitro. The application of monosodium glutamate induced either an excitatory, a biphasic or an inhibitory response. Application of NMDA induced an excitatory response in the majority of tested neurons. To determine the role of NMDA receptors in the response to glutamate application, the selective antagonist of NMDA receptors- AP-5, was applied to the incubation medium. The presence of AP-5 reduced the response of the IGL cells to focal application of glutamate and completely blocked their responsiveness to NMDA. To clarify whether GABAergic interneurons are involved in mediation of the inhibitory effects of glutamate, we repeated our experiments in the presence of bicuculline in the incubation medium. Since bicuculline did not influence the observed inhibitory effects, the involvement of GABAA receptors was excluded. The present study provides the first electrophysiological evidence that neurons in the rat IGL, respond to glutamate probably through NMDA receptors. However, our results also suggest that other types of glutamate receptors may play an additional role in mediating the action of this excitatory amino acid on the IGL neurons.

Qualitative changes in ultrasonic vocalization in rats after unilateral dopamine depletion or haloperidol: a preliminary study

The sensorimotor speech/voice deficits associated with Parkinson disease have been well documented in humans. They are largely resistant to pharmacological and surgical treatment, but respond to intensive speech therapy. The mechanisms underlying this phenomenon are not well understood and are difficult to systematically test in humans. Thus, we turn to the rat as a model. The purpose of this study is to compare the ultrasonic vocalization (USV) of rats in three conditions: control, haloperidol-induced transient dopamine depletion, and unilateral 6-hydroxydopamine (6-OHDA) induced moderately-severe degeneration of dopamine neurons. It was hypothesized that both dopamine-altered conditions would lead to a change in the features of the USV acoustic signal. Results demonstrated that bandwidth decreased in the dopamine-altered rats. This is the first study to document a degradation of the acoustic signal of frequency-modulated 50-kHz calls as a result of interfering with dopamine synaptic transmission in rats. The data suggest that mild transient dopamine depletion with haloperidol or even unilateral degeneration of dopamine neurons is associated with changes in the USV acoustic signal. Dopaminergic dysfunction influences USV quality without reducing the number of calls. This study provides a foundation to examine the role of dopamine in sensorimotor processes underlying USV production and potentially to explore treatments for dopamine deficiency-related impaired vocal outcome.

Should we treat the brain in glaucoma?

The loss of retinal ganglion cells in glaucoma may lead to blindness, and current therapies are directed at reducing pressure within the eye. Most of the retinal ganglion cell axon lies outside the eye, and evidence from experimental primate and human glaucoma suggests that axon injury extends from the optic nerve to visual pathways in the brain. Neurodegenerative changes in the central visual system may contribute to the pathology of glaucomatous progression. Thus, intraocular pressure-lowering strategies combined with neuroprotective therapies to protect visual neurons in the retina and brain may help to preserve vision in patients with glaucoma.

Changes in firing pattern of lateral geniculate neurons caused by membrane potential dependent modulation of retinal input through NMDA receptors

An optimal visual stimulus flashed on the receptive field of a retinal ganglion cell typically evokes a strong transient response followed by weaker sustained firing. Thalamocortical (TC) neurons in the dorsal lateral geniculate nucleus, which receive their sensory input from retina, respond similarly except that the gain, in particular of the sustained component, changes with level of arousal. Several lines of evidence suggest that retinal input to TC neurons through NMDA receptors plays a key role in generation of the sustained response, but the mechanisms for the state-dependent variation in this component are unclear. We used a slice preparation to study responses of TC neurons evoked by trains of electrical pulses to the retinal afferents at frequencies in the range of visual responses in vivo. Despite synaptic depression, the pharmacologically isolated NMDA component gave a pronounced build-up of depolarization through temporal summation of the NMDA receptor mediated EPSPs. This depolarization could provide sustained firing, the frequency of which depended on the holding potential. We suggest that the variation of sustained response in vivo is caused mainly by the state-dependent modulation of the membrane potential of TC neurons which shifts the NMDA receptor mediated depolarization closer to or further away from the firing threshold. The pharmacologically isolated AMPA receptor EPSPs were rather ineffective in spike generation. However, together with the depolarization evoked by the NMDA component, the AMPA component contributed significantly to spike generation, and was necessary for the precise timing of the generated spikes.

Noradrenergic antagonism of the P13 and N40 components of the rat auditory evoked potential

RATIONALE

Two rat auditory evoked potential (AEP) components P13 and N40 are suggested as analogues to the human P50, which has abnormal suppression properties in schizophrenia. However, P50 likely reflects neural activity from several different brain areas. Studies examining each of these components in the rat model have proposed circuitry that involves alpha2 norepinephrine (NE) receptors, and different disruption effects are predicted depending on whether effects are presynaptic or postsynaptic.

OBJECTIVES

The aim of this paper is to test differential effects of NE antagonism on disruption of normal P13 and N40 expression.

MATERIALS AND METHODS

AEPs were recorded simultaneously in alert, freely moving rats using the alpha2 antagonist yohimbine. Amplitudes of P13 and N40 elicited by 500-ms interstimulus interval click pairs were measured after administration of a placebo and three doses of the yohimbine.

RESULTS

A high dose of yohimbine yielded smaller P13 amplitudes to both clicks, consistent with presynaptic action. However, a moderate yohimbine dose yielded increased P13 amplitudes to both clicks. For N40, a moderate dose of yohimbine yielded increased amplitudes to the second stimulus, and a high dose restored normal suppression, which is consistent with previously reported findings.

CONCLUSIONS

This study demonstrated that noradrenergic activity differentially affects P13 and N40 components. As P13 and N40 are each models of human P50, these findings highlight the complex circuitry that likely underlies P50. An appreciation for these complexities is critical for understanding the mechanisms of the P50 suppression deficit in schizophrenia, which may be influenced by both trait and state factors.

The formation of auditory fear memory requires the synthesis of protein and mRNA in the auditory thalamus

The medial geniculate nucleus of the thalamus responds to auditory information and is a critical part of the neural circuitry underlying aversive conditioning with auditory signals for shock. Prior work has shown that lesions of this brain area selectively disrupt conditioning with auditory stimuli and that neurons in the medial geniculate demonstrate plastic changes during fear conditioning. However, recent evidence is less clear as to whether or not this area plays a role in the storage of auditory fear memories. In the current set of experiments rats were given infusions of protein or messenger RNA (mRNA) synthesis inhibitors into the medial geniculate nucleus of the thalamus 30 min prior to auditory fear conditioning. The next day animals were tested to the auditory cue and conditioning context. Results showed that rats infused with either inhibitor demonstrated less freezing to the auditory cue 24 h after training, while freezing to the context was normal. Autoradiography confirmed that the doses used were effective in disrupting synthesis. Taken together with prior work, these data suggest that the formation of fear memory requires the synthesis of new protein and mRNA at multiple brain sites across the neural circuit that supports fear conditioning.

Neonatal nicotine exposure impairs nicotinic enhancement of central auditory processing and auditory learning in adult rats

Children of women who smoke cigarettes during pregnancy display cognitive deficits in the auditory-verbal domain. Clinical studies have implicated developmental exposure to nicotine, the main psychoactive ingredient of tobacco, as a probable cause of subsequent auditory deficits. To test for a causal link, we have developed an animal model to determine how neonatal nicotine exposure affects adult auditory function. In adult control rats, nicotine administered systemically (0.7 mg/kg, s.c.) enhanced the sensitivity to sound of neural responses recorded in primary auditory cortex. The effect was strongest in cortical layers 3 and 4, where there is a dense concentration of nicotinic acetylcholine receptors (nAChRs) that has been hypothesized to regulate thalamocortical inputs. In support of the hypothesis, microinjection into layer 4 of the nonspecific nAChR antagonist mecamylamine (10 microM) strongly reduced sound-evoked responses. In contrast to the effects of acute nicotine and mecamylamine in adult control animals, neither drug was as effective in adult animals that had been treated with 5 days of chronic nicotine exposure (CNE) shortly after birth. Neonatal CNE also impaired performance on an auditory-cued active avoidance task, while having little effect on basic auditory or motor functions. Thus, neonatal CNE impairs nicotinic regulation of cortical function, and auditory learning, in the adult. Our results provide evidence that developmental nicotine exposure is responsible for auditory-cognitive deficits in the offspring of women who smoke during pregnancy, and suggest a potential underlying mechanism, namely diminished function of cortical nAChRs.

Some histological effects of chronic administration of chloroquine on the medial geniculate body of adult wistar rat

Some histological effects of chronic administration of chloroquine commonly used for prophylaxis or treatment of malaria. rheumatoid arthritis and lupus erythematosus on the medial geniculate body (MGB) of adult wistar rats was carefully studied. The rats of both sexes (n= 18), average weight of 184g were randomly assigned into treatment (n= 10) and control (n=7) groups. The rats in the treatment group received 2mg/kg body weight of chloroquine base dissolved in distilled water daily for fourteen days through the orogastric tube administration while the control rats received equal volume of distilled water daily through the same route. The rats were fed with rat pellets purchased from Topfeed Ltd. Sapele. Delta State. Nigeria and given water liberally and were then sacrificed on day fifteen of the experiment. The MGB were carefully dissected out and quickly fixed in 10% formal saline for routine histological study after H & E and thionin methods. The histological findings after H & E methods indicated that the treated sections of the MGB showed faintly reduced nuclei size, with the presence of many autophagic vacuoles and degenerative neurons when compared to the control sections. On the other hand. the thionin method indicated that the treated sections showed sparsely distributed neurons, which stain less intensely when compared with the control. The nissl substance in some of the neurons appeared degenerative while some hypertrophied with some vacuolations. These findings indicated that chronic administration of chloroquine has a deleterious effect on the neurons and nissl substance of the MGB. Chloroquine may probably have adverse effects on auditory sensibilities by its deleterious effects on the nerve cells and nissl substances of the MGB of the adult wistar rats. It is recommended that further studies aimed at corroborating these observations be carried out.

Infralimbic cortex projects to all parts of the pontine and medullary lateral tegmental field in cat

The infralimbic cortex (ILc) in cat is the ventralmost part of the anterior cingulate gyrus. The ILc, together with the amygdala, bed nucleus of the stria terminalis and lateral hypothalamus, is involved in the regulation of fear behavior. The latter three structures are thought to take part in triggering the fear response by means of their projections to the pontine and medullary lateral tegmental field (LTF). The LTF is a large region extending from the parabrachial nuclei rostrally to the spinal cord caudally. It contains almost all the premotor interneurons for the brainstem and for some upper spinal cord motoneurons innervating the muscles of face, head and throat. The question is whether ILc also projects to the LTF. Such a pathway would allow the ILc to influence the fear response by acting directly on these premotor interneurons. Anterograde tracer injections were made in the medial surface of the cortex in four cats. Only when the injection sites involved ILc were anterogradely labeled fibers observed throughout the rostrocaudal extent of the LTF. To verify whether these projections indeed originated from ILc, in two other cases retrograde tracer injections were made in the pontomedullary LTF. The results showed many retrogradely labeled neurons in ILc, but none in adjacent cortical regions. These results show that the ILc projects to the LTF in cat and can possibly modulate the fear response not only via indirect but also via direct routes to the premotor interneurons in the brainstem.

Brain-derived neurotrophic factor enhances expression of superior cervical ganglia clone 10 in lateral geniculate nucleus and visual cortex of developing kittens

Neuronal growth-associated proteins, including superior cervical ganglia clone 10 (SCG10) family molecules, play roles in neurite outgrowth and network formation as well as structural and functional plasticity. The present ontogenetic study revealed that the expression of neuronal growth-associated proteins in the visual cortex (VC) exhibited a sharp peak in the early postnatal period when growing lateral geniculate nucleus (LGN) axon terminals segregate into the ocular dominance columns depending on retinal activity. We then hypothesized that SCG10 family molecules, known for catastrophic factors of microtubules, play important roles in the formation of ocular dominance columns. To test this hypothesis, we studied whether: (i) monocular blockade of retinal activity changed the SCG10 expression in LGN and VC and (ii) brain-derived neurotrophic factor (BDNF) cortical infusion modified the expression of SCG10 family molecules and the number of excitatory/inhibitory cortical synapses. Using northern blot and in situ hybridization, we revealed that: (i) silencing retinal activity with tetrodotoxin eye injections dynamically reduced the expression of SCG10 mRNA and (ii) it was enhanced by BDNF in VC and LGN of kittens but not adult cats. These findings suggest that cortical infusion of BDNF and retinal activity up-regulate the expression of SCG10 in the LGN and VC and that up-regulated SCG10 in turn initiates marked reorganization of the microtuble network, eventually resulting in increase in synapse formation in the VC.

Cholinergic modulation of synaptic transmission and postsynaptic excitability in the rat gracilis dorsal column nucleus

Somatosensory information, conveyed through the gracilis nucleus (GN), is regulated by descending corticofugal (CF) glutamatergic fibers. In addition, the GN receives cholinergic inputs with still unclear source and functional significance. Using both the in vitro slice and intracellular recording with sharp and patch electrodes and in vivo extracellular single-unit recordings, we analyzed the effects of activation of cholinergic receptors on synaptic, intrinsic, and functional properties of rat GN neurons. The cholinergic agonist carbamilcholine-chloride [carbachol (CCh); 1-10 microM] in vitro (1) induced presynaptic inhibition of EPSPs evoked by both dorsal column and CF stimulation, (2) increased postsynaptic excitability, and (3) amplified the spike output of GN neurons. The inhibition by atropine (1 microM) and pirenzepine (10 microM) of all presynaptic and postsynaptic effects of CCh suggests actions through muscarinic M1 receptors. The above effects were insensitive to nicotinic antagonists. We searched the anatomical origin of the cholinergic projection to the GN throughout the hindbrain and forebrain, and we found that the cholinergic fibers originated mainly in the pontine reticular nucleus (PRN). Electrical stimulation of the PRN amplified sensory responses in the GN in vivo, an effect prevented by topical application of atropine. Our results demonstrate for the first time that cholinergic agonists induce both presynaptic and postsynaptic effects on GN neurons and suggest an important regulatory action of inputs from cholinergic neuronal groups in the pontine reticular formation in the functional control of somatosensory information flow in the GN.

Memantine protects neurons from shrinkage in the lateral geniculate nucleus in experimental glaucoma

OBJECTIVE

To determine whether memantine as a treatment for glaucoma prevents neuron shrinkage in the lateral geniculate nucleus, the major target for retinal ganglion cells.

METHODS

Sixteen monkeys with right-eye unilateral experimental glaucoma for 14 months were studied and treated with memantine (n = 9) or vehicle only (n = 7). Left lateral geniculate nucleus relay neurons (layers 1, 4, and 6) were examined following parvalbumin immunolabeling. Cell body cross-sectional areas and neuron numbers were assessed using unbiased methods. Memantine- and vehicle-treated glaucoma groups were compared using t tests and analysis of covariance.

RESULTS

Compared with vehicle-treated animals, memantine-treated animals showed significantly less mean +/- SD neuron shrinkage in layers 1 (-4.0% +/- 13.9% vs 28.2% +/- 17.4%; P = .001) and 4 (24.9% +/- 10.0% vs 37.2% +/- 12.3%; P = .04). For layer 6, the difference was not statistically significant (34.2% +/- 10.1% vs 45.3% +/- 14.5%; P = .10). Analysis of covariance results showed significantly less neuron shrinkage in the memantine-treated group for layers 1, 4, and 6 (P < .001; P < .02; and P < .04, respectively). This difference was greatest in layer 1. In each of these layers, neuron numbers did not differ significantly between groups.

CONCLUSION

Monkeys with glaucoma that were treated with memantine showed significantly less neuron shrinkage in the lateral geniculate nucleus than the vehicle-treated glaucoma group.

CLINICAL RELEVANCE

The finding that memantine protects adult visual neurons from transsynaptic atrophy in experimental glaucoma could have therapeutic value. Currently, memantine is being tested in an ongoing clinical trial as a treatment for glaucoma.

The Contribution of TWIK-Related Acid-Sensitive K+-Containing Channels to the Function of Dorsal Lateral Geniculate Thalamocortical Relay Neurons

A genetic knockout was used to determine the specific contribution of TWIK-related acid-sensitive K+ (TASK)-1 channels to the function of dorsal lateral geniculate nucleus (DLG) thalamocortical relay (TC) neurons. Disruption of TASK-1 function produced an approximately 19% decrease in amplitude of the standing outward current (ISO) and a 3 +/- 1-mV depolarizing shift in resting membrane potential (Vrest) of DLG neurons. We estimated that current through TASK-1 homodimers or TASK-1/TASK-3 heterodimers contribute(s) approximately one third of the current sensitive to TASK channel modulators in DLG TC neurons. The effects of the TASK channel blocker bupivacaine (20 microM), of muscarine (50 microM), and of H+ on ISO were reduced to approximately 60%, 59%, and shifted to more acidic pH values, respectively. The blocking effect of anandamide on ISO [30 microM; 23 +/- 3% current decrease in wild type (WT)] was absent in TASK-1 knockout (TASK-1-/-) mice (9 +/- 6% current increase). Comparable results were obtained with the more stable anand-amide derivative methanandamide (20 microM; 20 +/- 2% decrease in WT; 4 +/- 6% increase in TASK-1-/-). Current-clamp recordings revealed a muscarine-induced shift in TC neuron activity from burst to tonic firing in both mouse genotypes. Electrocorticograms and sleep/wake times were unchanged in TASK-1-/- mice. In conclusion, our findings demonstrate a significant contribution of TASK-1 channels to ISO in DLG TC neurons, although the genetic knockout of TASK-1 did not produce severe deficits in the thalamocortical system.

[The influence of dopamine injected into the lateral geniculate body on flash visual evoked response in rats]

PURPOSE

Numerous neurotransmitters and neuromodulators operating on all levels of the visual system, influence the parameters of cortical evoked response. The aim of this study was to find the influence of dopamine (DA) injected into the lateral geniculate body (nucleus) on flash visual evoked response (FVEP).

MATERIAL AND METHODS

The experiments were performed on adult Wistar rats. They were implanted with 1 caniula conducted into the lateral geniculate body and 2 electrodes: active under the skull on dura mater in occipital region of brain and reference one on the skull in the interorbital region. FVEPs curve before and after injection into the lateral geniculate body 0.9% NaCl and DA were analyzed.

RESULTS

DA in the lateral geniculate body causes increasing latency and amplitude of FVEP peaks.

CONCLUSIONS

Dopamine in the lateral geniculate body can improve visual perception in the occipital cortex.

Excitatory actions of synaptically released catecholamines in the rat lateral geniculate nucleus

The gating properties of thalamic relay neurons are influenced by the actions of a variety of neuromodulators in concert with the intrinsic properties of these relay neurons. In this study, we have investigated the consequences of synaptically released catecholamines on the excitability of neurons in the rat dorsal lateral geniculate nucleus. Tetanic stimulation of the optic tract, in which catecholamine fibers also course near or through, produced a strong depolarization that consisted of a fast and slow component. The fast excitatory postsynaptic potential was attenuated by ionotropic glutamate receptor antagonists and further unmasked the slow excitatory postsynaptic potential. The amplitude of the slow excitatory postsynaptic potential was dependent on the frequency and intensity of the tetanic stimulation. The alpha1-adrenergic receptor antagonist, prazosin, and the D1-like dopamine receptor antagonist, SCH23390, attenuated the slow excitatory postsynaptic potential; however, the slow excitatory postsynaptic potential was unaltered by metabotropic glutamate, cholinergic, alpha2-adrenergic, and beta-adrenergic receptor antagonists. On the other hand, tetanic stimulation of the optic radiations (corticothalamic axons) evoked a slow excitatory postsynaptic potential that was completely attenuated by metabotropic glutamate receptor antagonists. Our results suggest that tetanic stimulation of catecholamine fibers within the optic tract produces synaptic release of norepinephrine and dopamine that in turn activates both alpha(1)-adrenergic and D1-like dopamine receptors leading to a robust membrane depolarization. By altering the excitability of relay neurons, ascending activating systems may modulate the efficacy of information transfer through the thalamus.

Excitatory effects of thyrotropin-releasing hormone in the thalamus

The activity of the thalamus is state dependent. During slow-wave sleep, rhythmic burst firing is prominent, whereas during waking or rapid eye movement sleep, tonic, single-spike activity dominates. These state-dependent changes result from the actions of modulatory neurotransmitters. In the present study, we investigated the functional and cellular effects of the neuropeptide thyrotropin-releasing hormone (TRH) on the spontaneously active ferret geniculate slice. This peptide and its receptors are prominently expressed in the thalamic network, yet the role of thalamic TRH remains obscure. Bath application of TRH resulted in a transient cessation of both spindle waves and the epileptiform slow oscillation induced by application of bicuculline. With intracellular recordings, TRH application to the GABAergic neurons of the perigeniculate (PGN) or thalamocortical cells in the lateral geniculate nucleus resulted in depolarization and increased membrane resistance. In perigeniculate neurons, this effect reversed near the reversal potential for K+, suggesting that it is mediated by a decrease in K+ conductance. In thalamocortical cells, the TRH-induced depolarization was of sufficient amplitude to block the generation of rebound Ca2+ spikes, whereas the even larger direct depolarization of PGN neurons transformed these cells from the burst to tonic, single-spike mode of action potential generation. Furthermore, application of TRH prominently enhanced the afterdepolarization that follows rebound Ca2+ spikes, suggesting that this transmitter may also enhance Ca2+-activated nonspecific currents. These data suggest a novel role for TRH in the brain as an intrinsic regulator of thalamocortical network activity and provide a potential mechanism for the wake-promoting and anti-epileptic effects of this peptide.

Mosaic properties of midget and parasol ganglion cells in the marmoset retina

We measured mosaic properties of midget and parasol ganglion cells in the retina of a New World monkey, the common marmosetCallithrix jacchus. We addressed the functional specialization of these populations for color and spatial vision, by comparing the mosaic of ganglion cells in dichromatic (“red–green color blind”) and trichromatic marmosets. Ganglion cells were labelled by photolytic amplification of retrograde marker (“photofilling”) following injections into the lateral geniculate nucleus, or by intracellular injection in anin vitroretinal preparation. The dendritic-field size, shape, and overlap of neighboring cells were measured. We show that in marmosets, both midget and parasol cells exhibit a radial bias, so that the long axis of the dendritic field points towards the fovea. The radial bias is similar for parasol cells and midget cells, despite the fact that midget cell dendritic fields are more elongated than are those of parasol cells. The dendritic fields of midget ganglion cells from the same (ON or OFF) response-type array show very little overlap, consistent with the low coverage of the midget mosaic in humans. No large differences in radial bias, or overlap, were seen on comparing retinae from dichromatic and trichromatic animals. These data suggest that radial bias in ganglion cell populations is a consistent feature of the primate retina. Furthermore, they suggest that the mosaic properties of the midget cell population are associated with high spatial resolution rather than being specifically associated with trichromatic color vision.

Carbamazepine induced pitch shift and octave space representation

Octave-circular pitch perception, the repetition of pitch scale qualities when surpassing the octave interval, has been observed in behavioral data from humans and monkeys, but the underlying anatomy and physiology is still unknown. Here we analyze octave circularity in a concert pianist with absolute pitch, both under medication with the neurotropic drug carbamazepine (CBZ) and without medication. Analysis of 4619 responses in a pitch identification task revealed an internal tone-scale representation, based on the norm-tone scale re A4=440 Hz, with an octave-circular pattern of strongly and weakly represented tones. CBZ caused a global down-shift of pitch (ca. 1 semitone at 500 Hz), but no down-shift of the octave-circular pattern of tone characteristics. This pattern was similar in the six tested octave ranges (32.7-2093 Hz), both under the control and the CBZ condition. Pattern repetition always occurred at octave intervals and did not reflect the stretched octaves of piano tuning. The results indicate that CBZ influences pitch detection peripheral of an octave-circular pitch representation. Thus they support previous evidence for pitch detection in the auditory midbrain and for octave-circular pitch mapping in the auditory thalamus.

Chronic morphine exposure induces degradation of receptive field properties of LGN cells in cats

AIM

To investigate the effect of chronic morphine exposure on the receptive field properties of lateral geniculate nucleus (LGN) neurons in cats.

METHODS

Cats were injected with morphine (10 mg/kg) or saline twice daily, for 10 d. Subsequently, extracellular single-unit recording techniques were used to examine the sensitivity of LGN neurons to visual stimuli in chronic morphine-treated and saline-treated cats.

RESULTS

Compared with saline-treated cats (as controls), LGN neurons in morphine-treated cats had decreased signal-to-noise ratios (1.9 vs 3.1), and orientation and direction sensitivity (0.103 vs 0.135 and 0.074 vs 0.10, respectively), accompanied by significant increases in spontaneous (27.4 vs 17.5) and evoked activity (preferred: 42.2 vs 38.1; average: 28.1 vs 22.3).

CONCLUSION

Chronic morphine exposure can lead to the functional degradation of LGN neurons in cats, which might result from the effects of chronic morphine exposure on inhibitory neurotransmission.

Opposite effects of tetanic stimulation of the auditory thalamus or auditory cortex on the acoustic startle reflex in awake rats

The amygdala mediates both emotional learning and fear potentiation of startle. The lateral amygdala nucleus (LA) receives auditory inputs from both the auditory thalamus (medial geniculate nucleus; MGN) and auditory association cortex (AAC), and is critical for auditory fear conditioning. The central amygdala nucleus, which has intra-amygdaloid connections with LA, enhances startle magnitude via midbrain connections to the startle circuits. Tetanic stimulation of either MGN or AAC in vitro or in vivo can induce long-term potentiation in LA. In the present study, behavioural consequences of tetanization of these auditory afferents were investigated in awake rats. The acoustic startle reflex of rats was enhanced by tetanic stimulation of MGN, but suppressed by that of AAC. All the tetanization-induced changes of startle diminished within 24 h. Blockade of GABAB receptors in the LA area reversed the suppressive effect of tetanic stimulation of AAC on startle but did not change the enhancing effect of tetanic stimulation of MGN. Moreover, transient electrical stimulation of MGN enhanced the acoustic startle reflex when it lagged behind acoustic stimulation, but inhibited the acoustic startle reflex when it preceded acoustic stimulation. The results of the present study indicate that MGN and AAC afferents to LA play different roles in emotional modulation of startle, and AAC afferents are more influenced by inhibitory GABAB transmission in LA.

Morphological correlates of triadic circuitry in the lateral geniculate nucleus of cats and rats

We used an in vitro slice preparation of the lateral geniculate nucleus in cats and rats to study morphological correlates of triadic circuitry in relay cells. The three triadic elements involve a retinal synapse onto a GABAergic dendritic terminal of an interneuron, a synapse from the same retinal terminal onto a relay cell dendrite, and a synapse from the same interneuron terminal onto the same relay cell dendrite. We made whole cell recordings and labeled cells with biocytin. Previous methods were used to identify triadic circuitry based on evidence that the retinal terminal activates a metabotropic glutamate receptor on the interneuronal terminal. Thus application of (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (an agonist to that receptor) increases the rate of spontaneous inhibitory postsynaptic currents (sIPSCs) recorded in the relay cell, and if some of this increase remains with further addition of TTX (a TTX-insensitive response), a triad is indicated. We quantified the extent of the TTX-insensitive response and sought morphological correlates. In both rats and cats, this response correlated (negatively) with the number of primary dendrites and (positively) with polarity of the dendritic arbor. There was no correlation with cell size. Curiously, in cats, this response correlated with the presence of appendages at primary dendritic branches, but there was no such correlation in rats. These observations in cats map onto the X/Y classification, with X cells having triads, but it is not clear from our results if a comparable classification exists for rats.

Effects of conditioning during amygdalar inactivation on training-induced neuronal plasticity in the medial geniculate nucleus and cingulate cortex in rabbits (Oryctolagus cuniculus)

This study addressed the amygdala's role in avoidance conditioning in rabbits (Oryctolagus cuniculus). Intra-amygdalar muscimol infusion before 60 or 120 conditioning trials blocked training-induced neuronal activity (TIA) in the medial geniculate (MG) nucleus. One hundred twenty trials with muscimol blocked TIA permanently, during conditioning with muscimol and then later without muscimol; 60 trials with muscimol blocked TIA only when muscimol was present. Cingulate cortical TIA was blocked only when muscimol was present. Behavioral learning did not occur with muscimol, but later learning was facilitated (i.e., savings occurred) in rabbits initially given muscimol plus training. These results define the time period wherein amygdalar processes initiate TIA in the MG nucleus and suggest that distinct forms of amygdalar processes induce TIA in the MG nucleus and cingulate cortex.

Intraocular delivery of BDNF following visual cortex lesion upregulates cytosolic branched chain aminotransferase (BCATc) in the rat dorsal lateral geniculate nucleus

Visual cortex ablation in newborn rats determines the almost complete degeneration of neurons in the dorsal lateral geniculate nucleus (dLGN), as a consequence of the axotomy of the geniculo-cortical fibres. Death of dLGN neurons is massive and rapid, and occurs by apoptosis. We recently showed that exogenous administration of the neurotrophin brain-derived neurotrophic factor (BDNF) in the eye prevents the degeneration of dLGN neurons occurring after visual cortex lesion in newborn rats. To elucidate the molecular mechanisms of BDNF-mediated neuroprotection, we sought to identify novel genes regulated by BDNF in the rat dLGN after visual cortex lesion. By using mRNA fingerprinting, we isolated a cDNA fragment upregulated in the dLGN of lesioned rats treated with BDNF. This cDNA fragment shared 100% homology with the rat cytosolic branched chain aminotransferase (BCATc), a key enzyme of glutamate metabolism. Quantitative reverse transcription-polymerase chain reaction and in situ hybridization confirmed that BCATc mRNA is markedly overexpressed by exogenous supply of BDNF to axotomized dLGNs. Immunohistochemical analysis showed that upregulation of BCATc in the dLGN of lesioned rats treated with BDNF takes place in astrocytes. These results suggest that modulation of glutamate metabolism by astrocytes might play an important role in BDNF-mediated survival of axotomized dLGN neurons.

Short-term depression of synaptic transmission from rat lateral geniculate nucleus to primary visual cortex in vivo

Two forms of short-term synaptic plasticity, paired-pulse depression (PPD) and frequency depression, were prominent in the adult rat geniculo-cortical visual pathway in vivo. Iontophoresis of GABAa receptor antagonist (bicuculline methiodide) or GABAb receptor antagonist (2-hydroxy-saclofen) in the primary visual cortex significantly reduced the short-term synaptic depression. When NMDA or AMPA/kainate receptors were blocked, no obvious change of synaptic depression was observed. Application of high [Ca(2+)] enhanced the short-term synaptic depression. Our results suggest that the presynaptic Ca(2+)-dependent neurotransmitter depletion and postsynaptic GABAergic inhibition may be crucial for short-term synaptic depression in the geniculo-cortical pathway.

GABAC receptor mediated inhibition in acutely isolated neurons of the rat dorsal lateral geniculate nucleus

In the dorsal lateral geniculate nucleus (dLGN), GABA(C) receptors seems to be specifically expressed by local GABAergic interneurons. Although the presence of GABA(C) receptors has been demonstrated, a quantitative estimation of their contribution to inhibition in dLGN is lacking. Because the amount of inhibition mediated by these receptors might reflect their functional importance we performed whole-cell patch clamp recordings from dLGN cells acutely dissociated from brain slices. We focally applied the GABA receptor agonist muscimol and quantified effects mediated through either GABA(C) or GABA(A) receptors. Because their basic dendritic morphology was preserved, we tried to morphologically differentiate between thalamocortical cells and local interneurons. In the majority of multipolar cells, representing thalamocortical projection neurons, the specific GABA(A) receptor antagonist bicuculline completely blocked muscimol induced currents. In contrast, in most of the bipolar cells, representing interneurons, bicuculline blocked only 70-80% of the muscimol induced currents. The remaining currents were blocked by co-application of TPMPA, a specific GABA(C) receptor antagonist, or picrotoxin, an unspecific GABA(A) and GABA(C) receptor blocker. The latter neurons were also sensitive to the selective GABA(C) receptor agonist cis-aminocrotonic acid. These results indicate that in those dLGN neurons that express GABA(C) receptors, these receptors contribute considerably to GABAergic inhibitory inputs.

Synaptic Activation of Metabotropic Glutamate Receptors Regulates Dendritic Outputs of Thalamic Interneurons

Information gating through the thalamus is dependent on the output of thalamic relay neurons. These relay neurons receive convergent innervation from a number of sources, including GABA-containing interneurons that provide feed-forward inhibition. These interneurons are unique in that they have two distinct outputs: axonal and dendritic. In addition to conventional axonal outputs, these interneurons have presynaptic dendrites that may provide localized inhibitory influences. Our study indicates that synaptic activation of metabotropic glutamate receptors (mGluRs) increases inhibitory activity in relay neurons by increasing output of presynaptic dendrites of interneurons. Optic tract stimulation increases inhibitory activity in thalamic relay neurons in a frequency- and intensity-dependent manner and is attenuated by mGluR antagonists. Our data suggest that synaptic activation of mGluRs selectively alters dendritic output but not axonal output of thalamic interneurons. This mechanism could serve an important role in focal, feed-forward information processing in addition to dynamic information processing in thalamocortical circuits.

[Taurine effects on background impulse activity of the internal geniculate body neurons and mesencephalic inferior tubers of white rats]

Microelectrophysiological and computer techniques were used in the study of background impulse activity (BIA) of the internal geniculate body (IGB) neurons and mesencephalic inferior tubers (MIT) of white rats. Definite differences were found in BIA by regularity, dynamic types and modality of interimpulse histograms. Mean frequency of MIT neuron discharges was 16-17 Hz and was about 3 times higher than in neurons of the IGB. Intraperitoneal injection of taurin noticeably suppressed neuronal activity in both nuclei. The drug reduced mean frequency of background impulse discharges both in MIT and IGB. Thus, taurin produces primarily suppressing modulating effect on neuronal activity of IGB and MIT.

Distinct forms of cholinergic modulation in parallel thalamic sensory pathways

Mammalian thalamus is a critical site where early perception of sensorimotor signals is dynamically regulated by acetylcholine in a behavioral state-dependent manner. In this study, we examined how synaptic transmission is modulated by acetylcholine in auditory thalamus where sensory relay neurons form parallel lemniscal and nonlemniscal pathways. The former mediates tonotopic relay of acoustic signals, whereas the latter is involved in detecting and transmitting auditory cues of behavioral relevance. We report here that activation of cholinergic muscarinic receptors had opposite membrane effects on these parallel synaptic pathways. In lemniscal neurons, muscarine induced a sustained membrane depolarization and tonic firing by closing a linear K(+) conductance. In contrast, in nonlemniscal neurons, muscarine evoked a membrane hyperpolarization by opening a voltage-independent K(+) conductance. Depending on the level of membrane hyperpolarization and the strength of local synaptic input, nonlemniscal neurons were either suppressed or selectively engaged in detecting and transmitting synchronized synaptic input by firing a high-frequency spike burst. Immunohistochemical and Western blotting experiments showed that nonlemniscal neurons predominantly expressed M2 muscarinic receptors, whereas lemniscal cells had a significantly higher level of M1 receptors. Our data indicate that cholinergic modulation in the thalamus is pathway-specific. Enhanced cholinergic tone during behavioral arousal or attention may render synaptic transmission in nonlemniscal thalamus highly sensitive to the context of local synaptic activities.

Parallel colour-opponent pathways to primary visual cortex

The trichromatic primate retina parses the colour content of a visual scene into 'red/green' and 'blue/yellow' representations. Cortical circuits must combine the information encoded in these colour-opponent signals to reconstruct the full range of perceived colours. Red/green and blue/yellow inputs are relayed by the lateral geniculate nucleus (LGN) of thalamus to primary visual cortex (V1), so understanding how cortical circuits transform these signals requires understanding how LGN inputs to V1 are organized. Here we report direct recordings from LGN afferent axons in muscimol-inactivated V1. We found that blue/yellow afferents terminated exclusively in superficial cortical layers 3B and 4A, whereas red/green afferents were encountered only in deeper cortex, in lower layer 4C. We also describe a distinct cortical target for 'blue-OFF' cells, whose afferents terminated in layer 4A and seemed patchy in organization. The more common 'blue-ON' afferents were found in 4A as well as lower layer 2/3. Chromatic information is thus conveyed to V1 by parallel, anatomically segregated colour-opponent systems, to be combined at a later stage of the colour circuit.

Bursts in the medial geniculate body: a comparison between anesthetized and unanesthetized states in guinea pig

Thalamic high frequency bursts have long been described under anesthesia and during slow-wave sleep (SWS). More recently, studies in the lateral geniculate nucleus have pointed out that they are also present during waking (W). Here, we compared the bursts recorded in the medial geniculate body of guinea pigs under anesthesia or during periods of W and SWS. The tuning of single units was tested between threshold and 80 dB SPL in two conditions: (1) in restrained, undrugged, non-sleep-deprived guinea pigs (n=101 cells) and (2) under pentobarbital anesthesia (n=53 cells). Off-line analyses allowed us to distinguish single action potentials (APs) from bursts. A burst was defined as a group of APs with an interspike interval < or =4 ms, preceded by a silent period > or =100 ms. We found that auditory thalamus bursts occur in synchronized electroencephalogram states (SWS and anesthesia), but also during W. Although the burst characteristics did not differ among the three states, group data showed that the proportion of bursts within spike trains was the greatest under anesthesia. This observation resulted from two types of effects: (1) the percentage of non-bursting cells was lowest under anesthesia and (2) some cells under anesthesia exhibited up to 90% of bursts, whereas during W or SWS the highest proportion of bursts did not exceed 40%. The presence of these bursts is discussed with regards to the known fluctuations of membrane potential which occur in these various states.

Receptive field structure of burst and tonic firing in feline lateral geniculate nucleus

There are two recognised modes of firing activity in thalamic cells, burst and tonic. A low-threshold (LT) burst (referred to from now on as 'burst') comprises a small number of high-frequency action potentials riding the peak of a LT Ca(2+) spike which is preceded by a silent hyperpolarised state > 50 ms. This is traditionally viewed as a sleep-like phenomenon, with a shift to tonic mode at wake-up. However, bursts have also been seen in the wake state and may be a significant feature for full activation of recipient cortical cells. Here we show that for visual stimulation of anaesthetised cats, burst firing is restricted to a reduced area within the receptive field centre of lateral geniculate nucleus cells. Consistently, the receptive field size of all the recorded neurons decreased in size proportionally to the percentage of spikes in bursts versus tonic spikes, an effect that is further demonstrated with pharmacological manipulation. The role of this shrinkage may be distinct from that also seen in sleep-like states and we suggest that this is a mechanism that trades spatial resolution for security of information transfer.