Third-order Neurons Research Articles

The pathophysiology of migraine.

Migraine results from episodic changes in central nervous system physiologic function in hyperexcitable brain manifested by abnormal energy metabolism, lowered threshold for phosphene generation, and increased contingent negative variation. Human functional magnetic resonance imaging and magnetoencepholography data strongly suggest that aura is caused by cortical spreading depression. Brain hyperexcitability may be caused by low magnesium levels, mitochondrial abnormalities with abnormal phosphorylation of adenosine 5'-diphosphate, a dysfunction related to nitric oxide, or calcium channelopathy. Low magnesium can result in opening of calcium channels, increased intracellular calcium, glutamate release, and increased extracellular potassium, which may in turn trigger cortical spreading depression. Mitochondrial dysfunction has been suggested by a low phosphocreatine:Pi ratio and a possible response by migraine patients to riboflavin prophylaxis. Nitroglycerine administration results in a delayed migraine-like headache in migraine patients but not in control patients, and a nonspecific nitric oxide synthase inhibitor aborted migraine at 2 hours in the majority of tested migraine patients compared to controls. Many patients with familial hemiplegic migraine have a missense mutation in the P/Q calcium channel, so that this form of migraine, at least, is associated with a demonstrable calcium channelopathy. The generation of migraine occurs centrally in the brain stem, sometimes preceded by cortical spreading depression and aura. Activation of the trigeminovascular system stimulates perivascular trigeminal sensory afferent nerves with release of vasoactive neuropeptides, resulting in vasodilation and transduction of central nociceptive information. There is then a relay of pain impulses to central second- and third-order neurons and activation of brain stem autonomic nuclei to induce associated symptoms.

On-bipolar cells and depolarising third-order neurons as the origin of the ERG-b-wave in the RCS rat

In the retinas of Royal College of Surgeons (RCS) rats light induces an increase in distal extracellular potassium irrespective of the age, between days 19–24 and days 29–35 postpartum, but by days 29–35 the ERG b-wave has become reduced. The synaptic blocker 2-amino-4-phosphonobutyric acid (APB) causes the abolition of both the b-wave and the potassium increase at any age. MgCl 2 greatly reduces the b-wave at all ages and abolishes the potassium increase in older rats, but in younger rats the potassium increase is enlarged. Since this increase occurs in the absence of the b-wave it is unlikely that the on-bipolar cells are the only sources of the b-wave. Because the NMDA receptor blocker ketamine reduces the b-wave, third order neurons, which possess NMDA receptors, could contribute to the b-wave.

Retinal research using the perfused mammalian eye.

The effort to isolate and maintain alive in vitro an intact mammalian eye is rewarded by the full control provided over the arterial input and exclusion of systemic regulatory or compensatory mechanisms. Electrical recording of typical light-evoked field potentials from retina and optic nerve can be complemented by single-cell recording. Thus, light-induced electrical activity reflects the function of the retinal pigment epithelium, of the layers of the retina and of the ganglion cells or their axons. Retinal function in vitro is documented by electrophysiological and morphological methods revealing subtle features of retinal information processing as well as optic nerve signals that approach-at threshold stimulus intensity-the human psychophysical threshold. Such sensitivity of third-order retinal neurons is described for the first time. This well controlled in vitro preparation has been used successfully for biophysical, metabolic and pharmacological studies. Examples are provided that demonstrate the marked sensibility of the rod system to changes in glucose supply. Moreover, histochemical identification of glycogen stores revealed labeling of the second- and third-order neurons subserving the rod system, in addition to labeling of Müller (glial) cells in the cat retina. The glycogen content of the cat retina is augmented by prolonged anesthesia, largely depleted by ischemia after enucleation and enhanced by insulin. Pharmacological experiments using agonists and antagonists of putative retinal neurotransmitters are summarized and outlined using the muscarinic cholinergic agonist QNB as an example. Actions and uptake of the neuromodulator adenosine are presented in detail, including inhibitory effects on physiologically characterized ganglion cells. Neuronal effects of adenosine are distinguished from those resulting from vasodilatation and from glycogenolysis induced by the neuromodulator. To open the blood-retina barrier, a hyperosmotic challenge can be applied transiently. This process is monitored histochemically using FITC-albumin and with electrophysiological parameters. Changes in vitreo-scleral resistance and in the amplitude of the EOG-light peak appear to reflect the open/closed status of the barrier. This overview of the uses of the isolated perfused mammalian eye in retinal research concludes with a discussion of potential implications for clinically relevant topics.

Commissural effects in the otolith system.

We examined whether otolith-activated second- and third-order vestibular nucleus neurons received commissural inhibition from the contralateral otolithic macula oriented in the same geometric plane. For this purpose we performed intracellular recording in vestibular nucleus neurons after stimulation of the ipsi- and contralateral utricular and saccular nerves. More than half (41/72) of the utricular-activated second-order vestibular nucleus neurons received commissural inhibition from the contralateral utricular nerve. The remaining neurons (31/72) showed no visible response to contralateral utricular nerve stimulation. About half (17/36) of utricular-activated third-order neurons also received commissural inhibition from the contralateral utricular nerve. Approximately 10% (7/67) of saccular-activated second-order vestibular neurons received polysynaptic commissural inhibition, whereas 16% (11/67) received commissural facilitation. The majority (49/67) of saccular second-order vestibular neurons, and almost all (22/23) third-order neurons, showed no visible response to stimulation of the contralateral saccular nerve. The present findings suggest that many utricular-activated vestibular nucleus neurons receive commissural inhibition, which may provide a mechanism for increasing the sensitivity of vestibular neurons to horizontal linear acceleration and lateral tilt of the head. Commissural inhibition in the saccular system was less prominent than in the utricular system.

Amacrine and ganglion cell contributions to the electroretinogram in amphibian retina

The neuronal generators of the b- and d-waves of the electroretinogram (ERG) were investigated in the tiger salamander retina to determine if amacrine and ganglion cells contribute to this field potential. Several agents were used that affect third-order neurons, such as tetrodotoxin, baclofen, and NMDA agonists and antagonists. Baclofen, an agent that enhances light responses in third-order neurons, increased the d-wave and reduced the b-wave. In contrast, agents that decrease light responses in third-order neurons had the opposite effect of enhancing the b-wave and depressing the d-wave. The effect on the d-wave was particularly pronounced. The results indicate that third-order neuronal activity influences b- and d-waves of the ERG. The opposing actions suggest that the b-wave to d-wave ratio might serve as an measure of ganglion cell function.

Anterograde Axonal Transport, Transcytosis, and Recycling of Neurotrophic Factors

Traditional views of neurotrophic factor biology held that trophic factors are released from target cells, retrogradely transported along their axons, and rapidly degraded upon arrival in cell bodies. Increasing evidence indicates that several trophic factors such as brain-derived neurotrophic factor (BDNF), fibroblast growth factor (FGF-2), glial cell-line derived neurotrophic factor (GDNF), insulin-like growth factor (IGF-I), and neurotrophin-3 (NT-3), can move anterogradely along axons. They can escape the degradative pathway upon internalization and are recycled for future uses. Internalized ligands can move through intermediary cells by transcytosis, presumably by endocytosis via endosomes to the Golgi system, by trafficking of the factor to dendrites or by sorting into anterograde axonal transport with subsequent release from axon terminals and uptake by second- or third-order target neurons. Such data suggest the existence of multiple "trophic currencies," which may be used over several steps in neural networks to enable nurturing relationships between connected neurons or glial cells, not unlike currency exchanges between trading partners in the world economy. Functions of multistep transfer of trophic material through neural networks may include regulation of neuronal survival, differentiation of phenotypes and dendritic morphology, synapse plasticity, as well as excitatory neurotransmission. The molecular mechanisms of sorting, trafficking, and release of trophic factors from distinct neuronal compartments are important for an understanding of neurotrophism, but they present challenging tasks owing to the low levels of the endogenous factors.

Parasympathetic and sympathetic control of the pancreas: a role for the suprachiasmatic nucleus and other hypothalamic centers that are involved in the regulation of food intake.

To reveal brain regions and transmitter systems involved in control of pancreatic hormone secretion, specific vagal and sympathetic denervation were combined with injection of a retrograde transsynaptic tracer, pseudorabies virus (PRV), into the pancreas. After sympathetic or vagal transsection first-order neurons were revealed in the dorsal motor nucleus of the vagus (DMV) or in preganglionic spinal cord neurons (SPN), respectively. Careful timing of the survival of the animals allowed the detection of cell groups in immediate control of these DMV or SPN neurons. A far larger number of cell groups is involved in the control of DMV than of SPN neurons. Examples are given of a high level of interaction between the sympathetic and parasympathetic nervous system. Several cell groups project to both branches of the autonomic nervous system, sometimes even the same neurotransmitter is used, e.g., oxytocin neurons in the paraventricular nucleus and melanin-concentrating hormone and orexin neurons in the lateral hypothalamus project to both the DMV and SPN neurons. Moreover, the appearance of third-order neurons located in the sympathetic SPN after complete sympathectomy and in the DMV after complete vagotomy illustrates the possibility that motor neurons of the sympathetic and parasympathetic system may exchange information by means of interneurons. The presence of second-order neurons in prefrontal, gustatory, and piriform cortex may provide an anatomic basis for the involvement of these cortices in the cephalic insulin response. The observation that second-order neurons in both vagal and sympathetic control of the pancreas contain neuropeptides that are known to play a role in food intake indicates a direct association between behavioral and autonomic functions. Finally, the observation of third-order neurons in the suprachiasmatic nucleus and ventromedial hypothalamus shows the modulatory action of the time of the day and metabolic state, respectively.

Polysynaptic neural pathways between the hypothalamus, including the suprachiasmatic nucleus, and the liver

The suprachiasmatic nucleus of the hypothalamus is responsible for a 24-h rhythm in basal glucose levels in the rat. The neural pathways used by the suprachiasmatic nucleus to mediate this rhythm in plasma glucose have not yet been identified. In the present study we examined whether there are any connections between hypothalamic centers, including the suprachiasmatic nucleus, and the liver, which is the main site for glucose production and storage. Transneuronal virus tracing from the liver showed that after injection of pseudorabies virus, specific neuronal cell populations in the central nervous system were labeled retrogradely, suggesting that specific sites in the central nervous system may control liver metabolism. First-order neurons belonged to the sympathetic and parasympathetic system, while second-order and third-order neurons were present in both the brainstem and hypothalamus. The presence of third-order neurons in the suprachiasmatic nucleus suggests an involvement of the biological clock in the neural control of the liver.

Activation of renal afferent pathways following furosemide treatment: I. Effects of survival time and renal denervation

Three experiments were performed to determine whether renal afferent pathways were activated by the diuretic drug, furosemide. It was hypothesized that activated neurons of the renal afferent pathway would express the protein product Fos of the c- fos immediate early gene and be identified by immunocytochemical staining for Fos in the cell nucleus. In the first two experiments, rats were injected with either furosemide (5 mg) or vehicle solution (sterile isotonic saline) and sacrificed either 1.75 h (short-survival experiment) or 3.5 h (long-survival experiment) after injection. In both experiments, the furosemide-treated rats had significantly more Fos-positive cell nuclei than vehicle-treated rats in the subfornical organ (SFO), organum vasculosum lamina terminalis (OVLT), supraoptic nuclei (SON), and magnocellular region of the paraventricular nuclei (PVN) — areas previously shown to be activated by hypovolemia or peripheral angiotensin. In the short-survival experiment, the furosemide-treated rats had more Fos-positive cell nuclei in the nucleus of the solitary tract (NTS) and in the dorsal horn of the spinal cord at spinal levels T 11, T 12, and T 13. In contrast, furosemide treatment did not produce more Fos-positive cell nuclei in the NTS and dorsal horn of the spinal cord in the long-survival experiment. These results suggest that the activation of the SFO, OVLT, SON and PVN may be via a different mechanism than that of NTS or spinal cord dorsal horn. Based upon our previous work, we hypothesized that the NTS and spinal cord dorsal horn labeling was due to activation of sympathetic afferents originating in the kidney and labeling in forebrain structures was due to stimulation by angiotensin generated by renal renin release. To test this hypothesis, a third experiment was devised that was identical to the short-survival experiment, except that all rats had bilateral renal denervation surgery 1 week previously. In this experiment, furosemide administration increased the number of Fos-positive cells in the SFO, OVLT, SON and PVN, but not in the caudal thoracic spinal cord or NTS. These results together with the results of first two experiments lend support to our hypothesis that furosemide-induced neuronal activation in the thoracic spinal cord and NTS is due to activation of second- and/or third-order neurons of a renal sympathetic afferent pathway. Furosemide-induced activation in the SFO, OVLT, SON and PVN does not depend on renal innervation. It is hypothesized that activation in these forebrain regions depends on the action of angiotensin II that is generated after furosemide treatment. Our results indicate that both a hormonal pathway and a renal sympathetic afferent pathway conduct information from the kidney to the central nervous system (CNS) after furosemide treatment.

Contrasting molecular composition and channel properties of AMPA receptors on chick auditory and brainstem motor neurons.

1. Neurons in the brainstem auditory pathway exhibit a number of specializations for transmitting signals reliably at high rates, notably synaptic AMPA receptors with very rapid kinetics. Previous work has not revealed a common structural pattern shared by the AMPA receptors of auditory neurons that could account for their distinct functional properties. 2. We have used whole-cell patch-clamp recordings, mRNA analysis, immunofluorescence, Western blots and agonist-evoked cobalt uptake to compare AMPA receptors on the first-, second- and third-order neurons in the chick ascending auditory pathway with those on brainstem motor neurons of the glossopharyngeal/vagal nucleus, which have been shown to have very slow desensitization kinetics. 3. The results indicate that the AMPA receptors of the cochlear ganglion, nucleus magnocellularis and nucleus laminaris share a number of structural and functional properties that distinguish them from the AMPA receptors of brainstem motor neurons, namely a lower relative abundance of glutamate receptor (GluR)2 transcript and much lower levels of GluR2 immunoreactivity, higher relative levels of GluR3 flop and GluR4 flop, lower relative abundance of the C-terminal splice variants GluR4c and 4d, less R/G editing of GluR2 and 3, greater permeability to calcium, predominantly inwardly rectifying I-V relationships, and greater susceptibility to block by Joro spider toxin. 4. We conclude that the AMPA receptors of auditory neurons acquire rapid kinetics from their high content of GluR3 flop and GluR4 flop subunits and their high permeability to Ca2+ from selective post-transcriptional suppression of GluR2 expression.

Contribution to the kinetics and amplitude of the electroretinogram b-wave by third-order retinal neurons in the rabbit retina

The ERG b-wave is widely believed to reflect mainly light-induced activity of on-center bipolar cells and Müller cells. Third-order retinal neurons are thought to contribute negligibly to generation of the b-wave. Here we show that pharmacological agents which affect predominantly third-order neurons alter significantly both the kinetics and amplitude of the b-wave. Our results support the notion that changes in the amplitude and kinetics of light-induced membrane depolarization in third-order neurons produce similar changes in the amplitude and kinetics of the b-wave. We conclude that activity in third-order neurons makes a significant contribution to b-wave generation. Our results also provide evidence that spiking activity of third-order neurons truncates the a-wave by accelerating the onset of the b-wave.

Effects of prenatal exposure to ethanol on systems matching: the number of neurons in the ventrobasal thalamic nucleus of the mature rat

Following prenatal exposure to ethanol, rats have a 1/3 fewer neurons in the second order (principal sensory nucleus of the trigeminal nerve) and fourth order neurons (somatosensory cortex) of the trigeminal–somatosensory pathway than do controls. Based on the numerical matching hypothesis, we predict that the number of third-order neurons (in the ventrobasal nucleus of the thalamus; VB) also will show a similar effect of prenatal ethanol exposure. Stereological methods were used to determine the total number of neurons in the VB on postnatal day 30. Surprisingly, prenatal exposure to ethanol had no effect on the VB volume or on the number of VB neurons. Thus, prenatal exposure to ethanol induces numerical imbalances within the trigeminal–somatosensory system.

Zn2+ differentially modulates glycine receptors versus GABA receptors in isolated carp retinal third-order neurons

It was previously reported that strychnine-sensitive glycine receptors and GABAA receptors co-existed on carp retinal third-order neurons (amacrine/ganglion cells) (Li, P. and Yang, X.-L., Strong synergism between GABAA and glycine receptors on isolated carp third-order neurons. NeuroReport, 9 (1998) 2785–2789.). In this study, the effects of the divalent cation Zn2+ on these two receptors were studied in amacrine/ganglion cells acutely isolated from carp, with the use of the whole-cell patch clamp recording technique. The glycine-induced currents were steadily potentiated by Zn2+ of lower concentrations (0.1–10 μM), while being dose-dependently inhibited by Zn2+ of higher concentrations (>100 μM). Both the effects involved changes in apparent glycine affinity of the glycine receptor. In contrast, Zn2+ consistently suppressed the GABAA mediated currents of these cells, and did not show similar dual effect. The differential modulation by Zn2+ of glycine receptors versus GABAA receptors provides a versatility for regulating inhibitory glycinergic and GABAergic inputs converging on most amacrine/ganglion cells.

CNS innervation of vagal preganglionic neurons controlling peripheral airways: a transneuronal labeling study using pseudorabies virus

The CNS cell groups that project to vagal preganglionic neurons which innervate the most distal part of the airways were identified by the viral retrograde transneuronal labeling method. Pseudorabies virus (PRV) was injected into the lung parenchyma of C8 spinal rats and after 5 days survival, brain tissue sections from these animals were processed for immunohistochemical detection of PRV. Retrogradely labeled parasympathetic preganglionic cells (first-order neurons) were seen mainly in the ventral medulla oblongata: the compact portion of the nucleus ambiguus and the area ventral to it. Occasionally, a few labeled cells were seen within the rostral part of the dorsal vagal nucleus. This labeling pattern correlated well with the retrograde cell body labeling seen following cholera toxin β-subunit (CT-b) injections in the lung parenchyma. PRV transneuronally labeled neurons (second-order and/or presumed third-order neurons) were found throughout the CNS with the characteristic labeling in the brainstem. Labeled neurons were identified along and just beneath the ventral medullary surface, and in nearby areas: the parapyramidal, retrotrapezoid, gigantocellular and lateral paragigantocellular reticular nuclei, as well as the caudal raphe nuclei (raphe pallidus, obscurus, and magnus). Several nucleus tractus solitarius (nTS) regions contained labeled cells including the commissural, medial, and ventrolateral nTS subnuclei. The A5 cell group and a small number of locus coeruleus neurons were also labeled. PRV-infected neurons were present in the Kölliker–Fuse and Barrington's nuclei. In the mesencephalon, neurons within the ventral periventricular gray matter were labeled. Labeling was present in the dorsal, lateral and paraventricular hypothalamic nuclei, and within the amygdaloid complex. In summary, the parasympathetic preganglionic neurons that innervate the peripheral airways are controlled by networks of lower brainstem and suprapontine neurons that lie in the same regions known to be involved in central regulation of autonomic functions.

Strong synergism between GABA(A) and glycine receptors on isolated carp third-order neurons.

A strong synergistic interaction between the bicuculline-sensitive GABA receptor (GABA(A) receptor) and the strychnine-sensitive glycine receptor was observed in third-order neurons acutely isolated from crucian carp retina, with the use of the whole-cell patch-clamp recording technique. In 58 of 153 cells, 10 microM GABA or glycine separately applied to amacrine/ganglion cells failed to induce any responses or only induced small currents (<20 pA), while co-application of these two chemicals resulted in much larger responses (403.05+/-319.98 pA). The current induced by the co-application was mediated by chloride channels, and both GABA(A) and glycine receptors were involved in the potentiation. The underlying mechanisms of this interaction and its possible physiological role are discussed.

Solitarial premotor neuron projections to the rat esophagus and pharynx: Implications for control of swallowing

Background & Aims: The buccopharyngeal and esophageal phases of swallowing are controlled by distinct networks of premotor neurons localized in the nucleus tractus solitarius. The neuronal circuitry coordinating the two phases was investigated using a combination of central and peripheral tracing techniques. Methods: Using pseudorabies virus, a transsynaptic tracer, in anesthetized rats, third-order esophageal neurons (neurons projecting to premotor neurons) were identified. In a separate protocol that combined transsynaptic and retrograde fluorescent tracing, third-order esophageal neurons projecting to pharyngeal motoneurons (buccopharyngeal premotor neurons) were then identified. Results: Third-order esophageal neurons were identified in the interstitial and intermediate subnuclei of the nucleus tractus solitarius and in other medullary, pontine, midbrain, and forebrain nuclei. A subpopulation of these neurons (double labeled) in the interstitial and intermediate subnuclei were found to project to pharyngeal motoneurons (buccopharyngeal premotor neurons) and to be linked synaptically to esophageal premotor neurons. Conclusions: The synaptic link between buccopharyngeal and esophageal premotor neurons provides an anatomic pathway for the central initiation of esophageal peristalsis and its coordination with the pharyngeal phase of swallowing. This neural circuitry within the nucleus tractus solitarius is consistent with a complex central control mechanism for the swallowing motor sequence that can function independently of afferent feedback. GASTROENTEROLOGY 1998;114:1268-1275

Properties of saccular nerve-activated vestibulospinal neurons in cats.

Axonal pathways, projection levels, conduction velocities, and locations of the cell bodies of saccular nerve-activated vestibulospinal neurons were studied in decerebrated cats and anesthetized cats, using a collision test of orthodromic and antidromic spikes. The saccular nerve was selectively stimulated by bipolar tungsten electrodes. Three monopolar electrodes were inserted into the left and right lateral vestibulospinal tract (LVST) and medial vestibulospinal tract (MVST) of the C1 segment, to determine the pathway of axons. Three pairs of similar electrodes were positioned bilaterally in the C3-4, T1, and L3 segments to examine projection levels. Another monopolar electrode was placed in the oculomotor nucleus to determine whether saccular nerve-activated vestibulospinal neurons have branches ascending to the oculomotor nucleus. Of 145 vestibular neurons orthodromically activated by stimulation of the saccular nerve, 46 were activated from the C1 segment antidromically. Forty-three were second-order vestibulospinal neurons and 3 were third-order vestibulospinal neurons. Four saccular nerve-activated vestibulospinal neurons were also antidromically activated from the oculomotor nucleus. Sixty-three percent of the saccular nerve-activated vestibulospinal neurons descended through the MVST; one-third of these terminated in the upper cervical segments, one-third reached the lower cervical segments and the remaining one-third reached the upper thoracic segments. Thirty percent of the saccular nerve-activated vestibulospinal neurons descended through the ipsilateral LVST; most of these reached the upper thoracic segments. Seven percent of the saccular nerve-activated vestibulospinal neurons descended through the contralateral vestibulospinal tracts terminating in the upper cervical segments. Most of the saccular nerve-activated vestibulospinal neurons originated in the caudal part of the lateral nucleus and rostral part of the descending nucleus.

A network of tufted layer 5 pyramidal neurons.

Tufted layer 5 (TL5) pyramidal neurons are important projection neurons from the cerebral cortex to subcortical areas. Recent and ongoing experiments aimed at understanding the computational analysis performed by a network of synaptically connected TL5 neurons are reviewed here. The experiments employed dual and triple whole-cell patch clamp recordings from visually identified and preselected neurons in brain slices of somatosensory cortex of young (14- to 16-day-old) rats. These studies suggest that a local network of TL5 neurons within a cortical module of diameter 300 microns consists of a few hundred neurons that are extensively inter-connected with reciprocal feedback from at least first-, second- and third-order target neurons. A statistical analysis of synaptic innervation suggests that this recurrent network is not randomly arranged and hence each neuron could be functionally unique. Synaptic transmission between these neurons is characterized by use-dependent synaptic depression which confers novel properties to this recurrent network of neurons. First, a range of rates of depression for different synaptic connections enable each TL5 neuron to receive a unique mixture of information about the average firing rates and the temporally correlated action potential (AP) activity in the population of presynaptic TL5 neurons. Second, each AP generated by any neuron in the network induces a change (defined as an iteration step) in the functional coupling of the neurons in the network (defined as network configuration). It is proposed that the network configuration is iterated during a stimulus to achieve an optimally orchestrated network response. Hebbian, anti-Hebbian and neuromodulatory-induced modifications of neurotransmitter release probability change the rates of synaptic depression and thereby alter the iteration step size. These data may be important to understand the dynamics of electrical activity within the network.

Serial inhibitory synapses in retina.

Whole-cell voltage clamp in the retinal slice and intracellular current clamp in the intact retina were used to study inhibitory interactions in the inner plexiform layer. Picrotoxin or strychnine reduced inhibitory, light-evoked currents in a majority of ganglion cells. However, in nearly a third of the ganglion cells, each of these antagonists enhanced the inhibitory synaptic current. All inhibitory current was blocked by the addition of the other antagonist. This indicates a cross-inhibition between GABAergic and glycinergic feedforward pathways. Blocking of GABAARs with SR95531 shortened the time course of both excitatory and inhibitory synaptic currents in ganglion cells. Application of picrotoxin, which blocked both GABAARs and GABACRs, produced the opposite effect. Recordings in the intact retina indicated that the light responses of ON bipolar cells, sustained ON, and transient ON-OFF third-order neurons were all made more transient by SR95531 and made more sustained by picrotoxin. The data suggest that a GABAC feedback pathway to bipolar cells makes light responses more phasic and that this feedback is inhibited through a GABAAR pathway. Consequently, the balance between GABAAR and GABACR inhibition regulates the time course of inputs to ganglion cells.

The transneuronal spread phenotype of herpes simplex virus type 1 infection of the mouse hind footpad.

The mouse hind footpad inoculation model has served as a standard laboratory system for the study of the neuropathogenesis of herpes simplex virus type 1 (HSV-1) infection. The temporal and spatial distribution of viral antigen, known as the transneuronal spread phenotype, has not previously been described; nor is it understood why mice develop paralysis in an infection that involves sensory nerves. The HSV-as-transneuronal-tracer experimental paradigm was used to define the transneuronal spread of HSV-1 in this model. A new decalcification technique and standard immunocytochemical staining of HSV-1 antigens enabled a detailed analysis of the time-space distribution of HSV-1 in the intact spinal column. Mice were examined on days 3, 4, 5, and 6 postinoculation (p.i.) of a lethal dose of wild-type HSV-1 strain 17 syn+. Viral antigen was traced retrograde into first-order neurons in dorsal root ganglia on day 3 p.i., to the dorsal spinal roots on days 4 and 5 p.i., and to second- and third-order neurons within sensory regions of the spinal cord on days 5 and 6 p.i. HSV-1 antigen distribution was localized to the somatotopic representation of the footpad dermatome within the dorsal root ganglia and spinal cord. Antigen was found in the spinal cord gray and white matter sensory neuronal circuits of nociception (the spinothalamic tract) and proprioception (the dorsal spinocerebellar tract and gracile fasciculus). Within the brain stems and brains of three paralyzed animals examined late in infection (days 5 and 6 p.i.), HSV antigen was restricted to the nucleus subcoeruleus region bilaterally. Since motor neurons were not directly involved, we postulate that hindlimb paralysis may have resulted from intense involvement of the posterior column (gracile fasciculus) in the thoracolumbar spinal cord, a region known to contain the corticospinal tract in rodents.