Daniel J. Uhlrich

Thalamocortical projections to rat auditory cortex from the ventral and dorsal divisions of the medial geniculate nucleus

The ventral and dorsal medial geniculate (MGV and MGD) constitute the major auditory thalamic subdivisions providing thalamocortical inputs to layer IV and lower layer III of auditory cortex. No quantitative evaluation of this projection is available. Using biotinylated dextran amine (BDA)/biocytin injections, we describe the cortical projection patterns of MGV and MGD cells. In primary auditory cortex the bulk of MGV axon terminals are in layer IV/lower layer III with minor projections to supragranular layers and intermediate levels in infragranular layers. MGD axons project to cortical regions designated posterodorsal (PD) and ventral (VA) showing laminar terminal distributions that are quantitatively similar to the MGV‐to‐primary cortex terminal distribution. At the electron microscopic level MGV and MGD terminals are non‐γ‐aminobutyric acid (GABA)ergic with MGD terminals in PD and VA slightly but significantly larger than MGV terminals in primary cortex. MGV/MGD terminals synapse primarily onto non‐GABAergic spines/dendrites. A small number synapse on GABAergic structures, contacting large dendrites or cell bodies primarily in the major thalamocortical recipient layers. For MGV projections to primary cortex or MGD projections to PD or VA, the non‐GABAergic postsynaptic structures at each site were the same size regardless of whether they were in supragranular, granular, or infragranular layers. However, the population of MGD terminal‐recipient structures in VA were significantly larger than the MGD terminal‐recipient structures in PD or the MGV terminal‐recipient structures in primary cortex. Thus, if terminal and postsynaptic structure size indicate strength of excitation then MGD to VA inputs are strongest, MGD to PD intermediate, and MGV to primary cortex the weakest. J. Comp. Neurol., 2012.

Histamine-immunoreactive neurons and their innervation of visual regions in the cortex, tectum, and thalamus in the primate Macaca mulatta

The histaminergic system is involved in the control of arousal in the brain and may impact significantly on visual processing. However, little is known about the histaminergic innervation of visual areas, or the histamine system in the primate brain, in general. We examined in Macaca mulatta the location of histamine‐immunoreactive neurons and the innervation of important cortical and subcortical visual areas by histamine‐immunoreactive axons. Brain sections were treated with an antibody to histamine and processed with standard immunohistological procedures.

Nicotinic receptor-mediated responses in relay cells and interneurons in the rat lateral geniculate nucleus

We used the in vitro whole-cell recording technique to study the nicotinic responses of relay cells and interneurons in the adult rat dorsal lateral geniculate nucleus, the thalamic nucleus that conveys visual signals from the retina to the cortex. These geniculate relay cells and interneurons were identified by their physiological and morphological properties. We found that, in the presence of a muscarinic antagonist, atropine, acetylcholine induced a depolarization in relay cells. A similar depolarization was induced by application of nicotine. These depolarizations were completely blocked by a nicotinic antagonist, hexamethonium, but were little affected by bath solution that contained tetrodotoxin and/or low calcium concentration to block synaptic transmission. This suggests that the depolarization is mediated directly by nicotinic receptors in relay cells. Application of nicotine also induced a depolarization in geniculate interneurons. The interneurons continued to exhibit a response to nicotine in the presence of synaptic blockade, although the time-course of the response was altered. The nicotinic responses in relay cells and interneurons shared many similar properties. Both exhibited desensitization, although this characteristic was much more pronounced in the interneurons. In both cell types, the nicotinic response activated a relatively linear conductance with a slight inward rectification. The reversal potential for the conductance was about - 33 mV, which is consistent with a permeability to sodium and potassium ions. The reversal potential shifted negatively by 5-6 mV when the bath solution contained low calcium, which further suggests a permeability to calcium ions. Our results indicate that nicotinic receptors are present in both geniculate relay cells and interneurons. The nicotinic depolarization in relay cells may serve to enhance transmission of visual signals through the lateral geniculate nucleus as well as to contribute to a voltage-dependent shift in the response mode of geniculate relay cells from burst to tonic (single-spike) firing. The nicotinic depolarization in interneurons may provide an explanation for reports that activation of the cholinergic system can enhance inhibitory tuning in the lateral geniculate nucleus.

MUSCARINIC RECEPTOR SUBTYPES IN THE LATERAL GENICULATE NUCLEUS : A LIGHT AND ELECTRON MICROSCOPIC ANALYSIS

Neural activity in the dorsal lateral geniculate nucleus of the thalamus (DLG) is modulated by an ascending cholinergic projection from the brainstem. The purpose of this study was to identify and localize specific muscarinic receptors for acetylcholine in the DLG. Receptors were identified in rat and cat tissue by means of antibodies to muscarinic receptor subtypes, m1–m4. Brain sections were processed immunohistochemically and examined with light and electron microscopy. Rat DLG stained positively with antibodies to the m1, m2,and m3 receptor subtypes but not with antibodies to the m4 receptor subtype. The m1 and m3 antibodies appeared to label somata and dendrites of thalamocortical cells. The m1 immunostaining was pale, whereas m3‐positive neurons exhibited denser labeling with focal concentrations of staining. Strong immunoreactivity to the m2 antibody was widespread in dendrites and somata of cells resembling geniculate interneurons. Most m2‐positive synaptic contacts were classified as F2‐type terminals, which are the presynaptic dendrites of interneurons. The thalamic reticular nucleus also exhibited robust m2 immunostaining. Cat DLG exhibited immunoreactivity to the m2 and m3 antibodies. The entire DLG stained darkly for the m2 receptor subtype, except for patchy label in the medial interlaminar nucleus and the ventralmost C laminae. The staining for m3 was lighter and was distributed more homogeneously across the DLG. The perigeniculate nucleus also was immunoreactive to the m2 and m3 subtype‐specific antibodies. Immunoreactivity in cat to the m1 or m4 receptor antibodies was undetectable. These data provide anatomical evidence for specific muscarinic‐mediated actions of acetylcholine on DLG thalamocortical cells and thalamic interneurons. J. Comp. Neurol. 404:408–425, 1999.

Cellular mechanisms underlying two muscarinic receptor-mediated depolarizing responses in relay cells of the rat lateral geniculate nucleus

We used the whole-cell recording technique in an in vitro preparation to examine the electrophysiological actions of the muscarinic receptors on relay cells in the rat lateral geniculate nucleus. Drop application of the muscarinic agonist acetyl-beta-methylcholine resulted in a slow depolarization that persisted for several minutes. The response was insensitive to the nicotinic antagonist hexamethonium, but was blocked by atropine, a muscarinic antagonist. The response was also insensitive to blockade of synaptic transmission by tetrodotoxin, indicating a direct muscarinic effect. The muscarinic depolarization consisted of two components that were somewhat separated in time. The early portion of the muscarinic response was mediated by a large inward current with little change in input resistance, while the later portion was mediated by a small inward current associated with a large increase in input resistance. Pharmacological agents were used to distinguish the two components. Drop application of McN-A-343, an ml receptor agonist, could only mimic the later component of the muscarinic response. This was supported by the result that the later component was blocked by low concentrations of pirenzepine. These data suggest that the ml receptor only mediates the late component of the muscarinic response, while the early component is mainly mediated by the m3 receptor. The idea that both ml and m3 receptors were involved in the muscarinic depolarization was further supported by voltage-clamp analysis. This revealed that activation of the ml receptor was associated with a decrease in an inward potassium current, IKleak, while activation of the m3 receptor was likely associated with both a decrease in IKleak and an increase in the hyperpolarization-activated cation current Ih. In summary, our data suggest that muscarinic responses in geniculate relay cells result from the activation of two receptors, which modulate IKleak and Ih. Given the fact that the ascending aminergic systems also depolarize geniculate relay cells via two receptors acting on IKleak and Ih, we concluded that ascending activating systems use common mechanisms to enact the depolarizing form of arousal in relay neurons.

The histaminergic innervation of the lateral geniculate complex in the cat

The histaminergic innervation of the thalamic dorsal and ventral lateral geniculate nuclei and the perigeniculate nucleus of the cat was examined immunohistochemically by means of an antibody to histamine. We find histamine-immunoreactive neurons in the cat brain are concentrated in the ventrolateral portion of the posterior hypothalamus, confirming a previous report. However, this cell group also spreads into medial, dorsal, and extreme lateral regions of the posterior hypothalamus and extends as far rostral as the optic chiasm. Histamine-labeled fibers cover all regions of the lateral geniculate complex, but the density of labeling varies. The ventral lateral geniculate nucleus (vLGN) is most densely labeled, the A laminae of the dorsal lateral geniculate are sparsely labeled, and the geniculate C laminae and the perigeniculate nucleus show intermediate amounts of label. Thus, histaminergic fibers demonstrate a predilection for zones innervated by the W-cell system. Labeled fibers exhibit few branchings and numerous en passant swellings, lending a beaded appearance. The vLGN showed more instances of fibers with larger-sized swellings (up to 2 microns). Following injections of biotinylated tracers into the hypothalamus, we find labeled fibers throughout the lateral geniculate complex. The anterogradely labeled fibers resemble the histaminergic fibers in morphology, distribution, and relative bouton size. Thus, the hypothalamus appears to be the source of the histaminergic fibers in the lateral geniculate complex. Histamine-labeled fibers in the dorsal lateral geniculate nucleus (dLGN) exhibit uncommon ultrastructural morphology. Many extremely large, round, or elliptical vesicles fill the fiber swellings. Swellings are directly apposed to a variety of other dendritic and axonal profiles, but thus far no convincing synaptic contacts have been seen. The distribution and appearance of these histaminergic fibers resembles those reported for serotonergic fibers. Our results support the idea that histamine works nonsynaptically as a neuromodulator in the lateral geniculate complex, affecting the level of visual arousal.

Histaminergic and non-histamine-immunoreactive mast cells within the cat lateral geniculate complex examined with light and electron microscopy

Mast cells and their location in the cat lateral geniculate complex of the thalamus were examined by means of histamine immunohistochemistry and the mast cell stain pinacyanol erythrosinate. Brain sections from seven normal adult pigmented cats were processed for light or electron microscopy. Histamine-containing and pinacyanol erythrosinate-stained mast cells were widespread throughout the dorsal and ventral lateral geniculate nuclei and the surrounding regions. Mast cells were especially numerous rostrally in the complex and in the geniculate C laminae. The cells were found consistently in association with blood vessels, ranging from capillary size to vessels c. 150 microns diameter, and twice as often with arterioles as with venules. Large clusters of many mast cells associated with single blood vessels were seen. Individual mast cells were typically 8 microns in diameter and somewhat oval, although multipolar and crescent-shaped cells were also seen, up to twice as long. The amount of histamine labeling varied across cells. When histamine-labeled material was secondarily stained with pinacyanol erythrosinate, many mast cells were double labeled. In addition, there was a small population of mast cells that stained only with pinacyanol erythrosinate, but was otherwise identical to the histamine-immunoreactive mast cells. Electron microscopic examination showed that the mast cells lie on the brain side of the blood-brain barrier. Mast cells were found in close proximity to the thalamic neuropil, primarily apposed to the processes of astrocytes, but also apposed to neural elements. The distinctive electron-dense cytoplasmic granules in the fully granulated, mature state were largely amorphous in appearance and as large as 700 nm in diameter. Histamine was dispersed throughout some granules and contained within restricted areas of other granules. In degranulated mast cells, large, irregularly shaped, electron-lucent granules were seen fused with the cell membrane on the neuropil side, as well as the lumen side of the mast cell. More mast cells were observed at the electron microscopic level than were expected from the light level observations, which suggests that, despite the numbers of mast cells labeled, these results may still underestimate the total mast cell population present in this region of the thalamus. Mast cells, by their numbers, their distribution and the potent chemical substances they contain, may significantly influence vascular and neural function, directly and indirectly, in the cat lateral geniculate complex.

Cortical Function: A View from the Thalamus

Neuroscientists from across the country gathered at the University of Wisconsin, Madison in September to honor Ray Guillery and his seminal work on the thalamus. The meeting focused on three timely research topics, each of which inspired new thinking about thalamic function. Presentations on the organization and dynamic nature of thalamocortical pathways, the role of the thalamus in communication between cortical areas, and the relationship between sensory and motor pathways of the brain, including cognitive aspects of thalamocortical processing, made for lively discussions. The meeting revealed that communication between thalamus and cortex is so rich that we should no longer consider the operations of either structure separately from the other. Proceedings of the meeting will be published in Progress in Brain Research in 2005. In this report, we provide a general overview of the main themes of the meeting.

Chapter 9 GABAergic circuits in the lateral geniculate nucleus of the cat

Publisher Summary This chapter describes four sources of gamma-aminobutyric acid–releasing (GABAergic) innervation to the lateral geniculate nucleus (LGN) and the synaptic circuitry into which they enter; yet two observations suggest that this list is incomplete. First, the fact that the pretectum provides GABAergic innervation to the LGN suggests that other intermediate- or long-distance GABAergic projections to the LGN may exist. Second, the sources of many GABAergic terminals in the LGN have not been identified. Many are likely to be from intrinsic interneurons, such as the sources of F2 terminals, and other pre-synaptic dendritic profiles that contact geniculate Y cells. Physiological evidence suggests that GABA plays a major role in modulating the transmission of visual information through the visual thalamus. Anatomical studies reveal that the GABAergic circuitry in the LGN is complicated with a large cast of intrinsic and extrinsic players. The cast may be functionally expanded because some of these players assume more than one role.

Preferential effect of isoflurane on top-down vs. bottom-up pathways in sensory cortex

The mechanism of loss of consciousness (LOC) under anesthesia is unknown. Because consciousness depends on activity in the cortico-thalamic network, anesthetic actions on this network are likely critical for LOC. Competing theories stress the importance of anesthetic actions on bottom-up “core” thalamo-cortical (TC) vs. top-down cortico-cortical (CC) and matrix TC connections. We tested these models using laminar recordings in rat auditory cortex in vivo and murine brain slices. We selectively activated bottom-up vs. top-down afferent pathways using sensory stimuli in vivo and electrical stimulation in brain slices, and compared effects of isoflurane on responses evoked via the two pathways. Auditory stimuli in vivo and core TC afferent stimulation in brain slices evoked short latency current sinks in middle layers, consistent with activation of core TC afferents. By contrast, visual stimuli in vivo and stimulation of CC and matrix TC afferents in brain slices evoked responses mainly in superficial and deep layers, consistent with projection patterns of top-down afferents that carry visual information to auditory cortex. Responses to auditory stimuli in vivo and core TC afferents in brain slices were significantly less affected by isoflurane compared to responses triggered by visual stimuli in vivo and CC/matrix TC afferents in slices. At a just-hypnotic dose in vivo, auditory responses were enhanced by isoflurane, whereas visual responses were dramatically reduced. At a comparable concentration in slices, isoflurane suppressed both core TC and CC/matrix TC responses, but the effect on the latter responses was far greater than on core TC responses, indicating that at least part of the differential effects observed in vivo were due to local actions of isoflurane in auditory cortex. These data support a model in which disruption of top-down connectivity contributes to anesthesia-induced LOC, and have implications for understanding the neural basis of consciousness.