Comparison of cholinergic and histaminergic axons in the lateral geniculate complex of the macaque monkey

Mapping the primate thalamus: systematic approach to analyze the distribution of subcortical neuromodulatory afferents

Neuromodulatory afferents to thalamic nuclei are key for information transmission and thus play critical roles in sensory, motor, and limbic processes. Over the course of the last decades, diverse attempts have been made to map and describe subcortical neuromodulatory afferents to the primate thalamus, including axons using acetylcholine, serotonin, dopamine, noradrenaline, adrenaline, and histamine. Our group has been actively involved in this endeavor. The published descriptions on neuromodulatory afferents to the primate thalamus have been made in different laboratories and are not fully comparable due to methodological divergences (for example, fixation procedures, planes of cutting, techniques used to detect the afferents, different criteria for identification of thalamic nuclei…). Such variation affects the results obtained. Therefore, systematic methodological and analytical approaches are much needed. The present article proposes reproducible methodological and terminological frameworks for primate thalamic mapping. We suggest the use of standard stereotaxic planes to produce and present maps of the primate thalamus, as well as the use of the Anglo-American school terminology (vs. the German school terminology) for identification of thalamic nuclei. Finally, a public repository of the data collected under agreed-on frameworks would be a useful tool for looking up and comparing data on the structure and connections of primate thalamic nuclei. Important and agreed-on efforts are required to create, manage, and fund a unified and homogeneous resource of data on the primate thalamus. Likewise, a firm commitment of the institutions to preserve experimental brain material is much needed because neuroscience work with non-human primates is becoming increasingly rare, making earlier material still more valuable.

Water Drinking Behavior Associated with Aversive Arousal in Rats: An Integrative Approach

Cholinergic muscarinic stimulation of vast areas of the limbic brain induced a well-documented polydipsia in laboratory rats. This excessive water-drinking behavior has not received any convincing biological and physiological interpretation for the last 50 years. This review offers such an interpretation and suggests that cholinergically induced drinking response, mostly by carbachol, is associated with activation of the ascending mesolimbic cholinergic system that serves for initiation of emotional aversive arousal of the organism. The ascending cholinergic system originates from the laterodorsal tegmental nucleus, has a diffuse nature, and affects numerous subcortical limbic structures. It is proposed that the carbachol-induced drinking response is related to the state of anxiety and does not serve the regulation of thirst. Instead, the response is anxiety-induced polydipsia that might occur as a soothing procedure that decreases the aversiveness of the negative emotional state induced by carbachol. It is concluded that carbachol-induced water-drinking behavior is a rewarding process that contributes to alleviating the feeling of anxiety by bringing some relief from the cholinergically induced aversive state, and it is a homologue to anxiety-driven polydipsia in humans.

Neuromodulatory Control of Early Visual Processing in Macaque

Visual processing is dynamically controlled by multiple neuromodulatory molecules that modify the responsiveness of neurons and the strength of the connections between them. In particular, modulatory control of processing in the lateral geniculate nucleus of the thalamus, V1, and V2 will alter the outcome of all subsequent processing of visual information, including the extent to and manner in which individual inputs contribute to perception and decision making and are stored in memory. This review addresses five small-molecule neuromodulators-acetylcholine, dopamine, serotonin, noradrenaline, and histamine-considering the structural basis for their action, and the effects of their release, in the early visual pathway of the macaque monkey. Traditionally, neuromodulators are studied in isolation and in discrete circuits; this review makes a case for considering the joint action of modulatory molecules and differences in modulatory effects across brain areas as a better means of understanding the diverse roles that these molecules serve.

Immunocytochemical and ultrastructural organization of the taste thalamus of the tree shrew (Tupaia belangeri)

Ventroposterior medialis parvocellularis (VPMP) nucleus of the primate thalamus receives direct input from the nucleus of the solitary tract, whereas the homologous thalamic structure in the rodent does not. To reveal whether the synaptic circuitries in these nuclei lend evidence for conservation of design principles in the taste thalamus across species or across sensory thalamus in general, we characterized the ultrastructural and molecular properties of the VPMP in a close relative of primates, the tree shrew (Tupaia belangeri), and compared these to known properties of the taste thalamus in rodent, and the visual thalamus in mammals. Electron microscopy analysis to categorize the synaptic inputs in the VPMP revealed that the largest-size terminals contained many vesicles and formed large synaptic zones with thick postsynaptic density on multiple, medium-caliber dendrite segments. Some formed triads within glomerular arrangements. Smaller-sized terminals contained dark mitochondria; most formed a single asymmetric or symmetric synapse on small-diameter dendrites. Immuno-EM experiments revealed that the large-size terminals contained VGLUT2, whereas the small-size terminal populations contained VGLUT1 or ChAT. These findings provide evidence that the morphological and molecular characteristics of synaptic circuitry in the tree shrew VPMP are similar to that in nonchemical sensory thalamic nuclei. Furthermore, the results indicate that all primary sensory nuclei of the thalamus in higher mammals share a structural template for processing thalamocortical sensory information. In contrast, substantial morphological and molecular differences in rodent versus tree shrew taste nuclei suggest a fundamental divergence in cellular processing mechanisms of taste input in these two species.

Thalamic neuromodulation and its implications for executive networks

The thalamus is a key structure that controls the routing of information in the brain. Understanding modulation at the thalamic level is critical to understanding the flow of information to brain regions involved in cognitive functions, such as the neocortex, the hippocampus, and the basal ganglia. Modulators contribute the majority of synapses that thalamic cells receive, and the highest fraction of modulator synapses is found in thalamic nuclei interconnected with higher order cortical regions. In addition, disruption of modulators often translates into disabling disorders of executive behavior. However, modulation in thalamic nuclei such as the midline and intralaminar groups, which are interconnected with forebrain executive regions, has received little attention compared to sensory nuclei. Thalamic modulators are heterogeneous in regards to their origin, the neurotransmitter they use, and the effect on thalamic cells. Modulators also share some features, such as having small terminal boutons and activating metabotropic receptors on the cells they contact. I will review anatomical and physiological data on thalamic modulators with these goals: first, determine to what extent the evidence supports similar modulator functions across thalamic nuclei; and second, discuss the current evidence on modulation in the midline and intralaminar nuclei in relation to their role in executive function.

On the impact of attention and motor planning on the lateral geniculate nucleus

Although the lateral geniculate nucleus (LGN) is one of the most thoroughly characterized thalamic nuclei, its functional role remains controversial. Traditionally, the LGN in primates has been viewed as the lowest level of a set of feedforward parallel visual pathways to cortex. These feedforward pathways are pictured as connected hierarchies of areas designed to construct the visual image gradually - adding more complex features as one marches through successive levels of the hierarchy. In terms of synapse number and circuitry, the anatomy suggests that the LGN can be viewed also as the ultimate terminus in a series of feedback pathways that originate at the highest cortical levels. Since the visual system is dynamic, a more accurate picture of image construction might be one in which information flows bidirectionally, through both the feedforward and feedback pathways constantly and simultaneously. Based upon evidence from anatomy, physiology, and imaging, we argue that the LGN is more than a simple gate for retinal information. Here, we review evidence that suggests that one function of the LGN is to enhance relevant visual signals through circuits related to both motor planning and attention. Specifically, we argue that major extraretinal inputs to the LGN may provide: (1) eye movement information to enhance and bind visual signals related to new saccade targets and (2) top-down and bottom-up information about target relevance to selectively enhance visual signals through spatial attention.

Histamine and Schizophrenia

With the availability of an increased number of experimental tools, for example potent and brain-penetrating H1-, H2-, and H3-receptor ligands and mutant mice lacking the histamine synthesis enzyme or the histamine receptors, the functional roles of histaminergic neurons in the brain have been considerably clarified during the recent years, particularly their major role in the control of arousal, cognition, and energy balance. Various approaches tend to establish the implication of histaminergic neurons in schizophrenia. A strong hyperactivity of histamine neurons is induced in rodent brain by administration of methamphetamine or NMDA-receptor antagonists. Histamine neuron activity is modulated by typical and atypical neuroleptics. H3-receptor antagonists/inverse agonists display antipsychotic-like properties in animal models of the disease. Because of the limited predictability value of most animal models and the paucity of drugs affecting histaminergic transmission that were tried so far in human, the evidence remains therefore largely indirect, but supports a role of histamine neurons in schizophrenia.

Histamine receptors in mammalian retinas

Mammalian retinas are innervated by histaminergic axons that originate from perikarya in the posterior hypothalamus. To identify the targets of these retinopetal axons, we localized histamine receptors (HR) in monkey and rat retinas by light and electron microscopy. In monkeys, puncta containing HR3 were found at the tips of ON-bipolar cell dendrites in cone pedicles and rod spherules, closer to the photoreceptors than the other neurotransmitter receptors. This is the first ultrastructural localization of any histamine receptor and the first direct evidence that HR3 is present on postsynaptic membranes in the central nervous system. In rat retinas, most HR1 were localized to dopaminergic amacrine cells. The differences in histamine receptor localization may reflect the differences in the activity patterns of the two species.

Histamine modulates thalamocortical activity by activating a chloride conductance in ferret perigeniculate neurons

In the mammalian central nervous system only gamma-aminobutyric acid (GABA) and glycine have been firmly linked to inhibition of neuronal activity through increases in membrane Cl(-) conductance, and these responses are mediated by ionotropic receptors. Iontophoretic application of histamine can also cause inhibitory responses in vivo, although the mechanisms of this inhibition are unknown and may involve pre- or postsynaptic factors. Here, we report that application of histamine to the GABAergic neurons of the thalamic perigeniculate nucleus (PGN), which is innervated by histaminergic fibers from the tuberomammillary nucleus of the hypothalamus, causes a slow membrane hyperpolarization toward a reversal potential of -73 mV through a relatively small increase in membrane conductance to Cl(-). This histaminergic action appears to be mediated by the H(2) subclass of histaminergic receptors and inhibits the single-spike activity of these PGN GABAergic neurons. Application of histamine to the PGN could halt the generation of spindle waves, indicating that increased activity in the tuberomammillary histaminergic system may play a functional role in dampening thalamic oscillations in the transition from sleep to arousal.

Distribution of high affinity choline transporter immunoreactivity in the primate central nervous system

A mouse monoclonal antibody (clone 62-2E8) raised against a human recombinant high-affinity choline transporter (CHT)-glutathione-S-transferase fusion protein was used to determine the distribution of immunoreactive profiles containing this protein in the monkey central nervous system (CNS). Within the monkey telencephalon, CHT-immunoreactive perikarya were found in the striatum, nucleus accumbens, medial septum, vertical and horizontal limb nuclei of the diagonal band, nucleus basalis complex, and the bed nucleus of the stria terminalis. Dense fiber staining was observed within the islands of Calleja, olfactory tubercle, hippocampal complex, amygdala; moderate to light fiber staining was seen in iso- and limbic cortices. CHT-containing fibers were also present in sensory and limbic thalamic nuclei, preoptic and hypothalamic areas, and the floccular lobe of the cerebellum. In the brainstem, CHT-immunoreactive profiles were observed in the pedunculopontine and dorsolateral tegmental nuclei, the Edinger-Westphal, oculomotor, trochlear, trigeminal, abducens, facial, ambiguus, dorsal vagal motor, and hypoglossal nuclei. In the spinal cord, CHT-immunoreactive ventral horn motoneurons were seen in close apposition to intensely immunoreactive C-terminals at the level of the cervical spinal cord. CHT immunostaining revealed a similar distribution of labeled profiles in the aged human brain and spinal cord. Dual fluorescent confocal microscopy revealed that the majority of CHT immunoreactive neurons contained the specific cholinergic marker, choline acetyltransferase, at all levels of the monkey CNS. The present observations indicate that the present CHT antibody labels cholinergic structures within the primate CNS and provides an additional marker for the investigation of cholinergic neuronal function in aging and disease.

Neurotransmitters contained in the subcortical extraretinal inputs to the monkey lateral geniculate nucleus

The lateral geniculate nucleus (LGN) is the thalamic relay of retinal information to cortex. An extensive complement of nonretinal inputs to the LGN combine to modulate the responsiveness of relay cells to their retinal inputs, and thus control the transfer of visual information to cortex. These inputs have been studied in the most detail in the cat. The goal of the present study was to determine whether the neurotransmitters used by nonretinal afferents to the monkey LGN are similar to those identified in the cat. By combining the retrograde transport of tracers injected into the monkey LGN with immunocytochemical labeling for choline acetyl transferase, brain nitric oxide synthase, glutamic acid decarboxylase, tyrosine hydroxylase, or the histochemical nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase reaction, we determined that the organization of neurotransmitter inputs to the monkey LGN is strikingly similar to the patterns occurring in the cat. In particular, we found that the monkey LGN receives a significant cholinergic/nitrergic projection from the pedunculopontine tegmentum, gamma-aminobutyric acid (GABA)ergic projections from the thalamic reticular nucleus and pretectum, and a cholinergic projection from the parabigeminal nucleus. The major difference between the innervation of the LGN in the cat and the monkey is the absence of a noradrenergic projection to the monkey LGN. The segregation of the noradrenergic cells and cholinergic cells in the monkey brainstem also differs from the intermingled arrangement found in the cat brainstem. Our findings suggest that studies of basic mechanisms underlying the control of visual information flow through the LGN of the cat may relate directly to similar issues in primates, and ultimately, humans.