Differential effects of fasciculus retroflexus lesions on serotonin, glutamate and gamma-aminobutyrate content and choline acetyltransferase activity in the interpeduncular nucleus

Untangling the dorsal diencephalic conduction system: a review of structure and function of the stria medullaris, habenula and fasciculus retroflexus

The often-overlooked dorsal diencephalic conduction system (DDCS) is a highly conserved pathway linking the basal forebrain and the monoaminergic brainstem. It consists of three key structures; the stria medullaris, the habenula and the fasciculus retroflexus. The first component of the DDCS, the stria medullaris, is a discrete bilateral tract composed of fibers from the basal forebrain that terminate in the triangular eminence of the stalk of the pineal gland, known as the habenula. The habenula acts as a relay hub where incoming signals from the stria medullaris are processed and subsequently relayed to the midbrain and hindbrain monoaminergic nuclei through the fasciculus retroflexus. As a result of its wide-ranging connections, the DDCS has recently been implicated in a wide range of behaviors related to reward processing, aversion and motivation. As such, an understanding of the structure and connections of the DDCS may help illuminate the pathophysiology of neuropsychiatric disorders such as depression, addiction and pain. This is the first review of all three components of the DDCS, the stria medullaris, the habenula and the fasciculus retroflexus, with particular focus on their anatomy, function and development.

The habenular nuclei: a conserved asymmetric relay station in the vertebrate brain

The dorsal diencephalon, or epithalamus, contains the bilaterally paired habenular nuclei and the pineal complex. The habenulae form part of the dorsal diencephalic conduction (DDC) system, a highly conserved pathway found in all vertebrates. In this review, we shall describe the neuroanatomy of the DDC, consider its physiology and behavioural involvement, and discuss examples of neural asymmetries within both habenular circuitry and the pineal complex. We will discuss studies in zebrafish, which have examined the organization and development of this circuit, uncovered how asymmetry is represented at the level of individual neurons and determined how such left–right differences arise during development.

A conductor hidden in the orchestra? Role of the habenular complex in monoamine transmission and cognition

Influences of the habenular complex on electrophysiological and neurochemical aspects of brain functioning are well known. However, its role in cognition has been sparsely investigated until recently. The habenular complex, composed of medial and lateral subdivisions, is a node linking the forebrain with midbrain and hindbrain structures. The lateral habenula is the principal actor in this direct dialogue, while the medial habenula mostly conveys information to the interpeduncular nucleus before this modulates further regions. Here we describe neuroanatomical and physiological aspects of the habenular complex, and its role in cognitive processes, including new behavioral, electrophysiological and imaging findings. Habenular complex lesions result in deficits in learning, memory and attention, some of which decline during repeated testing, while others become worse, consistent with multiple roles in cognition. The habenular complex is particularly responsive to feedback about errors. Electrophysiological studies indicate a role in metaplasticity, the modulation of neuroplasticity. These studies thus reveal important roles of the habenular complex in learning, memory and attention.

Developmental changes in heterogeneous patterns of neurotransmitter receptor binding in the human interpeduncular nucleus

The interpeduncular nucleus (IPN) exhibits many complex features, including multiple subnuclei, widespread projections with the forebrain and brainstem, and neurotransmitter heterogeneity. Despite the putative importance of this nucleus, very little is known about its neurochemical development in the human. The human IPN is cytoarchitectonically simple, unlike the rat IPN, which displays considerable heterogeneity. In the following study, we hypothesized that the developing human IPN is neurochemically heterogeneous despite its cytological simplicity. The chemoarchitecture in this study was defined by neurotransmitter receptor binding patterns by using quantitative tissue autoradiography for the muscarinic, nicotinic, serotoninergic, opioid, and kainate receptors. We examined neurotransmitter receptor binding in the developing human IPN in a total of 15 cases. The midbrains of five midgestational fetuses (19-26 gestational weeks) and six infants (38-74 postconceptional weeks) were examined. The midbrain of one child (4 years) and three adults (20-68 years) were analyzed as indices of maturity. At all ages examined, high muscarinic binding was localized to the lateral subdivision of the IPN, high serotoninergic binding was localized to the dorsal IPN, and high opioid receptor binding was localized to the medial IPN. The developmental profile was unique for each radioligand. We report a heterogenous distribution of neurotransmitter receptor binding in the developing human IPN, which supports a complex role for it in human brain function.

Length of continuous cocaine exposure determines the persistence of muscarinic and benzodiazepine receptor alterations

The effects of varied durations of cocaine (1, 3 or 5 days) on muscarinic (MSC) and benzodiazepine (BZD) binding sites in striatum and hippocampus were investigated using homogenate receptor binding. The progressive alterations in these receptor sites from a 5 day cocaine administration were also examined 12 h, 2 days or 21 days after drug exposure. Neither a one nor a three day exposure to cocaine produced any long-term alteration in BZD binding in either structure whereas a 5 day administration produced significant increases in binding. Decreases in MSC receptor binding were apparent in striatum from either a 3 or 5 day cocaine exposure and in hippocampus from a 5 day exposure. The 5 day cocaine exposure produced immediate increases in striatal and hippocampal BZD binding which persisted for 21 days. Conversely, 5 days of cocaine produced a short-term increase in MSC receptor binding in both structures which then became significantly decreased 21 days later. Based on the divergent pattern of changes in BZD and MSC receptor types over time in these structures, it appears that cocaine may induce such changes via separate mechanisms. In addition, it is apparent that changes in the numbers of these receptor sites after cocaine exposure can be quite dynamic, changing rapidly over time.