Enzymatic evidence for a meso-limbic dopaminergic innervation in the olfactory tubercle of the rabbit

Embryonic and postnatal development of mouse olfactory tubercle

The olfactory tubercle (OT) is located in the ventral-medial region of the brain where it receives primary input from olfactory bulb (OB) projection neurons and processes olfactory behaviors related to motivation, hedonics of smell and sexual encounters. The OT is part of the dopamine reward system that shares characteristics with the striatum. Together with the nucleus accumbens, the OT has been referred to as the "ventral striatum". However, despite its functional importance little is known about the embryonic development of the OT and the phenotypic properties of the OT cells. Here, using thymidine analogs, we establish that mouse OT neurogenesis occurs predominantly between E11-E15 in a lateral-to-medial gradient. Then, using a piggyBac multicolor technique we characterized the migratory route of OT neuroblasts from their embryonic point of origin. Following neurogenesis in the ventral lateral ganglionic eminence (vLGE), neuroblasts destined for the OT followed a dorsal-ventral pathway we named "ventral migratory course" (VMC). Upon reaching the nascent OT, neurons established a prototypical laminar distribution that was determined, in part, by the progenitor cell of origin. A phenotypic analysis of OT neuroblasts using a single-color piggyBac technique, showed that OT shared the molecular specification of striatal neurons. In addition to primary afferent input from the OB, the OT also receives a robust dopaminergic input from ventral tegmentum (Ikemoto, 2007). We used tyrosine hydroxylase (TH) expression as a proxy for dopaminergic innervation and showed that TH onset occurs at E13 and progressively increased until postnatal stages following an 'inside-out' pattern. Postnatally, we established the myelination in the OT occurring between P7 and P14, as shown with CNPase staining, and we characterized the cellular phenotypes populating the OT by immunohistochemistry. Collectively, this work provides the first detailed analysis of the developmental and maturation processes occurring in mouse OT, and demonstrates the striatal nature of the OT as part of the ventral striatum (vST).

Localization of choline acetyltransferase in laminae of the rat olfactory tubercle

We report the distribution of choline acetyltransferase (ChAT) activity in the laminae of the rat olfactory tubercle. Within its posterior medical portion, the tubercle contains three parallel histological laminae that can be separated by cutting tangential sections from frozen tissue. ChAT was measured in homogenates of consecutive sections (16 micrometers) cut parallel to these laminae. The distribution of ChAT activity, as a function of tubercle depth, showed a broad peak centered at 500 micrometers from the ventral surface of the brain. Enzyme activity measured at this depth (85 pmol acetylcholine formed/microgram protein/h) was 2 1/2 times greater than that measured in the outermost, plexiform, layer. Stereotaxic injections of kainic acid (1 microgram in 1 microliter) made directly into the tubercle were used to eliminate intrinsic neurons. Three days after injection, histological examination revealed the almost total absence of neuronal cell bodies and the proliferation of glial cells. The greatest decreases in ChAT activity (50%) were seen at depths of 300-600 micrometers whereas no loss of activity occurred in the plexiform layer.

The distribution and origin of glutamate decarboxylase and choline acetyltransferase in ventral pallidum and other basal forebrain regions

The distribution of choline acetyltransferase (ChAT) and glutamate decarboxylase (GAD), and the histochemical reaction for acetylcholinesterase have been studied in the basal forebrain and globus pallidus of unoperated rats and in rats with an electrolytic lesion of the nucleus accumbens. ChAT was highly concentrated in the substriatal region, the neostriatum and the lateral part of the rostral substantia innominata. The strongest intensity of staining for acetylcholinesterase was found in the substriatal grey and the neostriatum. Very high GAD activity was found in the substantia innominata, being even slightly higher than that in the pars reticulata of the substantia nigra. The lateral preoptic area, the bed nucleus of the stria terminalis and the globus pallidus also showed high activity of GAD. After lesions of the nucleus accumbens the activity of GAD decreased significantly in the substantia innominata and in a restricted part of the rostroventral globus pallidus, but not in the other regions studied. ChAT activity and acetylcholinesterase staining were unaffected in all regions. The results indicate that a dense GABAergic projection originates in the nucleus accumbens and terminates in the rostral substantia innominata and rostroventral part of the globus pallidus. The study gives neurochemical support to the suggestion that nucleus accumbens may be regarded as a ventral part of the neostriatum and that the rostral substantia innominata may be regarded as a ventral part of the globus pallidus.

Olfactory pathway evoked potentials in response to hypothalamic stimulation

Ipsilateral and contralateral stimulation of lateral, ventromedial and posterior hypothalamic nuclei produced evoked responses in the olfactory bulb and in the prepyriform cortex. No differences in the latencies were found by stimulation of each nucleus in the homo and contralateral olfactory structures. The high amplitude of the fast component (N1) was obtained with stimuli applied to the ventral zones and the slow components (N2, N3) were obtained with more dorsal stimulation. An ipsilateral pathway is indicated at the supramammillary and posterior commissure level, since severing these structures abolishes the evoked responses. A bilateral projection is proposed for the olfactory bulb.

Biochemical changes accompanying unilateral 6-hydroxydopamine lesions in the rat substantia nigra

The biochemical consequences of a unilateral 6-hydroxydopamine injection into the substantia nigra of the rat brain were investigated. Projections of dopaminergic neurons from the A8-A9-A10 regions to a number of forebrain areas were confirmed. No innervation to the hypothalamus, including the median eminence, or to the brain stem, could be found with the present techniques. No destruction of serotonergic or GABAergic fibers could be demonstrated in the lesioned substantia nigra. Increases in glutamic acid decarboxylase activity were found restricted to the caudate and zona compacta of the substantia nigra ipsilateral to the lesion, indicating the possibility of a physiological interaction between GABAergic and dopaminergic systems. The neuroanatomical localization of the nigral dopamine-sensitive adenylate cyclase was also studied. No change in enzyme activity was found after destruction of a great proportion of the dopaminergic cells, suggesting that this enzyme has an extradopaminergic localization in the substantia nigra.