The GABA system in the mammalian habenula

Thirst Is Associated with Suppression of Habenula Output and Active Stress Coping: Is there a Role for a Non-canonical Vasopressin-Glutamate Pathway?

Water-homeostasis is a fundamental physiological process for terrestrial life. In vertebrates, thirst drives water intake, but the neuronal circuits that connect the physiology of water regulation with emotional context are poorly understood. Vasopressin (VP) is a prominent messenger in this circuit, as well as L-glutamate. We have investigated the role of a VP circuit and interaction between thirst and motivational behaviors evoked by life-threatening stimuli in rats. We demonstrate a direct pathway from hypothalamic paraventricular VP-expressing, glutamatergic magnocellular neurons to the medial division of lateral habenula (LHbM), a region containing GABAergic neurons. In vivo recording and juxtacellular labeling revealed that GABAergic neurons in the LHbM had locally branching axons, and received VP-positive axon terminal contacts on their dendrites. Water deprivation significantly reduced freezing and immobility behaviors evoked by innate fear and behavioral despair, respectively, accompanied by decreased Fos expression in the lateral habenula. Our results reveal a novel VP-expressing hypothalamus to the LHbM circuit that is likely to evoke GABA-mediated inhibition in the LHbM, which promotes escape behavior during stress coping.

Prenatal ontogeny of the GABAergic system in the rat brain: an immunocytochemical study

Prenatal development of the GABAergic system in the rat brain has been studied using an antiserum to GABA-glutaraldehyde-hemocyanin conjugates, specific for GABAergic neurons. The gamma-aminobutyric acid (GABA) system has been found to differentiate very early relative to other transmitter-identified neurons, such that by embryonic day 13 a well developed fiber network exists in the brainstem, mesencephalon and diencephalon, including a large projection in the posterior commissure and adjacent areas on the surface of the mesencephalon and tectum. Although no cell bodies are visible at this time, it appears that these fibers originate from the caudal brainstem and spinal cord. GABAergic cell bodies begin to appear on embryonic day 14 in the lateral cortical anlage. By embryonic day 16, they are also visible in the basal forebrain and in all regions of cortex where they are located in three zones: in layer I, below the cortical plate, and in the intermediate zone. Also contained in the outer part of layer I is a dense fiber plexus which stains intensely for GABA. These fibers may be part of the first contingent of cortical afferents to invade the telencephalic vesicle, an event which is thought to be a stimulus for the beginning of neuronal differentiation in this region. By E18, two bands of immunoreactivity are visible in layer I, which probably contain both cell bodies and fibers. The trajectories taken by growing GABAergic fibers in the brainstem, mesencephalon and diencephalon at embryonic day 13 and at subsequent stages of development are coincident with regions of both monoaminergic and peptidergic differentiation and appear to correspond to recently reported patterns of benzodiazepine receptors which appear slightly later. The early differentiation of the GABAergic system could indicate a trophic role for GABA in early brain development, possibly involving receptors for this neurotransmitter or related substances.

Prenatal and postnatal hypothyroidism abolishes lesion-induced noradrenergic sprouting in the adult rat

The effects of altered thyroid states on regeneration in the central nervous system are equivocal. This work was undertaken to examine the influence of propylthiouracil-induced (PTU-induced) pre- and postnatal hypothyroidism on collateral sprouting of noradrenergic (NA) axons in the habenula (Hb), following lesions in the stria medullaris (SM) of the adult rat. Ten pregnant dams were divided into control and PTU-treated groups. Control rats had free access to food and water and the hypothyroid group received, in addition, 0.05% (w/v) of PTU in their drinking water, beginning on day 12 of gestation and continuing post partum through lactation until the offspring were weaned at age 22 days. The pregnant dams received, in addition, daily injections of thyroxine (25 micrograms/kg, ip) and only male pups were used in the study. At weaning the pups were removed from the dams and placed in cages with free access to food and water. The hypothyroid offspring received 0.05% (w/v) PTU in the drinking water until sacrificed at 10 weeks of age. At 6 weeks, some rats in each group received bilateral lesions in the SM and the remainder were sham-operated. All rats were sacrificed 4 weeks after operation. Thus, four groups were formed: 1) control/sham, 2) control/lesion, 3) PTU/sham, 4) PTU/lesion. Sprouting of NA fibers in discrete regions of the Hb was identified by norepinephrine (NE) levels and by fluorescence histochemistry. Blood levels of thyroid stimulating hormone (TSH) were determined. The results show that pre- and postnatal induced chronic hypothyroidism abolished NA sprouting in the partially deafferented Hb. Furthermore, lesions of the SM resulted in a marked decrease in serum TSH levels.

Retrograde HRP tracing combined with a pharmacohistochemical method for GABA transaminase for the identification of presumptive GABAergic projections to the habenula

A pharmacohistochemical method for GABA-transaminase (GABA-T), the key enzyme for GABA metabolism, has been combined with retrograde tracing by horseradish peroxidase, to a study of projections to the lateral habenula. The results indicate that projections to the lateral habenula arise from both GABA-T intensive and non-GABA-T staining cells in the rostral entopeduncular nucleus and the lateral hypothalamus suggesting that these projections contain GABAergic elements, as indicated by previous work, but also involve other neurotransmitters. A few GABA-T intensive cells in the lateral preoptic area were also found to project to the lateral habenula by this double staining technique.

Experimental immunohistochemical studies on the substance P neuron system in the lateral habenular nucleus of the rat: distribution and origins

We examined the distribution and afferent projections of substance P-like immunoreactive (SPI) fibers in the lateral habenular nucleus of the rats by using the indirect immunofluorescence method. On the basis of the distribution, a dense plexus of SPI fibers in the lateral habenular nucleus (LHb) could be divided into two parts: medial (mLHb) and lateral (lLHb). The present study demonstrates that SPI fibers in the lLHb originate from SPI cells in the rostral entopeduncular nucleus (rEP) and the adjacent area, while those in the mLHb originate from SPI cells in the medial habenular nucleus. Moreover, we showed that the axons from SPI cells in the rEP reached the lLHB via the "pallidohabenular tract."

An immunohistochemical study on the location of GABAergic neurons in rat septum

Antisera against L-glutamate decarboxylase (GAD), the synthesizing enzyme of gamma-aminobutyric acid (GABA) were used to locate GABAergic neurons and nerve terminals in the septal complex of the rat by using the peroxidase-antiperoxidase method. Varying densities of immunoreactive terminals were observed in saline-treated rats but nerve cell bodies were only demonstrated after interventricular or intraseptal injections of colchicine. Small and medium-sized GAD-positive neurons were found in lateral septal nuclei, the largest number of these cells being in the pars dorsalis, and in the bed nucleus of the stria terminalis. Several GAD-immunoreactive neurons were located in the medial septal nucleus and the nucleus of the diagonal band of Broca (DB), where the cells were larger in the ventral than dorsal parts of the region. In the medial septal nucleus and in DB the GAD-positive cell bodies were distributed similarly to cholinergic neurons. Large GAD-positive neurons were also found in the septofimbrial nucleus. Intense immunoreactivity in nerve terminals was observed in the lateral septal nucleus, around the island of Calleja magna, between the DB and nucleus accumbens, and in the septofimbrial and triangular septal nuclei. In contrast, the medial septal nucleus, the DB, and the bed nucleus of the stria terminalis only showed weak to moderate immunoreactivity. These results provide direct morphological evidence for the presence of neurons capable of synthesizing GABA in septal nuclei. We suggest that there are two different GABAergic neuronal systems operating in the septum: a population of small cells in the lateral septal nucleus and a group of large cells in the medial septum and DB.

Cholinergic and GABAergic forebrain projections to the habenula and nucleus interpeduncularis: surgical and kainic acid lesions

The forebrain cholinergic and GABAergic projections to the habenula and nucleus interpeduncularis have been investigated by means of surgical and kainic acid lesions. Bilateral transection of the stria medullaris caused a 50% decrease of choline acetyltransferase in both the habenula and nucleus interpeduncularis, and a 65% decrease of glutamate decarboxylase in the habenula. Electrolytic lesions of the posterior septum (nucleus triangularis septi and nucleus septo-fimbrialis) accounted for at least 30-40% decrease of the cholinergic parameter in the habenula and nucleus interpeduncularis. Moreover, the choline acetyltransferase decrease in the habenula appeared restricted to the medial part of the nucleus. Kainic acid injections causing very large neuronal destruction in the nucleus of the diagonal band of Broca, and more than 70% decrease of choline acetyltransferase in the dorsal hippocampus, did not affect the cholinergic parameter in either the medial or lateral habenula or nucleus interpeduncularis. Kainic acid injections in the nucleus entopeduncularis resulted in a 40% decrease of glutamate decarboxylase in the habenula. Kainic acid injections in the nucleus of the diagonal band were accompanied by a 40% decrease of glutamate decarboxylase in the medial subdivision only. The present study points at the nuclei of the posterior septum as the source of a major cholinergic projection to the habenula and nucleus interpeduncularis, and reveals a previously unsuspected GABAergic input from the nucleus of the diagonal band to the medial habenula.

An electrophysiological study of convergence of entopeduncular and lateral preoptic inputs on lateral habenular neurons projecting to the midbrain

Extracellular recordings were made from single neurons in the lateral habenular nucleus of urethane-anesthetized rats. Single pulse stimulation of the entopeduncular nucleus influenced the spontaneous activity of 200 out of 293 (68%) lateral habenular neurons tested, with the most frequent response being suppression of activity and initial activation followed by suppression. Single pulse stimulation of the lateral preoptic area influenced the activity of 140 out of 165 (85%) lateral habenular neurons tested. Response were similar to those for entopeduncular stimulation. In a series of 137 lateral habenular units, 85 (62%) were influenced by stimulation of both sites indicating convergence of inputs from these two forebrain areas. Approximately half of the lateral habenular neurons which received converging inputs from entopeduncular nucleus and lateral preoptic area were activated antidromically by stimulation of the fasciculus retroflexus, indicating that they were output neurons. These electrophysiological findings support the suggestion from recent neuroanatomical studies that the lateral habenular nucleus is a site of integration for entopeduncular and limbic inputs and in turn sends signals to the midbrain.