Autoradiographic quantitation and anatomical mapping of GTP sensitive-galanin receptors in the guinea pig central nervous system

Galanin immunohistochemistry and electron microscopic studies in developing human fetal mammillary bodies

Development and maturation of nuclear groups in the mammillary complex of second and third trimester human fetal hypothalamus were studied using Nissl stain, galanin immunocytochemistry and transmission electron microscopy. While the identity of the supra and medial mammillary nucleus was established at 24 weeks of gestation (w.g.) in Nissl stained preparation, galanin immunoreactive (Gal-ir) neurons were seen in the supra and medial mammillary nucleus of 27 through 39 w.g. fetuses. Immunoreactive perikarya in the lateral mammillary nucleus appear later at 34 w.g. and show relatively meager population. Gal-ir neurons of the supramammillary nucleus were divisible in dorsal and ventral subgroups. There was a progressive increase in galanin expressing neurons in more and more ventral positions, along the medial margin of either mammillary body so that in term fetal specimens, the ventral subgroup appeared to be continuous with the medial mammillary nucleus. Galanin positive neurons were relatively sparse in the core of the mammillary bodies. Transmission electron micrographs revealed neurons with varying degree of indentation of the nuclear envelope. Vigorous synaptogenesis was seen in the supramammillary region of the mammillary bodies. The height and width of the synaptic complex also showed a progressive increase. Although galanin neurons were reported from the supramammillary nucleus of adult human mammillary complex, no immunoreactivity was detected in the medial and lateral components of the mammillary body. We suggest that expression of galanin in the medial and lateral components may be of transient occurrence and may serve a significant role in the synaptogenesis.

Degeneration of the basalocortical pathway from the cortex induces a functional increase in galaninergic markers in the nucleus basalis magnocellularis of the rat

The present work aimed 1) to evaluate whether an increase in galanin or galanin receptors could be induced in the nucleus basalis magnocellularis (nbm) by degeneration of the basalocortical neurons from the cortex and 2) to analyze the consequences of such an increase on cortical activity. First, a mild ischemic insult to the frontoparietal cortex was performed to induce the degeneration of the basalocortical system; galanin immunoreactivity, galanin binding sites, and cholinergic muscarinic receptors were quantified through immunocytochemistry and autoradiography. Second, galanin infusions in the nbm were undertaken to mimic a local increase of the galaninergic innervation; cortical acetylcholine release, cerebral glucose use, and cerebral blood flow were then measured as indices of cortical activity. As a result of the cortical ischemic lesion, the postsynaptic M1 and presynaptic M2 muscarinic receptors were found to be reduced in the altered cortex. In contrast, galaninergic binding capacity and fiber density were found to be increased in the ipsilateral nbm in parallel with a local decrease in the cholinergic markers such as the muscarinic M1 receptor density. Galanin infusion into the nbm inhibited the cortical acetylcholine release and cerebral blood flow increases elicited by the activation of the cholinergic basalocortical system but failed to affect acetylcholine release, cerebral blood flow, and cerebral glucose use when injected alone in the nbm. These results demonstrate that degeneration of the basalocortical system from the cortex induces an increase in galaninergic markers in the nbm, a result that might suggest that the galaninergic overexpression described in the basal forebrain of patients with Alzheimer's disease can result from a degeneration of the cholinergic basalocortical system from the cortex. Because galanin was found to reduce the activity of the basalocortical cholinergic system only when this one is activated, galanin might exert its role rather during activation deficits than under resting conditions such as the resting cortical hypometabolism, which is characteristic of Alzheimer's disease.

The effects of galanin on long-term synaptic plasticity in the CA1 area of rodent hippocampus

Learning and memory involve complex changes in neuronal excitability including long-lasting synaptic plasticity of glutamatergic synapses. The observation that the neuropeptide galanin affects performance in a number of behavioural models predicts that galanin should affect synaptic processes underlying learning. The present study in rat and mouse hippocampal slices now demonstrates that galanin inhibits long-term potentiation induced by both tetanic and theta-burst stimulation in both apical and basal dendrites of CA1 pyramidal neurones but does not affect long-term depression. This selective effect on long-term potentiation does not appear to be mediated through inhibition of N-methyl-D-aspartate or metabotropic glutamate receptor function, but likely resides downstream of receptor activation, possibly at the level of the kinase cascade that converts short-term into long-term potentiation. Our results indicate possible mechanisms by which the neuropeptide galanin may act at the molecular level to influence learning and memory in vertebrates.

Evidence for expression of galanin receptor Gal-R1 mRNA in certain gonadotropin releasing hormone neurones of the rostral preoptic area

Previous studies have shown that galanin plays an important role in the regulation of gonadotropin releasing hormone (GnRH) release. At present, it is not known if this role is exerted by direct or indirect interactions between galanin producing neurones and GnRH neurones. The objective of this study was to determine whether or not GnRH neurones could express galanin receptor Gal-R1 mRNA. Dual in-situ hybridization experiments were carried out with digoxigenin-labelled cRNA probes encoding GnRH in combination with 35S-labelled riboprobe encoding the galanin receptor Gal-R1. In order to detect possible variations in the expression of the Gal-R1 mRNA under different physiological conditions, male rats, intact female rats throughout the phases of the oestrous cycle, ovariectomized (OVX) and steroid-treated rats were analysed. The results show that many cells expressing Gal-R1 mRNA were present throughout the preoptic area. Gal-R1 mRNA-expressing cells were observed in very close proximity with GnRH neurones. In the female rat, some GnRH neurones located in the rostral preoptic area/vascular organ of the lamina terminalis expressed Gal-R1 mRNA. These double-labelled cells were observed at all times of the oestrous cycle, except during diestrus at 08.00 h and pro-oestrus at 18.00 h. Conspicuously, at oestrus 1800 h, we found that 21.6% of rostral GnRH neurones expressed the Gal-R1 mRNA. In addition, dual-labelled GnRH neurones were seen in OVX animals but not in oestrogen plus progesterone-treated ones. In the male rat, colocalization of GnRH mRNA and Gal-R1 receptor mRNA was not observed. In the medial preoptic area, no double-labelled GnRH neurones were detected regardless of the endocrine conditions. These results suggest that, in addition to a possible indirect action of galanin on GnRH cells via neurones located at close proximity, the effects of galanin on GnRH can be mediated by direct activation of galanin receptors in rostral GnRH neurones. This study also shows that expression of Gal-R1 mRNA in GnRH cells is influenced by the levels of circulating gonadal steroids.

Effects of estrous cyclicity on the expression of the galanin receptor Gal-R1 in the rat preoptic area: a comparison with the male

Variations in the number of galanin receptor (Gal-R1)-expressing cells and levels of Gal-R1 messenger RNA (mRNA) were determined in the preoptic area in intact female rats throughout the phases of the estrous cycle and compared with those in the male. Female and male Wistar rats were fixed by perfusion with 4% paraformaldehyde. Cryostat sections were hybridized with a 35S-labeled antisense Gal-R1 riboprobe. The number of Gal-R1 mRNA-expressing cells was lower in the rostral preoptic area than in the medial preoptic area. During the estrous cycle, the highest number of Gal-R1 mRNA-expressing cells in the rostral preoptic region was detected at 0800 h on proestrus, whereas in the medial preoptic area, the maximum number was observed at 1800 h on estrus. Gal-R1 mRNA levels in individual cells were low during diestrus and increased at estrus in both areas. In the male, the number of mRNA-expressing cells and the hybridization signal were significantly lower than those in females during estrus. The results demonstrate that Gal-R1 gene expression in the preoptic area varies during the estrous cycle and is low in males. Short term treatment of ovariectomized rats with estradiol plus progesterone caused significantly decreased preoptic Gal-R1 mRNA levels compared with those after treatment with estrogen only. These observations suggest that in the preoptic area, expression of Gal-R1 is influenced by progesterone. The variation in Gal-R1 expression is likely to influence the extent to which galanin can influence the preoptic cells implicated in the control of neighboring GnRH cells.