Comparative study of dopamine- and noradrenaline-immunoreactive terminals in the paraventricular and supraoptic nuclei of the rat

Quinelorane (LY163502), a D2 dopamine receptor agonist, facilitates seminal emission, but inhibits penile erection in the rat

Dopaminergic compounds have been shown to facilitate male sexual responses in various contexts. We investigated the effects of a specific D2 dopamine receptor agonist, quinelorane (LY163502), on sexual responses elicited in the restrained supine male rat (i.e., ex copula reflex tests). Penile erections, evoked by retraction of the penile sheath, were inhibited by systemic administration of 10 micrograms/kg quinelorane; however, the occurrence of seminal emission was dramatically increased. A smaller dose of 0.25 ng/kg was without effect. In a second experiment, intracranial microinjection of quinelorane was followed by ex copula reflex tests. The medial preoptic area (MPOA) has been previously implicated in the dopaminergic regulation of male copulatory behavior. The effects of an intra-MPOA injection of quinelorane on seminal emission and erectile responses were similar to those observed following systemic administration. These results are consistent with the hypothesis that DA receptors in the MPOA are important in the regulation of male sexual behavior and suggest that D2 receptors in the MPOA may decrease ejaculatory threshold while inhibiting erectile mechanisms.

The effects of intracranial administration of the dopamine agonist apomorphine on penile reflexes and seminal emission in the rat

Previous studies employing systemic administration of the dopamine agonist apomorphine have shown that the dose response curves for apomorphine's effects on penile reflexes and seminal emission differ, suggesting that experimentally separable populations of dopamine receptors regulate these two responses. The present experiments examined the locations of central nervous system DA receptors mediating genital responses in the restrained, supine rate by injecting apomorphine into the medial preoptic area and the paraventricular nucleus through chronic, indwelling cannulae. Medial preoptic area injections facilitated penile reflexes, but not seminal emission, while paraventricular injections facilitated seminal emission. These results suggest that systemically administered apomorphine may facilitate penile reflexes by acting on the medial preoptic area and may enhance seminal emission by acting on the paraventricular nucleus.

Facilitatory role of central dopamine in the osmotic release of vasopressin

In conscious rats, intracerebroventricular (i.c.v.) injection of the dopamine antagonist haloperidol (0.15 mumol) blocked increases in plasma vasopressin (AVP) and arterial pressure caused by i.c.v. administration of a hypertonic solution (990 mOsm/kg, 10 microliters), without affecting plasma osmolality, electrolytes or hematocrit. The application of the alpha-adrenergic antagonist phenoxybenzamine (0.15 mumol), however, did not significantly inhibit the responses of plasma AVP and arterial pressure to the hypertonic solution. We concluded that brain dopamine may play a facilitatory role in the osmotically stimulated AVP secretion.

Neuronal plasticity in the hedgehog supraoptic nucleus during hibernation

The purpose of the present study was to identify processes of plasticity in the receptive field of neurosecretory neurons of the supraoptic nucleus during hibernation in the hedgehog, in order to correlate them with the increased neurosecretory activity observed in this nucleus during this annual period. Using the Rapid Golgi method, a quantitative study was conducted in the receptive field of bipolar and multipolar neurons (the main components of the nucleus). Results indicate a generalized increase in the following characteristics: (1) number of dendritic spines per millimeter along the dendritic shafts; (2) degree of branching in the dendritic field; and (3) dendritic density around the neuronal soma. These data demonstrate modification of the dendritic field in the supraoptic nucleus during hibernation, a change undoubtedly related to functional conditions. Since the observed changes affect structures such as dendritic spines which are directly related to the arrival of neural afferences, the discussion is centered on the types of stimuli which may be responsible for the observed processes.

Axons containing neuropeptide Y innervate arginine vasopressin-containing neurons in the rat paraventricular nucleus. Dual electron microscopic immunolabeling

Synaptic connections between neurons immunoreactive for arginine vasopressin (AVP) and axon terminals immunoreactive for neuropeptide Y (NPY) were found in the magnocellular part of the paraventricular nucleus (PVN) in the rat hypothalamus. In pre-embedding double immunolabeling, NPY axon terminals labeled with diaminobenzidine (DAB) reaction product established synaptic junctions on the perikarya and neuronal processes of AVP neurons labeled with silver-gold particles. Ultrastructural morphology of the neurons was more suitably preserved by a combination of pre- and post-embedding procedures. The presynaptic NPY terminals contained many small clear vesicles and a few cored vesicles, and DAB chromogen (immunoreaction product) was located on the surface of the vesicular profiles and on the core. The postsynaptic AVP neurons possessed many large secretory granules labeled with gold particles. At the synaptic junctions, small clear vesicles were accumulated at the presynaptic membrane, and the postsynaptic membrane was coated with a dense accumulation of fine electron dense particles. The perikarya also received synapses made by immuno-negative axon terminals containing many small clear vesicles and a few cored vesicles. These terminals were found more frequently than those containing NPY.

Ultrastructural localization of neuropeptide Y and galanin immunoreactivity in the paraventricular nucleus of the hypothalamus in the rat

Preembedding immunoperoxidase staining methods were used to allow ultrastructural localization of neuropeptide Y (NPY) and galanin immunoreactivity (IR) in the paraventricular nucleus (PVH) of the rat hypothalamus. NPY-IR was localized exclusively in axons and axon terminals which could be grouped into three types: (1) symmetric axo-somatic contacts predominantly with parvocellular neurons, many of which displayed neurosecretory specializations, (2) predominantly asymmetric contacts onto larger dendritic processes, including some of magnocellular neurosecretory neurons, and (3) predominantly symmetric contacts with small dendritic and spine-like profiles. Galanin-IR terminals displayed a more limited distribution and formed both symmetric and asymmetric contacts with parvocellular neurons, and primarily asymmetric contacts with larger dendritic shafts. Numerous dendritic and somatic profiles, including those of some magnocellular neurosecretory neurons, were lightly stained for galanin IR. These results establish that NPY and galanin IR afferents form a variety of conventional synaptic contacts in the PVH. The two peptidergic terminal types differed with respect to the frequency of their interaction with various postsynaptic targets and/or their distribution upon them. Both peptidergic inputs arise at least in part from brainstem catecholaminergic neurons, and the relationship of the present results to the fine structure of catecholaminergic terminals is discussed.

Ultrastructural demonstration of the catecholaminergic innervation of vasopressin neurons in the paraventricular nucleus of the rat by double-labeling immunocytochemistry

The catecholaminergic innervation of vasopressin (VP) neurons in the paraventricular nucleus (PVN) of the rat was studied at the electron microscopic level by double-labeling immunocytochemistry combining the preembedding peroxidase-antiperoxidase method with the postembedding immunogold staining method. Tyrosine hydroxylase-like immunoreactive nerve terminals were found to establish synapses with neurophysin II-like immunoreactive neuronal perikarya and their processes. This provides morphological evidence for catecholaminergic control of the release of VP, at the PVN level.

Haemorrhage-induced release of noradrenaline, 5-hydroxytryptamine and uric acid in the supraoptic nucleus of the rat, measured by microdialysis

Concentrations of monoamines and metabolites and uric acid were measured in sequential 15-min microdialysis samples taken from the area of the supraoptic nucleus (SON) in 9 urethane-anaesthetized rats. A decrease in blood pressure of 40 mm Hg was induced by withdrawing 3-4 ml of blood from an indwelling jugular catheter. This decreased blood pressure was maintained for 30 min and produced a significant rise in the concentrations of noradrenaline (NA) and its metabolite 4-hydroxy-3-methoxyphenylethan-1,2-diol (MHPG) and also in 5-hydroxytryptamine (5-HT) and the purine metabolite uric acid (UA). When blood pressure was returned to normal by replacing the withdrawn blood, the concentrations of NA, MHPG, 5-HT and UA returned to pre-haemorrhage levels within 15 min. No significant changes in concentrations of dopamine and its metabolites were found, and in 7 control animals sampled 1.5-2 mm away from the SON, in the area of the olfactory tubercle, concentrations of NA, MHPG, 5-HT and UA were not significantly altered by haemorrhage-induced decreases in blood pressure.