Effects of discrete lesions of the sexually dimorphic nucleus of the preoptic area or other medial preoptic regions on the sexual behavior of male rats

The sexually dimorphic nucleus of the rat medial preoptic area (SDN-POA) has a volume five times larger in the adult male compared with that of the adult female. In the present study, the effects of discrete electrolytic destruction of the SDN-POA or other specific medial preoptic (MPOA) regions on masculine sexual behavior were determined in adult, sexually experienced male rats. Small lesions encompassing the SDN-POA had no effect on the maintenance of copulatory behavior. Lesions of similar size placed within the ventral or anterio-dorsal MPOA also did not consistently affect the display of masculine sexual behavior. However, animals that received small lesions within their dorsal MPOA showed a substantial, long-term decrease in number of mounts, intromissions, and ejaculations compared to these parameters in sham-lesioned control rats, thus indicating a lesion-induced disruption of those neural mechanisms mediating these behaviors. Collectively these data suggest that the SDN-POA is not critical for a full expression of male sexual behavior and that the dorsal MPOA may be more important than other MPOA regions for copulatory behavior.     

Steroid autoradiography of the sexually dimorphic nucleus of the preoptic area

Autoradiography was performed to determine if the neurons of the sexually dimorphic nucleus of the preoptic area (SDN-POA) in the adult rat accumulate estradiol (E2), testosterone (T), and/or dihydrotestosterone (DHT). Three days prior to steroid administration, adult male and female Sprague-Dawley rats were gonadectomized and adrenalectomized. Animals were then given either [3H]T, [3H]E2, or [3H]DHT through an indwelling jugular cannula. One hour later, animals were decapitated and brain sections processed for thaw mount autoradiography. The autoradiograms which contained the SDN-POA and an adjacent area of the medial preoptic area (MPOA) were quantitatively analyzed using the 3 times background, 5 times background, and Poisson criteria for labeled cells. In general, cells in the SDN-POA and the MPOA accumulate T, E2, or DHT. For both sexes, there is a greater percentage of labeled cells in the SDN-POA than in the MPOA, and a greater percentage of labeled cells following E2 exposure than following T or DHT exposure. In addition, there is a sex difference (male greater than female) in the percentage of labeled cells following T exposure. In summary, these data indicate that adult SDN-POA neurons do accumulate gonadal steroids.     

Neurogenesis of the sexually dimorphic nucleus of the preoptic area in the rat

The sexually dimorphic nucleus of the preoptic area (SDN-POA) of Sprague-Dawley rats is larger in volume in the male and hormone-dependent early in postnatal life. In the present study, we compared for each sex the time course of neuroblast proliferation which forms SDN-POA or adjacent medial preoptic area (MPOA) neurons. Additionally, we investigated whether there is a temporal gradient of production of neurons in relation to their final position within the SDN-POA. On day 14, 15, 16, 17, or 18 postfertilization (pf) pregnant rats were given a single injection of ³H-thymidine (*thy). At 30 postnatal days of age the pups were sacrificed and brain sections were prepared and processed for autoradiography. Three sections of the SDN-POA and an adjacent area just lateral to it in the MPOA wer also analyzed. In the MPOA and the SDN-POA the percentage (%) of labeled neurons decreases as the day of injection of *thy approaches the end of gestation, but the time period in which neuroblast divisions occurred is markedly different for the SDN-POA as compared to that for the MPOA. DNA synthesis occurs as late as day 18pf for neurons which form the SDN-POA but ceases on day 16pf for those destined for the MPOA. There is a sex difference in neuronal production on both day 14 and 17pf for neurons destined for the SDN-POA. After injection on day 14pf the % labeled neurons is larger in the female than in the male but after injection on day 17pf this is reversed. There are also significant sex differences as well as a temporal gradient associated with the % labeled neurons in the SDN-POA in relation to their final anterior-posterior position. In addition, this study confirms our previous results which justify labeling the SDN-POA a nucleus, since neuronal density in this region in the male and female is significantly greater than that in the surrounding MPOA. These data illustrate that the specific neurons which comprise the SDN-POA in both the male and female are being produced as late as day 18pf, whereas neurons located in the MPOA but not in the SDN-POA have all been born by day 16pf. Neuroblast division which produces the neurons of the SDN-POA may begin earlier and terminate sooner in the female than in the male. These differences in neuronal production may partially account for the sexual dimorphism seen in the volume and neuronal number of the SDN-POA of the adult rat.     

Ontogeny of opiate receptors in the rat medial preoptic area: Critical periods in regional development

Opiate receptor labeling was examined throughout the early postnatal period using autoradiography to localize and quantify [³H]naloxone binding to μ-type opiate receptors in the medial preoptic area (MPOA). This region begins to exhibit sexual dimorphism of volume and dendritic growth shortly after birth. A distinct concentration of opiate receptor labeling appears on postnatal day 3 in females: this labeling is directly associated with the sexually dimorphic nucleus of the preoptic area (SDN-POA). SDN-POA labeling becomes denser through postnatal day 10 in females and the densely labeled area increases in size to encompass and surround the SDN-POA. These changes in opiate receptor labeling occur only in females, since males show relatively uniform labeling across the region throughout the early postnatal period.The critical time of formation of dense MPOA opiate receptor labeling may be related to endogenous MPOA opioid function and to the critical period of dendritic growth of SDN-POA neurons. The timing of these critical periods and their focus in the SDN-POA are coincident. The possible role of MPOA opiate receptors in modulating growth of MPOA neurons is discussed.     

Whole-cell recordings of spontaneous synaptic currents in medial preoptic neurons from rat hypothalamic slices: mediation by amino acid neurotransmitters

Whole-cell recordings in hypothalamic slices from immature rats were used to test the hypothesis that inhibitory and excitatory amino acid neurotransmitters mediate fast synaptic currents in the medial preoptic area (MPOA). Bicuculline methiodide reversibly blocked spontaneous inhibitory postsynaptic currents (IPSCs), and 6-cyano-2,3-dihydroxy-7-nitroquinoxaline (CNOX) blocked spontaneous excitatory postsynaptic currents (EPSCs). These competitive antagonists act at γ-aminobutyric acid (GABA)_A and (NMDA) glutamate receptors, respectively, thus supporting the hypothesis that these amino acid receptors activate most if not all fast synaptic currents in the MPOA.     

Neurons of the bed nucleus of the stria terminalis: A golgi study in the rat

Neuronal morphology in the bed nucleus of the stria terminalis (BST) was studied using Golgi techniques. The principal neurons of the lateral subdivision of BST have ovoid perikarya and 4–5 dendrites that branch several times and exhibit a dense covering of spines. Adjacent to the internal capsule is a small region, termed the “juxtacapsular subdivision” of BST, that consists of small, spiny cells. Neurons of the medial subdivision of BST have ovoid perikarya and 2–3 dendrites that branch sparingly. Dendritic spine density varies from sparse to moderate. Dendrites in the dorsocaudal portion of the medial subdivision extend into a cell-sparse zone adjacent to the lateral ventricle. Cells in the lateral portion of the preoptic continuation of BST have dendrites oriented perpendicular to fibers of the stria terminalis which traverse this area while medially located cells are oriented parallel to fibers of the stria. Axons of BST neurons emit collaterals that arborize modestly near the cell of origin. Neurons in the lateral and medial subdivisions of BST resemble, respectively, cells in the lateral and medial subdivisions of the central amygdaloid nucleus. Neurons in the juxtacapsular subdivision of BST are similar to neurons of the intercalated masses of the amygdala.     

Morphology of neurons in area 4γ of the cat's cortex studied with intracellular injection of HRP

Neurons in laminae II, III, V, and VI of area 4γ of the cat motor cortex were studied following intracellular penetration with an HRP-filled microelectrode. Antidromic and synaptic responses produced by stimulation of the cerebral peduncles and/or of the ventrolateral nucleus of the thalamus were investigated. Horseradish peroxidase was then iontophoresed into the same neurons to allow examination of their detailed morphology. The morphology of pyramidal neurons whose somata were located in a particular lamina was similar but differed from that of pyramidal neurons in other laminae. The modified pyramidal neurons of lamina II had a truncated apical dendrite or did not possess an obvious apical dendrite, even though the ascending dendritic branches were longer and more extensive than the “basal” branches. As was the case for the pyramidal cells in other laminae, the axons of these lamina II modified pyramidal cells descended toward the white matter; their somata were generally pyramidal in shape; and their dendrites were spiny. All pyramidal neurons except some of lamina VI had ascending dendrites which terminated in a tuft in lamina I, subpially. No intracortical collaterals were seen originating from the axons of lamina II or of lamina VI pyramidal neurons. Lamina III pyramidal neurons had extensive short and long axon collaterals which contributed synaptic boutons to all laminae of the cortex. Pyramidal neurons of lamina V had fewer axon collaterals whose synaptic boutons were restricted to laminae V and VI. All somata of pyramidal tract neurons (PTNs), identified by antidromic responses from peduncular stimulation, were located in lamina V, except for one which was located in lamina VI. Recurrent collaterals of pyramidal neurons were activated by peduncular stimulation. Recurrent excitatory postsynaptic potentials (epsps) could be evoked in fast PTNs, slow PTNs, other pyramidal neurons of lamina V, and pyramidal neurons of lamina VI at latencies between 1.3 and 6.25 msec. In some slow PTNs, a recurrent inhibitory postsynaptic potential of long duration was the predominant response. Stimulation of the ventrolateral nucleus of the thalamus resulted in epsps in pyramidal neurons of lamina III, V, and VI at latencies between 1.0 and 5.0 msec.     

Simultaneous catecholamine histofluorescence and thymidine autoradiography of the sexually dimorphic nucleus of the preoptic area in the rat

The sexually dimorphic nucleus of the preoptic area (SDN-POA) in the rat represents a morphological substrate in which the influence of gonadal hormones on the process of sexual differentiation of the brain can be seen. Since the medial preoptic area (MPO) is a region rich in catecholamine (CA) terminals, it is possible that catecholamines may play a role either in the differentiation of the perinatal SDN-POA or in the function of this nucleus in the adult. It is not known whether catecholamine terminals exist within the SDN-POA or whether they can directly influence the activity of SDN-POA neurons. The present study was conducted to determine the extent to which catecholamines innervate this nucleus and further to elucidate the possibility of a potential sexual dimorphism in the innervation pattern. In order to determine which of the neurons in the MPO are within the SDN-POA we have utilized the fact that the SDN-POA has a prolonged period of neurogenesis in comparison to other neurons of the MPO. Thus, tritiated thymidine-labeled neurons can be used as a detection criterion for the SDN-POA. To conduct this experiment, timed pregnant Sprague-Dawley females were given a single injection of [3H]thymidine on Day 18 of gestation. Pups were killed as adults and prepared for fluorescence histochemistry of monoamines. Sections adjacent to those examined for catecholamine fluorescence were treated for autoradiographic localization of [3H]thymidine. Fluorescence innervation patterns were plotted within the boundaries of the nucleus following its identification from Nissl sections as well as from adjacent autoradiograms simultaneously viewed in a comparator bridge microscope with dark-field illumination.(ABSTRACT TRUNCATED AT 250 WORDS)     

The sex hormone-dependent development of opiate receptors in the rat medial preoptic area

The opiate receptor content of the sexually dimorphic medial preoptic area (MPOA) was examined in newborn and 5-day-old (D6) male and female rats. A significant increase of [³H]naloxone binding was observed in and around the sexually dimorphic nucleus of the preoptic area (SDN-POA) in D6 female rats, relative to newborn females. Opiate receptor labeling did not increase over this period in males, nor was labeling different between males and females at birth. This dramatic alteration of MPOA opiate receptor content was observed to occur in either sex in the absence of testosterone postnatally; that is, neonatally-castrated males exhibited the same increase of labeling by D6 as did normal females. Conversely, daily postnatal testosterone treatment of females from birth to D6 resulted in the development of male-like MPOA opiate receptor pattern. The sex hormone-dependence of MPOA opiate receptor development is discussed in relation to the sex hormone-dependent ontogeny of SDN-POA structure. The overlap of critical periods for the development of these structural and chemical sexual dimorphisms suggests a role for endogenous opioids in modulating MPOA development.     

Principal cells are the postsynaptic targets of supramammillary afferents in the hippocampus of the rat

Neurons of the supramammillary nucleus are known to fire phase-locked to hippocampal theta rhythm. Stimulation of this area induces theta activity in the hippocampus via the medial septum and facilitates perforant pathway stimulation-evoked population spikes in the dentate gyrus even if the medial septum is inactivated. This latter effect was suggested to be due to a direct inhibitory input from the supramammilary nucleus to hippocampal nonpyramidal cells resulting in disinhibition. In the present study, using anterograde tracing with Phaseolus vulgaris leucoagglutinin, we aimed to identify the types of neurons innervated by the supramammillary projection in the dentate gyrus and Ammons horn, with particular attention to the presumed postsynaptic inhibitory neurons, which may mediate the proposed disinhibitory action. Double-immunostaining for the tracer and different neuropeptides (somatostatin, cholecystokinin, neuropeptide Y) or calcium binding proteins (calretinin, parvalbumin, calbindin D_28k) present in different subpopulations of interneurons revealed no multiple contacts between supramammillary afferents and labeled inhibitory cells at the light microscopic level. Furthermore, postembedding immunostaining of electron microscopic sections for GABA demonstrated that none of the 68 PHAL-labeled supramammillary boutons examined and none of their postsynaptic targets were immunoreactive for the inhibitory neurotransmitter. We conclude, therefore, that most if not all postsynaptic targets of the supramammillary projection are principal cells both in the dentate gyrus and in the CA2-CA3a subfields. This suggests that a mechanism other than disinhibition is responsible for the facilitatory effect of this pathway on hippocampal evoked activity. © 1994 Wiley-Liss, Inc.     

Fluctuations in neuronal activity and sexual dimorphism in the medial preoptic nucleus of the hypothalamus during postnatal development of the rat

The medial preoptic nucleus of the hypothalamus of the rat is characterized by a sexual dimorphism of one of its substructures, the "sexually dimorphic nucleus of the preoptic area (SDN-POA)" (Gorski et al. 1980), which becomes light microscopically visible at lower magnification. The SDN-POA, showing up as a concentration of perikarya, is principally larger in males than in females. Furthermore, the sexual dimorphism of the medial preoptic nucleus is reflected in temporary significant differences of the nuclear size of its neurons, which again are discussed as the correlate of differences of the neuronal activities. Karyovolumetrically verified variations of activities of the medial preoptic nucleus were observed during continuous studies at totally 180 rats (90 females and 90 males), aged between 5 (P 5) and 60 (P 60) days. These are already known in other cerebral structures and also typical for the comparably studied neurons of the granular layer. They are especially distinct during the first 3-4 days of the postnatal development and characterized by larger amplitudes in male than in female rats. Typical are peaks of activities at P 10 and P 20 as also--especially in males--at P 45. The variations of the neuronal activities are discussed as summarizing reflection of gene activities, correlated with cytological processes of development and differentiation, which again are superimposed by a total of age related and sex specific performances of the Nucleus-preopticus-medialis-neurons. The karyovolumetrically verified sexual dimorphism of the medial preoptic nucleus, appearing as a differently developed SDN-POA, is a reflection of the sex specific morphological-functional differentiation of the central nervous system. A more for-reaching, specifying discussion of the results, complicated by the functional complexity of the medial preoptic nucleus, will be obtained by directed experimental studies.     

The morphology and divergent axonal organization of midbrain raphe projection neurons in the rat

The morphology of dorsal raphe neurons was examined using intracellular injections of horseradish peroxidase (HRP) and the Golgi technique. Light microscopic examination of HRP-labeled projection neurons revealed a neuron type with radiating, poorly branched and sparsely spined dendrites and terminal dendritic thickets. The stem axon of these neurons left the nucleus ventrally but gave off a beaded collateral while still within the parent cell's dendritic domain. Somatodendritic morphology from Golgi-Kopsch stained material coincided with intracellular HRP findings and the dorsal raphe may consist of varieties of one basic morphological type of neuron. Intracellular recordings made during the HRP injection experiments confirmed that stimulation of the ventral medial tegmentum elicited an antidromic action potential and an inhibitory postsynaptic potential in dorsal raphe projection neurons. The order of axonal projections arising from the midbrain raphe nuclei was examined using a double retrograde axonal tracing technique. After paired HRP and [3H] wheat germ agglutinin injections within certain projection targets of the dorsal and median raphe neurons (caudate-putamen, amygdala, hippocampus, substantia nigra and locus coeruleus), each target structure was found to have its own unique representation within a topographically distinct portion of one or more of the raphe subgroups. Neurons projecting to the caudate-putamen and substantia nigra occupied rather rostral portions. Neurons projecting to the hippocampus and locus coeruleus resided more caudally. Neurons projecting to the amygdala were situated intermediately. Overall, rostrocaudal topography in the intranuclear distributions of raphe projection neurons resulted in the formation of complex overlap zones where collateralized neurons always resided.     

Gamma-aminobutyric acid in the medial rat nucleus accumbens: ultrastructural localization in neurons receiving monosynaptic input from catecholaminergic afferents

Neurons containing gamma-aminobutyric acid (GABA) in the medial portion of the adult rat nucleus accumbens were characterized with respect to their ultrastructure, sites of termination, and catecholaminergic input. Antisera against GABA-conjugates and the catecholamine-synthesizing enzyme, tyrosine hydroxylase (TH), were localized within single sections by means of peroxidase-antiperoxidase (PAP) and immunoautoradiographic labeling methods. Peroxidase reaction product indicating GABA-like immunoreactivity (GABA-LI) was seen in medium-size (15-20 microns) perikarya containing either round and unindented or invaginated nuclear membranes. The cells with invaginated nuclei were few in number and usually exhibited more intense peroxidase reaction product in sections collected at the same distance from the surface of the tissue. Reaction product for GABA was also detected in proximal (1.5-3.0 microns) dendrites, axons, and terminals. Terminals with GABA-LI formed symmetric junctions on perikarya, proximal dendrites, and dendritic spines of neurons that usually lacked detectable immunoreactivity. Many of the GABAergic terminals also were apposed directly to other unlabeled terminals and to terminals exhibiting either peroxidase labeling for GABA or immunoautoradiographic labeling for TH. Many of the unlabeled terminals associated with the GABAergic axons formed asymmetric junctions on dendritic spines. From 138 TH-labeled, principally dopaminergic terminals that were examined in the medial nucleus accumbens, 4% were associated with the somata of GABAergic neurons and another 14% formed symmetric junctions with proximal dendrite showing GABA-LI. The remaining TH-immuno-reactive terminals either lacked recognizable densities or formed symmetric synapses on unlabeled dendrites and spines. A few of the unlabeled dendrites, as well as those containing GABA-LI, received symmetric synapses from both catecholaminergic and GABAergic terminals. We conclude that in the medial portion of the rat nucleus accumbens, GABA is localized to two morphologically distinct types of neurons, one or both of which receive monosynaptic input from catecholaminergic afferents, and that GABAergic terminals form symmetric synapses on other principally non-GABAergic neurons. The results also support earlier physiological evidence showing that GABA may modulate the output of other GABAergic and non-GABAergic neurons through presynaptic associations.     

Neuronal organization of the supracommissural ventral telencephalon and the nucleus preopticus periventricularis in the himé salmon (landlocked red salmon, Oncorhynchus nerka): a Golgi study

The neuronal organization of the supracommissural ventral telencephalon (Vs) and the nucleus preopticus periventricularis (NPP) of the medial preoptic area (MPOA) in the himé salmon (landlocked red salmon, Oncorhynchus nerka) was studied by means of the Nissl staining and the rapid Golgi methods. The Vs was composed of many laterally-orienting spiny neurons and a few sparsely-spined polymorphous neurons. Some of the laterally-orienting neurons extended their dendrites into the lateral forebrain bundle (LFB), suggesting that they may receive inputs from the telencephalofugal and telencephalopetal fibers in the LFB. The NPP consisted of five types of sparsely-spined neurons: laterally-orienting, bipolar, cerebrospinal fluid (CSF)-contacting, commissural, and multipolar neurons. Some of the laterally-orienting neurons extended their dendrites into the medial forebrain bundle (MFB) and regions near the LFB, suggesting that they may also receive inputs from the telencephalofugal and telencephalopetal fibers running there. The presence of the CSF-contacting neurons in the NPP suggests that the NPP may receive humoral informations from the CSF. Involvement of these CSF-contacting neurons in the control of the reproductive functions was discussed. The neuronal organization of the NPP in the himé salmon was compared with that of the MPOA in other vertebrates.     

Evidence for thyrotropin-releasing hormone and glucocorticoid receptor-immunoreactive neurons in various preoptic and hypothalamic nuclei of the male rat

Neurons containing thyrotropin-releasing hormone (TRH) and glucocorticoid receptor (GR) immunoreactivity (IR) were demonstrated by a two-colour immunoperoxidase method in coronal cryotome sections of the preoptic region and the hypothalamus of the male rat brain. All the TRH-IR neurons (TRH-IR) located in the dorsal hypothalamus - medial and dorsal parvocellular parts of the paraventricular hypothalamic nucleus and the dorsomedial hypothalamic nucleus - and in the anterior periventricular hypothalamic nucleus were strongly GR-IR. The TRH-IR neurons of the medial preoptic area, the perifornical nucleus and the medial tuberal area were mostly weakly GR-IR and some lacked GR-IR. These data indicate a differential regulation of diencephalic TRH-IR neurons by glucocorticoids. They also imply that the inhibitory effect of glucocorticoids on TSH secretion may involve a direct inhibition of TRH synthesis and/or release by a nuclear action in the TRH-IR nerve cells of the paraventricular hypothalamic nucleus projecting to the median eminence.     

Effects of testosterone on neuronal nitric oxide synthase and tyrosine hydroxylase

Male rat copulatory ability decreases dramatically following castration. This may be due in part to the impairment of medial preoptic area (MPOA) dopamine (DA) release. Previous studies showed that extracellular DA levels in the MPOA of castrates were lower than in intact males, both during basal conditions and in the presence of a receptive female. However, tissue levels of DA in the MPOA were higher in castrates than in intact males, suggesting that DA synthesis may be normal or increased in castrates, but that release may be compromised. The current study found that neither long term (2 months) nor short term (2 weeks) castration had any effect on the number of neurons in the DA A₁₄ area that were immunoreactive (ir) for tyrosine hydroxylase (TH), the rate limiting enzyme for DA synthesis. Therefore, castration may not affect DA synthesis in the MPOA. Tissue levels of neurotransmitter reflect release, as well as synthesis. We previously reported that nitric oxide (NO) may increase DA release in the MPOA. The present study tested whether castration affected the number of NO producing cells in the MPOA. Long term, but not short term, castration significantly decreased the number of NADPH-d (nicotinamide adenine dinucleotide phosphate diaphorase) positive neurons and brain nitric oxide synthase immunoreactive (bNOS-ir) neurons in the medial preoptic nucleus (MPN). This suggests that in gonadally intact animals testosterone may activate NOS, which increases the production of NO. Long or short term castration had no effect on the numbers of bNOS-ir neurons in the paraventricular nucleus (PVN) or medial amygdala. However, short term castration decreased bNOS-ir neurons in the bed nucleus of stria terminalis (BNST). Thus, one means by which testosterone promotes male sexual behavior may be by increasing production of NO in the MPOA, which increases local DA release.     

Androgen and estrogen concentrating neurons in chemosensory pathways of the male Syrian hamster brain.

The medial preoptic area (MPOA), bed nucleus of the stria terminalis (BNST), and medial amygdaloid nucleus (Me) are essential for male sexual behavior in the Syrian hamster. These nuclei received chemosensory stimuli and gonadal steroid signals, both of which are required for mating behavior. The objective of this study was to compare the distribution of androgen- and estrogen-concentrating neurons in MPOA, BNST, and Me in the adult male hamster using steroid autoradiography for estradiol (E2), testosterone (T) and dihydrotestosterone (DHT). Adult males (n = 4 per group) received two i.p. injections of tritiated steroid 4-7 days after castration. Six-microns frozen sections through the brain were mounted onto emulsion-coated slides, and exposed for 11-16 months. In MPOA, BNST, and Me, neurons were more abundant and heavily labelled after [3H]E2 treatment than after either [3H]T or [3H]DHT. Tritiated estradiol- and DHT-labeled cells were found throughout the rostrocaudal extent of Me, with a high concentration in posterodorsal Me. Tritiated testosterone treatment labelled cells largely within posterodorsal Me. In MPOA, the majority of E2-, T-, and DHT-labelled neurons were in the medial preoptic nucleus (MPN) and the preoptic continuation of the posteromedial bed nucleus of the stria terminalis (BNSTpm). Few T-labelled cells were present outside these subdivisions. In the BNST, E2- and DHT-labelled neurons were present in all subdivisions, whereas T labelling was confined to the antero- and posteromedial subdivisions of BNST. These results suggest that the distribution of androgen- and estrogen receptor-containing neurons overlap considerably in nuclei which transmit chemosensory signals in the control of mating behavior.     

Estrogen/progesterone treatment in adulthood affects the size of several components of the medial preoptic area in the male rat

The results of preliminary studies suggested that steroid and/or propylthiouracil (PTU) treatment of adult gonadectomized (Gxd) male rats significantly reduced the volume of the sexually dimorphic nucleus of the preoptic area (SDN-POA). Therefore, we designed a study to examine this effect in detail. Groups of adult rats were sham Gxd (intact) or Gxd, then treated with multiple injections of oil (males and females), or estrogen and progesterone (males). Gonadectomized estrogen/progesterone-treated males had a significantly smaller SDN-POA volume, smaller volume of the medial division of the medial preoptic nucleus (MPNm), smaller volume of the anteroventral MPNm (MPNav), and larger volume of the anteroventral periventricular nucleus (AVPv). The volume of the central division of the medial preoptic nucleus (MPNc) or of the suprachiasmatic nucleus was not affected. There were no differences between Gxd estrogen/progesterone-treated males vs the group that received PTU as well, indicating that the PTU treatment was unnecessary. The reduced volume of the SDN-POA was due to a reduced volume of the MPNav and of the portion of the SDN-POA located within the MPNm-exclusive of the MPNav and MPNc. In conclusion, estrogen/progesterone treatment in adulthood caused significant changes in the volume of several medial preoptic structures in two separate groups of Gxd males. Because the steroids produced no significant effects in intact males, testicular hormones appear to "protect" these structures from the effects of the estrogen/progesterone treatment.     

A depolarizing inhibitory response to GABA in brainstem auditory neurons of the chick

Neurons in the brainstem auditory nuclei, n. magnocellularis and n. laminaris, of the chick are contacted by terminals containing the inhibitory neurotransmitter γ-aminobutyric acid (GABA). In this report we describe the physiological response of these neurons to GABA using an in vitro slice preparation. In brainstem auditory neurons, GABA produced a depolarization of up to 20 mV and an associated decrease in input resistance. This depolarization was inhibitory; action potentials generated by orthodromic synaptic drive, antidromic stimulation and intracellular current injection were prevented by GABA application. The GABA response still occurred when synaptic transmission was prevented by perfusing the slice with a medium containing low Ca²⁺ and high Mg²⁺ concentrations. Thus, the effects of GABA were directly on the postsynaptic neuron and not via an interneuron. Whole-cell voltage clamp of neurons revealed that the reversal potential of the inward current was approximately −45 mV, suggesting that the channel responsible for this response is not selective for Cl⁻ or K⁺. Pharmacological analyses suggest that this GABA receptor has properties distinct from those typical of either GABA_a or GABA_b receptors. Although a similar response was observed with the GABA_a agonist, muscimol, it was not blocked by the GABA_a antagonist, bicuculline. The response was not evoked by the GABA_b agonist, baclofen, and was not blocked by the GABA_b antagonist phaclofen. This unusual depolarizing response is not a common feature of all brainstem neurons. Neurons located in the neighboring medial vestibular nucleus show a more traditional response to GABA application. At resting potential, these neurons show a hyperpolarizing or biphasic response associated with a decrease in input resistance and inhibition of their spontaneous activity. GABA-induced responses in the medial vestibular nucleus are blocked by bicuculline. These results suggest that an unusual form of the GABA receptor is present in the brainstem auditory system of the chick. It is possible that this form of GABA receptor provides an efficient mechanism for inhibiting the relatively powerful EPSPs received by brainstem auditory neurons, or it may play a trophic role in the afferent regulation of neuronal integrity in this system.     

Correlation of electrophysiological and morphological characteristics of enteric neurons in the mouse colon

We report on the first correlative study of the electrophysiological properties, shapes, and projections of enteric neurons in the mouse. Neurons in the myenteric plexus of the mouse colon were impaled with microelectrodes containing biocytin, their passive and active electrophysiological properties determined, and their responses to activation of synaptic inputs investigated. Biocytin, injected into the neurons from which recordings were made, was converted to an optically dense product and used to determine the shapes of neurons. By electrophysiological properties, almost all neurons belonged to one of two classes, AH neurons or S neurons. AH neurons had a biphasic repolarization of the action potential, and slow afterhyperpolarizing potentials usually followed the action potentials. S neurons had monophasic repolarizations, no slow afterhyperpolarization, and fast excitatory postsynaptic potentials in response to fibre tract stimulation. By shape, neurons were divided into Dogiel type II (28/136 neurons) and uniaxonal neurons. Dogiel type II neurons had large, smooth-surfaced cell bodies and several long processes that supplied branches within myenteric ganglia. All Dogiel type II neurons had AH electrophysiology; conversely, most AH neurons had Dogiel type II morphology. The majority of uniaxonal neurons had lamellar dendrites, i.e., Dogiel type I morphology. They projected to the circular muscle (circular muscle motor neurons), to the longitudinal muscle (longitudinal muscle motor neurons), and to other myenteric ganglia (interneurons) and in some cases could not be traced to target cells. All S neurons were uniaxonal. A small proportion of uniaxonal neurons (3/70) had AH electrophysiology. Fast excitatory synaptic potentials were only recorded from uniaxonal neurons and were in most cases blocked by nicotinic receptor antagonists. A small component of fast excitatory transmission in some neurons was antagonized by the purine receptor antagonist PPADS. Slow excitatory postsynaptic potentials were observed in both AH and S neurons. Slow inhibitory postsynaptic potentials were recorded from S neurons. We conclude that the major classes of neurons are Dogiel type II neurons with AH electrophysiological properties and Dogiel type I neurons with S electrophysiological properties. The S/Dogiel type I neurons include circular muscle motor neurons, longitudinal muscle motor neurons, and interneurons.