Biomedical subjects

B D Waterhouse

Publications and source records attributed to B D Waterhouse.

At least 37 records · Page 2Linked to original sources

PubMed1993

Topographic organization of rat locus coeruleus and dorsal raphe nuclei: distribution of cells projecting to visual system structures.

Previous reports from this laboratory and elsewhere have provided evidence that the locus coeruleus (LC) and dorsal raphe (DR) nuclei are topographically organized with respect to their efferent targets. Whereas most of these previous studies have focused on relationships between these monoamine-containing brainstem nuclei and cerebral cortex, basal ganglia, and limbic structures, they have not systematically examined the distribution of LC and DR cells that project to multiple structures with common sensory or motor functions. The goal of the present study was to characterize and compare the distributions of LC and DR cells which project to different visual areas of the rat central nervous system. Long-Evans hooded rats received unilateral pressure injections of the retrograde tracer wheat germ agglutinin-horseradish peroxidase in either the dorsal lateral geniculate, ventral lateral geniculate, or lateral posterior nucleus of thalamus; superior colliculus, cortical area 17, cortical area 18a/b; cerebellar vermis (lobules VI and VII); or paraflocculus. Transverse sections through the midbrain and pons were examined by light microscopy after performing routine tetramethyl benzidine histochemical procedures. For all cases studied, retrogradely labeled cells were observed throughout the rostrocaudal extent of the LC and DR; however, labeling patterns which were distinctive for different injection sites were noted in each of these brainstem nuclei. The major conclusion drawn from this work is that subsets of LC and DR cells which project to different target structures within the rat visual system are found in overlapping but not necessarily coextensive zones within these nuclei. These studies provide further evidence of a rough topographic ordering within both the LC and DR nuclei, as well as support a new hypothesis that the outputs from each of these nuclei are organized with respect to the sensory related functions of their efferent targets.

Animals↗

PubMed1993

Regulation of nerve growth factor mRNA in the hippocampal formation effects of N-methyl-D-aspartate receptor activation.

In the hippocampal formation, nerve growth factor (NGF) is produced in granule cells of the dentate gyrus and a few pyramidal cells of Ammon's horn. Both neuronal populations express N-methyl-D-aspartate (NMDA) receptors and receive putative glutamatergic afferents originating in the entorhinal cortex and projecting via the perforant path. We report in this study that intra-hippocampal or intraventricular injections of NMDA increase NGF mRNA levels in dentate gyrus granule cells as determined using in situ hybridization histochemistry and a solution hybridization assay. NGF mRNA induction is detected within 2 h following NMDA treatment and returns to control levels within 24 h. This NMDA effect is dose-dependent and blocked by pretreatment with 2-amino-5-phosphonopentanoic acid, a competitive NMDA antagonist. Finally, the induction of NGF mRNA is observed in the absence of detectable neurotoxicity or seizure activity. We postulate that normal physiological events associated with the activation of hippocampal NMDA receptors may regulate mRNA expression of this neurotrophic factor.

Animals↗

PubMed1991

Effects of systemically and locally applied cocaine on cerebrocortical neuron responsiveness to afferent synaptic inputs and glutamate.

The goal of the present study was to determine the effects of systemically or locally applied cocaine on rat somatosensory cortical neuron responsiveness to afferent synaptic inputs or putative transmitter application and to compare these results with previously observed actions of endogenous cortical monoamines on the same parameters of neuronal function. Individual cells in rat cortex were activated by stimulation of thalamocortical afferents or local iontophoretic application of glutamate. Extracellularly recorded responses to these stimuli were monitored before and after parenteral or microiontophoretic administration of cocaine. The results indicate that while high doses (greater than 2.0 mg/kg i.p.) of the drug can suppress both evoked and spontaneous activity of cortical neurons, low doses (0.5 mg/kg i.p.) can selectively enhance stimulus-evoked discharge. These facilitating effects can also be observed during iontophoretic application of cocaine directly onto recorded cells, thus suggesting that at least a component of the drug's influence on neuronal responsiveness is mediated by local actions at synapses within the cortex. Of the 3 major endogenous cortical monoamines whose synaptic reuptake is influenced by cocaine, the actions reported here mimic those described previously for norepinephrine but not those of dopamine or serotonin. As such these findings suggest that cocaine may enhance the responsiveness of sensory cortical neurons to afferent synaptic inputs via its ability to activate noradrenergic modulatory mechanisms within the cerebrocortical circuitry.

Amino Acids↗

PubMed1991

Cocaine actions in a central noradrenergic circuit: enhancement of cerebellar Purkinje neuron responses to iontophoretically applied GABA.

Many recent studies have implicated the mesolimbic dopaminergic pathway as the central neurotransmitter system which is most likely responsible for the euphoria and abuse potential associated with cocaine self-administration. Nevertheless, cocaine also has well established interactions with the norepinephrine- and serotonin-containing pathways of the brain. In order to begin assessing potential non-dopamine-mediated actions of cocaine in central circuits, we have initiated a series of experiments using the cerebellar Purkinje neuron as an electrophysiological test system. The strategy was to use the same experimental protocols employed in previous investigations of noradrenergic influences on putative amino acid transmitter action to examine the effects of exogenously applied cocaine on gamma-aminobutyric acid (GABA)-induced depressant responses of Purkinje cells. Accordingly, the inhibitory responses of Purkinje neurons to microiontophoretically applied GABA were examined before and after systemic or local iontophoretic administration of cocaine. Drug-induced changes in the spontaneous firing rate and GABA responsiveness of individual cells were assessed by quantitative analysis of perievent histograms. The results indicate that, like norepinephrine, cocaine at parenteral or iontophoretic doses subthreshold for producing direct suppression of spontaneous discharge can augment Purkinje neuron responses to GABA. Such potentiating effects of cocaine on GABA-mediated inhibition were not evident in animals pretreated with the selective noradrenergic toxins DSP-4. These findings indicate that cocaine can enhance central neuronal responsiveness to GABA in a manner identical to that shown previously for norepinephrine. Such actions in noradrenergic target circuits throughout the brain could contribute to the net behavioral response observed following cocaine administration.

Animals↗

PubMed1991

Noradrenergic potentiation of excitatory transmitter action in cerebrocortical slices: evidence for mediation by an alpha 1 receptor-linked second messenger pathway.

Considerable evidence from intact, anesthetized preparations suggests that norepinephrine (NE) can modulate the efficacy of synaptic transmission within local circuits of the mammalian neocortex; i.e. both iontophoretic application of NE and activation of the coeruleocortical pathway are capable of facilitating cortical neuronal responses to non-noradrenergic synaptic inputs and putative transmitter agents. In the present study, the effects of NE on somatosensory cortical neuronal responses to putative excitatory transmitters were characterized using in vitro tissue slice preparations. Somatosensory unit responses to iontophoretic pulses of acetylcholine (ACh) or glutamate (Glu) (10-60 nA; 5-25 s duration) were examined before, during and after a period of continuous NE (1-35 nA; 4-25 min duration) microiontophoresis. Quantitative analysis of per-event histograms indicated that both Glu- and ACh-evoked excitatory discharges were routinely (Glu 94%, n = 54; ACh 67%, n = 9) potentiated above control levels during NE administration. In 8 cells, NE revealed robust excitatory discharges to otherwise subthreshold iontophoretic doses of Glu. The alpha-specific agonist, phenylephrine, mimicked (n = 3), NE-induced potentiation of Glu-evoked discharges whereas the alpha antagonist phentolamine blocked (n = 5) enhancement of these responses. Moreover, activation of protein kinase C by iontophoretic application of phorbol 12,13-diacetate (5-15 nA, n = 4) mimicked the potentiating actions of NE on Glu-evoked excitatory responses. Results from other experiments further indicated that these facilitating actions of NE on Glu-evoked responses do not involve beta receptor activation or intracellular increases in cyclic AMP. In summary, these results demonstrate that NE can facilitate cortical neuronal responses to threshold and subthreshold level applications of putative excitatory transmitter agents. Moreover, it appears that, unlike noradrenergic facilitating influences on GABA-induced inhibition, these actions are mediated by an alpha adrenoceptor mechanism which may be linked to intracellular activation of protein kinase C. Overall, these findings reinforce the idea that noradrenergic modulatory actions on excitatory and inhibitory neuronal responses may involve the activation of separate receptor-linked second messenger systems.

Acetylcholine↗

PubMed1991

Modulatory actions of norepinephrine on neural circuits.

A spectrum of studies has been conducted on a single aspect of NE function in which, through a beta-one receptor activation, NE appears to mediate a degree of physiological control over the gain of GABA mediated inhibition. It is significant that this single effect has been observed in numerous interrelated preparations ranging from single isolated Purkinje cells from young rats to adult Purkinje cells in awake locomoting rats. With respect to the functional conse-quences of these effects, our best current speculation as to "what NE does" is that NE acts to regulate the strength of these tuned gating mechanisms in both cerebral and cerebellar cortices. There are numerous unanswered questions raised by the past work. One pressing issue is - when and for what reason in normal function does the modulation take place? When does NE release normally occur (is it phasic or tonic), and which of the demonstrated actions appears and for how long in relation to period of receptor activation? Does NE release cause the circuit to "react" to conditions which need "improved neurocomputation" or does NE stabilize the circuit to react predictably in the face of stress? Finally, what is the molecular sequence of events between receptor activation and an alteration of GABA receptor channel opening? What additional molecular control mechanisms exist and how can the diverse inhibitory and modulatory phenomena be reconciled, both short and long term? Issues are defined which need to be clarified at all levels of the current skeleton of basic understanding. Our prediction is that pursuit of these issues will benefit from an exchange of insight gained from investigations at all levels.

Animals↗

PubMed1991

The cerebellar norepinephrine system: inhibition, modulation, and gating.

A series of studies has been conducted to determine the mode of action on the cerebellar cortical circuitry of the norepinephrine (NE)-containing afferents from the locus coeruleus. NE has been known to exert an "inhibitory" action on the background firing observed in Purkinje cells, due presumably to a shift in conductances favoring hyperpolarization. An additional independent action at low threshold appears to be an enhancement of GABA, the inhibitory transmitter of cerebellar interneurons. Recent whole-cell patch-clamp studies on isolated Purkinje cells indicate that exposure to NE increases the chloride current caused by transient pulses of GABA applied iontophoretically. NE applied to Purkinje cells in the parafloccular lobule during stimulation by moving visual patterns revealed the capacity either to "gate" signals initially not expressed, or to amplify the gain of phasic excitations. The control of emergent circuit functions may be the functional consequence of the multiple modulatory functions of NE.

Action Potentials↗

PubMed1991

Second messenger-mediated actions of norepinephrine on target neurons in central circuits: a new perspective on intracellular mechanisms and functional consequences.

Ever since the initial demonstration of a widespread distribution of noradrenergic fibers to functionally diverse regions of the mammalian forebrain, there has been considerable interest in determining the electrophysiological effects of norepinephrine (NE) on individual neurons within these target areas. While early studies showed that NE could directly inhibit cell firing via increased intracellular levels of cyclic AMP, more recent work has revealed a spectrum of noradrenergic actions, which are more accurately characterized as neuromodulatory. More specifically, numerous experimental conditions have been described where NE at levels subthreshold for producing direct depressant effects on spontaneous firing can facilitate neuronal responses to both excitatory and inhibitory synaptic stimuli. The goal of this report is to review recent evidence which suggests that the various modulatory actions of NE on central neurons result from the activation of different adrenoceptor-linked second messenger systems. In particular, we have focused on the candidate signal transduction mechanisms that may underlie NE's ability to augment cerebellar and cortical neuronal responsiveness to GABAergic synaptic inputs. The consequences of such NE-induced changes in synaptic efficacy are considered not only with respect to their influences on feature extraction properties of individual sensory cortical neurons but also with regard to the potential impact such actions would have on the signal processing capabilities of a network of noradrenergically innervated cortical cells.

Action Potentials↗

PubMed1990

Modulation of rat cortical area 17 neuronal responses to moving visual stimuli during norepinephrine and serotonin microiontophoresis.

The present study was conducted to examine the actions of norepinephrine (NE) and serotonin (5-HT) on the multiphasic, visually evoked discharges of cells recorded from the visual cortex (area 17) of anesthetized Long-Evans pigmented rats. Visual responses of 51 cells, evoked by computer controlled presentation of moving visual stimuli, were examined before, during and after low level microiontophoretic application of NE (1-55 nA) or 5-HT (1-50 nA). Drug-induced changes in stimulus-evoked and spontaneous discharges were quantitatively assessed by computer analysis of peri-event histograms. In the majority of cases tested, NE produced a net enhancement of visually evoked responses by facilitating excitatory and inhibitory components of stimulus-bound discharges. By contrast, 5-HT tended to suppress stimulus-evoked excitation and inhibition in many cases to the extent that neurons were no longer responsive to appropriate visual stimuli. In selected cases we were able to demonstrate additional effects of NE and 5-HT on response threshold, direction selectivity and discrimination of receptive field borders. For example, in some cells NE was capable of revealing evoked responses to visual stimuli which were previously ineffective in eliciting stimulus-bound discharges. In other instances, changes in cell activity evoked by stimulus movement across the visual field were accentuated during NE application in such a way that unit discharges at receptive field borders were more sharply defined in comparison to control conditions. 5-HT, on the other hand, was capable of decreasing the contrast between spontaneous and visually evoked discharge at receptive field boundaries. In summary, these results suggest that endogenously released NE and 5-HT may modulate, by complimentary actions, the magnitude of responses of visual cortical neurons to afferent synaptic inputs. Moreover, these monoaminergic projection systems may also have the capacity to modify the threshold of detection of afferent signals within a neuronal network as well as alter feature extraction properties of the circuit.

Animals↗

PubMed1989

Noradrenergic potentiation of cerebellar Purkinje cell responses to GABA: evidence for mediation through the beta-adrenoceptor-coupled cyclic AMP system.

Previous in vivo studies from our laboratory have consistently shown that iontophoretically applied norepinephrine (NE) can potentiate gamma-aminobutyric acid (GABA)-induced depressant responses of cerebrocortical, cerebellar and hypothalamic neurons. Additional experiments have further suggested that this noradrenergic facilitating action is specific for GABA and results from the activation of a beta-type adrenoceptor. The goal of the present studies was to determine if the cAMP second messenger system might also be a component of the mechanism responsible for this NE modulatory action on GABA-mediated inhibition. In one set of in vitro experiments, we examined cerebellar neuronal responses to GABA before, during and after iontophoretic application of NE, 8-bromo-3',5'-cyclic AMP (BcAMP) or 3-isobutyl-1-methyl xanthine (IBMX) or bath application of forskolin (10-30 microM). In a second group of in vivo studies, extracellularly recorded responses of individual cerebellar Purkinje (P) cells to iontophoretic pulses of GABA or beta-alanine were examined before, during and after NE or BcAMP microiontophoresis. In 20 of 25 cerebellar cells recorded from tissue slices, iontophoretically applied NE markedly enhanced responses to GABA in a manner similar to that observed previously in vivo. In these in vitro preparations, bath application of forskolin was also capable of potentiating GABA-induced inhibition in each of 4 cases tested whereas dideoxy-forskolin was not. Iontophoretic application of IBMX further enhanced the facilitating effects of NE on GABA-induced inhibition in 10 of 11 cases tested. Furthermore, under in vitro conditions, BcAMP augmented inhibitory responses to GABA in all cerebellar neurons tested. In the intact rat brain, iontophoretic administration of BcAMP caused a marked NE-like augmentation of P-cell responses to GABA in 73% of the cells tested. As with NE, BcAMP was ineffective in enhancing P-cell inhibitory responses to beta-alanine, an agent which like GABA causes hyperpolarization, by increasing Cl conductance. In summary, these results indicate that a membrane permeant analog of cAMP, a phosphodiesterase inhibitor and an agent which directly activates adenyl cyclase can mimic the previously observed GABA-potentiating actions of NE. Thus, these findings provide further support for the contention that noradrenergic enhancement of GABA inhibition results from a cascade of transmembrane events which includes beta-receptor activation, adenyl cyclase stimulation and increased intracellular production of cAMP.

1-Methyl-3-isobutylxanthine↗

PubMed1988

Locally applied estrogens potentiate glutamate-evoked excitation of cerebellar Purkinje cells.

Ongoing studies in our laboratory have demonstrated that systemically administered sex steroids 17 beta-estradiol (E2) and progesterone (P) alter cerebellar Purkinje cell responses to the amino acid neurotransmitter glutamate (Glu) in the urethane-anesthetized, ovariectomized adult rat. In the present study, we have examined the effects of locally pressure ejected E2 (0.5 microM) on Purkinje cell responsiveness to microiontophoretically applied Glu. The inactive stereoisomer of E2, 17 alpha-E2 (0.5 microM), estradiol benzoate (EB, 0.5 microM), and estrone (E1, 0.5 microM) were also tested (vehicle: 0.01% propylene glycol-saline, pH 7.4). Extracellular activity of single Purkinje neurons was recorded using multibarrel glass micropipets. Spontaneous firing rate and neuronal responses to microiontophoretic pulses (10 s every 40 s at 10-50 nA) of Glu were examined before, during and after continuous local pressure application of the steroids (1-5 psi, 10-15 min). Local E2 administration increased Glu response by 86% within 2-3 min after the onset of steroid application, with no recovery apparent by 30 min after termination of steroid administration. As such, local E2 application mimicked the effect of systemic injection of this steroid. The inactive estrogen isomer, 17 alpha-E2, failed to significantly enhance Glu responsiveness. Both EB and E1, however, significantly potentiated Glu responsiveness in a manner similar to locally applied E2. In addition, EB administration produced long-lasting increases in background discharge, unlike E2, and eventual recovery of Glu responses to control pre-steroid levels. In summary, this study provides a demonstration of local sex steroid actions on neuronal responsiveness in a model extrahypothalamic CNS area. These effects were specific, as the inactive 17 alpha-E2 isomer did not alter neuronal physiology. The results presented here suggest that the neuronal effects of systemic estrogen may be mediated by local actions of E2 or E1.

Action Potentials↗

PubMed1988

Electrophysiological actions of norepinephrine in rat lateral hypothalamus. I. Norepinephrine-induced modulation of LH neuronal responsiveness to afferent synaptic inputs and putative neurotransmitters.

The present studies were conducted as part of an ongoing investigation of the effects of norepinephrine (NE) in neuronal circuits of the mammalian brain. In this report, we describe noradrenergic actions in the lateral hypothalamus (LH), an area which has been implicated in the central integration of cardiovascular regulatory mechanisms, fluid balance and ingestive behaviors. Microiontophoretically applied NE was interacted with extracellularly recorded responses of LH neurons to iontophoretically applied putative neurotransmitters gamma-aminobutyric acid (GABA), acetylcholine (ACh) and glutamate (Glu); and activation of known input pathways from the reticular thalamus (RT) and the lateral preoptic area (LPO). Peri-event histograms of cell responses were computed before, during and after NE microiontophoresis (5-50 nA) and used to quantitatively evaluate monoamine-induced effects on spontaneous and stimulus evoked activity of LH neurons. In 16 of 23 LH neurons, RT-stimulus-induced inhibition was markedly prolonged from a mean of 28.3 +/- 4.8 ms to 44.7 +/- 5.2 ms, during iontophoretic application of NE. In 22 of 38 LH cells, LPO-stimulus-induced excitatory responses were enhanced above control levels during NE administration. In further tests, inhibitory responses of LH cells to iontophoretic pulses of GABA were potentiated during NE administration in 69% (24 of 35) of the cases tested. ACh-induced excitation was potentiated in 9 of 21 cells. In 4 of these cases, otherwise subthreshold doses of ACh caused marked increases in cell firing during the period of NE administration. By contrast, Glu-evoked excitation was antagonized by NE iontophoresis in 65.5% (17 of 26) of LH cells tested. These findings indicate that, as in other noradrenergic target regions of the CNS, NE can facilitate synaptically mediated responses of LH neurons. Taken together these observations suggest that NE may play an important regulatory role in the synaptic transfer of information within LH circuits, and consequently exert considerable influence over the homeostatic functions mediated by this structure.

Acetylcholine↗

PubMed1988

Electrophysiological actions of norepinephrine in rat lateral hypothalamus. II. An in vitro study of the effects of iontophoretically applied norepinephrine on LH neuronal responses to gamma-aminobutyric acid (GABA).

The preceding studies demonstrated that norepinephrine (NE) can consistently augment synaptically mediated (70%) and gamma-aminobutyric acid (GABA)-induced (69%) inhibitory responses of lateral hypothalamic (LH) neurons in vivo. The present experiments further characterized the interactions of NE with LH neuronal responses to GABA in terms of alpha- and beta-receptor mechanisms and demonstrated the utility of the in vitro LH tissue slice preparation as a model for future extra- and intracellular studies of NE modulatory phenomena. Extracellular activity of LH cells was recorded from diencephalic slices (450 microns) incubated in artificial cerebrospinal fluid at 33 degrees C. Interactions between iontophoretically applied NE, isoproterenol (ISO) or phenylephrine (PE) and responses of LH neurons (n = 64) to GABA microiontophoresis were quantitated and characterized using computer-generated ratemeter and histogram records. This analysis revealed two distinct actions of NE on GABA-induced responses of LH neurons. In 8 of 32 cells tested (25%), locally applied NE markedly enhanced inhibitory responses to GABA iontophoresis in a manner identical to that observed in vivo. However, in 20 cells (62.5%), iontophoretic application of NE produced a clear antagonism of GABA responses. NE also exerted dual effects on the background firing rate of LH neurons, causing both inhibition and excitation. Overall, in those cells where NE administration increased spontaneous discharge, it either antagonized or had no effect on GABA-mediated inhibition. In contrast, spontaneous firing rate was never elevated above control levels in those cases where NE potentiated GABA responses. Additional experiments demonstrated that the GABA potentiating actions of the benzodiazepine, flurazepam, were preserved in LH tissue slice preparations. In addition, iontophoretic application of the beta-agonist, ISO, routinely suppressed the spontaneous activity of LH neurons and mimicked the facilitating action of NE on GABA. Likewise, microiontophoretic application of 8-bromo cyclic adenosine monophosphate (AMP) enhanced GABA-induced inhibition of LH firing rate in each of 11 cells tested. On the other hand, local administration of the alpha agonist, PE, routinely produced NE-like antagonism of GABA inhibition along with increases in spontaneous firing rate. Taken together these findings indicate that the commonly observed in vivo phenomena of NE augmentation of GABA and suppression of LH neuron spontaneous firing can be demonstrated in vitro, and most likely result from activation of beta adrenoceptors and subsequent elevation of cyclic AMP levels.(ABSTRACT TRUNCATED AT 400 WORDS)

8-Bromo Cyclic Adenosine Monophosphate↗

PubMed1988

New evidence for a gating action of norepinephrine in central neuronal circuits of mammalian brain.

Many previous studies have examined the effects of norepinephrine (NE) on neuronal responsiveness to synaptic inputs and putative transmitter substances and have described differential depressant actions of NE on stimulus evoked versus spontaneous discharge such that the "signal to noise" ratio of threshold responses was increased. In the present studies, similar experimental strategies employing a combination of microiontophoresis, single unit recording and afferent pathway stimulation in intact anesthetized and brain tissue slice preparations have revealed noradrenergic "gating" actions whereby weak or subthreshold synaptic stimuli can evoke threshold neuronal responses in the presence of iontophoretically applied NE or following electrical stimulation of the locus coeruleus. Overall, these results suggest that potentially threshold excitatory and inhibitory synaptic inputs may normally arrive at central neurons but appear weak or absent except during behavioral conditions favoring the synaptic release of NE. As such, these findings provide evidence that signal to noise ratio may not be the only potential modulatory action expressed by NE in noradrenergic target circuits of the mammalian brain.

8-Bromo Cyclic Adenosine Monophosphate↗

PubMed1987

Sex steroid effects on extrahypothalamic CNS. I. Estrogen augments neuronal responsiveness to iontophoretically applied glutamate in the cerebellum.

The purpose of this study was to test whether 17 beta-estradiol (E2) could alter neuronal activity or responsiveness to iontophoretically applied amino acid neurotransmitters in an area not reported to contain classical E2 receptors. Such a region is the cerebellum, which was selected as a model system for these studies because it has been well characterized electrophysiologically. Extracellular activity of cerebellar Purkinje neurons was recorded from urethane-anesthetized, adult, ovariectomized rats using multibarrel glass micropipets. Spontaneous firing rate and responses of single units to microiontophoretic pulses (10 s pulses every 40 s) of GABA (10-50 nA) or glutamate (GLUT, 3-40 nA) were examined before, during and after iontophoretic (0.25 mM 17 beta-estradiol hemisuccinate) or jugular i.v. (100, 300 or 1000 ng/kg 17 beta-estradiol) administration of E2. Both modes of E2 administration resulted in a significant increase in Purkinje cell excitatory responses to GLUT, independent of the direction of change in spontaneous firing rate. This effect was seen as early as one minute after iontophoretic application of E2 and 10-40 min following i.v. E2. In all cases, recovery to the control level of response was not observed by 2 h following E2 administration. 17 alpha-E2 (300 ng/kg) resulted in a less pronounced, transient increase in GLUT response, while a lower dose (100 ng/kg) did not have any effect. Prior administration of the anti-estrogen tamoxifen did not prevent any of the observed E2 effects. In addition, estrogen-priming did not alter E2-induced potentiation of GLUT responses. In contrast to the effect of E2 on GLUT responsiveness, GABA-mediated inhibition of Purkinje cells was either increased, antagonized or unchanged following E2 application. In summary, this study suggests the hypothesis that circulating levels of E2 may alter neuronal sensitivity to specific neurotransmitter substances within the cerebellar circuitry.

Action Potentials↗

PubMed1987

Sex steroid effects on extrahypothalamic CNS. II. Progesterone, alone and in combination with estrogen, modulates cerebellar responses to amino acid neurotransmitters.

In a preliminary report we have shown that both intravenous and local application of progesterone (P) are capable of increasing cerebellar Purkinje cell responsiveness to microiontophoretically applied gamma-aminobutyric acid (GABA) and decreasing responsiveness to glutamate (GLUT) in the urethane-anesthetized, ovariectomized adult rat. In the present study we have examined the time course of effects of several doses of P and different combinations of both E2 and P on responses of individual Purkinje cells to GABA and GLUT. Extracellular activity of single Purkinje neurons was recorded using multibarrel glass micropipets. Spontaneous firing rate and responses of neurons to microiontophoretic pulses (10 s pulses every 40 s) of GABA (10-50 nA) and GLUT (3-40 nA) were examined before and after jugular i.v. administration of P or E2/P combinations to ovariectomized rats. In some cases animals received s.c. injections of E2 (2 micrograms) at 24 and 48 h before the day of recording. This injection schedule results in maximal reproductive effects of P. Within 5-15 min after P administration (5,50 or 500 micrograms) to ovariectomized rats, Purkinje cell responses to GLUT were decreased by 87%, and inhibitory responses to GABA were increased by 50%, with no associated change in spontaneous firing rate. In addition, the magnitude of the change in amino acid response was directly proportional to the dose of P. In most cases, complete recovery was observed 20-45 min after P administration. E2 pretreatment did not alter these P-induced effects. Combinations of E2 (300 ng/kg) and P (50 or 500 micrograms) injected simultaneously resulted in effects on GLUT responsiveness which were similar to those seen with P alone, while effects similar to E2 alone were observed with administration of E2 plus P at 5 micrograms. The administration of a protein synthesis inhibitor, anisomycin (30 mg/kg, i.v.), 20 min before the recording session did not prevent any of the above steroid effects. These results indicate that sex steroids can act to alter neuronal responsiveness to putative neurotransmitters in a CNS region not known to contain steroid receptors and that the particular combination of steroids will determine the neuronal response. These findings further suggest that the observed steroid-induced alterations in Purkinje cell responsiveness do not appear to require genomic mechanisms.

Action Potentials↗

PubMed1987

Progesterone alters GABA and glutamate responsiveness: a possible mechanism for its anxiolytic action.

In this study, the neuromodulatory effects of progesterone were tested in an intact neuronal circuit of a model extrahypothalamic CNS area. Spontaneous discharge and responses of single cerebellar Purkinje neurons to microiontophoretically applied gamma-aminobutyric acid (GABA) and glutamate were monitored before, during and after either systemic injection, at physiologic doses, or local application of the steroid. By both means of administration, progesterone significantly enhanced inhibitory responses of Purkinje cells to GABA and suppressed glutamate excitation within 3-10 min post-steroid. These results are consistent with the anxiolytic actions of the steroid.

Animals↗

PubMed1987

Locally applied progesterone metabolites alter neuronal responsiveness in the cerebellum.

Ongoing studies in this laboratory have demonstrated that both systemically and locally administered sex steroids 17 beta estradiol (E2) and progesterone (P) alter cerebellar Purkinje cell responses to microiontophoretically applied amino acid neurotransmitters GABA and glutamate (GLUT) in the urethane-anesthetized, ovariectomized adult rat. In the present study, we have examined the effects of several locally pressure ejected P metabolites on Purkinje cell responsiveness to GABA and GLUT: 5 alpha-pregnane-20-one (5 alpha DHP), 5 alpha-pregnane-3 alpha-ol-20-one (3 alpha OH-DHP) and 5 alpha-pregnane-3 beta-ol-20-one (3 beta OH-DHP). GABA-induced inhibition was markedly enhanced immediately after onset of local application of 3 alpha OH-DHP or 5 alpha DHP, unaccompanied by alterations in background discharge. Both metabolites also attenuated excitatory responses to GLUT by 0-3 min after initiation of steroid application. In both cases, recovery to control levels of response was observed 6-9 min after termination of pressure application. These results are similar to those seen after local or systemic injection of P. In contrast, 3 beta OH-DHP did not produce any alteration in Purkinje cell responses to either amino acid. As 5 alpha DHP and 3 alpha OH-DHP can be localized in cerebellar tissue after P administration, the results presented here suggest that the neuronal effects of systemic P may be mediated by local membrane actions of P or its metabolites.

Animals↗