Manjit K. Sanghera

Buspirone, a non-benzodiazepine anxiolytic, increases locus coeruleus noradrenergic neuronal activity

It has been hypothesized that treatments which increase locus coeruleus (LC) noradrenergic neuronal activity produce anxiety, whereas treatments which decrease LC neuronal activity are anxiety-reducing. Although the benzodiazepine anxiolytic diazepam decreases LC neuronal impulse flow and norepinephrine metabolism, the non-benzodiazepine anxiolytic buspirone does the opposite. These data suggest that a reduction in LC output is not a necessary prerequisite for anxiolytic activity.

Projection of medal basal hypothalamic neurones to the preoptic anterior hypothalamic areas and the paraventricular nucleus in the rat

Abstract Electrophysiological techniques have been used to study the projections of medial basal hypothalamic neurones. Neurones whose cell bodies were situated in the arcuate, ventromedial and dorsal medial nuclei and the periventricular area were antidromically activated by stimuli applied to the preoptic and anterior hypothalamic areas. Neurones were also identified in the same regions following stimulation of the paraventricular nucleus. These neurones were often found adjacent to other neurones which could be antidromically activated only from the junction of the pituitary stalk with the median eminence, but a few neurones were activated from both the median eminence and the more rostral regions. The latencies of these rostral projections were in the same range as those for the median eminence projections and calculated conduction velocities were below 1 m/sec. The possible neuroendocrine significance of these projections is discussed.

Dopamine transporter mRNA levels are high in midbrain neurons vulnerable to MPTP

THE neurotoxin MPTP kills only certain midbrain dopaminergic (DA) neurons to produce a model of Parkinsons disease. The dopamine transporter (DAT) is important to MPTP toxicity because to be neurotoxic, an MPTP metabolite must first gain access to the DA neuron via the DAT. Also, MPTP is less toxic to DA neurons that contain the putative neuroprotective calcium-binding protein calbindin-D28k (CB). The present study examined the relative importance of DAT activity and CB for cellular vulnerability to MPTP-induced degeneration in the C57BL/6 mouse. Cells that were vulnerable to MPTP were found to contain high levels of DAT mRNA, whereas cells that were not vulnerable contained low levels. Also, the few substantia nigra cells remaining after a toxic dose of MPTP contained only low levels of DAT mRNA. However, there was not a strong relationship between cellular resistance to MPTP toxicity and cells containing CB. These data provide in vivo evidence for a direct correlation between midbrain cellular vulnerability to MPTP toxicity and the activity of the DAT.

Low dopamine transporter mRNA levels in midbrain regions containing calbindin

The dopamine transporter (DAT) is the site at which the neurotoxic metabolite of MPTP gains access to midbrain dopaminergic (DA) neurons. However, not all midbrain DA neurons degenerate following MPTP treatment. The midbrain DA neurons that contain the calcium-binding protein, calbindin-D28k (CALB), are relatively invulnerable to MPTP toxicity, compared with DA neurons that lack CALB. Using in situ hybridization and immunocytochemical staining techniques in the rat and mouse, we now report that there is as much as 10 fold less DAT mRNA in regions where DA neurons contain CALB compared with regions where DA neurons lack CALB. These data suggest that specific midbrain DA neurons are invulnerable to MPTP toxicity not only because they contain CALB, but also because they have relatively low DAT activity.

Effect of Medial Zona Incerta Lesions on the Ovulatory Surge of Gonadotrophins and Prolactin in the Rat

Bilateral electrolytic lesions of the incertohypothalamic A13 dopamine (DA) system were made on the morning of proestrus in female rats with regular estrous cycles under ether anesthesia. Hourly blood samples were withdrawn, via a jugular catheter from conscious, freely moving rats, between 14.00 h and 18.00 h on the afternoon of proestrus. Plasma LH, FSH and prolactin levels were determined by RIA. The preovulatory surges of LH and prolactin were blocked in animals in which the A13 DA nucleus was destroyed by more than 70%. Levels of FSH were not significantly different from those of control of sham-lesioned groups. Lesions that were dorsal, ventral or caudal to the A13 DA systems did not affect the preovulatory surges of LH and prolactin whereas anterior lesions caused variable changes in the level of all three hormones. These data suggest that the A13 DA region may have a role in the control of the preovulatory surges of LH and prolactin.

Ipsapirone and 1-(2-pyrimidinyl)-piperazine increase rat locus coeruleus noradrenergic activity

The effects of systemically administered ipsapirone, an aryl-piperazine compound, and its major metabolite 1-(2-pyrimidinyl)-piperazine (1-PP), on locus coeruleus (LC) noradrenergic activity was investigated. On an equimolar basis both ipsapirone and 1-PP were approximately equipotent in increasing LC neuronal activity. However, pretreatment with 1-PP caused a significantly greater parallel shift to the right of the dose response curve for the inhibitory action of the LC alpha 2-receptor agonist clonidine compared to ipsapirone. Biochemically, pretreatment with SKF 525A, a compound which prevents the formation of 1-PP from ipsapirone, diminished the ipsapirone-induced increase in MOPEG-SO4 levels in the brainstem and cortex. These data, together with the findings that 1-PP is more potent than ipsapirone in displacing 3H-clonidine from cerebral cortical membranes, suggest that the parent drug influences LC neuronal activity via the action of I-PP on LC alpha 2-adrenoceptors.

Measurement of 3-methoxytyramine by in vivo voltammetry: evidence for differences in central dopamine function in BALB/c and CBA mice

Differential pulse voltammetry (DPV) combined with carbon fibre electrodes allows selective detection of electroactive dopamine and serotonin metabolites in vivo. While usually employed in rats, we have now applied this in vivo technique in two inbred strains of mice: BALB/c and CBA. Three distinct oxidation peaks were recorded in vivo in the striatum of either BALB/c or CBA mice with a small shoulder occurring after the third peak at approximately +400 mV. Pargyline (150 mg/kg i.p.) potentiated this voltammetric shoulder into an easily measurable peak (Peak 4). In addition, Peak 4 was 2-3 times larger in BALB/c than in CBA mice. Homovanillic acid (HVA) and 3-methoxytyramine (3-MT), both catabolites of dopamine, oxidised at approximately +400 mV in vitro. Brain tissue levels of HVA and 3-MT, measured by high-performance liquid chromatography (HPLC) with electrochemical detection, demonstrated that pargyline treatment reduced striatal HVA, but increased 3-MT. These results support the view that Peak 4 recorded in the striatum of pargyline-treated mice in vivo is due to the oxidation of extracellular 3-MT. Thus, Peak 4 may be a useful index of dopamine release in situations where dopamine itself cannot be detected. Local infusion of KCl (2 microliters, 0.1 M) further increased the size of Peak 4 in the striatum of both BALB/c and CBA mice. However, the increase was approx. 3 times greater in BALB/c mice, supporting previous evidence of greater dopaminergic function of BALB/c compared with CBA mice. In addition these two inbred strains of mice provide model systems for investigating the comparative functional roles of nigrostriatal pathways.

Incertohypothalamic A13 Dopamine Neurons: Effect of Gonadal Steroids on Tyrosine Hydroxylase

In this study the effect of gonadectomy and steroid treatment on the dorsal component of the incertohypothalamic dopamine system or nucleus A13 was assessed by immunocytochemistry using an antibody raised to tyrosine hydroxylase (TH). A computer graphic system interfaced to a microscope was used to count and measure the diameters of TH-positive neurons and display the data in the three-dimensional space of the nucleus. In males, castration resulted in a dramatic decrease in the reaction product representative of TH. The number of TH-positive cells in the A13 DA nucleus decreased to 25% of intact levels. In females, ovariectomy also caused an impressive loss in the TH-immunostainable material, but this was not indicated by a change in the total number of TH-positive neurons. In both sexes the loss in TH immunostain was confined mainly to the mid-portion of the nucleus. Hormone treatment restored the TH immunostain (cell number and size) to and/or above intact levels in both sexes. These data suggest that A13 TH immunostain is stimulated by gonadal steroids in male and female rats.

Biochemical and in vivo voltammetric evidence for differences in striatal dopamine levels in inbred strains of mice

High performance liquid chromatography with electrochemical detection and differential pulse voltammetry were used to provide a direct measurement of tissue content of dopamine and its metabolites and extracellular dopamine levels, respectively, in the striata of BALB/c and CBA inbred strains of mice. We found that levels of striatal dopamine and its metabolite, dihydroxyphenylacetic acid, were significantly higher in the CBA strain than in the BALB/c strain, whereas levels of homovanillic acid were not significantly different between the strains. Levels of the dopamine metabolite 3-methoxytyramine, on the other hand, were higher in the BALB/c mice. Dopamine turnover rates were significantly higher in the CBA strain when the homovanillic acid/dopamine ratio was used as an index of dopamine activity. Voltammetric recording showed that the local infusion of K+ in pargyline-treated mice resulted in the immediate appearance of a peak at +85 mV, which has been shown to correspond to extracellular dopamine in the rat. The mean height of this peak detected in vivo following K+ stimulation corresponds to in vitro dopamine concentrations of 25 +/- 8 microM for BALB/c mice and 7 +/- 2 microM for CBA mice. K(+)-stimulated dopamine release in the BALB/c mice could be evoked every 10-15 min with similar magnitude. In contrast, very little dopamine release in CBA mice could be evoked after the first stimulation. Since striatal dopamine levels are higher in CBA mice, these data suggest that (a) BALB/c strain may have more dopamine in the readily releasable pool, whereas the CBA mice have a larger storage pool of dopamine, and/or (b) that dopamine uptake in the CBA mice is much more avid than in BALB/c.

Electrophysiological and pharmacological properties of putative A13 incertohypothalamic dopamine neurons in the rat.

A13 incertohypothalamic dopamine (DA) neurons were labelled with antibodies raised to tyrosine hydroxylase in the male rat. Electrophysiologically, these neurons could be distinguished from their neighboring non-DA cells by their wide action potentials (greater than 2 ms), slow firing rates (0-3.8 impulses/s) and by the ability of iontophoresed DA and systemically administered apomorphine to inhibit impulse flow. Low doses of the antipsychotic drug haloperidol attenuated DAs response and reversed the apomorphine-inhibition of impulse flow.