Electrophysiological and pharmacological characterization of identified nigrostriatal and mesoaccumbens dopamine neurons in the rat

The 5-HT1A receptor selective ligands, (R)-8-OH-DPAT and (S)-UH-301, differentially affect the activity of midbrain dopamine neurons

The effects of the selective 5-HT1A receptor agonist (R)-8-hydroxy-2(di-n-propylamino)tetralin [(R)-8-OH-DPAT] and the novel 5-HT1A antagonist (S)-5-fluoro-8-hydroxy-2-(dipropylamino)-tetralin [(S)-UH-301] were studied with regard to the firing pattern of single mesencephalic dopamine (DA) neurons with extracellular recording techniques in chloral hydrate anesthetized male rats. Neuronal activity was studied with respect to firing rate, burst firing and regularity of firing. In the ventral tegmental area (VTA) low doses of (R)-8-OH-DPAT (2-32 micrograms/kg i.v.) caused an increase in all three parameters. The effect on firing rate of DA neurons was more pronounced in the parabrachial pigmentosus nucleus than in the paranigral nucleus, the two major subdivisions of VTA. In the substantia nigra zona compacta (SN-ZC), (R)-8-OH-DPAT (2-256 micrograms/kg i.v.) had no effect on firing rate and regularity of firing and only slightly increased burst firing. High doses of (R)-8-OH-DPAT (512-1024 micrograms/kg i.v.) decreased the activity of DA cells in both areas, an effect that was prevented by pretreatment with the selective DA D2 receptor antagonist raclopride. (S)-UH-301 (100-800 micrograms/kg i.v.) decreased both firing rate and burst firing without affecting regularity of DA neurons in the VTA. In the SN-ZC, (S)-UH-301 decreased the firing rate but failed to affect burst firing and regularity of firing. These effects of (S)-UH-301 were blocked by raclopride pretreatment. Local application by pneumatic ejection of 8-OH-DPAT excited the DA cells in both the VTA and the SN-ZC, whereas (S)-UH-301 inhibited these cells when given locally. These results show that 5-HT1A receptor related compounds differentially affect the electrophysiological activity of central DA neurons. The DA receptor agonistic properties of these compound appear to contribute to the inhibitory effects of high doses of (R)-8-OH-DPAT and (S)-UH-301 on DA neuronal activity. Given the potential use of 5-HT1A receptor selective compounds in the treatment of anxiety and depression their effects on central DA systems involved in mood regulation and reward related processes are of considerable importance.

Iontophoretically administered drugs acting at the N-methyl-D-aspartate receptor modulate burst firing in A9 dopamine neurons in the rat

Extracellular single-unit recording and iontophoresis were used to examine the effect of N-methyl-D-aspartate (NMDA) and the competitive NMDA antagonist (+/-)-4-(3-phosphonopropyl)-2-piperazine carboxylic acid (CPP) on the firing rate and firing pattern of A9 dopamine (DA) neurons in the rat. Administration of NMDA produced a dose-dependent increase in firing rate (up to nearly 300% of baseline at the highest ejection current), which could be blocked by iontophoretic CPP. Low currents (less than 10 nA) were sufficient to induce apparent depolarisation inactivation in some neurons. In addition to this effect on firing rate, NMDA also caused a dramatic increase in burst firing, which was also dose dependent; cells made more bursts, and each burst consisted of more spikes. The only measured aspect of burst morphology that was not affected was the mean burst interspike interval. All nonbursting cells (n = 10) were converted to burst firing by the drug. CPP administered alone was found to reduce burst firing, without affecting the firing rate. These data suggest that a tonically active excitatory amino acid input to A9 DA neurons is responsible for inducing burst firing in vivo and that this input seems to operate via the NMDA receptor, possibly by virtue of its link to a Ca2+ ionophore.

Midbrain dopamine system electrophysiological functioning: a review and new hypothesis

The pathogenesis of a variety of neurological and mental disorders has been attributed to the malfunctioning of central dopaminergic systems. Twenty years of investigation concerning the way that dopamine systems function in the brain has yielded a great deal of information about the control and behavior of these systems. This collection of seemingly disparate facts is reviewed. A hypothesis is then presented that attempts to synthesize from these facts a view of the electrophysiological functioning of midbrain dopamine systems that may be helpful in understanding brain mechanisms underlying the pathology of such disorders as Parkinson's disease and schizophrenia.

Receptor selectivity of cholecystokinin effects on mesoaccumbens dopamine neurons

Extracellular recording techniques were combined with antidromic stimulation to examine the effects of C-terminal cholecystokinin (CCK) fragments and CCK antagonists on the activity of identified mesoaccumbens dopamine (MADA) neurons in chloral hydrate-anesthetized rats. These experiments were designed to determine the receptor selectivity of sulfated CCK octapeptide (CCK-8S) effects on MADA cells. Neither CCK tetrapeptide (CCK-4) nor unsulfated CCK octapeptide (CCK-8U) significantly altered MADA cell basal firing rate or responsiveness to the inhibitory effects of the D2 DA agonist quinpirole. As reported previously for ventral tegmental area DA cells, CCK-8S produced increases or decreases in the firing rate of most MADA cells sampled. CCK-8S also enhanced the sensitivity of MADA neurons to quinpirole-induced inhibition. This increase in sensitivity to quinpirole was blocked by pretreatment with the nonselective CCK receptor antagonist proglumide and the preferential CCK-A receptor antagonist CR 1409 but not by the preferential CCK-B receptor antagonist L-365,260. The inactivity of CCK-4 and CCK-8U in these tests and the results with the antagonists suggest that the effects of CCK-8S on MADA neuronal activity are mediated by CCK-A receptors.

Inhibition by talipexole, a thiazolo-azepine derivative, of dopaminergic neurons in the ventral tegmental area

A microiontophoretic study using rats anesthetized with chloral hydrate and immobilized with gallamine triethiodide was carried out to compare the effect of talipexole (B-HT 920 CL2:2-amino-6-allyl-5,6,7,8-tetrahydro-4H-thiazolo [4,5-d]-azepine-dihydrochloride), a dopamine autoreceptor agonist, on dopaminergic neurons in the ventral tegmental area (VTA) to non-dopaminergic neurons in the VTA. VTA neurons were classified into two types according to the responses to antidromic stimulation of the nucleus accumbens (Acc): type I neurons with a long spike latency (8.69 +/- 0.24 msec) upon Acc stimulation and low spontaneous firing rate (6.80 +/- 1.34/sec), and type II neurons with a short latency (2.76 +/- 0.20 msec) and high spontaneous firing rate (26.77 +/- 7.05/sec), probably corresponding to dopaminergic and non-dopaminergic neurons, respectively. In type I neurons, microiontophoretic application of talipexole and dopamine inhibited antidromic spike generation elicited by Acc stimulation, and talipexole-induced inhibition was antagonized by domperidone (dopamine D-2 antagonist). In type II neurons, however, the antidromic spikes were not affected by either talipexole or dopamine. Furthermore, spontaneous firing was also inhibited by iontophoretically applied talipexole and dopamine in most type I neurons, but rarely affected by either drug. Inhibitory effects of talipexole were antagonized by domperidone. These results suggest that talipexole acts on dopamine D-2 receptors, thereby inhibiting the dopaminergic neurons in the VTA.

Effects of B-HT 920 on nigrostriatal and mesolimbic dopamine systems in normosensitive and supersensitive rats

1. B-HT 920, a D2 dopamine receptor agonist, was tested for its ability to exert presynaptic actions in normosensitive rats, and for possible postsynaptic actions in rats made 'supersensitive' to apomorphine. 2. In normosensitive rats, B-HT 920 (0.01-0.3 mg kg-1, i.p.) increased dopamine concentrations and lowered metabolite levels to a similar extent in all four terminal regions examined (medial prefrontal cortex, olfactory tubercle, nucleus accumbens, caudate-putamen). Analogous effects were seen for 5-hydroxytryptamine and its metabolite 5-hydroxyindoleacetic acid. 3. Rats which received bilateral 6-hydroxydopamine (6-OHDA) infusions into the caudate-putamen showed signs of postsynaptic dopamine receptor activation (stereotyped behaviour) in response to B-HT 920 (0.1 and 1.0 mg kg-1, i.p.) and to apomorphine (0.2 mg kg-1, s.c.). Similarly, B-HT 920 (0.1 mg kg-1) induced contralateral circling in rats that had received unilateral 6-OHDA infusions into the medial forebrain bundle; the rate of circling increased gradually over several weeks. 4. In contrast, bilateral 6-OHDA infusions into the nucleus accumbens resulted in a supersensitive (locomotor stimulant) response to a low dose of apomorphine (0.1 mg kg-1, s.c.), but not to B-HT 920 (0.01 and 0.1 mg kg-1). 5. In intact rats, withdrawal of chronic haloperidol treatment induced behavioural supersensitivity to apomorphine but not to B-HT 920.

Statistical analysis of dose-response curves in extracellular electrophysiological studies of single neurons

The application of polynomial regression and the analysis of covariance (ANCOVA) to dose-response (DR) data derived from extracellular electrophysiological studies of midbrain dopamine neurons and noradrenergic locus coeruleus neurons in vivo is demonstrated and discussed. Third-order polynomial regression was found to be a better method for estimating ED50 values than probit analysis of linear regression. ANCOVA provides a more powerful statistical method than ANOVA for detecting significant differences in ED50 values or DR curves when a confounding variable such as basal discharge rate is present. The methods of analysis presented herein should be useful in the analysis of other types of neurons in electrophysiological studies.

(+/-)-3,4-Methylenedioxymethamphetamine-induced changes in the basal activity and pharmacological responsiveness of nigrostriatal dopamine neurons

The present study examined the effects of methylenedioxymethamphetamine (MDMA) on the basal activity and pharmacological responsiveness of rat nigrostriatal dopamine (DA) neurons. Under standard in vivo extracellular single-unit recording conditions, acute MDMA administered alone (i.v.) inhibited the firing rate of nigrostriatal DA neurons in a dose-dependent fashion. The potency of MDMA to elicit this inhibition was significantly reduced following depletion of either serotonin or DA. Acute MDMA pretreatments (10 mg/kg i.v., 90 s) also profoundly enhanced the sensitivity of nigrostriatal DA neurons to the rate-inhibitory effects of the D-2 DA receptor agonist quinpirole but not apomorphine. It has previously been demonstrated that the ability of quinpirole and apomorphine to inhibit nigrostriatal DA neuronal activity is dependent on the basal firing rate of the cell. Both acute MDMA and a single dose of MDMA (15 mg/kg i.p.) one week prior eliminated the rate dependency of quinpirole- and apomorphine-induced inhibition of the firing rate of these cells. These data suggest that, although MDMA is known to be a serotonergic neurotoxin, this compound may also exert direct functional effects on the nigrostriatal DA system.

Chloral hydrate anesthesia alters the responsiveness of identified midbrain dopamine neurons to dopamine agonist administration

Single-unit electrophysiological recording techniques were used to sample the basal activity of antidromically identified nigrostriatal and mesoaccumbens dopamine (NSDA and MADA, respectively) neurons and to examine the responsiveness of these cells to dopamine agonist-induced inhibition of cell firing rate in either chloral hydrate-anesthetized or paralyzed rats. Paralyzed rats exhibited a greater percentage of burst-firing cells (69%) than did anesthetized animals (37%). Furthermore, paralyzed rats were less sensitive to the mixed D1/D2 DA receptor agonist apomorphine and the selective D2 DA receptor agonist quinpirole. However, significantly higher doses of d-amphetamine were required in paralyzed animals only with respect to inhibiting MADA neurons. The abilities of apomorphine and quinpirole to inhibit NSDA cell firing were rate-dependent in both anesthetized and paralyzed rats, whereas d-amphetamine-induced inhibition was rate dependent only in anesthetized animals. In contrast, apomorphine- and quinpirole-induced inhibition of MADA neurons were rate-dependent only in anesthetized rats, whereas d-amphetamine-induced inhibition was rate-dependent only in paralyzed animals. These results suggest that general anesthesia exerts subtle effects on the basal activity and pharmacological responsiveness of midbrain dopamine neurons.

Dopamine-containing neurons in the mammalian central nervous system: electrophysiology and pharmacology

A decade of research culminated in the late 1950's with the demonstration that dopamine was a chemical neurotransmitter within the mammalian brain. Since this time, dopaminergic neuronal systems have been extensively studied using numerous techniques. This paper will review the last 14 years of electrophysiological investigation on neurochemically identified dopamine-containing neurons in the central nervous system. This will include an examination of both the electrophysiological and pharmacological characteristics in these cells, as well as the resulting insights into the regulation of dopamine cell electrical activity which is derived from this work.