A10 Dopamine Cells Research Articles

Chronic BRL 43694, a selective 5-HT 3 receptor antagonist, fails to alter the number of spontaneously active midbrain dopamine neurons

In this study, we examined the effect of chronic administration of the selective 5-HT 3 receptor antagonist BRL 43694 on the number of spontaneously active A9 (substantia nigra pars compacta) and A10 (ventral tegmental area) dopamine (DA) cells using the technique of extracellular single unit recording. Overall, chronic BRL 43694 (21 days, 5 mg/kg per day or 2.5 mg/kg q.i.d.) failed to alte teh number of spontaneously active DA cells in either area compared to saline-injected controls. Furthermore, the acute i.v. administration of BRL 43694 did not change A10 DA cell baseline firing rate. These data suggest that if BRL 43694 possesses antipsychotic activity, it is not the result of its action on midbrain DA neurons.

Dopamine agonists at repeated "autoreceptor-selective" doses: effects upon the sensitivity of A10 dopamine autoreceptors.

Previous reports have established the ability of dopamine (DA) agonists to stimulate inhibitory DA autoreceptors at doses which minimally stimulate postsynaptic DA receptors, suggesting that hyperactive DA transmission may be controlled clinically by treatment with DA agonists. Little is known, however, about the possible loss of autoreceptor sensitivity that may occur after repeated treatment with low doses of DA agonists. Extracellular single cell recording and microiontophoretic techniques were used to measure the sensitivity of impulse-regulating DA autoreceptors on A10 DA cells in the ventral tegmental area (VTA) of chloral hydrate-anesthetized rats pretreated for seven days with repeated subcutaneous (s.c.) doses of the DA agonist apomorphine (APO). The ability of intravenous (i.v.) administration of the potent D2 DA agonist quinpirole (QUIN) to inhibit the firing of A10 cells was not attenuated in rats pretreated with repeated low doses (2 x 50 micrograms/kg/day, s.c.) of APO for 7 days, although higher doses (2 x 250 or 500 micrograms/kg/day) did cause subsensitive responses to QUIN. In rats pretreated with repeated low doses of APO, microiontophoretic application of DA on A10 cells revealed somewhat subsensitive responses. However, ibotenic acid lesions of postsynaptic cells in the nucleus accumbens (NAc) prior to initiation of APO treatment (2 x 50 micrograms/kg/day) did not alter the response of A10 cells to systemic QUIN, contradicting the possibility that the feedback projection from the NAc to the VTA was compensating for autoreceptor down-regulation during systemic challenge with QUIN. In contrast, administration of the irreversible DA antagonist EEDQ (2 mg/kg, i.p.) to control and APO-treated rats (2 x 50 micrograms/kg/day) 24 hr prior to recording did reveal a difference in A10 cell sensitivity to systemic QUIN and to microiontophoretic DA between the two groups, suggesting that "spare" DA autoreceptors may have concealed the down-regulation of autoreceptors induced by repeated low doses of APO. Challenge of A10 DA cells with the partial DA autoreceptor agonist (-)-3-(3-hydroxyphenyl)-N-n-propylpiperidine [(-)3-PPP], for which an autoreceptor reserve should not exist, produced slightly attenuated responses in APO-treated rats (2 x 50 micrograms/kg/day). These findings provide evidence for the existence of spare somatodendritic DA autoreceptors on A10 DA cells with respect to potent DA agonists, suggesting that repeated administration of "autoreceptor-selective" doses of DA agonists may not result in a diminished inhibition of DA neuronal activity.

Spontaneous activity of A9 and A10 dopamine neurons after acute and chronic administration of the selective dopamine D-1 receptor antagonist SCH 23390

The effects of acute and chronic treatment with the selective dopamine (DA) D-1 antagonist SCH 23390 on the population response of midbrain DA cells were determined. One hour pretreatment with SCH 23390 (0.0125, 0.025, 0.05 mg/kg s.c.) caused a dose-related increase in the number of spontaneously firing DA neurons in both the A9 and A10 cell regions. Chronic (28 day) administration of SCH 23390 (0.05 mg/kg s.c.) caused depolarization inactivation of only A10 DA cells. These data suggest that SCH 23390 may have antipsychotic properties with a reduced likelihood of producing extrapyramidal side effects.

Firing patterns of midbrain dopamine neurons: differences between A9 and A10 cells.

Dopamine neurons of the substantia nigra (A9) and the ventral tegmental area (A10), giving rise to the nigrostriatal and mesolimbic dopamine pathways, respectively, are commonly supposed to show similar electrophysiological activity. There are, however, instances where the two systems are differently modulated. To assess possible physiological differences in the neuronal activity of A9 and A10 neurons, randomly sampled single cells were extracellularly recorded in the chloral hydrate-anaesthetized male rat. In addition to firing rate, the degree of burst firing and the regularity of firing were quantitatively analysed. Our results show that although A9 and A10 do not differ in firing rates, A10 neuronal activity is markedly less regular and shows a higher degree of burst firing, as judged from analysis of inter-spike time interval histograms. Mean burst firing values were 3% for the A9, and 23% for the A10 neurons. Regularity was described by variation coefficients of inter-spike time interval histograms. The mean variation coefficient was 38.4% in the A9 group and 63.8% in the A10 group, i.e. the A10 neuronal firing was less regular. The difference in regularity is partly, but not fully, dependent on the difference in burst firing. Previous biochemical and physiological studies strongly support the functional significance of modulatory changes in midbrain dopamine-cell firing patterns. Since the firing pattern of midbrain dopamine cells seems to be controlled by synaptic inputs, our results indicate a higher tonic modulatory influence on the A10 than on the A9 neurons. Thus the present results imply the pharmacological possibility of preferentially affecting A10 versus A9 dopamine cell function.

New potential antipsychotic drug BMY 20661: A profile of its effects on midbrain dopamine neurons

AbstractThe new potential antipsychotic drug BMY 20661 was subjected to a series of electrophysiological tests to determine its effects on midbrain dopamine (DA) neurons in the rat substantia nigra pars compacta (A9) and ventral tegmental area (A10). Acute intravenous (i.v.) administration of BMY 20661 exerted little effect on the basal spontaneous firing rates of either A9 or A10 DA neurons. Following suppression of DA unit activity by the DA agonist apomorphine (APO), BMY 20661 consistently increased the firing of A10, but not A9, DA cells. However, the rates of A10 DA cells seldom recovered to pre‐APO basal levels following BMY 20661 administration. The ability of BMY 20661 to partially reverse the APO‐induced rate suppression was apparently not due to blockade of DA receptors since pretreatment with BMY 20661 (6.3 mg/kg, i.v.) only slightly (nonsignificantly) reduced the ability of APO to inhibit the firing of A10 cells. Surprisingly, BMY 20661 significantly attenuated the effects of APO on A9 DA cells. These findings suggest that BMY 20661 influences A10 DA neurons by a non‐DA mechanism. Because BMY 20661 has been reported to bind to 5‐HT1A receptors, the acute effects of this compound were compared to those of 8‐hydroxy‐2‐(di‐n‐propylamino)tetralin (8‐OH‐DPAT), a known 5HT1A agonist. These compounds demonstrated similar profiles in their ability to reverse APO‐induced suppression of A10 DA cells but, unlike BMY 20661, 8‐OH‐DPAT was also active on A9 DA cells. Repeated administration of BMY 20661 for 28 days (either 5 or 10 mg/kg/day) failed to alter significantly the number of spontaneously active A10 DA cells but caused a small, but statistically significant, increase in the number of A9 DA cells/track. Because the ability to decrease the number of spontaneously active A10 DA cells has been proposed as a predictive model for antipsychotic drug action, the results obtained following repeated administration of BMY 20661 fail to support a potential antipsychotic profile for this new compound.

Inhibition of non-dopamine cells in the ventral tegmental area by benzodiazepines: relationship to A10 dopamine cell activity

Previous electrophysiological studies have demonstrated that non-dopaminergic (non-DA) neurons within the substantia nigra pars reticula (SNR) are extremely sensitive to the inhibitory effects of GABA and GABA-mimetic drugs, including benzodiazepines, whereas dopaminergic (DA) neurons in the substantia nigra pars compacta (SNC) are less sensitive to these compounds and may be influenced indirectly by SNR neurons. The interactions between A10 DA and non-DA neurons within the adjacent ventral tegmental area (VTA) are not as well characterized. In the present experiments, single unit recording and microiontophoretic techniques were used to determine the effects of benzodiazepines on DA and non-DA neurons in the VTA of chloral hydrate anesthetized rats. Diazepam, administered intravenously (i.v.), potently inhibited non-DA, SNR-like cells within the VTA. The effects of diazepam on A10 DA cells were more variable than those observed on non-DA, SNR-like cells in this region, but 77% of such cells showed moderate to marked excitation. Both of these effects were reversed by the benzodiazepine antagonist Ro 15-1788; on many cells, this agent produced marked rebound effects beyond the original basal firing rates. However, when administered alone, Ro 15-1788 exerted no effect on either cell population. Microiontophoretic administration of the benzodiazepines chlordiazepoxide and flurazepam resulted in marked inhibition of non-DA SNR-like cells, but produced either mild inhibition or no effect on A10 DA cells; excitation of DA cells was never observeb even though the same neuron was excited by i.v. diazepam. These findings suggest that benzodiazepines act directly upon non-DA, SNR-like cells in the VTA to produce inhibition of activity and a disinhibition of A10 DA cells. This relationship makes it unlikely that benzodiazepines would enhance feedback inhibition of DA cells following neuroleptic administration. In fact, when administered following haloperidol, i.v. diazepam failed to reverse haloperidol-induced increases of A10 DA cell firing; if anything, diazepam further depolarized the cell. If antipsychotic drugs produce their clinical effects, in part, by including depolarization inactivation of DA cells, then benzodiazepines might be a useful adjunctive therapy in the treatment of schizophrenia.

Organization of ventral tegmental area cells projecting to the occipital cortex and forebrain in the rat

Our horseradish peroxidase retrograde tracing study revealed a specific subpopulation of ventral tegmental area (VTA) neurons that send axons to the occipital cortex in the rat. A fluorescent retrograde tracing study demonstrated that neuronal populations in the VTA projecting to the occipital cortex are distributed in a manner separate from those projecting to forebrain structures such as the frontal/anterior cingulate cortices and nucleus accumbens. The scarcity of collateral projections from the VTA contrasts with the extensive collateralization of projection neurons in the substantia nigra pars compacta. Projections to the occipital cortex may define the distribution of cells comprising the VTA and thus the clear hodological separation of the A9 and A10 dopamine cell groups.

Differential effects of acute clozapine and haloperidol on the activity of ventral tegmental (A10) and nigrostriatal (A9) dopamine neurons

Extracellular single-unit recording techniques were used to evaluate the effects of acute intravenous (i.v.) clozapine (CLOZ) and haloperidol (HAL) on the activity of dopamine (DA) neurons of the ventral tegmental area (VTA or A10) and the substantia nigra pars compacta (SNC or A9). CLOZ increased the firing rate of A10 but not A9 DA cells, and drove 9/23 (39%) of A10 cells into an apparent depolarization blockade. HAL, on the other hand, produced a rate elevation and, at higher doses, depolarization inactivation in both subpopulations of DA neurons. Cell firing was restored in inactivated cells with i.v. apomorphine (APO) or iontophoretic GABA. CLOZ always fully reversed APO-induced suppression of A10 DA activity, but in many cases only partially reversed suppression of A9 DA neurons. Scopolamine did not mimic the effects of CLOZ on A10 neurons, and it also failed to block the activating effect of HAL on A9 units, indicating that the selective action of CLOZ cannot be interpreted simply by its anticholinergic properties. After hemi-transections of the diencephalon, which severed the medial forebrain bundle and other feedback pathways to the DA somata, CLOZ was still ineffective in altering A9 DA activity. This suggests that the lack of effect of CLOZ on A9 cells is not due to the inhibitory influence of forebrain feedback pathways. This hemi-transection also left intact the activation of A10 neurons produced by HAL and CLOZ, but it did prevent the excitatory action of HAL on most A9 neurons sampled. This indicates that forebrain feedback pathways are less critical in mediating the action of APDs on A10 DA neurons. Finally, iontophoretic application of CLOZ and HAL into the vicinity of DA cell bodies blocked the rate-reducing effects of locally applied DA, but not those of GABA. This suggests that both APDs block somatodendritic DA autoreceptors. However, HAL was considerably more potent than CLOZ in producing this blockade. It is suggested that the different pharmacological and clinical properties of HAL and CLOZ may be partially explained by a differential mode of action on the A10 and A9 subpopulations of DA cells. The data also provide pharmacological evidence that these 2 groups of DA cells are regulated by different mechanisms.

Effects of acute thioridazine, metoclopramide and SCH 23390 on the basal activity of A9 and A10 dopamine cells

Extracellular single-unit recording techniques were employed to examine the effects of various dopamine (DA) antagonists on the basal activity of spontaneously active DA cells. Metoclopramide and thioridazine were both effective in reversing apomorphine-induced suppression of A9 and A10 DA cells. SCH 23390 produced only a partial reversal of this suppression. When the antagonists were given without any pretreatment, thioridazine preferentially increased the firing rate of A10 DA cells, and was relatively ineffective in altering A9 activity. Metoclopramide, on the other hand, increased the activity of most A9 DA cells, but was less effective in doing so with A10 cells. SCH 23390 did not significantly affect the basal activity of either cell subpopulation. These data support the hypothesis that the so-called ‘atypical” antipsychotic drugs act preferentially on the A10 DA system. Taken together with previous results, they also suggest that the acute effects of DA antagonists on DA cell subpopulations coincide with their chronic effects.

Calcitonin gene-related peptide in the ventral tegmental area: selective modulation of prefrontal cortical dopamine metabolism.

The distribution of calcitonin gene-related peptide (CGRP) in the A10 dopamine (DA) cell group region of the ventral tegmental area (VTA) of the rat was examined using immunohistochemical techniques. CGRP-like immunoreactivity was localized to axons innervating the rostral and dorsal VTA. Direct administration of CGRP to the VTA of the rat resulted in a dose-related increase in DA utilization in the medial prefrontal cortex, but not other mesocortical, mesolimbic, or striatal DA terminal field regions. These data suggest that CGRP may function to selectively modulate the activity of VTA dopaminergic neurons which innervate the prefrontal cortex.

Acute and subchronic effects of Rimcazole (BW 234U), a potential antipsychotic drug, on A9 and A10 dopamine neurons in the rat

The effects of acute and subchronic Rimcazole administration on A9 and A10 dopamine (DA) neurons were examined using extra-cellular single cell recording techniques. Intravenous injections of Rimcazole did not prevent or reverse the inhibition of firing rates of DA cells produced by DA agonist apomorphine (APO). Single intraperitoneal injection of Rimcazole decreased the number of spontaneously active DA cells in A10, but not in A9; it had no effect on the firing rate of DA neurons in either A9 or A10. Following prolonged administration of Rimcazole, 25 mg/kg/day for 28 days, there was a significant increase in the number of spontaneously active A10 DA neurons, but not A9 DA cells. The firing rate of both A9 and A10 DA cells decreased significantly following prolonged Rimcazole administration; however, the firing pattern of these cells did not change. In addition, chronic Rimcazole did not affect the ID 50 of APO for DA neurons. These results suggest that Rimcazole has an indirect effect on DA neurons with a relative selectivity for A10 DA cells; it does not exhibit pharmacological profiles of previously reported antipsychtic drugs.

Ventral tegmental area infusion of substance P, neurotensin and enkephalin: Differential effects on feeding behavior

The neuropeptides substance P, neurotensin and [Met]enkephalin are found in the ventral tegmental area, site of the A10 dopamine cell bodies. Evidence suggests a functional interaction between these peptides and the dopaminergic neurons. All three peptides have been shown to exert an activating effect on these neurons. The present study analyzed the effects of ventral tegmental area infusion of neurotensin, substance P and d-ala-[Met]enkephalin on feeding behavior. These effects were studied in both food-deprived and satiated rats. During a 30 min test, the following parameters were registered: latency to eat, total food intake, food spillage, number of eating bouts and duration of eating. Similar measures were taken for drinking. In deprived rats substance P (0.5, 3.0μg) increased latency to eat but did not affect other parameters, and substance P did not affect eating in satiated rats. Neurotensin (0.5, 2.5 μg) increased latency to eat and markedly reduced food consumption in deprived rats and had no effect in satiated rats. d-Ala-[Met]enkephalin (0.1, 1.0 μg) stimulated feeding behavior in both deprived and satiated rats. These results show that although the different peptides are presumed to activate the dopaminergic A10 neurons, their effects on feeding behavior can be differentiated. The findings are discussed in terms of motor and motivational mechanisms, and the relative contributions of specific and non-specific influences on feeding are considered.

Regulation of mating behaviour in the female rat by gonadotropin-releasing hormone in the ventral tegmental area: effects of selective destruction of the A10 dopamine neurones

Microinfusions of gonadotropin-releasing hormone (GnRH) into the ventral tegmental area (VTA) potentiated lordosis behaviour in oestrogen-primed ovariectomised female rats. Facilitation was observed within 5 min after the infusion and lasted for about 90 min. When GnRH was infused into the VTA of oestrogen-primed animals which were previously subjected to 6-hydroxydopamine treatment (to destroy the A10 dopamine cells), it produced a marked facilitation of lordosis lasting for about 24 h. These results suggest that the A10 dopamine neurones in the VTA may be critically involved in the mechanisms by which GnRH may modulate midbrain circuits involved in the regulation of lordosis behaviour in the female rat. The lesion studies also imply that the A10 dopamine neurones function as inhibitory neurones regulating lordosis behaviour by suppressing the activity of those cells in the VTA which are sensitive to GnRH. Removal of this inhibitory input leads to an exaggerated response.

Substance K and substance P in the ventral tegmental area

A comparison was made between substance P (SP) and substance K (SK) in the ventral tegmental area (VTA) (A10 dopamine cell group) of the rat. Approximately equal densities of SP and SK-immunoreactive neuronal fibers were observed. However, while previous reports demonstrate negligible density of autoradiographically defined SP receptors in the VTA, we observed a high density of SK receptors. SK or SP was microinjected into the VTA, and changes in spontaneous motor activity were measured using a photo-cell apparatus. SK was found to be at least 10 times more potent than SP in producing an increase in motor activity. These data suggest that while both SK and SP are present in the VTA, SK may have a more significant physiological role in modulating dopamine neurons in the ventromedial mesencephalon.

Morphine-induced activation of A10 dopamine neurons in the rat

The effects of intravenous administration of morphine (MOR) on the spontaneous discharge rate of dopamine (DA) neurons in the ventral tegmental area (VTA or A10) and the substantia nigra pars compacta (SNC or A9) were compared. MOR (0.5–3.5 mg/kg) produced a marked increase in the spontaneous firing of both A10 and A9 DA neurons. Naloxone (NAL) reversed the MOR effects. Acute transection of the medial forebrain bundle (MFB) did not interfere with the observed MOR effects on either A10 or A9 DA neurons. However, following chronic lesions of the MFB (6 days), A9 DA neurons were no longer responsive to MOR whereas A10 DA cells were still activated by MOR. Neither radiofrequency lesions of the dorsal raphe nucleus (DNR) nor administration of the 5-HT 2 antagonist ketanserin affected the stimulatory effect of MOR on either A10 or A9 DA cells. Thus, it is confirmed that the effects of MOR on A9 DA cells depend on striatonigral feedback pathways. In contrast, it appears that the MOR-induced activation of A10 DA cells does not depend on afferents from the forebrain or on projections from the DRN, suggesting a more direct action of MOR on A10 DA cells. Microiontophoretic application of MOR or enkephalin analogues significantly increased the spontaneous activity of both A9 and A10 DA cells. However, these effects were not reversed by either iontophoretic or intravenous NAL. On the other hand, both intravenously (0.5–1.5 mg/kg) and iontophoretically administered MOR markedly suppressed the electrical activity of non-DA cells found in the vicinity of A10 DA neurons, and this effect was completely reversed by NAL. It is proposed that the MOR-induced activation of A10 DA cells could be mediated indirectly by non-DA cells.

Mesencephalic dopamine modulation of pituitary and central β-endorphin: Relation to food intake regulation

Pituitary and central β-endorphin have been implicated in the regulation of food intake. It has been suggested that an elevation in hypophyseal β-endorphin represents the genetic defect in the obese mutant Zucker rat. Both pituitary and central β-endorphin systems appear to interact with dopamine. We have therefore examined hypophyseal, hypothalamic and basal forebrain levels of β-endorphin in the obese Zucker rat, its lean littermate, and lean littermates sustaining neurotoxic lesions of the A10 dopamine cell group in the ventral mesencephalon. The obese mutant exhibits elevated pituitary, but not central, β-endorphin levels relative to lean littermates. A10 lesions result in a marked increase in both pituitary and hypothalamic β-endorphin levels, and tend to decrease the amount of the peptide in the basal forebrain. These lesions do not result in either increased food intake or body weight. These data therefore suggest that elevated pituitary β-endorphin levels do not mediate obesity in the Zucker rat, and also demonstrate that both central and pituitary β-endorphin are modulated by a dopamine system originating in the ventral mesencephalon.

Comparison of the effects of LSD and lisuride on A10 dopamine neurons in the rat

Extracellular single-unit recording techniques were used to compare the effects of d-lysergic acid diethylamide (LSD) and its non-hallucinogenic congener lisuride hydrogen maleate (LHM) on the activity of dopamine (DA) neurons in the A10 region of the rat ventral tegmental area. Lisuride hydrogen maleate potently suppressed the firing rates of A10 DA cells (ID 50 = 0.021 mg/kg). The DA antagonist haloperidol prevented, but did not reverse, the effect of lisuride. In addition, the DA agonist apomorphine was unable to suppress further the firing rates of A10 DA cells that had been partially suppressed by small doses of lisuride. These results suggest that lisuride acted as a non-competitive or irreversible DA agonist. In contrast to lisuride, LSD exerted mixed effects on A10 DA cells, partially depressing the firing rates of 54% of the cells tested, increasing the firing rates of 23% of the cells and exerting no effect on the other 23%. The response of a particular A10 DA cell to administration of LSD was dependent upon its baseline firing rate. Haloperidol readily reversed the LSD-induced partial suppression of the firing of A10 DA cells. Also, LSD effectively reversed apomorphine-induced suppression and increased the doses of apomorphine required to suppress firing of A10 DA neurons. These results suggest that, in addition to its weak DA agonist effects, LSD also exerted partial DA antagonist effects. Pharmacological and physiological manipulations of serotonergic neuronal systems failed to implicate a role for serotonin in the effects of LSD and lisuride on A10 DA neurons. These qualitative and quantitative differences between the effects of LSD and lisuride on A10 DA neurons suggest that the potent DA agonist actions of lisuride may contribute to its lack of hallucinogenic potential.

Comparison of the effects of chronic haloperidol treatment on A9 and A10 dopamine neurons in the rat

The effects of chronic haloperidol (CHAL) treatment on A9 and A10 dopamine (DA) neurons were compared using extracellular single cell recording techniques. CHAL caused a time-dependent reduction in the number of spontaneously active A9 and A10 DA cells and induced an irregular firing pattern in many of the DA cells that remained active. Both of these effects occurred earlier and to a greater extent in A10 than in A9. Intravenous injection of the DA agonist apomorphine reversed both the reduction of active DA neurons and the irregular discharge pattern, suggesting that both effects were due to the process of depolarization inactivation. Lesions of the nucleus accumbens (NAc) produced by ibotenic acid prevented the development of depolarization inactivation of A10 DA neurons, indicating that this process is mediated primarily by NAc — A10 feedback pathways. The results suggest that the slow development of depolarization inactivation of DA cells produced by CHAL may contribute to the delayed onset of the clinical effects of long-term treatment with antipsychotic drugs.

The mesolimbic dopamine system: Evidence for a high amine turnover and for a heterogeneity of the dopamine neuron population

By means of a new photographic method to quantitate catecholamine fluorescence in tissue sections it has been possible to demonstrate a high dopamine (DA) turnover within the DA cell bodies of the midbrain. The medially located small-sized DA cell bodies of the A10 DA cell group has a very high DA turnover which was significantly different from that found in the laterally located medium-sized DA cell bodies of the A10 cell group. Thus, the mesolimbic DA pathways may be divided into two systems, the medial system having a very high DA turnover in their cell bodies which may reflect a high functional activity in this system.

Absence of dopamine sensitive adenylate cyclase in the A10 region, the origin of mesolimbic dopamine neurones

Dopamine (DA) failed to stimulate the adenylate cyclase of the mesolimbic A10 DA nerve cell body area, in contrast to tis activating effect in the nigrostriatal A9 DA cell body area. The enzyme was stimulated by GMPPNP (a GTP analog) and NaF. This indicates the absence in the A 10 cell area of DA receptors with functional coupling on adenylate cyclase, in contrast to the A9 cell area where such DA receptors are believed to be located on afferent axon terminals.