D1 receptor activation enhances sciatic nerve stimulation-induced inhibition of nigrostriatal dopamine neurons

Study of the neural basis of striatal modulation of the jaw-opening reflex

Previous experimental data from this laboratory demonstrated the participation of the striatum and dopaminergic pathways in central nociceptive processing. The objective of this study was to examine the possible pathways and neural structures associated with the analgesic action of the striatum. The experiments were carried out in rats anesthetized with urethane. The jaw-opening reflex (JOR) was evoked by electrical stimulation of the tooth pulp of lower incisors and recorded in the anterior belly of the digastric muscles. Intrastriatal microinjection of apomorphine, a nonspecific dopamine agonist, reduced or abolished the JOR amplitude. Electrolytic or kainic acid lesions, unilateral to the apomorphine-injected striatum, of the globus pallidus, substantia nigra pars reticulata, subthalamic nucleus and bilateral lesion the rostroventromedial medulla (RVM), blocked the inhibition of the JOR by striatal stimulation. These findings suggest that the main output nuclei of the striatum and the RVM may be critical elements in the neural pathways mediating the inhibition of the reflex response, evoked in jaw muscles by noxious stimulation of dental pulp.

Chronic cold stress alters prefrontal cortical modulation of amygdala neuronal activity in rats

BACKGROUND

Recent studies suggest that long-term exposure to stress can sensitize animals to subsequent novel or acute stressors. Stressors affect amygdala activity, and the prefrontal cortex has been implicated in the regulation of responses to stress. Little is known, however, about how the physiology of amygdala neurons is altered by chronic stressors or the role of the prefrontal cortex in these changes.

METHODS

We used in vivo extracellular recordings from neurons in the rat central and basolateral amygdala nuclei to examine the effects of chronic stress on the basal firing and responses of amygdala neurons to a novel stressor. Additionally, prefrontal cortical afferents were severed to examine its role in the modulation of the response to stressors.

RESULTS

Chronic exposure to cold enhanced the sensitivity of central amygdala neurons to footshock. A portion of this may be due to enhanced basolateral amygdala output. Furthermore, prefrontal cortical regulation of this response is weakened by chronic stress.

CONCLUSIONS

The physiology of the amygdala is altered by chronic stress. Furthermore, the prefrontal cortical regulation of these responses may be weakened after chronic stress. This is a potential biological substrate for abnormal affect upon chronic stress and its effect on affective disorders.

Ascending afferent regulation of rat midbrain dopamine neurons

Standard, extracellular single-unit recording techniques were used to examine the electrophysiological and pharmacological responsiveness of midbrain dopamine (DA) neurons to selected, ascending afferent inputs. Sciatic nerve stimulation-induced inhibition of nigrostriatal DA (NSDA) neurons was blocked by both PCPA (5-HT synthesis inhibitor) and 5,7-DHT (5-HT neurotoxin), suggesting mediation by a serotonergic (5-HT) system. Direct stimulation of the dorsal raphe (which utilizes 5-HT as a neurotransmitter and inhibits slowly firing NSDA neurons) inhibited all mesoaccumbens DA (MADA) neurons tested. Paradoxically, DPAT, a 5-HT1A agonist which inhibits 5-HT cell firing, enhanced sciatic nerve stimulation-induced inhibition of NSDA neurons. MADA neurons were not inhibited by sciatic nerve stimulation and, therefore, could not be tested in this paradigm. In contrast to the dorsal raphe, electrical stimulation of the pedunculopontine tegmental nucleus preferentially excited slowly firing NSDA and MADA neurons. Thus, both excitatory and inhibitory ascending afferents influence the activity of midbrain DA neurons, and intact 5-HT systems are necessary for sciatic nerve stimulation to alter DA cell activity. However, the role that 5-HT plays in mediating peripheral sensory input remains unclear.

Electrophysiological effects of MK-801 on rat nigrostriatal and mesoaccumbal dopaminergic neurons

The electrophysiological effects of the non-competitive N-methyl-D-aspartate (NMDA) antagonist (+)-MK-801 (MK-801) on nigrostriatal and mesoaccumbal dopaminergic (DA) neurons were evaluated in chloral hydrate-anesthetized rats. MK-801 (0.05-3.2 mg/kg, i.v.) stimulated the firing rates of 14 (74%) of 19 nigrostriatal DA (NSDA) neurons and all 16 mesoaccumbal DA (MADA) neurons tested. Stimulatory effects of the drug were more prominent on MADA neurons. Interspike interval analysis revealed that MK-801 also regularized DA neuronal firing pattern. Acute brain hemitransection between the midbrain and forebrain attenuated the stimulatory effects of MK-801 on firing rate and blocked the effects on firing pattern. Similar to MK-801, hemitransection itself increased NSDA and MADA cell firing rates and regularized firing pattern. Both i.v. and iontophoretic MK-801 blocked the excitatory effects of iontophoretic NMDA but did not affect excitations caused by the non-NMDA glutamatergic receptor agonists quisqualate and kainate. Iontophoretic MK-801 had no effect alone. These results suggest that the excitatory effects of i.v. MK-801 on DA neuronal activity are not due to direct actions on DA neurons. Glutamatergic projections originating anterior to the hemitransection appear to play a role in the effects of MK-801 on DA neuronal activity.

Inhibition of nigral dopamine neurons by systemic and local apomorphine: Possible contribution of dendritic autoreceptors

Peripheral administration of low doses of dopamine agonist apomorphine induces a strong and short-latency inhibition of dopamine neurons in the substantia nigra, presumably via the activation of somatodendritic autoreceptors. We studied the site of action of apomorphine in anesthetized rats using volume-controlled pressure microejection combined with single unit recordings. Microapplication of apomorphine in the immediate vicinity of nigral dopamine neurons did not mimic the effect of intravenous administration of apomorphine (50 micrograms/kg), regardless of the concentration or volume used (10(-10)-10(-2) M, 10-100 nl). In contrast, the inhibition produced by systemic apomorphine was mimicked by drug application at a site 300 microns lateral and 600 microns ventral from the recording site in the zona reticulata of the substantia nigra, a region rich in dendrites of dopamine neurons. The inhibition induced by such a distant application of apomorphine could be reversed by systemic injection of D2, but not D1, receptor antagonists. Non-dopaminergic substances such as GABA, bicuculline or lidocaine were more effective when ejected close to rather than distant from the recording site, in a manner opposite to that of apomorphine. Similar to apomorphine, dopamine and D2 receptor agonists were more potent when intranigral applications were made at sites distant from, rather than close to, the recorded dopamine cells. Ejection of D2 antagonists in the substantia nigra zona reticulata attenuated the inhibitory effect of subsequent systemic apomorphine. Our results, together with other previous studies on the location of D2 receptors on dopamine neurons, suggest that peripheral administration of low doses of apomorphine inhibits nigral dopamine neurons by acting at D2 receptors located on the dendrites of these neurons.

The effects of the phencyclidine analogs BTCP and TCP on nigrostriatal dopamine neuronal activity

Extracellular single unit recording techniques were used to evaluate the effects of two phencyclidine (PCP) derivatives. N-[1-(2-benzo(b)thiophenyl)cyclohexyl]piperidine (BTCP) and N-[1-(2-thiophenyl)cyclohexyl]piperidine (TCP) on the electrophysiological activity of antidromically identified nigrostriatal dopamine (DA) neurons in chloral hydrate-anesthetized rats. I.v. BTCP produced a dose-dependent decrease in the firing rate of identified nigrostriatal DA neurons whereas TCP elicited a dose-dependent biphasic effect which was characterized by an activation of cell firing at low doses followed by a reversal of the response with larger doses. A hemitransection of the brain anterior to the substantia nigra significantly reduced the inhibitory effect of BTCP while this surgical procedure did not affect the response to TCP. However, iontophoretic application of BTCP induced a current-dependent inhibition of the spontaneous activity of cells while local application of TCP had no effect on the firing rate of these cells. These data indicate that PCP analogs are able to interact with the nigrostriatal DAergic pathway through distinct and opposing mechanisms. The results are discussed in light of recent observations that BTCP is selective for the DA uptake site while TCP is selective for the high affinity PCP binding site.

Dopamine neuron ontogeny: electrophysiological studies

The ontogeny of nigrostriatal dopamine (NSDA) neurons was examined with single-unit extracellular electrophysiological methods. The physiological and pharmacological characteristics of 2-, 4-, and 5-week-old rat pup NSDA neurons were compared with those of adults (8-10 weeks old). Although the basal discharge rate, conduction velocity, and firing pattern of NSDA neurons from 4- and 5-week-old rats were similar to adults, the 2-week-old-rats differed significantly in all three of these physiological characteristics. The conduction velocity and basal discharge rate were found to be significantly lower in the 2-week-old pups relative to adults. In addition, there were significantly fewer bursting NSDA neurons in 2-week-olds than there were in adults. Two and 4-week-olds exhibited significantly lower sensitivity to cumulative intravenous doses of apomorphine. In contrast, the sensitivity to cumulative intravenous doses of quinpirole was found to be similar across all age groups. It is evident that the pharmacological and physiological properties of NSDA neurons are in a dynamic state of flux during postnatal development. These electrophysiological findings are discussed in the context of the perinatal development of midbrain DA systems.