Research reportLocal pressure application of cannabinoid agonists increases spontaneous activity of rat substantia nigra pars reticulata neurons without affecting response to iontophoretically-applied GABA
Introduction
Marijuana has been used since antiquity for medicinal and recreational purposes. Its major active component, Δ9-tetrahydrocannabinol, and related compounds have been found to possess several potentially useful actions. Central nervous system (CNS) actions of cannabinoids include analgesia, drowsiness, antiemesis, and a decrement in psychomotor performance [24]. A significant advance in the understanding of cannabinoids has been brought about by the demonstration of brain cannabinoid receptors [5]and the cloning and characterization of the receptor, which was shown to be a G protein-linked receptor that inhibits adenylyl cyclase [23].
The distribution of cannabinoid binding sites in the CNS has been studied using the synthetic agonists [3H]CP 55940 16, 21and [3H]Win 55212–2 (WIN2) [17]]. Results showed that the distribution is not uniform, and that there are receptors associated with areas such as cerebral cortex and hippocampus, which may be involved in the cannabinoids' psychoactive effects. Dense labeling was also found in the basal ganglia, including areas innervated by the output neurons of the striatum, with the highest concentration being in the pars reticulata of the substantia nigra (SNpr) 15, 20. Lesion and binding studies involving striatonigral and nigrostriatal neurons have shown the receptor to be located on the presynaptic terminals of the striatonigral neurons [14]. The reported loss of cannabinoid receptors in the substantia nigra and the lateral globus pallidus in the brains of patients suffering from Huntington's disease is consistent with their presynaptic localization 11, 31. The function of these receptors has yet to be determined. Their localization in the basal ganglia suggests that cannabinoids may act to regulate striatal output, possibly playing a role in movement and posture. Considering the localization of the receptor on the terminals of GABAergic striatal output neurons, cannabinoid actions in SNpr are most likely to occur presynaptically, possibly involving a change in γ-aminobutyric acid (GABA) release or reuptake.
The effects of a synthetic cannabinoid, WIN2, on striatonigral transmission were studied by Miller and Walker [26], who found that systemic injections of WIN2 increased the rate of spontaneous action potential generation (‘firing rate’) of SNpr neurons, and also attenuated the slowing of SNpr discharge produced by electrical stimulation of the striatum. These findings, coupled with the reported presynaptic localization of cannabinoid receptors, strongly suggested that WIN2 modulated GABA release from striatal nerve terminals. Since the cannabinoid was injected systematically, and cannabinoid receptors exist in many brain areas, including the striatum [21]which is sensitive to cannabinoids [35], the site of action of WIN2 could only be inferred. Moreover, the results could not exclude the possibility that the SNpr neurons, either by direct or indirect WIN2 actions, were made less sensitive to GABA. Though local application of a cannabinoid (i.e., by micro-iontophoresis or pressure application) might provide more information, this is a difficult problem because of the extremely limited aqueous solubility of the cannabinoid agonists currently available.
In the present study, a technique for local application of cannabinoids by pressure application from a multibarreled glass electrode assembly is described. This technique was used to investigate the effects of two cannabinoid agonists, WIN2 and CP 55940, as well as the cannabinoid antagonist SR 141716A [32]on the spontaneous activity of SNpr neurons. The effects of Win 55212–3, the inactive isomer of WIN2 on SNpr neuron spontaneous activity was also examined. The interaction between the GABAA antagonist bicuculline methiodide and CP 55940 was also studied. To evaluate the possibility that cannabinoids might affect the responsiveness of SNpr neurons to GABA, the postsynaptic action of exogenously-applied GABA was also studied, using iontophoretic application of GABA during cannabinoid agonist application.
Section snippets
Rat preparation and electrical recording
Male Sprague-Dawley rats (250–300 g) were anesthetized with chloral hydrate (400 mg/kg) i.p. and placed in a stereotaxic instrument. The rats were kept at a constant level of anesthesia by i.v. infusion of 2.2 mg/kg/min chloral hydrate into the lateral tail vein [38]. The rat was placed on a constant temperature pad (Deltaphase, Braintree Scientific, Braintree, MA) to help maintain body temperature during the course of the experiment. A small diameter burr hole was drilled over the substantia
Effect of vehicle, WIN2 and CP55940 on spontaneous firing rate and GABA inhibition of SNpr cells
The effects of local pressure application of WIN2 and CP 55940 on the spontaneous firing rate of SNpr cells are shown in Fig. 1Fig. 2. Fig. 1 shows oscilloscope tracings and a representative ratemeter recording of the activity of a single SNpr cell and the effects of locally-applied WIN2. Locally-applied WIN2 and CP 55940 increased the spontaneous firing rate of SNpr cells (Fig. 2), but had no significant effect on the inhibition of the cells by iontophoretically-applied GABA (Fig. 3). The
Discussion
Locally-applied WIN2 and CP 55940 increased the spontaneous firing of SNpr neurons, but had no effect on iontophoretically-applied GABA. The lack of interaction between cannabinoids and GABA makes it unlikely that cannabinoids were acting on SNpr cells directly to alter the coupling between GABA binding and inhibition of neuronal activity, or by some indirect mechanism to change the susceptibility of the neuron to the inhibitory effects of GABA. The effects of the two cannabinoid agonists were
Acknowledgements
The authors wish to thank Pfizer, Inc. for the generous gift of CP 55940, and NIDA for the generous gift of SR 141716A. This work was supported by NIH Grant DA02194.
References (43)
- et al.
Neurobiology of marijuana abuse
Trends Pharmacol. Sci.
(1992) - et al.
Effects of cannabinoid receptor stimulation and blockade on catalepsy produced by dopamine receptor agonists
Eur. J. Pharmacol.
(1996) - et al.
The cannabinoid agonists WIN 55,212-2 and CP 55,940 attenuate rotational behavior induced by a dopamine D1 but not a D2 agonist in rats with unilateral lesions of the nigrostriatal pathway
Brain Res.
(1995) - et al.
Effects of bilateral injection of GABA into the substantia nigra on spontaneous behavior and measures of analgesia
Neuropharmacology
(1988) - et al.
Cannabinoid receptor down-regulation without alteration of the effect of CP 55,940 on adenylyl cyclase in the cerebellum of CP 55,940 tolerant mice
Brain Res.
(1996) - et al.
Evaluation of the role of antinociception in self-injurious behavior following intranigral injection of muscimol
Neuropharmacology
(1986) - et al.
Loss of cannabinoid receptors in the substantia nigra in Huntington's disease
Neuroscience
(1993) - et al.
Catalepsy induced by intrastriatal injections of Δ9-THC and 11-OH-Δ9-THC in the rat
Neuropharmacology
(1978) - et al.
Neuronal localization of cannabinoid receptors in the basal ganglia of the rat
Brain Res.
(1991) - et al.
Distribution of cannabinoid receptors in rat brain determined with aminoalkylindoles
Brain Res.
(1992)
The neostriatal inhibition of catalepsy, but not of muscle rigidity, evoked from the substantia nigra pars reticulata
Pharmacol. Biochem. Behav.
Localization of cannabinoid receptor in the human developing and adult basal ganglia. Higher levels in the striatonigral neurons
Neurosci. Lett.
Distribution of neuronal cannabinoid receptor in the adult rat brain: a comparative receptor binding radioautography and in situ hybridization histochemistry
Neuroscience
Effects of a cannabinoid on spontaneous and evoked neuronal activity in the substantia nigra pars reticulata
Eur. J. Pharmacol.
Motor disturbances induced by an acute dose of Δ9-tetrahydrocannabinol: possible involvement of nigrostriatal dopaminergic alterations
Pharmacol. Biochem. Behav.
The action of baclofen on neurons of the substantia nigra and of the ventral tegmental area
Brain Res.
Chronic cannabinoid administration alters cannabinoid receptor binding in rat brain: a quantitative autoradiographic study
Brain Res.
Changes in rat brain cannabinoid binding sites after acute or chronic exposure to their endogenous agonist, anandamide, or to Δ9-tetrahydrocannabinol
Pharmacol. Biochem. Behav.
Intrastriatal injection of cannabinoid receptor agonists induced turning behavior in mice
Pharmacol. Biochem. Behav.
GABAergic mechanisms in mediating muscular rigidity, catalepsy and postural asymmetry in rats: differences between dorsal and ventral striatum
Brain Res.
Substantia nigra and motor control in the rat: effect of intranigral α-kainate and γ-D-glutamylaminomethylsulphonate on motility
Brain Res.
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The Substantia Nigra Pars Reticulata
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2015, NeuroscienceCitation Excerpt :Furthermore, CB1RS are present in both glutamatergic excitatory pathways and on GABAergic inhibitory neurons (Freund et al., 2003; Castillo et al., 2012), suggesting a modulatory role in these neural networks. Studies of the striatonigral and nigrostriatal pathways have shown that CBRs are possibly located on presynaptic terminals of striatonigral neurons (Herkenham et al., 1991; Tersigni and Rosenberg, 1996; Chan et al., 1998). Their localization is most likely presynaptic in the SNpr, suggesting that eCBs may act by regulating GABAergic striatal outputs and possibly playing a role in GABA release or reuptake (Herkenham et al., 1991; Miller and Walker, 1995; Tersigni and Rosenberg, 1996; Chan et al. 1998).
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2011, NeuroscienceCitation Excerpt :At the same time, the paired pulse ratio is increased, indicating decreased vesicular GABA release. The presynaptic action of cannabinoids has been further confirmed by showing that CB receptor agonists neither affect the response of SNr neurons to exogenously applied GABA or a GABAA receptor agonist (Tersigni and Rosenberg, 1996; Chan et al., 1998; Wallmichrath and Szabo, 2002a,b) nor change the amplitude of Ca2+-independent miniature IPSCs (Wallmichrath and Szabo, 2002b). More important, CB receptor antagonists have been reported to increase evoked IPSC amplitude, suggesting tonic presynaptic modulation of GABAergic input by endogenous cannabinoids (Wallmichrath and Szabo, 2002a; Yanovsky et al., 2003).