Skip to main content
NCBI home page
The Journal of Physiology logoLink to The Journal of Physiology
. 2002 May 15;541(Pt 1):219–230. doi: 10.1113/jphysiol.2001.013404

Presynaptic modulation of synaptic transmission by opioid receptor in rat subthalamic nucleus in vitro

Ke-Zhong Shen *, Steven W Johnson *,†
PMCID: PMC2290302  PMID: 12015431

Abstract

Presynaptic modulation of synaptic transmission in rat subthalamic nucleus (STN) neurons was investigated using whole-cell patch-clamp recordings in brain slices. Evoked GABAergic inhibitory postsynaptic currents (IPSCs) were reversibly reduced by methionine enkephalin (ME) with an IC50 value of 1.1 ± 0.3 μM. The action of ME was mimicked by the μ-selective agonist [d-Ala2, N-Me-Phe4, Gly5-ol]-enkephalin (DAMGO), and was partially blocked by the μ-selective antagonists naloxonazine and d-Phe-Cys-Tyr-d-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP). Evoked GABAA IPSCs were also inhibited by the δ-selective agonist [d-Pen2,5]-enkephalin (DPDPE), but not by the κ-selective agonist (+)-(5α,7α,8β)-N-methyl-N-[7-(1-pyrrolidinyl)-1-oxaspiro[4.5]dec-8-yl]-benzeneacetamide (U-69593) and the orphan receptor agonist orphanin FQ/nociceptin (OFQ). DPDPE-induced inhibition was completely blocked by the δ-selective antagonist N,N-diallyl-Tyr-Aib-Aib-Phe-Leu-OH (ICI 174,864). ME, DAMGO and DPDPE increased the paired-pulse ratio of IPSCs. Evoked excitatory postsynaptic currents (EPSCs) were reversibly reduced by ME with an IC50 value of 0.35 ± 0.14 μM. Inhibition by ME was associated with an increase in the paired-pulse ratio of EPSCs. The action of ME was mimicked by DAMGO, and blocked by naloxonazine. DPDPE had little effect on evoked EPSCs. Neither U-69593 nor OFQ had any effect. ME significantly decreased the frequency of spontaneous miniature EPSCs (mEPSCs) without change in their amplitude. The action of ME was mimicked by DAMGO. DPDPE had no effect. The presynaptic voltage-dependent potassium conductance blocker 4-aminopyridine (4-AP, 100 μM) abolished the inhibitory effects of ME on evoked IPSCs and EPSCs. In contrast, 4-AP only partially blocked the actions of baclofen. These results suggest that opioids inhibit inhibitory synaptic transmission in the STN through the activation of presynaptic μ- and δ- receptors. In contrast, inhibition of excitatory synaptic inputs to the STN occurs through the activation of only μ-receptors. Both inhibitions may be mediated by blockade of voltage-dependent potassium conductance.

The subthalamic nucleus (STN) plays a critical role in regulating muscle tone and the dynamics of movement. The STN is thought to be the main relay structure in the so-called indirect pathway of basal ganglia. STN glutamatergic neurons receive GABAergic inputs from the globus pallidus externa and send excitatory projections to basal ganglia output nuclei: substantia nigra pars reticulata and globus pallidus interna (Albin, 1995). Recent anatomical and physiological studies have revealed far more complicated input and output connections to and from the STN than previously thought (Parent & Hazrati, 1995). The STN also receives projections from a variety of nuclei, such as the cerebral cortex, the substantia nigra pars compacta, the dorsal raphe nucleus, the pedunculopontine tegmental nucleus and parafascicular thalamic complex. Several other areas, including the hypothalamus, amygdala and locus coeruleus, project to the STN. The net activity of the STN neurons is determined not only by the inhibitory influence from globus pallidus externa, but also by the interplay of inputs which might be dopaminergic, serotoninergic, cholinergic, glutamatergic or opiatergic. Abnormal activity of the STN, which results from imbalances in these input systems, is associated with movement disorders, e.g. Parkinson's disease, bemiballism and chorea (Crossman, 1990; DeLong, 1990; Albin, 1995). Selective modulation of single or multiple synaptic inputs to the STN could be effective treatments for these disorders.

Parkinson's disease is thought to be classically associated with degeneration of nigrostriatal dopamine cells. However, involvement of other transmitters such as the opiate system, has received more attention in the past two decades in an attempt to elucidate the pathophysiology of Parkinson's disease and to understand the mechanisms underlying dyskinesia after dopamine replacement therapy in Parkinson's disease. In fact, opioid receptors and their endogenous ligands are present in high concentration in the basal ganglia (Gramsch et al. 1979; Emson et al. 1980; Zamir et al. 1984; Mansour et al. 1995). Alterations in endogenous opioids have been reported in postmortem brain from patients with Parkinson's disease (Taquet et al. 1983, 1985; Fernandez et al. 1984; Sivam, 1991). Opioid receptor agonists and antagonists have been shown to affect the behaviour of parkinsonian patients and animal models with parkinsonism (Diamond & Borisov, 1978; Trabucchi et al. 1982; Sandyk & Snider, 1986; Gropetti et al. 1990; Vermeulen et al. 1995). Most of these investigations have emphasized that striatal opiate pathways may play a role in the function of basal ganglia and in the pathological process of Parkinson's disease. However, several observations suggest the likely importance of the modulatory effect of opioids in the STN for normal or abnormal basal ganglionic function. First, ligand binding, in situ hybridization and immunohistochemical studies have revealed the expression of both presynaptic and postsynaptic opioid receptors in the STN (Wamsley et al. 1980; Delfs et al. 1994; Peckys & Landwehrmeyer, 1999; Florin et al. 2000). μ-Opioid receptor mRNA expression has been detected at unusually high levels in human subthalamic region, suggesting an involvement of opioid receptors in the STN in motor control (Raynor et al. 1995). Second, a significant decrease in μ-opioid receptor density associated with the development of vacuous chewing movements after long-term haloperidol treatment has been reported to be confined to the STN and ventrolateral thalamus (Sasaki et al. 1996). This observation suggests that μ-opioid receptor changes in the STN may play a role in the development or expression of oral dyskinetic syndromes after chronic neuroleptic exposure.

In the central nervous system opioids have two main cellular actions: first, they hyperpolarize cells by increasing membrane potassium conductance; and second, they inhibit synaptic transmission by reducing voltage-dependent calcium currents (North, 1993). Opioids have been reported to exert an inhibitory modulation of GABAergic and/or glutamatergic synaptic transmission in the basal ganglia circuitry, such as in the striatum (Jiang & North, 1992) and globus pallidus (Stanford & Cooper, 1999). However, the effects of opioids on cells, as well as on synaptic transmissions in STN, are unknown. Understanding the mechanisms underlying modulation of STN activity by the opiate system may help us to better understand the pathophysiology and neuropharmacology of relevant movement disorders.

METHODS

Tissue preparation

Horizontal slices of midbrain (300 μm thick) were prepared from male Sprague-Dawley rats (120-300 g; Bantin & Kingman, Seattle, WA, USA), as described previously (Shen & Johnson, 1997). Briefly, rats were anaesthetized with halothane and killed by exsanguination, in accordance with institutional guidelines. The brain was rapidly removed and slices containing caudal diencephalon and rostral midbrain were cut in cold (4 °C) physiological saline using a vibratome. A slice containing the STN was then placed on a supporting net and submerged in a continuously flowing solution (2 ml min−1) of the following composition (mm): NaCl, 126; KCl, 2.5; CaCl2, 2.4; MgCl2, 1.2; NaH2PO4, 1.2; NaHCO3, 19; glucose, 11; gassed with 95 % O2 and 5 % CO2 (pH 7.4) at 36 °C. Using a dissection microscope for visual guidance, the STN was located as grey matter approximately 2.7 mm lateral to the midline and 2 mm rostral to the centre of the substantia nigra reticulata (Paxinos & Watson, 1986).

Electrophysiological recordings

Whole-cell recordings were made with pipettes containing (mm): potassium gluconate, 130; MgCl2, 2; CaCl2, 1; EGTA, 11; Hepes, 10; ATP, 1.5; GTP, 0.3 (pH 7.3). Membrane currents were recorded under voltage clamp (-70 mV) and amplified with an Axopatch-1D amplifier. Data were acquired using a personal IBM compatible computer with a Digidata analog/digital interface and analysed using pCLAMP software (Axon Instruments, Foster City, CA, USA). Holding currents were recorded continuously using a MacLab analog/digital interface, Chart software (ADInstruments, Castle Hill, Australia) and a Macintosh IIVX computer. Series resistance was electronically compensated 50-80 % to 10-30 MΩ; membrane potentials have been corrected for the liquid junction potential (10 mV).

Synaptic currents

Bipolar stimulation electrodes (tip separation, 300-500 μm) were placed in the slice 300 μm rostral to the STN. Synaptic currents were either evoked by focal electrical stimulation of the brain slice or were recorded as spontaneous events. A single rectangular pulse (0.1 ms duration) of constant current was used to evoke a synaptic current every 10 s. The amplitude of evoked synaptic currents was measured from recordings which represent the average of three responses. An IPSC mediated by GABAA receptors was isolated pharmacologically by recording in the presence of (±)-2-amino-5-phosphonopentanoic acid (AP5, 50 μM) and 6-cyano-7-nitro-quinoxalone (CNQX, 10 μM) in order to block NMDA and non-NMDA receptors. An EPSC mediated by glutamate receptors was recorded in the presence of bicuculline (30 μM) to block GABAA receptors. Spontaneous miniature EPSCs (mEPSCs) were recorded in the presence of tetrodotoxin (TTX, 0.5 μM) using pCLAMP 8.0 (1 s sweep−1; Axon Instruments). For recording of spontaneous EPSCs, caesium was added to the internal solution instead of potassium. The amplitude and frequency of events were analysed automatically using Axograph 3.0 (Axon Instruments). For this analysis, a sliding variable amplitude template was used and erroneous events were visually examined and rejected before their amplitude and time occurrence were measured.

Drugs

All drugs were dissolved in aqueous stock solutions with the exception of CNQX, which was dissolved in dimethyl sulphoxide. Stock solutions of drugs were diluted at least 1:1000 to the desired concentration in superfusate immediately prior to their use. Dimethyl sulphoxide, diluted 1:1000 in superfusate, had no effect on either the holding current or synaptic currents. Approximately 30 s were required for the drug solution to enter the recording chamber. This delay was due to passage of the perfusate through a heat exchanger. Opioid agonists used were methionine enkephalin (ME; non-selective), [d-Ala2, d-Leu5]-enkephalin (DADLE; non-selective), [d-Ala2, N-Me-Phe4, Gly5-ol]-enkephalin (DAMGO; μ-receptor selective), [d-Pen2,5]-enkephalin (DPDPE; δ-receptor selective), (+)-(5α,7α,8β)-N-methyl-N-[7-(1-pyrrolidinyl)-1-oxaspiro[4.5]dec-8-yl]-benzeneacetamide (U-69593; κ-receptor selective) and orphanin FQ/nociceptin (OFQ; orphan receptor selective). Opioid antagonists used were naloxone (non-selective), d-Phe-Cys-Tyr-d-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP; μ-receptor selective), naloxonazine dihydrochloride (μ-receptor selective) and N,N-diallyl-Tyr-Aib-Aib-Phe-Leu-OH (ICI 174,864, δ-receptor selective). All agonists and antagonists were from Sigma Chemical Co. (St Louis, MO, USA) except CTAP, which was obtained from Peninsula Laboratories (Belmont, CA, USA), and OFQ, which was provided by Dr D. Grandy (Oregon Health Sciences University, Portland, OR, USA). Other chemicals including TTX, baclofen hydrochloride, bicuculline methiodide, 4-aminopyridine (4-AP), AP5 and CNQX were obtained from Sigma Chemical Co.

Data analysis

Numerical data in the text and error bars in the figures are expressed as means ± s.e.m. Student's two-tailed t tests were used to test for significant differences. In evaluating concentration-dependent drug effects, an IC50 was calculated for the responses from each neuron using KaleidaGraph curve-fitting program (Synergy Software, Reading, PA, USA) on a Macintosh computer. Concentration- response data were fitted to the Hill-Langmuir equation: y = ax/(x + b), where y is the magnitude of effect, a is maximum effect, x is the drug concentration and b is the concentration that inhibits the effects by 50 %.

RESULTS

Characteristics of STN neurons and postsynaptic effects of opioids

Most of the STN neurons fired spontaneous action potentials at zero holding current. Hyperpolarizing voltage steps of 400 ms duration from −70 to −140 mV revealed prominent inward rectification, moderate hyperpolarization-activated time-dependent inward current (Ih) and an inward tail current. Depolarizing voltage steps from −70 to −50 mV activated a persistent inward current (Shen & Johnson, 2000). Application of ME (10 μM) caused a small outward current (12.8 ± 2.5 pA at −70 mV) in 25 out of 49 STN neurons. ME had no effect on about half of the STN neurons observed (24 of 49 cells). The selective μ-agonist DAMGO (1 μM) mimicked ME, producing an outward current of 10.1 ± 1.5 pA at −70 mV in 10 out of 22 neurons. Application of the δ-agonist DPDPE (1 μM) or κ-agonist U-69593 (1 μM) did not cause a significant change in the membrane holding currents of STN neurons. However, the orphan receptor agonist OFQ (1 μM) produced an outward current of 10.6 ± 3.8 pA in 3 out of 5 STN neurons examined.

Opioid receptor-mediated inhibition of evoked IPSCs

Bipolar electrodes placed in the rostral region of the slice were used to evoke synaptic currents at a holding potential of −70 mV. After blockade of excitatory synaptic transmission by the addition of CNQX (10 μM) and AP5 (50 μM), a single stimulus evoked an IPSC in all STN neurons observed. This evoked IPSC was completely abolished by bicuculline (30 μM). As can be seen in Fig. 1A, ME reversibly reduced the amplitude of evoked IPSCs. The action of ME began within 1 min of perfusion, and reached its peak within 5 min. After removal of the peptide, the IPSC amplitude returned to baseline within 10-15 min. ME (10 μM) reduced the peak amplitude of evoked IPSCs by 42 ± 5 % (P < 0.01, paired t test; n = 13). In the presence of naloxone (3 μM), ME (10 μM) reduced the IPSC amplitude by only 6 ± 2 % (P < 0.01; n = 6; Fig. 1B). Naloxone alone had no effect on the holding current or IPSC amplitude. ME reduced the amplitude of evoked IPSCs in a concentration-dependent manner with an IC50 of 1.1 ± 0.3 μM (n = 7; Fig. 1C). The non-selective opioid agonist DADLE and the μ-selective agonist DAMGO mimicked the actions of ME. These two agonists also reduced the peak amplitude of evoked IPSCs in a concentration-dependent manner with IC50 values of 0.79 ± 0.15 μM (n = 6) and 0.10 ± 0.02 μM (n = 6), respectively. DADLE (10 μM) reduced the IPSCs by 46 ± 6 % (P < 0.01, paired t test; n = 6), and DAMGO (1 μM) reduced the IPSCs by 47 ± 7 % (P < 0.01, paired t test; n = 6). The δ-selective agonist DPDPE (1 μM) caused a smaller reduction in the amplitude of evoked IPSCs (by 19 ± 3 %; P < 0.01, paired t test; n = 5). The κ-selective agonist U-69593 (1-10 μM) had little effect on the amplitude of evoked IPSCs. U-69593 at 1 μM reduced the IPSCs by 5 ± 3 % (P = 0.18, paired t test; n = 3). OFQ (1 μM), the orphan opioid receptor agonist, had no effect (99 ± 3 % of control level, P > 0.05, paired t test).

Figure 1. Opioids inhibit evoked GABAergic IPSCs.

Figure 1

Open in a new tab

A, superimposed current traces showing that ME reversibly reduces GABAA IPSCs. ME at 10 μM caused about a 40 % reduction in the amplitude of IPSCs. B, the opioid receptor antagonist naloxone (3 μM) blocked the action of ME (10 μM). *Significant difference from ME alone (P < 0.05). C, concentration-response curves for percentage inhibition of GABAA IPSC amplitude by ME (•, n = 7), DAMGO (○, n = 6), DADLE (▪, n = 6) and U-69593 (□, n = 4). Each point represents the mean ± s.e.m. D, summary of results showing the percentage inhibition of IPSCs by ME (10 μM, n = 13), DAMGO (1 μM, n = 6), DPDPE (1 μM, n = 5), U-69593 (1 μM, n = 3) and OFQ (1 μM, n = 4).

The effect of ME on evoked IPSCs was reduced by the μ-antagonists naloxonazine and CTAP. As can be seen in Fig. 2C, ME (10 μM) reduced the amplitude of evoked IPSCs by 42 ± 5 % (at t = 10 min; n = 13). However, in the presence of either naloxonazine (1 μM) or CTAP (1 μM), ME reduced the IPSCs by only 12 ± 3 % (P < 0.01; n = 9) and 9 ± 3 % (P < 0.01; n = 6), respectively. Similarly, DAMGO (1 μM) reduced the amplitude of evoked IPSCs by 46 ± 5 % in the absence of antagonists. However, DAMGO reduced the IPSCs by only 4 ± 3 % (P < 0.01; n = 8) in the presence of naloxonazine (1 μM), and by 6 ± 3 % (P < 0.05; n = 4) in the presence of CTAP (1 μM) (Fig. 2D). As shown in Fig. 3, the effects of DPDPE on evoked IPSCs were completely blocked by the δ-selective antagonist ICI 174,864. In control, DPDPE (1 μM) reduced the amplitude of evoked IPSCs by 19 ± 5 % (n = 5). However, in the presence of ICI 174,864 (1 μM), DPDPE reduced the IPSCs by 2 ± 5 % (P = 0.0375; n = 7).

Figure 2. Inhibition of evoked GABAergic IPSCs by opioids is mediated by activation of μ-receptors.

Figure 2

Open in a new tab

A, time course showing the reversible reduction in IPSC amplitude by ME (10 μM) and the μ-selective agonist DAMGO (1 μM). Furthermore, the μ-selective antagonist CTAP (1 μM) blocked the actions of ME and DAMGO. B, representative IPSCs indicated by numbers correspond to the time points in A. C, summarized results demonstrating the inhibition of evoked IPSCs by ME in the absence (•) and presence of the μ-selective antagonists naloxonazine (□, 1 μM) and CTAP (▵, 1 μM). D, summarized results demonstrating the inhibition of evoked IPSCs by DAMGO in the absence (▪) and presence of naloxonazine (○, 1 μM) and CTAP (⋄, 1 μM). Each data point is the mean ± s.e.m. of 4-13 cells.

Figure 3. Stimulation of δ-opioid receptors inhibits evoked GABAA IPSCs.

Figure 3

Open in a new tab

A, current traces demonstrating the reversible reduction in IPSC amplitude by the δ-agonist DPDPE (1 μM), and blockade of DPDPE-induced inhibition of IPSCs by the δ-receptor antagonist ICI 174,864 (1 μM). B, summarized results showing the time course of the reduction in the amplitude of evoked IPSCs by DPDPE (•, 1 μM) and antagonism of the DPDPE-induced inhibition by ICI 174,864 (□, 1 μM). Each data point is the mean ± s.e.m. of 5-7 cells.

In order to investigate the site of action of ME, DAMGO and DPDPE, pairs of stimuli (30-50 ms apart) of identical strength were applied to characterize the effect of opioids on the paired-pulse ratio (IPSC2/IPSC1). Under control conditions, the mean paired-pulse ratio of the evoked IPSC amplitudes was 0.79 ± 0.02 (n = 15). Application of ME (10 μM) significantly increased the IPSC paired-pulse ratio to 0.87 ± 0.02 (P < 0.01, paired t test) in the same neurons (Fig. 4A). DAMGO (1 μM) mimicked ME by increasing the IPSC paired-pulse ratio from 0.74 ± 0.02 to 0.84 ± 0.03 (P < 0.01, paired t test; n = 9; Fig. 4B). Similarly, DPDPE (1 μM) also increased the IPSC paired-pulse ratio (Fig. 4C). Thus, inhibition of the evoked IPSC was associated with a reduction in paired-pulse depression, suggesting that presynaptic μ- and δ-opioid receptors modulate GABAergic transmission in the STN.

Figure 4. Opioids reduce paired-pulse depression of GABAA IPSCs.

Figure 4

Open in a new tab

A-C, left: superimposed IPSCs evoked by pairs of identical stimuli (50 ms apart) in the absence (Control and Wash) and presence of ME (10 μM; A), DAMGO (1 μM; B) and DPDPE (1 μM; C). Right: traces are the same as those shown on the left, except that the amplitude of the second IPSC has been normalized to match the amplitude of the first IPSC. Opioids increase the paired-pulse ratio.

Opioid receptor-mediated inhibition of evoked EPSCs

After blockade of inhibitory synaptic transmission by the addition of bicuculline (30 μM), a focal stimulus evoked an EPSC in all STN neurons observed. This evoked EPSC could be completely blocked by CNQX (10 μM) plus AP5 (50 μM). Application of ME reversibly reduced the amplitude of evoked EPSCs in a concentration-dependent manner with an IC50 value of 0.35 ± 0.14 μM (n = 5; Fig. 5A and B). ME (10 μM) reduced the peak amplitude of evoked EPSCs by 22 ± 2 % (P < 0.01, paired t test; n = 15). The effect of ME on evoked EPSCs was mimicked by the μ-selective agonist DAMGO (1 μM), which reduced the peak amplitude of evoked EPSCs by 19 ± 4 % (Fig. 5C and Fig. 6A; P < 0.01, paired t test; n = 10). The δ-selective agonist DPDPE had little effect on the amplitude of evoked EPECs (reduced by 7 ± 5 %; P = 0.658, paired t test; n = 5; Fig. 5C). The κ-selective agonist U-69593 (up to 10 μM) and the orphan receptor agonist OFQ (1 μM) had no effect (Fig. 5C). The effect of ME on evoked EPSCs was completely blocked by naloxonazine (Fig. 6B). In the presence of naloxonazine (1 μM), ME (10 μM) reduced the EPSC amplitude by only 5 ± 3 % (P < 0.01; n = 8).

Figure 5. Opioids inhibit evoked glutamatergic EPSCs.

Figure 5

Open in a new tab

A, current traces showing that ME reversibly reduces glutamate EPSCs. ME at 10 μM caused a 30 % reduction in the amplitude of EPSCs. In this and Fig. 6, each EPSC trace represents the average of 10 consecutive responses. B, concentration-response curve for the percentage inhibition of glutamate EPSC amplitude by ME. Each point represents the mean ± s.e.m. of 5 cells. C, summary of results showing the percentage inhibition of EPSCs by ME (10 μM, n = 15), DAMGO (1 μM, n = 10), DPDPE (1 μM, n = 5), U-69593 (1 μM, n = 4) and OFQ (1 μM, n = 3).

Figure 6. Opioid-induced inhibition of evoked glutamate EPSCs by activation of presynaptic μ-receptors.

Figure 6

Open in a new tab

A, summarized results showing the time course of reduction in EPSC amplitude by ME (▪, 10 μM, n = 15) and the μ-selective agonist DAMGO (□, 1 μM, n = 10). Note that DAMGO mimicked the action of ME. B, the μ-selective antagonist naloxonazine (1 μM) blocked the action of ME. *Significant difference from ME (P < 0.01). C and D, left: superimposed EPSCs evoked by pairs of identical stimuli (50 ms apart) in the absence (Control and Wash) and presence of ME (10 μM; C) and DAMGO (1 μM; D). Right: traces are the same as those shown on the left, except that the amplitude of the second EPSC has been normalized to match the amplitude of the first EPSC.

In order to determine the site of action, the effect of opioid agonists on the paired-pulse ratio (EPSC2/EPSC1) was examined (Fig. 6C and D). In the absence of ME, the mean paired-pulse ratio of the evoked EPSC amplitudes was 1.04 ± 0.03 (n = 12). However, in the presence of ME (10 μM), this EPSC paired-pulse ratio increased to 1.18 ± 0.04 in the same neurons (P < 0.01, paired t test). DAMGO (1 μM) mimicked ME by increasing the EPSC paired-pulse ratio from 1.04 ± 0.04 to 1.25 ± 0.06 in the same neurons (P = 0.007, paired t test; n = 6). These results indicate that presynaptic μ-opioid receptor activation inhibits glutamate release in the STN.

Effect of opioids on mEPSCs

Spontaneous EPSCs, recorded with pipettes containing caesium gluconate, were seen as transient downward deflections (inward currents), illustrated in Fig. 7A. In contrast, spontaneous inhibitory postsynaptic currents (upward deflections) were very rarely observed. The mEPSC recorded in TTX (0.5 μM) occurred at a mean frequency of 4.6 ± 0.7 Hz with a mean amplitude of 18.1 ± 3.2 pA at −70 mV (n = 7). These mEPSCs were completely abolished by adding CNQX (10 μM) to the superfusate. As illustrated in Fig. 7A and B, application of ME (10 μM) caused a significant reduction in the mean frequency of spontaneous mEPSCs to 1.8 ± 0.4 Hz (P = 0.002) without significant change in the mean amplitude (17.5 ± 2.8 pA) in the same neurons (P = 0.18; Fig. 7C). As shown in Fig. 7B and C, DAMGO exhibited a similar action on mEPSCs. In the presence of DAMGO (1 μM), the mean frequency of mEPSCs decreased from 5.4 ± 0.5 to 2.3 ± 0.4 Hz (P = 0.003; n = 8), whereas the mean amplitude was not significantly altered (from 12.5 ± 1.0 to 13.4 ± 1.1 pA; P = 0.152). In contrast, DPDPE (1 μM) had no effect on the mean frequency (from 2.1 ± 0.6 to 1.9 ± 0.6 Hz; P = 0.114; n = 4; Fig. 7B) and amplitude (from 12.1 ± 1.1 to 11.3 ± 1.2 pA; P = 0.088; Fig. 7C) of mEPSCs.

Figure 7. Opioids decrease the frequency of mEPSCs in the STN neurons.

Figure 7

Open in a new tab

A, two consecutive traces of mEPSCs before (left), during (middle) and after (right) application of ME (10 μM). All traces shown are from the same neuron, which was clamped at −70 mV. B and C, plots of mean frequency (B) and amplitude (C) of mEPSCs in the absence and presence of ME (10 μM, n = 7), DAMGO (1 μM, n = 8) and DPDPE (1 μM, n = 4). *Significant difference from Control (P < 0.05).

Effects of potassium conductance blockade on opioid inhibition of evoked synaptic currents

Activation of opioid receptors has been reported to open 4-AP-sensitive potassium channels and thereby to inhibit synaptic transmission in the rat eriaqueductal grey (Vaughan et al. 1997). Therefore, the actions of ME were tested by using the voltage-dependent potassium channel blocker 4-AP. Application of 4-AP (100 μM) alone dramatically increased the frequency of spontaneous EPSC events and the amplitude of both evoked EPSCs (by 110 ± 20 %, n = 10) and IPSCs (by 156 ± 21 %; n = 7). ME-induced inhibition of evoked synaptic currents was significantly reduced by 4-AP (Fig. 8). In control, ME (10 μM) inhibited the amplitude of the evoked GABA IPSCs by 42 ± 5 % (n = 13), and the amplitude of the evoked glutamate EPSCs by 20 ± 4 % (n = 15). In the presence of 4-AP (100 μM), ME-induced inhibition was reduced to 4 ± 2 % (n = 8) for evoked IPSCs and 2 ± 2 % (n = 3) for evoked EPSCs. As reported previously in this laboratory, baclofen also reduced evoked IPSCs and EPSCs in the STN via activation of presynaptic GABAB receptors (Shen & Johnson, 2001). However, baclofen-induced inhibition of evoked synaptic currents was only partially blocked by 4-AP. In control, baclofen inhibited evoked IPSCs by 69 ± 2 % (n = 8), and evoked EPSCs by 44 ± 6 % (n = 12). In the presence of 4-AP (100 μM), baclofen still caused 25 ± 3 % (n = 3) and 27 ± 6 % (n = 3) inhibition of evoked IPSCs and EPSCs, respectively.

Figure 8. Opioid inhibition of synaptic currents is mediated by a presynaptic potassium conductance.

Figure 8

Open in a new tab

A and B, summary bar graphs showing the effects of 4-AP on the ME-induced inhibition of IPSCs (A) and EPSCs (B). Note that the inhibition produced by ME (10 μM) was completely blocked by 4-AP (100 μM), whereas the inhibition caused by baclofen (10 μM) was only partially blocked by 4-AP.

DISCUSSION

The three principal findings in the present study are: (1) opioids inhibit evoked GABA release via presynaptic μ- and δ-receptors; (2) opioids presynaptically inhibit evoked and spontaneous mEPSCs only via μ-receptors; and (3) the actions of ME on glutamate and GABA release are mediated by modulation of a presynaptic 4-AP-sensitive potassium conductance.

Our pharmacological studies using selective opioid agonists and antagonists support the conclusion that μ-and δ-receptors appear to be involved in inhibition of the release of GABA in the STN. First, the action of ME was mimicked by the μ-selective agonist DAMGO and δ-selective agonist DPDPE. Second, the effect of ME or DAMGO was blocked by the μ-selective antagonists naloxonazine and CTAP. Third, the effect of DPDPE was blocked by the δ-selective antagonist ICI 174,864. Fourth, the κ-selective agonist U69593 and orphan receptor agonist OFQ had no significant effect on GABA IPSCs. The inhibitory action of opioids is not likely to result from their postsynaptic response because ME and DAMGO only caused very weak outward currents in about half of the STN neurons and DPDPE had no noticeable effect on membrane current in any STN neurons tested, while opioid-induced inhibition of GABA IPSCs was observed in all STN neurons examined. On the other hand, opioid-induced reduction in the amplitude of evoked GABA IPSCs has been associated with a decrease in the paired-pulse depression. This finding supports a presynaptic action of opioids to reduce the release of GABA (Davies et al. 1990). Opioids also inhibited the release of excitatory amino acids in the STN. Only μ-receptors appear to mediate this inhibition. This conclusion is based on (1) mimicry of the action of ME by DAMGO; (2) lack of effect of DPDPE, U69593 and OFQ; and (3) blockade of the effect of ME by μ-selective antagonists. Opioids reduced the amplitude of evoked EPSCs, while increasing the paired-pulse facilitation. Furthermore, opioids decreased the frequency of spontaneous miniature EPSCs without altering their amplitude. These observations indicate a presynaptic action of opioids to reduce the release of glutamate.

It can be interpreted, based on the present results, that presynaptic glutamate-containing fibres have predominantly μ-type receptors. However, it is impossible to determine from these experiments whether the μ- and δ-receptors are on the same or different presynaptic GABA-containing fibres. The maximal inhibition of GABA IPSCs observed with ME or DAMGO was about 50 % and the maximal inhibition of GABA IPSCs with DPDPE was only about 20 %, implying that at least a proportion of GABA-containing terminals have both μ- and δ-receptors. Interestingly, in several brain areas, excitatory or inhibitory synaptic transmission has been reported to be inhibited by activation of both μ- and δ-receptors (rat spinal cord, Glaum et al. 1994; rat striatum, Jiang & North, 1992; rat dentate gyrus, Piguet & North, 1993; rat lateral amygdala, Sugita & North, 1993; substantia gelatinosa of rat spinal cord, Kohno et al. 1999). In the mouse hypogastric ganglion, activation of μ- or δ-receptors has been demonstrated to inhibit the same single fibre inputs (Rogers & Henderson, 1990). These observations all suggest possible co-localization of μ- and δ-opioid receptors at the same presynaptic terminals. North et al. (1987) have reported that the selective agonist for μ- or δ-receptors increases the same potassium conductance, suggesting that the μ- and δ-receptors utilize the same effector mechanisms (North et al. 1987). If so, the effects of μ- and δ-receptors on GABAergic transmission in the STN would occlude each other. This hypothesis can be used to interpret why the maximal inhibition of IPSC amplitude by the selective μ-agonist DAMGO is comparable to that by the non-selective agonist ME. Presynaptic inhibition of the release of GABA or excitatory amino acids only through μ- or δ-receptors has also been reported for the rat anterior cingulate cortex (Tanaka & North, 1994), periaqueductal grey (Vaughan & Christie, 1997), arcuate nucleus and ventromedial hypothalamus (Emmerson & Miller, 1999), and globus pallidus (Stanford & Cooper, 1999). The present study provides the first physiological demonstration of the functional presence of opioid receptors in the STN. Several immunohistochemical and in situ hybridization studies in rat brain show a moderate number of enkephalinergic terminals in the STN (Delfs et al. 1994; Peckys & Landwehrmeyer, 1999; Wamsley et al. 1980). However, the origin of the enkephalin-containing fibres is not clear. Some of them may arise from enkephalin-containing neurons in striatum. However, there is no morphological evidence that striatum sends direct projections to the STN.

Potassium channels are responsible for setting the resting potential of nerve terminals and for repolarizing their membrane during action potentials. As a result of these actions, potassium channels play a role in regulating transmitter release. There are a number of types of different potassium channels, including calcium-activated potassium channels, ATP-sensitive potassium channels, and A-type channels. They are known to be present on nerve terminals in central and peripheral nervous systems. Activation of presynaptic opioid receptors causes these potassium channels to open. An increase in potassium conductance would limit calcium influx, thereby reducing the release of neurotransmitters. A-type channels are voltage gated and exhibit fast inactivation. They are blocked by 4-AP with an IC50 which varies from 100 μM to 2 mm (Mathie et al. 1998). Activation of presynaptic voltage-dependent potassium conductance has been reported to mediate opioid presynaptic inhibition of glutamate release at hippocampal mossy fibres (Simmons & Chavkin, 1996) and at the terminals to dopamine neurons in ventral tegmental area (Manzoni & Williams, 1999). Presynaptic inhibition of GABA release by opioids in periaqueductal grey has also been shown to result from the activation of this voltage-dependent potassium conductance (Vaughan et al. 1997). An immunohistochemical study demonstrated a high level of expression of Kv1 channel subunits in the STN, suggesting a possible role for A-type channels in regulating synaptic transmission in the STN (Chung et al. 2000). In this study, we found that the opioid-induced inhibition of both GABAergic and glutamatergic synaptic transmission in the STN was abolished by the voltage-dependent potassium channel blocker, 4-AP. This indicates that opioid inhibition of both GABA and glutamate release from terminals in the STN is mediated by activation of a presynaptic 4-AP-sensitive potassium conductance. However, the baclofen-mediated presynaptic inhibition was only partially blocked by 4-AP, suggesting that opioid and baclofen inhibition of GABA and glutamate release may not share a common mechanism. Our data using 4-AP do not really support the idea that potassium channels play a major role in mediating presynaptic inhibition of opioids. Transmitter release has been found to be mostly dependent on the regulation of calcium channels. It has been reported that 4-AP-induced glutamate release in rat striatal synaptosomes is highly calcium dependent and can be blocked by calcium channel antagonists (Hill & Brotchie, 1999). Further studies to compare the effects of calcium channel blockers with those of 4-AP on opioid inhibition of synaptic transmission in the STN would be worthwhile.

Interestingly, the STN neurons recorded from horizontal slices only exhibited spontaneous excitatory postsynaptic currents. Application of 4-AP (100 μM) caused a significant increase in the frequency of spontaneous EPSCs in all neurons tested, but still could not evoke spontaneous IPSCs in most of the neurons (data not shown). These results suggest that the excitatory inputs possibly play a more important role than GABAergic afferents in regulating neuronal activity in the STN. Regulation of STN by excitatory amino acids may be especially important for producing the hyperactivity of STN neurons during chronic dopamine depletion seen in Parkinson's disease. There are several lines of evidence supporting this implication. First, Hassani et al. (1996) reported that the increased neuronal activity and changes in firing pattern observed in the STN after nigrostriatal lesions are not reproduced by external globus pallidus lesions, which only cause a slight increase in the discharge rate of STN neurons. Second, blockade of glutamatergic transmission in the STN prevented the overactivity of the STN under dopamine depletion (Miwa et al. 1998) These results indicate that the disinhibition from the pallidal GABAergic inputs associates with an increase in excitatory drive from somewhere in the brain to cause overactivity of STN neurons following dopamine depletion. These excitatory glutamatergic drives may originate from the cerebral cortex, parafascicular thalamic complex and the pedunculopontine tegmental nucleus (Parent & Hazrati, 1995). Another important excitatory drive may come from the STN intrinsic axon collaterals (Iwahori, 1978; Chang et al. 1983; Afsharpour, 1985), which form the basis for a positive feedback that contribute to a long-term excitation of STN neurons. Although opioids inhibit GABA release more effectively than glutamate (maximal percentage inhibition: 40 % for IPSCs vs. 20 % for EPSCs), they are apparently more potent for inhibiting glutamate release (IC50 value: 0.35 μM for EPSCs vs. 1.1 μM for IPSCs). Based on the experimental findings reported above, selective reduction of glutamatergic transmission in the STN by opioids would be expected to alleviate symptoms of Parkinson's disease. However, effects of opioid analogues on motor behaviour observed in normal animal or parkinsonian patients and models are difficult to reconcile with this hypothesis. When administered to normal rats, morphine leads to locomotor activation at low doses, and induces locomotor depression at larger doses (see Baronti et al. 1991). In patients with Parkinson's disease and l-dopa-induced dyskinesia, morphine causes a decrease in dyskinesia at lower doses and an increase in akinesia at higher doses (Berg et al. 1999). Opiate agonist has also been reported to potentiate melatonin-induced attenuation of reserpine-induced catalepsy (Sandyk & Mukherjee, 1989). All these results suggest that further studies are needed to clarify the complicated actions of the opioid system in the STN as well as in basal ganglia in order for opioid analogues to be explored as possible therapeutic agents in the treatment of Parkinson's disease or side effects of neuroleptic drugs.

Acknowledgments

This study was supported by USPHS grant NS38715 (S.W.J.) and the Medical Research Foundation of Oregon (K.-Z.S.).

REFERENCES

  1. Afsharpour S. Light microscopic analysis of Golgi-impregnated rat subthalamic neurons. Journal of Comparative Neurology. 1985;236:1–13. doi: 10.1002/cne.902360102. [DOI] [PubMed] [Google Scholar]
  2. Albin RL. The pathophysiology of chorea/ballism and parkinsonism. Parkinsonism and Related Disorders. 1995;1:3–11. doi: 10.1016/1353-8020(95)00011-t. [DOI] [PubMed] [Google Scholar]
  3. Baronti F, Conant KE, Giuffra M, Davis TL, Brughitta G, Iadarola MJ, Berrettini WH, Chase TN, Mouradian MM. Opioid peptides in Parkinson's disease: effects of dopamine repletion. Brain Research. 1991;560:92–96. doi: 10.1016/0006-8993(91)91219-q. [DOI] [PubMed] [Google Scholar]
  4. Berg D, Becker G, Reiners K. Reduction of dyskinesia and induction of akinesia induced by morphine in two parkinsonian patients with severe sciatica. Journal of Neural Transmission. 1999;106:725–728. doi: 10.1007/s007020050192. [DOI] [PubMed] [Google Scholar]
  5. Chang HT, Kita H, Kitai ST. The fine structure of the rat subthalamic nucleus: an electron microscopic study. Journal of Comparative Neurology. 1983;221:113–123. doi: 10.1002/cne.902210110. [DOI] [PubMed] [Google Scholar]
  6. Chesselet MF, Delfs JM. Basal ganglia and movement disorders: an update. Trends in Neurosciences. 1996;19:417–422. doi: 10.1016/0166-2236(96)10052-7. [DOI] [PubMed] [Google Scholar]
  7. Chung YH, Shin C-M, Kim MJ, Cha CI. Immunohistochemical study on the distribution of six members of the Kv1 channel subunits in the rat basal ganglia. Brain Research. 2000;875:164–170. doi: 10.1016/s0006-8993(00)02586-5. [DOI] [PubMed] [Google Scholar]
  8. Crossman AR. A hypothesis on the pathophysiological mechanisms that underlie levodopa- or dopamine agonist-induced dyskinesia in Parkinson's disease: Implications for future strategies in treatment. Movement Disorders. 1990;5:100–108. doi: 10.1002/mds.870050203. [DOI] [PubMed] [Google Scholar]
  9. Davies CH, Davies SN, Collingridge GL. Paired-pulse depression of monosynaptic GABA-mediated inhibitory postsynaptic responses in rat hippocampus. Journal of Physiology. 1990;424:513–531. doi: 10.1113/jphysiol.1990.sp018080. [DOI] [PMC free article] [PubMed] [Google Scholar]
  10. Delfs JM, Kong H, Mestek A, Chen Y, Yu L, Raiaine T, Chesselet M-F. Expression of mu opioid receptor mRNA in rat brain: An in situ hybridization study at the single cell level. Journal of Comparative Neurology. 1994;345:46–68. doi: 10.1002/cne.903450104. [DOI] [PubMed] [Google Scholar]
  11. DeLong MR. Primate models of movement disorders of basal ganglia origin. Trends in Neurosciences. 1990;13:281–285. doi: 10.1016/0166-2236(90)90110-v. [DOI] [PubMed] [Google Scholar]
  12. Diamond BI, Borisov RL. Enkephalin and nigrostriatal function. Neurology. 1978;28:1085–1088. doi: 10.1212/wnl.28.11.1805. [DOI] [PubMed] [Google Scholar]
  13. Emmerson PJ, Miller RJ. Pre- and postsynaptic actions of opioid and orphan opioid agonists in the rat arcuate nucleus and ventromedial hypothalamus in vitro. Journal of Physiology. 1999;517:431–445. doi: 10.1111/j.1469-7793.1999.0431t.x. [DOI] [PMC free article] [PubMed] [Google Scholar]
  14. Emson PC, Arregui A, Clement-jones V, Sandberg BEB, Rossor M. Regional distribution of methionine-enkephalin and substance P-like immunoreactivity in normal human brain and in Huntington's disease. Brain Research. 1980;199:147–160. doi: 10.1016/0006-8993(80)90237-1. [DOI] [PubMed] [Google Scholar]
  15. Fernandez A, De Ceballos ML, Jenner P, Marsden CD. Neurotensin, substance P, delta and mu opioid receptors are decreased in basal ganglia of Parkinson's disease patients. Neuroscience. 1994;61:73–79. doi: 10.1016/0306-4522(94)90061-2. [DOI] [PubMed] [Google Scholar]
  16. Florin S, Meunier J-C, Costentin J. Autoradiographic localization of [3H]nociceptin binding sites in the rat brain. Brain Research. 2000;880:11–16. doi: 10.1016/s0006-8993(00)02669-x. [DOI] [PubMed] [Google Scholar]
  17. Glaum SR, Miller RJ, Hammond DL. Inhibitory actions of δ1-, δ2-, and μ-opioid receptor agonists on excitatory transmission in lamina II neurons of adult rat spinal cord. Journal of Neuroscience. 1994;14:4965–4971. doi: 10.1523/JNEUROSCI.14-08-04965.1994. [DOI] [PMC free article] [PubMed] [Google Scholar]
  18. Gramsch D, Hollt V, Mehraein P, Pasi A, Herz A. Regional distribution of methionine-enkephalin and β-endorphin like immunoreactivity in human brain and pituitary. Brain Research. 1979;171:261–270. doi: 10.1016/0006-8993(79)90332-9. [DOI] [PubMed] [Google Scholar]
  19. Gropetti A, Ceresoli G, Mandelli V, Parenti M. Role of opiate in striatal D-1 dopamine receptor supersensitivity induced by chronic L-dopa treatment. Journal of Pharmacology and Experimental Therapeutics. 1990;253:950–956. [PubMed] [Google Scholar]
  20. Hassani O-K, Mouroux M, Feger J. Increased subthalamic neuronal activity after nigral dopaminergic lesion independent of disinhibition via the globus pallidus. Neuroscience. 1996;72:105–115. doi: 10.1016/0306-4522(95)00535-8. [DOI] [PubMed] [Google Scholar]
  21. Hill MP, Brotchie JM. Control of glutamate release by calcium channels and κ-opioid receptors in rodent and primate striatum. British Journal of Pharmacology. 1999;127:275–283. doi: 10.1038/sj.bjp.0702523. [DOI] [PMC free article] [PubMed] [Google Scholar]
  22. Iwahori H. A Golgi study on the subthalamic nucleus of the cat. Journal of Comparative Neurology. 1978;182:383–398. doi: 10.1002/cne.901820303. [DOI] [PubMed] [Google Scholar]
  23. Jiang ZG, North RA. Pre- and postsynaptic inhibition by opioids in rat striatum. Journal of Neuroscience. 1992;12:356–361. doi: 10.1523/JNEUROSCI.12-01-00356.1992. [DOI] [PMC free article] [PubMed] [Google Scholar]
  24. Kohno T, Kumamoto E, Higashi H, Shimoji K, Yoshimura M. Actions of opioids on excitatory and inhibitory transmission in substantia gelatinosa of adult rat spinal cord. Journal of Physiology. 1999;518:803–813. doi: 10.1111/j.1469-7793.1999.0803p.x. [DOI] [PMC free article] [PubMed] [Google Scholar]
  25. Mansour A, Fox CA, Akil H, Watson SJ. Opioid-receptor mRNA expression in the rat CNS: anatomical and functional implications. Trends in Neurosciences. 1995;18:22–29. doi: 10.1016/0166-2236(95)93946-u. [DOI] [PubMed] [Google Scholar]
  26. Manzoni O, Williams JT. Presynaptic regulation of glutamate release in the ventral tegmental area during morphine withdrawal. Journal of Neuroscience. 1999;19:6629–6636. doi: 10.1523/JNEUROSCI.19-15-06629.1999. [DOI] [PMC free article] [PubMed] [Google Scholar]
  27. Mathie A, Wooltorton JR, Watkins CS. Voltage-activated potassium channels in mammalian neurons and their block by novel pharmacological agents. General Pharmacology. 1998;30:13–24. doi: 10.1016/s0306-3623(97)00034-7. [DOI] [PubMed] [Google Scholar]
  28. Miwa H, Nishi K, Fuwa T, Mizuno Y. Postural effects of unilateral blockade of glutamatergic neurotransmission in the subthalamic nucleus on haloperidol-induced akinesia in rats. Neuroscience Letters. 1998;252:167–170. doi: 10.1016/s0304-3940(98)00559-x. [DOI] [PubMed] [Google Scholar]
  29. North RA. Opioid actions on membrane ion channels. In: Herz A, editor. Opioids. Vol. 1. Berlin: Springer-Verlag; 1993. pp. 773–798. [Google Scholar]
  30. North RA, Williams JT, Surprenant A, Christie MJ. μ and δ receptors belong to a family of receptors that are coupled to potassium channels. Proceedings of the National Academy of Sciences of the USA. 1987;84:5487–5491. doi: 10.1073/pnas.84.15.5487. [DOI] [PMC free article] [PubMed] [Google Scholar]
  31. Parent A, Hazrati L-N. Functional anatomy of the basal ganglia. II. The place of subthalamic nucleus and external pallidum in basal ganglia circuitry. Brain Research Reviews. 1995;20:128–154. doi: 10.1016/0165-0173(94)00008-d. [DOI] [PubMed] [Google Scholar]
  32. Paxinos TJ, Watson G. The Rat Brain in Stereotaxic Coordinates. 2. San Diego: Academic Press; 1986. [Google Scholar]
  33. Peckys D, Landwehrmeyer GB. Expression of μ, κ, and δ opioid receptor messenger RNA in the human CNS: A 33P in situ hybridization study. Neuroscience. 1999;88:1093–1135. doi: 10.1016/s0306-4522(98)00251-6. [DOI] [PubMed] [Google Scholar]
  34. Piguet P, North RA. Opioid actions at μ and δ receptors in the rat dentate gyrus in vitro. Journal of Pharmacology and Experimental Therapeutics. 1993;266:1139–1146. [PubMed] [Google Scholar]
  35. Raynor K, Kong H, Mestek A, Bye LS, Tian M, Liu J, Yu L, Reisine T. Characterization of the cloned human μ opioid receptor. Journal of Pharmacology and Experimental Therapeutics. 1995;272:423–428. [PubMed] [Google Scholar]
  36. Rogers H, Henderson G. Activation of μ- and δ-opioid receptors present on the same nerve terminals depresses transmitter release in the mouse hypogastric ganglion. British Journal of Pharmacology. 1990;101:505–512. doi: 10.1111/j.1476-5381.1990.tb14112.x. [DOI] [PMC free article] [PubMed] [Google Scholar]
  37. Sandyk R, Mukherjee S. Attenuation of reserpine-induced catalepsy by melatonin and the role of the opioid system. International Journal of Neuroscience. 1989;48:297–301. doi: 10.3109/00207458909002173. [DOI] [PubMed] [Google Scholar]
  38. Sandyk R, Snider SR. Naloxone treatment of L-dopa induced dyskinesias in Parkinson's disease. American Journal of Psychiatry. 1986;143:118. doi: 10.1176/ajp.143.1.118a. [DOI] [PubMed] [Google Scholar]
  39. Sasaki T, Kennedy JL, Nobrega JN. Autoradiographic mapping of μ opioid receptor changes in rat brain after long-term haloperidol treatment: relationship to the development of vacuous chewing movements. Psychopharmacology. 1996;128:97–104. doi: 10.1007/s002130050115. [DOI] [PubMed] [Google Scholar]
  40. Shen K-Z, Johnson SW. Presynaptic GABAB and adenosine A1 receptors regulate synaptic transmission to rat substantia nigra reticulata neurones. Journal of Physiology. 1997;505:153–163. doi: 10.1111/j.1469-7793.1997.153bc.x. [DOI] [PMC free article] [PubMed] [Google Scholar]
  41. Shen K-Z, Johnson SW. Presynaptic dopamine D2 and muscarine M3 receptors inhibit excitatory and inhibitory transmission to rat subthalamic neurones in vitro. Journal of Physiology. 2000;525:331–341. doi: 10.1111/j.1469-7793.2000.00331.x. [DOI] [PMC free article] [PubMed] [Google Scholar]
  42. Shen K-Z, Johnson SW. Presynaptic GABAB receptors inhibit synaptic inputs to rat subthalamic neurons. Neuroscience. 2001;108:431–436. doi: 10.1016/s0306-4522(01)00424-9. [DOI] [PubMed] [Google Scholar]
  43. Simmons ML, Chavkin C. κ-Opioid receptor activation of a dendrotoxin-selective potassium channel mediates presynaptic inhibition of mossy fiber neurotransmitter release. Molecular Pharmacology. 1996;50:80–85. [PubMed] [Google Scholar]
  44. Sivam SP. Dopamine dependent decrease in enkephalin and substance P levels in basal ganglia regions of postmortem parkinsonian brains. Neuropeptides. 1991;18:210–207. doi: 10.1016/0143-4179(91)90148-c. [DOI] [PubMed] [Google Scholar]
  45. Stanford IM, Cooper AJ. Presynaptic μ and δ opioid receptors modulation of GABAA IPSCs in the rat globus pallidus in vitro. Journal of Neuroscience. 1999;19:4796–4803. doi: 10.1523/JNEUROSCI.19-12-04796.1999. [DOI] [PMC free article] [PubMed] [Google Scholar]
  46. Sugita S, North RA. Opioid actions on neurons of rat lateral amygdala in vitro. Brain Research. 1993;612:151–155. doi: 10.1016/0006-8993(93)91655-c. [DOI] [PubMed] [Google Scholar]
  47. Tanaka E, North RA. Opioid action on rat anterior cingulate cortex neurons in vitro. Journal of Neuroscience. 1994;14:1106–1113. doi: 10.1523/JNEUROSCI.14-03-01106.1994. [DOI] [PMC free article] [PubMed] [Google Scholar]
  48. Taquet H, Javoy-Agid F, Giraud P, Legrand JC, Agid Y, Cesselin F. Dynorphin levels in parkinsonian patients: Leu5-enkephalin production from either proenkephalin a or prodynorphin in human brain. Brain Research. 1985;341:390–392. doi: 10.1016/0006-8993(85)91081-9. [DOI] [PubMed] [Google Scholar]
  49. Taquet H, Javoy-Agid F, Hamon M, Legrand JC, Agid Y, Cesselin F. Parkinson's disease affects differently Met5 and Leu5-enkephalin in the human brain. Brain Research. 1983;280:379–382. doi: 10.1016/0006-8993(83)90071-9. [DOI] [PubMed] [Google Scholar]
  50. Trabucchi M, Bassi S, Frattola L. Effect of naloxone on the “on-off” syndrome in patients receiving long-term levodopa therapy. Archives of Neurology. 1982;39:120–121. doi: 10.1001/archneur.1982.00510140054014. [DOI] [PubMed] [Google Scholar]
  51. Vaughan CW, Christie MJ. Presynaptic inhibitory action of opioids on synaptic transmission in the rat periequeductal grey in vitro. Journal of Physiology. 1997;498:463–472. doi: 10.1113/jphysiol.1997.sp021872. [DOI] [PMC free article] [PubMed] [Google Scholar]
  52. Vaughan CW, Ingram SL, Connor MA, Christie MJ. How opioids inhibit GABA-mediated neurotransmission. Nature. 1997;390:611–614. doi: 10.1038/37610. [DOI] [PubMed] [Google Scholar]
  53. Vermeulen RJ, Drukarch B, Sahadat MC, Goosen C, Schoffelmeer AN, Wolters EC, Stoof JC. Morphine and naltrexone modulate D2 but not D1 receptor induced motor behavior in MPTP-lesioned monkeys. Psychopharmacology. 1995;118:451–459. doi: 10.1007/BF02245946. [DOI] [PubMed] [Google Scholar]
  54. Wamsley JK, Young WS, III, Kuhar MJ. Immunohistochemical localization in rat forebrain. Brain Research. 1980;190:153–174. doi: 10.1016/0006-8993(80)91166-x. [DOI] [PubMed] [Google Scholar]
  55. Zamir N, Skofitsch G, Bannon MJ, Helke CJ, Kopin IJ, Jacobowitz DM. Primate model of Parkinson's disease: alterations in multiple opioid systems in the basal ganglia. Brain Research. 1984;322:356–360. doi: 10.1016/0006-8993(84)90133-1. [DOI] [PubMed] [Google Scholar]

Articles from The Journal of Physiology are provided here courtesy of The Physiological Society

RESOURCES