Actions of a long-acting somatostatin analog SMS201-995 (sandostatin) on rat locus coeruleus neurons

Advances in understanding neuronal somatostatin receptors

It has long been considered that somatostatin acts as a neuromodulator in the mammalian central nervous system but its precise physiological roles remain elusive. Early studies to identify somatostatin-binding sites revealed a widespread heterogeneous pattern, especially in the CNS. More recently, a family of somatostatin receptors have been identified, of which five genes (sst(1-5)) have been cloned. In this review, we discuss current data describing the localisation of the five receptor types. Recent progress in understanding their function has been made using high-affinity, selective receptor ligands and transgenic animal technology. Finally, the therapeutic potential for somatostatin receptor-selective compounds as analgesics is considered.

Activation of rat locus coeruleus neuron GABA(A) receptors by propofol and its potentiation by pentobarbital or alphaxalone

The action of propofol on the rat locus coeruleus was examined using intracellular recording from in vitro brain slice preparations. Concentrations of propofol between 3 and 300 microM were tested. At 100 microM, propofol completely inhibited the firing of all neurons tested (n=34); this was associated with a 5.7-mV hyperpolarization (range 0-16 mV, n=33) and a 35.6% reduction in input resistance (range 7.3-66.1%, n=33). The propofol-induced responses were not affected by 2-hydroxysaclofen (50 microM) or BaCl(2) (300 microM), but were completely blocked by bicuculline methiodide (100 microM) or picrotoxin (100 microM), indicating that propofol acts on GABA(A) receptors. As assessed by inhibition of the spontaneous firing rate, propofol was 5.6-fold more potent than GABA (gamma-aminobutyric acid). Potentiation of the propofol effect by other general anesthetics or other drugs was also investigated. When pentobarbital (100 microM) was tested alone on locus coeruleus cells, no change in membrane potential or input resistance was seen and there was only a 20.3+/-7.2% (n=8) inhibition of firing rate; however, in combination with 30 microM propofol, it caused a 6.1-fold greater increase in membrane hyperpolarization and a 9.7-fold greater reduction in input resistance than 30 microM propofol alone. A relatively low concentration of alphaxalone (10 microM), when tested alone, had little effect on the membrane potential or input resistance and only produced a 46.0+/-8.9% (n=8) inhibition of firing rate; however, in combination with 30 microM propofol, it caused a 9.3-fold greater hyperpolarization and an 8.6-fold greater reduction in input resistance compared with 30 microM propofol alone. In contrast, diazepam caused no potentiation of either propofol- or GABA-induced responses. Our data also indicate that locus coeruleus neuron GABA(A) receptors possess distinctive pharmacologic characteristics, such as blocking of the propofol effects by zinc and insensitivity to diazepam and the direct action of pentobarbital. On the basis of these pharmacologic properties, we suggest that locus coeruleus neuron GABA(A) receptors do not contain the gamma subunit.

Cortistatin increase of a potassium conductance in rat locus coeruleus in vitro

1. In this study we examined the effects of cortistatin, a putative endogenous ligand for somatostatin (SRIF) receptors, on the membrane properties of rat locus coeruleus (LC) neurones in vitro, by use of intracellular and whole cell patch clamp recording. We have compared the actions of cortistatin with those of SRIF and the SRIF analogue D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP). 2. When LC neurones were voltage clamped to -60 mV, application of cortistatin caused an outward current in all cells examined (n = 44), with a pEC50 of 6.62. SRIF also caused an outward current in all cells examined (n = 43), with a pEC50 of 6.93. 3. The outward currents caused by cortistatin in 2.5 mM extracellular K+ reversed polarity at -106 mV, very close to the predicted K+ reversal potential of -105 mV. Increasing extracellular K+ to 10.5 mM resulted in a shift of the reversal potential of +38 mV, a shift consistent with a K+ conductance. The conductance activated by cortistatin showed mild inward rectification. 4. Continuous application of a high concentration of SRIF (1 microM) resulted in a decrease of the outward current to a steady level of 49% of the maximum response, with a t1/2 of 131 s. Application of a high concentration of cortistatin (3 microM) during the desensitized portion of the SRIF response did not result in any further outward current. Continuous application of a high concentration of cortistatin (10 microM) resulted in a decrease of the outward current to a steady level of 42% of the maximum response with a t1/2 of 114 s. Application of a high concentration of SRIF (3 microM) during the desensitized portion of the cortistatin response produced only a small outward current. 5. Continuous application of cortistatin (3 microM) also resulted in a decrease of the outward current (by 43%, t1/2 of 136 s) and application of a high concentration of CTOP (10 microM) during the desensitized portion of the cortistatin response did not produce any outward current. Continuous application of a high concentration of CTOP (10 microM) resulted in a decrease of the outward current to a steady level of 70% of the maximum response with a t1/2 of 143 s. Application of a high concentration of cortistatin (3 microM) during the desensitized portion of the CTOP response did not result in any further outward current. 6. The actions of cortistatin (300 nM-10 microM) were not affected by the opioid antagonist naloxone (10 microM). Application of met-enkephalin during the desensitized portion of the response to a high concentration of cortistatin (3 microM) produced an outward current similar to that produced by metenkephalin application alone. 7. Thus cortistatin efficaciously activates an inwardly rectifying K+ conductance in LC neurones. These actions appear to be mediated by a population of SRIF receptors, at which CTOP is also an agonist. Cortistatin does not appear to be a ligand for mu-opioid receptors in rat LC neurons.

Operational characteristics of somatostatin receptors mediating inhibitory actions on rat locus coeruleus neurones

1. In order to characterize somatostatin (SRIF) receptor inhibiting spontaneous firing of rat locus coeruleus neurones, and their transduction mechanism(S), extracellular recordings were obtained from a pontine slice preparation of rat brain containing the locus coeruleus (LC). LC neurones were identified by electrophysiological and pharmacological properties; spontaneous firing (characteristically 0.5-5 Hz) was reversibly and concentration-dependently inhibited by exogenously applied noradrenaline. 2. Spontaneous firing of LC neurones was reversibly and concentration-dependently inhibited by SRIF and the N-terminally extended form, somatostatin-28 (SRIF-28), with EC50 values of 15.1 and 19.4 nM, respectively. The synthetic SRIF analogues (octreotide, MK-678, BIM-23027 and L-362,855) also caused concentration-dependent inhibition of LC neurone firing with a rank order of agonist potencies compatible with actions at a receptor resembling the recombinant sst2 receptor. The putative sst3 selective agonist, BIM-23056, was without agonist or antagonist effect. 3. Addition of 100 nM desipramine significantly increased the efficacy of exogenously applied noradrenaline (EC50 values, 2.96 and 0.13 microM, absence and presence of desipramine, respectively) but did not significantly affect SRIF-induced inhibition (EC50 values, 15.6 and 8.0 nM, respectively). Furthermore, application of phenoxybenzamine (3 microM) abolished responses to NA, but did not affect responses to SRIF (EC50 = 14.1 nM). 4. Application of the cyclic AMP analogue, 8-bromoadenosine-cyclic monophosphate (8-Br-cyclic AMP; 500 microM), significantly increased the spontaneous firing rate of all neurones tested (223 +/- 24% over basal rate). Concentration-effect curves for SRIF constructed in the absence and presence of 8-Br-cyclic AMP had similar threshold concentrations, maxima and EC50 values. 5. Incubation of pontine slices in a modified artificial CSF containing 500 ng ml-1 pertussis toxin (PTX) for 18 h prior to extracellular recording affected neither the spontaneous firing of LC neurones, nor the inhibitory responses to muscimol (EC50 2.2 and 1.2 microM, absence and presence of PTX). However, inhibitory responses to SRIF were markedly attenuated. 6. We conclude that the inhibitory actions of SRIF on spontaneous firing of LC neurones are mediated directly by activation of somatodendritic SRIF receptors, and not indirectly by release of noradrenaline. The SRIF receptors involved appear to couple via a pertussis toxin sensitive G-protein, and elicit their response by a mechanism apparently independent of inhibition of cyclic AMP formation. The agonist profile of several selective and novel SRIF analogues suggests the identity of this receptor to be similar to the recombinant sst2 receptor.