Excitatory effects of thyrotropin-releasing hormone in the thalamus

Abstract

The activity of the thalamus is state dependent. During slow-wave sleep, rhythmic burst firing is prominent, whereas during waking or rapid eye movement sleep, tonic, single-spike activity dominates. These state-dependent changes result from the actions of modulatory neurotransmitters. In the present study, we investigated the functional and cellular effects of the neuropeptide thyrotropin-releasing hormone (TRH) on the spontaneously active ferret geniculate slice. This peptide and its receptors are prominently expressed in the thalamic network, yet the role of thalamic TRH remains obscure. Bath application of TRH resulted in a transient cessation of both spindle waves and the epileptiform slow oscillation induced by application of bicuculline. With intracellular recordings, TRH application to the GABAergic neurons of the perigeniculate (PGN) or thalamocortical cells in the lateral geniculate nucleus resulted in depolarization and increased membrane resistance. In perigeniculate neurons, this effect reversed near the reversal potential for K+, suggesting that it is mediated by a decrease in K+ conductance. In thalamocortical cells, the TRH-induced depolarization was of sufficient amplitude to block the generation of rebound Ca2+ spikes, whereas the even larger direct depolarization of PGN neurons transformed these cells from the burst to tonic, single-spike mode of action potential generation. Furthermore, application of TRH prominently enhanced the afterdepolarization that follows rebound Ca2+ spikes, suggesting that this transmitter may also enhance Ca2+-activated nonspecific currents. These data suggest a novel role for TRH in the brain as an intrinsic regulator of thalamocortical network activity and provide a potential mechanism for the wake-promoting and anti-epileptic effects of this peptide.

Figure 1.

TRH reversibly abolishes spindle waves in a dose-dependent manner. Extracellular multiunit recordings from the A-lamina of the LGNd. A, Spontaneous spindle waves occur approximately once every 20 s. Bath application of TRH (50 nm) results in cessation of spindle waves, which resume with washout. B-D, Expansions of portions of the recording in A showing the spindle waves before and after TRH and the lack of spindles during TRH application. E, Multiunit recording at different intervals after beginning of TRH (50 nm) bath application. With sustained delivery of the peptide, spindle waves reappear (20 min in TRH), albeit now less robustly. F, Bath application of a lower concentration (10 nm) of TRH causes a reversible increase in spindle-wave frequency. Top histogram shows number of spindle waves per 100 s; the recording was divided into 100 s periods, and the number of spindles waves within each period was counted by way of visual inspection. Spindles waves displayed a minimal duration of 2 s, and a full return to baseline was required to separate one spindle wave from the succeeding one. Bottom traces (a-c) are examples of the raw recording taken at different intervals as indicated in histogram. Rhythmic bursting is observed during application of 10 nm TRH (Fb); inserted expansion (Fb′) illustrates one such burst consisting of high-frequency discharge at ∼300 Hz.

Figure 2.

TRH inhibits the 3 Hz oscillation induced by GABA_A receptor blockade. Extracellular multiunit activity in the LGNd in which spindle waves and the bicuculline-induced slow oscillation are elicited by electrical stimulation of the optic radiation (•). A, Under control conditions, 7 Hz spindle waves appear in response to afferent stimulation. B, Bath application of the GABA_A receptor antagonist bicuculline methiodide (25 μm) slows the oscillation frequency to a 2-3 Hz network rhythm. C, When TRH (200 nm) is also added to the bath solution, the bicuculline-induced slow oscillation disappears, but it recovers after TRH is washed out (D).

Figure 3.

TRH application increases thalamocortical cell discharge but does not affect local interneuron firing rate. A, Simultaneous multiunit and single-unit recording in the LGNd. The single unit shows regular 12 Hz discharge of short-duration spikes that is transiently inhibited by local application of the muscarinic agonist methylcholine (MCh) (5 mm in micropipette). A, Bottom trace shows discharge frequency of the large (interneuron) unit; note reversible drop in frequency in response to MCh. These characteristics identify this cell as a putative local interneuron (Pape and McCormick, 1995). Local application of TRH (5 μm in micropipette) does not affect firing frequency of this putative interneuron. B, C, Expansions of bottom trace in A, showing interneuron (Int) single-unit activity in B and interneuron and presumed thalamocortical cell (Int+Relay) activity in C. D, Histogram shows spike incidence for multiple units determined at detection level indicated in C. (Bin size is 100 ms.) Note increased combined spike incidence in response to TRH; the application of TRH resulted in an increase in activity of the smaller multi-unit activity (sampled at a lower amplitude), which presumably represents the activity of a subpopulation of thalamocortical cells.

Figure 4.

TRH application switches PGN neurons from burst to tonic firing. A, Extracellular recording from the PGN (bottom trace); top histogram shows spike incidence at detection level indicated in bottom trace. Spindle wave-associated bursting is seen from a single unit at beginning of recording. Local application of TRH (1 μm in micropipette) results in abrupt cessation of spindle waves and the onset of tonic firing with additional units recruited. Recovery to baseline is characterized by successive cycles of tonic firing preceded by bursts. B, C, Expansions of bottom trace in A. Note rhythmic bursts preceding tonic discharge immediately after TRH application (B) and during the gradual recovery from TRH application (C).

Figure 5.

TRH depolarizes thalamocortical neurons and enhances the slow afterdepolarization after Ca²⁺ spikes. Intracellular recording from an LGNd thalamocortical cell. A, Under control conditions, rhythmic IPSP barrages elicit the rebound burst discharges typical of spindle waves. Local application of TRH (5 μm in micropipette) results in a gradual 3 mV depolarization. IPSP barrages are transiently abolished and show diminished amplitude and duration after return. During the later stages of the response to TRH, rhythmic IPSPs appear that are separate from spindle waves. B, Expansion of trace in A shows an intracellularly recorded spindle wave. C, Examples of rhythmic IPSPs occurring during the late stage of TRH action. D, Recovery of spindle waves. E, The application of TRH caused a small increase in apparent input resistance. TRH also enhances the slow afterdepolarization after a rebound low-threshold Ca²⁺ spike. (Action potential traces in A-E have been truncated for clarity.) F, Example of rhythmic IPSP barrages arriving at ∼2 Hz in a thalamocortical cell in response to TRH. Under control circumstances, IPSP barrages are rare. G, Expansion of one compound IPSP illustrating that it is composed of IPSPs arriving at ∼500 Hz, but in which the frequency slows down during the barrage.

Figure 6.

TRH enhances the slow afterdepolarization after Ca²⁺ spikes; Sr²⁺ abolishes this effect. Rebound bursts elicited by hyperpolarizing current pulses are followed by an ADP in 2 mm Ca²⁺. The amplitude of the current pulses was set to produce similar amplitudes of hyperpolarization in the absence and presence of TRH. Local application of TRH (5 μm in micropipette) results in an enhancement of the ADP (left trace). Substituting Ca²⁺ in the recording solution with Sr²⁺ blocks the slow ADP and prevents the TRH-induced augmentation (middle trace). When normal Ca²⁺ concentration is restored (Sr²⁺ washout; right trace), the ADP returns, and it is again enhanced by TRH. (Action potential traces have been truncated for clarity.)

Figure 7.

TRH depolarizes PGN neurons, shifting bursting discharge to tonic firing. A, Intracellular recording from a PGN neuron during local application of TRH. Spindle wave burst discharges are elicited in response to EPSP barrages arriving rhythmically before TRH. Local application of TRH (1 μm in micropipette) results in an ∼15 mV depolarization, causing sudden termination of spindle waves and the replacement of bursts by tonic discharge. As the effect or TRH declines, sudden hyperpolarizations appear that are followed by rhythmic burst discharge and tonic activity. This “hyperpolarization-rhythmic bursting-tonic activity” pattern recurs several times until the cell finally hyperpolarizes and resumes the normal spindle-wave pattern. B-D, Expansion of portions of A. Note intracellularly recorded spontaneous spindle wave with burst superimposed on EPSP barrages (B), TRH-induced tonic firing (12 Hz) (C), and tonic afterdischarge preceded by bursts (D). E, TRH causes an increase in apparent input resistance in PGN neurons, as demonstrated by an increased voltage deflection in response to hyperpolarizing current pulses and after compensation for the change in membrane potential with the intracellular injection of d.c.

Figure 8.

TRH induces an inward current with a reversal potential near E_K. A, Current versus voltage plot before and after the local application of TRH. B, Subtraction of the TRH plot from control reveals the TRH-induced current. TRH induces an inward current that exhibits a projected reversal potential of -115 mV, which is near E_K.