Histamine depolarizes cholinergic interneurones in the rat striatum via a H(1)-receptor mediated action

Abstract

1. Whole-cell patch clamp recordings were made from rat striatal cholinergic interneurones in slices of brain tissue in vitro. Bath application of histamine (EC(50) 6.3 microM) was found to rapidly and reversibly depolarize these neurones through the induction of an inward current at -60 mV. 2. The effects of histamine were mimicked by the H(1) receptor agonist 2-thiazolylethylamine (50 microM) and selectively inhibited by pre-incubation with the H(1) receptor antagonist triprolidine (1 microM). 3. Ion substitution experiments under voltage clamp conditions revealed that the histamine activated current was comprised of two components. One component was sensitive to the concentration of extracellular Na(+), whilst the other component was inhibited by intracellular Cs(+) or extracellular Ba(2+). 4. In situ hybridization experiments revealed that the majority of cholinergic interneurones in the rat striatum express the histamine H(1) receptor but few neurones express H(2) receptors. These findings were confirmed using single cell RT - PCR. 5. It is concluded that histamine depolarizes cholinergic interneurones in the rat striatum via a H(1)-receptor mediated mechanism.

Figure 1

Histamine receptor agonists depolarize cholinergic interneurones. (a) Continuous whole-cell current-clamp recording illustrating the depolarizing effect of 10 μM histamine on resting membrane potential. This depolarization was maintained after addition of 1 μM TTX and ionotropic glutamate antagonists to the bath. (b) The selective H₂ antagonist ranitidine (100 μM) and the potent H₃ receptor antagonist clobenpropit (100 nM) did not affect the histamine evoked depolarization, however the H₁ selective antagonist triprolidine (1 μM) was seen to block the response. (c) The effects of histamine could be mimicked by the H₁ selective agonist 2-thiazolylethylamine.

Figure 2

The histamine evoked current is concentration-dependent. (a) The effect of varying concentrations of histamine, applied at 40 min intervals, on the magnitude of the inward current at −60 mV. The large deflections are current responses to depolarizing voltage ramps. (b) Concentration-response curve for histamine. All points are means of three or four separate experiments. Vertical lines indicate s.e.mean. Where no lines are apparent, the error was within the size of the symbol.

Figure 3

The histamine-activated current is sensitive to extracellular [Na⁺]. (a) Ten minutes after replacing all the extracellular NaCl with Tris-HCl the response to histamine at −60 mV was diminished. Thirty minutes after returning to normal physiological saline, the response to histamine had returned. (b) Control response to 10 μM histamine after blockade of potassium channels by addition of 120 mM Cs⁺ to the electrode solution. Ten minutes after replacing all the extracellular NaCl with Tris-HCl the response to histamine was abolished in a reversible manner. (c) After complete replacement of external NaCl with Tris-HCl, voltage ramps were recorded before and after administration of 10 μM histamine. Under these conditions the histamine-activated current was seen to reverse at approximately −95 mV which is close to E_K. (d) After addition of 2 mM Ba²⁺ to normal physiological saline, voltage ramps were recorded before and after administration of 10 μM histamine. In this example extrapolation of the current traces using linear regression revealed that the histamine-activated current reversed towards 0 mV.

Figure 4

Expression of histamine receptor mRNA in striatal cholinergic interneurones. (a) The expression of H₁ histamine receptors was relatively widespread amongst cholinergic interneurones (white arrowhead) and is visible as black grains colocalized with large interneurones. In contrast, hybridization against H₂ receptor mRNAs was relatively scarce in cholinergic interneurones (white arrowheads). (b) Single cell RT–PCR analysis of histamine receptor expression in a cholinergic interneurone using primers specific for ChAT, H₁ and H₂ receptors. Lower panel shows a negative control in which the electrode was placed near the neurone but no cytosol was harvested.